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A111D3 fiia03E

NIST Special Publication 839

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Technical Digest

Symposium on Optical Fiber

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^Some elements at Boulder, CO 80303.
Technical Digest -Symposium on Optical^^^

Fiber l\/leasurements, 1992

Digest of a symposium sponsored by the

National Institute of Standards and Technology

in cooperation with the

IEEE Lasers and Electro-Optics Society

and the Optical Society of America

September 15-17, 1992

National Institute of Standards and Technology

Boulder, Colorado 80303-3328

Edited by

G. W. Day

D. L. Franzen

September 1992

U.S. DEPARTMENT OF COMMERCE, Barbara Hackman Franklin, Secretary

TECHNOLOGY ADMINISTRATION, Robert M. White, Under Secretary for Technology

NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY, John W. Lyons, Director
National Institute of Standards and Technology Special Publication 839

Natl. Inst. Stand. Technol. Spec. Publ. 839, 245 pages (Sept. 1992)

CODEN: NSPUE2

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1992

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, DC 20402-9325
PREFACE

The program for this seventh Symposium on Optical Fiber Measurements has been expanded to two

and a half days in order to include a total of 54 papers, 11 invited and 43 contributed.

The invited papers were selected from 43 that were nominated and reviewed by the committee. The

contributed papers were selected by the review of the committee from among the 56 submitted.

OTDR technology is an important topic of this Symposium, as it has been in most of the previous

ones. Measurements of fiber geometry, which reappeared in 1990 as important topic, continue to

be a significant issue. There is increased emphasis on the characterization of optical fiber amplifiers

and the measurement of polarization effects. Areas of emerging interest include nonlinear effects

in fibers and problems associated with field measurements in systems containing amplifiers and other

advanced components.

The Symposium continues to have a strong international character. Eleven countries are represented

in the program and more than half of the papers come from outside of the United States.

G.W. Day

D.L. Franzen

Boulder, Colorado

September 1992

Except where attributed to NIST authors, the content of individual sections of this volume has not been reviewed or edited by the National Institute of Standards and Technology. NIST therefore accepts no responsibility for comments or recommendations therein. The mention of trade names in this volume is in no sense an endorsement or recommendation of the National Institute of Standards and

Technology.

iii
SYMPOSIUM COMMITTEE

D.L. Franzen, NIST, General Chair

G.W. Day, NIST Program Chair

W.T. Anderson, Bellcore

C. Boisrobert, CNET

N. Channon, York

S. Furukawa, NTT

W.B. Gardner, AT&T Bell Laboratories

R.K. Hickernell, NIST

W.T. Kane, Corning

P.S. Lovely, Photon Kinetics

S.A. Newton, Hewlett-Packard

S. Pollitt, NPL

S.B. Poole, University of Sydney

W.A. Reed, AT&T Bell Laboratories

P.R. Reitz, AMP

C. Saravanos, Northern Telecom

G. Suchoski, United Technologies Photonics

iv
CONTENTS

PREFACE

SYMPOSIUM COMMITTEE

Testing strategies for modern fibre network architectures (invited)

Nick Lewis, Peter Keeble, David Ferguson, BT Laboratories 1

OTDR measurements through optical splitters

Felix P. Kapron, James D. Berardinelli, Bellcore 7

A 1.6-/im-band OTDR using a Raman fiber laser pumped by a Q-switched Er'^-doped

fiber ring laser

T. Horiguchi, T. Sato, Y. Koyamada, NTT-Ibaraki 11

The effect of OTDR measurement error on fiber uniformity metrics

Jane B. Clayton, Andrew L. Ingles, David A. James, Daryl Pregibon,

AT&T Bell Laboratories 15

An OTDR based combined end-reflection and backscatter measurement

A.F. Judy, AT&T Bell Laboratories 19

Factory measurement solutions to predictable field performance of singlemode fiber optic connectors (invited)

Bennett Wong, Halina Kosiorska, Northern Telecom 23

Measurements to assure fiber reliability

Hakan H. Yuce, Felix P. Kapron, Bellcore 29

Atomic force microscopy measurements to study optical fiber aging

H.H. Yuce, J.P. Kilmer, J.P. Varachi, Jr., Bellcore 33

Fiber microbending test method capable of predicting microbend loss in optical cables

Z. Pasturczyk, C. Saravanos, Northern Telecom 37

Measuring method for ribbon fiber skew

Ryuichi Matsuoka, Hiroshi Satoh, Nobukazu Kume, Furukawa Electric 41

Review of recent developments in fibre geometry measurements (invited)

J. Baines, K. Raine, NPL 45

Fiber cladding diameter measurement by white light interference microscopy

P.D. Hale, D.L. Franzen, NIST 51

Accurate measurements of fiber cladding diameter

Matt Young, Paul D. Hale, Steven E. Mechels, NIST 55

Measurement of fibre coating geometry by grey-scale analysis

Andrew G. Hallam, Julia H. Shaw, York Technology 59

V
Coating thickness unevenness measurement in fiber drawing process by using

reflected light detecting method

Akira Inoue, Shuji Shinoki, Yuji Kobayashi, Yasuji Hattori,

Sumitomo Electric Industries 63

Development of in-line coating thickness monitor for hermetic carbon coated fiber

Yoshinori Ishida, Yukio Kohmura, Shin-ich Aral, Furukawa Electric 67

Measurement of optical nonlinearity of transmission fibers (invited)

R.H. Stolen, W.A. Reed, K.S. Kim, K.W. Quoi, AT&T Bell Laboratories 71

Determination of non-linear effective areas for dispersion shifted fibres

J.V. Wright, E.S.R. Sikora, BT Laboratories 77

Beam current effects in cathodoluminescence spectroscopy

Graham R. Atkins, Mark G. Sceats, Simon B. Poole, University of Sydney 81

Relative dopant concentration profiling of germania, phosphorus and erbium doped

silica based optical fibres

J.M. Benson, NPL, C.J. Chunnilall, King's College London, J.C. Petersen,

Danish Institute of Fundamental Metrology 85

Design and characterization of dispersion compensating fiber (invited)

A.J. Antos, Corning 89

Measurement of group effective index of optical fibers and integrated waveguides

by the wavelength scan technique

S. Morasca, F. Pozzi, C. DeBernardi, CSELT . 95

Precision measurement of single-mode fiber group index

W.B. Gardner, A.D. Parker, AT&T 99

The direct measurement of highly absorbing bands in optical fibers

Robert M. Atkins, AT&T Bell Laboratories 103

Determination of stress optic effect in optical fibers

A.J. Barlow, EG&G Fiber Optics, W.H. Ficke, AT&T Bell Laboratories,

T. Voots, Photon Kinetics 107

Characterization of integrated optical components (invited)

P.G. Suchoski, Jr., United Technologies Photonics Ill

Real time measurement of mode partition noise in high speed data modulated laser

diodes, in presence of reflections

M. Puleo, P. Gambini, E. Vezzoni, CSELT 115

A complete measurement set for high performance optical connectors

M. Bottanelli, L. Duca, S. Pitassi, SIRTI 119

vi
A simple and accurate heterodyne technique for the characterization of active

and passive narrowband optical filters

N. Caponio, P. Gambini, R. Hui, M. Puleo, E. Vezzoni, CSELT 123

An automated method for measuring the radius of curvature of an optical fiber

A.L. Ingles, AT&T Network Systems 127

Recent progress in polarization measurement techniques (invited)

Brian Heffner, Hewlett Packard 131

Extinction ratio measurements of polarisation maintaining fibre

Y. Gu, L.J. Poyntz-Wright, York Fibers 137

Accurate measurement of polarization dependent loss over wavelength of single mode

optical fiber components

L.F. Stokes, Hewlett Packard 141

Polarization mode dispersion measurements in optical fibers (invited)

Yoshinori Namihira, Jun Maeda, KDD 145

Interferometric loop method for polarization dispersion measurements

Luc Th^venaz, Marc Nikl^s, Philippe Robert, Swiss Federal Institute

of Technology 151

Four interferometric methods for the determination of the birefringence and

polarization dispersion of polarization maintaining optical fibers

Ch. Chojetzki, Kabelmetal-electro, H. Schmitzer, Universitat Regensburg,

J. Vobian, W. Dultz, DBP Telekom 155

Low coherence reflectometry for the characterization of fiber and planar lightwave circuits

(invited)

Kazumasa Takada, NTT 159

Improved sensitivity for optical low-coherence reflectometry using reference power

attenuation

W.V. Sorin, D.M. Baney, Hewlett-Packard 163

Optical low coherence reflectometry: improving reflectivity accuracy in the presence

of chromatic dispersion

Harry Chou, Hewlett-Packard 167

Standardisation in fibre optic communications: Status and perspectives (invited)

P. DiVita, M. Artiglia, A. Cavaciuti, M. Potenza, A. Rossaro, CSELT 171

COST 217 intercomparison of OTDR measurements on single-mode fibres

M. Artiglia, COST 217 Group 179

COST 217 intercomparison of measurements on Er-doped fibres

P. Kiiveri, COST 217 Group 183

vii
Single-mode fiber geometry and chromatic dispersion: results of interlaboratory comparisons

Timothy J. Drapela, Douglas L. Franzen, Matt Young, NIST 187

Moderate-accuracy wavelength standards for optical communications

S.L. Gilbert, T.J. Drapela, D.L. Franzen, NIST 191

Characterization of fiber-optic amplifiers (invited)

Douglas W. Hall, Corning 195

Chromatic dispersion of erbium-doped fiber amplifier measured by Fourier transform

spectroscopy

Robert K. Hickernell, Kazumasa Takada, Makoto Yamada, Makoto Shimizu,

Masaharu Horiguchi, NTT 201

Noise measurements of erbium-doped optical fibre amplifiers: influence of source

excess noise

M. Artiglia, CSELT, P. Bonanni, SIRTI, A. Cavaciuti, M. Potenza, M. Puleo,

B. Sordo, CSELT 205

Accuracy of noise figure measurement for erbium-doped fiber amplifiers by the

optical method

T. Kashiwada, M. Shigematsu, M. Nishimura, Sumitomo Electric Industries .... 209

Automated measurement of gain characteristics of optical fiber amplifiers

Douglas R. Cole, Miles E. Vance, Corning 213

Cutoff wavelength measurement precision improvement by curve fitting

T.A. Hanson, K.A. Emig, Corning, C.S. Brown, AT&T Bell Laboratories,

R.J. Smith, AT&T Network Cable Systems 217

Determination of the LPn mode cutoff wavelength from the modal interference pattern

K. Abe, Y. Lacroix, Y. Cai, L. Bonnell, National Optics Institute 221

Length and curvature dependence of the effective cutoff wavelength in single-mode

fibers measured with an azimuthal filtering technique

D. Pagnoux, J.M. Blondy, Institut de Recherche en Communications Optiques et Microondes 225

Mode field diameter and cutoff wavelength profile measurement using dual wavelength OTDR

James Warder, Costas Saravanos, Northern Telecom 229

Far field MFD measurements from a reduced far field pattern

A. Chiantore, F. Cocchini, D. Cuomo, G. Ferri, Fibre Ottiche Sud 233

AUTHOR INDEX 237

viii
TESTING STRATEGIES FOR MODERN

FIBRE NETWORK ARCHITECTURES

Nick Lewis, Peter Keeble, David Ferguson

BT Laboratories

INTRODUCTION

Fibre networks have been used by telecom operators for many years. Up to the present, most fibre

based systems have been confined to the core network (communication between central offices).

These systems used point to point transmission

equipment. Recently, moves have been made to

introduce fibre into the access network (central

office to customer). These started with the provision to large customers using essentially core network systems. Provision to smaller customers has

required the use of different architectures such as passive optical networks [1]. In this paper we look

at the testing strategies for these new fibre network

architectures.

THE REASONS FOR TESTING

Testing is performed to ensure that a network

bearer can support a transmission system. If the

fibre network is found to be incapable ofsupporting

the transmission network then the network elements are individually tested to discover which of

the elements is not performing to the required

standard.

The amount of testing that is required in order

to satisfy the afore mentioned objective depends on

several factors. Firstly, the reliability of con:q)onents can vary in their susceptibility to their environment and the speed at which they naturally age.

This affects how often and in how much detail

testing has to be carried out. Secondly, networks are subjected to environmental stresses such as temperature changes, moisture and vibration. They

have different amounts of physical protection depending on whether they are ducted, aerial, deep

buried orin an area ofbuilding activity. Thirdly, the

customer is guaranteed a particular quality of service quoted as mean time between failure, out of

service time or average time to repair a fault. The

level of service depends on the speed of repair and

the amount of pre-emption of faults. Finally, a network may be shared by many users so that a network fault causes several service faults. All

these factors affect the amount of testing and the

type of testing that is required in order to instal and

maintain a fibre network.

TEST REQUIREMENTS DURING

INSTALLATION AND MAINTENANCE

Before addressing the testing strategy for networks,

the phases in the network life cycle must identified

in which testing is used or where it would be

beneficial if it were initiated. The network in this

context is defined as the fibre bearer that supports

transmission systems (i.e., the part of a transmission link where the traffic is in the optical domain).

There are several phases in the life of a network in

which testing is an important activity:

• Tests are made on a network that has just been

installed in order to commission it The network is

treated as a black box and commissioning entails

end to end tests of attenuation at one of more

wavelengths. It may also entail tests of fresnel

reflections. If the network fails the commissioning

tests then the fault and its location are found using

fault finding techniques.

• In the situation where a fault has been cured by

replacing or repairing a component then the individual component is commissioned. It may also be

necessary to perform end to end tests since the

distribution of performance of components may

1
well be defined statistically (depending on the

procurement strategy) and it is possible for a network containing only "in-spec" components to be

out of specification from end to end

• Some networks can be physically extended some

time after they have been initially installed. These

networks are already carrying hve traffic so they

can not be tumedoffwhen commissioning the new

parts of the network.

• Faultfindingisperformedwhenafaultisreported

by a customer, the transmission system that uses the

network or a network monitoring system. Fault

finding can also be performed after a network has

failed its commissioning. The purpose of fault

finding is to locate the component of the network

that is preventing the overall networkfrom meeting

the required specification.

• Network monitoring is not presentiy performed

on the bearer network. It has widely been appUed in

the transmission systems that use the bearer. The

advantage of using it in the bearer is that it may

detect faults before they become service affecting

(depending on the nature ofthe fault). Theycan also

give more information on where a fault is located

and why it has occurred.

Any test strategy must address the requirements of

a network in all ofthe phases described above. This

paper looks at test strategies for both present day

fibre optic networks and new types of fibre optic

network that are about to be deployed. In particular

it looks at some of the tools that have been developed in response to the demands of the new types

of network.

TEST STRATEGY FOR PRESErfT-DAY

NETWORKS

Fibre is widely usedin the core networkandto serve

larger business sites in the access network. These

are almost exclusively point to point networks. The

test requirements for point to point networks have

been studied over many years and are now well

understood. They do, however, act as a starting

point from which a test strategy for future network

archetectures can be developed.

Commissioning

Testing oftraditional point topoint networks can be

carried out using a light-source and a powermeter.

This tests only the loss parameter at the system

wavelength from one end of the network to the

other.

Duplex transmission systems — those that

transmit traffic in both directions on one fibre at one

wavelength — are sensitive to fresnel reflections

but in point to point networks the amplitudes of

fresnels are not significant in comparison to the

signal ampUtude. However, if a check is required

then it can beperformedwith aconventional OTDR.

Testing of the wavelength characteristics may

be required where a network bears systems at many

wavelenths. WDMs are only used in point to point

systems if the transmission system is a diplex

system (e.g., 1300nm TX, 1550nm RX). In this case the WDMsare usually incorporated in TX/RX

modules in the transmission equipment and not in

the network itself. If however tiie WDMis located

in the network then its wavelength parameters can

be tested using Ught sources at different wavelengths.

Fault Finding

The standard test tool forfault finding is the OTDR.

This tool has enough resolution to measure backscatter over even the longest fibre. Point losses

micro-bending losses and fresnel reflections are all easy to detect and locate with the OTDR. Some

OTDRs are now portable enough to be carried in

the palm of the hand. Some contain software that

detects the more obvious features automatically.

Monitoring

A network is always monitored but usually the

monitoring is performed indirectiy. The transmission system that uses the network as a bearer will

fail when serious network fault arises. If a fault can

be proved off the transmission equipment then the

fault must be in the fibre network. This is the

2
monitoring technique used by most present day

networks.

The disadvantage of this technique is that the

service using the network has been affected before

a fault is identified. With direct network monitoring, faults can be found before they become service

affecting. (In the UK, copper pairs are monitored

every night to see if certain parameters are within

limits). One monitoring technique that is sometimes used to test the loss of a network is optical

level monitoring. This monitors any changes in the

automatic gain control of a transmission system. A

level that falls slowly indicates a degrading fault

The problem can be identified before it affects the

bit error rate of services.

TEST STRATEGY FOR BRANCHED

NETWORKS

Conventional point to point fibre networks have

been used very successfully in the core network

wherethe highdemandsforbandwidth have ideally

suited fibre. Now fibre is being employed in the access network where the network requirements

are different Instead of the network nodes being

central offices they are individual customers who

have a relatively small but somewhat volatile demand for bandwidth.

Passive Optical Networks (PONs) have a

multipoint topologythat is ideally suited to this type

of bandwidth demand [2-5]. PONs use splitters in

order to share one network with many users. Power

launched at the head-end is divided out by the

splitters and only a small portion reaches each

remote-end. Similarly only a fraction of the power

launched from a remote end reaches the head-end — the splitters are symmetrical. The high loss

makes the transmission systems that operate over

the network vulnerable to any imperfections so

testing is more important for PONs than it is for

point to point networks. Testing is more important

but it is also more difficult because firstly the end to

end network losses are too great for some conventional test tools. Secondly, due to their multipoint

topology, PONs require testing to be performed on

them whilst live. Thirdly, any test tools used at the

head-end perceive all the paths ofthe network to be

superimposed and finally the short distances involved in the UK access network mean high resolution requirements.

Commissioning

PONsrequire commissioning tests to beperformed

at three phases of a network's life. Like point to

point networks, a PON requires commissioning

after it has been installed to ensure that it can

support the transmission systems that will use it, and after a faulty component has been replaced

[6,7]. Unlike point to point networks, a PON is not

necessarily installed in one go but may be extended

at a later date. This network extension must also be

commissioned before it can be used.

PONs can be commissioning after the network

is first installed by testing the end to end losses

between the head end and each remote end using a powermeterandlight-source in much the sameway

asforapointtopointnetwork. (Fig.l shows the link

over a PON between the head-end and a remoteend.) However, commissioning after a network

repair or a network extension must be performed

while the PONis live. The commissioning must not

interfere with the traffic on the network normust the

traffic interfere with the commissioning tests. Tocommissionanetwoik without affecting the

transmission system a separate wavelength known

as the "maintenance band" can be used. End to end

loss measurements can be performed at this wavelength (e.g., 1575nm) while traffic continues una- bated at the system wavelength (e.g., 1300) to and

from other users on the established parts of the

network. The optical receivers in the transmission

system have to be fitted with filters so that the light source at the test wavelength does not affect them.

Fault Finding

OTDRs are the best tools for fault finding on point

to point networks. Unfortunately conventional

OTDRs are unsuited to the fault finding of PONs.

PONs exhibit very high end to end losses so most

OTDRs can not see beyond a few splitters. Also

PONs are multipoint networks so OTDRs con- nected at the head-end see only an amalgam of all

the superimposed paths. Therefore, PONs require

3
other fault finding methods to be used. One way to

find a fault on any network is to directly measure a

power loss across it A light source can be connected at the head-end ofthe network and tests can

be made at various points in the network until a

discrepancy is found. This testing method requires

two tools; a light-source and a powermeter.

In duplex and diplex systems there is continuous

downstreampoweron all fibres so the transmission

wavelength could be used as the light-source. In

situations where the network is carrying simplex

transmission traffic a separate test light-source can

be connected at the head-end. It must be remembered, however, thatPONs are multipointnetworks

and some parts will be unaffected by a fault It is not

acceptable for the transmission system to be tumed

offin order to find afault Alight sourcecan be used

whilst the network is operating if it uses the maintenance wavelength. In this case amaintenanceport

that contains a connector and filter must be provided. (The filter prevents test equipment with the

wrong wavelength from affecting the transmission

system if connected in error.)

A conventional powermeter can not be used because it requires the network to be disassembled in

order to make ameasurement Thisis timeconsuming and halts the traffic flow. In order that the traffic

is not intermpted by the measurement process a

non-intrusive chp-on powermeter can be used [8].

The non-intrusive cUp-on powermeter is a handheld device that measures the power in a fibre

without affecting the traffic.

If the network fault is severe enough to cause

service faults then the location of these service

faults can implicate the location of the network

fault If the location of a fault can be narrowed to a

length offibre then an OTDR test can be used. The

clip-on launch technique used with a standard

OTDR enables a test to be performed without

breaking into the network. The clip-on launch

adaptor consists of an optical amplifier and a clipon launcher. It can achieve a performance approachingthatofadirecdyconnectedOTDRwhilst

allowing uninterrupted traffic.

All chp-on techniques have the disadvantage

that the fibre is manhandled fix)m the splice tray.

This can induce transitory micro-bending events

that interrupt the Uve traffic. In order to remove this

problem a new type ofspUce tray is being evaluated

within BT. This spUce tray contains several windows. Clip-on test tools with attachments that en- gage with these spUce tray windows enable a test to

be made without moving the spUce tray or removing the fibre from it (see Fig.2).

The use of end-on OTDRs to fault find on PONs is

not entirely excluded. The problems created by the

multipoint nature of PONs can be overcome by

connecting an end-on OTDR only at the remoteends of the network. In this way a unique path is

viewable. Theproblems created bythe high end-toend loss of PONs can be overcome if OTDRs

launch more power into the network. The launch

power can be increased by using an optical amplifier [5] . This enables the OTDRto resolve the backscatter all tile wayup the PONandtherefore directly

detect all features. (Fig.3 shows a schematic of an

optically amplified OTDR.) In tiiis application of

optical amplifiers all the power is launched into tiie network — the optical amplifier in the chp-on

OTDR was used only to compensate for losses

incurred injecting light into and extracting it fi*om

the network.

Monitoring

Monitoring of a PONrequires a technique that can

distinguish between different patiis of the network

and gather information about the state of each the

paths. However, the monitoring tool can only be

attached to one part ofthe PON. Ifa tool wasplaced

at the a remote-end of a PON then it could only

monitor one path. For the tool to monitor all paths

then the tool must be at the head-end. This rules out

conventional OTDR techniques (even high-power

OTDRs) because they can not distinguish the different paths of the PON.

A technique that can distinguish the paths of a PON is remote-end fiiesnel monitoring. In this

technique equipment monitors the reflections from

the ends of the network. The amplitude of any

4
reflection is a measure of the square of the loss

between the head-end a the remote-end that has the

reflection. The location of a fault can be implied

from which end reflections have been affected by a

change in amplitude. The reflections from the ends

ofthe networks are produced by the opto-electronic

components or filters in the transmission system

which must be specified accordingly. The OTDRs

are high cost so are shared over many networks.

They are connected to these networks via an optical

switch (see Fig.4). Pulse OTDR techniques could

be used but since the amplitudes of only specific

points are monitored, it is an ideal appUcation for a

correlating OTDR using a PRBS signal. Note that

in order to monitor simplex networks non-intrusively, these systems use their own optical sources

in the maintenance band.

TRENDS IN TEST TOOLS

It can be seen from the previous discussion that as new types of fibre network have been developed

and used by telecom operators, new types of test

tools have had to be developed.

Conventional point to point fibre networks

could be commissioned with a Hght source and

powermeter, faults could be found with an OTDR

and fibre monitoring was largely unnecessary. Second generation fibre networks present obstacles to

the use of some of these tools and require tests that

can not be achieved using others.

PONs present three obstacles to conventional

test methods; the need for tests to be performed on

tiiem whilst live, their high end to end powerlosses

and the multiple paths from the head-end. The first

obstacle can be overcome by using a dedicated

maintenance band The other two by using a highpower end-on OTDR or by performing localised

tests with a Clip-on OTDR or a Clip-on Powermeter.

The demand for Monitoring is developing at

the same time as developments are being made in

the network technology. Monitoring can improve

the reliabihty of fibre networks by making the

operator aware of degrading faults before they

become service affecting. Monitoring tools can be

shared over a number of networks using optical

switches. PONs can be monitored by analysising

the fresnel reflections that occur or are produced at

the. ends of PONs.

Test Tools for Future Networks

The architectures of fibre networks are still developing. The most sigruficant new element that will

be used in future networks is the optical amplifier.

This may be incorporated in a spUtter to provide

loss-less sputters or it may be spread over the entire

network. Other elements that may be used in future

fibre networks include comb WDMs modulation

dependent passive routers, ampUtude dependent

passive routers. Networks that are built around

these elements will all require new testing strate- gies and test tools.

CONCLUSION

As modem networks become more complex they

require more testing than previous ones and that

testing is more difficult to perform. It is anticipated

that in future a greater proportion of the cost of

networks will be spent on test tools. On the other

hand the time that is required formanual testing will

be gready reduced so the whole life cost of the

testing strategy will not be increased. The greater

automation and monitoring in future strategies will

increase perceived reliability and help fibre access systems to out-perform copper access systems in

terms of both price and quahty.

REFERENCES

1. Homung S, et al, 1990 "Single-Mode Optical

Fibre Networks to tiie Home", ICC'90.

2. Stem J R, et al, 1987 "Passive Optical Local

Networks for Telephony Applications and Beyond", Electronics Letters.

3. Oakley K A, et al, 1988 "Passive Fibre Local

Loop for Telephony with Broadband Upgrade"

Conference Proceedings ISSL/S, 179-183.

4. Hoppitt C E, et al, 1991 "Operations and Maintenance Experience of tiie Bishops Stortford Fibre

Trial", 3rd Conference on Telecommunications.

5. Hoppitt C E, et al, 1991 "Operations and Maintenance in tiie Bishop's Stortford Fibre Trial",

ISSL/S '91.

5
6. Finegan T, Hicks A M, 1990 "Prediction for the

optical performance of a novel passive branched

fibre network", IKH colloquium, London, March

1990.

7. Keeble P J, et al, 1991 "Optical performance

specification and measurement of local loop passive optical networks", ICC'91.

8. James S M, et al, 1989 " "Clip-on" —A Possible

Measurement Technique for Future Optical Networks" IWCS'89, 94-97.

9. Keeble P J, 1990 "Optical time domain

reflectometry performance enhancement using er- bium doped fibre amplifiers", OE/Fibres'90, September 1990.

FIGURES

Hsad-snd Cabte

Etectronic* FtexIbllHy ch«mb»r

Frama

Flexibility |_P

^^mb.r FlbraDtotrlbutlon cnambar "r"°**?!r'

p^j^^ Elactronlcs

Fig.l —PON Topology

OTDR

OTDR

pulse sync

. Coupler Acoustooptic switch TJ—

D-BH> B >-a—

'^^^

Lithium niobate switch

Optical Rbre

Isolator amplifier

Optical Fibre

Isolator amplifier

Fibre under

test

Timing and

driver circuitry

Fig.3 —High power OTDR Schematic

Detector Optical Switch

Fig.4 —Routing OTDR Schematic

Fig.2 —Clip-on Powermeter and Windowed

SpUce Trays.

6
OTDR Measurements Through Optical Splitters

Felix P. Kapron & James D. Berardinelli

Bellcore

445 South St., MRE-2K140

Morristown, NJ 07962-1910

201/829-5225, FAX 201/829-5965

1. Introduction

In an optical distribution system, a 1-by-^ optical splitter can passively route an optical signal from

one transmitter to N branch receivers. If there is a fault in a branch, can an OTDR be used at the splitter

input to measure fault loss and reflectance? Previous investigations concerned fiber breaks [1] and

utilized multi-wavelength OTDRs on splitters with embedded filters [2]. This paper is the first systematic

measurement utilizing a standard OTDR for loss and reflectance of a branch fault and conventional

splitter.

2. Splitter Properties Measured by an OTDR

2.1 Fiber Bacl<scatter

Consider an OTDR pulse of optical power P (dBm) launched into the splitter input branch at point 1

in Fig. 1. The power backscattered at any point of a fiber depends upon the pulse duration D and is

Bj) = B + mogioD (1)

down from the forward signal [3]. Normalized to a 1 ns pulse duration, the single-mode fiber backscatter

coefficient B is about -80 dB around 1310 nm, and about -82 dB around 1550 nm [3]. In Fig. 2, the

OTDR reading for point 1 is Pi=V2{P+Bn) . (2)

(The OTDR reading halves the dB power, and for relative power we ignore any additive constant.)

2.2 Splitter Losses

As it passes through the splitter, the pulse power suffers an excess loss E (positive dB). Then the

power divides among the N branches so that the level entering any one branch at point 2 just after the

splitter has been reduced by the intrinsic splitting loss

5 = -101ogio^ = 31og2^ . (3) N

The insertion loss of the splitter between points 1 & 2 is the sum

L,=E+S . (4)

At point 2, power (P -L^ +5/)) backscattered from each fiber re-enters the splitter, and experiences

the excess and splitting losses again. However, the latter is canceled out by all branch fibers now

contributing to total backscatter, so the effective recombination (inverse splitting) loss is zero. Thus the

OTDR reading for point 2 is P2-V2{P-L,+Bd-E) . (5)

The apparent insertion loss L„s is the OTDR reading drop from point 1 to point 2 in Fig. 2: Las=Px-P2=E^V2S . (6)

We have used Eqs. (2), (5), & (4). From Eqs. (6) & (4) one has the calculated insertion loss

L,=L,, + |log2iV . (7)

7
10

(9)

2.3 Splitter Reflection

If i?j is the splitter reflectance (negative dB), then at point 5 at the peak of the splitter reflection trace

in Fig. 2, the fiber-backscattered linear power corresponding to 2Pi of Eq. (2) adds to the splitterreflected linear power corresponding to (P +Rs)- The OTDR reading is then

P, = 51ogio 10 5 +10 10 , (8)

The pulse height above backscatter is

//,=P,-Pi=51ogio 1 + 10

using Eqs. (8) & (2). Along with Eq. (1) this gives the calculated splitter reflectance

rr — 1

'

7?, =B + 101ogio 10 ^ -1 D . (10)

This is the standard formula [3] for a localized reflectance, shown here to be valid for a splitter.

3. Branch Fault Properties Measured by an OTDR

3.1 Branch Fault Loss

Call A dB the fiber attenuation between point 2 and point 3 just before the fault, and Lf dB the fault

loss between point 3 and point 4 just after the fault. Power budgeting as above shows that the OTDR

reading of Fig. 2 for one branch at point 4 is P4=Pi-L,-A-Lf . (11)

To this must be added the signals from the - 1 branches with no faults. Using Eq. (11), the total OTDR

signal just after the fault is

P4, =P4 + 51og10 1+(N- 1)10 (12)

The total signal from just before the fault is P^t, i-e. P^t with Lf = 0. The apparent fault loss detected by

the OTDR is the drop

A,/ = -P3/ -^4/ = 51ogio

- (TV - 1) + 10 ^

(For a break one has = 00.) Solving leaves the calculated fault loss

N

(13)

' .hi.

'

Ly=-51ogio N 10 ^ -1 + 1 (14)

3.2 Branch Fault Reflectance

At point 3 the backward power '\s {P - Ls - A + Rf) due to fault reflectance Rj and (P - - A + B^)

due to fiber backscatter. As before, tracing the linear powers back to the OTDR gives the reading

P/ = P2 -A +51ogio

The height of the fault reflection is

1 + 10 10

Hf = Pf-P2,t .

Using Eqs. (16), (15), (2), & (8), the calculated fault reflectance is found to be

(15)

(16)

8
Rf = S +B + lOlogio 10 ^ - 1 D

4. Experimental Measurements

4.1 Test Setups

Both l-by-2 and l-by-4 uniform optical splitters were used in separate setups, each similar to Fig. 1. The OTDR pulse spatial width was about 50 m, so all features were resolvable from each other. All

branches were terminated reflectively or non-reflectively, with no differences in results.

4.2 Splitter Properties

In each setup with the splitter in place, the OTDR trace showed that = 0, so there were no apparent

splitter reflections. The concurrent apparent splitter loss was obtained in each case, and Eq. (7) leads

to a calculated insertion loss L^. The actual insertion losses were determined with a loss testset. The

results for the l-by-2 setup are given in Table 1; those for the l-by-4 setup are in Table 4. The calculated

and actual loss values are in agreement to within measurement error bounds for the l-by-2 splitter, but the

correlation is only fair for the l-by-4 splitter.

4.3 Splice Properties

The apparent splice losses L^/ were determined from the setups of Fig. 1, and Eq. (14) resulted in

calculated splice losses Lf. With the splitter removed in each setup, direct single-direction OTDR

measurements were made on the splices of the faulty branches. These showed larger features compared to those in the above traces. Again, losses summarized in Table 2 for the l-by-2 splitter agreed within error bounds, but the l-by-4 results in Table 5 were not as favorable.

The apparent reflection peaks //y with the splitter and the actual peaks without the splitter are given in

Tables 3a (l-by-2) & 6a (l-by-4). These were used in Eq. (17) to obtain the calculated and actual fault

reflectances given in Tables 3b (l-by-2) & 6b (l-by-4). Agreement is good in both cases.

5. Conclusions

We have given a procedure by which an OTDR can be used at the input port of a 1-by-A'^ uniform

splitter to measure the losses and reflectances of the splitter and of any faults in the output branches. A

theory outline was followed by experiments with l-by-2 and 1-by 4 splitters. With higher split ratios,

faults are expected to become more difficult to measure accurately.

6. References

[1] I. Sankawa et al., IEEE Photon. Technol Lett., vol. 2, no. 10, pp.766-768 (Oct. 1990)

[2] H. Yanagawa et al., Tech. Digest OFC'92, paper FA4, p. 263 (Feb. 1992)

[3] F. Kapron, et al., lEEEIOSA J. Lightwave Technol., vol. 7, no. 6, pp. 1234-1241 (Aug. 1989)

Dead-Zone Fiber (D2F)

OTDR

Figure 1. Schematic of a splitter connected to an OTDR,

defining several points at which power levels are referenced.

9
DZF fiber

fault

—I

distance

Figure 2. Schematic of an OTDR trace, with the reference points corresponding to Figure 1.

(In practice, points extrapolated backward

by the OTDR to account for pulse width.)

Table 1: 1x2 Splitter Loss (to ±0.2 dB) Table 2: 1x2 Splice Loss (to ±0.05 dB)

Apparent Calculated Actual

1.62 3.12 3.50

Table 3a: 1x2 Reflection Peaks (to ±0.1 dB)

Apparent Actual

Splice #1 7.60 8.46

Splice #2 8.20 9.22

Apparent Calculated Actual

Splice #1 0.26 0.56 0.48

Splice #2 0.32 0.69 0.60

Table 3b: 1x2 Reflectances (to ±0.4 dB)

Calculated Actual

Splice #1 -28.0 -29.3

Splice #2 -26.8 -27.2

Table 4: 1x4 Splitter Loss (to ±0.2 dB)

Apparent Calculated Actual

3.18 6.18 7.50

Table 5: 1x4 Splice Loss (to ±0.1 dB)

Apparent Calculated Actual

Splice #1 0.14 0.62 0.46

Splice #2 0.20 0.94 1.52

Table 6a: 1x4 Reflection Peaks (to ±0.1 dB)

Apparent Actual

Splice #1 6.74 8.72

Splice #2 5.16 7.70

Table 6b: 1x4 Reflectances (to ±0.4 dB)

Calculated Actual

Splice #1 -26.9 -28.8

Splice #2 -30.3 -30.9

10
A 1.6-|Lim-band OTDR using a Raman fiber laser pumped by a Qswitched Er3+-doped fiber ring laser

T. Horiguchi, T. Sato, and Y. Koyamada

NTT Telecommunication Field Systems R&D Center

Tokai, Ibaraki-ken, 319-11 Japan

1. Introduction

Optical time domain reflectometers (OTDR) operating at 1.6-|LLm-band are

a very important diagnostic tools for maintaining optical fiber transmission lines.

This is because the 1 .6-|im-band OTDR can find faults caused by fiber bending

with a higher sensitivity than OTDRs whose wavelength is the same as that of

existing optical transmission systems, 1.3 or 1.55 |Lim [1]. In addition, the 1.6-

|xm-band OTDR has the potential to investigate in-service subscriber Hnes of

1.3/1.5 |Lim WDM system in the planned.

One approach to extending the dynamic range of an OTDR is to use fiber

amplifiers or lasers. Of those currently available, the Raman fiber amplifier or

laser is the most promising for the 1 .6-|Ltm-band OTDR [2,3]. A Raman fiber

amplifier or laser requires a pump source at a wavelength that is one Stokes shift

shorter than the required amplification or lasing, but with significantly greater

power. Until recently, however, there was no light source with sufficiently high

power yet small enough to incorporate in a 1.6-|Lim-band OTDR for practical use.

Fortunately, the Er3+-doped fiber amplifier or laser has now been developed

which is compact, operates at around 1.55 |Lim as the pump for a 1 .6-|Lim-band

light signal and produces high peak pulse powers.

In our previous 1.6-iLim-band OTDR [3], a Raman fiber amplifier was used

which was pumped by the output of an Er3+-doped fiber amplifier. The output

peak power of the Er3+-doped fiber amplifier was limited to about 1.5 W, which

made it necessary to use a long Raman fiber of 20 km. In addition, the output

peak power of the Raman fiber amplifier was limited to 0.2 W. As a result, the

dynamic range of the OTDR was 25.7 dB with a relatively poor spatial resolution

of 100 m.

In this paper we report a 1 .6-|j,m-band OTDR with higher performance

which is based on a Raman fiber laser pumped by a Q-switched Er3+-doped fiber

ring laser.

11
2. Experiments

2.1 Q-switched Er^+.doped fiber ring laser and Raman fiber laser

The experimental set-up of the 1.6-|Lim-band OTDR, which includes a Qswitched Er3+-doped fiber ring laser and a Raman fiber laser, is shown in Fig.l.

The Q-switched laser had a ring cavity in which a 10 m Er3+-doped fiber with an

Er3+ concentration of 1800 ppm was pumped at 25 mW by a 1.48 |a,m laser diode

through a 1.48/1.55 |Ltm WDM fiber coupler. A polarization-independent optical

isolator was inserted into the ring cavity to attain unidirectional lasing. Qswitching was performed by a mechanical chopper in the cavity. The trace of the

output pulse of the Q-switched fiber laser is shown in Fig.2(a), indicating a pulse

duration of 170 ns and a peak power of as high as 16 W. The output pulse was

then launched into a Raman fiber as a pump. The Raman fiber was a specially

prepared single-mode fiber, whose Raman gain was enhanced by raising the

relative index difference of the fiber to 1.8 % by doping its core and cladding

with Ge02 [4] and F, respectively. The optimum length of the Raman fiber was

determined as 1.5 km, by observing the fiber length at which the backward

Rayleigh scattering of the stimulated Raman scattering due to the 1 6 W input

pulse in the Raman fiber reaches its maximum power. The transmitted pulse was

filtered with a long-wave-pass filter to eliminate the unwanted 1.55 |Lim pulse and

obtain a 1.6-|im-band pulse. The waveform of the generated pulse of the

stimulated Raman scattering is shown in Fig.2(b). Its duration and peak power

were 150 ns and 5.5 W, respectively, which are a significant improvement on the

results in [2]. Figure 2(c) shows the trace of the pump pulse transmitted through

the Raman fiber. It can be seen that the pump pulse was significantly depleted.

Figure 3 shows output pulse spectra from both the Q-switched Er3+-doped fiber

ring laser and the Raman fiber laser. Their center wavelengths were 1557 and

1672 nm, respectively. The wavelength difference was in good agreement with

the Stokes shift in silica optical fibers [5].

2.2 OTDR experiments

The experimental set-up of the 1 .6-|Lim-band OTDR is shown in Fig. 1 . A

1.6-|Lim-band probe pulse from the Raman fiber laser was launched into a 66 km

single-mode fiber which was composed of 6 fiber spools, each about 1 1 km long,

spliced in series. The input peak power of the probe pulse was reduced to 1 .2 W

to suppress the stimulated Raman scattering in the test fiber itself. The

backscattered Rayleigh scattering was received by an InGaAs avalanche photo12
diode (APD) through a 50/50 fiber coupler and an acoustooptic switch (AO-SW).

The AO-SW was used to prevent strong near-end Rayleigh backscattering from

entering the APD as this would otherwise distort the signal waveform when

measuring the weaker Rayleigh backscattering from the far-end of the test fiber.

The backscattered signal received by the APD was digitized and averaged with a

digital processing system. The probe pulses have jitter caused by fluctuations in

the revolution speed of the mechanical chopper blade. Therefore a timing signal

was produced from a portion of each probe pulse and supplied to the AO-SW and

the digital processing system to synchronize them.

Figure 4 shows a backscattered signal waveform which was measured with a

receiver bandwidth of 6 MHz and the 2 16 signal integrations. The dynamic range

of the 1.6-|Lim-band OTDR, which can be evaluated from the difference between the

near-end Rayleigh scattering level and the noise floor, was 24.5 dB. The spatial

resolution was confirmed to be 20 m by measuring the step-wise change in

backscattered power from the splice points. These results imply that the OTDR has

approximately the same dynamic range and 5 times better spatial resolution than the

previously reported OTDR [3]. The average loss of the test fiber including 5

splices was estimated to be 0.30 dB/km from the backscattered signal trace of Fig.4.

This value is in good agreement with results obtained by the cut-back method.

3. Conclusion

We have demonstrated a 1 .6-|Lim-band OTDR utilizing a Raman fiber laser

pumped by a Q-switched Er3+-doped fiber ring laser. The dynamic range of the

OTDR was 24.5 dB with a spatial resolution of 20 m. This performance is

superior to that of commercially available 1.3 or 1.55 |im OTDRs. The

demonstrated OTDR is also be able to operate at 1.55 |Lim by utilizing the output

from a Q-switched Er3+-doped fiber ring laser as the OTDR probe pulses.

References

[1] Y, Koyamada, N, Ohta, and N. Tomita, "Basic concepts of fiber subscriber loop operation

systems," in Proc. Int. Conf.Commun., 1990, vol.4, pp. 1540- 1544.

[2] T, Horiguchi, T. Sato, and Y. Koyamada, "Stimulated Raman amplification of 1.6-|im-band

pulsed light in optical fibers," IEEE Photon. Technol. Lett., vol.4, no.l, pp. 64-66, 1992.

[3] T. Sato, T. Horiguchi, and Y. Koyamada, "A 1.6 |a,m band OTDR using a synchronous Raman

fiber ampHfier," to be published in the August 1992 issue of IEEE Photon. Technol. Lett.

[4] S. Sudo, T. Hosaka, H. Itoh, and K. Okamoto, "High-An, small-core single-mode fibers for

efficient nonlinear optical effects," Electron. Lett., vol.22, no. 16, pp.833-835, 1986.

[5] R. H. Stolen, "Nonlinearity in fiber transmission," IEEE Proc. vol.68, no. 10, pp. 1232- 1236,

1980.

13
Fig. 2 Fiber laser data. (a)Pump pulse from Q-switched Fig. 3 Spectra of (a)Q-switched fiber

fiber laser, (b)output pulse from Raman fiber laser, (c) laser and (b)Raman fiber laser,

pump pulse after depletion. Time scale is 100 ns/div. Horizontal scale is 5 nm/div.

0 20 40

Distance (km)

Fig. 4 Rayleigh backscattered

power from a 66 km fiber.

14
The Effect of OTDR Measurement Error on Fiber Uniformity Metrics

Jane B. Clayton, Andrew L. Ingles, David A. James*, Daryl Pregibon*

AT&T Bell Laboratories

Norcross, GA 30071, *Murray Hill, NJ 07974-2070

1. INTRODUCTION

The Optical Time Domain Reflectometer (OTDR) which uses backscattered signals to measure power loss along the

fiber length and provides the basis for quantifying fiber uniformity is known to produce backscattered measurements

which are more variable the farther signals travel into a fiber; this increase in measurement variability adversely

affects the reliability of widely used nonuniformity metrics such as maximum individual segment loss (MISL) and

the difference between MISL and average loss (AVG).

Results firom an experiment characterizing the dependence of OTDR measurement variability on laser power unit,

pulse width, and number of signal averages show the primary determinants of OTDR measurement variabiUty are plugin and pulsewidth. The significance of these results is demonstrated in terms of the metric MISL-AVG.

2. EXPERIMENTAL ARRANGEMENT

Plugin type (high power versus low power), pulse width (25, 50, 250m), and number of signal averages (39, 100,

150, 200) were varied in a statistically designed experiment. A full factorial design was suggested since little prior

information on potential interactions was available, and this resulted in 24 (2x3x4) separate experimental conditions.

A single 30 km test fiber was used in our experiment, for which 100 OTDR traces were generated in succession.

Elapsed time for each set of traces was approximately 20 minutes (per wavelength). The 24 sets of 100 traces (each

either 579 or 2313 data points long, depending on the pulse width) are the basis for the subsequent analysis.

3. DATA PROCESSING

3.1 Estimating Effects of OTDR Settings on OTDR Variation

We examine the role of OTDR measurement variability on uniformity metrics by analyzing the variance of OTDR

measurements rather than directly analyzing the variance of derived metrics. Since the accuracy and precision of the

latter depends on the former, we do not lose any flexibility in doing so. What we gain is important insight into the

relationship of OTDR measurement variability and position along the fiber length.

OTDR measurement variability is quantified at each condition by simply computing the sample standard deviation

of the 100 repeat measurements by position. This results in 24 standard deviation traces which were smoothed using

the lowess algorithm. These smooth standard deviation traces capture the salient global relationship of measure- ment variation on fiber position. Since narrow pulse ffaces contain measurements every 12.5m as opposed to every

50m for the medium and wide pulses, we addressed the problem of unequal number of measurements by interpolating the smoothed traces every kilometer to obtain 30 variances per test condition.

We use an analysis of variance model'^' to quantify the relationship between OTDR measurement variability and

the experimental conditions. The model can be concisely written as:

Yip) = Xp(/7) + E(p)

where Y denotes the response variable, X the matrix of coded experimental conditions, P the effects of varying

parameter settings, and E the (independent) error terms with mean zero. The notation "(p)" conveys the fact that in our analysis each element of Y is a trace, i.e. a function of position "(p)" along the fiber. This induces position

dependence on the effects P(p) and the error terms E{p), but not on the coded experimental conditions X.

In our analysis, the logarithm of the interpolated smoothed standard deviation traces served as the response variable

since it better satisfies the assumptions underlying the analysis. The particular parameterization we chose for the

effects is given in Table 1.

4. DATA ANALYSIS

Figure 1 depicts the standard deviation of power measurements (in dBs) at positions 5, 15, and 30km (indicated at

15
the upper right-hand comer on each panel) for 1310nm and 1550nm. Intermediate positions can be obtained by lin- ear interpolation. On each panel, the horizontal line represents the overall average; the factors are displayed along

the horizontal axis, and the labels depict the average standard deviation at that level of the factor. Note that averages

for plugin are based on 12 observations each, those for pulse width on 8 observations, and those for number of signal

averages on 6 observations. Wide pulsewidths lead to relatively uniform OTDR measurement variability. For nar- row and medium pulse widths, 39 signal averages leads to noticeable increased baseline variation. Also a strong

effect due to the power of the test unit (plugin) at 1550nm is observed.

Figures 2 and 3 display the standardized effects^ by position for 1310nm and 1550nm. These plots exploit the continuity along fiber length of the raw data as a means to summarize estimated effects. At 1310nm, the only clear factor

driving measurement variability is pulse width. Other factors pale in comparison. At 1550nm, plugin is also important, and a small interaction between plugin and pulse is exhibited. The salient feature of the figures is the position

dependence (or lack thereof) of the estimated effects. For example at 1550nm, although pulse and plugin are the

largest two effects, they are increasingly so deeper into the fiber. In contrast, changing the number of averages from

39 times to 100 or more times is marginally important and not position dependent.

While this analysis suggest that plugin power is important only at 1550nm, we believe it to be important at both

wavelengths. The wider pulse widths contain higher power and we see consistent pulse width effects at both wavelengths, suggesting higher power is important at 1310nm and 1550nm. And upon further examination of the data,

we determined that there was essentially no difference in output power level between the "low" and "high" power

plugins at 1310nm; consequently, we should not have expected to see a plugin effect at 1310nm. The difference

between the actual output power levels was only observed at 1550nm. This misclassification of plugin type arose out of relying on the manufacturer's specification of "minimum output power" to characterize the plugins (the "low

power" plugin had higher power than its specification guaranteed). Future experiments will classify plugin type

based upon actual output power measurements.

5. EFFECT ON DERIVED UNIFORMITY METRICS

5.1 Modelling 'Perfect' Fiber

OTDR measurement variability in these experimental data deviates from measurement variability on a perfect fiber

in at least two ways: deviations due to nonuniformity in the test fiber and deviations caused by changes in the laser's

power output over the course of the experiment. To estimate "intrinsic" OTDR measurement variability, we esti- mated fiber nonuniformity and laser drift from the experimental data and corrected for them as follows.

Let P denote the matrix of power measurements for a given fiber at a given wavelength such that Pij is the measiu-ed

power at position j for the /th repeat. Let r, denote the mean of the iih row and cj the mean of the yth column. Average fiber loss at position predij), can be determined by the least squares fit of average power (cj) on position 0).

Fiber nonuniformity defined as the deviation of actual power from predicted power over the fiber is given by:

Cj = Cj - predU), j = 1,...,579 or 2313.

Assuming laser power is constant for each trace, laser drift, r-, is defined as the deviation of power from the average

power over the course of the experiment:

r'i = ri - meaneri), i = 1,..., 100.

We model "perfect" fiber from test fiber by simply subtracting the estimated laser drift from each trace (i.e., row)

and the estimated fiber nonuniformity from each column:

Pij = Pij — Ti — Cj.

This process yields 100 "perfect" fibers with identical average loss and OTDR launch power. These cannot have

arisen as OTDR measurements since the OTDR scales integer measurements by machine constants that effectively

rounds measured power to the nearest 0.01 dB or 0.02 dB. The final operation we perform to mimic OTDR measurements on "perfect" fiber thus entails rounding to the desired precision.

1. The precision of the coefficients P, varies with the number of factor levels in the corresponding term (see Table 1); the standardized

effects, however, have constant precisicm.

16
5.2 Assessment of Parameter Settings on Uniformity Metrics

The measurement variability affects the operating characteristics of derived metrics, such as MISL, in two important

ways: by changing the mean value and changing the spread (variance) about the mean. We now show how the best

and the worst settings translate into expected behavior of derived metrics on perfect fiber.

Upon "manufacturing" 100 perfect fiber OTDR traces as described earlier, 100 pseudo-traces are constructed by

sampling (by position) the power measurements. Another 100 pseudo-traces are similarly constructed, and these are "folded" to mimic 100 bidirectional (outside-in, inside-out) true attenuation U-aces. Each true attenuation pseudotrace is decomposed into individual 1 km sectional loss (ISL) values. The desired metric (MISL-AVE) is computed

for each ISL trace along with position of the MISL.

Figures 4 and 5 display the results of this procedure at 1310nm and 1550nm at the optimal parameter settings (wide

pulse, high plugin, and 100 averages). The top panel shows the variation in the one-kilometer ISLs as a function of

position; the vertical lines centered on the means depict two-a bars. There is a slight increase in the length of the

two-a bars at the fiber ends for 1310nm. This translates into a slight nonuniformity in the distribution of MISL locations towards the fiber ends. The induced distributions of MISL-AVG for 1310nm and 1550nm are unimodal with

varying degrees of asymmetry. As emphasized by us earlier , the distribution of MISL-AVE is not centered on zero, even for perfectly uniform fiber.

In comparison. Figures 6 and 7 display the corresponding results at the worst parameter settings (narrow pulse, low

power, 39 signal averages). The differences are striking, not only in the final distribution of the derived metric

MISL-AVE (which has shifted to the right and has larger spread), but more so in the ISL trace and the distribution of

MISL position. In effect, the fiber ends completely dominate the distribution of this metric when the parameter settings are not "optimally" set.

6. CONCLUSIONS

OTDR measiu^ement variability is minimized on existing OTDR devices, especially deeper into the fiber, by using

100 signal averages and wide pulse widths. Signal averages in excess of 100 times is marginal. Testing at wider

pulsewidths reduces measurement variability at the expense of reducing testing resolution and this tradeoff will need

to be considered before standardizing test procedures. High power plugin is shown to be important at 1550, and we

suspect this to be the case for 1310 as well.

7. REFERENCES

1. Chambers, J. M., W. S. Qeveland, B. Kleiner, and P. A. Tukey. (1983). Graphical Methods for Data Analysis. Belmont, CA; Wadsworth

International Group.

2. Box, G. E. P., W. G. Himter, and J. S. Hunter. (1978). Statistics for Experimenters, New Yoric. Wiley and Sons.

3. Chambers, J. M. and T. Hastie (editors). (1992). Statistical Models in S. Pacific Grove, CA. Wadsworth.

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18
An OTDR Based Combined End-Reflection and Backscatter

Measurement

A. F. Judy

AT&T Bell Laboratories

Norcross, Georgia 30071

1. INTRODUCTION. Because of Rayleigh scattering, optical fibers will reflect a portion of any

transmitted light back toward the input. The amount of this fiber backscatter (FBS) is important to

determine. It is needed for measuring splice and connector reflections with an OTDR, for predicting the

total feedback and double Rayleigh backscatter for transmission system design, and for measuring the

uni-directional OTDR splice loss between dissimilar fibers.

For step-index fiber profiles, the amount of FBS can be theoretically calculated but the accuracy is limited by uncertainties in the fiber parameters. For more complicated profiles even the theoretical

solution is uncertain. So a reliable measurement methodology is needed. This paper describes an OTDR

based method called the combined end-reflection and backscatter (CERBS) technique. It is an inherently

repeatable means for measuring FBS with potentially traceable accuracy.

2. DEFINITIONS AND THEORY. The amount of FBS is measured by the pulse reflectance, Rb,. This is

defined as the ratio of captured reflected power to the total pulse power, both evaluated at the scattering

location within in the fiber, /f^^ depends on wavelength, the fiber's index profile, and the optical input

signal. For a narrow rectangular pulse it is:

''^

Rbs=SaRPW, (1)

where is the total Rayleigh scattering per unit length, S is the fraaion of scattering captured by the

fiber, and PW is the spatial pulse width, i.e. the temporal width multiplied by half the group velocity. For

non-rectangular pulses, PW is the width of the rectangular pulse whose energy and peak power equals that

of the actual pulse. PW is assumed small enough that the fiber loss between the leading and trail edges

is negligible.

To calculate Rbs, the parameter S must be known. For a step-index fiber this is: -I (2)

where: X is the wavelength, rico is the refractive index of the fiber core, and co^ is the spot size (which also

depends on X,). For other than step-index profiles, equation (2) approximately holds if (Oq for the

"equivalent step index" is used. But the accuracy of this approximation is unevaluated and an

experimental measurement method is required to find Rhs.

3. CERBS MEASUREMENT METHOD. The CERBS measurement method (Figure 1) uses the principle

of creating a reference reflection (Rre/) at the fiber's end, measuring it, and measuring R^s with respect to

it Each of these three steps is described below.

3.1 CREATING THE REFERENCE REFLECTION. A large end-reflection can be generated by

perpendicularly placing the fiber-end against a front-surface mirror with a high, but not necessarily

known, reflectance. This is most easily done by terminating the fiber with a rotary-mechanical-splice

ferrule and using the polishing fixture to hold the ferrule perpendicular to the mirror. A cleaved fiber-end,

similarly positioned, should also woric. Any non-flatness of the fiber end or non-perpendicularity to the

mirror is unimportant because, as described below, the end-reflection is actually measured.

19
It is common to use the glass-air Fresnel reflection at an unterminated fiber-end as a "known" of

-14.7 dB. As will be shown, this is not always a repeatable method.

3.2 MEASURING THE REFERENCE REFLECTION. The CERBS method attains it accuracy by

actually measuring Rbj. This is done with an OTDR by considering the mirror as a splice and measuring

the 'splice loss'. That is, when viewed on the OTDR, the mirror will appear (Figure 2) as a reflection

with FBS both preceding it and following it.'^^ The FBS that follows it is the reflection of the fiber that

precedes it. It is generated by the OTDR pulse reflecting in the mirror and, as it propagates back towards

the OTDR, continuing to be backscattered. This 'backscatter' actually travels in the forward direction

(away from the OTDR) until it too hits the mirror and is reflected towards the OTDR. When it reaches

the OTDR its time delay wiU be greater then the round-trip delay to the mirror so it appears to follow the

mirror. Likewise, its amplitude will be reduced (in decibels) by 2/?„y. So Rref is found by measuring the

difference between the FBS before and after the mirror (points B and C in Figure (2), less the intervening

fiber loss). This is exactly what the OTDR 'splice loss' measurement perforais.

Three cautions must be observed in implementing this technique. First, the 'splice loss' measurement

may not be accurate unless the OTDR has a masking feature. This feature suppresses the mirror's large

Fresnel reflection that could cause the OTDR to distort the FBS that follows the mirror. Second, there

must be no other large reflections in the system, e.g. the OTDR panel connector, the optical attenuator,

etc.. Other large reflections, even if they are not near the mirror, can introduce errors by generating either

ghost reflections or additional double-reflected FBS. Third, the reference reflection must be large

(> -5dB). Smaller reflections cause too large a loss at the mirror for accurate OTDR measurement.

3.3 MEASURING THE DIFFERENCE BETWEEN Rref and Rbs- Since the difference between and

Rbs can be 50 dB or more, it probably cannot be directly measured on the OTDR. Even if it could, one

would have to verify the OTDR's accuracy over such a large range. For both reasons, a calibrated optical

attenuator is placed (Figure 1) between a reference fiber and the fiber under test. With the attenuator at its minimum loss, one measures the loss difference (L^a) on the OTDR between the FBS at the end of the

two fibers (Points A and B in Figure 2). Then the attenuator is increased an amount ATT until the end

reflection is about the same level as point B (see Figure 3), and the OTDR loss difference (L^d) between

the reference FBS and Rref (points A and D in Figure 3) is measured. The Rbs is then given by:

Rbs = 2(Rref+LAD-LAB-ATr) [dB ]

4. RESULTS. Using a commercial OTDR with a PW of 102 meters (1 ^.s), we measured both ends of

nine dispersion-shifted fibers at both 1.31 and 1.55 \im. Each fiber-end was terminated with three

different reference reflections. They were:

(1) A first-surface mirror (FSM) with a nominal 100% (0 dB) reflectance, and a measured Rref

that ranged from -0.06 to -0.80 dB.

(2) A neutral density filter (NDF) with a nominal density of 2.0, and a measured Rref that

ranged from -1.4 to -3.0 dB.

(3) The glass-air reflection (GAR) which is assumed to be -14.7 dB. Since the glass-air

reflectance is less then -5 dB, its true value could not be measured.

The test's repeatability can be gauged by comparing the results for each reference reflection at both

wavelengths. Any variations between the parameters of the eighteen fiber-ends should cause proportional

effects at the two wavelengths. This was observed (Figures 4 and 5) for the FSM and NDF but not for the

GAR (Figure 6). Thus the inherent repeatability of the CERBS method is high only when Rref is actually

measured, as is the case for the FSM and NDF.

The second repeatability test is to compare, at a given wavelength, the results for the three reference

reflections. At 1.31 )xm it is seen that the NDF and FSM data are highly correlated (Figure 7) but the

GAR and FSM are not (Figure 8). Similar results occurred at 1.55 iim .

20
The final statistics for all eighteen fiber ends is shown below:

Table 1. Rbj Summary Statistics

Reference

Reflection

1.31 \im 1.55 [un

X o

(dB) (dB)

X a

(dB) (dB)

FSM -46.2 .31 -49.8 .45

NDF -46.1 .30 -49.7 .48

GAR -46.1 .48 -49.2 .58

where x is the mean and o is the standard deviation. For each wavelength, the FSM and NDF statistics are

practically identical but the GAR results have a larger standard deviation and, at 1.55 jim, a different

mean. This demonstrates again the reliability of the CERBS measurement when the is actually

measured and the uncertainty that is introduced by assuming a known glass-air reflection.

5. THEORY AND MEASUREMENTS COMPARED. Both the theory and data show that R^s is a

function of wavelength. The measured mean values at 1.31 and 1.55 |im differ by 3.6 dB of which theory

predicts 2.9 dB is caused by the X"* dependence of a in equation (1) and the remainder is consistent with S

wavelength dependence. Although not obvious from equations (1) and (2), theory predicts that variations

in fiber parameters wiU cause a greater change in S at 1.55 ^mi than at 1.31. This causes the non-unity

slopes in Figures (4-6) and the larger o at 1.55 ^mi in Table 1. In fact, the slopes in Figures (4) and (5)

indicate which fiber parameters are varying. Numerical analysis shows that small variations in fiber core

radius and «co will yield slopes of .21 and .71, respectively. The measured slopes of 0.66 and 0.61

(whose difference is not statistically significant) appear to be dominated by rico variations. However, no

attempt was to made confimi this with independent measurement of core radius and

6. CONCLUSIONS. The CERBS measurement method is shown to be both repeatable and easily

implemented. Excellent agreement is obtained between different reference reflections, providing they are large enough for OTDR measurement. Using a glass-air reflection is less reliable, at least when attempted

wiUi polished ferrules.

REFERENCES

1. A. H. Hartog and M. P. Gold, "On the Theory of Backscattering in Single- Mode Optical

Fibers," /. Lightwave Tech., LT-2, 76, pp. 76-82, 1984.

2. F. P. Kapron, E. A. Thomas and J. W. Peters, "OTDR Measurements of Optical Return Loss,"

Technical Digest Symposium on Optical Fiber Measurements , NBS Publication 748 (Boulder

CO), pp. 35-38. 1988.

3. E. Brinkmeyer, "Analysis of the backscattering method for single- mode optical fibers,"

/. Opt. Soc. Am., vol. 70, no. 8, pp. 1010-1012, 1980.

4. V. A. Bhagavatula, "Estimation of Single-Mode Waveguide Dispersion using an EquivalentStep-Index Approach," Electron. Lett., vol. 18, no. 8, pp. 319-320, 1982.

5. E. L. Buckland and M. Nishimura, "Bidirectional OTDR Measurements Utilizing an Improved Folded-Path Technique," Technical Digest Symposium on Optical Fiber

Measurements , NBS Publication 748 (Boulder CO), p. 15-18. 1988.

21
OTDR

optical Reference

Attenuator f \ Reflection

Reference

Fiber

Test

Fiber

ND FILTER

B— lS.t4 »- .11 N- tl »• .174

-51 -50 -49 -48

1 .55um Rbs (dB)

Figure 1. CERBS Test Configuration. Figure 5. NDF comparison at 1.31 and 1.55 ^m.

58-

45

4e35-

Rttvnufttor R«f 1«ct ton

,'R«f 1 •ct Ion

Rof oronco

ribor

ribor

Undor Tot

.5 1 1.5 2 2.5 3 DISTRNCE (km)

Figure 2. OTDR display with 0 dB attenuation.

'-45

GLnss-niR

fl—27.3 B- .392 N- It R- .34H

-51 -50 -49 -48

1 .55um Rbs (dB)

Figure 6. GAR comparison at 1.31 and 1.55 urn.

5Bm 43- o

U 4 >

35-

Unmatkad

aranc*

'Raf lact ton

.5 1 1.5 2 2.5 3 DISTRNCE (km)

Figure 3. OTDR display with 25 dB attenuation.

-46 -45

FSM Rbs (dB)

Figure 7. Comparison of FSM and NDF at 1.31 nm.

FS MIRROR

ft— 13. IS B- .SS4 N- IB R- .8(14

|....|. .!.

-51 -50 -49 -49

1 .55um Rbs (dB)

Figure 4. FSM comparison at 1.31 and 1.55 ^m.

44i 1 . 3 1 urn n— 14.93 B« .St3 N- II R- .4317

-47 1 ... 4 ... I -47 -46 -45

FSM Rbs (dB)

Figure 8. Comparison of FSM and GAR at 1.31 ^m.

22
I

Factory Measurement Solutions to Predictable Field Performance of

Single-Mode Fiber Optic Connectors

by

Bennett Wong, Halina Kosiorska

Northern Telecom Canada Limited

Optical Cable Plant

P.O. Box 807, Saskatoon, Saskatchewan, Canada S7K-3L7

1.0 Introduction

Modem fiber optic systems are designed with ever decreasing margins in terms of loss and reflections. In

addition, high traffic densities require the systems to be both stable and reliable. These demands drive

manufacturers to design and produce connectors with optimized performance. A number of test methods

are used to verify performance during design and manufacture, but connectors measured in the factory

may give different results in the field. This is because factory measurements are performed using setups

and conditions that are different from those of installed connections. Also, as standards organizations are

yet to complete their work on optical and mechanical intermatability, products from different suppliers

may not be fully compatible.

2.0 Optical Performance

The three elements that contribute to insertion loss are fiber parameters, connector parameters, and

manufacturing process. In the category of fiber parameters, geometry plays the most important role, while

index and mode-field mismatch are of less significance. For the connector, geometry is again of primary

concern. In particular, it is the diameter and eccentricity of the ferrule microhole that has the greatest

impact to insertion loss. Finally, the manufacturing process must produce fiber and ferrule endfaces with

controlled profiles and pristine finishes. Through careful measurement and matching of components, and

process optimization, insertion losses of better than 0.5 dB are achievable in field connections.

Refiection at a connector interface is a function of index matching and scattering. If an air gap exists at the connection plane, a return loss of 14 dB results due to the air-glass mismatch. A common approach to promoting fiber contact is to polish the ferrule endface into a spherical profile. This allows the axis of the

two conection ferrules to readily mate, and routinely produces connectors with return losses in the range

of 30 dB. Higher return loss - in the range from 40 to 50 dB - is attained by eliminating high-index

layers' and scattering sites at the fiber end-face. A special class of connectors, utilizing a combination of

physical contact and angled fiber endfaces^-^, can achieve return losses in excess of 60 dB.

The most common straight-ferrule connectors use a combination of precision alignment and physical

contact to achieve low insertion loss and high return loss. However, the factory specified values often do

not match those experienced in the field, due to the methods in which the factory measurements are performed. As will be described in the following sections, predicting field performance of connectors

requires knowledge of other parameters as well.

2.1 Fiber and Connector Parameters

Connector eccentricity error results from fibers and ferrules with non-ideal geometry. Commercially

available fibers have a worst-case eccentricity of 1 [im and a diameter tolerance of ±2 i^m. Connector

23
ferrule microholes are typically 1 [im larger than the fiber diameters, in order to facilitate assembly.

Eccentricity of the microhole is specified to within 1 \im. The ferrule diameter is less critical because the

adapter sleeve element effectively compensates for small diameter variations. In the extreme case of

tolerance build up, there can be an offset as large as 8 ^m between the two fiber cores in the connection,

giving rise to an insertion loss of over 13 dB (Figure 1). In connectors designed for physical contact,

eccentricity combined with vertex offset can lead to fiber separation. Vertex offset is defined as the

distance between the apex of the spherical ferrule profile and the fiber axis. An air gap between the fibers can severely degrade both the insertion and return loss performances. Furthermore, the separation causes

interference to occur within the gap, so that wavelength drift or changes in the gap spacing (Figure 2) will

result in transmitted and reflected power fluctuations.

During manufacture, the overall offset in connectors is controlled through fiber and ferrule geometry

measurements. Accurate determination of the diameter, non-circularity, and eccentricity values permits

components to be pre-selected and matched prior to assembly, thereby improving consistency and yield.

Once the fiber-ferrule assembly is complete, a measurement of the overall eccentricity provides insight to the expected insertion loss of the connector, while vertex analysis is used to deduce return loss (Figure 3). Fiber geometry measurement with sub-micron accuracy and precision is achieved with grey scale"* and

interferometric^'^ methods. Ferrule geometry is tested using optical and mechanical techniques^ *, with

slightly lower accuracy and precision.

Geometry measurements can also help to reduce the impact of connector eccentricity through a concept

known as keying^. Here, the eccentricity direction of each fiber-ferrule assembly is measured and

identified. In the final assembly operation, the ferrules are mounted into connector bodies with the

eccentricities aligned to a consistent direction. For connectors that use oblique endfaces to achieve ultralow return loss, keying is imperative to ensure proper contact and alignment. In this case, the keying

feature fixes the orientation of the end-angle. The endface angle and orientation are found using beam

deflection'^ or optical microscopes. Connector keying is a simple solution to improving factory yield and

field performance, but as there is presently no industry standard defining the keying mechanism or keying

angle, performance is guaranteed only if all the parts in a connection are from the same manufacturer.

Factory insertion loss" and return loss'^'^ '"* are measured by mating the test connector to a reference

connector. The reference connector has overall eccentricity and vertex offset close to zero, so repeatable

test results are obtained regardless of the test connector's eccentricity direction. In the field, two mated

connectors may have their eccentricities pointing in the same or in opposite direction. These extremes can produce insertion losses as low as zero, or as high as four times the factory specified maximum. As

for return loss, the factory value gives no guarantee whatsoever of field performance. This is because a connector with finite vertex offset may make fiber contact with a reference connector having zero vertex

offset, but not with any other connector with non-zero vertex offset (Figure 4). To predict return loss in

the field, the ferrule vertex offset and radius of curvature must also be specified. These values are found

using interferometric'^-'^ techniques.

In the factory, the various test acceptance criteria can be adjusted to meet any required worst-case installed

performance. However, the feasibility and extent of this practice depend on the actual customer

requirements. For example, if the worst-case acceptable insertion loss is very low, it will be necessary to impose extremely stringent factory screening limits, thus increasing the rejection rate and cost of the

product. A more lenient approach is to specify typical field performance, which would allow for some

instances of marginal connections. The system designer must select the tolerance level for borderline

connections, based on the system margin and number of connections in each link. The manufacturer can

then apply statistical modeling' ' to the factory measured loss distributions to estimate the field

performance distribution (Figure 5).

24
2.2 Polishing Process

The polishing operation generates the proper ferrule end-face shape and finish for the connector. Even if connectors are made with zero eccentricity and vertex offset, a poor finish on the fiber can still render

unacceptable insertion and return losses. An non-optimized process may also cause over-polishing and

high index layers. An over-polished connector is characterized by the fiber being recessed from the

ferrule tip (Figure 6) because of the different rates of abrasion between the two materials. Recess, even in

the order of a few microns, is sufficient to prevent fiber contact and therefore give poor return loss

performance. High-index layers are formed due to compaction of the fiber during polishing, and can

degrade return loss by several decibels in contacting connections.

The end finish of cormectors is inspected after the polishing process using simple microscopes. Scratches,

debris, and epoxy smears are easily detected in this fashion'^. Since the surface finish of the fiber

contributes to the intrinsic loss of a connector, it affects a factory connection to the same extent as a field

connection. Therefore, factory loss testing is sufficient to prevent field failures caused solely by poor

finish. If over-polishing is present in a connector, direct return loss testing with a reference connector

alone is insufficient to guarantee field performance. This situation is similar to that of vertex offset,

where an over-polished connector may make contact with a reference connector in the factory, but not

with other over-polished connectors in the field. Over-polishing can be measured with sub-micron

resolution using the same techniques as for vertex offset and radius of curvature'^'^, and field

performance is further assured by limiting this parameter. High-index layers are detected by monitoring

changes in return loss as the ferrule end is chemically etched.

3.0 Mechanical Performance

The majority of equipment in which connectors are installed do not allow the cable leading from the

connector to exit in a straight path. In addition, the connector must withstand any pulling, bending, or

twisting of the cable during installation and removal. As such, the cormector must provide some form of

strain relief The strain relief mechanism is critical to a connector's performance (Figure 7), and is designed to restrict the bending radius of the cable so that bending induced losses are minimized.

Manufacturers incorporate mechanical testing in developing connectors, and apply sufficient margin to their designs to guarantee field performance.

A number of EIA/TIA test methods are available for verifying the mechanical performance of connectors.

These tests quantify a connector's design with respect to cable flexing'^, retention^^, twisting^', strength^^,

and installation^^. In addition to the standard test procedures, manufacturers often test connectors

according to Bellcore requirements^'*, which includes addition procedures for cable proof and loading

stability tests.

4.0 Reliability

Connector testing is not limited to optical or mechanical parameters. A number of techniques also exist

for studying reliability. In designing a connector, a manufacturer must perform extensive testing in harsh

environments to study and understand the long term reliability of the product. These tests help to identify

process or component related deficiencies, so that improvements can be implemented to reduce the risk of

in-service failures.

Depending on the design, components in a connector may place unusual stresses on the fiber when it is subjected to vibration, thermal variations, or aging. Any of these effects may induce high loss or even

fiber breakage. Connector stability and reliability are determined through accelerated environmental tests such as humidity aging2^ heat aging^^, and temperature cycling^^. Durability is checked by a series of

impact^*, vibration^^ and re-mating^° tests.

25
When fiber breaks occur, fi-acture analysis^' can highlight problems such as stripping tool damage, sharp

edges in the ferrule, or epoxy incompatibility. Fiber breaks are located using high-resolution OTDR or Xray^2 methods.

For physical contact coimectors, a substantial axial pressure exists at the fiber ends due to the small

contact area. Over time, the epoxy bond fixing the fiber to the ferrule may fail, resulting in the fiber

recessing^^ into the ferrule and eventual loss of contact. Fiber recess is identified by interferometric

techniques'^ ", while the epoxy bond, as part of the connector assembly, is tested during reliability testing.

5.0 Summary

High-performance connectors are typified by more than just low insertion loss and high return loss. Today's applications also require connectors to be robust and reliable. From a manufacturer's perspective,

designing a connector to satisfy these requirements means being able to accurately characterize the

determining parameters. Many excellent methods have been established for measuring connector optical,

mechanical, and reliability performance. These tests allow each manufacturer to design for - and impose

limits on - factors that affect field performance. However, this gives predictable in-service performance

only if the connectors are from the same manufacturer. To ensure performance when multiple vendors are involved, the connector industry must agree on acceptable limits for important parameters such as vertex

ofifset, ferrule end-face radius of curvature, and over-polishing, and resolve outstanding issues on optical

and mechanical compatibility. This work is ciurently the focus within international standards

organizations such as EIA/TIA, lEC, and CCITT.

6.0 References

1. J. B. Clayton, G. M. Alameel, G. F. DeVeau, "Low-reflection polish for ferrule-based mechanical

splices and straight-tip connectors," OFC '89, Houston, 1989, paper ThJ4, p 166.

2. W. C. Young, V. Shah, and L. Curtis, "Loss and reflectance of standard cylindrical-ferrule singlemode connectors modified by polishing a 10** oblique endface angle," IEEE Photonics Technology

Letters, Vol. 1, No. 12, December, 1989, pp 461-463.

3. N. Suzuki, "Physical contact technology suppresses light reflected from connectors," JEE, April, 1991,

pp 76-80.

4. EIAyTIA-455-176, "Method for measuring optical fiber cross-sectional geometry by automated greyscale analysis," in approval.

5. EIA/TIA-455-92, "Optical fiber cladding diameter and noncircularity by Fizeau interferometry," in

approval.

6. EIA/TIA-455-93, "Optical fiber cladding diameter and noncircularity by non-contacting Michelson

interferometry," in approval.

7. Z. Pasturczyk, B. Wong, C. Saravanos, "Fiber optic connector geometry test station," International

Wire and Cable Symposium Proceedings 1989, pp 623-625.

8. M. Ueki, Y. Hattori, "Accurate measurement of structural parameters of single fiber connector

ferrule," OFC '91, San Diego, 1991, paper ThAl, p 137.

9. Y. Ando, "Statistical analysis of insertion loss improvement for optical connectors using the

orientation method for fiber-core offset," IEEE Photonics Technology Letters, Vol. 3, No. 10, October

1991, pp 939-941.

10. EIAyTIA-455-57A, "Optical fiber end preparation and examination," Appendix A.

11. EIA/TIA-455-171, "Attenuation by substitution measurement ~ for short-length multimode gradedindex and single-mode optical fiber cable assemblies."

12. EIAyTIA-455-107, "Return loss for fiber optic components."

13. EIA/nA-455-8, "Measurement of splice or connector loss and reflection using an OTDR," in

approval.

26
14. S. K. Das, A. F. Judy, G. M. Alameel, R. M. Jopson, T. F. Adda, "Reflectance measurement in

lightwave systems: A comparison of various techniques," Technical Digest, Symposium on Optical

Fiber Measurements, pp 25-30, 1988.

15. Z. Pasturczyk, T. Chepyha, C. Saravanos, H. Wood, "Test methods for fiber optic connector

parameters directly affecting return loss," Technical Digest, Symposium on Optical Fiber

Measurements, pp 163-166, 1990.

16. K. Creath, "Step height measurement using two-wavelength phase-shifting interferometry," Applied

Optics, Vol. 26, No. 14, 15 July 1987, pp 2810-2816.

17. H. Kosiorska, B. Wong, C. Saravanos, "Statistical method for predicting connector loss in the field

using factory measurements,", OFC '91, San Diego, 1991, paper ThA2, p 138.

18. R. P. Drexel, J. A. Nelson, J. Schnecker, "New approaches to termination," Photonics Spectra, March

1988, pp 101-106.

19. E1A/T1A-455-1A, "Cable Flexing for Fiber Optic Interconnecting Devices"

20. EIA/TIA-455-6B, "Cable Retention Test Procedure for Fiber Optic Cable Interconnecting Devices."

21. EIA/TIA-455-36, "Twist Test for Fiber Optic Connecting Devices."

22. EIA/TIA-455-185, "Strength of Coupling Mechanism for Fiber Optic Interconnecting Devices."

23. EIA/TIA-455-187, "Engagement and Separation Force Measurement of Fiber Optic Connector Set." 24. Bellcore TR-TSY-000326, Issue 2, "Generic Requirements for Optical Fiber Connectors and

Connectorized Jumper Cable."

25. EIAyTIA-455-5, "Humidity Test Procedure for Fiber Optic Connecting Devices."

26. EIAyTIA-455-3, "Procedure to Measure Temperature Cycling Effects on Optical Fibers, Optical Cable,

and Other Passive Fiber Optic Components."

27. EIAyTIA-455-4, "Fiber Optic Component Temperature Life Test."

28. EIA/TIA-455-2, "Impact Test Measurements for Fiber Optic Devices."

29. EIA/TIA-455-1 1, "Vibration Test Procedure for Fiber Optic Connecting Devices and Cables."

30. EIA/TIA-455-2 1, "Mating Durability for Fiber Optic Interconnecting Devices."

3 1 . W. R. Wagner, "Extrinsic fiber damage and its effect on the reliability of optical fiber connectors and

splices," SPIE Vol. 1850, Fiber Optic Components and Reliability, 1991, pp 168-185.

32. T. Wei, D. J. Cotter, B. T. Devlin, W. D. Koenigsberg, "Nondestructive evaluation of optical fiber

splices and connectors," OFC '92, San Jose, 1992, paper ThIl, p 226.

33. R. Ziebol, H. Roberts, B. Daniel, "Permanent fiber withdrawal in fiber optic connectors," 8th Annual

NFOEC, Washington, D.C., 1992, pp 211-230.

20

dB

Insertion Loss Due to Rber/Ferrule Geometry

Fibef Ecc ^

Fiber OD

Fiber Ecc

Fiber Ecc +

Fiber OD + MicroholeEcc^

MicroholelD

Fibef Ecc +

Fiber OD + Micro hole Ecc

Insertion & Return Loss vs Separation

dB

T

Return Loss —

. f !:..:< 5 i '. ^ •• i J >. ;• ?

;• ' >.

:'. i > i - f. •[ -".^

II 0 "l " ? 'l • "I ' * " '•> M •' '• " I > ' 'll" .•'^.•••'l .,'<•'''•'' •" m . I'. ,....,1.1 II. ,.."'(, . ,. II' I. ...t.i .1.. , y ''^•lll,l,l'^" ,llllllIIII'l' Kil ^ Insertion Loss " ^

!• i

;; ^

"

'.' ' i . \' -

"

* '

-

) End Separation {\Mn) 25

Figure 1. Connector insertion loss contribution

due to accumulation of fiber and ferrule

geometry errors.

Figure 2. Insertion loss and return loss as a

function of fiber separation. Oscillations are due

to interference between the fiber ends.

27
—Pffi 0 o fr

° 1

_o] JJ

0 25 50 75 100 125 ISO

Vertex Offset (microns)

Figure 3. Effect of radius of curvature and

vertex offset on return loss. Rl < R2.

Field Field Field Connector Connector Connector

>—<

Factory Field Field Connector Connector Connector

(a) (b) (c)

Figure 4. A connector makes fiill contact during

factory testing (a) and in some instances, in a

field connection (b). However, contact may not

be achieved (c) if the radius of curvature and

vertex offset are not controlled.

Factory and Reld Loss Distributions

0.2 0.4 0.6 0.8

Insertion Loss (dB) 241

Figure 5. Insertion loss distributions for factory "^'S^'^ 6. Profile measurement showing fiber

and field (random) connections. over-polishmg.

20 r ^

Connector Radial Loading

Performance (1550 nm)

Figure 7. Cable bending-induced loss of

standard and enhanced SC connectors.

28
Measurements to Assure Fiber Reliability

Hakan H. Yuce & Felix P. Kapron

Bellcx)re

445 South St., Morristown, NJ 07962-1910

1. Fiber Environment

More than 10 Mkm of optical fiber has been installed in the U.S., some for a period approaching

15 years. To date, very few mechanical failures have been reported that are not related to a catastrophic

cable cut, fire, or earthquake. Most of the few failures are associated with defects in the fibers or cables.

Some failures are due to gradual fiber weakening under an extreme environmental condition (e.g., cable

tension, constant steam), or are due to handling the fiber at some termination point such as a splice or

connector. Why then is there an increasing interest in measuring fiber mechanical properties more

accurately? Why are stricter mechanical specifications being imposed on fiber? What new specified

parameters and measurements are necessary? This paper attempts to answer these questions.

Consider that until now most fiber has been used for trunking applications in which long lengths of

fiber cable are in a benign underground or underwater environment, with rather few terminations. As

fiber permeates closer to the customer, however, distribution cable lengths are shorter, and they are subject to increased handling at more close-packed termination points both during installation and during

network rearrangements. The fiber cable must endure a wider range of above-ground temperatures,

continuous vibrations in some applications, and terminations possibly exposed to humidity and corrosive

materials. Yet costs must be kept low, with few repairs required.

2. New Measurement Standards

The issues of fiber strength, aging, and lifetime have been addressed in the research literature for

some time [1]. So what's new? The above need for enhanced reliability requirements and the prospect of

additional fiber specifications have made it necessary to "fine tune" the measurement techniques so that

results are reproducible amongst testing facilities. Since this has commercial implications, precise

measurement standards are becoming necessary.

Several national and international standards organizations have recently become involved in testing

for mechanical reliability. In the U.S., this is resulting in several ANSI FOTPs (Fiber Optic Test

Procedures) [2-7] generated by fiber users and suppliers within the Telecommunications Industry

Association. An advantage of standards is that the mechanical properties of different fibers can now be

directly compared with each other in ways agreed to by the industry. Another is that estimates can be

made of fiber reliability (e.g., lifetime for a given service stress, or allowable stress for a desired lifetime)

in different service conditions.

A common field situation occurs when the fiber is subjected to a longterm residual applied stress, as within a cable or in a bends in a splice housing or on a payout bobbin. Three measurement deployment

conditions, somewhat relating to field deployments, are longitudinal tension with a gage length of about

1 m [2-4], mandrel-wrap uniform bending, or two-point bending of a few cm of fiber between parallel

plates [3, 6]. A particular crack fails when its decreasing inert strength during service or during a test

falls below the applied stress at that instant.

3. Fatigue Measurements

To be able to predict fiber lifetime, there needs to be a means of measuring it. One method is static

fatigue under constant applied stress [2, 3]. It would seem straightforward to obtain the times-to-failure as a function of several constant stresses, and then to extrapolate to smaller in-service stresses and longer

expected lifetimes. However, the distribution of flaws in a fiber is statistical both in position and in

29
strength. To take this into account, first measure the random failure times of many fiber samples at a fixed

applied stress. With a static fatigue measurement this is usually done in the worst-case environment that

the fiber is expected to experience, with failure times of days to months. Then plot the cumulative failure

probability F for these times on scaled Weibull paper to obtain the form of Fig. 1, where the slope is related to the ratio of mean over standard deviation for the distribution. Sometimes the plot consists of

two (or more) straight-line segments, characteristic of two (or more) flaw types. How to take this mto

account is still under study.

Repeat the measurements for several values of applied stress. The values of should not change

much, but plot the median failure times t^edian vs. these stresses as per Fig. 2. This yields the (static)

stress-corrosion coefficient (typically around 20), and an intercept tf{l). The testing parameters of gage

length, m^, tig, tf(l) and the parameters of service length, service stress, and acceptable failure probability can then be used in formulas [7] to estimate fiber lifetime or safe stress.

The above procedures can take a long time. Moreover, fiber aging can take place during the tests, and

the above crack parameters (apart from strength) will change. (This is starting to be addressed in a series

of draft FOTPs.) More convenient dynamic fatigue measurements are often used [4-6]. Here controlled

ambient conditions are common, so relating these to any more severe in-serve conditions is uncertain.

Moreover, any environmental aging must take place prior to the tests.

Similar to the above, first measure the random failure stresses of many fiber samples at a fixed applied

stress rate. Then plot the cumulative failure probability F for these stresses on scaled Weibull paper to obtain the form of Fig. 3; again, the slope nij may have more than one value. Repeat the measurements

for several values of applied stress rates, and plot the median failure stresses o^^i^n vs. these stress rates as per Fig. 4. This yields the (dynamic) stress-corrosion coefficient nj, and an intercept Of{\). The testing

parameters of gage length, m^, nj, Of{l) and the parameters of service length, service stress, and

acceptable failure probability can then be used in similar formulas [7] to estimate fiber reliability.

4. Prooftesting Measurements

Prooftesting is a basic screening that virtually all commercial fiber is subjected to as a part of the

manufacturing process. Figure 5 shows the stress history in which a proofstress is applied sequentially

along the full length of fiber [8]. The "measurement" aspects of the test are generally limited to determining the lengths of the surviving fiber. Unfortunately, it cannot be guaranteed that in the

surviving lengths there are no cracks with strengths below the proofstress level. Although this is the case

at the end of the dwell portion, further weakening occurs during unloading. Revisions are likely to occur

in the FOTP to limit this effect. In addition to the proofstress, the final minimum strength depends upon

the unload time, the stress-corrosion coefficient, and the crack "B-value". This last quantity is elusive,

and some controversy exists in the literature about how to measure it. Meanwhile, prooftesting also

distorts the Weibull failure distribution [9]. Although lifetime formulas appear in the literature [9],

studies in standards groups have not yet progressed sufficiently to allow a recommendation having

commercial impact.

5. Lifetime Prediction Example

Dynamic tensile fatigue measurements were made on standard 125-^m diameter fused silica fiber

coated with a 250-nm UV-curable polyurethane acrylate. The environment was 30 ± 1°C and 85 ± 5%

relative humidity. Strain rates ranged from 0.01 %/min to 10 %/min. At a gage length of 0.5 m,

31 samples were tested at each rate, as per Fig. 3, to obtain the median failure stress o^^ji^n and for

each rate. The lowest value of 30 for the latter was used. These median failure stresses were plotted

against the applied stress rate as per Fig. 4 to obtain the intercept and an value of 19.6.

Service conditions of 1 km and an allowed failure probability of 10"^ were assumed in the lifetime

formulae. Figure 6 shows the results, where a lb-force equals about 53 kpsi (common prooftest levels

range from 50 to 200 kpsi.) Based on these measurements and calculation, the fiber will survive several

decades so long as the service stress does not a exceed 2V2 lb-force. Fiber aging in harsh environments,

30
which will alter the parameter values (apart from strength) is not accounted for.

6. Conclusions

This paper has outlined why increased demands are being put on the mechanical properties of fiber,

and what standards-sponsored measurement techniques can be used to obtain required mechanical

parameters. Much work remains to complete these techniques and to relate the parameters to reliability

calculations. A useful example has been given of measurement results and the resulting estimates of

lifetime vs. applied stress.

7. References

[1] Review by J. E. Ritter, Jr., in Fracture Mechanics of Ceramics vol. 5, pp. 227-251, ed. by R. C. Bradt

et al (Plenum Press, N.Y., 1981), and references therein.

[2] Procedure for Measuring the Static Fatigue ofOptical Fiber in Tension, FOTP proposed,

[3] Procedure for Measuring the Static Fatigue of Optical Fiber in Two-Point Bending, FOTP-97.

[4] Methodfor Measuring Dynamic Fatigue ofOptical Fibers by Tension, FOTP-76.

[5] Methodfor Measuring Dynamic Tensile Strength of Optical Fibers, FOTP-28.

[6] Methodfor Measuring Dynamic Fatigue ofOptical Fibers by Two-Point Bending, FOTP-90.

[7] Power-Law Theory ofOptical Fiber Reliability, EIA/TIA draft.

[8] Fiber Tensile Proof Test Method, FOTP-31.

[9] Review by V. A. Bogatyrjov et al.. Optical Eng., vol. 30, no. 6, pp. 690-699 (June 1991), and

references therein.

log failure time log applied stress

Figure 1: Schematic plot of static Weibull Figure 2: Schematic plot of failure time

failure probability versus lifetime. versus applied stress.

31
log failure stress log applied stress rate

Figure 3: Schematic plot of dynamic Weibull Figure 4: Schematic plot of failure stress failure probability versus failure stress. versus applied stress rate.

4

applied

stress

proofstress

dwell

7 V

\l

tune

Figure 5: The prooftest cycle

32
Atomic Force Microscopy Measurements to Study Optical Fiber Aging

H. H. Yuce, J. P. Kilmer and J. P. Varachi, Jr.

Bellcore

445 South Street, Morristown, NJ 07962

INTRODUCTION

The critical mechanical properties of optical fibers are strength, fatigue resistance, and zero-stress

aging behavior. Understanding the effect of the chemical environment under zero stress on the

subsequent fracture strength of fibers is important because fibers in service are likely to encounter

water and other chemical species while deployed closer to subscriber premises. Zero-stress aging

refers to the loss of strength of high-strength glass fibers in the absence of stress in some corrosive

environments such as water.

In this work, the strength of fibers aged under zero-stress conditions has been measured and

compared to the surface roughness of the fibers. The roughening of the fibers from corrosion was

measured with atomic force microscopy (AFM) at intervals during aging duration. This paper

shows that AFM measurements can be used to image the surface of silica fibers at a nanometer

scale, opening fundamental understanding of the aging behavior of fibers.

2. EXPERIMENTAL PROCEDURE

Two commercially available silica optical fibers fibers 1 and 2, with a nominal glass diameter of

125 |a.m coated with UV-cured acrylate polymer to an overall diameter of 250 |J.m, were used.

Fibers 1 and 2 were subjected to aging in both 85 °C deionized water buffered at pH 7 and 85 °C,

85% RH for 1 day, 1 week, 1 month and 3 months at zero-stress. The aged and unaged fibers were

then examined by two techniques.

A computer controlled, two-point bending apparatus was used to determine the strength of the aged

and unaged fibers. The test apparatus (see Fig. 1) consists of two parallel platens, one fixed and one

moving, between which a fiber loop is bent until it breaks\ The moving platen is driven by a computer controlled stepper motor; the rate of closure can be varied so that the maximum stress in

the fiber loop changes at a constant rate^ Fiber failure is sensed by an acoustic transducer which

signals the computer to stop the moving platen and record the platen separation at the failure. The

stress at failure is calculated from the platen separation, the elastic modulus of the glass, and the

dimensions of the fiber. Because the effective gage length is less than 20 |J.m^ the probability of

measuring a fiber sample containing a large flaw is very small, so the results tend to be uniform and

unimodal. The technique is suitable for measuring strengths in different environments since the

fiber sample and jaws can be immersed in the environment of interest. Since there are no gripping

problems, uncoated fibers can be also tested. For these experiments, the fiber strengths were

measured in deionized water buffered at pH 7 and held at 23°C. Strength tests were made on 31

samples from each aging condition. Sufficient preconditioning in the test environment was imposed

on all fibers prior to testing.

The surface morphology the fibers in this study was characterized with a commercially available

atomic force microscope. It has given atomic resolution images of both conductors'* and

nonconductors^ The AFM was invented* in 1986 shordy after the development of the scanning

tunneling microscope (STM)'. The AFM offers the added advantage over the STM or the scanning

electron microscope (SEM) that the sample need not be a conductor. Mechanically, the AFM

consists of a stylus that is attached to a cantilever. The surface is scanned under the stylus and the

candlever is deflected as the stylus interacts with the surface. The stylus may be in direct contact

with the surface or may be vibrated above it. The latter method has been used to image magnetic

domains* and charge distributions'.

Images are produced by measuring the deflection of the cantilever. Three techniques for measuring

33
the deflection include: (1) tunneling to the back of the cantilever (the tunneling technique) (2)

measuring the phase change of light from an optical fiber reflected off the back of the cantilever

(the optical fiber interference technique)', and (3) measuring the change in angle of light reflected

off the back of the cantilever ( the optical lever technique )" '^

Our AFM combines a microfabricated cantilever'^ with the optical lever technique. It operates as follows (refer to Fig. 2): a Si3N4 cantilever with integrated tip, is allowed to gently touch the

surface. A small surface feature deflects the cantilever by a small distance 8z, which causes a laser

beam reflecting off the cantilever to deflect by an angle of 26z/L, where L is the length of the

cantilever, (j) is the angle between the surface and the cantilever; for us (j) is 15°. This deflection of

the reflected beam is detected as a change in the difference between light falling on separate

segments of a two- or four- segment photodiode. We can operate the microscope either by

measuring the deflection (variable deflection mode) or by using a feedback system to keep the

deflection constant (constant force mode). The constant force mode was used for all of the images

in this paper.

There are certain advantages to using the constant force mode. They are (1) the heights of the

surface features are accurately measured and (2) the repulsive force, from a few atoms at the tip of

the stylus, can be held constant at a low value (as small as 2 x 10-^° N for this paper). This repulsive

force plus the spring force of the cantilever counterbalances the attractive van der Waals force

between the stylus and the surface. Theoretically, the AFM can be operated with a vanishingly

small repulsive force by offsetting the attractive van der Waals force with the spring force of the

cantilever'^

The microfabricated cantilevers" that we used were 100 or 200 |J.m long with spring constants of

order 0. 1 N/m . The photodiode detector was approximately 4 cm from the cantilever. The sample

is placed on top of an xyz translator'^ and is enclosed in a sample cell that can be sealed with an O

ring. Sealing the sample cell allows us to deposit a variety of liquids or gases onto the sample. For

this paper, all the samples were imaged in air. None of the samples required any special

preparation. We used xyz translators with three different scan widths: 5 and 0.1 |J.m, Using

different translators allowed us to image with tremendous resolution (0.3 nm laterally and 0.1 nm

vertically), depending upon the sample. Scanning frequencies were in the range of 5 to 100 Hz with

the higher frequencies for smaller scan widths. The coating was removed before AFM examination

by dipping the fiber briefly in hot (200 °C) H2SO4 and rinsing in distilled water. To ensure

representative results, several samples from each aging condition were used. Each sample was also

examined with at least two different AFM tips, minimizing the possibility of misinterpreting tipinduced artifacts in the images.

RESULTS AND DISCUSSION

The median strength values of unaged and aged fibers are shown in Figure 3. The median strength

values for both aged and unaged fibers are shown in Figure 4 as a function of the depth of surface

roughness obtained from the AFM cantilever tip motion. Figures 3 and 4 show that the median

strength of fibers decreases and surface roughness of glass increases with increasing aqueous aging,

particularly with Fiber 1. The surface roughness of the fibers increased with increasing aging time

and was more dramatic in liquid water than in humidity.

Figure 5 show the AFM images of Fiber 1 where (a) is the glass surface of the unaged fiber and (b)

is the surface of the fiber aged for 1 month at 85 °C and 85% RH. The roughening that occurred

during the aging process is evident in this figure. The roughened surface showed an increase in

amplitude and wavelength of surface contour with increasing aging time, consistent with earlier

predictions" and findings

34
In summary, we have compared the strength and morphological properties of aged and unaged

fibers. The images in this study demonstrate the versatility of atomic force microscopes. The AFM

provides a new tool for studying long-term mechanical reliability of optical fibers.

REFERENCES

1. P.W. France et al., /. Mater. Sci. 15, 825(1980).

2. H. H. Yuce, et al., Proc. Seventh Int. Conf. on Integrated Optics 2 44(1989).

3. M. J. Matthewson, C. R. Kurkjian, /. Am. Ceram. Soc, 69 [1 1] 815(1986).

4. G. Binnig, et al., Europhys. Lett, 33, 128 1( 1987 ). 5. T. R. Albrecht and C. F. Quate, /. Appl. Phys., 62, 2599(1987)

.

6. G. Binnig, et al., Phys. Rev. Lett., 12, 930(1986).

7. G. Binnig, et al., Phys. Rev. Lett., 49, 57(1982)

.

8. H. J. Mamin et al., Phys. Rev. Lett., 53,1563 (1989).

9. J. E. Stem, et al., Appl. Phys. Lett. 53, 2717(1988).

10. Marti, et al., /. Vac. Sci. Technol. A6, 287(1988).

11. N. M. Amer and G. Meyer, Bull. Am. Phys. Soc. 33, 319(1988).

12. G. Meyer and N. M. Amer, Appl. Phys. Utt. 53, 1045(1988).

13. T. R. Albrecht and C. F. Quate, J. Vac. Sci. Technol, A6, 27 1( 1988).

14. A. L. Weisenhom, Appl. Phys. Lett. 54, 2651(1989).

15. D. P. E. Smith and G. Binnig, Rev. Sci. Instrum. 57, 2630(1986).

16. M. J. Matthewson, C. R. Kurkjian; J Amer Ceram Soc 71; 177 (1988).

17. H.H. Yuce, et al., OFC' 90 Postdeadline Paper, Paper PD14, (1990).

18. R. S. Robinson and H. H. Yuce; J Amer Ceram Soc 74, [4]; 814 (1991).

19. H. H. Yuce, et al.. Trans Glasses for Electronic Applications, (in print)

20. H. H. Yuce, et al., OFC' 92 Postdeadline Paper, Paper PD21, (1992).

STEPPER

MOTOR

COMPUTER

Stepper

Motor

Control

Fiber Sample

Movable Platen

Failure Detection

System

Acoustic Emission

Transducer

Fixed Platen

Fiber

V

Holder Platen

LIGHT

PHOTODIODE

CANTILEVER

XYZ

TRANSLATOR

Figure 1: Schematic diagram of

two-point bending unit

Figure 2: Schematic diagram of AFM

35
on

^400 r

00

200

O Fiber 1 [85 °C water]

Fiber 1 [85 "C, 85% RH]

A Fiber 2 [85 °C water]

O Fiber 2 [85 °C, 85% RH]

I I I I I I 1

1 10

Aging Time, days

100

800

600

00

Cu,

«400

l-H V2

200

O Fiber 1 [85 °C water]

Fiber 1 [85 °C, 85% RH]

A Fiber 2 [85 °C water]

O Fiber 2 [85 °C, 85% RH]

0.1 1 10

Surface Roughness, nm

100

Figure 3: Strength degradation after aging Figure 4: Strength versus surface roughness

(a) (b)

Figure 5: AFM image of surfaces of fiber 1

(a) unaged (b) aged at 85 °C and

85% RH for a month

36
Fiber Microbending Test Method Capable of

Predicting Microbend Loss in Optical Cables

Z. Pasturczyk C. Saravanos

Optical Cable Plant, Northern Telecom Canada Ltd.

Saskatoon, Saskatchewan, S7K3L7

1.0 Introduction

Fiber attenuation caused by small, randomly distributed perturbations along the axis of an

optical fiber is known as microbending loss. This microbending can be an important loss

mechanism of cabled fibers, and may dominate the overall loss if the fiber and cable

designs are not properly optimized. For cables at room temperature the microbending loss

is usually negligible. As the temperature decreases the fiber coating stiffens, while the

cable material contracts, allowing microbending effects to be transferred through the

coating to the glass fiber. At very low temperatures the microbending loss increases rapidly

and performance discrepancies of various fiber types and coatings become apparent. It is

important for a cable manufacturer and end user to be able to determine microbend

sensitivity of an optical fiber as a function of temperature, especially at low temperatures.

By developing the proper test method, it is possible to use uncabled fiber to predict the

microbending loss of a cabled fiber.

The microbending loss is a strong function of the fiber design and the coating material.

Even though the fiber design can reduce this loss significantly, the issue of protecting the

fiber axis from external lateral loads, is entirely up to the fiber coating. Dual coatings due

to their nature offer two levels of protection. A soft cushioning layer surrounding the fiber

and hard protective shell on the outside. Protection provided by single coatings is a

compromise between cushioning and shielding effects. The test method presented in this

paper determines how much protection against external forces is offered by a given coating

over the relevant cable operating temperature range.

Over the years the industry has developed two approaches to test the microbend induced

loss based on the fiber length. The respective examples of long and short length tests are

the basket weave l^l and FOTP-68 also known as the sandpaper test method I^l. The former

one works well at room temperature only and is not suitable at extreme temperatures or

over a wide temperature range. The sandpaper test is not very repeatable and, for that

reason, was recently rescinded by the TLA coating subcommittee. Various test methods

have been developed since, based on short fiber length principles 1^1. Even though the

repeatability of these methods has improved they have one common problem. The loss

induced by these tests in a short piece of fiber exceeds by orders of magnitude the worst

case microbend loss per unit length measured in cabled fibers. There are two different loss

mechanisms in these short and long fiber tests. A plastic deformation of the coating, in

short fibers, is known to cause a drastic increase in microbending loss In long fibers, the

coating is never deformed beyond its elastic limit. Only in this latter case does the loss

mechanism, and the resulting microbending loss, correspond to the actual cable-induced

fiber loss.

37
i

i

2.0 Proposed Test Method i The proposed test method is based on the well known basket weave method W. The | optimum fiber length is 2.5 kilometers. Shorter lengths give noisy results, while longer ones

do not offer any further improvements. The fiber is wound on a drum made of pure silica.

The choice of the material was made to guarantee its thermal compatibiUty to the optical

fiber and assure minimum fluctuations of the winding tension with temperature. The fiber

was wound on a 111 mm diameter drum with 2 mm pitch and under 70 g of tension. It should be noted that the diameter of the pure silica drum is large enough to eliminate any

macrobend induced loss.

Once on the drum, the fiber is placed inside a temperature chamber with one fiber end

cleaved and the other spliced to a 1550 nm OTDR. The temperature of the chamber is

cycled between -40°C and +80°C at a rate of one degree per minute. The slow rate is

necessary to prevent tension transients due to temperature gradients between winding

layers. The OTDR and the temperature chamber are computer controlled. Every two

minutes the computer receives the OTDR trace as well as the temperature readout. The

traces are processed by truncating 250 m from each end of the fiber and measuring the

attenuation of the middle 2 km section. Nonlinear portions of the OTDR trace,

corresponding to underwrap (next to the smooth drum surface) or overwrap layers (not

restrained from the top) are thus ignored. The middle section is always linear. The fiber

attenuation data are plotted versus temperature as shown in Figure 1.

3.0 Test Results

The measurement repeatability has been verified by rewinding and retesting the same

fiber. The results were repeatable to within ± 0.03 dB/km over the temperature range of - 40°C to +80''C. Even better repeatability of ±0.02 dB/km was observed when the same

fiber went through several temperature cycles without rewinding. The fact that the induced

loss mechanism is completely reversible indicates that there is no permanent (plastic)

deformation of the coating. Subsequent visual examination of the fiber confirmed this

finding.

Figure 1 shows the microbend sensitivity of three commercially available fibers. Fibers 1

and 3 are dual coated while fiber 2 is single coated. Only low temperature attenuations are

shown because for all the fibers tested the loss above +20°C was constant. At room

temperature fiber 2 is the most sensitive to microbending and fiber 3 the least. The coating

of fiber 3 provides almost perfect protection against microbending. The 0.2 [dB/km]

attenuation of fiber 3 is t^ical for 1550 nm and ideal winding conditions. As the

temperature decreases fiber 3 becomes more sensitive to microbending. Below -20*'C its

attenuation is significantly higher than the attenuations of fibers 1 and 2. The reason for

this behavior is due to reduced thermal stability of its coating system. The coating material

at low temperatures becomes stiff and the cushioning effect disappears.

The dual coating of fiber 1 provides low microbend sensitivity over a wide temperature

range. This type of dual coating outperforms single coating (fiber 2) at all temperatures.

The slightly higher attenuation of fiber 1 as compared to fiber 3 at temperatures above O^C

are made up for by the improvement at low temperatures. The small difference at room

temperature does not necessarily mean that the coating of fiber 1 is stiffer to start with. As

mentioned in the introduction, fiber design and mode field diameter affect microbend

sensitivity to a certain degree. Differences in the form of constant offset were observed

38
between two fiber samples having identical coatings, and were caused by variations in the

fiber parameters.

The cable performance of fibers 1 and 2 was tested by putting 12 one kilometer lengths of

each fiber type into a loose tube cable. Each tube contained equal count of both types of

fibers to eliminate potential tubing differences. The attenuation of each fiber was

measured as the temperature of the cable was cycled from -60°C to +60°C. The data

acquisition was performed according to Section 2.0. The low temperature range was

extended in order to compensate for the higher microbending loss induced by tight winding

on the glass drum. Lowering the temperature increased both, the cable-induced

microbending loss and fiber sensitivity to microbending.

The results of the cabled fiber experiment are shown in Figure 2. Each data point

represents an average of 12 fibers. The curves fitted to the data have similar shape and

relative position as the corresponding curves shown in Figure 1. Inside the cable, the dual

coated fiber (1) showed lower microbend loss than the single coated fiber (2) with the

improvement being more prominent at low temperatures. In addition, the performance of

fiber 1 was more consistent as seen from the standard deviation that was added to the

mean value and shown in Figure 2 as a thin line. It is seen that the attenuation of cabled

fiber is not as repeatable as the proposed test method. The microbending loss mechanism

inside a cable is more complex and difficult to reproduce rendering the results of a single

fiber test not representative.

4.0 Conclusions

The test method for determining fiber sensitivity to microbending over a wide temperature

range has been proposed and evaluated. The applied loss mechanism does not deform the

fiber coating beyond the elastic limit. Excellent repeatability of ±0.03 dB/km for

temperatures fi"om -40°C to + 80°C was achieved. Much less repeatable cable-induced loss

requires testing of many samples to draw valid conclusions. The good correlation between

the results of this test and cable test over a wide temperature range makes it possible to

predict performance of fibers inside the cable. This test method is an economical tool for

quick evaluation of new fiber coatings. Considerable fiber performance shortcomings,

impossible to detect at room temperature, are now possible to identify and quantify for the

critical low temperature region.

5.0 References

[1] A. Tomita, P.F. Glodis, D. Kalish, P. Kaiser, "Characterization of the bend sensitivity

of single-mode fibers using the basket-weave test". Tech. Digest - Symposium on Optical

Fiber Measurements 1982, Boulder CO, NBS S/P 641, pp. 89-92, 1982.

[2] FOTP-68 "Optical Fiber Microbend Test Procedure", ANSI/EIA-455-68-1988.

[3] P.A. Sutton, J.L.L. Roberts, A.T. Summers, A. Phoenix, D. Rees, "Development of a

non-destructive test for microbend loss mechanisms in cabled fiber". International Wire

& Cable Symposium Proceedings 1989, Atlanta GA, pp. 450-455, 1989.

[4] Y. Katsuyama, Y. Mitsunaga, C. Tanaka, T. Waki, Y. Ishida, "Optical loss stability of a

fiber cable under lateral force and optimum design of the coated fiber structure", Appl

Opt., vol. 21, pp. 1337-1341, 1982.

39
1.05

0.85 - m

o 0.65

"o

3

C

0) 0.45 - o

0.25 -

i I

«

1 „..L

i 1

\

\

\

J^iber 3

i i

X

X

\

^er 2 ,

i 1

Fiber 1

X

\

X

X

X

X

-40 -30 -20 -10 0 10 20

Temperature [deg C]

Figure 1. Microbend Loss of Fibers on Glass Drum

0.45 -I

E

^ 0.40

CD

T3

c 0.35

o

i 0.30

0)

o 0.25

in

0.20 -

Ni

: S

= > Fiber 1 —

solid lir |e

N

N.

Fiber

Mean

2 —

value

dashed —> thi

1 line

ick line

;t^<^

• •>

"X,

"S.

Xfc

^^

I

•x. •••^•h:

•X

Xk

X.

One s

X.

•X - «. ^

Fiber 2

igma

•X "x

--> thi h line

Fiber 1

' arroA.c

•m t

i

.'.•.J

-60 -50 -40 -30 -20 -10 0 10

Temperature [deg C]

Figere 2. Performance of Fibers inside a Cable

20

40
Measuring Method for Ribbon Fiber Skew

Ryuichi Matsuoka, Hiroshi Satoh, and Nobukazu Kume

The Furukawa Electric Co., Ltd.

Opto-Technology Laboratory

I. BACKGROUND

Recently, optical parallel transmission methods between computers have been

studied by various fields from the viewpoint of transmitting and processing

large volumes of information. The delay time difference (skew) of signals

between optical fibers has become an important factor in optical parallel transmission, requiring the skew to be minimized. In this respect, ribbon fiber is most

suitable among various types of optical fiber. The phase shift and pulse delay

methods in measuring skews of ribbon fiber have been compared and the result

of the comparison have been studied as reported below.

II. SKEW FACTOR ANALYSIS

The following three factors can be listed as the causes for skews which occur

with ribbon fiber:

1. Difference in signal propagation time caused by fiber parameter deviation.

2. Difference in residual distortion during ribbon fiber manufacture.

3. Difference in physical length caused by structural non-uniformity of ribbon

fiber.

II -1. Skew Caused by Fiber Parameter Deviation.

Using optical fiber parameters, the fiber delay time t can be given by the

following expression:

- "dF - ^1-"^ "dV"^ (1)

where ;5 is a propagation constant, b is a normalized variable of yS, v is a

normalized frequency, Na is a cladding group refractive index, and A is the

relative refractive index difference. This means that the ribbon fiber skew is decided by differences of the A value and the value of d(vb)/dv of each core.

II -2. Skew by Residual Distortion of Ribbon Fiber During Manufacture.

Residual distortion differences between cores are caused by difference of

supply-back tensions of uncoated fibers during the manufacture of ribbon fiber.

The skew ^t between cores in ribbon fiber of length L applied with distortion

£ 1 and £ 2 («1) can be expressed as follows.

At = U-l){£i- £2) — (2) c

where at is a photo-elasticity coefficient.

41
II -3. Skew by Structural Non-uniformity of Ribbon Fiber.

Physical lengths of optical fibers coiled on a bobbin or drum differ due to

distortion applied to cores if ribbon fiber has curves or misalignment. The

misalignment dimension d of the ribbon fiber is estimated and the value of skew

caused by misalignment based on the diameter r of the take-up bobbin (drum) is

calculated. The skew At between cores can be calculated as follows if the photo-elastic effects are taken into consideration:

At=ALX^ = (3)

c c r

ffl. SKEW MEASURING METHOD

Fundamentally, the ribbon fiber skew is measured by a method similar to that

applied to measuring chromatic dispersions of SM optical fibers. Utilizing the

apparatus for measuring chromatic dispersions, instead of changing of

wavelength, fibers to be measured are changed in succession to obtain relative

skews among fibers. As in chromatic dispersion measurement, skews can be

measured by the phase shift method (frequency domain method) and pulse delay

method (time domain method). A comparative study of the two methods has been

made as follows.

III -1. Phase Shift Method

The light sinusoidally modulated in power is driven into the optical fiber,

and the light emitted from the fiber is converted into an electrical signal by a

photodetector The phase angle difference of this output light to a reference

signal is measured for each fiber and determine the ribbon fiber skew.

Figure 1 Apparatus for measuring ribbon fiber skew by the phase shift method.

42
I

I

in -2. Pulse Delay Method

Skew are measured by the pulse delay method by the directly measuring the

signal delay time. An electrical signal generated by a pulse generator is

converted into an optical pulse by a laser diode (LD) and injected into the optical

fiber to be measured and the light emitted from the measurable fiber is

reconverted into an electrical pulse by an avalanche photodiode (APD). The time

and electrical level are measured by an oscilloscope. Delays are caused such as

responses by the pulse generator, LD and APD. However, skews between fibers

can be measured by changing optical fibers to be measured in succession.

test

ribbon fiber

time delay

generater

sampl ing

osci 1 loscope

Figure 2 Apparatus for measuring ribbon fiber skew by the pulse delay method.

IV. STUDY OF MEASUREMENT RESULTS

Skews on the same ribbon fiber were measured to compare the accuracy of

the two skew measuring methods, namely, the phase shift and pulse delay

methods. Figure 3 and 4 show the results of skew measurement on 4-core SM

ribbon fibers 100m in the length by the phase shift and pulse delay methods. The

results of the two methods were nearly identical except in deviation of measured

values. The deviation of measured values in three times by the phase shift

method was 13ps, compared with maximum 37ps by the pulse delay method. The

two measuring methods seemed to differ in terms of measurement accuracy.

Deviations of skew values by the two measuring methods were compared by

extensively changing the measurable lengths from 1000m to 10m in the length to

evaluate the accuracy of the two measuring methods. (See Figure 5.) The

measured skew values of ribbon fibers longer than 100m coincided with those of

ribbon fibers 100m in length, and those deviations also were small. Deviations of

measured skew values per 100m naturally increased when the measurable fiber

length was shorter than 100m. Deviations of measured skew values were

generally larger with the pulse delay method than with the phase shift method,

indicating that the pulse delay method would not be suitable for measuring

fibers shorter than 100m in length.

43
Measurement by the phase shift method involves processing of electrical

signals in a narrow band near a modulating frequency (200MHz) and the signal to

noise ratio of the measurement system can be increased relatively so that

measurement can be accurate. The pulse delay method is better than the phase

shift method in terms of measuring absolute values of the delay time. However,

the measurement system of it requires a high-speed response and time

resolution to the order of several picoseconds. As a result, effects of jitter by

each measuring equipment are expected to increase relatively. The measurement

system of the phase shift method is easy to operate and the cost of it will be low

compared with that of the pulse delay method. Taking these factor into

consideration, the phase shift method appears to be better than the pulse delay

method in measurement of ribbon fiber skews.

Figure 5 Deviations of Skew Values

44
REVIEW OF RECENT DEVELOPMENTS IN FIBRE GEOMETRY MEASUREMENTS.

J. Baines, K. Raine

National Physical Laboratory

Division of Mechanical and Optical Metrology

Queens Road

Teddington

Middlesex, TWll OLW

U.K.

REQUIREMENTS

Present specification requirements are for fibre diameter

to be determined to ±2jum with the possibility that this will be

reduced to ±l/im in the future. If we apply the calibration

standard rule of thumb then the standards laboratories are thus

required to make measurements with uncertainties of about O.ljum.

The standard deviation of grey scale mean diameter

measurements is typically 50nm and this has recently led to a

demand from the instrument manufacturers for a calibration

uncertainty of better than O.ljum. This contrasts with the view

of the fibre manufacturers who see little evidence to justify a

tightening of the specification that may follow from improved

measurement procedures

.

The grey scale instrument has become the most widely used

in industry because of reliability and operator independence.

Also used, but less widely, are the refracted and transmitted

near field, side view and image shearing methods. All of these

instruments require calibration and the standards laboratories

have investigated other methods, usually less convenient, which

are directly traceable to national length standards.

WORK OF THE STANDARDS LABORATORIES

Philosophically the units of length are not important

provided everybody uses the same unit. Since we require a

45
reference artifact or physical property it is best if the unit

is one which is easily realised; hence the use of the micrometre

which can be easily obtained from the wavelength of laser

sources

At the National Institute for Standards and Technology

(NIST), Matt Young, Steven Mechels and Paul Hale are using a

Mireau white light interferometer, a contact micrometer and a

scanning confocal microscope. These instruments use

interferometers for length measurement and the overall

uncertainty of the diameter measurement is 50nm (3a). They plan

to start selling transfer standard fibres by the summer of this

year.

At the National Physical Laboratory (NPL) we make traceable

measurements with a Michelson white light interferometer, an

image inverting microscope and a scanning confocal microscope;

in all cases the position of the sample stage is measured with a

length measuring interferometer. The edge setting offset of the

image inverting microscope is traceable to scanning electron

microscope measurements made with interferometrically measured

stage movement. We have some small doubts about the uncertainty

of the scanning confocal microscope; in our instrument, which

has a standard deviation of about 6nm, the measured diameter has

a small dependence, amounting to 30nm, on the pinhole size. As

far as we are aware this is not predicted by the theory.

At present these methods are used to calibrate an image

shearing microscope which is more convenient to use but it

requires some care to achieve the 0.15/im uncertainty we

typically quote when selling transfer standards.

The early indications are that the calibration stability of

grey scale instruments is good to about 0.1% over a period of

months. We are therefore developing a high resolution grey

scale instrument to provide cheap calibrations to customers with

uncertainty of 50nm or better. The instrument uses a high

resolution camera in which the pixel size corresponds to less

46
than 0.15/im in the object plane. We have performed sampling

calculations to show that with a 0.65 numerical aperture

objective this instrument should produce a diffraction limited

edge, between about 20% and 80% edge intensity levels, leading

to increased precision and a true edge location at the 25%

intensity level with coherent illumination of the object.

We have compared measurements on two fibres using our image

shear results with some results from NIST using the contact

micrometer. The agreement on diameter measurement for a fibre

with a non-circularity of 0.3% was O.OljLtm but for one with a

non-circularity of 1.5% it was only O.OSjum. Further work is

being done to try to resolve this discrepancy. There is an

ongoing exchange of information between the two laboratories to

ensure that calibration methods and reference materials are

widely acceptable with a high degree of confidence.

We have continued the investigation into using chromium on

glass masks for calibrating grey scale instruments to see under

what circumstances it is possible to obtain an accurate

extension, rather than just scale, calibration. Joint work

between the authors and Dave Levitt of BT has shown that it is

possible to calibrate back-lit grey scale instruments directly,

with an uncertainty of O.ljum or better, using a chrome circle

without the need to determine the edge setting offset using a

fibre.

INTERCOMPARISONS

r

The first CCITT round robin in 1989 [1] showed that there are

short range variations in fibre diameter of the order of 0.2/xm.

More recently John Colton and Andy Hal lam at York Technology

showed a worse case variation of about 2 . 5jum over lengths of 0.5

metre within their data set [ 2 ] . It is therefore clear that a

meaningful comparison between different laboratories can only be

47
made if all participants measure the same cleaved ends, unless

it can be shown that longitudinal variations are insignificant.

In order to try to preserve fibres during storage and

transportation for intercomparisons , Photon Kinetics have

developed a retractable fibre holder which it is hoped will

protect the fragile cleaved end from the attentions of Carriers

and Customs officials.

In the US the Telecommunications Industry Association (TIA)

has organized an intercomparison of grey scale cladding diameter

measurements amongst 8 participants with NIST as the

coordinating laboratory and it is intended that the results will

be presented by Tim Drapela at this meeting.

Within the European Cooperation in Science and Technology

framework, COST 217 organized a round robin between 12

participants using the side-view, grey scale, transmitted near

field, contact micrometry, image shear and refracted near field

technigues. For this intercomparison NPL circulated a fibre

with diameter variations along its length of less than O.lSjum.

The results confirmed that such instruments are capable making

measurements with an uncertainty of 0.15/im when used with

suitable calibration artifacts and measurement procedures

although the refracted near field measurements showed excessive

non-circularity as has been observed in earlier studies [3].

A second CCITT round robin was proposed last year and

preparations are under way to enable this to be started soon.

STANDARDS COMMITTEE ACTIVITY

Committees and working groups of the different standards

organizations have been active in various areas. A task group

has been set up under lEC TC86 WG4 to produce a calibration

procedure document for cladding geometry measurements but this

48
work is still at an early stage. One area of concern is the

current plethora of existing and emerging standards in this area

and the sometimes conflicting definitions of quantities to be

measured. For instance, cladding non-circularity is defined

differently in CCITT and TIA documents and, in the case of

multimode core diameter the so-called 'k factor' level at which

measurements are made is specified in EIA FOTP 58 as 2.5% but in

CCITT G651 as 5%, unless otherwise specified.

Also of current interest is the type of algorithm most

suitable for filtering and form-fitting to fibre edge data used

in the analysis of grey scale images. Fourier analysis, best

fit ellipse and cubic spline fits are all under consideration by

the TIA and CCITT. The performance of each algorithm depends on

the shape of the fibre and on the angular extent of any cleave

damage present. Andy Hallam and Neil MacFarlane of York

Technology have cleaves made with different cleave tensions to

investigate the robustness of the algorithms. They found that

as the amount of cleave damage, or hackle, on the fibre end

increased a difference of ljum between the filtered ellipse and

the Fourier algorithms was seen. Further, a cubic spline

developed by Tom Hanson of Corning showed a bias of between 0.5%

and 1% in non-circularity on these data sets. However, Tom has

recently shown that the spline fit may be modified to eliminate

this bias. In its current draft form the TIA test procedure

FOTP 176 allows the user to opt for any algorithm which passes a

defined quality of fit test.

NEW MEASUREMENT NEEDS

All of the above has referred to the geometry of bare

fibres. However, in the last year there has been interest in

developing traceable calibration methods for measurements of

diameter, thickness and concentricity of fibre coatings with an

uncertainty of 0.5jum for the primary coating diameter. This has

led to the development of new commercial instruments with

49
different calibration requirements.

The increasing use of ribbon cables is leading to demands

for accurate X-Y coordinate measurements of the positions of

individual fibres within the cable. It would appear that

requirement will be for an uncertainty of about O.S/im in 1cm for

ribbon fibre.

CONCLUSIONS

Standards labs are now capable making fibre diameter

measurements with an uncertainty of less than about 0.1/im or

better. At present we typically measure eighteen diameters each

with an individual uncertainty of O.lS/um but we hope to reduce

this to less than O.ljum in the near future. NIST plans to

provide reference test materials certified to 50nm by the summer

of 1992.

Recent work indicates that back-lit grey scale systems can

be directly calibrated with an uncertainty of O.l/xm or better

with an opaque chromium disk.

The dependence of the fibre diameter measurement on the

curve fitting routines and the type of cleave damage is not

fully satisfactory if accurate measurements are to follow

accurate calibration, whether by a reference fibre or a chromium

mask.

REFERENCES

CCITT Study Group XV, "Results of a round-robin study into the

measurement of the geometrical properties of single-mode

fibres", September 1989.

J. R. Colton, A. G Hallam, "Measurements on the uniformity of

fibre cladding geometry", in OFMC Optical Fibre Meas. Conf.

Digest, York, September 1991, ppl27-131.

J. G. N. Baines, A. G. Hallam, K. W. Raine, N. P. Turner, "Fibre

diameter measurements and their calibration", J. Lightwave

Tech., vol.18, no. 8, September 1990, ppl259-1268.

50
Fiber Cladding Diameter Measurement

by White Light Interference Microscopy

P. D. Hale and D. L. Franzen

National Institute of Standards and Technology

Electromagnetic Technology Division

325 Broadway, Boulder, Colorado 80303

Measurement of fiber cladding diameter with an overall uncertainty of

0.1 /im, or less, has been an ongoing project at NIST for the past three years.

The main purpose of this project is to produce standard reference fibers for

calibration of gray scale video microscopes. To help evaluate the systematic

errors of any one technique, we have constructed three absolute measurement

systems: a scanning confocal microscope, contact micrometer, and white light

interference microscope.

The white light interference microscope uses a Mirau interference

objective with a single surface in contact with the fiber. We chose a Mirau

objective over the Michelson objective used in previous works [1,2] because of

its stability, ease of operation, and higher magnification. Single-surface

contact was chosen because a knowledge of the fiber index profile is not

required [1] and material dispersion is avoided.

The system was constructed from commercial metallurgical microscope

parts and uses bright- field illumination from a halogen lamp, a binocular

eyepiece, and a CCD array video camera with 400x overall magnification. The

camera is connected to a video analyzer and monitor. An optical flat is held

perpendicular to the optical axis (±100 /zrad) with a high quality mirror mount

on a precise three-axis translation stage. The position of the flat is

monitored by a commercial interferometer with 1.2 nm least count. The

interferometer, translation stage, and video analyzer are all controlled by

computer, and repeated measurements can be made automatically (Fig. 1).

The mirror mount has been modified to hold a standard NIST fiber holder

along with a cantilevered brass weight which holds the fiber against the

optical flat. The fiber holder positions the fiber on top of the flat and can

be interchanged among all three NIST measurement systems. The brass weight

has a semi -circular cross section and is thin enough to fit between the fiber

and objective with the curved side toward the fiber. It has a 0.76 mm slot

through which the fiber and flat are viewed (Fig. 2). The weight can be

oriented using two precise 1/4-80 screws so that even pressure is applied to

the fiber from both contact points. The weight deforms the fiber by about 0.5

fim, but the deformation is localized to a region within 20 /zm of the edge of

the slot. Measurements of the fiber diameter are made 380 yum from both fiberweight contacts. Contact between fiber and flat is verified within 100 nm by

viewing contact white light interference fringes analogous to Newton's rings.

Surface contamination larger than 25 nm on or adjacent to the fiber can be

detected using colored fringes.

The top surface of the fiber or flat is located by scanning the stage

and recording white light fringes using the video analyzer while the position

of the stage is monitored. The central fringe of the interferogram is then

fitted to a parabola to average out random fluctuations in intensity and

position tracking (Fig. 3). The region of the video image to be sampled is

51
located using a set of cross hairs on the video monitor. Since the field of

view is a few fiber diameters in extent, no lateral motion of the translation

stage is required during a measurement. To measure the diameter of a fiber,

the flat is located, the top of the fiber is located, and the difference is

calculated. The flat is then relocated and the difference is again

calculated. The average of these two measurements is one datum which has been

corrected for linear drift. When this measurement is repeated without fiber

replacement, the standard deviation of the measured mean diameter is about 10

ran. When the fiber is removed and replaced between measurements the

uncertainty becomes 25 nra (all uncertainties are reported as 3a)

Placement of the cross hairs on top of the fiber is subject to

digitizing and random errors. Slight offset of the cross hairs makes the

measured fiber diameter too small. A 40x Mirau objective was used to estimate

the magnitude of this effect. Six measurements on a control fiber were made

alternating with measurements using the 20x objective. The 40x objective gave

values 6±12 nm larger than the 20x objective. If probability distributions

for placement of the cross hair are the same for both objectives, the mean

offset of the measured diameter using the 40x objective is 4 times smaller

than the offset for the 20x objective. Then, the correction factor is (4/3)x6

nm or 9±15 nm. All subsequent measurements include this additive correction

factor, giving a total uncertainty of 30 nm.

Finally we have compared measurements on fibers using the white light

interference microscope, a contact micrometer [3], and a scanning confocal

microscope [4] . Measurements were made by different operators without prior

knowledge of each others' results. The diameter of each fiber was measured at

two specific angular orientations, thus giving a comparison of eight measured

diameters (Fig. 4) . Results of the interference microscope and scanning

confocal microscope are plotted with respect to the contact micrometer. The

mean difference between the contact micrometer and interference microscope was

-1 nm and the rms difference was 15 nm. Similarly, the difference between the

contact micrometer and the scanning confocal microscope was 16 nm and the rms

difference was 29 nm.

Matt Young and Steven Mechels made the measurements with the contact

micrometer and the scanning confocal microscope, and provided many useful

insights on precise fiber measurements. Contribution of NIST, not subject to

copyright

References

[1] K.A. Emig, "A comparison of interferometric techniques for fiber cladding

diameter measurements," NIST Spec. Publ. 792, Technical digest- Symposium

on optical fiber measurements. 1990. ed. by G. W. Day and D. L. Franzen,

pp. 135-138, 1990.

[2] M. J. Saunders, "Noncontact interferometric determination of the outside

diameter of optical fibers," NBS Spec. Publ. 748, Technical digestSymposium on optical fiber measurements. 1988. ed. by G. W. Day and D.L.

Franzen, pp. 149-152, 1988.

[3] M. Young, "Fiber cladding diameter by contact micrometry," Digest, Optical

Fiber Measurement Conf . , York, U.K., 1991, pp. 121-126 (NPL, Teddington,

Middlesex, England).

[4] S. Mechels and M. Young, "Scanning confocal microscope for accurate

dimensional measurement," SPIE, Vol. 1660, 1992, in press.

52
Illuminator

Fiber

Weight

XYZ Stage

With Tilt

B&W

CCD

Camera

Y

Mirau

Objective

Optical

Flat

Video

Analyzer

Desktop

Computer

Differential

Distance

Measuring

Interferometer

Fig. 1. White light interference microscope system

based on Mirau objective.

Optical

Fiber

Optical

Flat

Mirau Interference

Objective

Coating

To

Laser

Interferometer

Fiber

Weight

Fig. 2. Schematic of fiber held in contact with

optical flat.

53
2.0

c

t-l OS

1.5

c 1.0

0.5

X Measured data —Parabolic fit

X X

X X

1.5 -1.0 -0.5 0.0 0.5

Position, um

1.0 1.5

Fig. 3. White light interf erogram with parabolic best

fit to central fringe.

B

o

a

50

40

30

20

10

0

o -10

««-( 2-20

50

-30 o CM

-40 — m IM

123.5 124.5 125.5

Fiber diameter, um

Fig. 4. Comparison of interference microscope (IM)

and scanning confocal microscope (CM) with respect

to contact micrometer. Measurements of fiber

diameter were made at two angular orientations on

four fibers.

126.5

54
ACCURATE MEASUREMENTS OF FIBER CLADDING DIAMETER

Matt Young, Paul D. Hale, and Steven E. Mechels

U. S. National Institute of Standards and Technology

325 Broadway, Boulder, Colorado 80303

This paper reports the development of an artifact standard for video microscopes devoted to measuring optical fiber geometry [1]. Specifically, we have developed three devices, a contact micrometer, a

scaiming confocal microscope, and a white-light interference microscope, that are capable of absolute

measurements with accuracy between 50 and 100 nm.

A video microscope dedicated to fiber geometry is called a gray scale system by the

Telecommunications Industry Association (TIA). Gray scale systems are typically used to determine the

outer, or cladding, diameter of a cleaved fiber end; the ovaUty or noncircularity of the cladding; and the

decentering, or concentricity error, between the core and the cladding. Measurements of ovality and

decentering do not require absolute accuracy, but cladding diameter must be measured within 0.1 /Mm or so

in order to manufacture efficient connectors that do not require manual adjustment. Measurements made

with video microscopes, however, may well suffer from a systematic error of a few-tenths micrometer [2].

The TIA subcommittees we work with therefore requested that NIST develop an artifact standard for

calibrating the gray scale systems. (The National Physical Laboratory in the U. K. has a similar program

[3].) The subcommittees were, however, reluctant to accept any standard other than an optical fiber

because the measured result is a function of illumination and because reflection from a metal film displays

phase shifts that are not present in reflection from a glass edge [1,4].

In this paper, we will describe the contact micrometer, scaiming confocal microscope, and whitehght interference microscope that we have set up to measure the cladding diameter within 0.1 /urn or less. Because of the ease of using the micrometer, it will become the instrimaent that we use regularly to prepare

standards. Because of the need for accuracy (as opposed to precision or repeatability), however, we require

the other methods to verify the accuracy of the micrometer.

Contact micrometer

We acquired a contact micrometer from our laboratory in Gaithersburg, Maryland. The

micrometer consists of a stationary, cylindrical anvil and a flat, moving spindle (Fig. 1). Both are steel and

have been pohshed with hard laps in an optical shop. The spindle is ahgned so that the pohshed end is accurately parallel to the anvil. It rides horizontally on an air bearing and is pressed against the fiber with a known force. Its position is measured by a commercial interferometer that has a least count of 1.25 nm [5].

The fiber and the anvil make a point contact, so there is measurable compression at that point. The

compression depends on the elastic constants of the materials and can in principle be calculated from a standard formula. The calculated value is then used as a correction to the measurement.

To test the formula, we measured the diameter of different fibers and steel thread wires as a fimction of the force the spindle exerts on the fiber. If the formula is correct, the measured diameter will

be constant, independent of force. Possibly because we do not know the elastic constants of the materials

accurately, the measured diameter is not constant (see Fig. 2). The formula does, however, display the

correct functional dependence on force: force to the two-thirds power. Therefore, instead of reljang on the

compression calculated by the formula, we measured the diameters of a number of fibers as a function of

force and calculated the slopes of curves similar to the "Raw data" curve in Fig. 2 in order to extrapolate to the values of the diameters in the absence of force on the fiber. Fortunately, all the fibers we tested gave

the same slope, within experimental uncertainty.

55
The statistical uncertainty of measuring the slope gives rise to a systematic uncertainty of the

diameter of about 3 mn (la). Additionally, the random uncertainty of a single set of 6 measurements is typically about 12 nm. Finally, a random uncertainty of 8 mn results from the roughness of the anvil.

Other xmcertainties we have been able to identify are less than 1 nm. Adding the random imcertainties in

quadrature and then adding the systematic uncertainty arithmetically, we find that the overall uncertainty of

the measurements is about 17 nm. We take the overall imcertainty of any measurement to be ±3 times this

value, or ±50 nm.

Confocal Microscope

We constructed a stage-scanning confocal microscope (Fig. 3) that also uses a conmiercial

interferometer to measure position [4]. The imagery in a confocal microscope is highly coherent, unlike that

in a conventional video microscope. We can therefore find the true location of the edge by calculating the

ampUtude image and seeking its inflection point (this procedure is equivalent to finding the 25 % intensity

point).

To help assess the accuracy of our instrument, we first measured widths of chrome-on-glass lines

that had previously been calibrated by our laboratory in Gaithersburg. Our measurements agree with the

Gaithersburg measurements within about 30 rmi, though the sign of the disagreement depends on whether

the line is clear or reflecting [4].

We measured diameters of carefully cleaved fibers and found a random uncertainty of 40 nm (3a).

Taking care not to rotate the fibers, we measured the diameters of the same fibers with the contact

micrometer. The results are plotted as open squares in Fig. 4, which shows the arithmetic difference

between the two measurements as a function of the diameter measiu-ed by the micrometer. That is, the

horizontal line represents the micrometer measurements, and the points represent differences from those

measurements.

On average, the confocal microscope measurements exceed the micrometer measurements by 20

nm; this is approximately the increment by which the confocal microscope measurements exceeded the

Gaithersburg measiu^ements of reflecting metallic lines. We cannot explain this discrepancy and do not

know whether it represents a systematic uncertainty in the confocal microscope. The single outiying point in

Fig. 4 is a measurement taken on a fiber that we think had very high stress and may have become deformed

when it was cleaved, but that is a subject for another time.

Interference micrn.scnpe

We also constructed a white-light interference microscope which uses a Mirau interference objective

and a partial contact method for locating the fiber surfaces [6]. We chose a Mirau objective over the

Michelson objective used in previous works [7,8] because of its stabUity, ease of operation, and higher

magnification. We chose a partial contact method because, otherwise, we would have to know the index

profile of a test fiber in order to perform absolute measurements [8].

The system consists of a commercial metallurgical microscope with bright-field illumination and a video monitor. An optical flat is held parallel to the Mirau objective, and the fiber is held against that flat by a semi-circular brass weight that contains a slot through which the fiber and flat are viewed. The

position of the flat is monitored with a commercial interferometer. The white-light fringes are monitored

with a video analyzer. All data acquisition and motion control are automated. Contact between fiber and

flat is verified by viewing white-Ught fringes analogous to Newton's rings; a separation of ~ 100 nm can be

detected by examining the central dark fringe visually. In addition, particles with diameters of the order of

25 nm can be detected by viewing white-light fringes from the Mirau objective.

56
Measurements are made by observing white-Kght fringes from the top of the fiber and then from

the flat. If the fiber is not removed, the random micertainty (la) of a set of measurements is about ±3 nm;

the uncertainty increases to ±8 nm when the fiber is removed and replaced between measurements.

We used the interference microscope to measure the diameters of the same set of fibers we

described with regard to Fig. 4. We had to remove the fibers from their holders and remount them, so we

remeasured their diameters with the micrometer, in case there had been rotation when the fibers were

remounted. We show the results as closed squares in Fig. 4; the mean difference between the interference

microscope and the micrometer is a few nanometers.

Discussion

We estimate the overall imcertainty (3 standard deviations of the mean) of the micrometer as 50

nm; of the confocal microscope as 40 nm; and of the white-light interference microscope as 30 nm. The

interference microscope and the micrometer agree remarkably well, whereas the confocal microscope may

display an additional systematic error of 20 nm. The microscopes are hard to use, so, as of the time of this

writing (May, 1992), we are preparing to use the micrometer to characterize Standard Reference Materials

that will shortly be made available from our offices in Gaithersburg [9].

Ted Doiron of NIST, Gaithersburg, Maryland, designed and constructed the micrometer. This

paper is based on a paper presented at the 1992 Conference on Precision Electromagnetic Measurements

and is a condensation of the paper to be pubUshed in the proceedings of that conference. Contribution of

NIST, not subject to copyright.

References

[1] M. Young, Standards for optical fiber geometry measurements," Natl. Inst. Stand. Technol. Spec.

Publ. 792, Technical digest-Symposium on optical fiber measurements, 1990, ed. by G. W. Day and

D. L. Franzen, pp. 129-133, 1990.

[2] S. Mechels and M. Young, "Video microscope with sub-micrometer resolution," Appl. Opt., vol. 30,

pp. 2202-2211, 1 June 1991.

[3] A. CoUyer, K. W. Ratne, and J. G. N. Baines, "Investigation into the use of a scanning confocal

microscope as a primary measurement method for determining optical fiber diameter," Digest, Optical

fiber measurement conference, York, U.K., pp. 143-146, 1990.

[4] S. Mechels and M. Young, "Scanning confocal microscope for accurate dimensional measurement,"

Proc. Soc. Photo-Optical Instrum. Engrs., vol. 1660, 1992, in press. S. Mechels and M. Young,

"Scanning confocal microscope for precise measurement of optical fiber diameter," Proc. Soc. PhotoOptical Instrum. Engrs., vol. 1556, pp. 164-170, 1991.

[5] M. Young, "Fiber cladding diameter by contact micrometry," Digest, Opticalfiber measurement

conference, York, U.K., pp. 123-126, 1990.

[6] P. D. Hale and D. L. Franzen, "Fiber cladding diameter measurement by white light interference

microscopy," this conference.

[7] K. A. Emig, "A comparison of interferometric techniques for fiber cladding diameter measurements,"

Natl. Inst. Stand. Technol. Spec. Publ. 792, Technical digest—Symposium on optical fiber

measurements, 1990, ed. by G. W. Day and D. L. Franzen, pp. 135-138, 1990.

[8] M. J. Saunders, "Noncontact interferometric determination of the outside diameter of optical fibers,"

Nat. Bur. Stand. (U.S.) Spec. Publ. 748, Technical digest-Symposium on optical fiber measurements

,

1988, ed. by G. W. Day and D. L. Franzen, pp. 149-152, 1988.

[9] National Institute of Standards and Technology, Office of Standard Reference Materials, Room B311,

Chemistry Building, Gaithersburg, Maryland 20899; 301-975-6776. Refer to SRM 2520, Optical Fiber

Diameter Standard.

57
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58
MEASUREMENT OF FIBRE COATING GEOMETRY BY GREY-SCALE ANALYSIS

Andrew G Hallam, Julia H Shaw

York Technology Ltd, School Lane,

Chandlers Ford, Hants, S05 3DG. UK

INTRODUCTION To preserve the mechanical strength of

newly-pulled optical fibres it is usual to apply one or more

protective polymer coatings to the fibre. Generally two coating

layers are applied; the inner coating to strip out cladding-modes

and also minimise microbending effects, the outer coating to

provide mechanical and chemical protection.

The thickness of the coatings is important to ensure the fibre's

mechanical and optical performance. The concentricity of the

coatings is particularly important in the manufacture of ribbon

cable. The established method (1) for measuring the geometry of

fibre coatings consists of positioning the fibre in an oil filled

cell, at right angles to the axis of a microscope, and measuring

the positions of the coating boundaries using a filar eyepiece.

The measurement repeatability of this method can no longer be

considered adequate in the light of recent improvements in fibre

glass geometry measurement technology (2) where repeatabilities

of O.lum are routinely achieved. A new method has therefore been

developed, based on established grey-scale analysis techniques.

EXPERIMENTAL APPARATUS An intensity profile obtained using a

conventional microscope illumination system is shown in figure 1.

A CCD camera was used to record the image profile. The positions

of the coating and cladding boundaries are difficult to locate

accurately and the image is not suitable for computer analysis.

In order to improve visibility of the boundaries a measurement

system employing dark-field illumination has been developed. Here

the illumination is arranged so that it is outside the collection

numerical aperture, NA, of the imaging objective. Thus when no

fibre is present no light reaches the camera. When a fibre is

inserted into the cell light is deviated by the fibre into the

collection NA of the objective and an image is formed at the

camera. Figure 2 shows the intensity profile obtained from a

double-coated fibre. The image consists of six bright peaks

corresponding to the coating and cladding boundaries. The peaks

are sufficiently sharp to allow computer optimisation and

analysis

.

The experimental set-up is shown schematically in figure 3, where

the central component of the system is a novel wedge-shaped

prism. Illumination at 850nm, provided by a large core fibre, is

collimated by a lens, and directed on to the front surfaces of

59
the prism. Light from the upper face, in the diagram, is

refracted downwards and then reflected by Total Internal

Reflection upwards towards the measurement cell. Similarly for

light incident in the lower half of the diagram. The two beams

cross in the centre of the cell, which is filled with oil, and

emerge through the front window of the cell with an NA which is

outside that of the imaging objective. The presence of a fibre in

the cell causes the light paths to be deviated, by the processes

of refraction and relection, into the collection NA of the

optical system. An image is thus formed which consists of a

series of bright peaks corresponding to the coating and cladding

boundaries

The practical implementation of this technique is shown in

figure 4. A plastic moulded collimating lens is located on the

projections at the wedged end of the prism. The measurement cell

itself is demountable to allow easy access for cleaning with an

0-ring seal and spring-clip retainers. The fibre is inserted into

the cell through a precision ferrule and is further located by a

second blind ferrule at the bottom of the cell. In order to fully

characterize the geometry of the coating, provision is also made

to rotate the fibre about its axis. The prism itself is

manufactured from perspex.

Further advantages of this technique are that the camera allows a

section of fibre about 0.5mm long to be seen on the monitor,

allowing the cleanliness of the cell to be assessed. Also the

presence of bubbles, which sometimes occur in fibres at the

interface between the inner coating and the cladding, can readily

be seen and their positions measured. Secondly, the system needs

no special imaging optics, the prism assembly can be used on

commercial grey-scale instruments.

MEASUREMENT PROCEDURE AND DATA ANALYSIS The geometrical

parameters of importance are the diameters and

non-circularities/ovalities of the coating layers, and the

concentricities with respect to the cladding. From these

parameters the thickness of the coating layers may also be

determined. The measurement procedure is as follows. The fibre is

inserted into the cell and located in the alignment ferrules. The

focus position is then optimised by analysing scan data from the

camera. Measurements are made at 30 degree intervals resulting in

a series of edge points for each boundary. Ellipses are fitted to

the sets of edge points giving the diameters and

non-circularities of each coating, and the concentricities are

computed from the relative centre positions of the ellipses. The

complete measurement, including re-optimisation of the focus for

each fibre position, takes less than 90 seconds.

RESULTS Some typical repeatability data are shown in figure 5.

The mean and maximum standard deviations, SD, from 9 sets of

60
measurements are shown, where each set consists of 12

re-insertions of the fibre under test. The mean SD on the

diameter of each layer was better than 0.2um with a worst case of

0.3 3um. The mean SD on the concentricity of each layer was better

than 0.25um with a worst case of 0.41um. These results are a

significant improvement on the conventional filar eyepiece

method

.

From the coating geometry values the thickness of the coatings

may also be determined. Measurements on a range of fibres have

shown some fibres with variations of up to 9um in the thickness

of the outer coating and Hum in the inner coating.

CALIBRATION To calibrate the side-view measurement system,

reference fibres or glass rods of certified diameters may be

used. In order to determine the effect of measuring fibres coated

with different materials a series of fibres from 11 manufacturers

was obtained. The refractive index of the inner and outer

coatings of these fibres were then measured using the refracted

near-field method. It was found that the use of 3 calibration

fibres, or glass rods, enabled a calibration accuracy on outer

diameter of 0.6um to be achieved over the range of fibres

investigated.

It was found that the calibration factor for the inner coating

diameter could be determined from the calibration factor for the

outer coating. This is due to the fact that the refractive index

differences between the inner and outer coatings on the range of

fibres measured were similar. An accuracy on inner coating

diameter of approximately lum may be achieved

The measurement of non-circularity is self-calibrating as it is

a ratio measurement. Concentricity calibration depends on only

nominal dimensional calibration of the optical system.

SUMMARY A practical system for fibre coating geometry

measurement based on the established side-view method has been

described. The new implementation makes use of computerised

control and data processing techniques to achieve typical

repeatabilities of 0.2um on diameter and 0.25um on concentricity.

REFERENCES

(1) Fibre Optics Test Procedure FOTP-173. 'Coating Geometry

Measurement for Optical Fiber, Side-view Method', TIA/EIA, 2001

Pennsylvania Avenue, N.W., Washington, D.C. 20006-1813, USA.

(2) 'The First International Fibre Geometry Round Robin', Kane

W.T., 'Optical Fibre Measurement Conference', York UK, Sept 1991.

61
INTENSITY PROFILE ,,....,....,.,..,...,,....|.,,.,....,....,....,,

zee 300

'•••'•'I I.

INTENSITY PROFILE

400 500 600

radial posftion. um

Fig 1. Conventional intensity profile.

208 300 400 500 600

radial position, um

Fig 2. Dark-field intensity profile.

CAMERA PRISM

OBJECTIVE

FIBRE

Fig 3. Optical set-up of the side-view system.

Parameter Mean std dev Max std dev

outer coating diam 0. 17um 0.33un

inner coating diam 0. 16un 0.33um

cladding diam 0. 1 lUffl 0.24um

outer coating non-circ 0. 157. 0.37/

inner coating non-circ 0. 23'/. 0.44%

cladding non-cir 0. 15% 0.23%

outer coating cone 0. 15um 0.26ui

inner coating cone 0. 23ua 0.41um

Fig 4. View of prism components. Fig 5. Repeatability of side-view system.

62
Coating Thickness Unevenness Measurement in Fiber Drawing Process

by using Reflected Ligtit Detecting Metiiod

Akira Inoue, Shuji Shinoki, Yuji Kobayashi, Yasuji Hattori

Sumitomo Electric Industries, Ltd,

1, Taya-cho, Sakae-ku, Yokohania, 244, Japan

Introduction

Optical fiber is coated with resin immediately after

fiber-drawing in order to keep its tensile strensth and improve its

endurance to environment. The coating state strongly influences the

transmission characteristics of a fiber, for example temperature

resistance and pressure resistance. So it is necessary to monitor the

coating state in the fiber-drawing process and to control fiber-drawing

condition upon the variation of the coating state.

B. R. E ichenbaum introduced coating thickness unevenness measuring

method by monitoring forward scattering patterns In this method,

coating thickness unevenness around a optical fiber is measured by

illuminating a side wall of a fiber under drawing with a laser beam and

detecting a forward scattering light pattern. This method is available

in the case that the light passing through both the resin portion and

the glass portion and the light passing through only the resin portion

can be discriminated from each other (Fig. 1(a)). When the coating

diameter is small and the resin portion is thin, there exists no light

that passes through only the resin portion and the coating thickness

unevenness can't be measured (Fig. 1(b)). In this paper, we discribe a new coating thickness unevenness measuring technique by detecting

reflected light from a fiber under drawing. Using this technique,

coating thickness unevenness of a fiber which has a thin resin layer can

be measured.

System Configuration

Fig. 2 shows the system configuration of the new coating

thickness unevenness measuring system. An linearly polarized He-Ne laser

and two reflected light detecting sections are located so that their

optical axes are perpendicular to the longitudinal axis of the coated

optical fiber under drawing. The direction of laser beam's polarization

is set perpendicular to the longitudinal axis of the fiber. Light

detecting sections are set symmetrically with respect to the fiber and

their optical axes are perpendicular to the optical axis of the He-Ne

laser. Reflected light beams from the fiber are detected by CCD cameras

with electronic shutters which are controlled by a personal computer and

monitored images of the reflected light beams are simultaneously stored

63
by image freezers. Each light detecting section has a diaphragm at the

focal plane of the receiving lens so as to limit its light-receiving

numerical aperture and enlarge its depth of focus. Diameters of the

fiber with resin portion and without resin portion are measured by laser

scanning micrometers and measured diameters are sent to the personal

computer.

Principle

Fig. 3 shows a cross sectional view of an optical fiber which

consists of a glass portion and a surrounding resin portion. The side

wall of the fiber is illuminated with parallel light. Light beams A and

B, which are reflected on the outer surface of the resin portion and

the boundary surface between the glass portion and the resin portion

respectively, are detected to measure a distance d of reflected light

beams, rl, r2 express the radius of the glass portion and the resin

portion respectively. nO, nl express the refractive index of the ambient

area of the coated optical fiber and that of the resin portion

respectively. If the center of the resin portion is selected as the

origin of the x-y coordinate system, coordinates of a reflection point

PO of the light beam A and a exit point P3 at which the light beam B

exits from the resin portion are calculated as follows.

PO = (r2//T, r2/^^Z)

P3 = (7r22 - {rl/Vr- d)^ . ^2/^^^- d)

The coordinate of the reflection point P2(P2x , P2y) of the light beam B

on the boundary surface satisfies the following equation.

P2y = P3y + tan (5 3- 5 2) * (P2x-P3x)

where ^3 = tan"MP3y/p3x)

e 2 = sin-^ (nO * sin5 3 / nl)

If it is assumed that the coordinate of the incident point PI, at which

the light beam B is incident on the resin portion, is (r2 * cosw , r2 * sino)), ^0. Q\ satisfy the following equations.

Q Q - Tl I 1 - <d)

6 1 = sin"^ (nO * sin ^ 0 / nl)

^1 = ^0- tan-i((Plx - P2x) / (Ply - P2y))

= ^0 - tan-U(r2 * cosw - P2x) / (r2 * sinw - P2y))

The center Cs of the glass portion is located on a bisector of an angle

formed at the reflection point P2 by the incident light and reflected

light, and is spaced from P2 by a distance of rl. Therefore, the

coordinate of Cg is expressed as follows.

Cs = (P2x - rl * COS0 , P2y - rl * sin0)

where 0 = { tan'M (Ply-P2y) / (Plx-P2x) ) nan-M (P3y-P2y) / (P3x-P2x) ) } / 2

The coordinate of Cs is expressed in the same manner as aboves by using

the distance d' of reflected light beams A' and B' which are reflected

64
to the opposite direction. If nO, nl are known, the coordinate of Cs can

be calculated from above mentioned equations by measuring rl, r2, d, d'.

The distance between the origin and Cs correspond to the eccentricity

of the center of the glass portion from the center of the resin portion.

Coating thickness unevenness is calculated from this value of the

eccentricity and the radii rl, r2.

Experimental Results

To estimate the accuracy of the measured coating thickness

unevenness, test fibers with coating thickness unevenness were

manufactured and their coating thickness unevenness were measured in the

fiber-drawing process by using the reflected light detecting method and

after fiber-drawing by using a microscope. The diameters of the fibers

are from 170// m to 210/im. To get the clear image of the reflection

point on the outer surface and on the boundary surface, high angular

selectivity and large depth of focus are needed to the light detecting

system. To achive these, the numerical apertures of the receiving lenses

are limited to 0.11 by the diaphragms. This value of numerical aperture

corresponds to 25 u m of the depth of focus. To eliminate the

degradation of the monitored image caused by the vibration of the fiber,

electronic shutters of the CCD cameras are drived at the speed of

l/2000sec.

Fig. 4 compares the coating thickness unevenness measured after

fiber-drawing by using a microscope with that measured in the

fiber-drawing process by using the reflected light detecting method.

The coating thickness unevenness is expressed as a ratio of a minimum

thickness of the resin portion to a maximum thickness of that around the

glass portion. The difference between the coating thickness unevenness

measured in fiber-drawing process and that measured after fiber-drawing

process is less than 10% .

Conclusion

A new technique to measure an optical fiber's coating thickness

unevenness in fiber-drawing process has been developed. The accuracy of

coating thickness unevenness measurement is better than 10%. Using this

technique, coating thickness unevenness is measured by detecting

reflected light from the fiber, so it is available for a fiber which has

a thin resin layer.

Reference

(1) . B. R. Eichenbau, "The Centering of Optical Fiber Coatings by

Monitoring Forward Scattering Patterns", Bell System

Technical Journal, Mar. 1980.

65
(a)

when a resin portion is thick

and the light passing through

only a resin portion exists

(b)

when a resin portion is thin and

the light passing through only a

resin portion doesn't exist

Fig.1 Light passing through

a coated optical fiber

L.S.M.

D

Coated fiber under drawing

-ILL

rOZ]

RL G..

I/O

i.p.u.

P.C.

LRLD

C

IFIPU PC

He-Ne Laser Receiving Lens Diaphragm CCD camera

Image Freezer Image Processing Unit Personal Computer

LSM: Laser Scanning Micrometer

Fig. 2 System configuration of the coating

thickness unevenness measuring system

Fig. 3 Reflection on the outer surface of the

resin portion and on the boundary

surface between the resin portion and

the glass portion

66

(%)

90 —

m n

c/i (U a> u

c

c L.

(U a. >

c c

3 —

z m 03

ta I. Q)-a c I U 01 —J3

80

TOGO

0°

60 70 80 90 (X) Coating thickness unevenness oeasured

after fiber-drawing process

Pig. 4 Coating thickness unevenness neasured

after fiber-drawing process and

neasured in fiber-drawing process
DEVELOPMENT OF IN-LINE COATING THICKNESS MONITOR

FOR HERMETIC CARBON COATED FIBER

Yoshinori Ishida, Yukio Kohmura, Shin-ich Arai

The Furukawa Electric Co., Ltd.

R&D Department Plant & Facilities Division

6,Yawata-kaigandori ,Ichihara,Chiba, 290, Japan

Tel. +81-436-42-1631 Fax. +81-436-41-6218

Introduction

Recently a hermetic carbon coated fiber(HCF) has been investigated intensively to suppress of hydrogen-induced losses and to improve the fatigue behavior.

Hermetic coating consists of an amorphous carbon which is deposited on the fiber

surface with thermal chemical vapor deposition during fiber drawing. After carbon

coating, polymer coating is formed on it, and then the fiber is wound up at high

speed as shown in Fig.l-(a).

The aforementioned features of HCF depends upon the thickness of the amorphous carbon coating. So, from the point of quality control, it is very significant

to measure the thickness of the full-length of fiber.

Because amorphous carbon is a conductor, the thickness of carbon coating has

relation to electrical resistance. So far, we have measured the electrical resistance experimentally after stripping the polymer coating. However, it is not a useful method for production because it is a destructive measurement. We must

develop a new method which can be applied to a production line and enables the

measurement of the thickness continuously.

Therefore, we have developed a thickness measurement technique for carbon coating beneath the polymer coating by using the eddy current measurement method.

Measurement technique

It is a common practice to analyze the change in complex impedance of an electoromagnetic circuit which is constituted by a pickup coil and a work, in the

area of the eddy current measurement of a metal wire and a metal tube. Amorphous

carbon of HCF can be regarded as a super thin film tube. Assuming that the quality of thin film, namely conductivity, and diameter of fiber are constant, the complex impedance is correlative only with the thickness of coating. We expect

that we can measure the thickness of carbon coating by the change in complex impedance. However there are some problems practically, and we must solve them.

Firstly, the fiber diameter is very thin, the thickness of carbon coating is very

thin(several tens of nanometers) , and the pickup signal is very weak. Secondly,

because of high running speed of fiber, mechanical vibration of fiber is large.

And finally, because of high sensitivity measurement, it is difficult to keep the

zero position during a long measurement.

67
We have solved aforementioned problems by the following method. We used MHz

order high frequency for the weak pickup signal. We vibrated a pickup coil

mechanically at a constant frequency to disregard the vibration of fiber and

keep the zero position electrically. This is bercause we ascertained experimentaly that the phase angle of impedance is determined only by electrical resistance in spite of vibration of the fiber and coil. Besides, we could compensate

the zero position using the OFF signal condition when a coil separates from a fiber. As a result, we could measure stably the thickness of carbon coating for

a long time.

Fig.l-(b) shows a schematic diagram of a mesuring system. High frequency

signal from oscillator excites a pickup coil which constitutes a part of bridge

circuit. Simultaneously, a phase shift circuit converts the signal to two

signals which have phase differences, 0° and 90°. The pickup signal which has

the information of carbon coating thickness is detected by a phase detector with

two phase difference signals. Each of detected signals corresponds to an imaginary part and a real part of complex impedance.

Results of measurement

Fig. 2 shows a result of measurement obtained by using this measurement method. It is plotted in a complex plane. The electrical resistance of the samples

were already measured by another method. It shows that there is high correlation

between the phase angle of impedance and the electrical resistance. When a coil

separates from a fiber, a signal is treated as the origin. When a coil approaches

a fiber, a signal aparts the origin and moves in a constant direction.

Fig. 3 shows the relation between electrical resistance and phase angle of

impedance. In this case, the phase angle means a relative value to a standard

sample. By convert in the electrical resistance to the thickness of carbon coating in Fig. 3, Fig. 4 was obtained. Presently the minimum measurable thickness

is 17.6nm,and the resolution is ±1.0nm(for the thickness 17.6nm).

Fig. 5 shows a result of a practical measurement in fiber drawing process. Total drawing length was 15Km. During the drawing, we changed the production condition for two times. The thickness of carbon coating and its fluctuation under each

condition are shown in the following.

condition thickness (nm) fluctuation (nm)

1 23.6 ±0.6

2 35.6 ±1.2

3 26.4 ±1.0

We could find a big fluctuation of measured values in Fig. 5 at the end of

condition 3. This fluctuation was caused by stopping the thickness control at

the end of drawing. Since the control was stopped, the reactor pressure was

affected by adhered carbon inside the reactor.

In this way, we are now able to control conditions necessary to keep the coating thickness constant. Those conditions are, for example, flow rate of gases,

pressure of reactor, and fiber drowing speed etc,.

68
Conclusion

We have developed an in-line and continuous coating thickness measurement technique by using eddy current measurement method. The measurement of phase angle

of impedance and the vibration of a pickup coil are key factors. In this way, we could realize the stable measurement of carbon coating thickness. Presently,

the minimum measurable thickness is 17.6nm, and the resolution is ±1.0nm ( for

the thickness 17.6nm ).

Using this thickness measurement technique, we can guarantee the features of

HCF along the full-length of fiber.

Reference

DK.Hirabayasi et al . , ' Characteristics of carbon coated optical fibers and

cables' ,IEICE JAPAN. OCS 91-16, P43-48 , 1991

2)' Handbook of non-destructive test'.

Edited by Association of Non-Destructive Test

Prefon red Bunnin{ fiber

Pickup coil Vlbritlon

UY curinc

furnace

Oscillator

I L_ DU»eler

ionllor

Bridie Aiplifier

Phase shifter

90"

Feedbaclc sifnal

Recorder

Phase detector

Calculation

circuit lan-'(I/X)

The thlcliness on llor

I ndicator

(a) (b)

Fig.l Schematic diagram of HCF drowing process (a), and monitor system (b)

Fig.

2

Output of electrical resistance

in a complex plane.

Electrical resistance(KQ/cm)

(a) 4.6 (b) 6.6

(c) 9.3 (d) 18.7

(e) 26.6

X

roX

I-. n

c

Real axis

69
X

Q.3O

10 20

Electrical resistance{KO/ci»)

30

Fig.

Output of monitor versus electrical resistance.

Output of monitor is converted by a function of tan''(Y/X),

X\>-

eo

B

O.so

10 20 30 40 50 60

The thickness of coating (no)

Fig. Output of monitor versus the thickness of coating,

Condition 1 Condition 2

w:

>- 5 50

^o

30

«

5

10

uO

c

4

20 - O 20

o

3

4)V o

2 15

C 30 - o. M

1 M O

12 V

_ e£ 0

Condition 3

The end of drawing

10 15

Drawing length (km)

Fig. 5 A result of measurement in fiber drawing process.

70
Measurement of Optical Nonlinearity of Transmission Fibers

R. H. Stolen , W. A. Reed^, K. S. Kim* and K. W. Quoi

AT&T Bell Laboratories

Holmdel*, NJ 07733 and Murray Hill-", NJ 07974

; +

1. ABSTRACT

The optical nonlinearity of a fiber is represented by the ratio of an effective nonlinear coefficient divided by

an effective core area. This ratio has been determined for transmission fibers at 1.55 um by using self-phase

modulation measurements in combination with mode-intensity calculations. The nonlinear index of silica can be

extracted and is in good agreement with earlier measurements in bulk silica.

2. INTRODUCTION

Optical nonlinearities limit the maximum power which can be transmitted through an optical fiber

transmission system [1]. This was not a serious limitation until optical amplifiers permitted the design of

ultralong transmission spans. It has therefore now become important that an accurate value for the nonlinearity

be available to the system designer.

The magnitude of the nonlinearity in the fiber depends on the nonlinear coefficient of the material, the power

in the fiber, and the mode confinement. We discuss here the basic issues related to measuring these quantities in transmission fibers. We will also argue that the measurement of the fundamental nonlinear coefficient in a fiber

will produce a more accurate value than previous techniques using bulk samples.

3. BACKGROUND

The optical nonlinearity of concern is a tiny increase of the refractive index, n, with optical intensity. This can be written as:

where n2 is the nonlinear coefficient, lambda is the vacuum wavelength, I is the optical intensity and 60 is the

phase shift.

The nonlinear index shows up as an intensity-dependent phase shift (50). Even though the nonlinear effect

is extremely small it doesn't take much power to produce a significant phase shift as the length L becomes large.

For example, a laser focused in a piece of silica glass requires a peak power of 10 MW to produce a k phase

shift. This drops to 20 mW of average power in a 70 km fiber transmission span between repeaters. For the

longest projected span using optical amplifiers (9000 km) a n phase shift is produced by an average power of

only 150 uW. '

The effect of the nonlinear phase shift is to produce a spectral broadening. The simplest way to treat this broadening is to look at the phase along an optical pulse. The phase will be delayed at the pulse maximum

relative to the wings. A phase modulated pulse has a broader spectrum than an ideal transform-limited pulse.

Although not immediately obvious, an equivalent approach is to treat the problem as a four-wave mixing

between all the spectral components of the input pulse. This generates new frequencies which spectrally broaden

the pulse. In general, the phase picture is most useful for optical pulses, while the four-wave mixing approach is

n = no + 6n(I)

5n(I) = njl (1)

50 =

27i5nL

71
more useful when discrete wavelengths are present such as in wavelength multiplexing.

The deleterious effects of nonlinear spectral broadening usually show up because of the simultaneous

presence of group-velocity-dispersion (GVD). GVD by itself broadens the pulse temporally while the

nonlinearity broadens the pulse spectrally. Increased spectral width then increases the temporal broadening by

GVD. This necessitates much greater precision in the control of GVD for a long fiber span. In amplified

systems there is also four- wave mixing between the signal and amplifier spontaneous emission[2]. The output

pulse is now not only longer but looks quite noisy. However, in spite of these complications, it appears that a

useful rule of thumb for the onset of trouble is still a k phase shift over the system length.

The effect of all this on a lightwave system is to create a minimum in the bit error rate as the power increases.

In a linear system, the errors always drop as optical power goes up. The inclusion of nonlinearities results in an

increase in the bit-error rate at high powers. There is now a minimum in the bit-error rate as a function of power.

Thus there is a maximum average power that can be injected into a system which in turn places a severe

limitation on amplifier spacing (See Fig. 2). Problems will also appear with wavelength multiplexing.

4. FIBER ISSUES

There are two issues that must be addressed before n2 can be measured in a fiber. First, the intensity (mode

power) is not constant across the fiber core, and second, the nonlinear coefficient is not necessarily the same in the core and the cladding because of different glass compositions. This is illustrated in the Fig. 3 where we show

an idealized index profile for a dispersion-shifted fiber along with the mode intensity.

To obtain the phase shift as a function of power it is necessary to average, in some way, over the intensity

profile. This is possible because there has to be a single value for the phase of a single frequency in a single

mode of the fiber. The average takes the form:

This average takes the form of an overlap integral with the dimensions of 1/area. Conveniently, this effective area is fairly close to the actual core area of the fiber[l,3].

Since the cores of fibers are usually doped with germanium and the nonlinear coefficient of Ge:silica is higher than for pure silica, n2 is a function of radius which must be included inside the integral of Eq. (2). The

result is that a fiber must be characterized by both an effective nonlinear coefficient and an effective area. The

phase shift out of the fiber then becomes:

From the equation it can be seen tiiat it is the ratio (n2/A)eff that is important for system design, and knowledge

of the individual quantities is not required.

5. METHOD OF MEASURING (n2/A)eff

The method chosen for measuring (n2/A)efr is that of self-phase modulation which is illustrated in Fig. 4[3].

A transform limited optical pulse is coupled into a single-mode fiber and the spectrum is observed at the output.

As input power is increased, the spectrum takes on various easily identified patterns. The maxima and minima of

the central peak occur at integer orders of k/2. Self-phase modulation can be viewed as an interferometer where

the shifted phase at the pulse maximum is referenced to the phase of the weaker pulse tails.

50 = (2)

60 = 4^ ill PL (3) ^ AL -"eff

72
The power is obtained by measuring the average power out of the fiber and correcting for the end reflection.

To get the peak power, a separate measurement of pulse shape and width is made. The errors in the

measurement can be further reduced by plotting phase shift vs power for a wide range of powers.

One might think that the major uncertainty would come from the extraction of the peak power from the

measurement of average power. Fortunately there turns out to be a secondary check on this number. An

independent value for peak power can be obtained from a knowledge of the phase shift and the 1/e spectral width

of the broadened pulse[4].

The ideal measurements are made in fibers that are short enough to neglect loss and dispersion. It is, of

course, possible to use numerical simulation to include these effects in the analysis.

6. MEASUREMENTS ON ACTUAL FIBERS

The actual measurements of (n2/A)eff are made using either the 1.318 |im or 1.3385 |im line of a modelocked Nd:YAG laser. For the measurements reported here we used the 1.3385 |Lim line although the 1.318 ^m

line will be used in future measurements. It is logical to ask why not try to work near 1.55 |im since this is the

wavelength of interest for system application? The reason is that the Nd:YAG laser is a convenient and available source of relatively high-power stable transform-limited pulses. The test fibers are typically 250 meters long so

that loss is negligible. Peak powers of more than 50 W are required so that several orders of nil can be seen in a

fiber of this length. Pulse lengths around 100 ps are relatively easy to measure and are not affected by dispersion

in a 250 m fiber. The measurements also require an isolator to eliminate degradation of the mode locked pulses

by light reflected from the front face of the fiber.

An independent calculation of effective area is necessary to translate the results for (n2/A)eff from 1.3 ^.m to 1.55 |im. The measured refractive index profile of the fiber is used as the input to a computer program that

models the optical transmission properties of the fiber and the validity of the model output is checked by

comparing calculated and measured far field pattems[5]. The effective area is calculated at both 1.3 ^m and 1.55

|im and the ratio is used to convert the measured value for (n2/A)eff to the desired wavelength of 1.55 jim. The

ratio of effective areas is not particularly sensitive to the germanium doping of the core or uncertainties in the

index profile. There is also good reason to believe that any frequency dependence of n2 between 1.3 and 1.55

|im is negligible.

A secondary goal of this work is to determine a value for n2 of pure silica. This ties into a large body of

existing work on the nonlinear index of bulk glasses which is driven by the self-focusing problem in high-power

lasers[6]. One of the central results of this earlier work is a relation between n2 and two-photon absorption[7]

which is similar to the Kramers-Kronig relation relating linear absorption and refractive index. It is this relation

which leads us to believe that the frequency dependence of n 2 is well understood and is negligible over the range

1.3 - 1.55 ^im. The existing studies of bulk materials have produced good relative values of n2 for various

materials but the absolute uncertainty is around ±15%. We believe that the present fiber measurements have the

potential of giving a value for n2 of silica with an accuracy of ±5%. The ultimate limitation is the power

measurement.

To extract the absolute value for n2 of silica requires considerably more precision in the effective area than is needed for the simple ratio of area between 1.3 and 1.55 fim. Also, a correction for the Ge-doping of the core becomes essential. Fortunately, there are some data on the variation of the third-order susceptibility (X3) for

germanium concentrations between pure silica and pure germania[8]. The third-order susceptibility of pure

germania is four times that of silica and the concentration dependence lies somewhere between linear and

quadratic. The nonlinear index is proportional to and the relation between refractive index and Ge

concentration is known[9]. Thus, given the index profile, we can generate a reasonable estimate for n2(r). We

represent n2(r) as n2f(r) where f(r) is the increase of the nonlinear index due to the germania, and n 2 is the

nonlinear index of pure silica. We have also used several forms of f(r) in an attempt to use the n 2 results to gain

73
information about the effect of Ge concentration on n2. In the end, if everything is correct, we should extract the same value for n2 from measurements on different fibers of widely differing doping levels. The overall accuracy

is not yet sufficient to obtain any meaningful information but the different forms of f(r) provide support for our

confidence in the accuracy of the ratio of effective areas between 1.3 and 1.55 |im. The different forms of f(r) have almost no effect on the ratio; in fact, neglecting the effect of Ge on n2 altogether (f(r) = 1.0) has less than

1% effect on the ratio of the effective areas.

Measurements on three fibers are shown in Table I. One fiber (Std.) is a standard transmission fiber with

at 1.31 |im. The other two fibers (DSl and DS2) are dispersion shifted (Xq = 1.55 pm). The first column is (n2/A)eff measured at 1.3385 |UJn. Columns 2-4 contain the effective areas calculated from the measured index

profiles assuming that the n2 for germania is equal (+), twice (++), and three times (+++) the n 2 of silica. The

last three columns contain the n2's, which are the product of the measured ratio and the calculated effective

areas. Table I Effective Areas and n2 at 1.3385 |im

(n2/A)eff* Effective Area n2+ n2++ n2+++

xlO"^* cm^AV-^im^ + ++ +++ X I0"'^cm2/W

Std. 4.13 59.6 57.8 56.1 2.46 2.38 2.32

DSl 8.07 32.0 29.9 28.0 2.58 2.41 2.20

DS2 6.99 33.5 31.4 29.7 2.34 2.19 2.08

Notes:

# measured value

+,++,+++: assuming n2 for Ge02 is 1, 2, and 3 times n2 of silica

The calculated wavelength ratios of the effective areas, Ai 55/Ai 3335 are 1.17, 1.48 and 1.46 respectively for

the three fibers assuming n2 is the same for germania and silica. As can be seen, the values for n2 for silica at

this point are consistent to about 10% but little can be said about the n2 for germania. It is clear that additional

measurements are required if our goal of ±5% is to be achieved.

The effect of polarization must be included when comparing n2 as measured in a fiber to n 2 as measured in bulk glass. Typically, bulk measurements are made with linear polarization while standard telecommunications

fibers average over all possible states of polarization. The ratio between the fiber and bulk values for n2 should

be 8:9[10]. Our experimental result for 1.3 \im becomes 2.62x10"^^ cm^AV which is obtained by converting the

n2++ data of Table I to linear polarization. This compares to the accepted value for bulk silica of 2.74x10"'^

cm^AV at 1.064 ^im [6].

7. STANDARD METHODS FOR MEASURING (n2/A)eff

There is a growing desire by people in the area of systems design and modeling for a standard method for

measuring (n2/A)eff in a non-laboratory environment. The present method is not particularly suitable for this

application. We suggest two possible approaches. One method would use standard techniques to measure the

index profile of the fiber and (n2/A)eff would then be calculated using the known value for n2 of silica, the effect on n 2 of the Ge doping, and the Agff calculated from the index profile. A convenient experimental method

would use a comparison with a standard fiber of known (n2/A)efT. This method would modulate a 1.55 (xm cw

laser and observe the growth of the modulation sidebands due to four-wave mixing[ll]. A comparison between

the test and reference fibers combined with a measurement of output powers would give (n2/A)eff.

74
8. SUMMARY

Using self-phase modulation measurements in combination with mode- intensity calculations we have

obtained a value for the nonlinearity of transmission fibers at 1.55 urn. The value of the nonlinear index for silica which we obtain is in good agreement with earlier measurements in bulk samples. The present uncertainty is estimated to be ±10% which we anticipate can be reduced to around ±5% by projected improvements in various

aspects of the measurements.

9. REFERENCES

[1] R.H. Stolen, "Nonlinearity in fiber transmission", IEEE J. Quantum Electron, QE-68, 1232 (1980); D.

Marcuse, A.R. Chraplyvy, and R.W. Tkach, "Effect of fiber nonlinearity on long distance transmission",

J. Lightwave Tech. 9, 121 (1991); G.P. Agrawal, "Nonlinear Fiber Optics", (Academic Press, N.Y.,

1989).

[2] D. Marcuse, "Single-channel operation in very long nonlinear fibers with optical amplifiers at zero

dispersion", J. Lightwave Tech. 6, 356 (1991).

[3] R.H. Stolen and Chinlon Lin, "Self-Phase Modulation In Silica Optical Fibers", Phys Rev. A14, 1948

(1978).

[4] C.H. Lin and T.K. Gustafson, "Optical pulsewidth measurement using self-phase modulation", IEEE J, Quantum Electron. 8, 429 (1972).

[5] W. J. Stewart, "Optical Fiber and Preform Profiling Technology", IEEE J. Quant. Electron. QE18, 1451

(1982); and W. T. Anderson and D. L. Philen, "Spot Size Measurements for Single-Mode Fibers - A

Comparison of Four Techniques", J. Lightwave Tech. LT-1, 20 (1983).

[6] D. Milam and M. J. Weber, " Measurement of nonlinear refractive- index coefficients using time-resolved

interferometry: Application to optical materials for high-power neodymium lasers", J. App. Phys. 47,

2497 (1976).

[7] M. Sheik-Bahae, "Dispersion of bound electronic nonlinear refraction in solids", IEEE J. Quantum

Electron. 27, 1296(1991).

[8] W.E. Torruellas, "The optical cubic susceptibility dispersion of some transparent thin films", Ph.D.

Thesis, Univ. of Arizona (1990).

[9] J. W. Reming, "Material Dispersion in Lightguide Glasses", Electron. Letters 14, 326 (1978).

[10] S.G. Evangelides, L.F. Mollenauer, J.P. Gordon, and N.S. Bergano, "Polarization multiplexing with

solitons", J. Lightwave Tech. 10, 28 (1992).

[1 1] R.W. Tkach, Private communication

75
IN OUT

jLrm- oo

10 10

UJ- io-«

o

10 14

1 Error/week

Power (dB)

Fig 1. Optical pulse in and out of a fiber for: a. SPM

but no GVD; b. SPM plus GVD; c. SPM plus GVD

plus mixing with amplifier noise.

Fig 2. Bit error rate vs. optical power in the presence

of nonlinearity.

MODE INTENSITY

at 1.31

at 1.55 jim

Aa> = 0

-6 -4 -2 0 2 4 6

FIBER RADIUS (MICRONS)

a

A(I> = 1.5n

AO = 3.5n

FREQUENCY

Fig 3. Index profile and mode intensity for an

idealized dispersion-shifted fiber.

Fig 4. Spectral output of self-phase modulated pulse

for different peak powers.

76
Determination of Non-linear EfiFective Areas for Dispersion Shifted Fibres

J V Wright and E S R Sikora

BT Laboratories, Martlesham Heath

Ipswich, IP5 7RE, UK

1. Introduction

The non-linear effective area of a fibre is an important parameter for any system where optical nonlinearities become important. These will include: long haul optically amplified systems operating with

NRZ formats such as those proposed for the Atlantic and Pacific Oceans and any soliton systems. In

general, we observe that the refractive index of silica fibre is dependent on the intensity of the optical

signal according to

n = Hi + nil (1)

where ^2 is the non-linear Kerr coefficient with a value of 3.2xlO"^°m^/PF. Although the non-linear

term is very small compared to the conventional refractive index term, «i, a significant non-linear

phase shift can accumulate over the very long distances experienced in transoceanic cable systems.

This is given by

lim^Iz 27m2P(t)z , ^

where z is the total distance and A is

^

the

=

optical

-r

wavelength.

^

The non-linear effective area, A^^ allows

the transmitted optical power, P (?), with units of watts to be converted into the intensity, /, with units

of watts per square meter. We can see that the non-linearity is proportional to the product

«2

and so for a given power level, a fibre with the smaller value of effective area will experience the

greater non-linearity. For a long haul NRZ system we will wish to minimise the non-linearity since

this is responsible for various system impairments and so it is desirable to use as large a value ofA^ff

as practicable. However, non-linearity is essential for soliton systems where A^ff now determines the

exact optical power required to support correct soliton operation. Even in the case of long haul

soliton systems, it may still be desirable to utilise a large value of Agff since this will require a larger

optical power to support the soliton and thus improve the resultant signal to noise ratio.

2. Non-linear Effective Area

The non-linear effective area is given by

0 dA

^eff (3)

0 dA

where F{r) is the field distribution of the fundamental mode of the fibre at radius r [1]. The

integration is over the entire cross-sectional area of the fibre with dA = Inrdr, For example, if we

make a gaussian approximation such that

77
F(r) = exp(-^) (4)

where Wq is the 1/e width of the field, ie the spot-size radius, then the effective area can be integrated

analytically to give A^jf = twq- In the case of a typical dispersion shifted fibre with a spot-size

diameter of 8.10 ± 0.65fjm [2], we should expect ~ 52 ± 8fjm^. However, the field shape is never

quite gaussian, particularly for a dispersion shifted fibre where the refractive index profile is deliberately designed to control the variation of spot-size with wavelength. For the detailed design

and assessment of potential transoceanic systems, we require to know the exact value of^l^j^ and also

its likely variations. In previous work, A^jf has been measured for step-index fibres using

measurements of Raman gain. However, in this paper we wiU determine Agff from the refractive

index profiles measured for a range of commercially available dispersion shifted monomode fibres.

This wiU require establishing the exact field shape for a selection of fibres so that (3) can be used to

give the effective areas.

2.1 Measured profiles

The refractive index profile has been measured on twelve different samples of Corning dispersion

shifted fibre. An example of one measurement is shown in Figure 1 showing a distinctive triangular

shaped core with triangular shoulders. The profiles have been digitised and used as an input to a

finite element program which can estabUsh the field shape of a mode given a circularly symmetric

refractive index profile, n (r). However due to experimental limitations, the measured profiles are not

strictly symmetrical and so two separate data sets were derived from each profile measurement,

representing the profiles on the left and right-hand sides of each fibre axis. For this process, the fibre

axis was selected at a point where the profile and its mirror image achieved the greatest overlap over

the central triangular region.

The calculation of Aeff and hence the refractive index profile is required at a wavelength

corresponding to the erbium gain peak, say 1555nm. However, the profile measurements were made

with a helium-neon laser and so the measured index differences were corrected by a simple factor of

0.982. This was determined from the ratio of 8n between germania-doped and pure silica fibre at the

wavelengths of 1555 and 632nm, see Table 1 which has been compiled from data contained in [3].

TABLE 1. Refractive index of silica and germania doped silica

633nm 1555nm

Si 1.45756 1.44439

Ge 1.47907 1.46552

Sn .02151 .02113

2.2 Calculation

The field shapes for all twelve fibres were evaluated at 1.555/im using the wavelength corrected

refractive index profiles. Using profiles on both the left and right hand sides of the fibre axes gave a

total of twenty four field shapes. The effective areas were then calculated from (3) using numerical

quadrature and the results shown in Table 2. The difference between the left and right hand profiles

is tabulated as A and most likely gives an indication of the measurement accuracy rather than the

fibre geometry. An unbiased estimate of the variance on each measurement is A^/2 which can be

summed over the twelve fibres to give an average standard deviation of 0.9ym^. A histogram of these

results is shown in Figure 2 which shows a distribution with Agjf ~ 60 ± 5fjm^. This variation is considerably larger than that expected from the standard deviation and suggests that this is caused by

a real variability from fibre to fibre. Particularly as there is one isolated peak at ~72fjm^ which is attributed solely to fibre E.

78
TABLE 2. Estimates ofA^ff at 1.555/im

Fibre Id. Effective area fim^ A ion^

Left profile Right profile

A 59 5 60.6 1.1

B 62 8 63.6 0.8

C 57 2 56.4 0.8

D 62.4 61.5 0.9

E 73.0 72.8 0.2

F 57.9 58.1 0.2

G 55.3 57.0 1.7

H 59.7 63.0 3.3

I 6L2 61.4 0.2

J 60.5 62.0 1.5

Y 56.7 58.8 2.1

Z 58.9 61.0 2.1

2.3 Discussion

The variability in observed values of A^^f is broadly in agreement with the initial estimate from the

gaussian approximation. However, the mean value of 60/im^ is somewhat larger than that expected

from the gaussian approximation. This is not surprising since the field shape varies as the Bessel

function K^iwr) within the cladding which decays far less rapidly than a gaussian. Figure 3 shows the

field shape for the left profile of fibre A superimposed on its refractive index profile. This shows the

important feature that the field shape is picked up by the shoulders of the index profile for a dispersion shifted fibre when A is increased towards the dispersion zero, Aq, again resulting in non

gaussian behaviour. The difference between the calculated field shape and the gaussian

approximation is also shown in Figure 3 by plotting the gaussian which matches the calculated field at

the 1/e radius

3. Conclusions

Non-linear effective areas have been determined for a number of commercially available dispersion

shifted monomode fibres directly from their measured refractive index profiles. These show values of

Aeff which are somewhat larger than those expected by applying the gaussian approximation to the

spot-size. There is a real variability from fibre to fibre with a standard deviation of 5/im ^ but this is not inconsistent with the published variability in the spot-size. Finally, it is interesting to note that

fibres such as E can produce such large values of A^^ since it would be beneficial if every fibre could

be made like this. The interplay between large ^1^^ and other fibre parameters is being investigated.

4. References

[1] Nonlinear Fibre Optics: G P Agrawal, Academic Press, 1989.

[2] Corguide® Product Information: SMF/DS CPC3 Single-Mode Dispersion-Shifted Optical

Fibre (Issue 9/88).

[3] Material dispersion in light-guide glasses: J W Fleming, Electron. Lett. 14, p326, 1978.

79
r

fibre A

Fig. 1 Refractive index profile for dispersion shifted fibre

CDE

=3C

effective area in square microns

Fig. 2 Histogram of effective areas

^5

radius in microns

Fig 3. Field shape of dispersion shifted fibre

80
Beam current effects in Cathodoluminescence Spectroscopy

Graham R Atkins, Mark G Sceats* and Simon B Poole

Optical Fibre Teciinology Centre

University of Sydney , NSW 2006, Australia

Abstract

Cathodoluminescence spectra of gennanosilicate and silicate glass in MCVD and VAD

preforms are presented, and the major peaks assigned to E' and the so-called drawing-induced

defects. The effects of the electron microscope beam cuiTent on the spectra obtained is

analysed and limitations observed.

Introduction

It is well known that bonding defects in the glass network structure have an important

influence on both the linear' and non-lineai" transmission properties of optical fibres. More

recently, such defects have been shown to play a role in the formation of gratings by both

visible^ and ultraviolet'' excitation. However the identification and the mechanism of

formation of such defects is not well understood. The most common probe of defects in

optical fibres has been by optical spectroscopy in the fibre or prefonn. However, the majority

of results in this ai'ea give little or no spatial infonnation regarding the location of the

defects.

We have recently shown'''^ that the spatial distribution of GeE' defects in gennanosilicate

optical fibres and prefonns can be accurately and simply profiled using cathodoluminescence^

(abbreviated to CL). Here, the preform sample is placed in an electron microscope and

irradiated with low-cuiTent electron beams with a (typically) 20kV accelerating voltage. The

technique (Fig. 1) enables the determination of not only the CL but also other information

such as the secondary and backscattered electron emission which allows an accurate location

of the region under investigation.

We have also shown'* that the addition of phosphorus as a co-dopant greatly reduces the E'

concentration, whilst other co-dopants, such as boron or fluorine, reduce the concentration of

the so-called 'drawing-induced defects'^ However, since it is known^ that the use of highcurrent electron beams can damage silica glasses sufficiendy to cause compaction of the glass

to produce waveguides, it is impoitant to understand the effect that the electron beam current

may have on the sample under investigation. We report here the results of a study to

understand these effects in which a sample of gennanosilicate glass is subject to repeated

measurements and the change in the obseived spectrum recorded. The results show that, at

the low beam currents typically used in these measurements, there is negligible effect on the

cathodoluminescence spectrum.

81
Experiment

The CL specU'a presented here were obtained using a Jeol 35C scanning electron microscope

(SEM) equipped with an Oxford Instruments monoCL dedicated CL spectroscopy system.

The CL emitted by the sample was collected by a pai-aboloidal mirror, passed through a

grating monochromator and detected with a Hamamatsu R2228 photomultiplier tube (with a

multialkali extended red photocathode, range 300 to 900 nm, peak response 650 nm). A

schematic of the system is shown in Figure 1; note that the electron beam passes through a hole in the paraboloidal mirror. For purposes of clarity, the specti'a shown here were not

corrected for detector response; however the quoted band positions and widths were obtained

from the corrected spectra. This point has been discussed in detail elsewhere*. The CL was

excited by a 20 kV electron beam, with beam current of approximately 1 nA. The effect of

the beam current on the CL spectra was monitored by acquuing and comparing a series of

spectra obtained with the above beam current, but with different exposure times.

Results and Discussion

A CL spectmm of MCVD germanosilicate glass (Si:Ge ratio approximately 10:1) is shown

in Figure 2(a). Two bands ai'e observed: a lai-ge band at 425 nm, FWHM 55 nm, assigned

to the GeE' defect, and a smaller band an 650 nm, FWHM 65 nm, assigned to the GeDID

defect. Similar spectra are obtained from pure silica glass, although the strength of both the

SiE' and SiDID CL signals ai'e considerably weaker due to the better stoichiometry of pure

silica glass. The broad, weak band at around 800 nm, which is visible in Fig 2(a) has not

been assigned but appears to be present only in gennanium-doped glass.

It has been shown that co-dopants and fabrication conditions affect the CL spectra, and these

changes have yielded a great deal of information regai^ding the defects and their

transformations'^'^. As an example of the information which can be obtained by this

technique, the CL spectrum of boron co-doped germanosihcate glass is shown in Figure 2(b).

It can be seen that the presence of boron has removed the GeDID band and shifted the GeE'

band to 410 nm.

Figure 3 shows a series of four CL spectra collected sequentially from the same region of

germanosilicate glass. The spectra each took approximately five minutes to collect, so that

the total exposure time was around 20 minutes. It can be seen that while there is a slight

decrease in the intensities of both bands, the changes are not significant. The bands have

clearly maintained their integrity, and no new bands have appeared as a result of electron

beam damage. The small reduction in CL intensity may be due either to the emission being

absorbed by the deposition of surface contamination (a common problem in electron

microscopy), or to actual physical transfomiations of the defect species. This question is

under continuing investigation.

82
Conclusions

The cathodoluminescence spectrum of gennanosilicate glass in MCVD preforms has been

presented. Two broad bands were observed and assigned: a strong violet band centred at 425

nm, assigned to the GeE' defect; and a weak red band centi-ed at 650 nm, assigned to the Ge

drawing-induced defect. The influence of co-dopants, such as boron, on the defects present

can also be determined by this technique. Finally, the effects of electron beam exposure on

the measured defect profiles have been investigated and shown to be insignificant at the beam

currents used in the measurement technique.

Acknowledgments

GRA acknowledges the support of the Australian Postgraduate Reseai'ch Award. This work

was carried out with funding from Siemens Ltd, through a Generic Industrial Research and

Development (GIRD) program. AOTC is acknowledged as a Sponsor of the Optical Fibre

Technology Centre.

References

1. L. J. Poyntz-Wright, M. E. Fermann and P. St-J. Russell "Nonlinear transmission and

color-center dynamics in gennanosilicate fibers at 420-540 nm", Opt. Lett. 13, 1023-

1025, (1988).

2. U. Osterburg and W. Margulis, "Dye laser pumped by Nd: YAG laser pulses frequency

doubled in a glass opucal fiber". Opt. Lett. 11, 516-518, (1986).

3. K. O. Hill, Y. Fujii, D. C. Johnson and B. S. Kawasaki, "Photosensitivity in optical

fiber waveguides: Application to reflection filter fabrication", Appl. Phys. Lett. 32,

647-649, (1978).

4. G. Meltz, W. W. Morey and W. H. Glenn, "Fonnation of Bragg gratings in optical

fibers by a ti'ansverse holographic method". Opt. Lett. 14, 823-825, (1989).

5. G. R. Atkins, S. B. Poole, M. G. Sceats, H. W. Simmons and C. E. Nockolds,

"Defects in optical fibres in regions of high stress gradients". Electron. Lett. 11, 1432-

1433, (1991).

6. G. R. Atkins, S. B. Poole, M. G. Sceats, H. W. Simmons and C. E. Nockolds, "The

influence of co-dopants and fabrication conditions on germanium defects in optical

fibre preforms", IEEE Photon. Technol. Lett. 4, 43-46, (1992).

7. A. D. Trigg, "A high efficiency cathodoluminescence system and its application to

optical materials". Scan. Elect. Microsc. 1985/III, 1011-1022, (1985).

8. G.R.Atkins, Z. H. Wang, D. R. McKenzie, M. G. Sceats, S. B. Poole and H. W.

Simmons, "Control of Defects in Optical Fibres - a Study Using

Cathodoluminescence Spectroscopy" Submitted to IEEE Journal of Lightwave

Technology.

9. D. Bai'bier, M. Green, and S. J. Madden, "Waveguide fabricadon for integrated optics

by electron beam irradiation of silica", IEEE J. Lightwave Technol. 9, 715-720,

(1992).
cooled

photomultiplier

tube

grating

mirrors

electron beam

parabolloidal

mirror 7

monochromator sample electron microscope

Figure 1 . Experimental arrangement for cathodoluminescence.

25000

g 15000 B

c

5000

5000

9CCC

7000 -

5000

toc

0} 3000

1000

•COO 300 400 500 500 "00 300 900 '000

(a) wavelength / nm

300 400 500 500 "?0 300 900

(b) wavelength / nm

Figure 2.

Figure 3.

Cathodoluminescence spectra of (a) germanosilicate glass and (b) boron co-doped

germanosilicate glass.

140-

300 400 500 600 700 800 900

wavelength/nm

Successive cathodoluminescence spectra of germanosilicate glass. Curve 1 is the initial

measurement and curve 4 is the final spectrum.

84
RELATIVE DOPANT CONCENTRATION PROFILING OF GERMANL\,

PHOSPHORUS AND ERBIUM DOPED SILICA BASED OPTICAL FIBRES

J M Benson^* and C J Chunnilall^ and J C Petersen^

^Division of Quantum Metrology, National Physical Laboratory, Queen's Road, Teddington,

Middlesex TWll OLW, UK

^Department of Physics, King's College London, Strand,

London WC2R 2LS, UK

^Danish Institute of Fundamental Metrology, Bldg 307, Lundtoftevej 100,

2800 Lyngby, Denmark

1. Introduction

The propagation properties of an optical fibre critically depend on the material nature of the

glass from which it is manufactured. The majority of optical fibres are made from silica glass,

which is doped to produce the raised refractive index core, the profile of which can be

manipulated to modify the dispersion properties of the fibre. Deviation from the optimum

dopant profile seriously degrades the fibre's operation. Therefore, it is important to measure

dopant profiles of fibre after manufacture. In addition, the fibre core can be doped in order

to produce fibre lasers and amplifiers. For efficient operation of these active devices, the

mode shape of the pump laser must overlap efficiently with the concentration profile of the

active component. Also, for accurate modelling of active fibres, a detailed knowledge of the

spatial distribution of the active component is required. The techniques of micro-Raman and

micro-fluorescence spectroscopies are ideal for obtaining these profiles, as they are nondestructive and can be used on the fibre as opposed to the preform. This paper presents

relative dopant profiles from multimode and single mode telecommunications fibre as well

as preliminary results of relative concentration profiles of rare earth ions in the cores of

erbium and praseodymium doped fibre.

2. Experiment

Optical fibre slices were prepared from standard telecoms fibre, 50 |xm thick and flat to better

than 1 |xm across the whole diameter of the fibre. Raman spectra were recorded using a Dilor

XY multichannel spectrometer with a microscope attachment and 488 nm laser radiation. A

stepper stage enabled precise movement of the fibre samples, allowing spatial resolution on

the 1 \im scale. Refractive index profiles were measured interferometrically [1].

Slices from active fibres were not available at the time of measurement therefore these fibres

were profiled from the side. The laser was focused at the core/cladding interface and the fibre

stepped horizontally in 1 jxm steps, such that the laser sampled a diameter of the core. Fluorescence spectra were recorded for each fibre position. It was assumed to a first

approximation that the focal position of the laser did not vary with fibre position.

85
3. Results

3.1 Standard Fibre

Fig.l shows Raman spectra measured at 2 \im intervals across the diameter of a slice from

a typical multimode telecommunications fibre. These spectra can be understood in terms of

Raman scattering from silica, with varying amounts of germania (GeOj) and P2O5 doping [2].

The dominant feature for P2O5 doping which is at approximately 1325 cm"\ is associated with

-P=0 stretching [2] and can be profiled using band intensities. The case for germania

profiling is more complex. Silica glass is understood in terms of strong coupling between

Si04 tetrahedra [3], leading to the band-like Raman spectra. The replacement of Si atoms by

Ge atoms therefore affects the entire spectrum, modifying some features more than others.

This means that a more precise technique is required to measure the profiles. Factor analysis

[4] has been found to be effective in extracting the germania profile.

Fig. 2 shows the relative concentration profiles of germania and P2O5 obtained by factor

analysis of the spectra in Fig. 1, along with the measured refractive index profile of the fibre.

There is close correlation between the germania profile and the refractive index profile as expected because the fibre is doped with germania to raise the refractive index of the core

region. The sharp dip in the refractive index profile is mirrored in the germania profile,

although reduced resolution makes it less prominent. A uniform distribution of P2O5 is

observed which is consistent with its purpose to modify the mechanical properties rather than

the optical properties of the fibre. Profiles for a number of different types of optical fibre

have been measured [5].

Fig.3 shows the measured variation of germania across the core of a step index single mode

fibre extracted from Raman spectra using factor analysis.

3.2 Active Fibre

Because of the nature of the rare earth ions within a silica fibre, distinct Raman features

associated with these dopants are not observed. However, characteristic fluorescence spectra,

associated with transitions within the 4f manifold of the Er^* and Pr^"^ ions can be measured.

Fig. 4 and Fig. 5. show the fluorescence features associated with the ^liii/2>'^S2f2-%5/2

transitions in Er'* and ^D2-^H4 transition in Pr^* respectively. Saturation of these fluorescence

features has not been observed for the 10 mW laser excitation power used in this experiment

and therefore the relative intensity of these features is proportional to the concentration of the

respective rare earth at each sampling position across the core of the fibre.

Fig. 6 and Fig. 7 show the relative rare earth concentration profile for the erbium and

praseodymium doped fibres respectively. In both cases the rare earth is observed to be

localised within the core region.

86
4. Conclusion

The techniques of micro-Raman and micro-fluorescence spectroscopies have been

demonstrated to be useful tools in the characterisation of dopant variations in both passive and

active fibres. Improved spatial resolution is required for the accurate measurement of smallcored active fibres. This will be achieved by the use of a confocal microscope.

5. References

1. K Raine, J Baines and D Putland, J Lightwave Tech. 7, 1162 (1989).

2. N. Shibata, M Horigudhi and T. Edahiro, J. Non-Cryst. Solids 45, 115 (1981).

3. P. N. Sen and M. F. Thorpe, Phys. Rev. B15, 4030 (1977).

4. H. Martens and T Naes, Multivariate Calibration, John Wiley & Sons Ltd, London

5. J Benson and C J Chunnilall, Proceedings of the Fibre Optics Measurement Conference,

York, UK (1991).

Fig. 1. Spatially resolved Raman spectra recorded at 2 ^im intervals across the core of a

multimode fibre slice

• awmtinl* ProAl* X pads Pronto

147a

147S

1474

-1472

1470

14S8

14M

1404

14«2

-14M

#14SS

10 20 M 40 Displacement / um M

ecd O.flOO7^ 10.0 12.6 le.O 17.6 Displacement / um

20.0 22.S 26.0

Fig. 2. GeOj P2O5 and refractive index profiles Fig. 3. Ge02 profile of a single mode fibre,

of a multimode fibre.

87
3000 2500 2000 MavenuBOer (c»-lp

Fig. 4. Fluorescence spectrum associated with ^iiu/2>'^^3/2~%5a transitions in erbium

doped fibre.

5000 4500 4000 3500 HavonuBbsr (cb-1>J

Fig. 5. Fluorescence spectrum associated with ^D2-^H4 transition in praseodymium doped

fibre.

asT.s

380.0

S82.e

376.0-

r 360.0

UJ•f

s

346.0

337.6

33ao-. —I

1 1 1 1

1 10 12 14 13 18 30 8 10 12

Fig. 6. Relative distribution of Er in the core of Fig. 7. Relative distribution of Pr in the core of

Er doped fibre. Pr doped fibre.

88
Design and Characterization of Dispersion Compensating Fiber

A.J. Antos

Opto-Electronics Group, Corning Incorporated

Corning, NY 14831

Abstract

Dispersion compensation provides a method for upgrading 1310 nanometer (nm) optimized

systems to run at 1550 nm. Design criteria for an all-optical, fiber-based method are presented together

with optical results. System tests demonstrating 10 Gbit/s performance over 150 kilometers (1cm) of

standard single-mode fiber (SMF) also are shown.

Introduction

Erbium-doped fiber amplifiers (EDFAs) have reduced the importance of loss as a Icey system

design limitation. One issue associated with EDFAs is the need for operation in the 1550 nm window.

The best solution for new installations is the use of dispersion-shifted fibers. However there is a very

large installed base of fiber optimized for 1310 nm operation. These systems are dispersion limited at 1550 nm except for short lengths and low bit rates. To fully utilize the flexibility of EDFAs in already

installed systems and to realize high data rates over long unrepeatered lengths, a technique to compensate

for the chromatic dispersion of the fiber is required.

There are several solutions to this problem; they can be roughly divided into active and passive

classes. The active solutions include external modulation [1], narrow line-widtii sources, and prechirp

techniques [2,3] . Passive solutions include dispersion compensation using the large waveguide dispersion

of the LP„ mode [4] and dispersion compensation directly by the LP(n mode [5-7]. The balance of this

paper concentrates on dispersion compensation by the LP^i mode with some comparison to the LP^

dispersion compensation method.

Theory and Design

The dispersion compensator concept is shown in Figure la and b. Figure lb shows that after

travelling through 100 km of standard singlemode fiber, a 1550 nm signal has broadened by 1.7 ns/nm.

The signal is fed into the dispersion compensator (DC) module, consisting of a length of dispersion

compensator fiber and an EDFA to make up for the extra loss. The total dispersion of the system can be written as: where Ismf and iDC are the lengths of the standard fiber and dispersion compensating fiber.

At = DsmfIsmf + Due Idc,

respectively. The system is perfectly compensated when At = 0. The larger the negative dispersion of

the DC fiber, the shorter the length of fiber that is required to compensate for a given link length. The

length-dependent loss (not including splice losses) of the system can be written as:

Loss - asMFlsMF + aoclDc

= (asMF - (udcIDdc) Dsmf) x Ismf

= (asMF - Dsmf/FOM) x Ismf

89
where FOM = Doc/aDC. The figure of merit (FOM) defines the efficiency of the DC fiber. The loss

due to the DC fiber is made up by the EDFA. Because EDFAs have certain limitations to the amount

of gain available, a larger negative FOM denotes a better fiber.

Another key parameter is the dispersion slope around 1550 nm. One can imagine a flat or even

an inverted dispersion slope which, when concatenated to the SMF fiber, produces a flatter total system

dispersion, facilitating wavelength division multiplexing at several wavelengths around 1550 nm.

Unfortunately, the dispersion slope of the DC fiber required for total system dispersion flattening is not

easily achieved in practice.

Several profile designs have been explored to achieve large negative dispersion. Computer

modeling was done to predict optical properties, especially dispersion as a function of key profile

parameters. Table 1 lists five classes of fibers and the potential fiber properties achieved by each. Fiber

1 is a high delta step-index fiber. Fibers 2, 3, and 4 all are of a segmented core profile design. At 1550

nm dispersions of between 0 to -85 ps/nm/km have been demonstrated. The FOM for these fibers range

between -100 and -140 ps/nm/dB. Figure 2 shows both attenuation and dispersion as a function of

wavelength for a selected fiber. As can be seen from this figure, the dispersion of this fiber at 1550 nm

is -77 ps/nm^/km and the dispersion slope is inverted at -0.015 ps/nm/km.

Fiber 5 is an early attempt at dispersion compensation [4] that uses the dispersion of the LPn

mode to compensate for the dispersion of the standard fiber. Although light propagating in the LPu mode

in this fiber experiences the largest effective negative dispersion, the FOM is much smaller than the other

fibers. Clearly this is the result of high attenuation in this fiber; it is unknown whether this is intrinsic.

This technique also has issues associated with polarization dispersion and the need for complete LPn to

LPii mode conversion.

System Tests

Two key fiber and EDFA tests have been completed using the LPn DC module. The first [6],

shown schematically in Figure 3, involved DC fiber that had -65 ps/nm/km dispersion and an attenuation

of 0.48 db/km at 1550 nm. To compensate for the dispersion resulting from 150 km of standard singlemode fiber, 39 km of DC fiber was used. The loss of the DC fiber was overcome by an EDFA. Two

other EDFAs were used to overcome the loss of the transmission fiber. A uniform-grating DFB laser

diode was the optical source; the laser was driven with a 10 Gbit/s NRZ waveform. Eye patterns of the

optical waveform after 0 km, after 12 km without dispersion compensation and after 150 km with

dispersion compensation are shown in Figure 4. Without dispersion compensation the system is inoperable

after 12 km of SMF fiber. However, with the fiber-based optical compensator, a receiver power penalty

of less than 1 dB was obtained after 150 km of SMF fiber as shown in the BER graph of Figure 5. A

second test [7] was done, demonstrating the simultaneous transmission of both 1310 and 1550 nm

wavelengths over 60 km of SMF fiber with a 1310/1550 WDM at the 2.5 Gbit/s SONET rate.

In summary, the concept of dispersion compensation using a simple all-optical, fiber-based

method has been demonstrated. 10 Gbit/s has been transmitted through 150 km of standard single-mode

fiber at 1550 nm. This technique for upgrading already installed systems optimized to run at 1310 nm

has many potential applications from long-haul telephony systems and interoffice networks to AM video

for cable TV.

90
Acknowledgments

The author would like to thank Alcatel Network Systems and Bellcore for their assistance® in

doing the systems tests. The author also would like to thank D. Smith and D. Hall for computer

modeling and useful discussions.

[1] A.H. Gnauck, et al., "Optical Equalization of Fiber Chromatic Dispersion in a 5 Gbit/s Transmission

System," IEEE Photonics Technology Letters, vol. 2, no. 8, pp. 585-587.

[2] N. Henmi, et al., "10 Gb/s, 100 km Normal Fiber Transmission Experiment Employing a Modified

Prechirp Technique," Paper Tu02, Technical Digest, Conf. on Optical Fiber Comm., 1991.

[3] A.D. Ellis, et al., "Dispersion Compensation in 450 km Transmission System using Standard Fibre,"

Electronic Letters, vol. 28, No. 10, pp. 954-955.

[4] CD. Poole, et al., "Broad-Band Dispersion Compensation Using the Higher-order Spatial-mode in a Two-mode Fiber," Paper PD13, Technical Digest, Conf. on Optical Fiber Comm., 1991.

[5] Chinlon Lin, H. Kogelnik, and L.G. Cohen, "Optical-pulse Equalization of Low Dispersion

Transmission in Single-mode Fibers in the 1.3-1.7 /^m Spectral Region," Optical Letters, vol. 5, no. 11,

Nov. 1980.

[6] M.J. Dugan, et al., "All Optical Fiber-Based 1550 nm Dispersion Compensation in a 10 Gbit/s, 150

km Transmission Experiment over 1310 nm Optimized Fiber,"- Paper PD14, Technical Digest, Conf.

on Optical Fiber Comm., 1992.

[7] H. Izadpanah, et al.. Dispersion Compensation for Upgrading Interoffice Networks Built with 1310

nm Optimized SMF Using an Equalizer Fiber, EDFA's and 1310/1550 nm WDM," Paper PD15,

Technical Digest, Conf on Optical Fiber Comm., 1992.

Table 1. Dispersion Compensating Fiber Properties

References

Fiber

Type

Dispersion

(ps/nm/km)

Dispersion

Slope

(ps/nM2/kM)

Nominal

Attenuation

(dB/km)

Figure of

Merit (FOM)

(ps/nm/dB)

Single-mode Propagation

F 1 -45 to -60

F 2 -55 to -80

F 3 -60 to -80

F 4 -80 to -100

0.07

0.0

-0.01-.02

0.07

0.4

0.5

0.6

0.6

112 -150 [7]

110 -160 [6]

100 -133

133 -200

LPn Mode Propagation

F 5 1 60 to -240 4.9 33 - 50 [4]

91
Applications - SMF Upgrade

1.3 ^fn

»mtr

35-45 km

1.55

junli

1300/1550 nm

WDM

1.3 1.3 rcvr xmtr

DC fiber

1.55

pulse shapes

t

A

t t

A

2000 -r

1500 -

1000 -

500 -

-500 --

1000 --

-1500 --

-2000

Dispersion Compensation

Total System Dispersion

100km of SMF

DC fiber

Pred. Sum

1 1

1300 1400 1500

Wavelength (nm)

Figure 1. Dispersion compensator concept.

92

1600
-80-) 1 1 1 ;

1 1 1 1: 1 1

1

1 p 1 p

1150 1250 1350 1450 1550 1650

Wavelength (nm)

Figure 2. Dispersion and attenuation of dispersion compensating fiber.

lOGbit/s

PATTERN

GENERATOR

DFB

LASER EDFA-1

.-^^^ i 6.4 km

ISOLATOR I DC FIBER I

VARIABLE

ATTENUATOR

150 km

STANDARD

FIBER

EDFA-2

r

1

1.2 nm. -

EDFA-3

22 km

I DC FIBER

3 nm

O/E

CONV

11 km

DC FIBER

BER CLOCK RECOVERY. J

RECEIVER RETIME. & DEMUX

Figure 3. Block diagram of experimental set-up.

93
' ^^^^

—

CD

I UJ

I UJ

I UJ

t I LU

I LU

CO

ru

I

ru

I 0 "e

cTj m 1 ID

I cumI

m1 inm

I

COcn

I

I UJ

UJ

o

UJ

UJ

u

UJ

a:

J3

3O

T3

O

N

O CO

D

O

c

6

a- W

o

c

t

o

OliVH UOHfciH ilB

es .2 "I

6 o

o to

o ^

§1^

o o

N 52

.

. B ®|

ti o

o

.JJ ^ a, o e 8

3

PL, 94
MEASUREMENT OF GROUP EFFECTIVE INDEX OF OPTICAL FIBERS AND

INTEGRATED WAVEGUIDES BY THE WAVELENGTH SCAN TECHNIQUE

S. Morasca, F. Pozzi, C. De Bernardi

CSELT S.p.A. - Via G. Reiss Romoli 274 - 10148 Torino, Italy

ABSTRACT

The technique of Fourier analysis of the spectral transmission function of a resonant

cavity formed by a waveguide has been applied for the first time to single-mode optical fibers

and semiconductor waveguides, to determine directly the group effective indices of the guided

modes. On single-mode fibers, accuracy to four decimal places is demonstrated, and for the

integrated guides differences between TE and TM indices are also evidenced.

1. - Introduction

The determination of the effective indices of the guided modes has a fundamental role

in waveguide technology: precise values of these parameters are needed both for the design of

optical guided-wave devices, and to test the validity of the theoretical models used to

calculate the waveguide propagation constants from the geometrical and material parameters

of the guide; this applies to optical fibers as well, with added applications, e.g. to calibrate

the distance scale of OTDR instruments.

A technique based on a wavelength scan of a resonant cavity, formed by the guide under

measurement, was introduced to this purpose [1] and has been successfully implemented. In the

present work we report experimental results obtained on single mode fibers and InP/InGaAsP

optical waveguides. The method is particularly suitable for semiconductor waveguides, whose

cleaved facets provide sufficient reflectivity without any further preparation, but can be used

on any type of waveguide. Its accuracy is discussed, as well as evolutionary lines for further

improvements.

2. - Prir\ciple of the technique

The transmittance T of a Fabry-Perot resonator as function of the incident wavelength X

has a well known form [2], which for a lossy medium can be expressed as:

- a L

T=C ?

where a is the cavity internal loss, L its length, n the refractive index of the medium filling the

cavity, R the cavity mirror reflectivity and C a constant.

The period P of eq. (1) depends only on X, L and n (provided that a and R are not rapidly

varying with X), and is given by the following relationship:

2

P = (2)

2L(n-;i^)

dX

where the medium dispersion has been taken into account. From eq. (2) the group index

ng=(n - X dn/dX) can be obtained as a function of P and X, if L is also known. Using a tunable,

narrow linewidth laser source it is easy to cover a large number of periods of the cavity, while

monitoring the wavelength by means of a wavemeter, and therefore to determine adequately

95
the period itself; in the case of a waveguide, the effective indices Ueif ar\d neff g of the guided

mode substitute indices n and ng respectively.

A Discrete Fourier Transform algorithm is particularly suitable to determine P, when

the sampled points are irregularly distributed [3], as in this case; the analysis provides the

power density in the frequency spectrum of the cavity transmission function, so that peaks

appear corresponding to its periodic terms; actually, several peaks may appear, due to the

harmonics of the fundamental period, to terms coming from the distribution of sampled points,

and to their sums and differences. Since the terms from the sampling distribution can be

calculated, or a first approximation of the significant period is already known, the peak

corresponding to the real cavity term can be univocally determined. For samples of centimeter

size, the cavity length L can be easily measured to about ±1 |j.m, so that the effective index can be accurately calculated.

3. - Experimental set-up

The experimental configuration we have used [4] is shown schematically in Fig. 1: a tunable laser source (in this case an external-cavity, grating-tuned semiconductor laser diode) is coupled to the guide under measurement via a collimator-polariser-focuser system; a portion of

the power emitted by the source goes to a wavemeter (with a resolution of 1 pm) to provide

continuous monitoring of the source wavelength, while another fraction provides a reference

power signal for normalization of the power transmitted by the guide; this transmitted power is collected by a microscope objective and read by a power meter. The entire set-up is under

computer control, for data acquisition and processing. The presently available laser source has a linewidth below 200 kHz and is usefully tunable in the range 1.52 to 1.58 jim, although rather

discontinuously due to laser mode jumps; for guide lengths of about 1 centimeter this amounts to thousands of periods of the corresponding resonant cavity, which allow a very precise period

determination. On the other hand, over such a wide wavelength range the chromatic dispersion

may be significant (e.g. for semiconductors), and the corresponding "chirp" on P would degrade

its determination; therefore, the whole spectral interval is divided into subsections of less than

10 nm, so that an appropriate balance between dispersion and Fourier analysis baseline is realised; in this way FiX) and the guide group dispersion are obtained as well.

A modification of this set-up is also used, in which a current-tuned DFB laser is used in

place of the external-cavity source; although presently the tuning range, at a rate of 9.6

pm/mA, is only about 0.2 nm and corresponds to a few p>eriods of the cavity, the dense spectral

sampling attained allows the determination of a first-approximation value for the period

itself and hence an independent check of the analysis on the wider spectral baseline.

4. - Results and discussion

The technique has been applied to short sections (25 to 35 mm, measured to within 1 |im)

of standard single mode fiber; to enhance the fiber reflectivity, a thin (6 nm) gold layer was

coated on the end faces. The results from different lengths (ngff g = 1.4682 ± 0.0003 around 1.555

|im) are in excellent agreement with the declared value for the fiber, and their uncertainty

indicates the present level of accuracy of the method; measurements over subsections of the

1.52+1.58 |im range evidence the group dispersion clearly.

The application to integrated waveguides has been tested on InGaAsP/InP guides

having a Moderately Diluted Multi Quantum Well Structure (MD-MQW), schematically

shown in Fig. 2 (this structure has the purpose to effectively "dilute" the InGaAsP refractive

index by the lower-index InP, to get a low core-cladding index difference and therefore a better

modal match to optical fibers). Guides 3 to 10 ^im wide have been measured, about 15.1, 8.4 and

6.7 mm long, obtained by cleavage of a single chip; the individual guide lengths have been

measured to better than 2 ^im. A typical wavelength scan is shown in Fig. 3: although no

periodic behaviour is apparent in the distribution of points, due to the low sampling density

(less than one point per period), the period shows up prominently in the corresponding frequency

power spectrum, reported in Fig. 4. An average effective group index n^ff g (TM) = 3.394 ± 0.002

96
around 1.555 fim has been measured, with no significant difference between different guide

widths. On the other hand, a conspicuous, repeatable difference is observed between TE and TM

indices, about 0.009±0.002, evidencing a marked guide birefringence; scans over subintervals of

the available 1.52-1.58 jim window show clearly the group chromatic dispersion effect as well,

with a decrease from 3.410 to 3.393 (TE polarisation) between 1.535 and 1.575 (im. Work is in

progress on other waveguides, both on glass and on other semiconductor structures.

The present high level of accuracy (about 10'^ for fibers and 10"^ for semiconductor

waveguides) is a significant improvement in the effective index determination, compared to the

previously available results. At the moment it is limited mainly by the following factors:

a) low sampling density due to the currently available tunable laser (it provides only 5 to 15

stable wavelengths per nanometer); b) power instability of the used laser; c) thermal and

mechanical drifts (e.g. creep of piezo positioners) of the set-up during the measurement time

(currently about 30 min, mostly due to the settling time of the source); d) resolution of the

available wavemeter. In fact, the latter parameter should be much smaller than the cavity

wavelength spacing, and thus its value of 1 pm poses an upper limit of few centimeters to the

usable cavity length.

For fibers, increasing the sample length to decimetric values would enhance the

measurement accuracy in two ways: increasing proportionately the number of periods and the

associated resolution of the Fourier analysis over a given spectral interval, and reducing the

relative error on the measure of the sample length.

For the case of integrated waveguides, the current uncertainty can be attributed mainly

to the higher impact of the thermal and mechanical drifts during the measurement.

Improvements under investigation include the use of stable, multielectrode

semiconductor lasers, rapidly and continuously current-tunable over a few nanometers, to reduce

the effect of factors a), b) and c), and concurrent refinements on cavity length measurements;

improving also the resolution of wavelength measurements, enhancements in accuracy by nearly

an order of magnitude are expected, for both fibers and integrated waveguides.

5. - Conclusions

A new, simple technique has been tested for the measurement of the effective group

index of both single-mode fibers and integrated optical waveguides. The method, based on a Fourier analysis of the transmission function of a resonant cavity composed by the guide or by a short (a few centimeters) fiber section, has a demonstrated accuracy up to the fourth decimal

place, detecting the difference in group index for fibers between 1.52 and 1.58 |im. Furthermore,

the method has margins for further improvements by at least an order of magnitude, limited

mostly by the spectral characteristics of tunable lasers, while the simplicity of the method

allows a routine determination of the group index even for each fiber reel. As to integrated

guides, the accuracy limits are set mainly by their overall length and longitudinal uniformity,

and are expected to improve one extra decade from the current third decimal place.

Acknowledgements

The authors wish to thank G. Galliano for the useful discussions, D. Re for coating the fiber

samples, G. Schiavini and A. Stano for fabricating and providing the semiconductor guides.

References

[1] C. De Bernardi, S. Morasca, "Waveguide loss and effective indices determination by

optical frequency scan integrated resonant cavities", VI Symposium on Optical Fiber

Measurements, Boulder, NIST Special Publication 792, pp. 67-70, 1990.

[2] M. Bom, E. Wolf, "Principles of optics", 5*^ Ed. (Pergamon Press, London 1975), pp. 323-329.

[3] E.P. Belserene, "Rhythms of a variable star". Sky & Telescope, pp. 288-290, Sept. 1988.

[4] Patent pending.

97
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98
PRECISION MEASUREMENT OF SINGLE-MODE

FIBER GROUP INDEX

W. B. Gardner

A. D. Parker

AT&T

2000 Northeast Expressway

Norcross,GA 30071

1. INTRODUCTION

Single-mode fiber attenuation is frequently measured and reported with an

uncertainty of .001 dB/km. The fiber length is usually measured with an Optical

Time Domain Reflectometer (OTDR). If one wishes to keep the uncertainty in this

length measurement from contributing significantly to the uncertainty in a .350 ± .001 dB/km loss measurement, then the group velocity Vg of the

fundamental mode must be known to better than ± 1 part in 350, say to ± 0.07%.

Since the group index N is c/Vg (where c is the speed of light in vacuum), this

implies that N must also be known to an accuracy of ±0.07%, which is ± .001.

Methods having this capability have been reported.

^"^ However, these methods

generally require facilities which are not readily available, such as IR fiber

interferometers,^''^ 50 meter benchmarks surveyed with 2 mm accuracy or nanosecond modulation of broad-band sources.'*'^ Because N will in general be

different for each fiber design from each supplier, there is a need to make frequent

precision measurements of N to keep pace with evolving fiber designs and the

increasing accuracy of attenuation measurements. This paper shows how the

desired accuracy can be achieved using only an OTDR and a suitable fiber winder.

2. MEASUREMENT METHOD

The time t for a round trip transit of a fiber of length L is given by t = IL/V^. The

group index is then N = ct/2L. ± .001 accuracy in N can be achieved with

sufficiently careful measurements of t and L.

A fiber approximately 1.2 km in length was wound in a single layer onto a metal

drum whose circumference was measured to be 878.0 ± 0.5 mm, using a precision

steel rule as the length reference. The number of turns was recorded to a fraction

of a turn by an optical encoder during winding. The encoder was mounted on the

winder's headstock to avoid errors due to slippage in belts and pulleys.

99
The transit time t was measured by an OTDR whose pulse width was 3 nsec. A

point on the reflected pulse 3 dB down from the peak was chosen to represent the

fiber end. By judicious choice of matching fluids, the reflection from the far end of

the 1.2 km test fiber could be made equal in magnitude to the reflection from the

end of the 600 m fiber test lead that was spliced to the test fiber. In this way, the

transit time reproducibility was ±0.1 nsec, which corresponds to ± 1 cm. To check

for calibration errors, one of the fibers was measured on two different OTDR's.

With the group index set to 1.47, the two OTDR's gave lengths of 1220.01 m and

1220.11 m. This uncertainty is Hsted as iii) in Sec. 4.

3. WINDING TENSION

The fiber tension during winding was measured to be 35 gm. After the

measurements, several of the fibers were paid off the metal drum into a loose pile.

The transit time was then re-measured with the fiber in this "zero-tension"

condition. The decrease in transit time was typically 0.04%, which calls for the N

measured under tension to be increased by .02%. The ± .01% uncertainty in this

correction is item vi in Sec. 4. After winding a fiber onto the drum, the drum

radius measurement was repeated over the wound fiber. The increase in this radius

equalled the coated fiber diameter to within ± .03%. This is item ii in Sec. 4, and

indicates that compression of the coating and/or drum due to the winding tension is

small.

4. ERROR ANALYSIS

This section summarizes the known sources of error. The impact of each is given

in parenthesis as a percent of the measured group index.

i. Circumference of empty drum (± .06%)

ii. Uncertainty in the distance of the fiber core from the drum surface (± .03%)

iii. OTDR Calibration (± .01%)

iv. Reproducibility of OTDR length measurement by the Sec. 2 procedure

(±.001%)

v. Change in the transit time due to a ± 5°C change in the fiber temperature

(calculated from Ref. 6) (± .005%)

vi. Uncertainty associated with a 35 gm winding tension (see Sec. 3) (± .01%)

vii. ± 25 cm uncertainty in fiber length resulting from multiple end preparation

(± .02%)

viii. ± 30 nm uncertainty in the OTDR laser wavelength (± .006%)

Assuming that the above errors are uncorrelated, an rms addition gives a total

uncertainty of ± .07% or ± .001 in the measured group index.

100
MEASUREMENT RESULTS

The results of applying this technique to seven AT&T single-mode fibers are given

in Table I. The apparent fiber-to-fiber variations are explainable by the ± .001

measurement uncertainty. Actual fiber-to-fiber variations have been shown^ to be

on the order of .0003. For comparison. Table II gives some previously reported

group index values. As expected, the group index is always lowest near the zero- dispersion wavelength.

Table I — Group indices of conventional single-mode (SM) and dispersion-shifted

(DS) single-mode AT&T fibers. The 1 a uncertainty of all values is .001.

Fiber N(1.31 um) N(1.55 um)

SM#1 1.468 1.469

SM#2 1.466 1.467

SM#3 1.467 1.468

SM#4 1.467 1.468

DS#1 1.470 1.469

DS#2 1.471 1.470

DS #3 1.471 1.470

Table n — Some previously reported group index values. Only the measurements

in Ref. 1 were performed on AT&T fibers.

Fiber N(1.31 um) N (1.55 um)

SM(Ref. 1) 1.4659 1.4666

SM (Ref. 3) 1.4675 1.4681

DS(Ref. 3) 1.4718 1.4711

DS (Ref. 7) - 1.475

Z-Fiber (Ref. 7) - 1.462

ACKNOWLEDGMENTS

The authors are grateful for the assistance of C. W. Jackson, J. A. Hudson, and R.

B. Kummer.

101
REFERENCES

1. M. J. Saunders and W, B, Gardner, "Interferometric Determination of Dispersion

Variations in SM Fibers", J. Lightwave Tech., Vol. 5, pp. 1701-1705 (1987).

2. Y. Namihira, Y. Iwamoto, T. Murakami, and S. An, "High Speed and High

Accurate SM Fiber Dispersion Measuring Equipment Using Modified

Interferometric Method", NIST Symp. off Optical Fiber Measurements, Boulder,

Sept. 9-10, 1986, pp. 19-22.

3. J. J. Carr, S. L. Saikkonen, and D. H. Williams, "Refractive Index Measurements

on SM Fiber as Functions of Product Parameters, Tensile Stress, and

Temperature", NIST Symp. on Optical Fiber Measurements, Boulder, Sept. 11-

12, 1990, pp. 59-62.

4. M. Horiguchi, Y. Ohmori, and T. Miya, "Evaluation of Material Dispersion Using

a Nanosecond Optical Pulse Radiator", Appl. Opt., Vol. 18, pp. 2223-2228 (1979).

5. L. Thevanaz and J. P. Pellaux, "Group Delay Measurement in SM Fibers with True

Picosecond Resolution Using Double Optical Modulation", J. Lightwave Tech.,

Vol. 6, pp. 1470-1475(1988).

6. L. G. Cohen and J. W. Fleming, "Effect of Temperature on Transmission in

Lightguides", Bell Sys. Tech. J., Vol. 58, pp. 945-951 (1979).

7. Y. Namihira, Y. Horiuchi, H. Wakabayashi, T. Oshimi, K. Kitagawa, and T. Ooka,

"155 km Long-Range OTDR Measurement at 1.55 |J.m Wavelength Region", NIST

Symp. on Optical Fiber Measurements, Boulder, Sept. 20-21, 1988, pp. 11-14.

102
THE DIRECT MEASUREMENT OF HIGHLY ABSORBING BANDS IN OPTICAL

FIBERS

Robert M Atkins

AT&T Bell Laboratories

Murray Hill

NJ 07974

Abstract

A technique has been developed to directly measure the attenuation of optical

fibers even in the region of very highly absorbing bands. An example is given of the

measurement of the UV spectrum of a singlemode germanosilicate core fiber between

200nm and 300nm, where the absorption band at 241nm due to GeO shows attenuation

of ca. 230dB/mm.

Introduction

Conventional cut-back techniques for measuring the optical attenuation of fibers

become impractical when optical losses become greater than a few dB/ cm. Though it is

in principle possible to fusion splice a very short section of test fiber into a longer length

of non-attenuating fiber, the extreme temperatures involved in the fusion splice can

radically alter the concentration of some highly attenuating species. Some species present

in fibers can give rise to optical absorptions as high as several hundred dB/mm and thus

attenuation can only be measured on fiber lengths of several hundred microns. One such

species, whose measurement will be discussed in detail in this paper, is GeO, formed by

the reduction of GeOj in germanosilicate fibers, which has an intense UV absorption

band near 241nm'^l GeO is of particular importance in view of its reported role in the

formation of UV written refractive index gratings in optical fibers^^. Fusion splicing

results in very high levels of GeO at the splice and so cannot be used in the

measurement of GeO in short lengths of fiber. The alternative splicing techniques

developed and reported here have enabled direct, accurate measurements to be made

in fiber test lengths as short as 40/x.m with attenuation values >400dB/mm and at

wavelengths as short as 190nm.

Experimental

This technique relies on the precise alignment of a very short section of the fiber

to be measured (typically <200/xm) with two pigtail fibers. The test section can be

fabricated by cutting and polishing short sections of fiber, or with some care, such short

sections can be cleaved directly saving a considerable amount of time and effort. The

pigtail fibers must be transparent over the wavelength range of interest. In this case,

since GeO absorbs in the short wavelength UV, with a peak near 241nm, the pigtail

fibers must be UV transparent and thus pure silica core fibers must be used. The fiber

section to be measured and the pigtail fibers are precisely aligned by placing them in a

"V" groove etched into a small silicon block as shown in figure 1. The accuracy with

which silicon can be etched ensures a straight and uniform "V" groove. It is also essential

that both test and pigtail fibers have low eccentricity in order to ensure core alignment.

103
and that the core size and numerical aperture of the pigtail fiber is no larger than that

of the test fiber in order to ensure that light is launched only into the test fiber core. For

fibers with small cores it was found necessary to fabricate non-standard small core silica

fibers for use as pigtails. In order to obtain minimum splice loss an index matching fluid

is required. In this case, since measurements were required down to 190nm, the index

matching fluid must not only match the index of silica (ca 1.55 at 200nm), but must also

be transparent down to 190nm, at least for path lengths of the order of the gaps in the

mechanical splice, i.e. <10/i,m. Though not a perfect match, cyclohexane was found to

work quite well in this role. Alternative materials are cyclooctane, cyclooctanol and

bromoform, though these materials, while providing a slightly better index match, have

higher short wavelength optical absorptions as well as potentially higher toxicity and in

practice have no advantage over cyclohexane. The volatility of cyclohexane may mean

that repeated applications are required if measurement times are more than a few

minutes. The assembly of pigtail and test fibers was secured in place in the silicon "V"

grove by means of a spring loaded cover glass. Index matching fluid is drawn into the

splice region by capillary action.

To obtain the absorption spectrum of the test fiber the system shown in figure 1

was used. Light from a monochromator, illuminated by a 75W Xe arc lamp, was

launched into one of the fiber pigtails. To avoid problems associated with the chromatic

aberrations and optical absorption of lenses, the light was launched into the fiber by

simply placing it at the exit slit of the monochromator. The light exiting from the second

fiber pigtail was launched into a second monochromator by positioning the fiber at the

entrance slit. The purpose of this second monochromator was to eliminate fluorescence

radiation from the test fiber. The two monochromators were set to track each other in

wavelength during measurements. Light exiting the second monochromator was detected

using a thermoelectrically cooled photomultiplier tube. The same procedure was then

repeated, but this time using only a length of pigtail fiber equal to the total length of the

two pigtails used in the first experiment. The ratio of these two measurements can then

be used to determine the loss of the test section of fiber. It may be necessary to make

small offset adjustments to take into account wavelength independent losses (such as

splice losses). If measurements on the test section of fiber are extended into a wavelength

region at which losses (as determined by other measurement techniques) are known,

then corrections for wavelength independent losses can be made with confidence.

A few words of caution are appropriate in the discussion of these measurements.

GeO is well known to fluoresce at ca. 400nm when illuminated near its absorption bands

at 241nm and 330nm. For high accuracy measurements it is of critical importance to

ensure that no fluorescence radiation reaches the detector, and that no light can bj^ass

the test fiber section via propagation by cladding modes. In a typical measurement of

a high GeO fiber optical transmission at 241nm of a 150/xm test section falls to a very

low level (ca. 35dB below the launched light level). If any extraneous light falls on the

detector (either from light propagating in cladding modes, or fluorescence from the core

itself) considerable underestimation of the peak attenuation will result, sometimes

without an obvious distortion of the resultant peak shapes. Such problems have lead

erroneous reports of the UV absorption spectrum of optical fibers^^^, later corrected when

these problems were addressed'^l A check that such problems are not distorting

experimental results can be made by measuring the attenuation per unit length of several

104
different lengths of test fiber. If the attenuation per unit length is not constant, it is a

very strong indication of measurement problems due to propagation of Hght via

cladding modes or core fluorescence. If this occurs it may be necessary to use pigtails

with a smaller core diameter, lower eccentricity and /or to improve the filtering of

fluorescence radiation.

Results

An examples of measurements made using the technique described here is given

in figure 2 which shows the absorption spectrum of an AT&T Tethered Vehicle fiber (a

single mode fiber with a 6.5 jum core containing about 10 mole% germania). The small

fiber core required the fabrication of pure silica core pigtail fibers with a core diameter

around 5/xm for accurate measurement. The GeO peak near 241nm was measured on a

160/i,m length of fiber and is seen to be ca, 230dB/ mm. Despite efforts to minimize errors

due to the effects discussed previously, an earlier reported measurement made using

larger core pigtail fibers evidently suffered from some propagation of light via cladding

modes and the resulting reported loss value of 80dB/mm for the GeO band in an AT&T

Tethered Vehicle fiber was significantly too low^^l After the formation of a refractive

index grating in the fiber by 242nm UV exposure, the absorption spectrum of the fiber

showed considerable changes as shown in figure 2. Analysis of such changes can give

insight into the mechanisms involved in grating formation^^l

Conclusions

A technique has been presented which allows absorption measurements to be

made in very highly attenuating fibers. The specific application of the measurement of

GeO in fibers has been described, though the technique is generally applicable to any

highly attenuating species. The technique has the advantage over fusion splicing that the

fiber is not subjected to high temperature extremes which can radically affect the fiber

attenuation under some circumstances.

Acknowledgements

The author would like to thank K. T. Nelson, D. DiGiovanni, J. W. Fleming, F. V.

DiMarcello, W. A Reed, A. Moesle, R. G. Huff, V. Mizrahi and K. L. Walker for their

assistance in this project.

References

r

1 - M. J. Yuen, Appl. Opt. 21, 136 (1982).

2 - G. Meltz, W. W. Morey and W.H. Glenn, Opt. Lett. 14, 823 (1989).

3 - D. L. Williams, S. T. Davey, R. Kashyap, J. R. Armitage and B. J. Ainsle, Appl. Phys.

Lett. 59, 762 (1991).

4 - D. L. Williams, S. T. Davey, R. Kashyap, J. R. Armitage and B. J. Ainsle, Elect. Lett.

28, 369 (1992).

5 - R. M. Atkins, Opt. Lett. 17, 469 (1992).

6 - R. M. Atkins and V. Mizrahi, Paper JTuA3, Technical Digest, Conference on Lasers

and Electro-Optics, May 10-15, 1992, Anaheim, CA.

105
Xe Arc Lamp

Section of Grating

Monochromator Moncchromator ck)oled PMT

Pigtail Rbers

COVER GLASS

INDEX MATCHING

FLUID

SlUCON BLOCK

PIGTAIL FIBER TEST SECTION

FIGURE 1 - Sytem used to measure the attenuation of very short lengths of fiber

190 210 230 250 270 290

WAVELENGTH (nm)

FIGURE 2 - UV attenuation of an AT&T Tethered Vehicle fiber and changes

observed after exposure to 242nm radiation

106
DETERMINATION OF STRESS OPTIC EFFECT IN OPTICAL FIBERS

A J Barlow*, W H Ficke**, T Voots***

* EGScG Fiber Optics, Wokingham, UK

** AT&T Bell Laboratories, Norcross, GA, USA

*** Photon Kinetics, Beaverton, OR, USA

I ntroduct i on

Qualifying a fiber cable design, to verify that premature

failure caused by excessive fiber strain will not occur during

cable service, requires the accurate determination of its fiber

strain/ cable load characteristics. Measurement of this characteristic typically involves using a phase shift method [1] to

observe the change in optical propagation delay of the fiber [2]

in a loaded cable. Conversion of the delay change to a physical

fiber strain (or elongation) requires two constants: the fiber

group index [3] and its stress-optic characteristic [4], [5].

In this paper, a refined phase shift method to accurately

determine the "stress-optic correction factor" is presented. The

new method uses standard fiber strain measurement and tensile

test equipment.

The propagation delay change 6t(X) of a strained fiber of

initial gauge length L and group index N(X) at wavelength \, is

given by [4] :

6t(.\) = ( N(\).6L + L.6N )/c --(1)

where: 6t(\) is the relative delay change in the fiber when

strain G is applied;

6L is the physical elongation of the fiber;

6N is the term representing the stress-optic effect on

the group index N(\) due to axial tension;

c is the speed of light in vacuo.

Applying stress-optic theory [4], equation (1) becomes :-

6xiX) = N(\). 6L. K /c --(2)

where: K = [ 1 - NiAl^ • {Pl2-cr(Pll+Pl2 ) > 1 --(3)

2

K is the "stress-optic correction factor" for a fiber under axial

tension. Other correction factors would be needed to correct any

thermo-optic [4], [5] or pressure effects on N [4].

The strain G is then given by:-

G = 6L/L = 6t(\) .c .(1/K) --(4)

(N(\).L)

Group index N and the associated stress-optic correction factor K

both directly affect strain measurement accuracy. Evaluation of K

using fused silica values [4] of P^ i=0 . 121 , pi2=0 • 27, a=0.17, and

N=1.468 yields a value of K= 0.78. However, it is the object of

this work to experimental ly determine the actual K value for a

typical single mode unshifted fiber to high accuracy.

107
Determination of The Stress Optic Correction Factor

Light

Source

X elongation

6L

test fiber

on strain jig

Detector

sig Phase

Meter

6x —>Out

ref

Signal

Generator

By definition, K can only be determined [4] by straining a fiber

and measuring 6L, 6T using standard tensile test jigs [2] (see

figure above). It would be desirable to minimise measurement

uncertainty, since the K value obtained will likely be used

systematically and repeatedly in subseguent fiber tests. Obviously, the fiber used in the determination should be of the same

type as the fiber used in the cables under investigation.

Similarly, the fiber should be tightly buffered (eg Nylon or

Hytrel), in order that a deterministic strain can be applied, at

the same time ensuring negligible slippage at the anchoring

points. The fiber leads to and from the measurement instrumentation must be kept short to minimise the effect of delay changes

arising from these leads. We assume that K is almost constant up

to a few percent strain [5]. To avoid potential non-linear

elasticity effects at high strains, the fiber length L must be

sufficiently large to provide sufficient 6L and 6r at only moderate percentage strain levels.

Assuming the use of a phase shift based measurement system, the

phase (delay) detector must be well calibrated and of sufficient

linearity. In many such systems, the phase detection linearity

may only a percent or so [3]. The Quadrature Phase shift method

[1] widely used in the industry greatly reduces this to levels

that are negligible for normal tensile measurements. However, for

the purpose of determining K or N [3], some means of circumventing any phase meter non-linearity is required.

The proposal of this paper is to circumvent the non-linearity

and other error of the phase detection system by using a simple

modification [4] of the existing technique. Basically, by always

operating the phase meter at the same (zero, or null) phase

point, nonlinear ity is never invoked. Operation in this mode may

for example, be obtained using an electrical variable phase

shifter inserted into the phase meter reference path, but such

devices are themselves generally non-linear. Alternatively, a

variable air-path optical delay line fitted in the optical measurement system could bring the phase meter indication back to

zero for any 6-c, the delay line length yielding 6z . Such a method

is not easily adapted to routine use. Instead, a third possibility is proposed.

The normal response curve of any phase meter repeats every

2n of phase change, no matter what the linearity or scale factor

error of the phase meter is within the 2n range. Hence if the

elongation 6L is increased such that the delay change 6t is

108
always close to one period of the source modulation frequency f

(i.e. the phase change is always close to 2n) , the effect of any

non-linearity or scale factor error in the phase meter is greatly

diminished. In effect, the phase meter reading is only taken

close to two identical fixed points on the phase meter curve for

both the unstrained and strained fiber measurements. We set out

to show that this simple variation incorporated into the technique yields excellent accuracy and precision of K using standard commercial instrumentation.

Results and Discussion

An unshifted standard telecommunication single mode fiber

(AT&T) with a 0 . 9mm Hytrel jacket was used. The phase shift

measurement system used was a standard, unmodified commercial

strain measurement unit (EG&G SPL3 / PK 2300), capable of automatically measuring 6t; with a precision better than Ips. A wavelength of ISlOnm and source linewidth of 4nm was used. The fiber

strain jig used conformed to the requirements of TIA FOTP-33/-38

[2].

First, the fiber group index N(\) was determined using the

cut-back method, again modified as above [3] for increased precision. N was found to be 1.468 +/-0.001 by this method [3], at a

temperature of 23 degrees Celsius.

The fiber was placed onto the strain jig, using several loops

wrapped around each 50cm-diameter mandrel, one of which was movable. The separation between the mandrels and the total fiber

gauge length were approximately 17m and 215m respectively. The

mandrel movement relative to a fixed calibrated graduated scale

was determined using an anti-parallax sight marker on the mandrel

stage, from which 6L was calculated.

A small, arbitrary amount of initial tension was applied to

ensure all fiber loops were taut and that slippage and other

movement on the mandrels (which would result in non-linear take

up of the tension when the mandrel initially moves) was negligible. The strain measurement cycle was started and the fiber load

gradually increased until a total of approximately 2n incremental

phase change (in this case corresponding to 20.000ns) had developed due to the fiber elongation. At a point close to 2ti phase

change, the corresponding 6"c and 6L values were simultaneously

recorded. The total elongation was of the order of 5 m, giving a

2-3% strain level, well below the proof test level for this

fiber. The stress-optic correction factor, K, was determined

from equation (2) using the N value obtained previously.

Several measurements of K were performed to establish the

repeatability of the method. Results are summarised below:

Fiber unshifted, single mode

Group Index 1.468 +/- 0.001

Temperature 23 +/-2 OC

Wavelength 1310 +/- 0.05 nm

Linewidth 4nm

Number of measurements = 20

Mean K value = 0.8074

Std Deviation = 0.0044

109
The random uncertainty in K is made up of contributions from 6L

(~15mm) and 6x (Ips), and residual slippage on the mandrels. The

Systematic uncertainty in K (0.003) largely originates from the

mechanical measurement of 6L, with smaller contributions from 6x

and N. 6L uncertainty could be significantly improved by using

electronic or opto-electronic methods of detecting the mandrel

movement

Thus the stress optic correction factor for this fiber is 0.807

+/-0.009, to 95% confidence. This value compares with the value

predicted above (0.78). In a single mode fiber of a different

construction, group index measurements versus strain [5] gave a

derived value for K of 0.816 +/- 0.04 at 1295 nm. Further results will be presented at the conference.

Conclusion

A simple modification of the existing phase shift method for

measuring stress-optic correction factor in single mode optical

fibers has been described. The modified method appears to offer a

substantial improvement in uncertainty over the standard implementation of the phase shift method [1], [4] or comparable time

delay techniques [5] . The improvement has been achieved by arranging for the phase detection system used to be operated close

to a fixed phase value, in order to avoid inherent phase detection non-linearities. The fixed phase value is accomplished by

arranging the fiber elongation to be approximately that length of

fiber that produces one complete 2 n phase shift, a length

typically of the order of five meters.

Using commercial equipment commonly available in cable/ fiber

manufacturing plants, where most strain measurements are made, a

95% confidence interval of correction factor K of 0.009 has been

obtained, a value limited mainly by the mechanical determination

of the fiber elongation. Such an uncertainty (~1%) will permit

more accurate determination of cable strain performance and

thereby help to improve cable design and quality control.

References

1) A Barlow, "Quadrature Phase Shift Technique For Measurement of

Strain, Optical Power Transmission, and Length in Optical

Fibers", Jnl . Lightwave Technology, 7,1989, ppl264-1269.

2) . EIA/TIA-455-33 'Fiber Optic Cable Tensile Loading and Bending

Test' , and EIA/TIA-455-38 'Measurement of Fiber Strain in Cable

Under Tensile Load'.

3) A J Barlow, S Dodd and N Ives, " Measurement of Group Index

using Fiber Cut Back" OFMC Proceedings, 1991, pp. 16-19.

4) M Brininstool, "Measuring longitudinal strain in optical

fibers". Optical Engineering, 26, 1987, pplll2-1119.

5) J J Carr, S L Saikkonen and D H Williams, "Refractive Index

Measurements on Single-mode Fiber as Function of Product Parameters, Tensile Stress and Temperature", Proc. Tech. Digest Symp.

on Optical Fiber Measurements, Boulder CO, USA, 1990, pp. 59-62.

110
CHARACTERIZATION OF INTEGRATED OPTICAL COMPONENTS

PAUL G. SUCHOSKI, JR.

UNITED TECHNOLOGIES PHOTONICS

1289 BLUE HILLS AVENUE

BLOOMFIELD, CT 06002

(203) 769-3012

United Technologies Photonics (UTP) currently offers a family of

integrated optic products for application in guidance systems,

CATV distribution, antenna remoting, telecommunications, and

instrumentation. The devices are fabricated on x-cut LiNb03

using the annealed proton exchange (APE) process and are

available with either single mode or polarization-maintaining

fiber pigtails. Devices are available for operation at 830,

1320, and 1550 nm wavelengths.

Due to the relative immaturity of this technology, standardized

test procedures do not exist for LiNb03 integrated optical

components. Thus, it has been necessary for UTP to work with

customers to develop a set of test procedures for characterizing

those device parameters that impact system performance. As a

component manufacturer, UTP is faced with the challenge of

developing a minimum test set that will guarantee that all

devices meet specifications. The test set must be relatively

easy and inexpensive to implement as well as being suitable for

automation. At the same time, the test set must be accurate

enough to provide the customer and the manufacturer with useful

data that can be used for statistical process control, future

device design iterations, and system modeling.

In this report, we initially list the critical specifications for

the various devices along with typical performance values. We

then describe the test set that has been developed for each

device type. Detailed descriptions of the various tests will be

presented at the conference and are available as appendices to

this report.

Fiber Optic Gyro f FOG^ Circuits

A three-port FOG circuit consists of a Y- junction power splitter

with independently-addressable phase modulators on each output

arm. The device is pigtailed with polarization-maintaining fiber

on all three output ports. Critical performance parameters and

typical performance are listed below:

111
Insertion loss 2.0-3.5 dB

Split ratio 52:48 to 48:52

Polarization extinction 60-75 dB

Polarization crosstalk

(waveguide-fiber interfaces) -30 to -40 dB

Back reflections <-60 dB

Half-wave voltage 3-4 Volts

Intensity modulation 0.01-0.05 %

FOG systems are typically configured using broadband sources such

as ELEDs or SLDs. Due to the limited power of these sources, it

is difficult to make accurate measurements on FOG circuits. We

therefore opt to use high-power (10 mW) multimode laser diodes at

830, 1320, and 1550 nm for the measurements. The only parameter

that is impacted by source spectral width is the intensity

modulation which is typically 2-5 times lower when measured with

a broadband source. Thus, the intensity modulation measurements

made using a multimode laser diode source represent worst-case

values

The following test set is used to characterize the FOG circuits:

1. Launch optical power into the input fiber. Measure the power

from the two output fibers (PI and P2). The split ratio is

defined as P1/(P1+P2 ) :P2/(P1+P2 )

2. Measure polarization crosstalk of the two output fibers using

a strain-free colimating objective and a polarizer.

3. Measure the half-wave voltage of the two phase modulators by

interfering the optical signals from the two output fibers.

4. Measure the intensity modulation by driving each phase

modulator with a 2000 Hz sinusoid of amplitude (+-) 2Vpj^.

5. Measure the polarization extinction of the device.

6. Using the cut-back method, measure the insertion loss which

is defined as (PH-P2 ) /Pj^^^.

7. Excite one of the output fibers. Measure the polarization

crosstalk of the input fiber.

Phase Modulators

Phase modulators are provided with polarization-maintaining fiber

on the input and either polarization-maintaining or single mode

fiber on the output. Critical performance parameters and typical

performance are listed below:

Insertion loss

Polarization crosstalk

Back reflection

Half-wave voltage

Intensity modulation

2.0-3.5 dB

-30 to -40 dB

<-60 dB

Depends on wavelength and

frequency response

0.01-0.05%

112
The following test set is used to measure the phase modulators.

High-power multimode laser diodes are used as the light sources.

1. Launch optical power into the input fiber. Measure

polarization crosstalk of the output fiber.

2. Measure the intensity modulation of the device with a 2 kHz

(+-) 10 Volt sinusoidal signal.

3. Measure the half-wave voltage of the device by interfering

the fiber ouput with a reference optical beam.

4. Using the cut-back method, measure the insertion loss of the

device

5. Launch power into the output fiber. Measure the polarization

crosstalk of the input fiber.

UHF Intensity Modulators

Two types of UHF intensity modulators are currently offered:

Mach Zehnder interferometers with input and output Y- junctions;

and Y-fed balanced bridge modulators with an input Y- junction and

an output 3-dB directional coupler. The devices have both highspeed (600 and 860 MHz 1-dB points) and bias electrodes. The

Mach Zehnder output power varies sinusoidally with applied

voltage while the YBBM provides two complementary optical outputs

that vary sinusoidally with applied voltage. The devices are

typically pigtailed with polarization-maintaining input fiber and

single mode output fiber. Critical performance parameters and

typical performance are listed below

Insertion loss

On/off extinction

Back reflection

Frequency response

Flatness

Half-wave voltage

The following test set is used

modulators

1. Launch power into the input fiber. Measure power from the

output fiber(s) while applying a DC voltage to the bias port.

Adjust the DC voltage to obtain minimum and maximum output

powers. On/off extinction is defined as Pjpax/^min*

2. Measure the half-wave voltage of the bias and RF electrodes.

3. Measure the frequency response (amplitude and phase) using a

network analyzer. Extreme care must be taken in this measurement

since device flatness is comparable to or better than the best

available electronic test gear.

4. Using the cut-back method, measure the insertion loss of the

device, which is defined as Pmax/^in*

3.0-4.5 dB

25-40 dB

<-60 dB

-1 dB >600, 860 MHz

0.5 dB amplitude,

2 degrees phase

4-5 Volts

to characterize the UHF

113
Microwave Intensity Modulators

The microwave intensity modulators are similar to the UHF

intensity modulators except that traveling-wave electrodes are

utilized to achieve bandwidths (-3 dB electrical) of 3, 8, 12,

and 18 GHz. The critical performance parameters and test

procedures are similar to the UHF modulators with the exception

that

1. the swept frequency technique is used to measure the

frequency response and

2. S-.1 is measured using a network analyzer.

Conclusions

It is important to note that the test sets described above are

performed as final tests on every packaged, pigtailed component

that is delivered by UTP. Since it is difficult to integrate

into production, back reflection is currently not measured on

every device. It is experience that back reflection is

consistently <-65 dB and is insensitive to device design and

fabrication conditions. The low back reflection is obtained by

polishing the optical fibers at 15 degrees and the LiNb03 device

at 10 degrees so that the back reflection is well outside the

collection apperture of the optical fiber.

In addition to the tests described above, there is extensive

testing, inspecting, and tracking of parts and materials

associated with quality control that is performed during device

fabrication and assembly.

114
REAL TIME MEASUREMENT OF MODE PARTITION NOISE IN HIGH SPEED DATA

MODULATED LASER DIODES, IN PRESENCE OF REFLECTIONS.

M. Puleo, P. Gambini, E. Vezzoni

CSELT - Centro Studi E Laboratori Telecomunicazioni S.p.A. - Torino - Italy

Abstract

A measurement set-up is described that allows the real time observation of the

instantaneous optical spectrum of a multimode laser diode, modulated by a data stream;

statistical analysis on the temporal evolution of this spectrum leads to the evaluation of the

mode partition coefficient k, in realistic operating conditions. Different lasers have been

evaluated, with high speed (622 Mbit/s) modulation, evidencing the effects of data pattern

and optical reflections.

Introduction

The power distribution among the different longitudinal modes of a multimode laser

diode continuosly fluctuates, while keeping constant the total power. This behaviour is a severe cause of impairment for system performance at high bit rates, when the different

wavelengths experience different propagation conditions due to chromatic dispersion [1]. To

characterize the spectral behaviour a mode partition coefficient k was defined as follows [1]:

k= — -_ fori^j (1)

i j>i

where aij are the instantaneous amplitudes of each mode, normalized to the total emitted

power at a given time; the summation being carried out over all modes.

A few methods have been proposed so far to characterize this parameter: some of them

separately measure the fluctuations of the different modes, neglecting, under proper

assumptions, the cross-correlation terms [1]; other techniques simultaneously observe all

modes, by applying pulsed modulating waveforms to the laser source [2].

The proposed technique allows the simultaneous, real time observation of several

longitudinal modes of the laser spectrum corresponding to a selected bit, while the source is continuosly modulated by a pseudorandom bit stream at 622 Mbit/s; the average and cross- correlation terms aj and aiaj, needed to apply (1), are then easily derived. The mode partition

noise coefficient has been measured under different operating conditions, as a function of

bias current, bit pattern and external optical reflections.

Experimental set-up

The experimental set-up is schematically shown in fig. 1. The laser under test is

temperature controlled, biased by a high stability current source and modulated by a 622

Mbit/s PRBS data stream. A controlled optical reflection is provided by a variable backreflector, in order to check the device sensitivity to external reflections; uncontrolled

reflections from the following optical set-up are rejected by a polarization independent

pigtailed isolator. The modulated optical signal is then fed to a LiNbOs Mach-Zehnder

intensity modulator, acting as an optical gate, which allows beam transmission for a short

time interval only (8 ns gating time, corresponding to 5 bit times of the 622 Mbit/s

modulating signal, has been used in the experiment), sinchronously with the data sequence

entering the gate. The output beam is collimated on a diffraction grating, which spreads the

light coming from the different modes in different directions; a suitable lens system with

variable magnification (zoom) then focuses each beam on an array of 5 equally spaced fibres,

positioned by a V-grooved silicon substrate. The zoom is adjusted so that each fibre collects

the light coming from a single, different mode of the laser. The optical system works as a multiple output grating monochromator, whose output resolution is set by the core diameter

of the collecting fibre. As the fibre spacing is fixed at 160 |im, and the fibre core diameter is 50

115
|im, a wavelength resolution of about 0.3 nm is obtained (which should be large enough to

accept mode spreading due to wavelength chirp).

JU-ITTL

grating

622Mbit/s

FRBS

generator

waveform

digitizer

differential optical delays

Fig.l - Experimental set-up

At each fibre input in the array, the same (gated) pulse sequence, with different

wavelengths, is present simultaneously; to avoid the need of multichannel waveform

acquisition, the fibre lenghts in the array are different, such that the differential delay (one

gating period plus a guard interval of 2 ns) allows to detect the same sequence at different

wavelengths, without overlapping. After detection and amplification by low noise APDFET

receivers, the pulse sequences are electrically combined and then displayed by a single

channel waveform digitizer (Tektronix 7912AD), operating in single shot mode; the

simultaneous temporal evolution of the 5 observed modes, inside the 8 ns gate period, is then displayed in a single sweep of the digitizer; due to the syncronous operation of the data

and gate generator circuitry, the laser spectral behaviour corresponding to any particular data

sequence in the PRBS stream can be observed. The waveform digitizer is then connected to a workstation, to performs data acquisition and processing. Multiple acquisitions can be carried

out in a relatively short time (400 ms/sweep), allowing a further statistical analysis of the

measurement data.

150-

-50 J ^ ^ . , 1 0 10 20 30 40 50

Time [ ns ]

Fig.2 - Waveforms acquired in 100 consecutive sweeps, corresponding to a 01010 bit sequence

extracted from a 215-1 bits long PRBS

116
The overall bandwidth of the receiver and digitizing system is higher than 500 MHz,

thus allowing the observation of very fast amplitude fluctuations; the waveforms acquired in

100 consecutive sweeps, corresponding in this particular case to a 01010 bit sequence

exctracted from the 2^5-] bits long PRBS, are displayed in figure 2; these data are then

corrected for the gate and receiver pulse responses, (previously measured using a fast DFB

laser source), and the amplitude of each mode at the same instant is computed. Some optical

spectra, corresponding to the peaks of the first and second "1"( upper and lower row

respectively) of consecutive waveforms taken from the display of fig. 2, are shown in fig. 3.

Fig. 3 - Measured laser optical spectra, corresponding to mark peaks in the 01010 sequence

Repeated observations of the instantaneous spectrum allow to carry out a statistical

analysis on the spectral behaviour of the laser under test; the mode partition coefficient k is then evaluated by applying (1).

It should be noted that the operation of the laser under test is not affected by the

measurement procedure, nor special modulation waveforms are required; this spectral

analysis could be carried out even at the output of standard optical transmitters, provided

that a suitable timing signal for the gate operation is available.

Results

Different Fabry-Perot devices have been evaluated in realistic operating conditions. A

commercial pig-tailed laser diode (#1) has been measured first. 0.4-

° 0.2

co

a.

X3o

0. 1-

G 0 1 I 11 1 sequence

^ h 01010 sequence

1 ^ _—1 1 1 -G0 -50 -40 -30 -20 -10 0

Optical reflection [ dB ]

Fig. 4 - Measured values of the k parameter vs. optical reflection for laser#l, and for: a) a long

sequence of marks, and b) a 01010 sequence, extracted from the PRBS pattern.

117
This device has a relarively narrow optical spectrum, with few significant modes. It is biased at threshold and modulated with a data stream at 622 Mbit/s; the peak-to-peak driving

current is 40 mA and the peak output power is about 1 mW. The measured values of k are reported in fig. 4, as a function of the optical reflection level, corresponding to a long

sequence of Is (a) and a 01010 sequence (b), respectively, extracted from the PRBS pattern. At

low reflection levels the mode partition coefficient is lower for sequence a), when the laser is kept turned on for a long time, than for sequence b), where the source is continuosly

switched on and off; as the reflection level increases the two curves converge to higher

values of k (0.3-0.4).

A different laser (#2), with broader spectrum, has been tested as well, with comparable

results. The change of the mode partition coefficient with bias current is shown in fig. 5 for

reflection values of -8 and -40 dB; as expected k increases when the laser is biased below

threshold and is relatively stable above threshold. As the measurement was carried out in

correspondence of many transitions, reflections have little effect on k value.

0.5 -

c 0.4 -

CDU

O

U

I 0-2 -

^ + ^0.1--

IDO

SI 0^

8 10 12 14 IB 18 20

Bias current [ mfl ]

Fig. 5 - Variation of the mode partition coefficient, k, with the bias current of laser #2

Conclusions

A specialized set-up for the real time measurement of the instantaneous spectrum of a laser diode, operating in realistic modulation conditions and in presence of optical

reflections, has been developed. Statistical processing of the acquired spectra has lead to the

evaluation of the mode partition coefficient, k. Different multimode devices have been

investigated; a strong pattern dependence of the k coefficient was found at low reflection

levels, while at higher reflection levels k is less affected by data pattern.

This technique can be applied as well to investigate mode partitioning effects between

the main and the highest side-modes in nominally single mode DFB laser diodes; higher

dynamic range can be obtained by using EDFAs.

Acknowledgements

The authors gratefully acknowledge Dr. B. Sordo for his helpful assistance and Dr. M.

Potenza for useful discussions. This work has been supported by SIP and Italtel-SIT.

References

[1] K. Ogawa,R.S. Vodhanel - Measurement of mode partition noise of laser diodes- IEEE

Journal of Quantum Electronics. Vol. QE-18, No 7, July 1982, pp. 1090,1093.

[2] I.D. Henning, D.A. Frisch - Real time measurement of semiconductor laser spectra

IEEE Journal of Lightwave Technology, Vol. LT-1, No 1, March 1983.

threshold

-+ ref 1 ect 1 on -8 d

reflection -40 dB

118
A COMPLETE MEASUREMENT SET FOR HIGH PERFORMZU^CE OPTICAL CONNECTORS

M. Bottanelli, L. Duca, S. Pitassi

SIRTI S.p.A., Cables and Optical Technologies/

Via G. Vida 19, 20127 Milano, ITALY

ABSTRACT

This paper illustrates how to assemble and characterize high performance

connectors by means of a complete measurement set on connector parts and

mounted connectors. Some of these measurements are known and are reported

here for completeness only, while other ones are new or very little

experimented

INTRODUCTION

The development of optical telecommunication systems has a strong impact

in the implementation of very accurate and effective optical passive

components. In recent years, the growing of the [bit rate ]x [distance] product

towards higher and higher values has forced connector manufacturers to

improve dramatically the quality of their products.

Nowadays, a good single-mode optical connector ensures less than 0.2 ds

insertion loss and more than 40 dB return loss. High performances can only be

obtained by a perfect matching between the two fibers involved in the

connection. Sub-micrometric precisions are therefore required in measuring

and mounting the main parts of connectors, and manufacturers need the

availability of fast, high precision measuring set-ups.

Figure 1 shows the scheme of a modern cylindrical single-mode

interconnection [1]. Two fibers are inserted and glued into two ferrules and

polished to obtain high quality end surfaces. The two ferrules are then

coupled into an alignment sleeve and pushed axially one against the other by

the action of two calibrated springs. In this situation, the transverse

offset between the fiber cores, being the main cause of loss, is due to the

composition of the following parameters:

- the eccentricity between the fiber core and the fiber cladding;

- the clearance between the fiber cladding and the ferrule hole;

- the eccentricity of the ferrule hole.

Additionally, the transverse offset between the top of the ferrule end face

convexity and the theoretical axis of the connection is another contribution

to the global transverse offset. This convexity is a necessary result of the

polishing procedure to improve the physical contact between the fibers.

FIBER GEOMETRICAL PARAMETERS AND MEASUREMENTS

For connector manufacturers, the cladding diameter and the core-cladding

concentricity error are the main geometrical parameters of a fiber. In 1989,

the First International Fibre Geometry Round Robin [2] suggested a 125 ± 1 jum

and 0.5 jam upper limit respectively as the future target specifications for

these parameters; but it also showed high interlaboratories measurement sigma

and a substantial need for calibration and transfer standards. Recent years

researches are giving interesting results especially in the field of

calibration standards ([3], [4], [5]), while it has not been yet determined

the best measurement method among the possible ones. Recently, the "Gray

Scale" method, which uses incoherent illumination, video microscopy and image

digitalization, seems to receive manufacturers' attentions, especially

because of its intrinsic simplicity and usefulness. Figure 2 illustrates, for

instance, a reproducibility test for a Gray Scale set-up, calibrated by a high precision reticle, with very good result obtained in a short measuring

time. At present, the Gray Scale method is classified among the CCITT

alternative test methods, even without the approval of specific algorithms

[6], waiting for the results and suggestions of the next CCITT Fibre Geometry

Round Robin. In the meantime, Gray Scale method, contact micrometry, manual

selection of the ferrule with respect to fiber diameter and autocentering

epoxy resins are simultaneously used to limit the effect of the clearance

between the fiber cladding and the ferrule hole, while the current quality of

the fibers ensures fiber core-cladding concentricity error mean value of

about 0.3 fum.

FERRULE GEOMETRICAL PARAMETERS AND MEASUREMENTS

Many ferrule geometrical parameters may be considered, but only a few ones

are really important because of their direct impact on the optical

performances of connectors; among these the outer diameter, the hole diameter

119
and the hole eccentricity are the most important ones.

A large difference between the two ferrule outer diameters may not only influence the global transverse offset in a connection, as stated above, but

also increase dramatically tilt angle losses by widening the alignment sleeve

in a non uniform way. For such reasons, a tight tolerance of ± 0.5 ;jm on the

nominal value is nowadays recommended for this parameter. Figure 3

illustrates a fast and effective optical set-up for ferrule outer diameter

measurement. The ferrule is held by a precision mandrel, turned around its

axis and shifted forward and backward in the light band of a laser scanning

gauge by microposit ioning motors. A whole accurate mapping of the ferrule

cylinder can thus be obtained in a very short time. Figure 4 illustrates a reproducibility test for this set-up, considering the maximum ferrule outer

diameter only.

In measuring ferrule eccentricity, the main difficulties consist in the

ratio of 1 to about 20 between the hole and the ferrule diameter, and their

different focal planes due to the frontal chamfering. Many different methods

have been proposed to overcome these difficulties ([7], [8], [9], [10]), but

nowadays it seems to be impossible to obtain good resolution measurements

without turning the ferrule around its axis. Figure 5 shows the scheme and

the principle of an optical set-up for ferrule eccentricity and hole diameter

measurement. The ferrule hole is back-lighted, the ferrule is turned six

times on a V-groove and the spot projected by the hole is taken for each of

the different positions. The hole diameter is the mean value of the six spot

diameters; the eccentricity is the radius of the circumference fitting the

six spot centers. Figure 5 illustrates a reproducibility test for this setup, with very good results.

GEOMETRICAL PARAMETERS OF POLISHED FERRULES AND RELEVANT MEASUREMENTS

The optical parameters and the stability of the PC (Physical Contact)

connection between two mated ferrules, polished in the convex shape, mainly

depends on three geometrical parameters ([11], [12]):

- eccentricity between the top of the ferrule end face convexity and the

ferrule axis (convexity vertex eccentricity)

- fiber withdrawal with respect to the spherical surface best fitting the

ferrule end face;

- curvature radius of the ferrule end face;

Such three parameters are subjected to some restrictions in order to preserve

the PC of the fibers. In fact, if two ferrules with high convexity vertex

eccentricity are connected to each other, the fiber ends could be separated

by an extra gap above the fiber withdrawals, resulting in poor return loss

and insertion loss performances. Besides the geometrical parameters of the

ferrule, however, the quality of the PC strongly depends on the compression

forces between the fibers and the elasticity characteristics of the ferrule

material

Both mechanical and optical methods can be used to control those

geometrical parameters. The vertex eccentricity can be measured by a Michelson interferometer supported by a signal controller processing the

image of the interference patterns. The most important source of inaccuracy

is the angular offset between the ferrule axis and the interferometer axis.

This error can even introduce a measurement variability of a few tens of

microns (a typical vertex eccentricity mean value is about 20 jum) . If the

ferrule axis and the interferometer axis are parallel, the geometrical locus

described by the center of the fringes with the rotation of the ferrule is a circumference centered on the ferrule axis. The radius of this circumference

is the vertex eccentricity: in other words, the vertex eccentricity measured

value is the same for every angle of rotation. Let's suppose that the ferrule

has its axis tilted of an angle 6 in comparison with the interferometer axis.

From simple geometrical considerations, the locus of the center of the

fringes, obtained by rotating the ferrule, is an ellipse with the center

shifted of a quantity R»sin6 with respect to the fiber center (R being the

curvature radius of the ferrule end face). Because of the small value of 6,

the ellipse can be approximated with a circumference whose radius represents

the vertex eccentricity without angular errors. Following these

considerations, a measurement set-up based on a Michelson interferometer has

been developed by which it is possible to turn the ferrule around its axis,

to determine the convexity vertex positions and to fit them with a circumference. The radius of this circumference is the real vertex

eccentricity. This procedure improves measurement accuracy above 2.5 ;jm.

Figure 7 shows a final plot for a vertex eccentricity measurement: the

biggest circumference is the fiber, while the smaller one is the

circumference fitting three different convexity vertex positions for three

120

4
different rotations of the ferrule.

Fiber withdrawal and ferrule end face curvature radius can be measured by

a mechanical contact prof ilometer . The measurement cord, followed by the

sensitive pick-up, is usually 500 jum long. Figure 8 shows the typical

transverse profile of a ferrule end face after polishing procedure: the

irregular line is the surface profile along the measurement cord, while the

straight line is its mathematical average; the fiber end face is about 0.04

;jm above this line. Withdrawals lower than 0.1 jam are necessary for optimum

return loss performance.

CONCLUSIONS

The main geometrical parameters of cylindrical optical connectors and a complete measurement set on these parameters have been described from the

connector manufacturer's point of view. Less than 0.2 dB insertion loss and

more than 40 dB return loss can be obtained by few, fast and very effective

measurement set-ups.

ACKNOWLEDGEMENTS

We would like to thank U. Rossi and E. Serafini for their invaluable

remarks and suggestions.

REFERENCES

M. Bottanelli, E. Serafini

[1] MISURA DEI PARAMETRI GEOMETRICI

DI CONNETTORI OTTICI CILINDRICI

Atti del 2° Convegno Nazionale

"Strumentazione e Metodi Ottici"

- Firenze (ITALY) - 25-27 May

1992 - pp. 281-284

J. G. N. Baines, A. G. Hallam, K.

W. Raine, N. P. Turner

[2] FIBER DIAMETER MEASUREMENTS AND

THEIR CALIBRATION

Journal of Lightwave Technology,

vol. 8, No. 9, September 1990,

pp. 1259-1267

W. T. Kane

[3] THE FIRST INTERNATIONAL FIBRE

GEOMETRY ROUND ROBIN

Optical Fibre Measurement

Conference - York (UK) - 17-18

September 1991 - Conference

Digest - pp. 119-122

M. Young

[4] FIBER CLADDING DIAMETER BY

CONTACT MICROMETRY

Optical Fibre Measurement

Conference - York (UK) - 17-18

September 1991 - Conference

Digest - pp. 123-126

A. Collyer, K. W. Raine, J. G. N.

Baines

[5] INVESTIGATION INTO THE USE OF A

SCANNING CONFOCAL MICROSCOPE AS A

PRIMARY MEASUREMENT METHOD FOR

DETERMINING OPTICAL FIBRE

DIAMETER

Optical Fibre Measurement

Conference - York (UK) - 17-18

September 1991 - Conference

Digest - pp. 143-146

f61 CCITT TEMPORARY DOCUMENT 39

(XV/5)

Geneva - 4-15 May 1992 - Report

of the expert meeting on Q.ll/XV

- pp. 1-2

G. Warnes, C. A. Millar

[7] A STUDY OF CORE CONCENTRICITY

ERROR MEASUREMENTS

- SPIE vol. 468 - Fibre Optics

'84 (Sira) 1984 - pp. 138-144

T. Tanabe, A. Ushirogawa, M.

Takashima

[8] HIGHLY ACCURATE MEASURING

EQUIPMENT FOR THE OPTICAL

CONNECTOR FERRULE

Measurements, vol. 4, No.' 1,

Jan. -Mar. 1986, pp. 23-27

Z. Pasturczyk, B. Wong, C.

Saravanos

[9] FIBER OPTIC CONNECTOR GEOMETRY

TEST STATION

International Wire & Cable

Symposium Proceeding 1989, pp. 683-626

M. Ueki, Y. Hattori

-l-O] ACCURATE MEASUREMENT OF

STRUCTURAL PARAMETERS OF SINGLE

FIBER CONNECTOR FERRULE

OFC '91 - Thursday, February 21,

p. 137

T. Shintaku, R. Nagase, E. Suqita

[11] CONNECTION MECHANISM OF PHYSICALCONTACT OPTICAL FIBER CONNECTORS

WITH SPHERICAL CONVEX POLISHED

ENDS

Applied Optics, vol. 30, No. 36, December 1991, pp. 5260-5265

Z. Pasturczyk, T. Chepyha, C.

Saravanos, H. Wood

[12] TEST METHOD FOR FIBER OPTIC

CONNECTOR PARAMETERS DIRECTLY

AFFECTING RETURN LOSS

Symposium on Optical Fiber

Measurements, 1990 - Technical

Digest - NIST Special Publication

792 - pp. 163-156

121
fiber 1 ferrule 1 ferrule 2 fiber 2

sleeve tfieoretical axis ; of connection

Spring action

Fig. 1

- Scheme of a modern cylindrical single-mode Interconnection

<2B.»

IBSJ

Stindird Dav - O 0732 f;m Vilua - 12S.274 MIn Vilua « 129 012

2 3 4

Fig. 2 - Reproducibility test for a Gray Scale optical set-up

Ypostoner

<^'~'°'

\ ferrule

laser beam \ mandrel

\

\ ^——-1 \i iP

X positioner

MMn Viki* - 24Se.7f 7//ni

Stindwd D«v. ° 0 0192 pm

Mlx.VlIu*^ 2408.75 f/m Mln.VlIU*^ 24M 6euni

Fig. 3 - Scheme of a set-up tor ferrule outer diameter measurement Rg. 4 - Reproducibility test for the measurement set-up showed In fig. 3

ferrule j_L ob|ectlve holder

Bflfl n

monitor computer

Fig. 5 - Scheme and principle of an optical set-up for ferrule

eccentricity and hole diameter measurement

VERTCX CCCCNTmCfTT * 14JMim

ISO 200

El •

Fig. 7 - Rnal plot for a convexity vertex eccentricity measurement

o.E Mean Valua • 0.234 Sttndanj D«v. - 0.031 MeK. Value - 0.29^

MIn. ValuB 0.19 /jm

J I I I 1_ -1 I ] L

1 2 3 4 G 8 7

Maaauramanta

8 9 10

Rg. 6 - Reproducibility test for the measurement set-up showed In fig. 5

+0.02 pm

+0.00 fjm

-0.04 /urn —

500 /im

Rg. 8 - Typical transverse profile of a ferrule end face

after polishing procedure

122
A SIMPLE AND ACCURATE HETERODYNE TECHNIQUE

FOR THE CHARACTERIZATION OF ACTIVE AND PASSIVE NARROWBAND

OPTICAL FILTERS

N. Caponio, P. Gambini, R. Hui, M. Puleo, E. Vezzoni

CSELT - Centre Studi E Laboratori Telecomunicazioni S.p.A.

Via G. Reiss Romoli, 274

10148 TORINO - ITALY

ABSTRACT

An effective technique has been developed to characterise the spectral

transmission of narrowband (=GHz) fiber-coupled optical filters, both passive and active.

The measurement principle is based on optical heterodyne detection of the emission of a frequency-swept source, passing through the filter under test. This allows the

straightforward and accurate identification of the sweep frequency.

INTRODUCTION

High density wavelength division multiplexed (HDWDM) optical transmission

systems, operating with channel spacings in the order of a few tens GHz or less, result in the

efficient exploitation of the wide transmission bandwidth available in single mode optical

fibres [1]. Channel selection can be performed directly in the optical domain by means of an

optical filter, followed by a direct detection receiver. Passive tunable filters based on

Fabry-Perot or Mach-Zehnder interferometers have been proposed and used, as well as

active tunable filters based on DFB or DBR structures [2].

The precise knowledge of the shape of the filter transmission curve is essential for

an accurate design of the receiver and for system performance evaluation. To characterise

the transmission curve of these narrow-bandwidth filters, conventional grating

spectrometers cannot be used, due to their limited resolution. A usual technique is based on

sweeping the optical frequency of a single wavelength, narrow-linewidth laser source

across the wavelength range of interest, and on measuring the transmitted power after the

filter. Yet, when a laser diode is used as a current-tunable source, the relationship between

emission wavelength (or frequency) and injection current is usually unknown and nonlinear,

and therefore an in-line frequency monitor with sub-GHz resolution (etalon, wavemeter...)

has to be used, to scale the optical frequency.

In this paper an alternative and effective technique for the accurate spectral

characterisation of optical narrowband filters, allowing direct frequency reading, is

proposed and demonstrated; experimental results for narrowband (==GHz) passive FabryPerot and active DFB filters are presented.

PRINCIPLE OF OPERATION AND EXPERIMENTAL SET-UP

Fig. 1 shows a schematic of the measurement set-up that has been implemented;

two narrow linewidth, wavelength matched and temperature controlled DFB lasers are biased with low noise current generators. Their emission is fed to a 2x2 fibre coupler; high

performance optical isolators are used to prevent spectral instabilities due to

backreflections. The emission wavelength of laser #1 is fixed, while a sawtooth

123
modulation current is fed to laser #2, resulting in a swept emission wavelength. The device

under test (i. e. the filter) is inserted between laser #2 and the corresponding input branch of

the coupler. The transmission response of the filter (i.e. the amplitude vs. frequency

information) is simply obtained from the beat signal at the output of the photodiode,

displayed by means of a RF spectrum analyser (SA). A fiber polarisation control is used to

match the state of polarization of the two fields on the photodetector, to achieve the

maximum heterodyne detection efficiency. Although it could be possible to synchronize the

start of the sweep of the SA and of the DFB laser frequency, it would be however difficult

to have an exact time coincidence of the beat frequency and of the SA central frequency, due

to the above mentioned DFB nonlinear frequency vs. current behaviour. For this reason we

have chosen two very different sweep times for the SA (xga) and for the laser frequency

(xi). Two choices are possible: either Xsa » ti or Xga « ^1. In both cases the ratio between Xsa

and XI should be much larger than the number of points to be displayed on the SA screen

(typically 801). In the second case the SA is used in the "peak-hold" mode. We usually

adopt the first approach as it gives cleaner traces, not being subject to the "worst case"

noise.

In the present set-up the beat linewidth of the sources exceeds the SA resolution

bandwidth (3 MHz), causing a signal decrease. The use of very narrow linewidth lasers [3],

would easily overcome this problem.

To calibrate the measurement set-up, the small power and linewidth variations

due to the DFB laser current sweep and the frequency response of the set-up are obtained in a separate measurement, bypassing the filter. The upper limit to the optical frequency span depends on the laser tuning

characteristics and on the frequency response of the spectrum analyser and photodetector. In

the present set-up it is limited to 22 GHz (=0.2 nm at 1.55 )im ) by the SA. If necessary, this

limitation can be overcome by performing different measurements in different, adjacent

windows; this can be simply obtained by step tuning the source wavelengths adjusting their

temperatures. In this way the tuning characteristic and the spectral response of threeelectrode DFB filters has been measured over more than 60 GHz.

The sensitivity of the measurement is mainly limited by the thermal noise of the

receiver and spectrum analyzer. It could be improved, if required, by a lock-in technique, as described in [4]; chopping the "local oscillator" beam would eliminate the spontaneous

emission noise, in the case of active filters.

It must be noted that the main advantage of this technique is the accurate

determination of the frequency, which results in the accurate evaluation of the lineshape

of the filter under test.

EXPERIMENTAL RESULTS

Figure 2 shows the measured transmission characteristic of a Fibre Fabry-Perot

filter (FWHM width: 2.9 GHz), resulting in a regular, symmetrical response. Fig. 3 refers to

the transmission characteristic of a bulk-optics fiber-coupled tunable etalon, which,

suffering from spurious reflections at the interfaces, gives a highly distorted shape.

The measurement of active filters is performed inserting a variable optical

attenuator and another polarization control before the filter in the set-up. Fig. 4 shows the

spectral gain characteristic of a Distributed FeedBack (DFB) laser amplifier (FWHM

width: 5.1 GHz), biased just below the lasing threshold. The shape is asymmetrical, as

predicted by theory for moderate input powers.

CONCLUSIONS

In conclusion, a simple and accurate technique for the characterisation of

narrowband optical filters, based on optical heterodyne detection of a frequency-swept

source, has been demonstrated. This approach allows an easy identification of the

(relative) sweep frequency by the RF spectrum analyser, which results in the precise

124
I

evaluation of the filter transmission shape. Both passive (Fabry-Perot) and active (DFB

lasers biased below threshold) filters have been characterized.

AKNOWLEDGEMENTS

This work has been partially supported by SIP and Italtel.

REFERENCES

[1] H. Toba, K. Oda, K. Nakanishi, N. Shibata, K. Nosu, N. Takato, M. Fukuda, "100-

channel optical FDM transmission/distribution at 622 Mbit/s over 50 km", Proc. OFC90,

paper PDl, 1990

[2] G. Magari, H. Kawaguchi, K. Oe, Y. Nakano, M. Fukuda, "Optical signal selection with

a constant gain and gain bandwidth by a multielectrode distributed feedback laser

amplifier", Appl. Phys. Lett., Vol. 51, No 24, pp. 51-52, 1987

[3] J. Charil, A. Ougazzaden, J.C. Bouley, H. Nakajima, A. Gloukhian, J. Landreau, A.

Mircea, "Sub-MHz spectral linewidth in 1.5 [im strained quantum well DFB BRS LDs",

Proc. 17th ECOC - 8th lOOC, pp. 641-644, 9-12 sept. 1991

[4] M. Puleo, P. Gambini, E. Vezzoni, "Characterisation of laser diode intensity noise at

microwave frequencies with high sensitivity", SPIE Vol. 1175, Coherent Lightwave

Communications, pp.24-32, 1989

Temperature

controller

Current

generator

~ , Polarisation t^oiarisation contr. _ Optical

. OOP

DFB #1

Ramp

generator

Current

generator

Temperature

controller

Optical

isolator

Filter

under test

•^~^3 dB

coupler

PIN

RF spectrum

analyser

DFB #2

Fig. 1: Experimental set-up for the characterisation of narroband optical filters.

125
ma 18 a

LIGH5fMflgE ELEC^RICfiLi

RB 3.B8 RHz 'W 380 Ht

SPAN ii.M CRi

ST 73.33 flpc

j-RL -8.88 dBa

RB 3.88 tlHi 'UB 388 Ht ST 73.33 sec

Fig. 2: Measured transmission characteristic of Fig. 3: Measured transmission characteristic of

a fiber Fabry-Perot filter. a fiber coupled bulk-optics Fabry-Perot etalon.

•Rl ^ BH dB« HTTEH Ifi id 2.88 dB/D!U m fUB -7.8 dSi BEFEBEMCE LEVEL

^.8B dB>

START B It2 RB 3 BB flH^ 'VB 3B8 Hz

LICHJUflUE ELECTRICAL:

5T0P IB.B8 GHz

ST 53,33 aec

Fig. 4: Measured transmission characteristic of a DFB filter/amplifier.

126
An Automated Method for Measuring

The Radius of Curvature of An Optical Fiber

A. L. Ingles

AT&T Network Systems

Norcross, GA

INTRODUCTION

An uncoated, unconstrained length of optical fiber can have sufficient

curvature to cause alignment problems in automated mass fusion

splicing equipment. The mass splicing techniques, which require the

simultaneous alignment of as many as 24 fiber ends (12 splices), can

tolerate limited offset between fiber ends. This paper describes a

system for use in factory production which quickly and accurately

measures the radius of curvature of a fiber, from which the maximum

offset at any length can be calculated.

DESCRIPTION OF THE METHOD

Typically, a mass fusion splicer has an unconstrained distance of

about 3 mm and can tolerate a fiber end which is offset about 3.5

microns from where a perfectly straight fiber would be. By accurately

determining the radius of curvature of the fiber, we can insure this

condition is met. The technique is to measure the fiber's offset from

a straight line at a point 2 0 mm from where the fiber becomes

unconstrained. The principle of the method is shown in Figure 1. By

rotating the fiber 360 degrees and viewing the fiber transverse to the

axis of rotation, the maximum offset y is obtained. The radius of

curvature of the fiber (R) is then

R=(x2+y2)/(2*y). (1)

DESCRIPTION OF THE HARDWARE IMPLEMENTATION

The system consists of a computer, a CCD camera, a light source, a

stepping motor, and a fixture to hold the sample. A diagram of the

system is shown in Figure 2. A fixed ceramic ferrule with an inside

diameter of 127 microns is used to hold the fiber during rotation.

The fiber sample, 6 inches long with 4 inches of coating mechanically

removed, is inserted into the end of the ferrule. The sample is then

rotated one full revolution stopping every 9 degrees to analyze a

backlighted transverse image of the uncoated fiber at a distance,

x=20mm, from the point where the ferrule allows the fiber to bend

freely

.

Precision fiber centers obtained from the analysis represent a

series of y values. The set of fiber centers is analyzed in two ways.

127
First a simple calculation of one half the separation of the highest

and lowest center positions is the offset, y, in equation 1 above.

Also, y is obtained by fitting a sine wave to the centers. Very

little difference has been observed between the two methods.

The computer controls the stepping motor and analyzes the video image.

A video frame memory card is used to capture the images from the

camera. Sample preparation and insertion takes about 20 seconds while

the rotating and analysis take 10 seconds.

ACCURACY AND REPEATABILITY

Ideally a measurement system should have accuracy and repeatability a

factor of 20 smaller than the the limit or range of interest. In this

case, we want to insure that the offset is less than 3.5 microns at 3

mm, so we need accuracy and repeatablility to be less than +/- .175

microns at 3 mm.

The magnification of the video imaging system and the distance from

the end of the ferrule to the point of measurement are the two

parameters which must be accurately set and maintained to insure an

accurate measurement. Methods have been developed to insure that both

of these requirements are met.

To determine the magnification of the system a stage micrometer is

scanned and analyzed with a computer program. The error associated

with this determination of the magnification is +/- .16 %, so that an

offset of y = 10 microns would have an uncertainty of +/- .016

microns. This equates to an uncertainty at 3 mm of +/- .00036

microns

To set the measurement distance, a special calibration rod has been

fabricated. The rod, which has a small transverse hole in it, is

inserted into the fixture at the opposite end from the ferrule. The

end of the rod touches the end of the ferrule and the hole is located

20 +/- .01 mm from the end of the rod. The fixture is adjusted so

that the hole is in the center of the video image. This is accurate

to better than +/- 10 microns, which can contribute +/- .040 microns

maximum error in the offset at 20 mm. At a distance of 3 mm from the

end of the ferrule this equates to +/- .0009 microns, which is

insignificant . Adding these two sources of error we get a total +/-

.0013 microns of uncertainty. This is well within our desired limits.

An experiment was performed to determine the measurement

repeatability. Twenty five samples were measured on the test system

and then retested. Figure 3 shows the data in a control chart format.

128
From these results the one sigma error of measurement is calculated to

be +/- 1.28 microns offset for a 20 mm measurement distance. Since the

y2 term in equation 1 is much smaller than , the error at 3 mm is

error = ( 1.28 ) * ( 32 / 202 )

= .03 microns

Thus, in total, the three sigma uncertainty due to accuracy and

repeatability is 3 *

J"( .001^ + .03^ ) = +/- .1 microns.

SUMMARY

A system has been developed and implemented in production which

measures fiber curvature with accuracy and repeatability sufficient to

insure the desired product performance. The technique is simple and

measurement time is about 30 seconds including sample preparation.

ACKNOWLEDGMENTS

I would like to acknowledge S. A. Jacobs for providing the sine wave

fitting algorithm, and S. L. Pelts, J. M. Palmquist and C. L. Bice for

helpful consultations.

Figure 1 . Measurement Method

129
1 0x objective

/

ferrule

fiber clamp

fiber under test

white light source

Figure 2. System Block Diagram

130
Recent Progress in Polarization Measurement Techniques

Brian L. Heffner

Hewlett-Packard Laboratories, 3500 Deer Creek Road 26M, Palo Alto, CA 94303-0867

Introduction

Attempting to measure polarization through single-mode fiber might at first seem foolish: Aside from the problem of getting the light of interest into a single-mode fiber, any

real fiber will significantly transform the state of polarization (SOP) of the light passing

through it, and that transformation will change as the fiber is flexed. In spite of these constraints, a fiber-based polarimeter has proven extremely useful for a wide variety of measurements. In many applications the light of interest is only available through a fiber, and

many measurements are insensitive to the constant, unitary polarization transformation presented by a stationary fiber. When it is necessary to measure a SOP in an absolute coordinate system, a linear polarizer can be used to establish a known reference frame on the laboratory bench-top, in effect calibrating out the effects of the fiber to the polarimeter.

The capabilities of a polarimeter are dramatically augmented by the addition of a

simple polarization synthesizer. The synthesizer allows stimulus-response measurement of

devices and networks, allowing calculation of Jones matrices and complete characterization of linear, time-invariant networks. This capability, along with automated operation and

fiber connectors, can make polarization measurements comparable to microwave measurements in their levels of convenience, flexibility, speed and sophistication of analysis.

Measurement of Stokes vectors

Accuracy, convenience and measurement speed are of primary importance to the usefulness of Stokes vector measurements, especially when a large number of Stokes vectors

are required, for example, to characterize polarization dispersion as a function of wavelength. Measurement of a SOP is traditionally performed by rotating a waveplate and a polarizer to four pairs of angular orientations to obtain four optical intensities transmitted

through the polarizer. These intensities form a vector which is linearly related to the SOP.

Alternatively, by splitting the optical beam into four beams (Fig. 1), the same four optical

powers can be measured in parallel through a suitable set of fixed polarizing filters, allowing measurement of the complete SOP at speeds Hmited only by electronic data acquisition.

Figure 1 A four-detector

polarimeter. A divided concave

mirror images the output of a single-mode fiber onto four

photodiodes through a set a fixed

polarizing filters.

131
Measurement of Jones matrices

R. C. Jones gave an explicit algorithm for experimentally determining the forward

transmission Jones matrix T of an unknown linear, time-invariant optical device [1]. The

restriction of time invariance applies only to the polarization transformation caused by the

device, and does not include the absolute optical phase delay, so this technique can be used

to characterize fiber networks even when the phase delay through the fiber is drifting during the measurement.

Any Jones vector v can be completely specified by a magnitude, an absolute phase,

and a unit vector v which locates the SOP on the Poincare sphere. To measure the Jones

matrix of a device, a stimulus optical field of linear polarization parallel to the x axis is first

A

generated, and the resulting response unit vector h is measured through the device. Similarly, stimulus fields of linear polarization parallel to the y axis, and parallel to the bisector

A A

of the angle between the positive x and y axes result in response unit vectors v and q , respectively. Three complex ratios independent of the intensities of the three stimulus fields

A A A A A yv, can now be formed from the x and y components of h , v , and q : = h^ / h^ , k2 = v^ /

Vy , and = q^ / ^ . A fourth ratio k^ = (k^ - k2)/(k^ - k^) is then found. To within a

complex constant /3, the transmission Jones matrix T is then given [1] by

T = ^ (1)

Measurement of polarization-dependence of the loss through a device

The polarization dependence of the gain or loss through a DUT is often measured by

monitoring the output power of the DUT with a polarization-independent detector or optical power meter while the input SOP is deliberately varied over all possible values. This

technique can try the patience of the measurement technician and suffers two fundamental

limitations. One is the difficulty of arbitrary SOP generation, as the control inputs to a polarization transformer do not relate to the output SOP in a simple way. Moreover, the output intensity of a SOP synthesizer is usually a weak function of the control inputs, and this

variability in intensity translates directly into errors in measurement of the polarization dependence of transmission. A second, more serious disadvantage is the necessity of a

search algorithm. At constant power, the state of a purely polarized source (i.e. with unit

degree of polarization) has two degrees of freedom, so the synthesized SOP must be varied

over a two-dimensional space while searching for the global minimum and maximum transmission. The search algorithm must not mistakenly identify local extrema of transmission

as the desired global values. Other sources of error include polarization sensitivity of the

power meter, unstable source amplitude or unstable device transmission, and, unless a separate polarizer is used, partial polarization of the source. These problems yield to an efficient solution once it is recognized that the global minimum and maximum intensity transmission coefficients T^j^^ and T^^^ through a DUT

132
SOLENOID-MOUNTED POLARIZERS

FOR INSERTION INTO OPEN BEAM

r

OPTICAL

SOURCE

[A] DEVICE

UNDER

TEST

POLARIMETER

D

0

Figure 2 Apparatus used for singular value

calculation of polarization-dependent

transmission. L: lens; P: linear polarizer;

PT: polarization transformer.

m

2. 1.1 c

UJ 1

o I- z ^

UJ X

UJ ^ og

If

a. O

ISOLATOR A

1.0-

0.2-

0.1

0.0

ISOLATOR B

10 SAMPLE

NUMBER

Figure 3 Measurements of two pigtaUed optical isolators. Shaded areas indicate the manually measured values including uncertainty of

±0.03 dB; plotted points are singular value measurements for different settings of PTl and PT2.

over all SOPs can be determined without resorting to a search over polarization space.

The polarization responses to only three input SOPs completely determine the polarization

sensitivity of a linear, time-invariant optical device. Although not generally recognized, it can be shown [2] that T • and T are given by the squares of the singular values of the mm max o j o

Jones matrix associated with the DUT.

Suppose the only access to the device under test is through fiber pigtails (Fig. 2).

While direct measurement of the device matrix A is then impossible, the matrix B = VAU,

where U and V are unitary Jones matrices representing the pigtails, is easily determined to

within a complex scalar constant /3 using (1) and the method previously described. Be-

•cause its loss is independent of polarization, each pigtail can be represented by a unitary

Jones matrix times a scalar. Having measured C = ^B, we can compute the singular values s-(C) = |i8 I Sj(B) . Finally, using the fact that singular values are invariant under unitary transformation, we obtain an expression for the polarization dependence of

transmission in terms of the singular v^alues of the measured Jones matrix C :

T„,i„ s2(A) 5^2(8) q^o

T„,ax S2'(A) 52(B) (C)

Experimental results comparing the manual method to the singular value method for

measurement of the polarization sensitivity of two optical isolators with single-mode fiber

pigtails are shown in Fig. 3. The shaded areas indicate the manually measured polarization

sensitivity of the two test devices using the apparatus of Fig. 1, including a measurement

uncertainty of ±0.03 dB caused by residual polarization dependence of the power meter

and slight drifts in the intensity of the polarized source. The singular value measurement

133
was performed ten times for each device, with both the input and output polarization transformers shown in Fig. 2 randomly reset for each of the ten measurements to present a different unitary polarization transformation before and after the device each time a Jones

matrix was measured. The grouping of the plotted measurement points experimentally verifies the invariance of the singular value ratio over a variety of unitary input and output

transformations, demonstrating that the polarization transformations caused by the pigtails

do not impair measurement accuracy.

Measurement of polarization mode dispersion

Thorough characterization of the optical components intended for high-speed transmission links requires accurate, repeatable measurement of polarization mode dispersion

(PMD). PMD is completely characterized by a wavelength-dependent, three-dimensional

polarization dispersion vector, or equivalently by the specification of a pair of principal

states of polarization (PSP) and a differential group delay At as a function of wavelength.

A general device or network has associated with it a pair of input principal

states x(a)) which, as the SOP is held constant while the optical frequency w is changed a

small amount, result in a pair of output principal states whose unit vectors are invariant to

first order over w. For a general transmission Jones matrix T{o)), we can express an output principal state as a magnitude cr(w) and absolute phase 0(w) times a unit vector y(w)

which specifies the SOP of the output PSP:

>S i0(w) A /-,N y(a)) = T(a)) x(a,) = a(w) e y(w) . (3)

A

a(w) and 0(w) may vary with w, but y(cj) is frequency-invariant to first order by definition

of the output PSP. If the network is not perfectly polarizing, its transmission matrix T is

A _1

nonsingular and the input can be expressed in terms of the output as x = T y . Using

primes to denote differentiation with respect to w, differentiation of (3) leads to the eigenvalue relation

10

o c y = I y = 0 , (4)

where y' is set to zero because it locates the output PSP . The imaginary parts of the eigenvalues of the matrix product T'T'-*^ are the group delays associated with the PSPs, and the

differential group delay At = r g^i-Tg 2 which leads to PMD is given by the difference of

the imaginary parts of the two eigenvalues.

Measurement of T' and T, including measurement of the absolute phase, would allow

direct calculation of the two group delays and At , but in practice two restrictions are imposed by the Jones matrix measurement technique previously described. Instead of measuring T' directly, we must approximate it as T' ~ [T(a) + Aa)) - T(w)]/A£o for a finite Aw.

If the frequency interval Aw is small enough so that each output PSP suffers nearly the

same loss at w and w + Aw, then a 'Aw / a ^ 0 and (4) can be rewritten as

134
T(w + Aa))T'^(w) - (1 + iTgAo))! y = 0 (5)

The second restriction arises from the fact that T(ci)+Aw)T'-^(w), and therefore its associated eigenvalues p i and P2, can be determined only to within a complex constant, preventing

determination of the two group delays individually. When TgAw is small we can approximate the eigenvalues as pj^ = 1+ iTg i^Aw « exp( iTg ^Aw ), and the differential group delay At can be expressed as [3]

Arg(Pi/P2)

At 'g,i - V Acj

(6)

where p^ and P2 are the eigenvalues of T(w+Aco)T"^(a)) and Arg denotes the argument

function. In fact, the requirement that 7"g k Aw be small can be substantially relaxed: Since

all the fundamental measurements are completely insensitive to absolute optical phase, only

the quantity At Aw need be small enough to allow an exponential approximation of the

eigenvalues. The exponential approximation is exact when the loss of the DUT is independent of polarization, in which case only the condition At Au < ti need be satisfied in order

to avoid the ambiguities of the multiple-valued argument function

The experimental apparatus is shown in Fig. 4. A tunable laser source was directed

through the polarization synthesizer and through the DUT to the polarimeter through short

lengths of single-mode fiber which are assumed to introduce negligible polarization dispersion. At any given optical frequency , the polarization synthesizer generated three

stimulus SOPs and the polarimeter measured the corresponding three response SOPs, resulting in a Jones matrix T(w^) given by (1). The process is repeated for each w in a sequence from n = l to n = N. N-1 values of At at frequencies {o)^+cd^_^)/2 are then

calculated from the eigenvalues of successive matrk products T(£Oj^)T"'^(Wj^^) using (6).

The eigenvectors of T(a)^)T"-^(w^^) locate the output PSP as a function of frequency.

SOLENOID-MOUNTED POLARIZERS

FOR INSERTION INTO OPEN BEAM

D

0

TUNABLE LASER

SOURCE

DEVICE OR

NETWORK

UNDER TEST

' L

|c

' L

I.

POLARIMETER

7

1

01

Ol

3

3

Figure 4 Apparatus used for measurement

of polarization mode dispersion by Jones

matrix eigenanalysis. L: lens; P: linear

polarizer; C: crystal sample; OI: optical

isolator; F: 2-km single-mode fiber.

135
Figure 5 Differential delay of a 2-km spool of

fiber measured at two different temperatures.

Figure 6 Differential delay measured using a

similarity technique. A: isolator; B: isolator and

2-km fiber; C: fiber measured through isolator

using similarity technique; D: fiber alone.

Summary

A fiber-based, automated approach to polarimetry can greatly reduce the tedium of

polarization measurement while offering speed, ruggedness, accuracy and convenience.

Addition of a simple polarization synthesizer allows complete stimulus-response measurement of the polarization-transforming properties of linear devices and networks, affording

powerful characterization and analysis capabilities. Measurement of a Jones matrix depends on no assumptions about the device or network under test except that it must be linear and its polarization transformation must be constant over a period of several seconds.

This system makes possible two new analysis techniques which offer unique measurement advantages. Singular value calculations provide a deterministic, analytically complete means to characterize the variation in transmission of devices such as isolators or directional couplers over all SOPs. By eliminating the search over polarization space it allows a completely specified test suitable for comparisons and standards. Jones matrix

eigenanalysis offers measurement of both differential group delay and orientation of the

PSP as a function of optical frequency, providing a complete measurement of polarization

mode dispersion. This technique affords temporal accuracy of a few percent down to a

limit of several femtoseconds, even for networks exhibiting polarization-dependent loss.

References

[1] R. C. Jones, "A new calculus for the treatment of optical systems. VI. Experimental

determination of the matrix,"/. Optical Soc. Am., 37, pp. 110 - 112, 1947.

[2] B. L. Heffner, "Deterministic, analytically complete measurement of polarizationdependent transmission through optical devices," IEEE Photonics Technol. Lett., PTL-4,

1992.

[3]B. L. Heffner, "Automated measurement of polarization mode dispersion using Jones

matrix eigenanalysis," submitted to IEEE Photonics Technol. Lett.

136
I

EXTINCTION RATIO MEASUREMENTS OF POLARISATION MAINTAINING FIBRE

Y. GU, L. J.POYNTZ-WRIGHT

York Fibers Ltd . , York House , School Lane

,

Chandler's Ford, Hampshire SOS 3DG, U.K.

INTRODUCTION

Polarisation maintaining (or high birefringence) fibre is an

important component in many optical communication and sensor

applications, including fibre optic gyroscope systems^^^. However,

agreement has yet to be reached as to how fibre performance

should be specified. On exciting only one of the two orthogonal,

linearly polarised modes, the fibre is capable of sustaining the

transmission of stable, linearly polarised light over long

lengths. How well the fibre performs depends not only on the

fibre birefringence but also other factors such as coating

properties and the severity of external perturbations.

While the beat length characterises the intrinsic birefringence

of the fibre, in practical terms a more useful parameter is the

overall polarisation holding ability of the fibre. The degree of

cross-talk between the two polarised modes is commonly specified

by the h-parameter^^' , a measure of the average rate at which

power in the excited polarisation mode is coupled to the

orthogonal mode. Alternatively, the ratio of the powers in the

two modes can be expressed in dB, known as the extinction ratio.

However, the validity of quoting extinction ratio or h-parameter

measurements to describe commercially available fibres has been

questioned due to the large dependence on external stresses.

Failure to pay attention to this point results in an h-parameter

specification which is not a test of the quality of the fibre,

but of the winding conditions.

Here we present results of an investigation into the

repeatability of extinction ratio measurements, and the effects

of winding conditions.

EXPERIMENT

A schematic of the measurement station is shown in Figure 1. A

Glan-Thompson polariser is used at the input, and a Woolaston

beam splitter at the output, both of which have extinction ratios

of greater than 50 dB. The source used is a super luminescent

137
diode, with a central wavelength of 820nm and bandwidth of 35nm.

Using this arrangement, extinction ratios in fibre can be

measured up to 39 dB (although figures of up to 60dB can be

obtained when measuring fibre polarisers).

A YORK FK-11 cleaver was used in all the experiments in order to

ensure the guality and repeatability of the fibre ends. The

alignment technique is the standard iterative approach of

searching for the best cross-talk ratio, and rotating the

polariser through increasingly small steps (the use of the beam

splitter at the output simplifies this considerably). With this

method the angular position of the input axes can be defined with

an accuracy of better than 10 minutes.

To assess the repeatability of the system, a 500m length of

HB800, wound on a 164mm drum under constant tension, was measured

a number of times without altering the winding conditions. The

results of a set of 5 measurements for each eigenaxis excitation

had a mean value of 27.3 dB, with a standard deviation of 0.34

dB. The system could be improved by replacing the polariser at

the input which deviates the beam slightly, resulting in varying

launch conditions between measurements. This is believed to be

the main cause of errors in our results.

The same length of fibre was then rewound onto different diameter

mandrels, under varying tensions. The mandrels ranged from 60mm

to 300mm diameter, and winding tensions from 20g to 60g. A set

of measurements for each eigenaxis was taken for each of the

winding conditions. The results are summarised in Table 1.

DISCUSSION

The experimental results presented in Table 1 show that the

standard deviation for the majority of measurements did not

exceed 0.44 dB. This is only slightly higher than the system

repeatability, indicating that the winding conditions are

constant and that coils can be produced repeatably.

It can also be seen from Table 1 that the extinction ratio values

vary significantly with winding conditions. As expected, the

higher the tension, the greater the cross-talk. There is also a

large dependence on coil diameter, the extinction ratio of the

68mm diameter coils being on average 11 dB lower than that of the

320mm diameter coils.

It is also observed that the effects of winding tension are not

so significant for larger diameters. In fact, the values obtained

for the 3 20mm diameter coil are similar to the free standing

value of 35.5 dB. Thus these measurements on the larger drum are

a better indication of the inherent fibre quality than tighter

coils.

138
I

I

It should be noted that all these measurements were undertaken

using mandrels of the same material. It is expected that

different results would be obtained for, say, an aluminium

mandrel compared to a similar one covered in foam. Therefore,

although the results are repeatable, they can only be used to

compare fibres wound under exactly the same conditions. It would

be extremely difficult to extrapolate any results to determine

fibre performance under any other conditions, and so can in no

way be used to guarantee performance of a fibre for a particular

application.

CONCLUSIONS

The results presented here show that it is possible to measure

the polarisation holding ability of a fibre repeatably. However,

great care has to be taken when quoting h-parameter or extinction

ratio measurements to describe commercially available fibres,

since they are only appropriate for the conditions under which

the measurement was conducted. From the end-users point of view

it is desirable for fibre manufacturers to standardise on

measurement conditions if any comparisons are to be made.

REFERENCES

[1] R.A.Bergh, H.C.Lefevre, and H.J.Shaw, Journal of Lightwave

Tech., tT-2, 1984, pp91-107.

[2] S.C.Rashleigh, Journal of Lightwave Tech., 1, 1983, pp3l2-

331.

139
Martock

(3-axl8 + rotation) Objective Polariaer

Light

Source

(3-axis * rotation) Beam-Splitter

Detectors

Figure 1. Schematic of Measurement Arrangement

Coil Winding Conditions Ext. Ratio

Spool Dia

(mm)

Tension

(g)

Mean

(dB)

Std Dev

(dB)

320 56 34.0 0.67

22 34.7 0.23

164 56 27.8 0.44

32 30.6 0.34

22 31.3 0.39

68 56 21.4 0.4 0

32 25.3 0.4 0

22 26.6 0.41

Table 1. Experimental Results

140
ACCURATE MEASUREMENT OF POLARIZATION DEPENDENT LOSS

OVER WAVELENGTH OF SINGLE MODE OPTICAL FIBER COMPONENTS

L. F. STOKES

HEWLETT-PACKARD COMPANY

ROHNERT PARK, CALIFORNIA

Introduction

Polarization dependent loss (PDL) is defined as the maximum change in insertion loss,

expressed in dB units, of an optical fiber component as the input polarization is varied over all

possible states. Accurate PDL measurements have become important recently. A long haul fiber

transmission system, for example, may contain many fiber components between transmitter and

receiver. Each component may have a very small polarization dependent loss. However,

cumulative effects could produce a large fluctuation in received power as polarization randomly

varies with the environment. Consequently, it is desired to measure the very low (0.1 dB or

lower) polarization dependent loss of each component.

Traditionally, a polarized laser has been used as a source to measure PDL. The laser

polarization is varied with a polarization state controller located before the component under test,

and power exiting the component is measured. The ratio (difference in dB units) between the

maximum and minimum power is the polarization dependent loss. This technique can be used

with a wavelength tunable laser to measure PDL over wavelength, although it is quite time

consuming if many wavelength points are desired. The technique described in this paper uses a white light source and an optical spectrum analyzer (OSA) to quickly measure PDL over wide

wavelength ranges (greater than that possible with a single tunable laser) with great precision.

Optical Source

White light sources coupled to single mode fiber cannot begin to match the intensity of lasers

coupled to single mode fiber. To illustrate this, consider a black body radiator (such as a hot

lamp filament) imaged to the end of an optical fiber. The traditional Planck black body radiation

law can be modified [1] to show that the maximum optical power in the fiber per spatial mode

per unit optical frequency, dP/df, is

dP/df = ehf/[exp(hf/kT) - 1]

where e is the material emissivity (<1), h is Planck's constant, f is the optical frequency, k is

Boltzmann's constant, and T is the black body temperature in deg K. Take f = 231 THz

(1300 nm wavelength), df = 178 GHz (1 nm bandwidth), e = 0.3 for a tungsten filament, and

T = 3300 deg K. With the above formula, we calculate a power per spatial mode of 0.294 nW

for the 1 nm bandwidth. Equivalently, at 1300 nm, we have a power spectral density of

141
0.294 nW/nm, or -65.3 dBm/nm, in single mode fiber. It should be noted that the power

spectral density calculated here is for one polarization only, as if a lossless polarizer were

inserted between the lamp and the optical fiber. In general, single mode fiber supports two

polarization modes, and the power spectral density will be 3 dB greater (-62.3 dBm/nm) without

a polarizer present.

Optical Spectrum Analyzer

The very small amount of power available from a white light source is not a deterrent to

accurate PDL measurements over wavelength because of the new generation of high sensitivity

optical spectrum analyzers. For example, the HP 71451 A has a typical sensitivity (noise level)

of -90 dBm at 1300 nm, independent of resolution bandwidth. A PDL measurement with 1 nm

resolution would have about -65 dBm signal and 25 dB dynamic range available. The noise is

about 0.3% of the signal, and represents a PDL measurement uncertainty of 0.014 dB. A PDL

measurement with 10 nm resolution would have 10 times the signal (-55 dBm) and 35 dB

dynamic range.

Measurement Technique

The simplest PDL measurement using a white light source and an optical spectrum analyzer

would be configured as follows: An unpolarized single mode fiber white light source would be

followed by a single mode fiber polarizer, a single mode fiber polarization state controller, the

component under test, and finally the optical spectrum analyzer. The OSA sweeps over the

wavelength range of interest using the desired resolution bandwidth. Simultaneous MAX HOLD

and MIN HOLD traces are selected, and the polarization state controller is adjusted, over many

OSA sweeps, until no larger MAX or smaller MIN can be found over wavelength. Finally,

subtracting (in dB units) the MIN trace from the MAX trace gives the desired PDL over

wavelength. Note that no source flatness calibration is necessary, as it is common to both traces.

A major problem with the above measurement technique is that the PDL of both the

component under test and the OSA has been measured. Typically, OSA's have a polarization

sensitivity of at least 0.5 dB (due to polarization dependence of diffraction gratings). This

sensitivity limits the PDL sensitivity and accuracy to 0.5 dB. Indeed, replacing the component

under test with a short section of single mode fiber, and performing the above measurement, will

yield the polarization sensitivity over wavelength of the OSA.

To avoid this problem, the OSA monochromator is placed directly after the unpolarized

white light source, as shown in Figure 1 . The fiberized output of the OSA monochromator, now

partially polarized due to the polarization sensitivity of the grating, passes through the fiber

polarizer. As long as this piece of fiber is relatively short (eg., 1 meter) and is not disturbed

during the measurement (so as not to change whatever state of polarization is incident on the

polarizer), the polarization dependence of the OSA will be common to both the MAX and MIN

traces. Trace subtraction will eliminate this dependence leaving just the PDL of the component

under test.

One minor drawback to this technique is that no OSAs are presently available with single

142
mode fiber at the monochromator output. Consequently, multimode fiber is used between the

white light source and the monochromator input, and between the monochromator output and the

polarizer. The output of the polarizer is single mode fiber. The same power of -65 dBm/nm

(minus the monochromator insertion loss) is available as a signal, and the same dynamic r<mge

exists. However, some speckle will be present, due to the multimode/single mode fiber

interface, which gives a variation of power over wavelength. Again, this variation is common

to both MAX and MIN traces and is eliminated with trace subtraction.

Measurement Results

Figures 2 and 3 show the precision of this PDL technique. A 3 meter section of single mode

fiber was chosen as the component under test, and it should have no PDL. Figure 2 shows the

MAX and MIN traces virtually overlapping. About 10 monochromator sweeps were taken, while

the polarization state controller was adjusted continuously looking for MAX and MIN response.

Total measurement time was 400 seconds. Note the 1 dB power variation over wavelength due

to speckle described above. Figure 3 shows the subtraction of the two traces of Figure 2. The

lowest measurable PDL is limited by the residual of 0.05 dB in this case. The residual results

from the speckle pattern variation in time, the insertion loss variation of the polarization state

controller, and the polarization dependence of the photodetector.

Figures 4 and 5 show PDL of a 1310 nm "polarization independent" isolator. The MAX and

MIN traces of Figure 4 took about 10 sweeps to generate, again representing 400 seconds

measurement time. Figure 5, the trace subtraction of Figure 4, shows a PDL of 0.27 to 0.39 dB

over the 100 nm span. Again, note how the speckle effect is common to both traces of Figure 4,

and subtracts out quite well.

Summary

A simple, accurate method of measuring polarization dependent loss over wavelength has

been described. A low cost white light source and a high sensitivity optical spectrum analyzer

allow quick (a few minutes) determination of very low polarization dependent loss over

wavelength to high accuracy.

Acknowledgements

The author appreciates helpful discussions and assistance from J. P. Hamilton-Gahart and

J. R. Stimple.

References

1. W. R. Trutna, Jr., Hewlett-Packard Laboratories, personal communication.

143
WHITE LIGHT

SOURCE

HP 71451A OPTICAL SPECTRUM ANALYZER

Light

Input

Multimode

Fiber

Single Mode

Fiber

Monochromator

Double Pass Transfer Switch

Optional Current Source

Ptiotodetector

Input

Monochromator Output

Control

Transimpedance

Amplifier MSIB HPHB

Transimpedance D'splay

Input (Rear Panel)

Multimode

Fiber

COMPONENT

UNDER

TEST

n

Single Mode

Fiber

POLARIZATION

STATE

CONTROLLER Single

n

Mode

Fiber

< POLARIZER

Fig. 1. PDL measurement configuration.

RL -58.52 dBn tIKR tl Wl 1359.9 na

•SENS

jB.EB

-90 dB/Dl

dBin S rn/RS 1 58. H' dBn —r

JCENTER 1310.0 nm •R6 10 nn UB E Hz

SPRN 100.0 nm •ST>39 sec

Fig. 2. MAX and MIN traces of fiber.

RL -G0.H2 dBn flKR «1 Wl 1310.0 na

CENTER 1310.0 nn

>RB IB nn UB S Hz

SPRN 100.0 nn

•ST>39 sec.

Fig. 4. MAX and MIN traces of isolator.

•RL 0.00 dBn NKR «1 mi 1359.9 nn

•SENS 0.05

-90

dB/Dl

dBn S

U

th/rs > 0.0] dBn

tixmm

•RB 10 nm UB 2 Hz S •ST>39 sec

Fig. 3. PDL sensitivity.

RL 8.00 dBn UKR Hi Wi 1310.0 nn

•SENS

0.10

-90 dB/D

dBn S Vth/rs 0.3e dBn

I1RRK :r 1310 0.30

0 nn

dBn

1

•RB 10 nn UB 2 Hz S •ST)39 sec

Fig. 5. PDL of isolator.

144
POLARIZATION MODE DISPERSION MEASUREMENTS

IN OPTICAL FIBERS

Yoshinori NAMIHIRA and Jun MAEDA (*)

KDD R&D Laboratories,

2-1-15, Ohara, Kamifukuoka, Saitama, 356 Japan

I. INTRODUCTION

In the single-mode optical fibers (SMFs), there exists a modal birefringence due to geometrical

core deformation and external stress which give rise to polarization mode dispersion (PMD). The PMD

may set a limit to the use of ultra high bit rate long haul optical fiber amplifier systems, especially when

the chromatic dispersion effects become very small by tuning the signal wavelength near the zerochromatic dispersion wavelength. So far, the PMD has been measured by mainly two different

procedures. One is the time domain measurement and the other is the frequency domain measurement.

The former method is based on the interferometric [l]-[3] and pulse methods [3], [4]. The latter method is

based on the evolution of the states of polarization (SOP) as a function of the frequency (wavelength) and

measures the differential group delay (or PMD) x between the principal SOP [3]-[6].

This paper presents die comparison of various PMD measurement technologies ; (1) Interferometric,

(2) Optical pulse, (3) Fixed analyzer, (4) Poincar^ sphere, and (5) SOP- PMD measurement methods,

respectively.

II. TIME DOMAIN PMD MEASUREMENTS

A. Interferometric PMD Measurement Method

Fig.l shows the experimental set-up for the highly accurate interferometric PMD measurement

method [l]-[3]. The principle of the measurement is based on the Michelson interferometer [l]-[3]. The

interferenced output optical powers (visibility curves) can be obtained if the group-delay difference

between the two orthogonal polarized waves is within the coherence time of a light source. The center

wavelength of semiconductor laser amplifier (SLA), the spectral line width and single-mode fiber pigtail

output power were -1.548 |im, -80 nm (~ 10 THz) and - -15 dBm, respectively. The measurement

accuracy is depending on the mechanical accuracy (- 1.0 p,m) of optical delay line, and the PMD

measurement accuracy is - 0.003 ps [2] . The PMD x is given by [2]

X=2d/c (1)

where, 2d and c are the optical path length (= 2d ; two times of delay line moving length d) and the light

velocity in free space, respectively.

The example of the interferometric PMD measurements of the polarization independent optical

isolator and the 1.3 [im optimized SMF (L^IO km) are shown in Figs.2-(a) and (b), respectively.

In Fig.2, it is confirmed that a small PMD values of ~ 0.012 ps of die polarization independent

(*) : University of Electro-Communications, Chofu, Tokyo, 182 Japan

145
optical isolator can be measured successfully by using the interferometric PMD measurement method.

Also, it is found that there is great difference between the two visibility curves of Figs. 2-(a) and (b).

Many interference points in the visibility pattern of the SMF are interpreted as a results of the random

polarization mode couplings along the fiber length. The other results are summarized in Table 1.

B. Optical Pulse PMD Method

Fig. 3 represents the experimental set-up for the optical pulse PMD measurement method. In the

PMD method, a pulsed DFB-LD and an optical sampling oscilloscope were used. Incident polarization

with rotation angle of 45° was launched into the fiber under test. The output optical pulse was split into

two pulses (fast and slow waves). The interval of the two optical pulses is equivalent to the averaged

PMD value. Fig.4 shows an example of PMD measurement result of the polarization maintaining fiber

(PMF 2, L= 200 m). The PMD is found to be ~ 44 ps (~ 0.22 ps/m) as shown in Table 1.

The measurement accuracy depends on the optical pulse width, and its value is ~15 ps when the pulsed

DFB-LD was used in the experiment.

III. FREQUENCY DOMAIN PMD MEASUREMENTS

A. Fixed Analyzer PMD Measurement Method using Optical Spectrum Analyzer

Fig. 5 shows the experimental set-up for the fixed analyzer PMD measurement method using optical

spectrum analyzer (OSA). The PMD measurement results of the PMF4 (L^ 6.4 m) and the SMF (L =

10 km) are shown in Fig.6-(a) and (b), respectively. The interval oi the peak to peak powers is

equivalent to the phase difference of In. The PMD T is expressed as T = N( 1 /Af) = N(Xi Xn/cAX) (2)

where, N, Af, and AX represent the numbers from peak at Xl (fl) to peak at Xn (fn), the frequency

difference ( fn - fl ) and the wavelength difference (Xn-Xl ), respectively. The PMF4 (Fig.6-a) shows

almost periodic curve, meanwhile the SMF (Fig.6-b) indicates somewhat irregular periods. The other

results are summarized in Table 1.

B. PMD Measurement Methods using Stokes Analyzer

In the PMD measurement method using a real time Stokes analyzer, there are two different PMD

measurement methods such as conventional Poincare' sphere method [3],[5]-[8], and new SOP method.

B.l Poincare Sphere PMD Measurement Method

A schematic diagram of the experimental set-up for the PMD measurement using a real time Stokes

analyzer [3], [8] is shown in Fig.7. A tunable 1.55 }im external cavity semiconductor laser (ECL) with a

spectral linewidth of -65 kHz at 1.55 |im was used as a light source. The wavelength was tuned by

adjusting the angle of the external-grating automatically. It is possible to measure in 1.51 ~ 1.59 ^im

wavelength range at 0.2 nm (~ 25 GHz) wavelength steps corresponding to the longitudinal mode spacing

of the laser diode (LD). The SOP can be determined by the Stokes parameters (So, Si, S2 and S3).

So, Si, S2 and S3 relate to the total optical power, the linear polarization of 6 = 0°, the linear

polarization of 9 = 45° and the right circular polarization, respectively. In Fig.8, ( o ) and ( x ) are SOP

arcs of measured values due to PMD as a function of wavelength X. Here, Pa-a' is principal SOP [8].

The PMD T is given by [8]

146
X = (A(t)/27c)( 1/Af) = (A^/2n)(XiXn/cAX) (6)

where, A<j) is the phase difference (Stokes vector arc on the Poincare sphere). Figs. 8 -(a) and (b) show

the example of Poincard sphere representation of PMF4 (L ^6.4 m) and SMF (L^IO km). PMF4

(Fig.8-a) shows almost periodic curve, meanwhile the SMF (Fig.8-b) indicates somewhat irregular

periods. The other results are summarized in Table 1

B.2 SOP PMD Measurement Method

After the polarization fluctuation was measured by Stokes analyzer (or Rotatable analyzer) , it can be

transformed into the SOP curve as a function of wavelength (frequency); The SOP is expressed as SOP = (1 - Tl2)/(1+ Tl2) (7)

where, r[ = tan [0.5 tan -1

{ S3/ VSi^TSi^}]

Here, t] is the polarization ellipticity, S 1, S2 and S3 are Stokes parameters. In Fig.9, the peak to peak

of SOP curves is equivalent to the phase difference of k. The PMD x is given by

X= (N/2)(l/Af) = (N/2)(?ii?in/cA?i) (8)

VI. CONCLUSIONS

The comparison of the various PMD measurement methods is shown in Table 1. From Table 1,

it was found that the following results;

(A) Birefringent optical devices (PMFs etc.) ; (1) Not a great difference was remarked among all PMD

measurement methods. (2) Interferometric method was best for optical components having very small

PMD values because of highest measurement accuracy (-0.003 ps). (3) Optical pulse method was useful

for only large PMD values (> -15 ps).

(B) Optical fibers (SMF, DSF and DFF) ; (1) The time domain methods (Interferometric and Optical

pulse method) were not useful due to SOP fluctuations during measuring time. (2) The PMD values of

the Interferometric method were slightly smaller than those of the Frequency domain methods. (3) In the

Frequency domain methods, the PMD values of the Poincare' sphere method are slightly scattered than

those of the SOP method. (4) Although, PMD values of the Fixed analyzer method were slighdy larger

than those of the SOP method, these two methods (Fixed analyzer method and SOP method) were better

than the Poincare' sphere method for PMD measurements in optical fibers. Finally, we would like to

thank Dr. K. Ono, Dr. T. Yamamoto and Dr. Y. Kushiro and Mr. H. Wakabayashi of KDD R&D Labs

for their encouragement, and also thank Mr.T. Kawazawa of KDD R&D Labs, for his cooperation.

REFERENCES :

[1] Y. Namihira et al,

IEEE, Photonics Letters, Vol.1, pp.329-331, Oct.,1989.

[2] Y. Namihira et al.,

J. of Optical Communications, Vol.12, No. 1, pp.1-8, 1991.

[3] Y. Namihira et al.,

lEICE of Japan, Technical Report, OCS90-56, pp.31-38, 1990.

[4] C. D. Poole et al., : Optics Letters, Vol. 13 pp.155-157, 1988.

[5] C. D. Poole et al.,

IEEE, JLT, Vol.6, No.7, pp.1 185-1 189, 1988.

[6] N. S. Bergano et al. : IEEE, JLT, Vol.5, No.ll, pp.1618-1622, 1987.

[7] D. Andresciani et al., : Optics Lett., Vol.12, No. 10, pp.844-846, 1987.

[8] Y. Namihira et al., :: Electron . Lett., Vol.28, No.9, pp.881-883,1992.

147
SLA ^

or b-^

EDFA r

ks US urn

P2

r>ci Pi [±1Q

Fibtr Under

Test

i BS H ( )

Comer

Cube (CO

Lens

InGaAs A -PD AmpliHer

Lens

II

J_L cc

P ; rolarizer, A : Analvzfr, BS : neam Splitter Q . Quarter Wave Plale, H ; Hair Wave Plate

Micrth

Stage

High Speed

Signal Processing Sub-Svstem

Fig. 1 Experimental set-up for Interferometric

PMD measurement method

Pulsed

DFB-LD

Polarization

Controller

Fiber

Under Optical Sampling

Test Osciloscope

-4- Q H 0 m

A =1.535 )un O O O

P : Polarizer Q : Quarter Wave Plate H : Half Wave Plate

Delay Unit

Fig. 3 Experimental set-up for Optical pulse

PMD measurement method

DFD - LD

- ^ S 1.535 Itiml- L 2 200 (ml Intensity : 10 iiiw/divi Time : 100 Ijnicii.! ACCOM : SO

Fig. 4 Optical pulse PMD measurement results in PMF2 (L e 200 m)

Fiber

Under

Test

or i_oM-M

SLA J ^

Polarization Controller P „ Fiber

Coupler

Polarization Independent

Optical Isolator T = 0.012 ps

-200 -100 0 100 200

Optical Dela\ Length (^m)

(a)

SMF

L = 10 ktn

T = 0.103 ps

(0.033 ps/Vkm)

-200 100 II 100 200

Optical Dchiv Li niilh (nm)

(b)

Fi g. 2 Interferometric PMD measurement results in

(a) Polarization independent optical isolator,

(b) SMF (L^ 10 km)

0

_ -4

CQ•a

PMF4

(PANDA)

L = 6.4 m

t = 9.819 ps

(1.534 ps/m)

1550 1552 1554 1556 1558 Wavelength X. (nm)

(a)

1560

0

-10

SMF

L = 10 km

T = 0.213 ps

(0.067 ps/Vkim)

Optical

Spectrum

Analyzer

P : Polarizer A : Analyzer Q : Quarter Wave Plale H : Hair Wave Plate BPF : Band Pass Filter Optical

BPF

Fig. 5 Experimental set-up for Fixed anlyzer

PMD measurement method using Optical

spectrum analyzer (OSA)

Optical

Power

Meter

s:

u

-20 -

OC -30 -

icul L

Q. c

-40 I

-50 L 1450 1490 1530 1570 1610 1650

Wavelength X (nm)

(b)

Fig. 6 Fixed anlyzer PMD measurements in

(a) PMF 4 (L s 6.4 m), and (b) SMF

148
Real Time Stokes Analyzer

Polarization Controller P

Tunable-LD (ECU)

-ibtr —

0

Flbtr >+ Coupler

Wavelength

Meter

X

U\ 13 12 [pd] Ppd] [TdI

^- ^- El m BS BS PBS

P : Polarizer Quarter Wave Plate H: Half Wave Plate

Personal I Computer I

Fig.7 Experimental set-up for PMD measurement method using Real time

Stokes analyzer

(a) (b)

Fig. 8 Poincard sphere representation of PMD measurements in

(a) PMF 4 (L s 6.4m) and (b) SMF (L ^ 10km)

Fig. 9 State of polarization (SOP) representation of PMD measurements in

(a) PMF 4 (L 6.4m) and (b) SMF (L^ 10km)

149
Table 1. Comparison of Various PMD Measurement Methods

Time Domain Frequency Domain

PMD

IMeasuremeDt Methods

Interferometric Method

Optical Pulse Method

Fixed

Analyzer

Method

Poincare' Sphere

Method

SOP Method

Optical Source

SLA or / edfa\

\LED /

Pulsed-LD

EDFA or / sla\

\LEDy

Tunable-LD (ECL) or DFB-LD (Temperature Control)

Equipments

Michelson

Interferometer

Ontiral Sampling

Oscilloscope

Spectrum

Analyzer

Stokes Analyzer

Stokes Analyzer

or Rotatable Analyzer

Measurement

Accuracy

> - 0.003ps > ~ 15 ps > - 0.05 ps > ~ 0.05 ps > - 0.025ps

PolarizatloD Independent

Optical

Isolator

T= 0.012 ps

PMFl

(L = 0.245 m)

T = 0.048 ps (0.196ps/m) o = 0.001 ps

X = 0.051 ps

(0 208 ps/m) o 0.001 ps

T B 0.056 ps

(0.229 ps/m) a = 0.002 ps

T = 0.051 ps

(0.208 ps/m)

O = 0.003 ps

PMF2

(L s 0.646 m)

X = 0.134 ps

(0.207 ps/m)

O = 0.002 ps

T = 44.0 ps

(0 270 ns/ml

(L = 200 m)

1 a 0.135 ps

(0.209 ps/m) o = 0.001 ps

0.128 ps

(0.198 ps/m) o = 0.004 ps

T= 0.139 ps

(0.215 ps/m) o = 0.002 ps

PMF3

(L = 0.64 m)

T = 0.970 ps (1.516 ps/m)

0 = 0.002 ps

T = 0.969 ps

(1.514 ps/m) o = 0.001 ps

T = 0.842 ps (lJ16ps/m) o -s 0.003 ps

T = 0.972 ps

(1.522 ps/m) o = 0.004 ps

PMF4

(L = 6.4 m)

T = 9.772 ps

(1.503 ps/m)

0 = 0.003 ps

T = 9.792 ps

(1.506 ps/m) o = 0.019 ps

T = 10.538 ps

(1.621 ps/m) c = 0.284 ps

T = 9.772 ps

(1.503 ps/m)

O = 0.141 ps

DFF

(L = 6.4 km)

T = 0J4S ps (0.136ps/\'km)

OS 0.001 ps

T = 0J68 ps (0.145ps/Vkm)

0 = 0.008 ps

-I = 0.294 ps

(0.066ps/\'km)

0 a 0.006 ps

-c = 0.251 ps (0.099ps/Vkm) o = 0.006 ps SMF

(L= lOkm)

T = 0.094 ps (0.030ps'\'km) o = 0.006 ps

1 = 0.175 ps (O.OSSps/Vkm)

C = 0.004 ps

-c = 0.160 ps

(0.051ps/\'km)

0 = 0.012 ps

T = 0.130 ps

(0.041 ps/Vkm) o = 0.007 ps

DSF

(L = 20 km)

T= 0.110 ps (0.02Sps/Vkm)

0 = 0.041 ps

T = 0.264 ps

(0.059ps/\'km) o = 0.005 ps

Ts 0.184 ps

(0.041ps \ km)

0 = 0.013 ps

T = 0.224 ps

(0.050ps/\'km) o = 0.006 ps

Application

PMFs,

Birefringent - OpticalComponents, and

Short-Optical- Fibers (SMF,

CSF.DSF.DFF)

etc.

Long-PMFs,

HighBirefringentOpticalComponents

etc.

Optical Fibers (SMf , CSF. DSF, DFF), PMFs,

Optical Components and Optical AmpliTier Systems etc.. P.MF : Polarization Maintaining Fiber SMF : Standard Single Mode Fiber CSF : Cutoff Shifted Fiber DSF : Dispersion Shifted Fiber DFF : Dispersion Flattened Fiber SOP : State of Polarization SLA : Semiconductor Laser Amplifier

EDFA : Er-Doped Fiber Amplifier

Ellipitical PANDA SMF DSF

150
interferometric Loop Method for Polarization Dispersion Measurements

Luc Thevenaz, Marc Nilcles, Philippe Robert

EPFL. Metrology Laboratory, Swiss Federal Institute of Technology of Lausanne

CH-1015 Lausanne. Switzerland

Abstract: A novel method for polarization dispersion measurements using an interferometric loop is presented. It can be achieved using a particularly simple setup and provides a representation of the probability distribution of the

polarization dispersion.

Introduction

A single-mode fiber is not truly single-mode since it can

propagate two degenerate modes that are orthogonaly

polarized. Weak residual birefringence together with

environmental perturbations makes the polarization

degeneracy to be actually removed leading to pulse

broadening or Polarization Mode Dispersion (PMD).

The combined effects of weak birefringence and mode

coupling due to external perturbations result in a

stochastic behavior of the polarization dispersion and

consequently of the output polarization state. The socalled principal states, a particular set of orthogonal

polarization states, experience the maximum propagation

delay and are therefore very convenient for the

description of PMD [1-5]. Dynamic equations show

that polarization in fibers is a randomly varying

quantity and its probability density function is a Maxwell distribution [2,4,5].

Several methods have been reported so far for the

measurements of PMD in long single-mode fibers. A

technique using a tunable laser source and analyzing

paths over the Poincard sphere provides a measurement

of the instantaneous polarization dispersion [6,7]. This

requires repeated measurements to obtain the delay

statistical distribution. Another technique uses

interferences produced by a low-coherence source to map

the actual distribution of possible delays [8], so that a

single measurement yields the delay statistical

distribution and makes the determination of the PMD

expectation value possible.

In this contribution we report a novel method that

enables the measurement of PMD using a particularly

simple setup, without polarizing elements or retardation

plates. A single measurement also provides the PMD

expectation value.

Theoretical background

In an interferometric loop, counterpropagating

lightwaves propagate along the same optical path, so

OUT

Fig.l Example of non-reciprocal propagation in an

interferometric loop through a 90 deg twisted

linearly birefringent fiber.

151
that the propagation phase difference is zero at the

recombination point for any optical path length, except

when light experiences a non-reciprocal effect such as Sagnac effect (rotation) or Faraday effect (magnetic

field).

A non-reciprocal propagation may also occur when the

optical medium is birefiringenL Our measurement method

uses this property to evaluate the PMD.

For instance a linearly birefiingent fiber experiencing a 90 deg twist in the loop gives rise to a complete nonreciprocal propagation, as shown in Fig.l. This way,

light from the same input polarization state splits and

propagates along two distinct polarization modes of the

fiber. Their phase difference 5(p is given by

Sd = 2^AnL

X

where L is the fiber length and An the birefringence.

This phase difference may actually be changed by

varying the wavelength X, so that a succession of

constructive and destructive interferences is observed

when the wavelength is scanned, resulting in a periodic

variation of the interferometer output intensity. For a given fiber length L, the higher the birefringence An, the

faster the periodic variations, so that the period

measurement straightforwardly yields the value of PMD.

Actually this period is simply calculated by performing

a Fourier transform.

For a long standard single mode fiber, the random nature

of PMD makes the expression for interference intensity

more difficult to be carried out. In addition, the set of

principal states depends on the propagation direction.

After some lengthy calculations using Jones calculus,

the interferometer output intensity turns out to be for

any input light polarization state:

(out = /<, [l -I- cos^Q cos2<l> - sin^e siri^An l)]

where 0 and 0 are the angles defining the rotation on the

Poincar6 sphere to perform the vector base change from

the principal states for one direction to the set of

principal states for the other direction. When the

wavelength is scanned. An is expected to be constant

only over a limited range, so that interference periodicity

randomly varies over the spectrum. The wavelength scan

makes the fiber experience a great number of polarization

situations, so that it may be equivalent to an ensemble

average, provided that the statistical distribution is unchanged over the spectrum. With this assumption the

distribution obtained by performing the Fourier

transform is a measure of the actual PMD statistical

distribution. The angles 9 and 0 also vary when the

wavelength is changed, but it was demonstrated

elsewhere that they don't fluctuate at a higher rate than

PMD[3,4], so that their fluctuations induce variations

of the output intensity with longer periods. The

resulting distribution function of delays has still to be

determined, even though its asymptotic nature is expected to be Gaussian.

Test

Fibre

3dB

Coupler

Fig.2 Block diagram of the interferometric loop

method.

Experimental results

The interferometer output intensity does not depend on

the input light polarization as stated above, so that

neither polarization preparation nor tuning is required

using polarizing elements or retardation plates.

Consequently, even unpolarized light may be used. The

optical circuit is therefore particularly simple, as shown

in Fig.2. Hie only needed optical component is a 3dB

single mode coupler, that has to be preferably

wavelength-independent

The most critical element is the tunable source. Three

configurations were actually tested:

- A white light halogen lamp, whereas detection is performed using an optical spectrum analyzer. This

configurati(Ni provides the largest wavelength coverage,

a very uniform and smooth spectrum, but has a poor

dynamic range. It is also low-cost and quite suitable for

the measurement of less than 10-km fibers [9].

152
- A set of LEDs, detection still performed using an

optical spectrum analyzer. The dynamic range is larger,

enabling the measurement of up to 30-km fibers. The

drawback is a narrower and less uniform spectral

covoage.

- A tunable external cavity laser diode. This advanced

configuration enables the best performances, with an

ideal dynamic range and a convenient spectral coverage.

However it suffers fi-om the drawback of the present

high cost of such a source.

2.5 T 1

Wavelength, nm

Fig.3 Interferometric loop output intensity as a

function of wavelength. The loop is a 0.5m

highly birefringent fiber.

The results obtained using any of these three

configurations were identical. As a first test, a sample of

0.5 m highly birefringent fiber was measured, using a

configuration equivalent to Fig.l. The output intensity

periodically varies as the wavelength is scanned, as shown in Fig.3. The interference contrast was observed

to change fi'om a maximum to a minimum value when

either fiber end was rotated. The period of the intensity

spectral variations is obtained by performing a Fourier

transform, so that the PMD corresponds to the peak

position in the transformed distribution, as shown in

Fig.4.

2.5 1 1

1275 1285 1295 1305 1315 1325

Wavelength, nm

Fig.5 Interferometric loop output intensity as a

function of wavelength. The loop is a 3600m

standard single modefiber.

The observed intensity variations are quite different

when a several kilometers standard single-mode fib^ is measured, as shown in Fig.5. The delay between

polarization modes is wavelength-dependent making the

0 1000 2000 3000 4000

Polarization Dispersion, ps/km

Fig.4 Fourier transform of the measurements shown

in Fig.3. One frequency is dominant,

corresponding to the polarization dispersion

value.

a 0.12

0 2 4 6 8 10 12 14 16 18 20

Polarisation Dispersion, ps

Fig.6 Fourier transform of the measurements shown

in Fig.5. The obtained Gaussian-like

distribution is directly related to the statistical

distribution ofpolarization dispersion.

153
period of intensity variations to fluctuate over the

spectrum. The Fourier transform of such a measurement

is not just one peak, but a distribution of numerous

peaks, as shown in Fig.6. With the "wavelength scan" =

"ensemble average" assumption, this distribution is a

close representation of the actual delay statistical

distribution.

The minimum observable delay corresponds to an

interference period equal to the spread of the

measurement spectral range, so that the larger is this

spread, the better is the resolution. Hence:

where Xfnin, ^max the lower and upper bounds of

the measurement wavelength range, respectively.

On the other hand, the maximum observable delay is bounded by the source spectral width, because this delay

must remain shorter than the coherence time in order to

obsCTve interferences. Hence

Olmax —

where AX is the spectral width and A the center

wavelength {AX corresponds to the minimum

wavelength step when using a tunable laser source).

Actual obtained figures are shown in the table below:

Source S^min S^max

White lamp 0.008 ps 1.4 ps

LEDs 0.016 ps 9.6 ps

Tunable laser 0.075 ps 375 ps

Conclusion

Reproducibility tests were successfully carried out using

this method and the scattering of the obtained PMD

expectation value was below 10%, even though the fiber

was displaced between the measurements and the

intensity versus wavelength curves look totally

different. Results were also compared to those obtained

using another measuring technique [10] and the

agreement was excellent for expectation value below

Ips. For larger values, great differences are observed and

are still unexplained. This method has the great

advantage to be very easy to implement, because most of

the setup elements are available in any optics lab, so

that most of them can already afford the implementation

of this technique.

References

[1] C. D. Poole, R. E. Wagner, Phenomenological

Approach to Polarization Dispersion in long

Single-Mode Fiber, Electron. Lett., 22, pp 1029-

1030.

[2] F. Curti, B. Daino, G. de Marchis, F. Matera,

Statistical Treatement of the Evolution of the

Principal States of Polarization in Single-Mode

Fiber, IEEE J. Lightwave Technol,, 8, pp 1162-

1166.

[3] S. Betti, F. Curti, B. Daino, G. de Marchis. E.

lannone, F, Matera, Evolution of the Bandwidth of

the Principal states of Polarization in SingleMode Fiber, Opt Lett., 16, pp 467-469.

[4] C. D. Poole, J. H Winters, J. A. Nagel, Dynamical

Equations for Polarization Dispersion, Opt. Lett.,

16, pp 372-374.

[5] G. J. Foschini, C. D. Poole, Statistical Theory of

Polarizarion Dispersion in Single-Mode Fiber,

IEEE J. Lightwave Technol., 9, pp 1439-1456.

[6] D. Andresciani, F. Curti, F. Matera, B. Daino,

Measurement of the group-delay difference

between the principal states of polarization on a

low-birefringence terrestrial fiber cable. Opt.

Lett, 12, pp 844-846.

[7] C. D. Poole, N. S. Bergano, R. E. Wagner, H. J. Schulte, Polarization Dispersion and Principal

States in a 147-km Undersea Lightwave Cable,

IEEE J. Lightwave Technol., 6, pp 1 185-1190.

[8] N. Gisin, J.-P. von der Weid, J.-P. Pellaux,

Polarization Mode Dispersion of Short and Long

Single-Mode Fibers, IEEE J. Lightwave Technol.,

9, pp 821-827.

[9] L. Th6venaz, Ph. Robert, Simple method for

Polarization Dispersion Measurements in Long

Single-Mode Fibres, Proc. ECOC '90

(Amsterdam), Vol.1, pp. 581-584.

[10] L. Th6venaz, M. Nikl6s, N. Gisin, Ph. Robert,

Overall Polarization Dispersion after

Propagation through Different Fibres, Ptoc.

EFOC/LAN "91 (London), pp 156-158.

154
Four interferometric methods for the determination of the

birefringence and polarization dispersion of polarization

maintaining optical fibers

Ch. Chojetzki, kabelmetal-electro, Stadthagen

H. Schmitzer, Universitat Regensburg

J. Vobian, W. Dultz, DBP Telekom

Germany

Polarization-maintaining optical fibers are destined to gain

importance in optics and optical communications since they

combine the properties of a waveguide with those of a mechanically stable two-beam interferometer. Interferometric measuring techniques are the most precise in physics. Thus they

suggest themselves, to characterize polarization-maintaining

optical fibers with respect to their physical parameters.

The transmission behaviour of these fibers is determined by

polarization dispersion,, but since this parameter depends on

the character of the spectral and temporal features of the

transmitted optical information flow, the birefringence is the

primary physical quantity in spite of the fact that it combines waveguide and material properties in an obscure blend.

Unfortunately, instead of the birefringence B (x) the beat

length Lb(X) has been adopted to characterize polarizationmaintaining fibers in optical communications

S = -bTxT' ^^^^ = - ' ^s,f

=

Cs,Cf are the phase velocities and ng nf the refractive indices of the slow and fast axis of the ^iber. Lg has an intrinsic dependence on the wavelength X, while the birefringence B

only depends weakly on X . To introduce pulse propagation in

this picture, the modified beatlength Lg* and the modified

birefringence B are defined with respect to the group

velocities Vgg^f

Lu* = -E^i B*(X) = n - n v . =— . (2) B* B* gs gf gs,f n

Equations (1) and (2) are connected by the dispersion relation

for the wave vector k(X)

V k . = ^".^.-^.f . (3)

From (1) , (2) and (3)

155
can be derived /I/. Equation (4) allows the calculation of the

modified birefringence B , when the spectral dependence of the

birefringence B(X) is known.

We compare four different experimental techniques to measure

beatlength and modified beatlength of a polarization-maintaining optical fiber. The pressing method (P) , see Fig. 1

above, uses the Lyot-filter principle /2/. Linearly polarized

light (parallel to one of the two orthogonal polarization

axes) is launched into the polarization-maintaining fiber

under test. A pressing wheel couples light coherently into the

second channel of the fiber. At the end of the fiber the two

polarization modes are resolved by a polarizer and interfere.

During the movement of the wheel along the fiber, a detector

shows alternate constructive and destructive interference

maxima and minima at a distance of one beatlength LgCx^.

(a)

(b)

POLARIZER

MONOCHROMATIC LIGHT

WHITE LIGHT

PRESSING WHEEL

f

ANALYZER DETECTOR

3-H 5t

MONOCHROMATOR

V l II''

(c)

REFERENCE FIBER AIR GAP

Fig. 1 Measurement set-up of the

(a) pressing method (P)

(b) wavelength-scanning method (WLS)

(c) Mach-Zehnder int er f er ometric method (MZl)

The wavelencfth scanning method fWLS) , see Fig 1 in the center,

is also based on the Lyot-filter principle. In this case the

length 1 of the Lyot filter element is fixed, and the wavelength of the light is changed. The number of beats in fiber

piece 1 changes with wavelength

N = Knf - n.q) ^

1- B (5)

\ X

156
and this becomes apparent in a periodic detector reading with

period AX . It was shown that ax determines the modified beatlenth Lr* -i Lg. = -i-AX (6)

The WLS-method allows the easy measurement of the modified

beatlength Lg* and the modified birefringence B in a wide

spectral region. Unfortunately, it needs one additional

measurement of the beatlength Lb(A.) with the pressing method

at a single wavelength to determine the beat length Lb(X) from

(4) and (1) by integration, since the integration constant has

to be known. This supplementary measurement with a different

method can be avoided if the number N of beats Lg of the Lyot

filter element is changed in a known way by introducing a

birefringent plate with a definite retardation in the optical

path behind the fiber. From the shift dX of the maximum N can

be determined

From equation (5) we get the birefringence B. We call this the

modified wavelencfth scaninq method (MWLS) .

The last technique we discuss is the Mach-Zehnder interferometric method (MZI) /3/, see Fig. 1 below. The white light

interference fringes of the light from the two polarization

channels of the polarization maintaining fiber with the light

from a normal single mode reference fiber are determined with

a variable air gap. The distance Ax is equal to the optical

path length difference 1*B (X) of the polarization-maintaining

fiber. As in the case of the WLS-method, one measurement with

the pressing method is necessary to determine B and Lg.

beat- -BERTLENGTH VERSUS NRVELENGTH

' [!ilm]'''-FIBER : YORK c u t o f -F =G 00n m

1 . 8

--

1 . G

--

1 . 2

-- + be at 1 eng th

mod i f i ed

o beat 1 ength

700 900

wavelength [nm]

Fig. 2 Spectral dependence of the beatlength and the

modified beatlength of a YORK bow-tie fiber

157
In Table 1 the measurement results of the modified beatlength

Lg* and birefringence B* (3 polarization-maintaining fibers of

COST 217) obtained by means of the WLS and MZI method are

compared. An excellent agreement can be stated, with the

exception of the AT&T fiber.

X = 1300 nm \ = 1500 nm

»« Lb in mm

MZI

Lq in mm WLS

B 10

"

MZI

B • 10 "

WLS

Lb in mm

MZI

Lb in mm WLS

B • 30

"

MZI

* -A B • 10

"

WLS

PANDA

f iben 3. 01 2. 99 4, 32 A. 34 3. 42 3. 405 4. 39 4. 41

YORK

Bow-t ie

fiber

1. 97 1. 99 6. 60 6. 53 2. 25 2. 25 6, 57 5. 67

PM fiber

AT S T

5. 58 5. 72 2. 33 2. 27 6. 45 6. 44 2. 35 2. 33

Table 1 Comparison of the test results of the modified beatlength and birefringence of the MZI- and WLS method

We demonstrate the high precision of these interferometric

techniques on a YORK-fiber with a cutoff wavelength of 600 nm.

As an example, the beatlength and the modified beatlength of

this fiber which were measured with the wavelength scanning

method together with the pressing method are shown in Fig. 2.

Note the interesting opposite behaviour of the phase and the

group beatlength, the latter becoming constant above 950 nm.

This study was partly performed in the framework of the COST

217/241 action.

References

/I/ CH. CHOJETZKI, J. VOBIAN, W. DULTZ; J. Opt. Comm. 1992

/2/ K. TAKADA, J. NODA, R. ULRICH; Appl. Optics, 24 (1985)

p. 4387

/3/ J. VOBIAN, W. DULTZ; Proc. Interferometry 89,

Warsaw (1989), SPIE Volume 1121, p. 325

158
Low Coherence Ref 1 ectomet ry for the Characterization of

Fiber and Planar Lightwave Circuits

Kazumasa TAKADA

NTT Opto-electronics Laboratories,

Tokai, Ibaraki-ken, 319-11, Japan

Ab s t r a c t : M i c r ome t e

r - r e s o 1 u t i o n low coherence ref 1 ectomet ry has

been demonstrated using high-power broadband sources. We describe

recent advances in the field with particular attention to characterization of the fiber and planar lightwave circuits.

Introduction : Recent progress in planar lightwave circuit (PLC)

technology [1] requires a nondestructive diagnostic method that

can provide, for example, a loss level diagram and the backreflection distribution in a PLC. M i c r ome t e

r - r e s o 1 u t i on r e f 1 e c t ome t r

y

shows promise as a solution to PLC diagnosis. Fontaine et al.

first demonstrated the m i c r ome t e

r - r e s o 1 u t i on r e f 1 e c t ome t r y using

ultrashort light pulses and the optical intensity correlation

technique based on second harmonic generation [2]. Due to the

limited dynamic range, the most promising r e f 1 e c t ome t r y technique

for PLC diagnosis is now considered to be optical low coherence

ref 1 ectomet ry (OLCR) [3-5] using a CW broadband source. We describe the performance of OLCR and its application to waveguide

character izat ion.

Performance of OLCR : The basic OLCR configuration, consisting of a broadband source and a Michelson interferometer with a movable

arm, is shown in Fig. 1 [6]. The output light is split into a reference and a probe light. The probe light is launched into the

waveguide under test. The backreflected light from the test waveguide is combined with the reference light. The intensity of the

beat signal is measured for reflection measurement, while an

inte r f e rog ram is used for dispersion measurement. Optical modulation is often applied to either the reference or reflected light

using a piezoelectric stretcher or an optical frequency shifter

to reduce the noise at the lower frequency.

The main factors limiting the sensitivity of the OLCR are

excess photon noise, beat noise and shot noise. The excess photon

noise of the reference light is removed by using the balanced

detection technique [6-8]. Fresnel reflection produces beat noise

due to beating of the various Fourier components within the reference and reflected light spectra [9]. Figure 2 shows the measured

minimum detectable reflectivity at unit bandwidth versus total

mean photocurrent at four different Fresnel reflections R from the

test waveguide. The source was the 1 . 3-|jm InGaAs s u pe r 1 urn i ne s c e n t diode [10]. At R = 0 the minimum reflectivity decreases by 10

dB/decade of current, indicating that the shot noise limited the

sensitivity. The optimum value was -140 dB at a 3 Hz detection

bandwidth. As R increases, the reflectivity approaches a definite

noise floor. A reduction in the facet reflection with matching oil

or AR coating is indispensable for shot-noise limited operation.

159
Spatial resolution depends on the reflection distribution

profile. With point-like distribution, the resolution is determined by the coherence length divided by the waveguide refractive

index [3]. Typical values for LED's and SLD's range from 15-40 jum.

With Rayleigh bac ksc at t e r i n g, coherent jagging occurs owing to the

speckle-like phenomenon induced by the number of scattered waves

contributing to the detected signal [11]. The signal is randomly

jagged according to the law of Rayleigh distribution and does not

change with external perturbations. Signal smoothing, which reduces the resolution, can reveal the desired profile for waveguide diagnosis. Figure 3 shows one example of the revealed backscattering signal profile for a bent portion of the fiber. The

actual resolution determined by smoothing was 400 ;jm. There is a

3 dB change in intensity after the signal propagates through the

bent portion, thus indicating a bending loss of 1.5 dB.

Reflection Profiles : Reflection profiles for an integrated GaAs

Mach-Zehnder interferometer [12], and straight and curved silicabased waveguides [13] have been measured. Figure 4 describes the

results for the GaAs interferometer. Bac k r e f 1 ec t i on at the splitting section (Y) and Fresnel reflections at both facets (F and B)

are observed, from which the average loss coefficient and the

coupling coefficient are estimated. The profile for a curved

(3-mm- rad i us) silica glass waveguide is shown in Fig. 5, where the

light is launched from the end of region A into the waveguide. The

intensity decreases by 5 dB during the back-and-f or t h propagation

from region A to region C, indicating a transmission loss of 2.5

dB. Rare-earth-doped supe r f 1 uorescent fiber sources provide an

output power in a range of more than 10 mW and a spectral linewidth of more than 10 nm [14, 15]. An OLCR coupled to the source

has achieved a minimum detectable reflectivity of less than -140

dB [16, 17], as shown in Fig. 6. The dynamic range is 25 dB, indicating that a waveguide loss of up to 12 dB can be evaluated. Such

a large dynamic range is required for semiconductor waveguides

without injection current, or with absorbed regions.

Dispersion Measurement : Fourier transform spectroscopy has long

been known to be a powerful tool for measuring the dispersive

properties of bulk optical samples [18]. Recently, dispersive

spectroscopy has been applied to the measurement of guided wave

properties, including chromatic dispersion [19] and the location

of index discontinuities with fraction of a wavelength precision

[20]. Based on the principles of Fourier spectroscopy, the phase

of the Fourier transform of the dispersive i nte r f e rog ram is equal

to the optical phase difference between the two arms in the interferometers. The second derivative of the phase spectrum with

respect to the wavenumber leads to the chromatic dispersion spectrum. Measuring the phase slope of the Fourier components in the

frequency domain makes it possible to locate the positions of

reflections with nanometer precision even in the presence of

sample dispersion. Precise dispersion measurement requires that

the i n t e r f e r og r am be sampled without distortion. Coherent laser

light is launched into the interferometer and separated at the

160
output with a wavelength-division multiplexer. Clock pulses for

inte rf e rog ram sampling are generated by zero crossings of an accoupled high coherence interference signal [21]. Since the signal

and reference light are co-propagating, distortion of the sampled

in t e r f e r og r am due to temperature changes and scanning arm vibrations is minimal.

Conclusion : Recent progress in optical low coherence r e f 1 e c t ome t r

y

(OLCR) for nondestructive waveguide diagnosis has been described.

High-power broadband sources such as rare-earth-doped superfluorescent fiber sources provide sensitivity in the -140 dB range and

a dynamic range of more than 25 dB in Rayleigh backseat te r i ng

measurement. Most of the problems regarding OLCR have now been

overcome and its practical application to the diagnosis of fiber

and planar lightwave circuits will provide useful information for

circuit development.

Acknowledgment : The author would like to express his sincere

thanks to M. Nakahara, M. Horiguchi, K. Yukimatsu and A. Himeno

for their continuous encouragement.

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Tech. Dig. Symp. Opt. Fiber Meas. , 1988, 7-10.

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Chichester 1979.

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Technol. 7, 500-513 (1989).

20. B. L. Danielson, and C. Y. Boisrobert, Appl. Opt. 30, 2975-2979 (1991).

21. K. Takada, M. Kobayashi, and J. Noda, Appl. Opt. 29, 5170-5176 (1990).

161
LIGHT SOURCE

MIRROR M

I

, ' DELAY

LINE

BS

^ ^robe ^ P

DETECTION

SYSTEM

SAMPLE

-110

X -120

m

>

o

UJ

_J

u.

lU

a.

-130

-140

-150

"T t—I—r I I r r| 1 1—r—i i i 1

1

1

phase noise

shot noise

—

I—I—I I nT

R=0.13

10 100

PHOTOCURRENT (tiA)

1000

Fig.l. Schematic arrangement of

OLCR (Ref. 6). Rp,RQ: end

reflection coefficients.

Fig. 2. Minimum detectable reflectivity

vs total mean photocurrent

Points represent experimental

values.

ffl

<

z

o

to

Io<00o_i

i-1

80

90

100

TEST FIBER

X

STRAIGHT BENT

NOISE UMIT -128 dB-

-15 -10 -5 0 5

POSmON (mm)

10 15

1 2

mirror translQfion Icml

Fig. 3. Backscatter signal change by

fiber bending.

Fig. 4. Backscatter signal from GaAs

Mach-Zehnder interferometer

(Ref. 12).

0 12 3 4 POSITION, cm

m

o

COz

UJ

<

z

(A

QC

UJ<

o

(A

be: o

<

CO

-90

-100

-110

-120

-130

-140

-150

-160

-121dB

25dBdj

-146dB

-4 -3 -2 -1

POSITION (cm)

Fig . 5 . Backscatter signal from curved

silica-based waveguide.

Fig. 6. Backscatter signal from fiber

in index matching oil.

162
Improved Sensitivity for

Optical Low-Coherence Refiectometry

using Reference Power Attenuation

W. V. Sorin and D. M. Baney

Hewlett-Packard Laboratories

3500 Deer Creek Road, Palo Alto, California 94303

Introduction

Optical Low- Coherence Reflectometry (OLCR) is a measurement technique capable of spatially

resolving optical reflections separated by tens of microns with reflection sensitivities demonstrated

as low as -148 dB [l]-[3]. Initial work consisted of a simple Michelson interferometer with a variable

time delay in one arm [4]. It was soon realized that reflection sensitivities obtained using the

simple Michelson interferometer were limited by the intensity noise associated with low-coherence

sources. Minimum reflection sensitivities were set by intensity noise and did not improve as source power was increased. Using a more complex interferometer arrangement, it was later shown [2]

that sensitivity limitations due to intensity noise can be removed. This arrangement consisted of

a combined Michelson and Mach Zehnder interferometer allowing use of a balanced receiver to

subtract out the intensity noise [l]-[3].

In this paper a simple modification to a Michelson interferometer based OLCR is proposed allowing

for improvement of reflection sensitivities to a value similar to that achieved using the more complex

balanced receiver approach. The main concept for the modification is the reduction of intensity

noise at the optical detector via reference path attenuation. As wiU be shown, this improves the

detected signal to noise ratio (SNR).

Theory

The Michelson interferometer with incorporated reference path attenuation is shown in figure 1. The

interferometric beating between reflected powers from the reference and DUT (device under test)

arms of the interferometer is detected as a function of the position of the translatable retroreflector.

The following expression shows how the signal to noise ratio (SNR) of the photocurrent depends

on the reflected powers from each arm.

c D PrefPdut

bis R (X

Pref + KiPj^p Kt

The reflected powers incident onto the photodetector, Pdut a^nd Pref from the upper DUT

and lower reference arms, respectively. This result assumes that Pdut <C Pref which is usually

the case when attempting to obtain the greatest reflection sensitivity. The three terms in the

denominator represent the shot noise, the intensity noise (RIN), and the receiver noise contributions,

respectively. The parameters Kj and Kt a-re constants independent of the actual powers returning

to the receiver. For reference powers on the order of 1 /xW or more, the source intensity noise term

KI Pref becomes dominant. For this condition, the above expression shows that the signal to noise

163
ratio can be improved by attenuating the reflected reference power. This effect occurs since the

detected noise decreases at a faster rate than the interferometric signal.

Figure 2 shows a plot of how the signal to noise ratio degrades as the returning reference power

is varied using the attenuation method illustrated in figure 1. It is assumed that Pdut •C Pref

for all values of Pref- The shot noise limit is the best SNR that can be obtained for a given source power. Figure 2 assumes a receiver noise equivalent to the thermal noise from 1 Mfi, a photodiode responsivity equal to 1 A/W, and a source RIN equal to -121 dB/Hz. For large values

of the returning reference power, it can be seen that a 10 dB attenuation of Pref equates to a 10

dB improvement in the SNR. Eventually, as Pref is made too small, the receiver noise starts to

become dominant and the SNR decreases with increased attenuation. The optimized value for the

reference power occurs when the noise contributions due to the receiver and source RIN become

equal. For the case when the above assumption of small Pdut is not valid, then the best SNR

occurs for Pdut ~ Pref- In this case, additional methods designed to remove the intensity noise

should not be too useful since the effects of phase noise [5] should be of a similar value.

Experiment

Figure 1 describes the experimental arrangement used to demonstrate the sensitivity improvement

possible using reference power attenuation. The low-coherence signal wa5 obtained from an erbium

doped fiber noise source. An unpolarized output power of 16 mW was generated at 1.55 /xm

with a spectral width of about 20 nm. The source RIN was measured to be -121 dB/Hz at the

detection frequency of 20 KHz. The polarization controller in the lower reference arm was adjusted

to maximize the interference signal at the photodetector. Attenuation of Pref was achieved by

adjusting the spacing between two angle polished fibers. The DUT consisted of a fiber cleaved at

an angle to minimize the reflectivity from the fiber end. A phase modulator Wcis used to translate

the Doppler beat frequency to about 20 KHz to avoid the increased intensity noise that occurs at

lower detection frequencies.

The measurement sensitivity with and without attenuation of the reference power is shown in figure

3. The upper and lower curves correspond to reference powers of Pref = 2 mW and Pref =

200 nW respectively. The increased reflectivity of the lower curve, for distances of less than 70

mm, corresponds to Rayleigh backscattering (RBS) from the DUT fiber. For the upper curve,

corresponding to no attenuation, the RBS is completely masked by intensity noise. The difference

in sensitivity of about 34 dB is in good agreement with that predicted from figure 2. The spatial

resolution within the optical fiber was measured to be 20 /im, which for the RBS level of -120 dB

gives a normalized value of -73 dB/m for the DUT fiber. Data points were measured at 5 /im

intervals and each 100 points were spatially averaged to reduce amplitude uncertainty. A reflection

sensitivity of -134 dB as shown by the lower curve was achieved in a 30 Hz detection bandwidth. A

sensitivity penalty of 5 dB was also present due to the detection scheme that measured the strength

of the first order Bessel function of the sinusoidally phase modulated Doppler frequency. Taking

the bandwidth difference and detection scheme into account, this sensitivity result is approximately

equal to that obtained using the more complex balanced detection technique [1],[3].

164
i

Summary

This reference power attenuation technique offers several advantages over the more complex balanced detection technique. First, for a given translation range of the retroreflector, this new

technique allows the use of a Michelson interferometer which gives twice the measurement range

since the reference signal experiences a double pass through the corner cube retroreflector. Second,

since only one receiver is required, accurate balanced detection does not have to be maintained and

the implementation of a polarization diversity receiver is much easier.

By attenuating the reference power in a simple Michelson interferometer based OLCR, it has been

shown that reflection sensitivity can be improved to values similar to that obtained using more

complex approaches requiring two balanced receivers. Using this technique, a 34 dB improvement

in reflection sensitivity was demonstrated when using a high power erbium doped fiber noise source.

References

[1] W. V. Sorin and D. M. Baney, "Measurement of Rayleigh backscattering at 1.55 ^m with a 32 /xm spatial resolution," Photonics Tech. Lett., vol. 4, 374 (1992).

[2] K. Takada, K. Yukimatsu, M. Kobayashi and J. Noda, "Rayleigh backscattering measurement

of single-mode fibers by low coherence optical time-domain reflectometer with 14 fxm spatial

resolution," Appl. Phys. Lett., vol. 59, p. 143, 1991.

[3] K. Takada, M. Shimizu, M. Yamada, M. Horiguchi, A. Himeno and K. Yukimatsu, "Ultrahighsensitivity low coherence OTDR using Er^"*"-doped high-power superfluorescent fibre source,"

Electron. Lett., vol. 28, p. 29, 1992.

[4] B. L. Danielson and C. D. Whittenberg, "Guided-wave reflectometry with micrometer resolution," Appl. Opt., vol. 26, p. 2836, 1987.

[5] K. Takada, A. Himeno and K. Yukimatsu, "Phase-noise and shot-noise limited operations of

low coherence optical time domain reflectometry," Appl. Phys. Lett., vol. 59, p. 2483, 1991.

Figure 1. Experimental set-up for OLCR measurements using the reference path attenuation technique. PC: polarization controller, PZT: piezoelectric phase modulator, T^: optical

transmission of attenuator.

165
40

I \ I I I I I L_

IQS IQS 107 10-6 IC* 10^ 10-2

Pref (Watts)

Figure 2. The effect of reference power on OLCR noise performance as limited by source

intensity noise and receiver thermal noise.

-60

-70

-80

-90

-100

-110

-120

-130

-140

-150

7

SMF

Without Attenuation

34 dB

With Attenuation

20 40 60 80

Distance (mm)

100 120 140

Figure 3. OLCR measurement of optical backscatter near the end of a clea\ed single-mode

fiber (SMF) with and without reference power attenuation. Detection bandwidth = 30 Hz. source limited resolution = 20 //m, resolution after spatial a\eraging = 340 //m.

166
Optical Low Coherence Reflectometry: Improving

Reflectivity Accuracy in the Presence of Chromatic Dispersion

Harry Chou

Hewlett-Packard Co.

1400 Fountaingrove Parkway

Santa Rosa, CA 95403

Introduction

Optical low coherence reflectometry (OLCR), also known as white light interferometry, is an

important non-destructive technique for probing small optical devices. [1-3] Its main advantages are

high resolution and excellent dynamic range. In a typical system, chromatic dispersion can play an

important role due to the broadband source needed to achieve high resolution. A mismatch between

the dispersion properties of the two arms of the interferometer will result in the broadening of the

interferogram and a reduction in its peak magnitude. This can lead to an error in the measurement of

reflectivity if the peak magnitude is used to calculate reflectivity. A number of researchers have

examined the effects of chromatic dispersion in OLCR. [4-7] The general problem of recovering the

true signal from the dispersion-broadened signal has been treated by Brinkmeyer et. al.[4,5] In this

paper, we present data on the dependence of the reflectometer signal as a function of the length of

dispersion mismatch, and demonstrate reasonable agreement with a Gaussian model. We have used

this model to improve the accuracy of reflectivity measurements.

Experimental Apparatus

The experimental setup is a Michelson interferometer, as shown in Fig. 1. Only the 1550 nm

source (an edge-emitting light emitting diode) is used in this work. A 3-dB directional coupler splits

the light into the reference and test arms of the interferometer. The light exiting the reference arm

fiber is polarized by a polarizer and collimated by a lens. The collimated beam is reflected from a

flat gold mirror after going through a corner cube, which is movable to provide 400 mm of maximum

delay. In the receiver arm, a polarization diversity receiver is used to reduce the sensitivity of the

interference signal to the polarization transformations of the test arm fiber. This is achieved by first

setting up the polarization split in the receiver such that the optical power returned from the reference

arm alone is about equally divided between the two orthgonal polarizations of the receiver. Then the

interferograms of the two polarization channels are envelope-detected, digitized, and mathematically

167
combined to produce a peak value proportional to the reflectivity of the interface under study.

Chromatic dispersion, however, will decrease this peak value and lead to an apparently lower

reflectivity if uncorrected.

Experimental Results

To obtain the variation of the peak signal with length of dispersion mismatch, a set of four

fiber cables of different lengths with polished ends was used to sample four points along the 400 mm

range of the interferometer. The fiber ends had been measured in a power meter-coupler setup to

have a return loss of -14.73 +/- 0.085 dB. The cables were then each connected to the test port of

the reflectometer and the reflectivity signal was measured. In each case, the connection loss at the

test port and the change in the power returned from the reference arm were also measured and taken

into account. The peak signal measured for the shortest cable was taken to be 0 dB and all the others

were referenced to it. To check for consistency, the same measurement was made on three systems

of the same block diagram, and the data are shown in Fig. 2.

We model the free-space coherence envelope of the optical field as a Gaussian with a full 1/e

width of Wq. After going through a dispersive medium, the pulse is broader in width and lower in

peak amplitude. [8] The quantity of interest is the peak magnitude squared of the correlation function

between the unbroadened and dispersion-broadened optical fields, since it is used to compute the

reflectivity. By carrying out the calculation of the correlation function, we obtain the following

dependence

where L is the length of the dispersion mismatch, S(L) the apparent reduction in reflectivity due to

dispersion broadening (S(0)= 1), D the dispersion coefficient, and AX the full width at half maximum

of the source spectrum. Wq is related to AX by

Model

168
where X is the peak wavelength of the source and c the free-space speed of light. The following

parameter values yielded a good fit to the experimental data:

D = 17.5 ps/(nm.km)

AX = 60 nm

as shown in Fig. 2. These are reasonable values for the sources and fibers used in these

measurements.

Conclusions

We have demonstrated reasonable agreement between experimental data and a Gaussian model

to account for the dependence of the peak signal of a low coherence reflectometer on the length of

dispersion mismatch. We have used this model to improve the accuracy of reflectivity measurements.

Acknowledgement

The author wishes to thank Wayne Sorin and Steve Newton for helpful discussions and

Howard Booster and Roger Wong for their support.

References

1. K. Takada, N. Takato, J. Noda, and Y. Noguchi, Optics Lett. 14 706 (1989)

2. B.L. Danielson and CD. Whittenberg, Applied Optics 26 2836 (1987)

3. R.C. Youngquist, S. Carr, and D.E.N. Davies, Optics Lett. 12 158 (1987)

4. E. Brinkmeyer and R. Ulrich, Electron. Lett. 26 413 (1990)

5. A. Kohlhaas, C. Fromchen, and E. Brinkmeyer, J. Lightwave Technology 9 1493 (1991)

6. P. Merrit, R.P. Tatam, and D.A. Jackson, J. Lightwave Technology 2 703 (1989)

7. P.-L. Francois, M. Monerie, C. Vassalo, Y. Durteste, and F.R. Alard, J. of Lightwave

Technology 7 500 (1989)

8. A. Yariv, Optical Electronics. Third Ed., p. 42 (Holt Rinehart & Winston, Orlando, 1985)

169
HP SSmfl OVERRLL BLOCK DIflGRRn

r

UGHTWAVE SECTION

SOURCE SLOCK

CUMKMT ORNC L

RKEKER BLOCK

1

I— DISPLAY PROCESSOR

1

11UNSLA1DN STAGE

AHUM-TO

Fig. 1 Schematic of the experimental apparatus.

.J

ffl •D

(A

100 200

Length of Dispersion Mismatch (air mm)

300 400

Fig. 2 Dependence of the reflectivity signal on the length of dispersion mismatch. The data points

are for three systems of the same block diagram as shown in Fig. 1.

170
STANDARDISATION IN FIBRE OPTIC COMMUNICATIONS:

STATUS AND PERSPECTIVES

P. Di Vita, M. Artiglia, A. Cavaciuti, M. Potenza, A. Rossaro

GSELT - Via G. Reiss Romoli, 274 Torino (Italy)

Abstract

The activity of standardisation in ttie field of fibre optic communications is reviewed with

emphasis on new developments in optical fibres, optical amplifiers and on new trends in the

telecommunication network.

1. Introduction

Standardisation is one of the major aspects for the development of every industrial field, since it mirrors the confrontation and the agreement between customers and manufacturers requirements and

brings to a rational and widespread diffusion of the industrial products. This is particularly true in the

field of fibre optic communications where developments are going increasingly rapid. New applications

for optical components and new functions and enhanced capabilities for the optical network are coming to light. New network architectures can be introduced thanks to the availability of optical

amplifiers and particularly Optical (active) Fibre Amplifiers (OFAs) with their high efficiency and

versatility. Standardisation of components and subsystems represents a key condition for a rational

development of the optical network all over the world.

The aim of this activity is two-fold: specify the physical parameters featuring the performance of

each device and characterize it in terms of such quantities through a univocally defined and well

assessed set of measurements. This in turn will allow the optimized interfacing of the various

components and the exploitation of the full system capabilities and would be of great help in designing

the systems themselves. On the other hand it is also evident that a premature full standardisation

could, to a certain extent, prevent the proper evolution of the technology. With the rapid pace of

development of optical communications it is not always easy to understand where the proper border

between research and standards should be placed. This is the hard task of the standardisation bodies,

who should reach the ideal trade-off between these two opposite needs.

To understand the importance and complexity of this problem, it is worth mentioning the groups

of standardisation bodies at international level (see Figs. 1 and 2) such as the CCITT (Com. XV) and

lEC (TC 86). These groups are supported by some regional bodies, such as the EIA/TIA and the T1

Committee in the USA, the TTC of Japan, the Asian ISDN Council in the Far East (although primarily

commerce oriented) and in Europe the ETSI (stc TM1) and the Cenelec/CECC (WGs 26, 27, 28) that

bring to the attention of the international community the needs emerging from local demands that have

to be harmonised in the framework of a global project and in the meantime provides a mean of diffusing

the standards agreed upon at international level. Last but not least, standardisation bodies also

represent a forum for confronting and harmonising the requirements of such differently motivated

interests as those of users and suppliers of industrial products. Important working tools for the

standardisation activity are studies carried out by individual companies and institutions as well as

interlaboratory comparisons at regional and international levels, which have been of great help in identifying the characterisation parameters and the crucial aspects of the pertaining test methods.

In the following section we shall review the present status of standardisation activities in the field

of single-mode fibres including the latest developrhents. Section 3 will treat possible lines of evolution

of the optical network motivated by the introduction of optical amplification . Finally, Section 4 will briefly cover some aspects of system and component standardisation issues.

2. Standardisation of single mode fibres

The preferred physical carrier in today telecommunication systems is the single-mode fibre

(SMF) with its very low attenuation and huge bandwidth. Standardisation of SMF is in progress since

the middle of 70s and has reached a good level of maturity. As a result, relevant documents [1-6] on

SMFs have been published which unambiguously define proper parameters featuring the fibre

behaviour, give accurate descriptions of their test methods and fix recommended range of values.

171
Within SMFs, CCITT actually gives recommendations for three types of fibres: SMFs dispersion

optimized around 1310 nm covered by Recommendation G.652 [2]; dispersion shifted SMFs,

dispersion optimized around 1550 nm, in correspondence of the absolute attenuation minimum,

covered by Rec. G.653 [3]; and SMFs loss minimized (but not dispersion optimized) around 1550 nm,

covered by Rec. G. 654 [4], for use in the third transmission window. Up to now, dispersion flattened

fibres are not considered in CCITT Recommendations, but work Is in progress in lEC.

A synthetic specification of SMFs through the mode field diameter, the cutoff wavelength, the

attenuation coefficient, the chromatic dispersion parameters and the geometric and dimensional

parameters had been chosen. In the following some evolution trends concerning these parameters

and the relative test methods are discussed. CCITT considers two categories of test methods: the

Reference Test Methods (RTMs) and the Alternative Test Methods (ATMs). RTMs ensure the

maximum accuracy and are directly related to the definition of the parameter. ATMs may be more

practical than RTMs but unambiguously related to them.

2.1 Mode field diameter

The MFD [1] provides information about micro- and macro-bending sensitivity of the SMF and

about joint performances [7,8]. Actually all standardisation bodies have adopted the MFD definition

based on the inverse of the rms angular width of the far field pattern of the fibre [8,9]. Correspondingly,

the recommended RTM is the far field scan of the fibre exit radiation pattern, while the variable aperture

technique, the knife-edge scan and the near field scan have been considered as ATMs [1]. The

transverse offset technique, once very popular, has been discarded from the set of test methods

because of the intrinsically poor reliability and repeatability, as demonstrated by the results of various

Round Robin Tests (RRT) [10,11].

As previously said, the MFD influences microbending losses. However, a very accurate

prediction depends also on other parameters as the mode propagation constant and the statistic

parameters of the microbending perturbation, which are difficult to measure. Some Committees (lEC,

CECC) are considering standards on microbending loss tests directly and proposals are under study.

2.2 Cutoff wavelength

The Cutoff Wavelength (CWL) of the first higher order mode determines the spectral region of

effective single-mode operation of the fibre. Its operational definition from a measurement of the

power transmitted by the fibre in a given deployment condition, referenced to that of the fibre Itself with a small bend inserted or a multimode fibre has been widely accepted since the middle of 80s. At

first, only the uncabled CWL was considered, as measured on a fibre sample 2 m long bent on a loop of

140 mm radius [1]. Though this quantity favoured enhancement of the agreement among

manufacturers and users, it was felt that this parameter only was too conservative in view of long

distance applications. For this reason the cabled CWL has been introduced, as measured on a cable

section 22 m long, including two uncabled fibre tails 1 meter long [1] with small loops - diameter 80 mm

- to simulate the effect of splice organisers (RTM); however, the definition of this parameter raises

some problems. In the first place, as far as the test method is concerned, the cabled CWL

measurement is heavily destructive. To circumvent this problem, an ATM has been proposed where

22 m of uncabled fibre are suitably deployed. Some debate is in run concerning the diameter of the

small loops at the fibre ends: only recently lEC and CCITT reached an agreement on the value of 80

mm. In second place, the cabled CWL can depend on the cable type and is application-dependent. A

possible way out of this problem is empirical and consist of developing mapping functions relating on a

statistical basis the cabled CWL for a given cable structure to the uncabled CWL of the used fibres: also

this aspect is actually matter of study.

Finally, international intercomparisons have shown that the reproducibility of uncabled CWL

measurements Is in some cases severely affected by the presence of unpredictable humps in the

spectral transmission curve [12,13] which seem to be due to coupling between the radiating higher

order mode and modes of the fibre cladding. They are very much sensitive to the fibre refractive Index

profile and curvatures. Work is in progress to get a deeper understanding of the phenomenon so as to

devise means of eliminating their uncontrollable effect on the determination of CWL. The use of

suitable fitting functions is under study.

2.3 Attenuation coefficient and losses

The standardisation of SMF attenuation is generally well assessed. The RTM recommended by

CCITT Is the cut-back method, which is partially destructive and requires access to both fibre ends.

172
A very popular technique overcoming the aforementioned drawbacks is the backscattering (OTDR)

technique, recommended by CCITT as an ATM, which is very practical and can also be used for routine

measurements and testing of installed plants.

Presently the discussion is open on how to set properly attenuation specifications for practical

system applications. While a description in terms of attenuation coefficient at the center of the various

transmission windows is generally accepted, it can be of interest to consider attenuation properties over a certain spectral range around the nominal ones. This fact could permit to reach an economical

trade-off between system performance and cost with reference to actual applications. This could allow,

for example, to tolerate a wider spread of source emission wavelengths in short range links, such as

the subscriber loop; moreover, consideration of WDM transmission systems also pushes in the

direction of an attenuation standard given on a suitable range of wavelengths.

A convenient way of setting more flexible attenuation specifications in a more applicationoriented form, seems to be that of spectral attenuation modelling. This should be based on the

measurement of the attenuation coefficient at a given small set of predictor wavelengths (say 1300,

1330, 1370, 1380, 1550 nm) and evaluating the attenuation coefficients for a larger set of

wavelengths (say 10 nm spaced wavelengths in the range 1260-1600 nm) by means of a suitable

matrix (and eventually of a correction vector) whose elements have to be calculated on a statistical

basis for each fibre type and manufacturer, due to the dependence of the attenuation spectra on the

actual fabrication process. This procedure is actually reported as an Appendix to CCITT Rec. G. 652

[2]. Need is felt to consider a similar algorithm for G.653 fibres too. A loss parameter of increasing importance is related to the longitudinal uniformity of the fibre.

The problem of its definition and testing appears among the questions for the next CCITT/XV study

period 1993-1996: a possible solution could be based on the analysis of the backscattering traces of

the fibre under test. In fact, some of the fibre parameters, such as the MFD and the CWL, can be

measured only at the ends of a fibre section; for some other parameters, such as attenuation and

dispersion, global values on the whole length are measured: a longitudinal uniformity criterium should ensure that also these parameters are uniform all along the sample: this aspect is important when the

factory lengths of fibre are much longer than the cable units obtained from them.

2.4 Dispersion

Chromatic dispersion (CD) sets a limit to the transmission capacity of a SMF. Presently it is standardised in terms of the wavelength and slope of the dispersion curve versus wavelength at the zero dispersion point. Spectral fitting functions (different for dispersion-unshifted and dispersionshifted fibres) are also provided which give limits for the CD coefficient in terms of minimum and

maximum allowable values for the zero dispersion wavelength and of maximum allowable value for the zero dispersion slope [2,3].

The recommended RTM for the determination of CD parameters is the phase shift technique,

while ATMs are the pulse delay technique and the interferometric method. The latter is intrinsically the

most accurate (assuring time resolution in the order of 0.1 ps) but can be applied to short fibre lengths

(2-5 m) only. Therefore its reliability depends on the longitudinal uniformity of the fibre itself. All methods giving the CD coefficient from measurements of the spectral time delay share a common delicate aspect, i. e., the choice of the fitting function to interpolate the experimental data.

The various Committees have recommended standard functions such as the three-term Sellmeier

expansion for dispersion-unshifted fibres and the parabolic fit for dispersion-shifted ones, to which the

spectral fitting functions mentioned at the beginning of this Section are related. However, if dispersion-flattened fibres will be considered by standardisation bodies, a new suitable fitting function

should be found: this question is still matter of study.

A characteristic of SMFs of special interest in long-haul high-bitrate systems for which the need

for standardisation is emerging, is the Polarisation-Mode Dispersion (PMD). It originates from the fact

that, contrary to what happens in ideal fibres, the two orthogonal polarisations of the fundamental

mode of an actual fibre do not propagate with the same group velocity, because of fluctuations of fibre

geometric and optical parameters or random stresses caused by the cabling process, which induce

unwanted birefringence. The resulting statistical distribution of group velocities along the fibre causes

a broadening of the launched pulse. This effect becomes relevant for long distance, high capacity

systems where ultra-low chromatic dispersion fibres and very narrow linewidth laser sources are used;

in this application PMD can represent the ultimate limitation to the system performance. Problems

could also arise in systems making use of optical isolators, such those employing optical amplifiers, see

173
Sect. 3. Proposals are under study at CCITT to define proper parameters to describe PMD and suitable

test methods: the question has been inserted in the list of arguments for the next study period.

2.5 Geometry

The relevant geometrical parameters of SMFs are: the MFD, the mode field concentricity error, the cladding diameter and the cladding non-circularity. Leaving aside the MFD which has been already

considered in Sect. 2.1, the RTM for the other geometric parameters is the transmitted near-field

technique. Actually recommended ATMs are the refracted near field, the side-view and the imaging

transmitted near field. The present status of fibre geometry specifications is not yet satisfactory

because both the RTM and the ATMs suffer from relatively large reproducibility errors, as

demonstrated by various international measurements intercomparisons [10,14], which only in part can

be attributed to local geometry fluctuations along the fibre sample.The question of fibre geometry has

assumed a high level of importance for related industries, because of the wide use of mechanical

connectors expected in optical links such as those involved in the project Fibre-To-The-Home (FTTH).

The necessity of keeping losses and costs of connectors and mechanical splices to a minimum,

pushes for a tighter tolerance of fibre geometry. Considerable efforts have been done to improve the

accuracy of existing methods and look for new ones; interferometric and micrometric methods are

gathering increasing interest [14,15]. Presently a new extensive RRT of geometry measurements is in run organised within CCITT/XV with the aim to identify the most reproducible methods and algorithms

for experimental data analysis.

2.6 Mechanical properties

Specifications in this area are intended to characterise the in-service reliability of optical fibres

with respect to environmental and deployment conditions. Because of the ever increasing use of

optical fibres in aerial, buried and submarine cables, the interest for the aspect of predictability of fibre

lifetime is now crucial. Test methods for strength testing have been set by lEC [5,6], and recently

revised, by CCITT. A parameter measuring the fibre resistance to stress corrosion, which is empirically

related to the dependence of crack growth on applied stress, has been included in CCITT Recs. G.650

to G.654. The introduction of further parameters is under study.

3. Future trends In the optical network

The development of optical amplifiers of various types has quickly led to impressive

enhancements of the performance of optical communications systems. Ultimately it can be envisaged

to arrive to substantially transparent and lossless links, allowing new network architectures and

increased capability for provision of new services. Moreover, non linear transmission techniques, such

as soliton based systems, have demonstrated their practical viability and paved the way to long-haul

non-repeatered links, because of the dispersion compensation provided by non linearity. Besides

this, a more efficient exploitation of the huge fibre bandwith could be achieved by means of

wavelength multiplexing techniques, that would result useful also for enhancing the throughput of

switches. These features strongly push towards an all-optical telecommunication network where

unprecedented and unparalleled possibilities are offered to system and network designers to meet

the requirements of the telecommunication market. New functionalities to the optical network could

also comprehend distribution of radio-satellite signals (CATV, HDTV) and provision of a high-capacity

framework for mobile radio systems, the use of which is rapidly growing world wide.

In the light of these trends, entirely new fields of activity are opened to standardisation bodies.

Fig. 1 shows how CCITT is planning to face these new items in the next study period.

3.1 Standardisation of active fibre amplifiers

The standardisation activities on OFAs are worth of particular mention. In fact, in very few years,

very rapid technological developments have made it possible to transform Erbium doped OFAs from

laboratory to commercial devices. The enormous potentials of OFAs and their near use in

telecommunication systems around 1550 nm have brought the problem of their standardisation to an

international level, in order to fix uniform and homogeneous requirements on OFA performances and

supply adequate informations for users in the choice of suitable devices for the various applications (inline amp., pre-amp., booster).

In its last meeting (May 1992), CCITT has in fact prepared Rec. G.661 [16] concerning the

definition of the relevant parameters of an OFA, seen as a black-box (with at least two optical ports and

electrical connections for power supply), characterizing its transmission and operation properties.

174
As far as the test methods are concerned they will be covered together with reliability and

environmental characteristics by the I EC Generic Specification on Optical Fibre Amplifiers, currently

under development, to which Rec. G. 661 will make reference. lEC (TC86 - WG6) has also developed

a Technical Report (not a standard) on OFA components containing the definitions of the relevant

parameters of the essential components inside the OFA. As future activities, CCITT has planned to develop by 1994 two new Recommendations; the first on the generic characteristics of OFAs, the

second on system aspects of OFAs. lEC will complete the aforementioned documents by 1993 and in parallel will begin to develop section and blank detail specifications, for each particular application of

OFAs. Simultaneously a considerable activity is going to be developed on the system specifications in a way to take into account the introduction of OFAs.

4. Standardisation of systems and components

Because of the high flexibility of the optical network which allows its use at various levels from

small private LANs to broadband services on an international scale, there will be in the immediate future

an enormous demand for optical components and for standards to efficiently interface systems at different function levels. In fact, standardisation of systems and components is mandatory in order to allow users, network and service suppliers to design compatible links and system interfaces using

components from a variety of unrelated suppliers. In addition to this, effective management of complex

telecommunication networks requires sophisticated management systems. Again network suppliers are free to buy equipment from several manufacturers, leading to increasing demand for

standardisation in the fields of operation, management and maintenance too [17]. Some examples are given by FDDI and IEEE 802 standards as regards high-speed data communication LANs, based on

f^M fibres. The ETSI MAN standardisation activity covers aspects of architecture and protocols within

the European public networks and the specification of a public connectionless data service. ETSI has

also set up a MAN Working Party with the purpose of defining MAN architectures, protocols and

services suitable for an evolution towards the future B-ISDN [18]. At CCITT, standardisation activity is in progress for regulating the evolution from ISDN to B-ISDN. These efforts are very important because

they are setting out the framework for the development and introduction of future broadband

networks, regulating the introduction of new sets of services such as CATV or HDTV, or providing

standard access to the existing ones. This would bring about cost benefits for both manufacturers and

service suppliers. However, up to now this is perhaps the least mature area of the standards process.

Some problems have been solved, setting some standards for digital networks, such as SONET and

SDH, but analog (such as CATV) and distribution (such as PONs) network standardisation is still far

behind.

At the level of single components some specification issues are still open. As far as fibre passive

components are concerned, activities are in progress at lEC (SC86B), ETSI (TM1) and CECC (WG27).

In particular, various subgroups of IEC-SC86B (see Fig. 2) are concerned with generic, mid-level and

detailed specifications of joints and connectors (WG1), branching devices, attenuators, multiplexers

(WG2), and relevant test methods, reliability and quality control (WGs 3-5). Work is also in progress to set specifications on switches and optical isolators. It is worth noting that passive optical components

are extensively employed also in OFAs manufacturing.

Particular care requires the characterisation of general branching-devices that can be done in terms of the insertion loss matrix, from which parameters such as excess loss, coupling ratio, directivity,

return loss and isolation can be derived. An important feature of all kinds of passive components, and

particularly connectors, is the Optical Return Loss (ORL), which characterises the amount of backreflected light; presently the specification of ORL is under study in the various Committees. Also the

polarisation sensitivity of branching devices and optical isolators is of concern.

5. Conclusions

The globalisation of economy and the needs for all telecommunication actors (network

operators and manufacturers) to deploy common solutions, ask for the availability of internationally

recognised standards. This is becoming a precondition to be able to successfully offer new services

and the corresponding equipments. Standardisation plays an important role also in the regulation

process and is crucial in the evolution towards an open telecommunication market. Same criteria of

non-discrimination among suppliers, international networking, uniform availability with homogeneous

quality of services and interworking or interchangeability of different equipments, lead to the

conclusion that larger markets are only accessible for services which are either full-standardised or, at least, make use of standardised elements necessary for compatible communications. This should

175
indicate a rapid introduction of standards, in order to avoid problems associated with inconnpatible

implementations.The ISDN represents a clear case in which the lengthy process for harmonisation and

standardisation is one of the factors that have so far delayed the time schedule for its availability. On

the other hand there are some new technologies (e.g. the OFAs) which are still under development,

although already introduced in the market. In this case a judicious, progressive setting of standards will permit an harmonised development of the application without preventing further progress of the

research in the corresponding fields.

References

[I] CCITT Recommendation G.650, "Definition and test methods for the relevant parameters of

single-mode fibres"

[2] CCITT Recommendation G.652, "Characteristics of a single-mode optical fibre cable"

[3] CCITT Recommendation G.653, "Characteristics of a dispersion shifted single-mode optical

fibre cable"

[4] CCITT Recommendation G.654, "Characteristics of a 1550 nm wavelength loss minimised

single-mode optical fibre cable"

[5] International Standard I EC 793/1 , "Optical fibres - Part 1: Generic specifications"

[6] International Standard I EC 793/2, "Optical fibres - Part 2: Product specifications"

[7] CSELT Technical Staff, "Fiber Optic Communications Handbook", TAB/McGraw-Hill, Blue Ridge

Summit (PA), (1990).

[8] M. Artiglia et al. , "Mode Field Diameter ^Measurements in Single-h/lode Optical Fibers", J. Lightwave Technol. Vol. 7, No. 8, pp. 1139-1152, (1989).

[9] K. Petermann, "Constraints for fundamental-mode spot size for broadband dispersioncompensated single-mode fibres", Electron. Lett. Vol. 19, pp. 712-714, (1983).

[10] COST 217 Group, "Interlaboratory measurement campaign on single-mode fibres", lEE Proc.

Pt. J, Vol. 136, No. 6, pp. 307-314. (1989).

[II] B. Walker, COST 217 Group, "COST 217 intercomparison and analysis of fibre mode field

diameter measurements", lEE Proc. Pt. J, Vol. 138, No. 6, pp. 373-378, (1991).

[12] G. Kuyt, COST 217 Group, "COST 217 interlaboratory comparison of uncabled and cabled

fibre cut-off wavelength measurements", Tech. Dig. of the Optical Fibre Measurement Conference,

York (U.K.), Sept. 17-18. pp. 12-15 (1991)

[13] S.A. Jacobs, D. W. Peckham, "The anomalous structure observed in single-mode fiber cutoff

wavelength measurements: theory and solutions", Tech. Dig. of the Symposium on Optical Fiber

Measurements, Boulder (CO), pp. 49-54, (1990).

[14] W.T. Kane. "The first international fibre geometry round robin". Tech. Dig. of the Optical Fibre

Measurement Conference, York (U.K.), Sept. 17-18, pp. 119-122 (1991)

[15] M. Young, "Fiber Cladding diameter by contact micrometry", Tech. Dig. of the Optical Fibre

Measurement Conference. York (U.K.), Sept. 17-18, pp. 123-126 (1991)

[16] CCITT Recommendation G.661, "Definitions and test methods for the relevant generic

parameters of optical fibre amplifiers"

[17] CCITT Recommendation M.30, "Principles for a Telecommunications Management Network"

[18] ETSI NA 5 Working Party on MAN, Report of the meeting , Stockholm, Sweden (October 28-

November 2, 1990).

176
CCITT - COM. XV

Transmission Systems and Equipments

Question Dl/XV:

Characteristics and Test Methods of Optical Fibres and Cables

Question D2/XV:

Characteristics of Optical Systems for Use in Local Access Network

Question D3/XV:

Characteristics of Optical Systems for Inter-Office and Long-Dislance Networks

Question D4/XV:

Transmission-Related Characteristics of Optical Components and Subsystems, Including Optical

Amplifiers

Question D5/XV:

Characteristics of Optical Fibre Submarine Cable Systems

I Question D6/XV:

Reliability and Availability of Optical Systems

Fig. 1 - Proposed structure for Studies on Fibre Optic Communication in CCITT

for the next Study period (1992-1996)

lEC - TC86

FIBRE OPTICS

.WGl: Fibre optic

terminology and

symbology

WG2: Fibre optic

safety aspects

—WG4: Fibre optic lest

.WG6: Optical fibres amplifiers

SC86A

OPTICAL FIBRES

AND CABLES

WGl: Optical

fibres

WG2: Optical cables

SC86B

FIBRE OPTIC

INTERCONNECTING

DEVICES AND PASSIVE

COMPONENTS

SC86C

FIBRE OPTICS

SYSTEMS

SPECIFICATIONS

WGl: Interconnecting devices for optical

fibres and cables

• WG2: Fibre optic passive

components

WG3: Quality assessment

procedures and optical

fibres interconnections WG4: Standard Tests and Measurement Methods for Fibre Optic Interconnecting devices and Passive Components

WG5: Reliability of Fibre Optic Interconnecting Devices and

Passive Components

WGl: Fibre optic

subsystem

WG2: Fibre optic sensors

Fig. 2 - Structure of lEC Groups for standardisation on Fibre Optic Communication

177
1

i

i

I

I

I

I

I

I

178
COST 217 INTERCOMPARISON OF OTDR MEASUREMENTS

ON SINGLE-MODE FIBRES

M.Artiglia, COST 217 Group

*

Abstract

The results of an extensive intercompan'son of OTDR measurements on three types of

single mode fibres, organised in the framework of the European Project COST 21 7, are presented

and discussed. Seventeen laboratories from all over Europe participated to the intercompan'son.

The results show an acceptable agreement, particularly in attenuation measurements. However the

overall reproducibility is still too high and the need emerges of effective calibration procedures for

OTDR machines.

1. Introduction

The backscattering technique, or OTDR [1], is very popular since it provides a powerful and

practical tool for testing optical fibre attenuation, loss of fibre joints and for locating fibre defects or

faults in an installed line. The key of its success is that OTDR is in principle very simple and suitable

for field measurements, because it requires only one fibre-end access. OTDR machines are commercialy available since some years and product engineering has reached an advanced

degree of maturity. In view of industrial testing of SMF OTDR is expected to play a very important

role; in fact it has been recommended as ATM by CCITT for attenuation measurements. In this

perspective, the participants to Working Group 1 of the European Project COST 217 (Optical

measurements techniques for advanced optical fibre devices and systems) have recently

organised and carried out an extensive interlaboratory intercomparison to assess the

reproducibility of OTDR measurements in the laboratories all over Europe. Twenty-three

laboratories from ten Countries (B, CH, D, E, I, ML, 8, SF, UK and TK) took part to the

intercomparison. For the measurements two sets of three fibre bobbins and a special fibre bobbin

were prepared and the participants divided in two groups so as to allow the groups to work in parallel. The fibres were provided by the participants and were: two bobbins of Matched Cladding

fibre (fibres MCI and MC2, about 10 km long), two bobbins of Dispersion Shifted fibre (fibres DS1

and DS2, about 6 km long) and two bobbins of Dispersion Flattened fibre (fibres DF1 and DF2,

about 6 km long). Besides these very good quality fibres, a seventh fibre bobbin with joints, fibre

disuniformities and short length samples, particularly suited to test OTDR measurement set-ups

was prepared and circulated. To avoid direct handling of the fibre spools the fibres were closed in sealed boxes and the fibre ends connectorised with FC/PC connectors. The two fibre sets

(circulated in March 1990) were intended to be used also for a comparison of Chromatic Dispersion

measurements, the results of which have been discussed elsewhere [2]. The special bobbin

(circulated in march 1991) consisted of six fibres about 3 Km long in a loose tube; two of them were

joined together in order to obtain a line 6 km long (referred to in the following as the long line), and

one was made joining together four fibre pieces about 0.8 km long (the short line), see Fig. 1. Presently, the CD and OTDR measurements on the two set of fibre bobbins are still in run whereas

those on the special bobbin are almost concluded. The main results obtained so far will be now

briefly discussed. To evaluate the quality of the measurements the Interlaboratory Reproducibility

(I.L.R.) defined as the standard deviation of all the measurements in the various laboratories will be

used in the following.

2. Discussion of the results

The participants were asked to provide mean value, standard deviation and maximum

deviation for each measured parameter; moreover detailed information was required concerning

the set-up used (assessed accuracy, type of instrument and so on) and the configuration adopted

for the measurements. In all measurements described below the effective refractive index value

neff = 1-4670 was used, independently of the fibre type. All the participants performed their

measurements using commercial instruments. The average reproducibility of the used set-ups

M. Artiglia, P. Di Vita, A. Rossaro, CSELT, Turin (I); M. Blonde!, E. Jaunart, Faculte Polytechnique de Mons,

Mons (B); H. Gilgen. B. Perny, Swiss PTT Bern (CH); J. Vobian, FTZ - DBP, Darmstadt (D); P. Blasco, CCSA,

Zaragoza (E)\; J. Attn, Telefonica, Madrid (E); G. De Marchis, F. Matera, FUB, Roma (I); D. Cuomo, FOS,

Battipaglia (I); E. Uboldi, SIRTI, Milano (I); F. Esposito, Teleco Cavi, Roseto degli Abruzzi (I); G. Kuyt, Philips

Optical Fibre, Eindhoven (NL); E. Sundberg, Telia, Haninge (S); K. Sjdiin, Ericsson Cables, Hudiksvall (S); L. Oksanen. Nokia Cables, Vantaa (SF); B. Walker, NPL, Teddington (UK); Z. Ziya Ozturk, MRC-TObitak, Gebze

(TK)

179
(computed from the figures claimed by the participants) on the various parameters resulted to be as

follows:

att. coeff: Aa = ±0.015 (max values <0.03) dB/km, fibre length: AL = ±5 m,

joint loss: Aajoint = ±0.015 dB, joint location: ALjoint = ±7 m

As far as the measurements on the two sets of fibre bobbins are concerned (23 Labs), the

following parameters were to be measured:

attenuation coefficients at 1300 nm and 1500 nm (a), Fibre Length (L)

The results are summarised in Table 1. As far as the attenuation coefficient is considered a

quite good agreement was found for all fibres at both wavelengths. The I.L.R is less than 0.02

dB/km and is comparable with the average reproducibility of the used set-ups. The only exception

is given by fibre DF2 at 1300 nm; in this case a laboratory measured a very high attenuation

coefficient; by neglecting this value the corresponding reproducibility is substantially improved.

Also for fibre length an acceptable agreement among the laboratories was found. For fibres DS1

and DF1 the I.L.R. is comparable with the claimed average reproducibility of the instruments. Worse

I.L.R.s were found for fibres MC1, MC2, DS2 and DF2, due to the fact that three laboratories

measured values somewhat below average. By neglecting these results the I.L.R.s becomes

smaller than ±7 m. As far as the measurements on the fibre bobbin specially conceived for testing OTDR set- ups are concerened (17 Labs), the participant were asked to measure the following parameters:

long line : attenuation coefficient of fibres 1 and 2 in both transmission windows, total length of

the line, joint loss, joint location

short line : attenuation coefficient of fibres 1, 2, 3 and 4 in both transmission windows, loss of

joints 1, 2 and 3, location of the three joints

Fibres and joints were counted starting from head 1 of the lines (see Fig 1). The results are summarised in Table 2 and 3. Considering first the results of the long line (Table 2), a fairly good agreement at both 1300

nm and 1550 nm (I.L.R. < ±0.02 dB/km for both fibres) was found for the attenuation coefficient

(Figs. 2 and 3). In fact the I.L.R.s are comparable to the average reproducibility of the set-ups

claimed by the participants; the I.L.R. of the second fibre seems to be slightly worse. Total length

of the line results are in good agreement at 1550 nm (I.L.R.= ±5 m); but some problems exist at 1300 nm (I.L.R.= ±18 m) were two laboratories gave results somewhat below the average. By

neglecting these results the agreement is considerably improved. Also for Joint loss (Fig. 4) an

acceptable I.L.R. is found at both wavelengths (note that three participants did not provide

sufficient information to calculate the joint loss). Joint location measurements are in fairly good

agreement at 1300 nm (I.L.R. = ±10 m), but at 1550 nm the agreement is quite poor because one

laboratory measured a value which is far below average; neglecting it the agreement becomes

acceptable.

As far as the short line is considered, a slightly worse agreement is found for the attenuation

coefficients at both 1300 nm and 1550 nm (Figs. 5 and 6), this could be due to the short length of

the fibre samples. The I.L.R. ranges from ±0.02 to ±0.04dB/km. A substantially good agreement at both wavelengths was found for the total lenath of the line (I.L.R. ='±6 m) and the ioint loss, which

shows I.L.R.s comparable to attenuation, see Figs. 7 and 8. For the ioint location a fairly good

agreement is found at 1300 nm (I.L.R. = ±10 m) but some problems exist at 1550 nm where one

laboratory obtained values that are higher than average; by neglecting it the agreement is considerably improved.

3. Conclusions

In conclusion the OTDR instruments have reached a good level of maturity and showed a

fairly good reproducibility. However, the reproducibility values obtained and those claimed by most

of the participants on attenuation (around 0.02 dB/km) are too high to meet the requirements of

fibre producers for quality control and of typical contractual specifications imposed by PTTs to fibre

producers (<0.005 dB/Km). Further work is therefore needed to improve the agreement. Some

Laboratories have to correct their set-up for systematic errors. This issue will be the subject of

scheduled activities in the framework of the recently started Project COST 241 (ideal continuation

of COST 217 which expired few months ago); in particular, calibration of OTDR set-ups will be

180
investigated; this is a very important topic in view of ttie use of this techique on industrial scale for

SMF characterisation.

References

[1] CCITT Recommendation G. 650

[2] COST 217 Group: "COST 217 intercomparison of chromatic dispersion measurements

on single-mode fibres", Proc. of EFOC/LAN '92, Paris, June 24-26, 1992, Paper 12

Table 1 - OTDR mesasurements on long fibre bobbins: mean values and standard deviations (ILR)

no= 1.467 MC1 MC2 DS1 DS2 DF1 DF2

a @ 1300 nm (dB/km) 0.366 0.363 0.425 0.405 0.433 0.458

ILRa (dB/km) 0.006 0.008 0.010 0.077 0.019 0.085

a @ 1550 nm (dB/km) 0.223 0.227 0.255 0.253 0.270 0.271

ILRa (dB/km) 0.004 0.019 0.006 0.053 0.026 0.020

L @ 1300 nm (m) 10172.2 12578.1 6616.3 6382.1 6468.1 6370.9

ILRl (m) 10.8 13.7 7.3 15.2 6.4 12.6

Table 2 - OTDR mesasurements on the special bobbin (long line):

mean values and standard deviations (ILRs)

n=1 .4670 1300 nm 1550 nm

fibre 1 att.(dB/Km) 0.346 0.224

ILRl (dB/Km) 0.015 0.019

fibre 2 att.(dB/Km) 0.508 0.420

ILR2 (dB/km) 0.026 0.025

Total Length (m) 6101 6104

ILR (m) 18 5

Joint Loss (dB) 0.073 0.07

ILR (dB) 0.01 0.01

(*) Joint Location (m) 3056 3064

ILR (m) 10 21

(*) Measured from head 1 of the line

Table 3- OTDR mesasurements on the special bobbin (short line):

mean values and standard deviations (ILRs)

n=1 .4670 1.300 nm iLRnoo 1550 nm ILR1550

fibre 1 att. (dB/Km) 0.34 0.03 0.22 0.04

fibre 2 att. (dB/Km) 0.34 0.02 0.21 0.02

fibre 3 att. (dB/Km) 0.34 0.02 0.20 0.02

fibre 4 att. (dB/Km) 0.37 0.03 0.23 0.03

Total Length (m) 3055 5 3054 6

Joint 1 Loss (dB) 0.63 0.03 0.46 0.03

Joint 2 Loss (dB) 0.75 0.03 0.52 0.03

Joint 3 Loss (dB) 0.24 0.02 0.16 0.02

(*) Joint 1 Loc.(m) 746 1

746 10

(*) Joint 2 Loc.(m) 1393 10 1398 (1395^^) 16(7^)

(*) Joint 3 Loc.(m) 2299 9 2304 (2300^^) 17(9^^)

(*) Measured from head 1 of the line

C") Worst value excluded

181
At

FtbrtaI

JointitFll>r»2

Long lino (*t>oat 6 km)

Fibrt I

Short Une (ibout y tm)

* ^

V Tttn* i Flbf• ) ribr* t

F/p. 7 - Schematic of the measured fibre lines

OTDR BOBBIN LONG UNE

FIbr* *tt*nu*llon eo«fflcl*nl •! 1300 iwn

Fig 2 - Results of attenuation measurements on

the long line @1300 nm (fibres 1 and 2)

Fig 3 - Results of attenuation measurements on

the long line (g>1550 nm (fibres 1 and2)

Fig 4- Results of joint loss measurements on the

long line @1300 nm and 1550 nm

Fig 5- Results of attenuation measurements on Fig 6 - Results of attenuation measurements on

the short line (§>1300 nm the short line (S>l550nm (fibres 1 to 4)

OTDR BOBBIN SHORT UNE

Joint lot* tt 1300 nm

Fig 7 - Results of joint loss measurements on

the short line @1300 nm (joints 1 to 3)

OTDR BOBBIN SHORT UNE

Joint Lo«* tl 1S50 nm

1 2 9 4 S ( 7 I « 10 II 12 13 14 15 IC 17

Fig 8 - Results of joint loss measurements on

the short line @1550 nm (joints 1 to 3)

182
COST 217 INTERCOMPARISON OF MEASUREMENTS ON

ER-DOPED FIBRES

p. Kilveri, COST 217 Group (*) VTT, Technical Research Centre of Finland, Otal<aari IB. 02150 Espoo (SF)

(*) C.A. Millar BTRL, (UK ); M. Monerie, J.F. Bayon, CNET. (F); M. Artlglla, CSELT, (I); M.A. Rebolledo, University of Zaragosa, (E); M. Settembre FUB, (I); J. Voblan, FT2, (D); K. Dybdal Jutland Telephone. (DK); B. Walker NPL, (UK); M. Babellowsky, RTF

Research, (NL); S. Wingstrand, Telia. (S); N. Gisin University of Geneva, (CH); P. KJIveri, VTT (SF); J. Atin, Telefonica, (E); W.E.

Helnlein (University of Kaiserlautem, (D)

Abstract

An intercomparison of measurements on three Er-doped fibres has been organised in the working group 3 of the

European COST 217 project (optical measurement techniques for advanced optical fibre systems and devices). A set

of fibre parameters, including core diameter, refractive index difference, mode field diameter, cut-off, absorption,

fluorescence peak wavelength, time constant, doping concentration and gain, have been measured using samples

from three fibres made by three participating laboratories. The measurement results and the properties of the three

fibres with different core compositions are compared.

1 Introduction

The COST 217 group has characterized three laboratory made Er-doped fibres, of which almost no data was

available before the measurement campaign. Most of the fibre parameters were measured by several participants

and could be compared. Results of some parameters that were measured only by one or two participants are given to complete the fibre parameter sets. The information of the fibre parameters is needed e.g. for understanding and

modelling the operation of the amplifiers, optimising the amplifier structures and for designing better active fibres.

2 Used Er-doped fibres

The measured samples were taken from three different erbium doped fibres. The long fibres were cut to short

samples with original lengths of 5.5m (samples from fibre I), 14.5m (from fibre II) and 13m (from fibre III). Each of the

14 participating laboratories got a sample from every fibre. The homogeneity of the fibre 111 was checked using the

OTDR before the fibre was cut to samples. The homogeneity of the fibre I and fibre II was not checked. The fibres were made by the British Telecom Research Laboratories BTRL (UK), Technical Research Centre of Finland VTT

(SF) and Centre National d'Etudes des Telecommunications CNET (F).

2.1 Fibre compositions

The three fibres had different core compositions. The fibre I had Ge/AI/Er doped core, fibre 1! had Al/Er doped

core and fibre III had Ge/Er doped core. One laboratory was able to measure the Er-concentrations from fibre

samples using SEM/EPMA (scanning electron microscopy/electron probe micro analysis) technique (table 1 ).

3 l\/leasured fibre parameters (passive)

3.1 Core diameter

The core diameter results showed that the fibre II had elliptical core (with eccentricity of about 0.6). The standard

deviations (S.D.) of the core diameter results of the three fibres were about 3% of the mean values, except the S.D.

of the short axis of the fibre II, which was 8.4% (tables 2-4).

3.2 Refractive index difference

The S.D. of the refractive index difference of the fibre I was 9.4%, of fibre II It was 6.8% and of fibre III 3.0% of the

mean value (fig. 1., tables 2-4). It is Interesting to notice that the S.D. is worse with the cores with higher An and Al- doplng than the S.D. of the Ge-doped fibre III with lower An. Because the reproducibility of the cut-off measurements

was good (the S.D. of all three fibres was < 1%), it is probable that the high S.D. of the An is not only connected to the variations of the refractive index difference along the fibres but also to the accuracy of the measurements.

3.3 Mode field diameter

The mode field diameter (MFD) measurements were done using commercial devices or laboratory measurement

set-ups calibrated during the earlier COST 217 standard singlemode fibre measurement campaign [1,2]. Petermann II definition was used for the mode field diameters. The MFD results of the participant N (tables 2-4) are not included in the standard deviation, because the measured values were systematically higher than the values of the other three

participants. The standard deviations of the mode field diameters (excluding N results) at 1.3|j.m / 1.55p.m

183
wavelengths were: fibre I 2.2% / 1.4%, fibre II 2.9% / 0.0% and fibre III 1.7% / 1.9%. These values can be

compared to the COST 21 7 dispersion shifted fibre MFD measurements, where the S.D. was 3.5% / 4.0% when

several measurement methods were used [2].

3.4 Cut-off

The cut-off measurements have been made using the standard bending method. The S.D. of the cut-off

wavelength measurements was less than 10 nm (<1% of the mean values) for all three fibres. The standard

deviations are about the same as in previous COST 217 matched cladding fibre measurements (S.D.=11 nm), but

smaller than in dispersion shifted fibre measurements (S.D.=16.7 nm) [3].

3.5 Absorption at the peak near 1.53 \im

The absorption values were measured using cut-back method. The absorption mean values and standard

deviations (%) of the three fibres at 1.53 \im (fig. 2 and tables 2-4) were: 4.4 dB/m, 5.1% (fibre I); 6.3 dB/m, 2.4%

(fibre II) and 12.3 dB/m, 5.8% (fibre III). These absorption results could be compared with the absorption values

calculated using the measured Er-concentrations (table 1 ), but the accuracy of the concentration measurements is possibly too low for a meaningful comparison.

4 Measured fibre parameters (active)

4.1 Fluorescence peak wavelengths

The measured fluorescence peak wavelength mean values were: Fibre I: 1531.5 nm (number of results N=2,

S.D.=0.5nm), Fibre II: 1530.9 nm (N=5, S.D.=0.9nm) and Fibre III: 1537 nm (N=4, S.D.=0.5nm). Pumping

wavelengths of 514 nm, 980 nm and 1481 nm with different pumping powers and sample lengths from 0.1m to 14m

were used. Due to the aluminium doping in the fibre I and II cores the fluorescence peak wavelengths of those fibres were shorter than the peak wavelength of the fibre III (fig. 2).

4.2 Fluorescence time constants

The fluorescence time constants (1/e life times) were measured by one participant and only from fibres II and III. The values were 1 1 .5±0.2 ms (fibre II) and 12.0±0.2 ms (fibre 111). Pumping wavelength was 514 nm and fibre lengths

2 cm.

4.3 Small signal gain

The gains of short fibre samples (L=2m) were measured by one participant. The gain of the fibre III (Ge-core) was

smaller than the gain of the fibre I and 11 (Al-doped cores) at 1536 nm (fig. 4). The gain with long samples of the fibre

II was measured by three participants. The signal wavelengths varied from 1536 nm to 1550 nm and the pumping

wavelengths from 1478 nm to 1486 nm. Also the signal powers varied. It is obvious that the gain measurement set- ups should be standardised to get comparable results (fig. 3).

5 Summary and conclusions

A measurement campaign with three laboratory made erbium doped fibres with different core compositions has

been completed. The results are summarised in the tables 1-4 and in figures 1-4. Similar measurement methods as

for standard telecommunication fibres were used for MFD, cut-off, core diameter, absorption and refractive index

measurements. For cut-off and absorption measurements non-destructive methods could be preferable, because

active fibres are quite expensive. The interlaboratory reproducibility of the MFD, cut-off, and fluorescence peak

wavelength measurements was quite good (S.D. <3%). The interlaboratory reproducibility of the refractive index, core diameter and absorption measurement results was lower (S.D. 5.8%-9.4%). There were not enough data for

comparison of the Er-concentration and fluorescence time constant measurements. Er-concentration analysis from

fibre samples appeared to be difficult due to the low Er-concentration levels used.

Because the gain depends on pump and signal wavelengths, standard pumping and signal wavelengths for

measurement set-ups should be defined to make the gain measurements comparable.

References

[1 ] COST 21 7 Group. "COST 21 7 Interlaboratory measurement campaign on single-mode fibres", lEE Proc., vol. 136. R. J, No 6, pp. 307-314, Dec. 1989.

[2] COST 21 7 Group. "COST 21 7 Intercomparison and analysis of fibre mode field diameter measurements", lEE

Proc, vol. 1 38, No 6, pp. 373-378, Dec. 1991.

[3] G. Kuyt, COST 217 Group. "COST 217 Interlaboratory comparison of uncabled and cabled fibre cut-off

wavelength measurements". Optical Fibre Measurement Conference, York, UK., 1991, pp. 12-15.

184
APPENDIX i

Delta N: 0.67E 2 Delta N: 0.840 2 Delta N: 0.S81 2

UicI II II C 4 I a i 4 C I It 12

a) Rbre I b) Rbre II

Fig. 1 . Refractive Index profiles of the three Er-doped fibres.

l.'llt

\.<in

l.4(]B

i.<tn

l.ttIB

i.<(n

i.<s»

i.»n

l.<S7t

4^

. l :..l M . h i.l ; .,i,:,l..:i::i l .,,l,

la)c} IJ II I C ' ; e ? 4 E 8 IB I?

c) Fibre 111

7.00

Fluotaicanc*

a) Fibre I b) Rbre II c) Fbre III

Fig. 2. Absorption and fluorescence curves of the Er-flbres.

Fibre II gain results Gain resutts L=1.9...2.2 m

14.Sm, U66nm

Gws1536nm,

P8=-34.1dBm

- \ZBm, 1481nm, 6Wo1537.9nm,

Pte-24.1dBm

- L»:13m, 1476nm

GWr>1530nm

- U13m, 1478nm ew=1550nm

- Rbfo 1, 1.9m, Ps=-3S.7dBm

' Rbre II, Z2m,

Ps=-32.1dBm

• Rbf« 111. 2.0m. P6=-35.6dBm

Pump power (mW) Pump power (mW)

Fig. 3. Gain of the fibre II as a function of coupled

pumping power.

Fig. 4. Gain of the short fibre samples as a function

of coupled pumping power. Signal wavelength was

1536nm.

185
APPENDIX II

Table 1. Measured Er-concentrations in the fibre I, II and III cores.

Measured by Fibre Er3+/wt% Er203/wt%

F 1 0.04 0.04 F II 0.05 0.05 F III 0.07 0.08 B* III 0.13-0.20

') Estimated from the absorption values

Table 2. Measured fibre I parameters.

MEASURED <t> core MFD®1.3/1 .55 An Cut-Oft Att @1.53

BY: pm pm e(-3) nm dB/m

A 5.5 13.5 1290 4.2 B 5.7 11.0 1290 4.2

1 6.2 1308 4.5 K 6.2 7.1 1309 4.5 L 5.6 / 6.0 6.5 6.9 11.9 1309 4.4 M 1308 4.8 H 5.5 / 5.7 f6.81 [8.01 11.7-12.2 1290 4.1 O 5.8 9.9 MEAN 5.7 6.3 7.0 11.7 1301 4.4 S.D. 0.16 0.14 0.10 1.1 9.2 0.22 S.Dymean (%) 2.9% 2.2% 1.4% 9.4% 0.7% 5.1%

Table 3. Measured fibre II parameters.

MEASURED <t>x/4>y core MFD@1.3/1.55 An Cut-off Att @1.S3

BY: |jm Mm e(-3) nm dB/m

M [4.31

* 11.5-11.8* 6.1 B [4.21 11.8-13.3 1086 6.3 C 1100 6.3 F 4.0 / 5.1 12.2 1090

1 5.6 1093 6.0 K 6.0 6.7 1090 6.4 L 4.5 / 5.5 5.7 6.7 10.5-12.2 1088 6.2 M 1088 6.2 N 3.8 / 5.1 [6.91 [8.41 11.7-12.0 1096 6.5 O 3.6 / 5.1 10.4 MEAN 4.0 / 5.2 5.8 6.7 11.7 1091 6.3 S.D. 0.33/ 0.17 0.17 0.0 0.8 4.4 0.15 S.DVmean % 8.4% / 3.3% 2.9% 0.0% 6.8% 0.4% 2.4%

*) Measured from ttie preform

Table 4. Fibre III parameters.

MEASURED <t> core MFD®1.3/1.55 An Cut-off Att @1.53

BY: pm pm e(-3) nm dB/m

B 6.4 8.0 1125 12.6 C 1150 11.9 D 12.5 F 6.0 8.5

1 7.3 1138* 12.9 K 7.2 8.1 1142 11.3 L 6 7.0 7.8 8.8 1155 11.1 M 1143 13.2 N 5.9/6.3 [7.41 [8.81 8.2-8.3 1155 12.7 O 6.1 8.3 MEAN 6.1 7.2 8.0 8.4 1144 12.3

S.D. 0.18 0.12 0.15 0.25 9.9 0.71 S.DVmean (%) 2.9% 1.7% 1.9% 3.0% 0.9% 5.8%

*) Bending reference r=4cm

186
SINGLE-MODE FIBER GEOMETRY AND CHROMATIC DISPERSION:

RESULTS OF INTERLABORATORY COMPARISONS

Timothy J. Drapela, Douglas L. Franzen, and Matt Young

National Institute of Standards and Technology

325 Broadway

Boulder, CO 80303

The National Institute of Standards and Technology (NIST) recently completed two interlaboratory comparisons

with members of the Telecommunications Industry Association (TIA). Participants included most major fiber and

cable manufacturers in North America. We evaluated test procedures for geometry and chromatic dispersion. In both cases, the measurement spread in the industry was due mostly to systematic differences among laboratories.

Measurement agreement would be significantly improved through the use of reference fibers.

GEOMETRY

A previous international geometry comparison (1989), organized through CCITT and coordinated in North

America by NIST, gave a spread (1 a) of 0.38 /zm for cladding diameter. An interim report concluded that the

agreement should be improved if the industry wanted to further reduce tolerances. Participants in the comparison

received fiber samples consecutively cut from fiber spools; consequently, sample nonuniformity could have

influenced the results. In the present study, participants used the gray scale method to measure cladding diameter, noncircularity, and

concentricity error. The gray scale method, based on video microscopy, is the most common method used by

industry and is described in TIA test procedure FOTP-176.' A near-field image of a cleaved fiber end is obtained

with a video microscope. The outside boundary of the cladding is digitized, and an "edge table" is compiled.

Cladding diameter is then determined by curve-fitting to the edge table data. Calibration is accomplished by

measuring reticles and/or reference fibers of known dimension.

Participants in the TIA comparisons measured the same cleaved fiber ends, so sample uniformity was not an

issue. An individual fiber was contained in a protective metal housing designed to fit into a gray scale system.

Fig. 1 . In this configuration, the fiber is extended for measurement; the brass barrel can rotate and is indexed to indicate a specific angular orientation.

Results from TIA members were compared to the diameters measured at NIST with a contact micrometer and

a scanning confocal microscope (SCM).^ The contact micrometer measures fiber diameter approximately 1 mm from

the cleaved end. The micrometer jaws consist of a fixed cylinder (the anvil) and a moveable spindle, both made

of polished, hardened steel. Differential displacement of the spindle is measured by an optical interferometer. In the SCM, the fiber endface is scanned across a focused HeNe laser beam, and the location of the cladding edge is determined by observing the reflected light. The contact micrometer and SCM have been compared, and cladding

diameter measurements agree within 30 nm. When measuring the comparison fibers, NIST measured cladding

diameter at four angular orientations: 0°, 45°, 90° and 135°.

Comparison of cladding diameters of five fiber samples are shown in Fig. 2. The measurement spread (1 a),

averaged over all fibers, for the eight participants is 0. 15 /zm. This is a substantial improvement over the previous

comparison and reinforces the need to measure the same fiber ends. Results shown in Fig. 2 are the differences

for each measured fiber with respect to the NIST contact micrometer (solid horizontal line). The NIST value for a given fiber is the average of the 0°, 45°, 90°, and 135° diameters. The square data points are the four-diameter

participant averages, whereas the circular points are the participant values using the curve-fitting method described

in FOTP-176. With the exception of Participant 4, there is no statistical difference between the two diameters.

Figure 2 indicates that the overall measurement spread is largely due to systematic differences among participants;

that is, a given participant's results are closely grouped. Table 1 is the average offset and standard deviation of the

offset for each participant with respect to NIST. The average offset standard deviation among gray scale participants

is 0.027 pm. If each participant's data were corrected for an offset, the overall measurement spread would

dramatically decrease; this emphasizes the need for reference fibers. Comparison results were also obtained for

noncircularity, with a worst-case measurement spread (1 a) of 0.07 %, and for concentricity error, with a worstcase measurement spread (1 a) of 0.04 /zm.

187
CHROMATIC DISPERSION

The comparison of test procedures for measuring chromatic dispersion in single-mode fibers was carried out over a two-year period, from mid-1989 through late 1991. Seven fibers, donated by various fiber manufacturers, were used in the comparison. Five of the fibers were dispersion unshifted; two were dispersion shifted. The fibers were color coded for identification and put into a loose-tube buffer. This cable was then cut into 5.1 km and

2.7 km lengths. Participants were asked to measure the zero-dispersion wavelength Xq, the dispersion slope at that

wavelength, Sq, and the dispersion coefficient D(A,) at 1310 nm, for both lengths of fiber.

Six participants used the differential phase shift method, in which D(A.) is determined from the differential

group delay between two closely spaced wavelengths. Xq and Sq are determined by curve-fitting to the dispersion

data. The source is typically one or more sinusoidally modulated LEDs filtered by a monochromator. Further

details of this method are given in FOTP-175.'

Three participants used the phase shift method, in which spectral group delay data are obtained from measuring

the relative phase shifts of modulated sources at three or more wavelengths. Xq, Sq, and D(k) are determined from

equations fitted to the spectral group delay. The sinusoidally modulated source can be either multiple laser diodes or filtered LEDs. Further details of this method are given in FOTP-169.'

Three participants used the time domain method described in FOTP-168.' In this method, a Nd:YAG fiber

Raman laser or several laser diodes are used, and the spectral group delay data at several wavelengths are obtained

directly in the time domain. Dispersion parameters are obtained by fitting to the spectral group delay data.

For dispersion-unshifted fibers, the fit to the group delay for all three methods is a 3-term Sellmeier equation;

the dispersion is the first derivative of this equation.

We restrict our analysis here to the disp>ersion-imshifted fibers. Overall results have been calculated, in the

form of mean values ± 1 a for each fiber. Long and short lengths exhibit very similar behavior in these statistics. The overall measurement spreads (la) averaged over the five long dispersion-imshifted fibers, are 0.93 nm for Xq,

0.0016 ps/(nm2-km) for So, and 0.08 ps/(nm-km) for D(1310 nm).

We also examined the relative offsets among participants. Figure 3 shows the offsets from the average

measured zero-dispersion wavelength value for each fiber, for each of the participants. Each point for a given

participant represents one of the five long dispersion-unshifted fibers. Similar plots have been made for Sq and

D(1310 nm). These three graphs are similar in the conclusions that can be reached. There are no readily

identifiable systematic offsets between measurement methods. There are, however, clear systematic offsets among

participants.

Analysis can be done on the statistics of these offsets. An average offset taken over all five dispersionunshifted fibers and the standard deviation of the offset is calculated for each participant. This standard deviation

is related to the participant's random error. Table 2 shows the average value of the zero-dispersion wavelength

offset and the offset standard deviation for each participant, for the long fibers; the average offset standard deviation

is 0.16 nm. For the short lengths, the corresponding value is 0.35 nm. In this case, the length of the fiber has

a significant effect. The average offset standard deviations for the long fibers are 0.0004 ps/(nm^-km) for So and

0.016 ps/(nm km) for D(1310 nm). For the short fibers, these values increase to 0.0008 ps/(nm^ km) for Sq and

0.03 ps/(nm-km) for D(1310 nm). The average offset standard deviations are all roughly twice as high for the short

lengths than for the long lengths. This implies larger random error for shorter lengths. The main reason for

including long and short lengths in this study was to verify this effect. Analysis of the dispersion-shifted fibers is more complicated but is worth brief mention. The overall statistics for these fibers compare favorably with the previous five fibers. The offset statistics, on the other hand, are not

as precise. The average offset standard deviations are roughly double.

This comparison shows acceptable overall spread in dispersion measurements by three different methods, with no apparent method-dependent offsets. Also, for these lengths of fiber, differences are due to systematic offsets

among participants, with the random error small. If better agreement is desired, calibration fibers could be used

to significantly reduce the offsets. This possibility is currently being examined.

ACKNOWLEDGMENTS

We thank the following organizations for their participation in these comparisons: Alcatel Cable Systems,

Inc.; AT&T Bell Technologies; Bellcore; Coming, Inc.; GTE Laboratories, Inc.; Litespec, Inc.; Northern Telecom;

Photon Kinetics; Raynet Corporation; Siecor Corporation; Spectran Corporation; York Technology, Inc. We thank

Casey Shaar of Photon Kinetics for providing the design of the fiber housings.

188
REFERENCES

1 . Fiberoptics Test Procedures (FOTPs) can be obtained from: Telecommunications Industry Association-Electronic

Industries Association, 2001 Pennsylvania Avenue, N.W., Washington, D.C. 20006.

2. M. Young, S. E. Mechels, P. D. Hale, "Optical Fiber Geometry: Accurate Measurement of Cladding

Diameter," paper presented at Conf. on Precision Electromagnetic Measurements, Paris, June 9-12, 1992; IEEE

Trans. Inst, and Meas. (submitted).

FRONT

VIEW -5.7^ l-e

Cleaved

Fiber End

SIDE VIEW

Fiber Extended

Brass Barrel/

Plunger

1

71.2

Figure 1 Diagram of fiber housing used in the geometry comparison, showing brass barrel pushed forward and

hence, fiber end extended. (Dimensions are in millimeters.)

U

0

-(-> 0)

6

O

u

o a

0)

cdu

CO

(d

o t3

-l->

0.3 n

0.2 -

0.1 -

0.0

-0.1 -

-0.2 -

-0.3

CLADDING DIAMETER

00000 Diameter from Curve Fit

ODD Ave. of 4 Diameters (0°. 45°. 90°. 135°)

Gray Scale Participants

1 I

"3 \ I I

Participant

Scanning

Confocai

Microscope

Figure 2 Participant offsets from NIST contact micrometer average values of cladding

diameter, for five comparison fibers. Gray scale offsets of best-fit and 4-diameter average

values are shown. SCM values are shown for comparison.

189
CLADDING DIAMETER

GRAY SCALE BEST-FIT OFFSETS FROM

NIST CONTACT MICROMETER

4-DIAMETER AVERAGE

Partici nant Average

Offset, /im

Standard

Deviation, /xm

1 -0.127 0.041

2 -0.131 0.025

3 -0.246 0.012

4 -0.229 0.029

5 -t-0.021 0.029

6 -1-0.052 0.019

7 -0.017 0.028

8 -1-0.163 0.030

Avg. 0.027

Table 1 Average and standard deviation of offsets

of gray scale best-fit values from NIST contact

micrometer four-diameter average values of

cladding diameter, taken over five housed fiber

samples, for each participant.

ZERO-DISPERSION WAVELENGTH

OFFSETS FROM AVERAGE VALUES

LONG FIBERS (5.1 km)

Participant

Average

Offset, mn

Standard

Deviation, nm

1 -0.86 0.08

2 -»-0.21 0.11

3 -0.28 0.12

4 -1-0.85 0.08

5 -1.10 0.15

6 -1-0.43 0.13

7 -1-1.09 0.08

8 +0.26 0.14

9 -1-1.04 0.06

10 -1.79 0.16

11 +0.61 0.51

12 -0.45 0.35

Ave. 0.16

Table 2 Average and standard deviation of offsets

from average values of zero-dispersion wavelength,

taken over five long dispersion-unshifted fibers, for

each participant.

ZERO-DISPERSION WAVELENGTH

Five dispersion unshifted fibers — 5.1 km length

differential phase shift phase shift . time domain

1 I ? T

Peirticipant

Figure 3 Participant offsets from average values of zero-dispersion wavelength,

for long lengths of five dispersion-imshifted fibers. (Five points per participant

correspond to measurements on five fibers.)

190
Moderate-Accuracy Wavelength Standards

for Optical Communications

S.L. Gilbert, T.J. Drapela, and D.L. Franzen

National Institute of Standards and Technology

Electromagnetic Technology Division

325 Broadway, Boulder, Colorado 80303

INTRODUCTION

Wavelength standards in the 1.3 and 1.5 regions are important for proposed

optical communication schemes involving wavelength and frequency division multiplexing.

Such systems will increase the information-carrying capacity of a fiber by simultaneously

transmitting many different wavelengths. The first systems that will utilize this type of

multiplexing will probably have channel separations on the order of 1 nm (» 130 GHz). To

define each channel and prevent channel overlap, a moderate-accuracy wavelength reference

(accuracy of about 0.01 nm, or 7 parts in 10^) will be needed for each channel. Such

references could also be used to calibrate optical spectrum analyzers in these wavelength

regions. Wavelength references with higher accuracy will be required for systems with

smaller channel spacing.

We have designed an apparatus for moderate-accuracy wavelength standards which

uses molecular absorption lines as references. For absolute wavelength standards, a stable,

reproducible reference is necessary. Natural resonances of atoms or molecules are ideal,

since their wavelengths are not subject to large variations due to environmental changes such

as temperature or pressure. Unfortunately, atomic and molecular reference lines in the

1.3 /wm and 1.5 jum regions are not common; atomic transitions initiate from excited states,

and molecular lines are weak overtone or combination bands.

We probe molecular resonances by illuminating a gaseous sample of molecules with

light from an LED and observe the characteristic absorption lines using an optical spectrum

analyzer. The system is simple and compact, and provides a series of reference lines in the

1.5 fxm region. We are presently working on a similar approach for the 1.3 /xm region.

1.5 fim APPARATUS

Our 1.5 fjLm wavelength reference apparatus is based on the absorption of light by the

acetylene molecule. Of the molecular absorbers identified to date, acetylene has the

strongest lines in this region. The Vi + vibrational-rotational band of the '^CjHj acetylene

molecule has a clean spectrum containing approximately 40 clearly distinguishable lines with

191
spacings of about 0.5 nm between 1.51 /im and 1.54 ^m.^ A similar spectrum shifted about

8 nm toward longer wavelengths can be obtained from '^CjHj.^ If additional lines are

required, the '^C'^CHj spectrum' could also be used.

A schematic diagram of the apparatus is shown in Fig. 1. Light from a fiberpigtailed 1550 nm edge-emitting LED is collimated and passed through a sealed absorption

cell containing ^^CjHj acetylene gas. The transmitted light is coupled into an optical fiber

and is then observed on a diffraction-grating optical spectrum analyzer. The acetylene

absorption lines appear as dips on the 70 nm wide LED emission spectrum. Figure 2 shows

a scan over the acetylene absorption spectrum using this apparatus. The absorption lines are

labeled according to the molecule's rotational-vibrational structure. The vacuum wavelengths

of these lines, given in Table 1, have been measured to an accuracy of ±0.001 nm.'

The system was designed to be rugged, compact, and stable. With the exception of

the path through the acetylene cell, the LED light was guided in single-mode fiber.

Coupling of the light into and out of the acetylene cell was accomplished using fibers which

were fused to graded refractive index (GRIN) rod lenses. The first lens produced a

collimated beam of light which passed through the 5 cm long acetylene cell and was coupled

into the output fiber by a second GRIN lens. The acetylene cell was secured in an aluminum

frame, and the GRIN lenses were held in aluminum plates which were rigidly locked to the

cell frame after alignment. A simple electronic circuit using 5 VDC supplies and resistance

networks supplied current for the LED and its thermoelectric cooler. The cell was

constructed out of glass tubing with windows fused onto the ends and a small tube on the

side for filling. It was first evacuated and then filled to a pressure of 4 x 10^ Pa (300 Torr).

Figure 3 shows a scan over part of the acetylene spectrum on an expanded scale. The

linewidths of the absorption lines are limited by the 0.1 nm instrumental resolution. The line

centers can be easily determined to an accuracy of 0.01 nm. Sakai, Sudo, and Ikegami^ have

studied the effect of temperature and acetylene pressure on the center wavelengths of the

acetylene absorption lines. They concluded that the temperature sensitivity is less than

100 kHz/K and the pressure sensitivity is less than 1.5 kHz/Pa. This sensitivity is very small

compared with the desired accuracy of0.01nm(-' 1.3 GHz).

1.3 fim APPARATUS

We are presently investigating the use of methane (CHJ as a reference for the 1.3 fim

region. The P2 +21/3 band, centered at 1.33 fim, is very weak and requires a longer

absorption path. We have constructed an apparatus which is very similar to that described in

the previous section with the substitution of a 15 cm long absorption cell. With a methane

pressure of 4 x 10* Pa, we observed an absorption of 15% on the 1.331 /xm Q branch line'*

of this band. Other molecular candidates for references in the 1.3 /xm region are ammonia

and hydrogen fluoride.

192
CONCLUSIONS

We have constructed a simple apparatus for producing a series of reference lines in

the 1.5 /im region. We are investigating a similar approach for the 1.3 ^m region. These

LED/molecular absorber systems provide wavelength references with accuracies of 0.01 nm,

which is sufficient for wavelength division multiplexed optical communication systems with

channel spacings of about 1 nm.

REFERENCES

1. P. Varanasi and B.R.P. Bangaru, "Intensity and Half-Width Measurements in the

1.525 ^m Band of Acetylene," J. Quant. Spectrosc. Radiat. Transfer 15, 267 (1975).

2. Y. Sakai, S. Sudo, and T. Ikegami, "Frequency Stabilization of Laser Diodes Using

1.51 - 1.55 ^m Absorption Lines of ^^€2^2 and '^C2H2," IEEE J. Quantum Electron.,

28, 75 (1992).

3. A. Baldacci, S. Ghersetti, and K. N, Rao, "Interpretation of the Acetylene Spectrum

at 1.5 nm," J. Mol. Spectrosc. 68, 183 (1977).

4. K. Chan, H. Ito, and H. Inaba, "Optical Remote Monitoring of CH4 Gas Using LowLoss Optical Fiber Link and InGaAsP Light-Emitting Diode in 1.33-/im Region,"

Appl. Phys. Lett. 43, 634 (1983).

FIGURE 1 Schematic diagram of the 1 .5 /zm wavelength reference apparatus.

Aluminum Frame

193
1511.0 1526.0 1541.0

Wavelength (nm)

FIGURE 2 Scan over the acetylene Vi + absorption band with a resolution of

0.1 nm. Wavelengths for the labeled lines are given in Table 1. The intermediate

lines {R22/ R20' ••) are also accurately measured.

TABLE 1 Vacuum Wavelengths of Selected

Lines in Figure 2 (from Ref. 3)

Line Line (nm)

R23 1513.97 PI 1525.76

R21 1514.78 P3 1526.87

R19 1515.59 P5 1528.01

R17 1516.44 P7 1529.18

R15 1517.31 P9 1530.37

R13 1518.22 P11531.59

R11 1519.14 P13 1532.83

R9 1520.08 P15 1534.10

R7 1521.06 P17 1535.39

R5 1522.06 P19 1536.71

R3 1523.09 P21 1538.06

R1 1524.14 P23 1539.43

1519.1 1520.1

Wavelength (nm)

1521.1

FIGURE 3 Scan in the vicinity of R9

(0.1 nm resolution).

194
Characterization of Fiber-Optic Amplifiers

Douglas W. Hall

Opto-Electronics Group, Coming Incorporated

Coming, NY 14831

The telecommunications industry is on the verge of a third fiber-optic revolution. The

first revolution occurred in the early '70s with the invention of low-attenuation fiber itself.

Multi-mode fibers, PIN detectors, and LED light sources allowed communication over guidedwave media with bandwidth-length products approaching those of free space links. Eventually,

with the improvement in fibers and the advent of reliable semiconductor lasers, signal dispersion

in the media, rather than attenuation, became the factor limiting increased bandwidth-length

product systems. The second revolution occurred in the mid-1980s with the wide-spread

deployment of single-mode fiber systems. The use of a single spatial mode reduced dispersion,

making attenuation once again the limiting factor in system bandwidth-length product. The third

revolution is the upcoming deployment of fiber-optic amplifiers; their use will shift the limit of

a system's bandwidth-length product back to dispersion.

As with any rapidly developing technology, ideas and their scientific demonstration have

outstripped the detailed study and consensual agreement of measurement techniques that is

required before widespread acceptance of the technology occurs. The international standards

bodies, such as CCITT and lEC, to whom we normally look for definition of standard

measurement techniques, are only now defining what a fiber-optic amplifier is and what its

important parameters are, let alone techniques on how to reliably obtain values of those

parameters. This paper describes the Coming group's view of the state of measurement

development and highlights some of the remaining challenges.

First we must define the beast we are trying to characterize. For example, an lEC

subcommittee created to study optical amplifiers has preliminarily agreed to define a fiber-optic

amplifier using the concept of a "black box" with two optical ports; this allows manufacturers

latitude to bring different and evolving technologies to bear on solving optical, environmental,

and reliability problems, without the need to specify the internal workings of their devices. The

simplest manifestation of a fiber-optic amplifier consists of a pump laser, a wavelength division

multiplexer (WDM) and a length of Er-doped fiber. In order to have a complete survivable

amplifier, other elements are required, such as pump-laser controllers, power supplies, monitor

optics (e.g. tap couplers) and control circuitry, optical isolators, telemetry circuitry and

telemetry signaling method. Different manufacturers are currently offering many combinations

of these elements as the market-place decides what assortment of elements it wants to define as an optical-fiber amplifier. The optical schematic diagram of one version of an amplifier

specified for a power amplifier application is shown in Fig. 1

.

195
The primary parameters that characterize the two-port black box are common to electrical as well as optical amplifiers: gain, bandwidth, noise, and distortion. Other secondary

parameters of importance that must be specified to understand the impact of a fiber-optic

amplifier in various applications include amplified spontaneous emission (ASE) optical power

spectral density, the amount of pump-light leakage to the amplifier ports, the return loss of

signal light from the input and output port, and the out-of-band insertion loss. For two of the

primary parameters, gain and noise, measurements based on optical and electrical-based

techniques have appeared in the literature. It is our group's opinion that all four of the primary

measurements must be made in the electrical domain since the purpose of lightwave systems,

amplified or not, is to transport electrical signals. In other words, measurements of optical

amplifiers should employ techniques that are most directly related to what system designers need

to know. Secondary parameters can be measured either optically or electrically since they may

be manifestations of the uniquely optical nature of the signal transmission. All the primary

amplifier parameters can, in principle, be measured using a single experimental set-up, shown

schematically in Fig. 2. In practice, it is unlikely that a single laser light source will combine

the features of wavelength tunability, low noise, and high-linearity required for the bandwidth,

noise, and distortion measurements, respectively. In the following, issues associated with some

of the primary measurements are discussed.

Gain

In a gain measurement, one must separate the optical power associated with the signal

from the co-propagating ASE power. This is most easily done by modulating the input signal

and determining the modulated or AC gain of the amplifier. Using the equivalence of AC and

DC gain of Erbium amplifiers that occurs for modulation frequencies above 10 kHz, the average

signal power out of the amplifier is the input power times the AC gain.

While the principle of a gain measurement is fairly straightforward, the high gain of fiber

amplifiers can lead to erroneous results in the small-input power regime unless great care is

taken to reduce reflections within the measurement system. Reflection of either signal or ASE

back into the amplifier can cause saturation of the amplifier's gain with signal powers well below

the saturation level. An example of the impact of reflection at the output of amplifiers with

small-signal gain of 42 dB and 31 dB is shown in Fig. 3, where the apparent small-signal gain

is plotted vs. reflection.

Another challenge that has not yet been addressed is the measurement of the gain of an

amplifier for a specific signal wavelength in the presence of other wavelengths or bands of

wavelengths. This information is necessary for the design of WDM systems and also singlewavelength systems consisting of chains of amplifiers where ASE may build up along the chain

to a level where it affects gain at signal wavelength(s).

196
Noise

Noise measurements are perhaps the least understood and most difficult of the primary

amplifier measurements. The goal is to obtain a figure-of-merit that quantifies an amplifier's

contribution to a system's total noise. That figure-of-merit presently used is the amplifier's

noise figure, defined as the logarithm (in dB) of the ratio of the electrical signal-to-noise ratio

(SNR) at the input of an amplifier to the electrical SNR at its output. Two important caveats

are often forgotten in this definition: 1) the input light source is meant to be shot-noise limited

and 2) the detector is meant to be a noiseless detector with unity quantum efficiency. Part of

the problem in measurement is that noise contributed by an amplifier is often so small that it is

difficult to measure the noise added by the amplifier in the presence of noise from a real light

source and detector.

Our preferred method for measurement involves determining the carrier-to-noise ratio

(CNR) of the system in Fig. 2 with and without the device under test while changing the

attenuation of the output attenuator to keep constant average signal power on the detector. This

yields the noise added by the amplifier and thus enables calculation of the amplifier's noise

figure.

It is well known that two types of noise added by an amplifier are signal-spontaneous (ssp) beat noise and spontaneous-spontaneous (sp-sp) beat noise. The first results from

heterodyning between the signal and spontaneous radiation at the detector, while the latter results

from a similar heterodyning between various spectral components of the ASE. The electrical

noise spectral density, N(f) with units of A^/Hz, resulting from these phenomena can be

expressed in terms of two optically measurable quantities: the received signal power P^, with

units of W, and the ASE optical power spectral density pase(j'), with units of W/Hz, where f

represents signal (base-band) frequencies and v represents optical frequencies.

Nsp-sp = /

[P«,.(v)f

The danger of using an optical technique to obtain P^ and pase(v) and then calculating the

electrical noise spectral density is that noise contributions of the amplifier due to other

phenomena are not accounted for. Examples of other phenomena that could possible occur in

an amplifier are phase-noise to intensity-noise conversion and noise resulting from leakage of

light into the signal path from a nominally 1480 nm pump.

Distortion

It has been established that distortion due to gain saturation that occurs in semiconductoroptical amplifiers does not occur in fiber-optic amplifiers [1]. Distortion due to other processes

can occur but at a level which does not have impact on digital systems. The distortion can have

197
significant impact in subcarrier multiplexed AM transmission, such as that used in transmission

of CATV signals, where parts per million distortion is unacceptable. One mechanism of

distortion is the interaction of laser chirp with the variation of gain with wavelength, also known

as gain slope. While the gain slope of an amplifier can be quite small in a highly saturated

amplifier, effects are often visible in a CATV signal's composite second-order (CSO) distortion.

Since the magnitude of the effect is so dependent on the characteristics of the source, alternative

techniques such as measurement of the small-signal gain spectrum in the presence of an

unmodulated signal have been performed [2]. An established, reliable technique has not,

however, been proposed.

Summary

While amplifier products rush through development toward deployment, measurement

techniques have not kept pace; work to develop and perfect these measurements must be

accelerated before widespread implementation of amplified systems takes place. Areas that need

immediate attention are the measurement of gain with multiple-wavelength signals, measurement

of noise properties, and measurement of distortion for AM-VSB applications.

Acknowledgment: It is a pleasure to acknowledge the contributions of R. P. Blaszyk, L. J. Button, D. R. Cole, R. A. Modavis, E. F. Murphy, and M. A. Newhouse to my understanding

of optical amplifier parameters and their measurement.

References

1. R.I. Laming, L. Reekie, P.R. Morkel, and D.N. Payne, Electron. Lett., 25, p. 455

(1989).

2. N. Kwong, J. Pastaski, H. Blauvelt, and I. Ury, "Measured spectral dependence of

distortion in erbium-doped fiber amplifiers," in Digest of Conference on Optical Fiber

Communication, 1992 OSA Technical Digest Series, Vol. 5 (Optical Society of America,

Washington, D.C., 1992) p. 191.

198
Erbium

Fiber

a

Gain Shaping

Filter

Pump

Laser

Pump

Filter WDM

Coupler Isolator

Fig. 1 Optical schematic of one implementation of a single-pumped erbium fiber-optic amplifier.

Tunable or Single Frequency

Laser Light

Source

I 1 Device under

Test (DUT)

Electric Modulator

Detector and

Electrical Pre-amp

Electrical

Spectrum

Analyzer

Fig. 2 Schematic of set-up required for measuring the four primary amplifier quantities: gain,

bcindwidth, noise, and distortion.

199
40-

m;uc

S

o>« 35

g>

«

E

CO

30-

Reflection coefficient (dB)

Fig. 3 Impact of back-reflection on the apparent small-signal gain of an amplifier with maximum

small-signal gain of 40 dB.

200
Chromatic Dispersion of Erbium-doped Fiber Amplifier

{Measured by Fourier Transform Spectroscopy

'Robert K. Hiclcernell, Kazumasa Takada, Makoto Yamada, Makoto Shimlzu, and Masaharu Horiguchi

NTT Opto-electronics Laboratories, Tokai, Ibarakl-ken, 319-1 1 Japan

*guest researcher, from National Institute of Standards and Technology, Boulder, Colorado USA

Abstract: We report the measurement of group index and chromatic dispersion in an erbium-doped

fiber amplifier by Fourier transformation of low-coherence interferograms. Resonant absorption-induced

changes in dispersion due to erbium ions in a germania co-doped fiber are as large as 25 ps/(km-nm),

and gain-induced changes in dispersion are as large as -12 ps/(km-nm). The measurements confirm

theoretical calculations based on a Kramers-Kronig transformation of measured absorption and emission

spectra.

Introduction :

- Erbium-doped fiber amplifiers are increasingly used in a variety of components for 1 .5 /jm

telecommunications. The dispersion changes which accompany the resonant gain and absorption in

rare-earth doped fibers may have a significant effect on short pulse propagation in devices such as

soliton lasers. A quasi-three-ievel system description has been used to model the complex susceptiblity

of erbium-doped fiber amplifiers, and refractive index changes were calculated from the Kramers-Kronig

transformation of measured absorption and emission spectra as a function of pump power. ^ A change

in phase index on the order of 1 0 ' due to pumping of a germania co-doped fiber was measured using

a fiber interferometer with a tunable laser source.^ Noise in the measured phase data concealed the

dispersion characteristics.

Fourier transform spectroscopy is a powerful tool for measuring the absorptive and dispersive

properties of bulk optical samples.^ Recently, dispersive spectroscopy has been applied to the

measurement of guided-wave properties, including dispersion,^ the location of index discontinuities

with fraction-of-a-wavelength precision,^ and the correction of dispersion-induced interferogram

broadening.®

We present here the measurement of group index and dispersion in an erbium-doped fiber amplifier

using dispersive spectroscopy. Resonant index perturbation is separated from resonant loss and gain

by Fourier transformation, greatly simplifying the interferogram analysis. The measurements are compared with the theoretical predictions of the three-level laser system model.

Background : Consider a Mach-Zehnder interferometer with a movable reference arm and a broadband

source. The phase of the Fourier transform of the interferogram with respect to the change in optical

path is equal to the optical phase difference between the two arms, as a function of wavenumber.^

The phase change due to a test sample in one arm can be measured (within an unknown constant) by

subtracting the phase of the interferogram with the sample removed from that of the sample

interferogram. The first derivative of the phase with respect to wavenumber yields the group index

of the sample, and the second derivative is proportional to the dispersion. If the length through which

the scanning arm can be moved is shorter than the path delay of the sample, a calibrated reference

fiber can be inserted in the scanning arm and its phase delay subtracted in the analysis.

Experimental setup : A fiber-optic Mach-Zehnder interferometer was constructed as shown in Fig. 1 and consisted primarily of standard single-mode fiber. A segment of dispersion-shifted fiber in the test

arm balanced the dispersion of the two arms. The signal source was an LED having a center

wavelength of 1 .53 jjm and a 3-dB bandwidth of 70 nm. Signal power in the erbium-doped test fiber

was maintained at -30 dBm to avoid gain saturation. The scanning arm contained a polarization

controller to maximize interference visibility. Optical path difference was changed by translating a

linear dc-motor-driven stage on which collimating lenses and a cube-corner retroreflector were

mounted. The optical phase was modulated at 40 kHz by a piezoelectric fiber stretcher in the test arm.

201
and lock-in detection with a 100 Hz bandwidth extracted the signal interferogram. The test arm

contained a fiber wavelength-division coupler to inject 1 .48 fjm counter-propagating pump light into

the erbium-doped test fiber. Optical isolators in each input and output port and a fiber connector return

loss of greater than 45 dB prevented back-reflection of pump light and laser oscillation of the fiber

amplifier.

Clock pulses for interferogram sampling were generated by the zero crossings of an ac-coupled,

high coherence interference signal.^ Light from a stabilized 1 .31 97 jjm DFB laser was injected into the

interferometer through the second input coupler port and separated at the output with a dichroic

mirror. Since the 1 .3 jjm reference light and 1 .55 jjm signal light were copropagating, distortion of

the sampled interferogram due to fiber temperature changes and scanning arm vibration was minimal.

A free-space, white-light Michelson interferometer was constructed with its scanning stage in common with that of the Mach-Zehnder interferometer to provide a start trigger for each sampling run.

Its fixed arm was mounted on a stepping motor to calibrate the relative position difference between

sample and reference measurements, as required for the determination of group index.

Measurement and Analysis : We made initial measurements on standard single-mode, dispersionshifted, and high-NA fibers. The maximum length of fiber for which an absolute measurement of group

index could be made without a dummy reference fiber was 25 cm. The stage was scanned at a rate

of 20 ;wm/s through an optical path length change of 5.4 mm to sample 8192 points. Fast Fourier

transformation (FFT) yielded the phase with a wavelength resolution of 0.4 nm near 1 .53 /vm. After

subtraction of the residual system phase without the test fiber, a fourth-order polynomial fit to the

sample phase as a function of wavenumber was made over the region 1 .5 //m to 1 .6 //m. Our

measurements of 1 m lengths of fiber agreed to within ± 0.6 ps/lkmnm) with measurements by a commercial instrument.

A 729 ± 0.5 cm length of erbium-doped, germano-silicate optical fiber with a relative core index

difference of 1 .2% was studied.^ The average erbium concentration in the core was 6.8 x 10'^' cm'^,

and loss at the absorption peak was 1.16 dB/m. The 730 cm dummy reference fiber had a core index

of 1 .2% and a dispersion of -21 ps/(kmnm) at 1 .535 /jm. Figure 2 shows the sampled interferograms

without pumping and for 25 mW coupled pump power. Increase in the overall magnitude of the signal

due to amplifier gain is easily observed, but the complex change in shape of the interferogram is undecipherable without the use of Fourier transform analysis. After subtraction of the system phase

and correction for the group index and dispersion of the reference fiber, a fourth-order polynomial fit of phase vs. wavenumber over the entire region between 1 .5 //m and 1 .6 //m yields only the broad

spectral variation of group index and dispersion of the host fiber.

To resolve the resonant dispersive features due to the presence of erbium-induced absorption and

gain, we performed a quadratic curve fit over a sliding, seven-point data window. Figure 3 is a plot

of the dispersion as a function of wavelength for different coupled pump powers. The peak

perturbation of dispersion of the unpumped fiber is 25 ps/lkmnm) at 1 .537 //m, large enough to

counteract the background dispersion of -15 ps/(kmnm), also plotted in Fig. 3. The sign of the

perturbation reverses as the pump power increases through the threshold value, coincident with the

transition from loss to gain. The effect of population inversion saturation is seen near the largest pump

power of 25 mW, which was 9.4 times the threshold power of 2.7 mW, as calculated from gain data.®

Maximum pumped dispersion perturbation was 10 ps/(kmnm) at 1.535 fjm and -12 ps/(kmnm) at

1 .537 |/m. The noise is due in part to polarization-dependent source artifacts.

We measured the emission and absorption lineshapes of the erbium fiber in the wavelength range

from 1 .40 fjm to 1 .65 pm with 0.5 nm resolution. The normalized lineshapes were used to calculate

the respective cross sections, and a Kramers-Kronig transformation produced the corresponding

dispersive part of the cross sections.^ We scaled the emission and absorption spectra to the measured

peak absorption coefficient to account for the overlap of the signal mode and the erbium concentration

profile. Populations of the upper and lower energy levels as a function of distance in the fiber were

calculated from the pump absorption and threshold power. The calculation included the saturation

term due to stimulated emission, ° based on the average pump absorption and emission coefficients

given in Ref. 8. The calculated index change integrated over the length of the fiber had peak values

202
of -1 . 1 X 10^ and +1.6x10"^ without pumping, and -0.4 x 1 0"^ and +0.8 x 1 0 * with a pump power

of 25 mW.

The calculated change In dispersion, also using a sliding, seven-point quadratic fit, is shown in Fig. 4. The experimentally measured perturbation from the host fiber dispersion is plotted in the same

figure for comparison. Agreement between the measurement and calculation is very good. Noise in the theoretical calculation is due to noise in the measured absorption and emission spectra. The

measured and calculated perturbation of group index are plotted in Fig. 5 over a wider wavelength

range than in Fig. 4. The resonance near 1 .552 //m is due to the secondary erbium absorption peak.

Conclusion : We conclude that Fourier transform spectroscopy with a fiber interferometer is a powerful

technique for measuring resonant dispersive changes in rare-earth-doped fibers. The change in chromatic dispersion due to absorption and gain in an erbium-doped fiber can be significant compared

to the background dispersion of the host fiber, particularly in the case of a narrow gain spectrum.

Acknowledoments : The authors express their thanks to Y. Takeuchi for supplying the fiber couplers

and to T. Ikegami, T. Izawa, and M. Nakahara for their support of this work. R.K. Hickernell expresses

his appreciation to the NTT staff for their help during his visit.

References

1. E. Desurvire, J. Lightwave Technol. 8, 1517-1527 (1990).

2. S.C. Fleming and T.J. Whitley, Electron. Lett. 27, 1959-1961 (1991).

3. J. Chamberlain, The Principles of Interferometric Spectroscopy. John Wiley & Sons, Chichester

1979.

4. P.-L. Francois, M. Monerie, C. Vassallo, Y. Durteste, and F.R. Alard, J. Lightwave Technol. 7,

500-513(1989).

5. B.L. Danielson and C.Y. Boisrobert, Appl. Opt. 30, 2975-2979 (1991).

6. A. Kohlhaas, C. Fromchen, and E. Brinkmeyer, J. Lightwave Technol. 9, 1493-1502 (1991).

7. K. Takada, M. Kobayashi, and J. Noda, Appl. Opt. 29, 5170-5176 (1990).

8. M. Yamada, M. Shimizu, M. Horiguchi, and M. Okayasu, IEEE J. Quant. Electron. 28, 640-649

(1992).

9. E. Desurvire, IEEE Photon. Technol. Lett. 1, 293-296 (1989).

1.53

LED

1.3 ^lm

DFB-LD

WDM 1 -48

coupler pump LD

dichroic

mirror 1.53 ^m ^ —

reference

fiber

polariz

control. 0

do linear /t<> motor

J

stage ^

I (

stepper

motor

stage

40 kHz lock-in

clock

generator

£3—eH>

halogen

lamp

lock-in

A/D

converter

Fig. 1 . Experimental setup of low-coherence interferometer.

203
(a)

12 3 4 Optlcil ptth chings ()

2 3 4 Optical ptth chtnge (m)

Fig. 2. Low-coherence interferogram signal as a function of optical path change:

(a) unpumped; (b) pump power, 25 mW.

20

10

-10

n

c

o

"5I -20 M

5

-30

-401.525

25 mW\

16 \ 9.6 Vv

4.6

; li,

0.27 ^O' \^

0 "'^ — » —' —

1.529 1.533 1.537 Wavelength (^m)

1.541 1.545

Fig. 3. Measured dispersion of erbium-doped

fiber for different pump powers: below pump

threshold power (dashed); above pump

threshold power (solid); host fiber dispersion

(dotted).

1.529 1.533 1.537

Wavelength (^m)

1.541 1.545

Fig. 4. Pumped and unpumped perturbation of

dispersion due to erbium ions: measured

(solid line, dot); calculated from KramersKronig transformation of absorption and

emission data (dashed line, box).

204
NOISE MEASUREMENTS OF ERBIUM-DOPED OPTICAL FIBRE AMPLIFIERS:

INFLUENCE OF SOURCE EXCESS NOISE

M.Artiglia, P. Bonanni (*), A. Cavaciuti, M. Potenza, M. Puleo. B. Sordo

CSELT - Centro Studi E Laboratori Telecomunicazioni SpA

Via G. Reiss Romoli, 274 - 10148 TORINO (Italy)

(*) SIRTI - Via Vida. 19 -20127 MILANO (Italy)

Abstract: This paper will consider the influence that the presence of input excess noise

may have on the noise degradation figure of an Erbium-doped fibre amplifier. A procedure is

suggested to correct the measured data for the excess noise and relate them to the ideal noise

figure.

1. Introduction

Erbium Doped Fibre Amplifiers (EDFAs) are very attractive for use in optical

telecommunication systems operating in the third window. They can in principle bring to dramatic

enhancements of the performance of optical links and their potentials in various applications are being intensively explored worldwide. One aspect of EDFAs which is of great interest is the

degradation of the signal to noise ratio (SNR) they cause because of the amplified spontaneous

emission (ASE) generated by the device itself. In the literature this feature is characterised by the

noise figure Fof the amplifier, defined as the ratio of the input SNR to the output SNR [1]. A crucial

assumption of this definition is that the input signal must be coherent, that is quantum noise

limited. This assumption, however, does not hold true in practice where the noise figure is calculated from measured SNRs. In fact, laser diodes used as optical sources for the

characterisation and the applications of EDFAs are affected by excess noise, such as the Relative

Intensity Noise (RIN), back-reflection induced noise and, for external cavity lasers, the noise

contributed by suppressed side-modes. This excess noise at the input is amplified too and

influences both optical and electrical noise measurements since it affects the measured

fluctuations and the level of noise floor. The aim of this paper is to show that this effect contributes

in a non negligible way to the measured noise characteristics of the device. Throughout the

attention will be focussed on the RIN of DFB lasers. In particular noise dependence on input power

level will be investigated, both theoretically and experimentally. A good agreement of experimental

data, measured on an EDFA pumped at 1480 nm, and theory is found. A procedure is suggested

to correct the measured data for the excess noise and relate them to the ideal noise figure F. This

could be useful in many practical situations, in which very low noise sources are not available.

The definition of the noise figure Fof an optical amplifier is related to the evaluation of the

intrinsic limits of the device in contributing noise to an input signal affected only by the unavoidable

quantum noise [2]. For an ideal amplifier F cannot be lower than the theoretical quantum limit of 3

dB. In most practical applications, where the input signal can be not-quantum limited, F becomes

unrelated to the actual situation and the degradation of the SNR is dependent on the excess noise

at the input. In particular, the noise degradation of the amplifier due to spontaneous emission

decreases as the excess input noise increases. Therefore, in practical applications, care must be

exercised in defining the noise properties of EDFAs.

The SNR for the input signal beam can be expressed as:

in terms of the mean photon number n/n and of the relative fluctuations Zin^. If the noise properties

of the signal are quantum limited (i.e., the signal is in a coherent state) Zin^= njn and (1) assumes

the well-known form used to compute noise figures. In the following we will consider a laser beam

affected by a non negligible RIN and calculate the SNR degradation due to the amplifier. Ihis Is considered as a cascade of an input device (characterised by a loss T/n), of an erbium-doped active

fibre (characterised by its total internal gain G and spontaneous emission factor nsp) and of an

output device (with loss Fout)- The input and output devices actually can consist of connectors,

WDMs and isolators. At the end of the cascade, the SNR will be again given by (1), but with the

pertaining output values, riou/and Eout^- Defining the noise degradation figure, Np, as:

2. Theory

(1)

in {S/N\ (2)

out

205
and passing to electrical quantities, we will have:

/ \ 1 G-l

Gr

(3)

where the following quantities have been introduced: detector responsivity 77, RIN spectral density

<^RIN^' mean input photon flux Ijp (associated to nip and the signal band), optical filter bandwidth

Afo and number rrit of "transverse" modes contributing to ASE {mt = 2 in the present case). The

various terms in braces of Eq. (3) are respectively the contributions of the signal shot noise, the

spontaneous emission shot noise, the signal-spontaneous emission beat noise, the spontaneous

emission beat noise with itself and the excess noise to the output SNR. In particular, the latter term

does not appear in formulas giving the amplifier noise figure F(where GRif^- 0).

2. Experimental set-up and discussion of the results

The used experimental set-up is shown in Fig.1. The light exiting a DFB laser is passed

through a pigtailed variable attenuator and then injected into the EDFA. An optical isolator prevents

backward ASE to enter the DFB laser. The output of the EDFA is then sent to a PIN photodiode for

detection. The average DC component of the photocurrent is measured by a calibrated

picoamperometer, while the AC component, carrying the fluctuations, is amplified and fed to an

electrical spectrum analyser. In the measurements discussed here no optical filter was inserted at the output of the amplifier to reduce the detected ASE; this explains the somewhat high values of

the noise degradation measured at low input power but has no influence on the validity of the

presented approach. The used EDFA has been developed by the authors. The active fibre,

pumped at 1480 nm with a copropagating scheme, has a relatively large core radius and is a little longer than the optima! length so as to obtain a flat gain spectrum even in the small to mid signal

range. Measured spectral gain curves for three different input power levels are shown in Fig. 2; gain variations are less than 4 dB even in the small-signal regime. An optical isolator is inserted

internally before the output port of the amplifier. Both input and output ports are FC/PC

connectorised. As a source a commercial DFB laser of very high quality was used, operated in two

different conditions, such that the measured RIN values were less than -165 dB/Hz in one case and

-145 dB/Hz in the other. Emission wavelength was 1530 nm. The RIN was measured using the

method described in [3].

The curves in Fig. 3 a) show the behaviour of the measured noise degradation figure Npi^) versus the input signal level for the two operating conditions. The high values of Nf^^) are due both

to the absence of any optical filter to reduce ASE level and to the fact that the active fibre was not

fully inverted. The lowest curve corresponds to a source RIN value of -145 dB/Hz, the other to a RIN of -165 dB/Hz. Fig. 4 shows curves as calculated from Eq. (3) for RIN equal to -170 dB/Hz, -165

dB/Hz (these cases are hardly distinguishable), -150 dB/Hz, -145 dB/Hz from top to bottom. The

upper curves of Fig. 4 a) correspond to experimental conditions: no optical filter inserted, Fm = Fout = - 1 dB, G given by the measured values of the fibre internal gain (the small-signal gain at 1530 nm signal wavelength was 25 dB for a 25 mW pump power) as a function of input power. The

detector quantum efficiency was put equal to unity, according to Ihe normalisation used to obtain

the experimental curves in order to eliminate the influence of photodiode characteristics.

reasonable agreement is found between experimental data and theoretical results. An even better

agreement is expected through a more accurate estimation of the behaviour of /isp versus input

signal level. Theoretical calculations show that the -1 70 dB RIN-curve practically coincides with the

behaviour of the F-curve (no excess noise present). Therefore, optical sources with RIN lower than

that value, can be considered as quantum limited for practical purposes.

Since so low RIN figures are not commonly encountered, attention must be paid to excess

noise of optical sources. In fact, coming to Fig. 3 b), let us consider the upper (experimental) curve which represents the amplifier noise degradation figure for the -145 dB/Hz case assuming that the

input signal is quantum limited. The discrepancy with the lowest curveof Fig. 3 a) is evident in the

saturation region, where the corresponding behaviours are quite different. This is because also

RIN contributes to both the input and the output SNRs: neglecting its presence before the

amplifier necessarily leads to an overestimation of the device noise degradation figure. On the

other hand, starting from the same assumption of Poissonian input signal, but correcting the

measured SNR at the output by subtracting from the measured fluctuations the amplified RIN

206
contribution [last term in braces in Eq. (3)], one obtains a Nf(^) (dashed line) in good agreement

with that measured for the RIN equal to -165 dB/Hz [lowest curve in Fig. 3 b)] which in turn is very

close to the ideal behaviour of F[see Fig. 4 a)]. To support the validity of the proposed analysis,

Fig. 4 a) also shows theoretical curves in different operating conditions: the lower family of plots

refers to the presence of an optical filter of 1 nm bandwidth. It is interesting to note that the

behaviour of Nf(^)\s mainly influenced, besides rigp, by the optical filter in the small-signal regime

and by the RIN of the source in the large-signal regime. Finally, Fig 4 b) shows the noise

degradation for a highly inverted fibre (small-signal ngp of around 1 .2) with optical filter. A

considerable reduction of A/^^j is seen as expected, but the influence of RIN is still clearly evident.

3. Conclusions

The influence of source excess noise on SNR degradation measurements of EDFAs has

been investigated experimentally. Excess noise is capable of altering considerably the behaviour

of the (S/N) degradation versus input power level, with respect to the ideal case. A simple

theoretical model which fits quite well the experimental data has been presented. Finally, a simple

procedure for relating the measured data to the ideal noise figure has been proposed, based on

the knowledge of the excess noise (RIN) of the laser diode used as an optical source for the

measurements. Effectiveness of the procedure has been demonstrated.

References

[1] See for example the Tech. Dig. of the Topical Meeting on Optical Amplifiers and

applications, Snowmass (CO), July 24-26, 1991 and references therein quoted.

[2] T. Okoshi, K. Kikuchi: "Coherent Optical Fiber Communications",KTK-Kluwer, 1988

[3] P. Gambini et al., "Characterization of laser diode intensity noise at microwave

frequencies with high sensitivity", SPIE OE/FIBERS Conf., Boston (USA), September 1989.

optical isolator

DFB laser

amperometer

variable

attenuator

Fig. 1 - Used experimental set-up

electrical spectrum

analyser

o

1.53 1.535 1.54 1.545 1.55 1.555 1.5B

Mavelength [ ]

Fig. 2 - Gain spectra of the used EDFA measured at different input power levels: -30 dBm

(dashed line), -20 dBm (dotted line), -8 dBm (continuous line)

207
a) b)

co

IV

CD

a>

<u

CO

-40 -30 -20 -10

Input Optica! Power [ dBm ]

T3

co

C3

o

-15 -10 -5

Input Optical Power [ dBm ]

Fig.3 a) Measured values of Nf^^); continuous line: source RIN - 165 dB/Hz; dashed line: - 145

dB/Hz;

b) lower curve: Npi^) at - 165 dB/Hz; upper curve: noise figure without correction; dashed

line: corrected noise figure.

a) b)

30

00

co

a

-aa

i.O)V

-o

I)

oz

20

10

0

-40

without optical filter

with optical filter

quantum limit

10

-30 -20 -10

Input power (dBm)

m

c

o>

a

-ae

u

a>uo

u

0

-40

//

quantum limit

1

-30 -20 -10

Input power CdBm)

0

Fig.4 - a) Upper curves: calculated values of Nf^^) after (3) (from top to bottom: source RIN - 1 70

dB/Hz; -165 dB/Hz; - 150 dB/Hz; -145 dB/Hz) ; lower curves: same as before but with a

1 nm optical filter; b) same RIN levels as a) but with lower spontaneous emission factor (i.e. at higher pump

levels) and 1 nm optical filter.

208
Accuracy of noise figure measurement

for erbium-doped fiber amplifiers by tlie optical method

T. Kashiwada, M. Shigematsu and M. Nishimura

Sumitomo Electric Industeies, Ltd.

1, Taya-cho, Sakae-ku, Yokohama, 244, Japan

1. Introduction

Noise figure (NF) is one of the most important parameters of Er3+ -

doped fiber amplifiers (EDFAs). In general, there are two types of techniques

for measuring the NF: the optical and electrical methods. (0(2) Although the

optical method is relatively simple, accuracy of the NF measured in the

saturation region has been questioned. Recently, a new technique was proposed

to improve accuracy of the optical method by utilizing the polarization

properties of the signal and the amplified spontaneous emission (ASE).(3)(4) It seems, however, that the accuracy and limits of the measurements have not yet

been analyzed quantitatively. In this paper, we examine the errors in the NF

measurements, and determine the accuracy of each setup for measurement.

2.0ptical method of NF measurement

The NF is given by the following equation, when the signal-spontaneous

beat noise is dominant

]\jF ^ Pase

hvAvG

^ where Pase is the ASE power within the optical bandwidth of Av, v is the

signal frequency and G is the gain of the EDFA. The NF can be determined by

measuring Pase and G using an optical spectrum analyzer.

In the saturation region, however, the ASE power level becomes small

and comparable to amplified sideband components of the signal and/or noises

due to the limited dynamic range of the spectrum analyzer. Therefore, it is

necessary to eliminate or suppress them for accurate measurement of Pase • Possible techniques for the purpose are (a) add a narrow spectral width

bandpass filter to the light source, and (b) polarize the signal at the output port

of the EDFA using a polarization controller and suppress it by an

orthogonally aligned polarizer. (3)(4)

209
3. Error analysis

Each technique was tested experimentally. Figure 1 shows the

experimental setup. Four cases were examined : Case 1 -without a bandpass

filter (BPF) or polarization control (PC), Case 2-with BPF only, Case 3-with

PC only and Case 4-with both BPF and PC. The same curve fitting technique

was used in all cases to interpolate Pase at the signal wavelength. The spectral

resolution of the spectrum analyzer was Inm. Figure 2 shows the gain and the

NF as a function of the input signal power measured in each case. In Case 3,

the NF could not be determined because of wavelength-dependent undesirable

noises in the ASE measurement. In the low input signal power region, the NFs

of the other three cases were in good agreement. On the other hand, in the

high input signal power region, the NF measured in Case 4 was considerably

smaller than those in the other cases.

In order to quantitatively determine the error in each case, the noise of

the measurement system itself was examined. Figure 3 shows the measured

optical spectrum for each setup with the Er3+-doped fiber (EDF) replaced by a

normal single mode fiber having the same length. The noise level relative to

the main signal in the adjacent wavelength region could be determined from

Fig. 3. The signal-to-noise ratio (S/N) in Case 4 was as high as 69dB, while the

S/N in Case 1 was only 39dB.

From the S/N values determined above, it was possible to predict

measured values of Pase in each case by theoretically calculating G (thus the

output signal power) and Pase^^) and then adding the system noise, assuming

that the S/N of the system was conserved with existence of the EDF.

Comparison between the prediction and actual measurements is demonstrated

in Fig.4. Agreement is excellent.

4.Discussion

It is worth noting that neither Case 2 nor Case 3 showed a significant

improvement in the system S/N, while Case 4 v/as dramatic as seen in Fig.3.

The polarization control is effective for suppressing the amplified main signal

to substantially increase the dynamic range of the spectrum analyzer, but the

amplified sideband components of the signal still remain, because their

polarization state is different from that of the main signal after passing

through several optical components in the EDFA. The BPF is essential for

reducing those sideband components.

210
Measurement accuracy of the improved method was investigated based

on the system S/N determined from Fig. 3. Figure 5 illustrates the relation

among the output signal power, the system noise and the ASE power level. In

order to achieve 0.1 dB accuracy of the NF measurement, the system noise

must be 17dB lower than Pase or less. It can be concluded that the

conventional optical method (Case 1) is accurate only when the input signal

power is smaller than -20dBm, and that the improved method (Case 4) can be

applied to a wide input signal power range up to nearly +10dBm.

5. Conclusion

Accuracy of the NF measurement for EDFAs by the optical technique

was investigated quantitatively. The system noises of different setups were

determined and the improved method employing a bandpass filter and the

polarization control technique was proved to be accurate even in the deep

saturation region.

References

(1) P.R.Morkel, et al, Opt. Utt., Vol.14, ppl062-1064 (1989)

(2) C.R.Giles, et al., Photon. Technol. Lett., Vol.1, No.U, pp367-369

(1989)

(3) J. Aspell, et al.,Technical Digest on OFC'92, paper ThA4 (1992)

(4)T.Kashiwada, et al., lEICE National Conf., paper C-309 (in Japanese)

(1992)

(5) K.Kikuchi, Electron. Lett., Vol.26, No.22, ppl851-1853 (1990)

Attenuator

Optical isolator—i —H- ATT

A =1552nin

3+

A =1470nm

WDM Er-doped fiber

coupler

Polarization

Controller

/ —CD

a

Optical

spectrum

analyzer

Polarizer

Fig.l Measurement setup for NF

211
Gain

(dB)

30

201.

10

Pump power 40inW

Gain

IVF(Casel) *

N.^ 4 NF (C«se2)

NF(Case4)

0

-30 -20 -10 0

Input signal power (dBm)

Fig. 3 System noise levels relative

to the main signal measured

for different setups

NF

15

10

^ 5

0

10

sa

ea_ -40

-50

9

-60

-70

Fig.2 Gain and NF measured

with different setups

Casf 1 rag- 2 \

\ \ I /

•'• U 1

••

•

t /

V. W ** n*

1 u 1/ *

V'

1

•

' t

1 1 1 1 1 ,

Wavelength (nm)

Input signal power (dBm)

Fig. 4 Comparison between the ASE

power levels actually observed (marks)

and theoretically predicted based

on the system noises (curves)

I

u

0

Output signal power

-39dB -«9dB

-20 -17dB Casel.

* c; ~ "

"

-40 Required* * ^

noise level

-60 ' -30 -20 -10 0 10

Input signal power (dBm)

Fig.5 Relation among the output

signal power, the system noise

and the ASE power level

212
AUTOMATED MEASUREMENT OF GAIN CHARACTERISTICS

OF OPTICAL FIBER AMPLIFIERS

Douglas R. Cole and Miles E. Vance

Corning Incorporated

Opto-Electronic Components

Corning, New York 14831

INTRODUCTION

In this paper we describe and evaluate an automated system for

the measurement of gain, output powers, amplified spontaneous

emission (ASE), and unabsorbed pump power of an erbium-doped

fiber amplifier (EDFA), as a function of input power and

wavelength

.

In order to make accurate gain measurements it is necessary to

separate the signal from the ASE. One solution to this problem

is provided by intensity modulation of the signal laser followed

by AC detection of the amplified output. The measurement system

utilizes RF modulation and an RF spectrum analyzer, which

provides the requisite accuracy and precision. We chose,

wherever possible, to use commercially available measuring

instruments and equipment which could be easily interfaced with

the control and data processing computer.

The system obtains values of modulated input and output optical

powers, as well as total average input and output optical powers.

These measurements are made first on a non-amplifying standard

single mode reference (REF) fiber, then on the EDFA to be tested.

The optical power gain G is then given by ratios involving only

the modulated powers:

where P . is the modulated input optical power and P

""^ is the

modulated output optical power. The total average optical input

signal power P. is then measured and multiplied by the gain to

get the average optical output signal power P^:

P = G P . , ( 2

)

o 1

Note that P^ does not include ASE, but is the true average signal

power for a given average P.. At high signal levels, P becomes

essentially equal to the measured total average output power,

since ASE becomes negligible.

OPTICAL AND ELECTRICAL HARDWARE SET-UP

The system's signal wavelength can be tuned from 1520 nm to 1550

nm. Before splicing in an EDFA at the device under test (DUT)

location, as shown on Fig. 1, the input fiber is spliced to the

output fiber and a REF measurement is run. Our calibration

specifications require a REF gain stability of +/- 0.25 dB.

213
An EDFA booster amplifier is used to provide roughly 18 dB of

gain for the tunable laser diode signal, thereby obtaining high

input power through the entire tuning range. The tunable

bandpass filter (TBF) has a FWHM bandpass of 3.5 nm and passes

the signal, while blocking most of the booster amplifier's ASE.

A variable optical attenuator (VOA 1) steps the attenuation of

the input signal. VOA 2 and VOA 3 are appropriately adjusted to

prevent signal saturation of the input and output receivers.

Coupler 1 is used to tap the input signal. A ratio of the actual

input signal delivered to the DUT and the input receiver signal

is established. Backward ASE is also captured at this point.

Couplers 2 and 3 allow unabsorbed 980 nm pump power (P ) to be

measured for either backward or forward pumping.

Reflected light traveling through the gain fiber of an EDFA

reduces the overall amplification of the signal. Therefore, it

is important to minimize back reflections to about -60 dB . Low

back reflection devices, isolators and angled physical contact

connectors (FC/APC) are used.

Polarization variations in signal light are minimized by

stipulating minimum polarization sensitivity of critical optical

components and placing most system fibers in a fixed position.

The signal generator provides an RF signal, at 50 MHz with 1 mW

of electrical power, to the tunable laser diode. The optical

signal modulation index is about 16 percent.

PINFET receivers are mounted on test fixtures which provide RF

outputs to the RF switch and spectrum analyzer combination and

photocur rents to picoammeters

SOFTWARE

A General Purpose Interface Bus (GPIB) connects a Macintosh IIFX

computer to the equipment. Instrument control and data

acquisition are performed by a National Instruments software

application called LabVIEW 2.

Each real instrument has a stand alone control program or

'virtual instrument' (VI) which writes set-up commands to and

reads data from the instrument via a GPIB cable. Each

instrument's operating sequence and data acquisition is governed

by the main VI program which 'software wires' the devices

together

For a given signal wavelength the input signal is attenuated in 3

dB or other increments from 0 dBm to - 45 dBm. Instruments are

polled at each step and data is stored in one of 21 arrays.

Next, the main VI program completes the following functions. A

raw data file is created in a spreadsheet format. Data is

normalized to account for receiver respons i vi ty , variable and

214
fixed optical insertion losses and attenuation of fixed filters.

This normalized data is plotted, along with a fit curve, as shown

on Fig. 2, as well as a test sheet listing pertinent test

conditions and data table.

RESULTS

Typical gain curves of Fig. 2 were all obtained in 20 minutes

after splicing in the DUT. Measurement time can be dramatically

reduced by selecting fewer input signal values, especially at the

low-input end, and by sacrificing precision.

The basic repeatability of consecutive measurements, with no

changes made between measurements, was +/- 0.39 dB (3 SIGMA) for

small-signal gain and +/- 0.25 dB (3 SIGMA) for output power.

According to Eq .

( 1 ) , long-term variation of the input and output

attenuations and signal-processing systems does not produce an

error in G, and we thus expect day-to-day repeatability of G to

be only slightly worse than +/- 0.39 dB (3 SIGMA) because of

small differences in splice loss when the amplifier is exchanged

for the REF fiber.

From Eq .( 2 ) , long-term variation of the input attenuation and

signal-processing system produces a proportional error in . This variation was found to be +/- 0.21 dB (3 SIGMA) over a

period of 4 months. Thus we may expect long-term repeatability

of +/- 0.33 dB (3 SIGMA) plus the small splice-loss variations.

A test of the linearity of the RF and DC signal-processing

systems is the equivalence of the calculated P and the measured

total optical average power at high signal levels. Typically we

find these to agree within 0.1 dB

.

The absolute accuracy of P depends on that of the power meter

used for calibrating the P. measurement, stated by the

manufacturer as +/- 3.5 % tl].

CONCLUSIONS

We have assembled a convenient and versatile automated system for

the measurement of gain, output power, ASE, and unabsorbed pump

power for a wide range of input powers and wavelengths. Computer

control permits data acquisition and processing that is executed

with minimal operator intervention. Repeatability of the

small-signal gain and the output-power measurements is about +/-

0. 4 dB (3 SIGMA)

.

REFERENCE

1. Hewlett Packard, Operating Manual Optical Head HP81521B

Part No. 81522-90011, April 1989.

215
hp 8656B

s 1 gna 1 generator

f mod. = 50 MHz

c 5 JDS FC/APC

4 dBm

ItunaDle LD

Santec TSL-80

10 dBm

ILX lex: 3722 laser diode controller aoaltor

VOA 3

hp 81S21B meter

OPTICAL QAIM NBASURSNSin* STSTBf

I power met ^ eatter

Keithley

i485 plcouneter

CORNING

rig. 1

CUSTOMER TEST SHEET CORNING INC

file date; measurement

sample commenta

69112086.

«11« I data I PIBERGAINrOATA GAIN ; P4-16 : P4 - 16 1532 ran 3/16/92

Monday. March 16, 1992 12:12 PM Coming's optical Gain; version 3B; CRIB 1 P4-16; £ii>er to fiber gain w/ output isolator #

rXBBIGAXll;DATA OAXNt P4-16 ; P4-16 1532 nm 3/U/92

Ibnday. March It, 1992 12:12 m

Coming's Optical Gain; vsriion 38; CRIB L P4-1C: Cibar CO Cibar gain w/ output isolator •

to

PIBERGAIN MODULE TEST PARAMETERS

signal wavelength (nm)

PUMP LASER »1 CONDITIONS

forward current (mA)

TEC temperature (C)

TEC current 1mA)

pump PIN monitor I 9 -5v bias (uA pump monitor (mw) :

POWER PERFORMANCE

P out ( P in a 0 dBm ) : P out ( P in :i -3 dBm ) : P out ( P in . -5 dBB ) ;

1532.0

200

15 236, 304,

1,

11,83 (d 11.64 (d 11.49 (d

15,25 (mW) 14,58 (mW) 14.11 (mW)

.

j

'

j

'

!

^

OUTFUT pown (

SMALL SIGNAL PERFORMANCE gain (dB) : 38,11 forward ASE (mM) : 4.15 backward ASE (mW) : 3.94

unabs. pump towards source (mW) : 0,001

unabs. pump towards receiver (mW) : 0.000 ae

CURVE PITTING PARAMETERS

model: G(Po) laO' (l-expta2* IPo-al)

aO . 38.11 al . 12.47 a2 . Chi - squared: 7.648

P in P out GAIN

(dBa) IdlBal (dB) 1.31 11 91 10.60

-1.80 11 64 13.45

-4.82 11 47 16.30

-7.88 11 J2 19.20

10.89 11 06 21.95

13 .95 10 30 24.75

16.96 10 39 27.35

20.02 9 78 29.80

23.04 9 16 32.20 26.10 7 95 34.05

29.14 6 66 35.80

32.32 5 03 37.35

35.27 2 52 37.80

38.15 0 40 38.55

40.86 -2 17 38.70

44.03 -5 34 38.70 46.64 -8 19 38.45

iNWT rcmm (4»m)

0 1 0

nmn Mwm (aiai

0 1 0

Fig . 2

216
Cutoff Wavelength Measurement Precision

Improvement by Curve Fitting

T.A. Hanson, Coming, Inc. K.A. Emig, Coming, Inc. C.S. Brown, AT&T Bell Labs

R.J. Smith, AT&T Network Cable Systems

Abstract

The existence of spurious, transient, long-wavelength transition structure (humps) in transmitted

cutoff wavelength measurement curves has been previously documented and well studied over the

last few years. This transition structure creates a large increase in the variability of the

measurement when using the present test procedure recommendations of national and

international standards bodies. A method of fitting the data to a rigorous model, which minimizes

the effect of the structure, is presented. Application of the fitting algorithm to two common fiber

designs, and to the two most popular measurement methods, shows significant improvement in

precision.

Introduction

The theoretical basis for long-wavelength transition structure in the transmitted cutoff

wavelength measurement, as well as empirical methods to eliminate the structure, have been

previously reported [1] [2] [3]. The general conclusions from these studies are: a) the transition

structure results from coating materials which have a different refractive index than the fiber's

cladding, b) small levels of microbending, index-matching the cladding, or increased sample

length drastically reduces or eliminates the structure, and c) the existence of the structure is more a metrology problem than a functional one since the low levels of bending in practical

applications are sufficient to eliminate the structure.

In 1988, Okafor and Peckham [3] presented a model for fitting the transmitted cutoff wavelength

curve in the transition region. To accomplish this, a transform was derived which, when applied to the measurement data, forms a straight line function in wavelength:

1) \ogma--Kc) = Y = 101og[-(10/A)log[(10(«/10)-l)/p]]

where a is the measured attenuation

p is found to be equal to 2

A= 101og[p/(100-01-l)]

log(x) is unknown

and is the cutoff wavelength

The transform is constructed such that the wavelength at which the line crosses zero is the cutoff

wavelength as defined by the 0.1 dB intercept definition [4] [5] [6]. By applying a linear

regression to this equation, the slope, log (x), and the intercept, A-c, can be determined. The

recommended least-squares method of performing the regression [3] works well for cutoff

wavelength curves which do not exhibit transition structure. However, since the transition

structure is always positive in direction, it causes a unidirectional fit error on those cutoff plots

exhibiting the structure. To deal with the transition structure, a linear regression which limits

the allowable negative errors has been developed.

217
Algorithm

The basic idea of the algorithm proposed in this paper comes from observation that the transition

structure is made up of data points with positive deviation from the ideal. To illustrate this, a 2-

meter fiber sample was deployed per TIA FOTP-80, i.e., straight deployment with a single, 140-

mm radius turn, and measured for cutoff wavelength. The coating was then removed and the sample

remeasured in index -matching oil. The results are shown in Figures 1 and 2, respectively, and

demonstrate that the transition structure consists of a positive deviation from the ideal curve

causing the cutoff value to err in the direction of longer wavelength.

When the data from the above curves are plotted as -Y vs. X, after transformation according to Eq.

1, the transition structure causes positive deviations from a straight line. A method of defining

this straight-line fit is to minimize the sum of the absolute values of error, subject to a constraint

that all errors are positive. This is accomplished through the use of a Simplex Algorithm [7]. A

further refinement of the regression is to allow for a small level of negative error equivalent to the magnitude of the electro-optic noise demonstrated in the long-wavelength portion of the curve. Appropriate formulas are applied to determine the negative error constraint. The advantage of

this refinement is it minimizes overcorrection of the curve-fit by allowing an appropriate level of

negative fit-error.

This model is designed to apply only to the cutoff transition region. Applied to data generated

with the bend reference technique, the upper wavelength limit of the transition region is defined

as the maximum wavelength at which the attenuation is greater than 0.1 dB, plus 10 nm. The lower

wavelength limit is defined as the wavelength where a local maximum, less the next local

minimum, is greatest.

For the multimode reference technique, a preceding step is required before the model's upper and

lower limits are determined. The data from the long wavelength region beyond the transition

region forms a sloped line, sometimes called the backslope. The backslope is fit by starting at the

minimum attenuation wavelength and using the next 150 nm of data towards longer wavelengths.

Non-negative error regression, followed by adjusting the intercept by the median value of the

errors, reduces the effect of humps on the straight-line fit. The difference between the measured

data points and a straight line projected from the backslope toward shorter wavelengths is calculated, yielding a data set similar to that obtained using the bend reference technique. The

model limits are now determined in the same manner as described above.

Results

The measurements resulting in Figures 1 and 2 were described earlier. Cutoff values calculated

from the raw data for the coated fiber and the index-matched fiber were 1200 and 1180 nm,

respectively. Application of the curve-fitting algorithm to both data sets result in calculated

values of 1178 nm for both measurements. In the case of the index-matched fiber, where little trace of transition structure exists, the curve-fit value is essentially the same as that obtained

from the raw data. In the case of the coated fiber exhibiting significant structure, the curve-fit

value is equivalent to the value obtained without structure.

Figure 3 shows a comparison of the model value vs. the standard definition on a population of

depressed-clad design fibers measured by the bend reference technique. In this case, 104

samples were measured using both calculations. The average difference between the two

measurements is only 4 nm. The effect of transition structure on the standard definition of cutoff

is to create measurement outliers in the direction of longer wavelength.

218
Figure 4 shows a repeatability study for matched-clad design fiber using the multimode reference

technique. A total of 64 measurements were made on 16 different fibers, and the cutoff values

calculated using the standard algorithm and the proposed curve-fit algorithm. Cutoff

measurements exhibiting multiple 0.1 dB crossovers were eliminated from this data set. The

average difference between the two calculation methods was 3 nm. Pooled standard deviations for

the standard calculation and the curve-fit were 6 nm and 2 nm, respectively, indicating a significant improvement in measurement precision.

Conclusions

The results of this work leads us to three basic conclusions. First, the effects of transition

structure on the measured cutoff wavelength value can be significantly reduced or eliminated by a combination of the appropriate theoretical model and a fitting algorithm that controls the negative

fit-errors. Secondly, the improved precision resulting from the use of the fitting algorithm

reduces the potential for specification compliance issues, as well as decreasing the variability in a variety of cutoff-related models. Lastly, a universal method of controlling the effects of humps

on the measured cutoff value by alternative deployment configurations have been sought, but no

practical configuration has been found. The software solution presented here yields nearly the

same value as would be reported if the structure wasn't present, but with much less measurement

variability.

References

[1] "The Anomalous Structure Observed in Single-mode Fiber Cutoff Wavelength Measurements:

Theory and Solutions," S.A. Jacobs and D.W. Peckham, Symposium on Optical Fiber

Measurements, NIST SP 792, 1990, pp. 49-52.

[2] "Elimination of the Primary Sources of Measurement Variability in Single-mode Fiber

Transmitted Cutoff Wavelength," K.A. Emig, Journal of Lightwave Technology, vol. 17, No. 10,

Oct. 1989, pp. 1466-1472.

[3] "An Accurate Method of Determining Single-mode Fiber Cutoff Wavelength," C.N. Okafor and

D.W. Peckham, Symposium on Optical Fiber Measurements, NBS SP 748, 1988, pp. 119-122.

[4] TIA/EIA (Telecommunications Industry Association / Electronic Industries Association),

RS-455-80

[5] CCITT (International Telegraph and Telephone Consultative Committee), Recommendation

G.652.

[6] lEC (International Electrotechnical Commission), Method lEC 793-1-C7

[7] Linear and Nonlinear Programming. V.A. Sposito, Iowa State University Press, 1975.

219
0

O - 2 .

3

; . 4

-

• 5 .

• 6

0.1 <a Fit Line

Raw Data, k = 1200 ran

Curve n, k = 1178 ran

o

Wavelength (nm)

0 V

o - 2

-3 .

4

-5

-6 '

H h

— Raw Data, k = 1180 ran ~ Cunt Fit, Ic = 1178 nm H 1 1 1—

o o

o o

Wavelength (nm)

Fig. 1 - Coated Fiber Cutoff Plot Fig. 2 - Index -matched Cutoff Plot

n : 104 Meas.

1 170 1190 1210 1230 1250 1270 1290 1310 1330 1350

Standard Cutoff (nm)

n = 64 Meas,

1 100 1120 1 140 1160 1 180 1200 1220 1240 1260 1280 1300

Standard Cutoff (nm)

Fig. 3 - Model vs. Standard Cutoff

Comparison on Depressed-clad Fiber

Fig. 4 - Model vs. Standard Cutoff

Comparison on Matched-clad Fiber

220
DETERMmATION OF THE LP,i MODE CUTOFF WAVELENGTH

FROM THE MODAL INTERFERENCE PATTERN

K.ABE, Y.LACROIX, Y.CAI AND L.BONNELL

NATIONAL OPTICS INSTITUTE

369 RUE FRANQUET, SAINTE-FOY, QUEBEC, CANADA, GIP 4N8

INTRODUCTION

In order to eliminate the modal interference (MI) between LPqi and LPj, modes in the

1300 nm region [1] and the consequent system noise [2] which could be induced by a short

section of fiber (e.g. stub, patchcord), it is desirable to select a fiber whose cutoff wavelength (Xj

for the LPii mode is lower than the minimum operational wavelength. However, the rigorous

definition and determination of require an accurate knowledge of the index profile which is

not trivial to measure for fiber. Alternatively, attempts to measure the parameter directly have

encountered difficulties due to the poorly defined LPj, mode near cutoff, for example, the

extreme sensitivity of the power level within the mode to the fiber lay-out condition. For the

purpose of specification, the EIA cutoff wavelength {X^(EIA)) is commonly being quoted based

on the definition and the test method recommended by the EIA standards committee [3].

However, the relation between ^^.(EIA) and the real has not been established. Furthermore,

measurement difficulties are often encountered with matched cladding fibers when the test

method is used [4].

Instead of the direct determination of X^, references [5,6] suggested the measurement of

the equalization wavelength (Xj from the MI pattern, where X^ is the wavelength at which the

wavelength derivatives of the propagation constants are equal for LPq, and LPj, modes.

Assuming a step index profile, the following relation can be used to determine the cutoff

wavelength, X^iEQ):

X,(EQ)/X, = WJV, ~ 1.26, (1)

where the normalized frequencies ¥^=2.405 by definition and Vg~3.03 for a typical step index

"single-mode" fiber.

During the investigations of MI for commercially available matched cladding fibers [1],

it was found that equation ( 1 ) is not applicable to these fibers due to the deviation of the index

profile from a step profile and also that a reasonably accurate X^ value can be estimated from the

MI pattern in the 1300 nm range without relying upon the power level in the LPn mode.

EXPERIMENTAL RESULTS

The experimental set-up consisted of a white light source, a fiber with a stub spliced in

the middle, and a spectrum analyzer. The MI was measured for the stub lengths between 10 mm

and I m over the wavelength range from 900 to 1550 nm. For long stub lengths, the coating was

kept on the stub and the splice sections were immersed in an index-matching oil in order to

minimize the cladding modes. For short stub lengths, the coating was stripped and the stub fiber

221
was immersed in an index-matching oil. An example of MI is shown in Fig.l.

The phase of MI is given by (}) = Ap-/ [6], where Ap = PorPn. ^ is the stub length, and

Poi and are the propagation constants for the LPqi and LPu modes, respectively. The

oscillation period (AA,) provides us with die information on the wavelength derivative of A^:

d(Ap)/d?. = 27C/AX-/ (2)

The oscillation periods and the corresponding wavelengths were measured from the plot and the

-2%/Ak-l values were plotted as a function of the wavelength, as shown in Fig.2. Note that

d(AP)/dA, is negative in the wavelength range. The measurements were repeated several times

for different stub lengths and the results were combined to make the plot.

In any text books, theoretical plots of P and its wavelength derivative (or group delay)

vs. V can be found for infinite cladding. As pointed out in references [1,7] however, the

propagation constant near cutoff indicates a continuity to that of the cladding modes which are sensitively affected by the fiber coating and the lay-out condition. For this reason, we can not

expect the abrupt termination of the propagation constant at the cutoff as the theory predicts,

unless the cladding mode is totally eliminated without disturbing the LPu mode. In spite of this

experimental difficulty, the cutoff wavelength can be estimated with reasonable accuracy from

Fig.2. As the wavelength increases, the plot indicates a monotonic downward trend with an

increasing slope. The theory predicts that the plot should come to the end at cutoff. However,

the effects of the residual cladding modes manifest itself as the irregular continuing points, as demonstrated in the figure. The cutoff wavelength (X^CMI)) was estimated from the transition

point. As compared to the long stub measurements, short stub lengths were found to be more

convenient for the MI measurements in the wavelength range near and beyond the cutoff, because

the S/N ratio could be enhanced due to the higher power level in the LPu mode and the large

oscillation period which allowed us to use a large slit width.

For four matched cladding fibers, the X,c(MI) values were determined in this manner. The

results are listed in Table 1 (fibers-A, B, C and D) together with other fiber parameters. In the

table, the MFDs were measured by using a far-field scanning method. The ^^(EIA)

measurements encountered the problem commonly known as "bumps and humps" for all the fiber

samples. Therefore, ranges of the estimated cutoff values were given. The values were

measured from the MI pattern in the 1000 nm region using the stub length of 1 m, and the

A,c(EQ) values were determined using equation (1). Note that ^^(MI) is consistently larger than

Xc(EQ) for all the sample fibers. This discrepancy was suspected as being caused by the index

profile which was slightly graded, as shown in Fig. 3. The refracted near-field technique was

used for this measurement. In order to confirm this interpretation, similar measurements were

performed for a depressed cladding fiber made by the MCVD process. The results of the MI

measurements are shown in Fig.4 and the index profile in Fig. 3. The cutoff parameters for this

fiber are listed in Table 1 (fiber-E), With this fiber, a good agreement was found between the

values estimated from the two methods due to the fact that the core-cladding boundary was

step-like, a characteristic of fibers made by the MCVD process. However, the appearance of the

plot shown in Fig.4 is slightly different from those for matched cladding fibers, e.g. Fig.2.

Tentatively, the more straight plot of Fig.4 was interpreted as due to the high index outer

cladding. Currently, an attempt is being made to fabricate a fiber with a near perfect step index

profile and to compare the theoretically calculated cutoff wavelength with the measurement.

222
CONCLUSION

For the purpose of selecting a fiber to ensure that the LPn mode cutoff is below the

operational wavelength, the EIA cutoff was found to be unreliable because of the measurement

problem associated with the matched cladding fibers and because of the lack of correlation

between the real cutoff and this parameter. Alternatively, the cutoff wavelength estimated from

the equalization wavelength was also found to be unreliable due to the breakdown of the step

index assumption for the fibers: note fiber-D which indicated relatively low X^iEQ) in spite of

the high A<^,(MI) and fiber-A with the opposite trend. However, it has been demonstrated that the

LP,i cutoff wavelength can be estimated by measuring MI and plotting the -2n/AX'l value

against X. Even though the accuracy of the measurement is not great, the test method has the

advantages of being experimentally simple and of being independent of the power level in the

LPn mode which is sensitive to the fiber length and its lay-out condition. For a depressed

cladding fiber made by the MCVD process, a good agreement between X^iEQ) and A-^CMI) was

demonstrated due to the step-like index profile in the core-cladding boundary.

REFERENCES

[1] K.Abe, Y.Lacroix, L.Bonnell and Z.Jakubczyk, "Modal interference in a short fiber

section: fiber length, splice loss, cutoff, and wavelength dependences", J. Lightwave Tech.,

Vol.10, No.4, April 1992, pp.40 1-406.

[2] D.G.Duff, F.T.Stone and J.Wu, "Measurements of modal noise in single mode lightwave

systems". Optical Fiber Communications Conference, 1985 Technical Digest, (Optical

Society of America, Washington DC), pp.52.

[3] EIA standard, FOTP-80, "Cutoff wavelength of uncabled single-mode fiber by transmitted

power", EIA-455-80, Electronic Industries Association, October 1988.

[4] S.A.Jacobs and D.W.Peckham, "The anomalous structure observed in single mode fiber

cutoff wavelength measurements: theory and solutions", Technical Digest Symposium on

Optical Fiber Measurements, Boulder, 1990, pp.49-54.

[5] A.C.Boucouvalas and S.C.Robertson, "Simple technique for measurement of equalization

wavelength in single mode optical fibres", Elec. Lett., Vol.23, 1987, pp.215-216.

[6] J.L.Archambault, R.J.Black, H.Bures, F.Gonthier, S.Lacroix and C.Saravanos, "Fiber core

profile characterization by measuring group velocity equalization wavelengths", IEEE

Photonics Tech. Lett., Vol.3, 1991, pp.351-353.

[7] P.J.B.Clarricoats, "Optical fibre waveguides-a review", Progress in Optics, (North Holland,

1976), Vol.XIV, pp.329-400.

NOTE for Table 1

"M" and "D" for the cladding design stand for matched and depressed cladding, respectively.

"Quoted" means that the values were quoted by the fiber supplier. "^^.(EIA), X^(EQ), and ^c(MI)"

are the cutoff wavelengths determined by the EIA test method, the LPqi-LPh equalization

wavelength, and the modal interference pattern in the 1300 nm range, respectively.

223
Table 1: LIST OF PARAMETERS FOR THE FIBERS

Fiber: A B C D E

Cladding design: M M M M D

MFD(quoted), urn: 8.96 9.44 9.32

MFD(measured), iim: 9.40 9.32 9.42 9.40

A,c(EIA)(quoted), nm: 1238 1211 1319

\, nm: 1020 1000 980 985 1150

A,c(EIA)(measured), nm: 1235 1230-60 1225 1310-40 1260

^c(EQ), nm: 1290 1265 1240 1246 1450

?ij.(MI), nm: 1320 1340 1310 1350 1440

Fig.l: Example of MI pattern;

fiber-A, / = 10 cm.

Fig.2: -liObX-l vs. \ plot;

fiber-A, / = 10 cm (O), 1.5 cm (•).

13oo i2o i3bo WAVELENGTH (om)

CM

I <E

I

1

aia

-4

•€

-8

-10

-12

-14

-16

o

0300

CD • • •

• •

'

' <^ 1000 1100 1200 1300 1400

WAVELENGTH (rwn)

1500 1600

Fig.3: Fiber index profile;

solid line; fiber-A,

dotted line; fiber-E.

85 90 i5o i55~ DISTANCE (urn)

Fig.4:

t

eco

-2tz/AX'1 vs. A, plot;

fiber-E, / = 1.5-3.0 cm.

1000 1100 1200 1300 1400

WAVELENGTH (nm)

1500 1600

224
LENGTH AND CURVATURE DEPENDENCE

OF THE EFFECTIVE CUTOFF WAVELENGTH IN SINGLE-MODE FIBERS

MEASURED WITH AN AZIMUTHAL FILTERING TECHNIQUE

D. PAGNOUX J.M. BLONDY

Institut de Recherche en Communications Optiques et Microondes

123, avenue A. Thomas - 87 060 Limoges - FRANCE

1. Introduction

The theoretical cutoff wavelength X,q of a single-mode fiber determines the theoretical frontier between the

single-mode and the bimodal propagation regions. On the other hand, the effective cutoff wavelength Xg is the

practical limit of the single-mode operation domain of the fiber. It takes into account the effects of all the

perturbation parameters (bendings, microbendings, core diameter fluctuations, etc.). Xq also strongly depends on the

length of the fiber and is always lower than X^.

When the operating wavelength X is lower than Xq, the first higher-order mode can propagate, inducing modalnoise and pulse dispersion. So, for an installed single-mode fiber, Xq must be accurately determined. Many techniques

have already been proposed, among which : the near-field shape observation method [1], the mode interference

technique [2], the field coherence degree measurement [3] and the short pulses propagation experiment [4]. The

technique recommanded by the International Advisory Committee mi Telegraph and Telephone (CCITT) is the bend

loss technique [5]. Most of these previous methods need a measurement reference or can work only with specific

lengths of fiber.

In this paper, we compare the length and curvature dependence of the effective cutoff wavelength for two

different kinds of single-mode fibers. We make use of an azimuthal filtering self-referenced technique, allowing precise

measurements of Xq for lengths of fiber between some centimeters and several kilometers. This technique will be

described in the next section, and then the experimental results will be given and analysed.

2. The azimuthal filtering technique [6]

The experimental set-up is shown in figure 1. The light beam from a 0.5 nm spectral width monochromator,

chopped at a f^ fi"equency equal to 70 Hz, is launched into the fiber under test. The output far-field pattern of the fiber

is scanned by mean of a rotating "bow-tie" slit, which is rotated at a fg frequency equal to 4 Hz. The part of the light

crossing the slit is focused by a concave mirror on the input face of a large core diameter multimode fiber. The optical

power detected by a cooled Ge-detector at the output of the multimode fiber is displayed on a FFT spectrum analyser.

In the single-mode domain, the part of the optical power crossing the rotating slit is constant and a single-line

spectrum at the f^ frequency is displayed by the analyser. In the bimodal region, a modulation due to the azimuthal

properties of the propagating LPj i mode is introduced in the detected power. This modulation creates two lateral lines

at the fj frequency, with fi = fc ± 2fs, in the spectrum of the displayed signal.

Pj 1 and Pqi are the optical powers carried by the LPj j and the LPqi modes, respectively. The ratio Pj i/Pqi

225
expressed in dB is simply related to and A], the amplitudes of the lines at the and fj frequencies respectively, by:

rXX) = lOlog

11

01

= lOlog

2A,

2Ai

(1)

In the monomode region, Pn = 0 and then D(K) = -<». in the bimodal region, in the case of an equal

repartition of the power between the LPqi mode and the LP21 mode, D(K) becomes equal to 0. At last, assuming no

fundamental mode in the fiber, D(k) would tend to +<». D(K) exhibits very large variations when X crosses Xq.

In the bend loss technique, Xq is the wavelength for which the total transmitted power level is 0.1 dB higher

than the power transmitted by the pure LPqi mode. In our technique, this criterion corresponds to a D(X^ value equal

to -17dB. So, Xq is simply determined by scanning the wavelength domain until D(X) reaches -17dB.

3. Experimental results

Length dependence of Ag : The bend loss technique is not adequate in order to study the length dependence of the cutoff wavelength over approximatively 10 m. An alternative method, based on the measurement of the losses of a

splice realized between the tested fiber and a monomode fiber with low Xq, has already been used [7].

With our method, we have measured the length dependence of Xq for a slightly depressed cladding single-mode

fiber (DC fiber), from 1 meter to 2.15 kilometers. The fiber under test was submitted neither to bends with less than

200 mm radii, nor mechanical stress. The experimental results are reported in figure 2.

Xq is a linear function of the logarithm of the length, for lengths from Im to 2 km. For a 2 m reference

sample, the value of Xq is 1 105 nm. The average shift of Xq is approximatively 40 nm per decade. Our results are in

good agreement with previously published results. Over 2 km, because of the differential attenuation, the optical

power guided by the LPjj mode becomes negligible compared to the power guided in the LPqi mode, and this

phenomena induces large imprecisions in the determination of Xq . Same effects have been mentioned in [7].

A quadruple-clad dispersion flattened fiber (QC fiber) has then been tested. In order to avoid difficulties due to

special propagation properties [8], we have first verified that the LPji mode was the first higher-order mode in this

fiber. As shown in figure 3, Xq exhibits a remarquable linear variation versus fiber length, until 25 m. In the first

25m, the down slop of the curve is 10 nm/m. In the following 2.5 km, no significant variation can be noticed. The

length dependence observed for this kind of fiber is very different from the typical variations obtained with the DC

fiber. The cuivature dependence of Xq for the two different fibers is studied in the next section.

Curvature dependence of X^

: The measurements have been made on 2m reference samples of the tested fibers. Every

bend was one complete loop around a drum with a constant curvature radius. For the DC fiber, experimental results

are reported in figure 4. The curve decreases linearly versus 1/R, where R is the bend radius. Xq falls down from

1110 nm when R = 35 mm to 875 nm when R = 4 mm. For values of R larger than 40 mm, no significant

variations of Xq have been observed, as reported in [9].

226
In the case of the QC fiber, is directly an increasing linear function of R up to 150 mm, as shown in

figure 5. is very sensitive to large bends radii, which are somewhat slight perturbations of the guide. The losses of

the LPi 1 mode are then largely increased by small perturbations such as these bends, but also microbends or core

diameter variations. This can partially explain the strong length dependence of in the QC fiber, in the very first

meters of propagation. More detailed results will be presented at the conference.

4. Conclusion

The proposed azimuthal filtering technique, easy to set-up, allows a very quick and accurate determination of

Xq owing to the very abrupt variations of the parameter D(K) in the cutoff wavelength region. The precision of the

measurement is better than 1 nm. It does not require any reference measurement and is also insensitive to source power level and detectOT sensitivity drifts. The method can be used for any length of fiber as well as for installed fiber

links. To our knowledge, the only disadvantage of this technique is that it does not work with some special multiclad fibers, for which the first higher-order mode is the LP02 mode instead of the LPj j mode.

Length and curvature effects have been measured for DC and QC fibers. Particularly in the QC fiber, the value

of Xq measured on a short reference sample is largely higher than the value measured for an operating link, due to the

rapid decrease of with length and curvature. When designing the fiber, this behaviour must be taken into account in

order to reduce the losses of the LPqi mode by keeping the practical value of sufficiently close to the operating

wavelength.

5. Acknowledgements

The authors are grateful to Pr. P.H. FACQ and M. CLAPEAU for their major contribution in the conceiving

of the proposed technique, and for helpful discussions.

6. References

[1] Y. MURAKAMI, K. KAWANA and H. TSUCfflYA : "Cutoff wavelength measurements for single-mode

optical fibers", App/. Opt.. 1979, 18, pp. 1101-1105.

[2] E. BRINKMEYER and S. HECKMANN : "Cutoff wavelength determination in single-mode fibers by mode

interference". Opt. Lett., 1984, 9, pp. 28-30.

[3] P. SPANO, G. DE MARCHIS and G. GROSSO : "Coherence properties and cutoff wavelength determination

in dielectric wave-guides", App/. Opt., 1983, 2 2, pp. 1915-1917.

[4] Y. KATO, K. KITAYAMA, S. SEIKAI and N. UCHIDA : "Effective cutoff wavelength of the LP^ i mode in

single mode fiber cables", IEEE J, quantum Electron., 1981, QE-17, pp. 35-38.

[5] Y. KATSUYAMA, M. TOKUDA, N. UCHIDA and M. NAKAHARA : "New method for measuring V-value

of a single-mode optical fibre". Electron. Lett., 1976, 1 2, pp. 669-670.

[6] J.M. BLONDY,A.M. BLANC, M. CLAPEAU, and P. FACQ : "Azimuthal filtering technique for effective

LPj I cutoff wavelength measurement in optical fibres". Electron. Lett., 1987, 23, n°10, pp. 522-523.

[7] W.T. ANDERSON and T.A. LENAHAN : "Length dependence of the effective cutoff wavelength in singlemode fibers", /owr/i. ofLigh. Techn., 1984, 2, n°3, pp 238-242

[8] P.L. FRANCOIS, J.F. BAYON, F. ALARD : "Design of monomode quadruple-clad fibres". Electron. Lett.,

1984, 20, n°17, pp 688-689

[9] V. SHAH : "Curvature dependence of the effective cutoff wavelength in single-mode fibers", Journ. of Ligh.

Techn., 1987, 5, n°l, pp 35-43

227
Azimuthal

mode filter

Mirror

Chopper

FFT

spectrum analyser

Halogen

lamp Vfonochromator

Ge-detector

figure 1 : experimental set-up

1.15

1.1 DC fiber

1.05

1.

0.95

+ +

L(m)

1. 10. 100. 1000. 10000.

figure 2 : length dependence of the effective cutoff

wavelength of a depressed clad single-mode fiber.

13

1.2

1.1

1.0

0.9

QC fiber

+

L(m)

' '—ri '

15 25A //J " so //Jl '100 //-J. "700 // ' //.A — 1000 400 2400

figure 3 : length dependence of the effective cutoff

wavelength of a quadruple clad single-mode fiber.

1

1.1

1.0 DC fiber

0.9

0.8

1

• ,1/R (cm"')

0.5 I.O 1.5 2.0

)Xq (urn)

1.2

1.1

1.0 QC fiber

0.9 R(cm)

2.5 5. IS la 12.5 ^

figure 4 : curvature dependence of the effective cutoff figure 5: curvature dependence of the effective cutoff

wavelength of a depressed clad single-mode fiber. wavelength of a quadruple clad single-mode fiber.

228
Mode Field Diameter and Cutoff Wavelength Profile Measurement

using Dual Wavelength OTDR

by James Warder and Costas Saravanos

Northem Telecom Canada Limited

Optical Cable Division

Saskatoon, Saskatchewan, Canada

Introduction

Mode field diameter and cutoff wavelength are two very important parameters in the characterization

of single mode optical fibers. Currently, single mode optical fibers are characterized for mode field

diameter of die fijndamental (LPqi) n^ode and cutoff wavelengtii of the LP^^ mode using destructive

test methods in vAach 2 meter long fiber samples are measured fi'om each fiber end.

The most widely used techniques for measuring mode field diameter are described in FOTP-164

(Single mode fiber, measurement of mode field diameter by far field scanning) and FOTP-167 (Mode

field diameter measurement, variable aperture method in the far field). The fiber cutoff wavelength is

measured according to FOTP-80 (Cutoff wavelength of uncabled single mode fiber by transmitted

power). Unless a fiber is cut into many 2 meter sample lengths (a completely destructive ^proach),

these methods yield values of mode field diameter and cutoff wavelength for ^e fiber ends only.

The test method presented in tiiis p^er demonstrates an ^proach to characterizing the absolute mode

field diameter and cutoff wavelength in a single mode fiber along its entire length using a switchable,

dual wavelength OTDR, with accuracies of ±0.3)im and ±20nm respectively.

The main advantages of this method are:

- non-destructive in nature

- provides measurements of mode field diameter and cutoff wavelengtii along the entire

fiber length

- determines several parameters from the same data set, reducing the time required for

fiber characterization

Principles of operation

It has been shown [1] that OTDR fiber signatures, acquired using a two direction measurement

method, are separated into two constituent components, representing fiber loss and fiber imperfection.

It has been fiirther determined [2] tiiat the fiber imperfection component, I(A.), is related to the mode

field diameter fluctuations by

:

I(X,z) = 201ogio

W(X, zr)

0) lW(X,z)J

The test method presented here, based on an empirical equation developed by Marcuse [3], relates tiie

229
fiber cutoff wavelenglh, X^f, and tiie ratio of mode field diameters, W(Xi) and W(X2), measured at two

distinct wavelengths, X.^ and X2, [4]

R =

W(Xi) 0.65 + 0.434 M

(3/2)

+ 0.0149 M

W(>^)-

0.65 + 0.434

(3/2)

-K).0149 M

(2)

Given tiie ratio of mode field diameter profiles (at two distinct wavelengths) the cutoff wavelenglh

profile is generated. A combination of the above methods determines absolutely the profiles of mode

field diameter and cutoff wavelength, along the entire fiber length, if W(X,, Zj) is known. But because

Ae two-direction OTDR technique provides data for tiie test fiber only, tiiese profiles are relative

measurements, not absolute values. To make absolute measurements of ^e mode field diameter and

cutoff wavelength profiles an appropriate reference must be provided to determine W(X,, Zj),

Experimental

The test mediod presented here provides this reference with tiie test setup illustrated in figure 1. In diis

setup the input port of a 1x2 optical switch is spliced to the dual wavelength OTDR input, vAale the

output ports are connected to two 1.5 km long alignment fibers. When the OTDR signature is measured fi'om both directions, each single-ended trace contains data from the alignment fiber, the

alignment points, and the fiber under test. By combining the two OTDR signatures, the loss and

imperfection components for the alignment fibers, alignment points and test fiber are separated. With

this information we reference the imperfection profiles to a known value, that is, one of die two

alignment fibers. These fibers now provide the reference from wdiich the absolute values of mode field

diameter and fiber cutoff wavelength are determined.

The test equipment used for ^s work included a single mode 1310/1 550nm dual waveleng^ OTDR, a

1x2 optical switch and the two 1500 meter long alignment fibers. The OTDR is connected to a

computer for data acquisition and analysis. The cutoff wavelength and mode field diameter of the

alignment fibers are measured using the methods described in FOTP-80 and FOTP-167.

During data analysis it is important to be able to identify the fiber end point within the data field. This

is achieved by using a highly reflective optical switch, positioned within the OTDR "dead zone" to

prevent any forward reflections from appearing within the fiber span.

The ends of the test fiber are aligned to fibers A and B using an X-Y-Z positioner and index matching

oil. The OTDR then acquires the fiber signatures from each direction with the aid of the optical switch,

(figure 2a and 2b). Subsequently, they are analyzed for fiber loss, mode field diameter (figure 2c and

2d) and cutoff wavelength profiles (figure 2e).

To verify this technique several fiber types were analyzed, with the results tabulated in Tables 1 and 2.

As can be seen, the accuracy and repeatability of this technique agrees with established methods to

within ±0.3|im for mode field diameter and ±20nm for cutoff wavelength.

230
Conclusion

The technique outlined here provides an alternate test method for tiie absolute measurement of mode

field diameter and cutofiT wavelength profiles in single mode fibers with accuracies of ±0.3^m and

±20nm respectively.

The simplicity and relative speed of this technique, coi^led with die ability to measure loss,

discontinuity, mode field diameter, and cutoff wavelength with one procedure, makes this method a

powerful tool for both process development and production monitoring.

Acknowledgements

The assistance received from Ming-jun Li and Ken Kowaliuk in the preparation of this paper is greatly

^preciated.

References

[1] P. Divita, V. Rossi; Backscattering Measurements in Optical Fibers: Separation of Power

Decay From Imperfection Contributions. Electronic Letters 15, P. 467, (1979),

[2] M.S. O'Sullivan, RS. Lowe; Interpretation of SM Fiber OTDR Signatures. Proceedings

SPIE '86 Optical Testing and Metrology (1986).

[3] D. Marcuse; Loss Analysis of Single Mode Fiber Splices. Bell System Techbical Journal,

May-June (1977).

[4] K.W. Kowaliuk, J. Femer; A Technique to Estimate the Cutoff Wavelength Profile in Single

Mode Fibers Using a Switchable Dual Wavelength OTDR. Proceedings NIST '88

Symposium on Optical Fiber Measurements (1988).

Table 1 - Comparison of FOTP and OTDR Test Methods

Fiber OTDR FOTP-80 OTDR FOTP-167

Type Cutofif Cutoff MFD MFD

Fiber A 1288nin 1297nm 8.98|xm 8.90^m

Fiber B 1260nm 1250nm 8.88^m 8.69ixm

Fiber C 1222nm 1216nm 9.10^un 9.11nm

Table 2 - OTDR Test Method Accuracy / Repeatability

Accuracy (3ct) Repeatability (3a)

MFD (1310nm) < 0.3 < 0.1

Cutoff Wavelength < 20 nm < 10 nm

231
13lO/l650nm

OTDR

Switch

Alignment Fiber

Fiber A Alignment

Test

Fiber

KAllgnment Fiber

Fltaer B Alignment

figure I - OTDR Test Method Setup

5

.5 -

5 B

-.8

-1

.« a

•3 -'^

-i

.5

10.5!:

J*! 10.5

I

^ 10.4!-

(d)

1' I ' 1 1 1 1 1 1 1 1 1 11 1 1 I ll 11 1 11 1 1 1 1 11 1

I 2 3

figure 2 - OTDR Test Results

(a) Bidirectional traces @ UlOnm

(b) Bidirectional traces @ ISSOnm

(c) MFD Profile @ 1310nm

(d) MFD Profile @ 1550nm

(e) Fiber Cutoff Wavelength Profile

232
Far Field MFD measurements from a reduced far field pattern

A. Chlantore, F. Cocchini, D.Cuomo, G. Ferrl

F. O. S. S. p. A. , Battlpaglla CSA3, ITALY

1., Iptrod uct ion

Far field technique, for Mode Field Diameter (MFD) measurements, consists in measuring the far field pattern of a fibre,

i.e. measuring the radiated power from a fibre on a given (symmetric) angular range at a given distance from the output end of the

fibre. From the wave propagation theory, ^^th^ far field

(Fraunhofer) zone is any distance greater than S /X, where \ is

the operating wavelength, and S is any dimension representative of

the radiating object; for optical fibres X = 1300 nm while S <MFD)

% 10 A^m, so that a distance x is in the far field zone if:

H > > -^^76 Aim » 0. 076 mm

To fulfil this prescription, one usually choose x % 1 cm

(i.e. X 100 S /X), not exceeding this limit in order to balance

the far field request with the limited amount of power available

from solid states ligth sources.

The receiver, at the distance x from the output end of the

fibre, is capable to rotate on an angular range the amplitude of

which is a function of the MFD (i.e. of the refractive index

profile) of the fibre: the lower the (near field) MFD of the

fibre, the greater the angulare range. For instance, to reconstruct the MFD of a standard step index fibre for telecommunication an angular range of at least ± 22 is necessary; moreover,

this range must be spanned (in order to obtain the required

accuracy) with a step of 0.5 degrees: the obtained power data (F)

can be consequently used as input for the so called Petermann II

definition of MFD:

. r /^q»F'(q)dq

The aforementioned procedure, even though accurate and easy

to follow, is very expensive in terms of time, requiring the

acquisition of typically 90 data per measure. This main problem

could be avoided reducing the number of necessary data, i.e.

trying to reconstruct the MFD from a reduced set of experimental

points. The set can be identified, when the pattern to be reconstructed is very regular, following a method CID based on a model

suggested by T. Hanson for spectral attenuation modeling ZS.1.

This paper describes the results of a study devoted to the

determination of such a reduced set of data.

g. Theoret ical back ground

Given "m" far field pattern corresponding to "m" different

fibres, let a., be the data read by the receiver at the angle "i"

233

1
for the J-th fibre: for instance, a^

^ is the power at -££

for the l~st fibre. Our task is to reconstruct the whole data set

a. (j = 1 to «, i = -2£ to +££) represented by a matrix ft with

dimension m x n (n being the number of sampling angles for the far

field pattern), through a matrix B containing the reduced set of

data for the same m fibres (so that its dimension is m x p, with p

« n) and an unknown matrix S, the dimension of which is of course

p X n. In formulae:

ft « B S

so that S is derived in the least squares sense ass

T —IT

S = (B B) B ft

T

B being the transpose of B. Depending on the statistical significance of the matrix ft from which S is obtained, the same matrix S

can be used to derive the far field pattern of any other fibre,

Just measuring the far field in the reduced number of points "p".

The accuracy of this procedure has been inferred:

a) a priori, choosing a data set (matrix ft) the distribution

of which closely resembles the distribution of the whole

populat i on

b) a posteriori, optimizing the prediction of the MFD's:

using a linear regression technique to obtain the elements of

S, the average error (measured values - simulated values) in

the prediction of the MFD by definition tends to zero, so

that a more appropriate parameter to be minimised is the

standard deviation (ty) of those errors.

fts mentioned, the s. . (elements of S) were determined

applying a standard linear regression technique on 50 known

(measured) complete far field patterns of as many fibres. However,

the identification of the s. . cannot prescind from the determination of the proper number of elements of B <b_), i.e. one has to

find how many and which far field data have to be in any case

measured. The criterium used was, first to fix only one angle (and

the symmetric one with respect to the fibre axis) and minimize <y as a function of its position; then, once the position of the

first angle has been found, to look for the position of the second

angle and so on, until cr reaches a given value.

3,. Experimental set-up and result

In order to determine the a. s a ("home-made") direct far

field bench was used, with a dynamic range of about -60 dB,

measuring 50 step index single mode OVD fibres (SM-R, with MFD'

ranging between 9.0 and 9.6 yjm i?1300 nm) and 50 segmented core

dispersion shifted DVD fibres (SM-DS, with MFD' s ranging between

7.6 and 8.4 /jm (ai540 nm) : the close similarity among the far field

patterns is clearly shown in figg. 1 (SM-R) and £ (SM-DS), also

Justifying the use of the method pr^eviously described.

The first step consisted in assuming that the data necessary

to correctly reconstruct the MFD can be reduced to only one angle,

and in determining its value: in figg. 3a, 4a, 5a the standard

234
deviation of the errors as a function of the angle value is given,

showing that a minimum is obtained for an angle equal to ± 8 for

SM-R fibres (MFD ©1301, fig. 3a and (£"1540, fig. 4a), and to ± 14*^

for SM-DS fibres (MFD ©1540, fig. 5a). Once the first angle was so

identified, next step was to find the position of the second

angle, and so on: finally, we found that using four angles (8

measured points) the standard deviation of the errors was reduced

down to (or less then) 1"/-. This is clearly shown by figg. 3, 4, 5,

which should have to be used as follows:

1) curves a) show the trend of a when the MFD is

reconstructed from one angle only;

£) once the first angle position has been fixed (e.g., for

the MFD ©1301 nm of a SM-R fibre, it's ± 8*^), looking at

curves b) the position of the second angle is found (in the

example above, ±0.5 );

3) at this stage, an accuracy of about 1.5 % has been

achieved: depending on the target, one can continue with the

same procedure, or stop; in our simulation, four angles (8

measurements) are necessary in order to obtain an accuracy of

about IS. It is clearly seen from the figures that the

minima are more evident for the first angles (curves a,

b, c) : increasing the number of angles (curves "d" and

possibly other curves for 5, 6 or more angles) one tends to

the intrinsic uncertainty in MFD evaluation, so the curves

become more flat; however, in these situations the minima are

readily found from the numerical data, not from the graphs.

Even better results than \% of uncertainty can be achieved

with five or more angles.

4. Conclusions

In this paper, a method has been suggested to obtain the MFD

of SM~R and SM-DS fibres starting from a reduced set of measured

far field power values: the reason for such a kind of study was in

the attempt to reduce the (very long) time necessary for a direct

far field measurement, which is the most accurate and reliable,

without loosing too much in terms of precision.

We have shown that, knowing a certain numbev^ of complete far

field patterns representative of a given category of fibres, it is

possible to reproduce the MFD with an accuracy of less than \%

reducing the number of necessary points from about 90 to 8. Even

better accuracies can be obtained with 10, 1£ or more data

acquisition points: however, we point out that a precision of \%

could be accepted, if compared with the tolerance (lOS) suggested

by CCITT 1133 on the nominal value of SM fibres.

S-L. Re ferences

C13 M. Ohashi aX. : "Loss Model for singleraode fibres". Elect.

Lett. 28, 6, 1932.

C2D T. ft. Hanson: "Spectral attenuation modeling with matrix

methods", OFMC Proc. , York (UK), 1991.

C33 CCITT Rec. G. 652, Blue Book, Melbourne 1988

ftckn ow l ed gem ent s

We gratefully acknowledge FOS General Management for the kind

permission to publish this paper.

235
FF PATTERNS SM-R 130 1 nm 0 0000

-6 0000

-12 0000

-18 0000

-2^ 0000

-30 0000

-a -36 0000 o

0000

n

-18 0000

-51 0000

-60.0000

l''F I'ATTbJRNS SM-DS 15-10 iiin

CM — —

ro OD —. —. OJ

ingular displacement (*) [degrees]

Fig. 1

angular displacement {&) [degrees]

FiQ. £

0.5000

0.2500

0.2000

0. 1000

0.0500

0.0250 0.0200

0.0100

0.0050

SM-R 1301 nm 0.5000

0.2500

0.2000

6^ 0.0500

>e6

oo

5; 0.0250 u 0.0200

0.0100

0.0050

SM-R 1540 nm

* • -./- :-s^^- . . . ye^-<-^:. . . -r

' m—

V

CD QO —• K%

angular position [degrees]

Fig. 3

O (NJ -J" to do o OJ O r^j -a- (D 00 —• ro

angular position [degrees]

Fig. 4

SM-DS 1540 nm

^ a 0- ^^^^ ' ^

""'-a 1fir—

A

angular position [degrees]

Fig. 5

Legend* for Flgg. 3, 4, 5

error o for MFD reconstruction froi

curve a - (•) - one angle;

curve b - (A) - two angles;

curve c - (A) - three angles;

curve d - (9) - four angles.

236
AUTHOR INDEX

Abe, K. 221 Hale, P.D. 51,55 Potenza, M. 171,205

Antos, J. 89 Hall, D. 195 Poyntz-Wright, L.J 137

Arai, S. 67 Hallam A G 59 Pozzi, F. 95

Artiglia, M. 171,179,205 Hanson T A 217 Pregibon, D. 15

Atkins, G.R. 81 Hattori, Y. 63 Puleo, M. 115,123,205

Atkins, R.M. 103 Heffner, B. 131

Hickernell, R.K. 201 Quoi, K.W. 71

Baines, J. 45 Horiguchi, M. 201

Baney, D.M. 163 Horiguchi, T. 11 Raine, K. 45

Barlow, A.J. 107 Hui, R. 123 Reed, W.A. 71

Benson, J.M. 85 Robert, P. 151

Berardinelli, J.D. 7 Ingles A L 15,127 Rossaro, A. 171

Blondy, J.M. 225 Inoue, A.

' 63

Bonanni, P. 205 Ishida Y 67 Saravanos, C. 37,229

Bonnell, L. 221 Sato, T. 11

Bottanelii, M. 119 James, D.A., Jr. 15 Satoh, H. 41

Brown, C.S. 217 Judy, A.F. 19 Sceats, M.G. 81

Schmitzer, H. 155

Cai, Y. 221 Kapron, F.P. 7,29 Shaw, Julia H. 59

Caponio, N. 123 Kashiwada, T. 209 Shigematsu, M. 209

Cavaciuti, A. 171,205 Kiiveri, P. 183 Shimizu, M. 201

Chiantore, A. 233 Kilmer, J. P. 33 Shinoki, S. 63

Chojetzki, C. 155 Kim, K.S. 71 Sikora, E.S.R. 77

Chou, H. 167 Kobayashi, Y. 63 Smith, R.J. 217

Chunnilall, C.J. 85 Kohmura, Y. 67 Sordo, B. 205

Clayton, J.B. 15 Kosiorska, H. 23 Sorin, W.V. 163

Cocchini, F. 233 Koyamada, Y. 11 Stokes, L.F. 141

Cole, D.R. 213 Kume, N. 41 Stolen, R. 71

Cuomo, D. 233 Suchoski, P. 111

Lacroix, Y. 221

DeBernardi, C. 95 Lewis, N. 1 Takada, K. 159,201

DiVita, P. 171 Th^venaz, L. 151

Drapela, T.J. 187,191 Maeda, J. 145

Duca, L. 119 Matsuoka, R. 41 Vance, M.E. 213

Dultz, W. 155 Mechels, S.E. 55 Varachi, J. P. 33

Morasca, S. 95 Vezzoni, E. 115,123

Emig, K.A. 217

r Vobian, J. 155

Namihira, Y. 145 Voots, T. 107

Ferri, G. 233 Nikl^s M 151

Ficke, W.H. 107 Nishimura, M. 209 Warder, J. 229

Franzen, D.L. 51,187,191 Wong, B. 23

Pagnoux, D. 225 Wright, J.V. 77

Gambini, P. 115,123 Parker, A.D. 99

Gardner, W.B. 99 Pasturczyk, Z. 37 Yamada, M. 201

Gilbert, S.L. 191 Petersen, J.C. 85 Young, M. 55,187

Gu, Y. 137 Pitassi, S. 119 Yuce, H.H. 29,33

Poole, S.B. 81

237
I

BL-114A

(5-90)

11^ nFPARTMFNT OF POMMPRPF

NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY

1. PUBUCATION OR REPORT NUMBER

NIST SP 839

2. PERFORMING ORGANIZATION REPORT NUMBER

BIBLIOGRAPHIC DATA SHEET B92-0221

3. PUBUCATION DATE ^ September 1992

4. TITLE AND SUBTITUE

Technical Digest -- Symposium on Optical Fiber Measurements, 1992

5. AUTHOR(S)

Gordon W. Day and Douglas L. Franzen

6. PERFORMING ORGANIZATION (IF JOINT OR OTHER THAN NIST, SEE INSTRUCTIONS)

U.S. DEPARTMENT OF COMMERCE

NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY

BOULDER, COLORADO 80303-3328

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Final

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (STREET, CITY, STATE, ZIP)

Sponsored by the National Institute of Standards and Technology, in cooperation with the IEEE

Laser and Electro-Optics Society and the Optical Society of America.

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10. SUPPLEMENTARY NOTES , Previous symposia were held in 1980, 1982, 1984, 1986, 1988 and 1990. Technical Digests of

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11. ABSTRACT (A 200-WORD OR LESS FACTUAL SUMMARY OF MOST SIGNIFICANT INFORMATION. IF DOCUMENT INCLUDES A SIGNIFICANT BIBUOQRAPHY OR

UTERATURE SURVEY, MENTION IT HERE.)

This digest contains summaries of 54 papers presented at the Symposium on Optical Fiber , Measurements, held September 15-17, 1992, at the National Institute of Standards and

Technology, Boulder, Colorado. 1

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12. KEY WORDS (6 TO 12 ENTRIES; ALPHABETICAL ORDER; CAPITAUZE ONLY PROPER NAMES; AND SEPARATE KEY WORDS BY SEMICOLONS)

fiber optics; instrumentation; measurements; optical fiber; reviews

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