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NATL INST OF STANDARDS & TECH R. .C

A1 11 01 786798

Symposium on Optical/Technical digest - QC100 U57 NO.S97 V1980 C.1 NBS-PUB-R 19 NBS SPECIAL PUBLICATION 597

—ec

U.S. DEPARTMENT OF COMMERCE / National Bureau of Standards

Technical Digest - Symposium on Optical Fiber

Measurements, 1980

Sponsored by the National Bureau of Standards

in cooperation with the IEEE Transmission Systems

Sub-Committee on Fiber Optics (COMMSOC)

and the Optical Society of America.

100

• U57

no i 597

1930
NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards' was established by an act of Congress on March 3, 1901. The Bureau's overall goal is to strengthen and advance the Nation's science and technology

and facilitate their effective application for public benefit. To this end, the Bureau conducts

research and provides: (1) a basis for the Nation's physical measurement system, (2) scientific and technological services for industry and government, (3) a technical basis for equity in trade, and (4) technical services to promote public safety. The Bureau's technical work is per- formed by the National Measurement Laboratory, the National Engineering Laboratory, and

the Institute for Computer Sciences and Technology.

THE NATIONAL MEASUREMENT LABORATORY provides the national system of

physical and chemical and materials measurement; coordinates the system with measurement

systems of other nations and furnishes essential services leading to accurate and uniform

physical and chemical measurement throughout the Nation's scientific community, industry,

and commerce; conducts materials research leading to improved methods of measurement,

standards, and data on the properties of materials needed by industry, commerce, educational

institutions, and Government; provides advisory and research services to other Government

agencies; develops, produces, and distributes Standard Reference Materials; and provides

calibration services. The Laboratory consists of the following centers:

Absolute Physical Quantities2 — Radiation Research — Thermodynamics and

Molecular Science — Analytical Chemistry — Materials Science. THE NATIONAL ENGINEERING LABORATORY provides technology and technical ser- vices to the public and private sectors to address national needs and to solve national problems; conducts research in engineering and applied science in support of these efforts; builds and maintains competence in the necessary disciplines required to carry out this research and technical service; develops engineering data and measurement capabilities;

provides engineering measurement traceability services; develops test methods and proposes

engineering standards and code changes; develops and proposes new engineering practices;

and develops and improves mechanisms to transfer results of its research to the ultimate user. The Laboratory consists of the following centers:

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Engineering and Process Technology 2 — Building Technology — Fire Research —

Consumer Product Technology — Field Methods.

THE INSTITUTE FOR COMPUTER SCIENCES AND TECHNOLOGY conducts

research and provides scientific and technical services to aid Federal agencies in the selection, acquisition, application, and use of computer technology to improve effectiveness and

economy in Government operations in accordance with Public Law 89-306 (40 U.S.C. 759), relevant Executive Orders, and other directives; carries out this mission by managing the Federal Information Processing Standards Program, developing Federal ADP standards

guidelines, and managing Federal participation in ADP voluntary standardization activities; provides scientific and technological advisory services and assistance to Federal agencies; and

provides the technical foundation for computer-related policies of the Federal Government.

The Institute consists of the following centers:

Programming Science and Technology — Computer Systems Engineering.

1 Headquarters and Laboratories at Gaithersburg, M D, unless otherwise noted; mailing address Washington, DC 20234.

: Some divisions within the center are located at Boulder, CO 80303.
Technical Digest - Symposium on

Optical Fiber Measurements, 1980

Digest of a Symposium sponsored by the

National Bureau of Standards

in cooperation with the

IEEE Transmission Systems Sub-Committee on Fiber Optics (COMMSOC)

and the Optical Society of America

October 28-29, 1980

National Bureau of Standards

Boulder, Colorado 80303

Edited by

G. W. Day

and D. L. Franzen

Electromagnetic Technology Division National Engineering Laboratory

National Bureau of Standards

Boulder, Colorado 80303

C

U.S. DEPARTMENT OF COMMERCE, Philip M. Klutznick, Secretary

Luther H. Hodges, Jr., Deputy Secretary

Jordan J. Baruch, Assistant Secretary for Productivity, Technology and Innovation

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

tioxaL BCUAU

Or ITAMPABM

UM/urr

OCT 1 0 1980

Issued October 1980
Library of Congress Catalog Card Number: 80-600141

National Bureau of Standards Special Publication 597

Nat. Bur. Stand. (U.S.). Spec. Publ. 597, 148 pages (Oct. 1980)

CODEN: XNBSAV

U.S. GOVERNMENT PRINTING OFFICE WASHINGTON: 1980

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D C. 20402

Price $5.50

(Add 25 percent for other than U.S. Mailing.)
PREFACE

As optical communications technology matures, the need for better characterization of components becomes more pressing. The requirements arise in

three partially distinct areas. R and D laboratories need measurement techniques to support product development. Trade and commerce require product specifications that can be established by measurements that are precise (reproducible) and economical. And for the system designer, data on each component

must be such that the performance of an entire system can be predicted. Some

measurement problems, particularly the determination of those parameters most

directly related to system performance, are common to all three areas, though

with different emphasis. Others relate to only one area and within each there

are more specialized problems.

This situation is not unique to the optical communications industry. It

does seem at this time, however, that several unique characteristics of optical waveguides, notably mulitmode propagation, make many measurements on these

components more difficult than on analogous components in other communication

technologies. One consequence of differential mode effects is the difficulty

encountered in predicting the performance of a link from data on individual

transmission elements and joints. Another is that measurement requirements in

all the areas cited above become very dependent on the intended application of

the component. Measurements designed to predict the performance of long con- catenated systems using laser diodes, for instance, are of little use in predicting the performance of short systems using light emitting diodes.

Optical fiber measurements are a frequent topic in the technical literature and have been the subject of sessions at several meetings. Several

groups are now developing standards for fiber and fiber measurements. The

importance of the issues being considered in these forums as well as their

difficulty and diversity has led the National Bureau of Standards to sponsor

this Symposium as an opportunity for intensive discussion of fiber measurement

problems.

The program includes eight invited papers: four on technical topics,

three on standards, and a meeting report. A total of twenty-one contributed

papers was accepted by the committee. These cover the technical areas of

attenuation, bandwidth/distortion, index profile and geometric measurements,

joint/defect characterization, single mode fibers, and applied measurements.

The distribution of papers submitted suggests the current emphasis of work in

various laboratories. The most frequent topic was attenuation followed by

joint/defect characterization. Within these two areas a number of papers

involving the application of optical time domain reflectometry (OTDR) to very

long systems were submitted. The number of papers concerning single mode

fiber measurements suggests a growing emphasis in this area.

A particularly important feature of the Symposium is the time allocated

for discussion. One-hour workshops have been scheduled on each of the following topics: attenuation, bandwidth/distortion and index profile, joint/

defect characterization, and field measurements. In addition, extended discussion of fiber measurement standards has been scheduled.

iii
Twenty-four different laboratories are represented among the total of

twenty-nine papers. Twelve papers involving fifteen different laboratories

originated outside the United States.

The National Bureau of Standards is indebted to the IEEE Transmission

Systems Subcommittee on Fiber Optics (COMMSOC), the Optical Society of

America, and members of the conference committee for their assistance in

organizing this Symposium.

G. W. Day

0. L. Franzen

Boulder, Colorado

October 1980

iv
TABLE OF CONTENTS

Preface iii

Conference Committee ix

Attenuation Measurements

Multimode waveguide attenuation measurements, (invited)

R. M. Hawk, Corning Glass Works 1 Loss measurements of graded-index fibers:

Accuracy versus convenience,

P. Kaiser, Bell Labs., Holmdel 11

An accurate method for the measurement of

fiber attenuations, A. B. Sharma, E. J. R. Hubach,

S. J. Halme, Helsinki Univ. of Technology 15

A fiber concatenation experiment using a standardized loss measurement method,

A. H. Cherin, E. D. Head, Bell Labs., Norcross 19

Automated loss measurement set for optical cables,

L. C. Hotchkiss, Bell Labs., Norcross 23

Comparative measurements regarding the attenuation of

optical fibres for the Eindhoven-Helmond field trial,

A. Diekema, Netherlands Postal and Telecommunications

Services, L. H. M. Engel , Philips Telecommunicate

Services, G. A. M. Goltstein, NKF Kabel B.V., P. Matthijsse,

Netherlands Postal and Telecommunications Services,

J. W. Versluis, Philips, Product Glass Div 27

Index Profile and Geometric Measurements

Profile characterization of optical fibers

and preforms (invited), H. M. Presby, D. Marcuse,

Bell Labs., Crawford Hill 31

Linearity and resolution of refracted

near-field scanning technique, M. Young,

NBS, Boulder 37

A comparison of techniques to measure the

diameter of lightguide fiber, D. H. Smithgall,

C. M.Schroeder, Western Electric, Princeton 41

An automatic inspection system for

single fiber connector pluqs, N. K. Cheung,

N. M. Denkin, Bell Labs., Holmdel 45

Bandwidth/Distortion Measurements

Bandwidth measurement in multimode

optical fibers (invited), I. Kobayashi,

Yokosuka Electrical Communication Lab., N.T.T 49

An interrelationship between loss and dispersion in

multimode fibers, L. G. Cohen, Bell Labs., Crawford Hill,

S-J. Jang, Western Electric, Princeton 55

v
Optimization of concatenated fiber bandwidth via

differential mode delay, M. J. Buckler,

Bell Labs., Norcross . 59

Attenuation and pulse broadening along concatenated

fiber links, F. P. Kapron, F. M. E. Sladen,

P. M. Garel-Jones, D. G. Kneller, Bell-Northern Res 63

Joint/Defect Characterization

Difficulties encountered in the measurement of optical

fiber interconnection performance (invited),

K. S. Gordon, F. M. E. Sladen, Thomas and Betts 67

Loss characterization of bi conic single-fiber connectors,

P. Kaiser, W. C. Young, N. K. Cheung, L. Curtis,

Bell Labs. , Holmdel 73

Contribution to splice loss evaluation by the

back scattering technique: a statistical comparison

with insertion loss data, A. LeBoutet, CNET 77

Optical cable fault location using a correlation

technique, K. Okada, I. Kobayashi, K. Hashimoto,

Yokosuka Electrical Communication Lab., N.T.T.,

T. Shibata, T. Kosugi , Y. Nagaki, Anritsu Electric Co 81

Optical time domain ref1 ectometry by photon counting,

P. Healey, P. Hensel , British Telecom Research Labs.,

Ipswich 85

Measurements on Single Mode Fibers

Characterisation of single mode fibres,

K. I. White, B. P. Nelson, J. V. Wright,

M. C. Brierly, A. Beaumont, British Telecom Research

Labs., Ipswich 89

Effect of curvature on the cutoff wavelength of

single mode fibers, P. D. Lazay, Bell Labs., Murray Hill 93

Optical time domain ref1 ectometry on single mode

fibers using a Q-switched Nd:YAG laser, D. L. Philen,

Bell Labs., Norcross 97

Interferometric technique for the determination of

dispersion in a short length of single mode optical

fiber, W. D. Bomberger, J. J. Burke, Univ. of Arizona 101

Applied Measurements

Industrialized .system for the automated measurements of

the optical properties of waveguide fibers,

E. F. Murphy, R. W. Lapi erre, C. F. Laing,

Corning Glass Works 105

Wavelength meters for optical cable transmission systems,

K. Sano, K. Okada, Yokosuka Electrical Communication

Lab., N.T.T., T. Oki, Ando Electric Co., Ltd 113

vi
Report on Optical Fiber Measurement

Conference in Berlin

Fiber measurement techniques in West Germany (invited),

J. Feldman, Deutche Bundespost, Berlin 119

Standards for Fiber Measurements

The preparation of standards for "Optical fibers and

cables" within the International Electrotechnical

Commission (invited), M. P. Smid, NKF Kabel 121

CCITT studies on optical fibres cables measurements

(invited), F. Bigi, CCITT Secretariat-ITU, G. Bonaventura,

AAST, Ministry of Communication, Italy 129

Waveguide fiber standards (invited), R. E. Love,

Corning Glass Works 135

Index of Authors 144

Except where attributed to NBS authors, the content of individual sections

of this volume has not been reviewed or edited by the National Bureau of Standards. NBS 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 Bureau of Standards.

vii

CONFERENCE COMMITTEE

D. L. Franzen, NBS, General Chairman

G. W. Day, NBS, Program Chairman

R. L. Gallawa, NTIA/ITS

Peter Kaiser, Bell Laboratories, Holmdel

F. P. Kapron, Bell -Northern Research

D. B. Keck, Corning Glass Works

Tingye Li, Bell Laboratories, Crawford Hill

0. Szentesi, Siecor Optical Cables

ix

Multimode Waveguide Attenuation Measurements

Robert M. Hawk

Research and Development Division

Corning Glass Works, Corning, New York 14830

Several multimode optical waveguide attenuation measurement

techniques are described in terms of the basic phenomenological

propagation model. Previous interfacility variations can be

explained primarily on the sensitivity of modern low- loss waveguides to the effective mode volume of the launch conditions and

the state of mode coupling in the test length. Direct measurement

of EMV transfer and EMV dependent attenuation is seen to be a

possible method of reducing variations.

1
Attenuation measurement in optical waveguides is probably the

measurement most widely studied over the longest period of time

because of its tremendous functional impact on waveguide systems.

With the evolution of the technology and the continually decreasing

attenuation, now in the 0.5 to 1 dB/km range, variations in the

measurement of this parameter, both intra- and inter- facility , can

be a substantial fraction of this value. The purpose of this paper

is to examine various attenuation measurement techniques using a

recently developed phenomenological model of the propagation

characteristics. The necessity of some practical model has been

demonstrated clearly by the results of Figure 1 where measurements

from various laboratories differ by over 1 dB/km for the same

waveguide. To be useful, the framework should not only explain

these results but indicate ways for reducing these interfacility

variations

There are two basic views of OWG attenuation measurements.

One holds that the waveguide should be characterized by an insertion

loss value (i.e. dB loss over a fixed length). The other asserts

a simple exponential length dependence of power (i.e. dB per unit

length) . These viewpoints are probably the natural result of the

two roles of user and manufacturer where the latter produces unit

lengths for use in arbitrary length installations. Of course these

views are great oversimplifications of the physics involved in

optical waveguide power propagation. The fact is that both viewpoints are partially incorrect because they use the common viewpoint

that the waveguide can be thought of as a two terminal device. It

is now becoming increasingly clear that multimode waveguides should

2
be treated as at least an N terminal interconnected network where

N is the number of propagating mode groups in the waveguide.

While the basic multimode propagation model has not changed,

the quality of waveguides available has improved significantly.

(2 3) Recent papers v

' 7 indicate that the strength of coupling between

mode groups can be very low. Thus the power launched into a mode

group tends to propagate in that group with loss characteristics

that may be quite different from other mode groups. This differs

(4) from past results v 7 that power was redistributed among mode groups

in a fairly short length of waveguide (.5-2 km). Now, however,

attenuation measurements must take into account the entire model

including at least three factors : (1) differential model attenuation

rates, (2) mode coupling, and (3) input modal power distribution.

If these can be controlled or measured then interfacility variations

should be significantly diminished. The techniques that exist '

for directly measuring differential attenuation have been primarily

used by research and engineering laboratories to study propagation

phenomena and are generally expensive.

One practical approach to the variability problem is to

describe an equilibrium modal power distribution. If a stochastic

balance exists involving high-order mode loss and mode coupling,

then after some length a valid attenuation rate can be measured,

which will be independent of excitation conditions . For this

description to be useful, however, equilibrium must be obtained in

(2)

a reasonably short length. Recent measurement by Holmes v 7 have

shown that the attenuation rate of the 10th 1-km length of a concatenation experiment is still dependent on the launch conditions into

the 1st kilometer length, therefore, this situation may not exist.
A similar approach is used to measure "steady state" properties

where the "steady state" is defined as a condition where the modal

power distribution near the input of the test length is the same

as obtained at the long length. ' Rigorous attenuations are thus

obtained. One drawback of this method is that design constraints

for a particular system may not allow the flexibility of choosing

a source that will launch the same "steady state" distribution.

Possibly more serious is that mode coupling or conversion at splices

or in-line devices such as splitters or couplers will effectively

re-excite a non-steady-state distribution causing unpredictable

results for the attenuation of subsequent lengths of waveguide.

(2) The empirical technique proposed by Holmes v 7 is another

attempt at minimizing measurement variations with the added value

that predicted attenuations for long length systems of graded index

waveguide can be accurately made from short (1 km) measurements.

The basic method used is the standard cutback technique where the

effective attenuation rate is determined by measuring through-put

power of a 1-km length and a power after 2-m and calculating the

rate as - "^L^°^ ^long^short^ ' Additionally, a quantity called

effective mode volume (EMV) is calculated from the measured FWHM

diameter of the near-field power density distribution, ^> e ff> and

the sine of the angle at the HWHM of the measured far- field radiant

? intensityJ distribution, NA The EMV is (D ^-NA and is ' eff eff eff

defined for both long and short fiber lengths. The short length

EMV can be varied by changing the aperture limits of the excitation spot size (aperture A) and excitation NA (aperture B) as

shown in Fig. 2. Figure 3a and 3b show the measured long length

4
EMV and attenuation rate for a typical Corning graded index waveguide as a function of the short length EMV respectively. Also

shown are measurements of the same waveguide with moderate mode

coupling introduced by a microbending perturbation. As one would

expect, a small launch EMV is broadened and a large one reduced.

A long length of such a perturbed waveguide would rather quickly

approach an equilibrium EMV of about 5.

Using the measured EMV transfer and measured EMV attenuation

rate on 9 , 1.1 km lengths of waveguide which were subsequently

fusion spliced, a prediction of the concatenated attenuation was

made and compared with the actual measurement. The predictions

are shown in Fig. 4 and the measured values in Fig. 5. Based on

other data the deviations from predicted values can be explained

completely by "weak" mode coupling at the fusion splices. Note

that this method is nearly source- independent since it is based

on measured distributions within the waveguide.

Based on the cummulative probability distribution, 80?o and

987o of the absolute losses are predicted within 0.5 and 1.0 dB

of the measured values for any launch and/or length condition.

While this is a demonstration of what should be possible, a major

drawback of this technique is the cost of the large number of

measurements required to specify the EMV transfer and attenuation

rate dependence

.

Much cost advantage would be gained by choosing 2 or 3 launch

EMV's if the general shape of the attenuation and mode volume

transfer could be assumed constant. Figure 6 shows results of

5
EMV transfer measurements for 7 Corning waveguides. A total of

about 50 waveguides has been done with the results agreeing with

this figure. Corning has instituted spot size and NA controls in

2 all measurement facilities that launch an EMV of about 9 (ym) .

In addition to closely approximating the EMV for which the attenuation rate is nearly constant an added benefit has been to reduce

the 957o confidence level for interfacility agreement from 0.75 to

0.5 dB/km.

In conclusion it is clear that the la = + 1 dB/km variations

of Fig. 1 can be explained by varying mode coupling, differential

modal attenuation and launch power distributions among several

laboratories. As indicated by the close agreement between predicted

and measured attenuation values it also appears that the effective

mode volume is the relevant parameter to track and/or specify in

order to minimize these inter- and intra-facility variations. It

also appears possible to select a single EMV in some cases for

which the attenuation characteristics remain stationary over fairly

long distances. What remains unclear is whether a single launch

EMV will be suitable for all types of fibers as well as satisfy the

needs of systems designers, sales engineers and purchasing agents.

If this single EMV cannot be found then more extensive transfer

measurements will be required which can generate extremely good

concatenation prediction but at a cost to the technology. This

trade off between increased measurement cost and increased system

design precision must be evaluated as the technology matures.

6
REFERENCES

1. G. W. Day and G. E. Chamberlain, IOOC '79, Amsterdam, 1979.

2. G. T. Holmes, 6th ECOC, York, 1980.

3. K. Kitayama, S. Seikai, and N. Uchida, IEEE Journal of Quantum

Electronics, V. 16, No. 3, March 1980.

4. R. Olshansky, M. G. Blankenship , and D. B. Keck, Proc. 2nd

ECOC, IV. 5, Paris, 1976.

5. R. Olshansky and S. Oaks, Appl. Opt. 17, 1830 (1978).

6. K. Kitayama, M. Tateda, S. Seikai, and N. Uchida, IEEE Journal

of Quantum Electronics V. 15, No. 10, Oct. 1979.

7
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1

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1 1 1 1 l

LABORATORY

Fig. 1. Typical interfacility

variations. (after Day et al

,

Amsterdam, 1979)

xopper pinhole adjustable microscope

|

, APERTURE L£NS6S

3 $ MONOCHROM -

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INPUT -OUTPUT EMV TRANSFER

FIBER 21217

25.0

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INPUT EMV Fig. 3a. EMV transfer measurements for 1 km.

ATTENUATION RATE

1 r- FIBER 21217

6.

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£ 2.L a i a

# „ " PERTURBED WITH MICRO BENDING

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5.0 10.0 15.0 20.0 25.0 30.0 35.0 40. INPUT EMV

Fig. 3b. EMV dependent attenuation in 1 km.

P
I I I I t I I I I I I

1 234 56 789 10 11 LENGTH (km)

Fig. 4. Predicted attenuation

rate for 10 km concatenation.

1 23 4 56 7 8 9 10 11 LENGTH (km)

Fig. 5. Measured attenuation

rate for 10 km concatenation.

INPUT EMV

Fig. 6. Measured EMV transfer for 7 Corning waveguides.

9

LOSS MEASUREMENTS OF GRADED- INDEX FIBERS: ACCURACY VERSUS

CONVENIENCE

P. Kaiser

Bell Laboratories, Holmdel, N. J. 07733

INTRODUCTION

Because of differential mode attenuation, the transmission

losses of multimode graded-index fibers depend on the mode distribution propagating in the fiber, and accurate loss data which

can be extrapolated to arbitrary fiber lengths are only achievable

with an equilibrium mode distribution (EMD) . The EMD establishes

itself as a result of the selective loss of the various mode

groups and the replenishing of their power via coupling from other

modes. Since the distance within which this occurs may amount to

more than 10 km's in recent low-mode-mixing fibers, techniques

have been proposed to approximate this distribution by using restricted-numerical-aperature and spot-size excitation-*- ' 2 / 3 , mode

filters 4 , mode scramblers^ , 6 , 7 , an(j long reference or dummy fibers 8

. However, the use of most of these techniques, per se, does not

guarantee the creation of an EMD. For example, a low-mode-mixing

fiber excited with lower-order modes does not establish the EMD

even for a many-km fiber length- A necessary condition for the

existence of the EMD is the constancy of the far-field radiation

pattern (FFP) along the fiber as has first been utilized for the

length- independent loss measurements of plastic-clad silica fibers^.

HATCKED-BEAM EXCITATION

In order to establish the EMD in graded-index fibers, it is

essential to control both the launch numerical aperture (NA) , as

well as the spot size of the focused beam as can be accomplished

with the set up shown in Fig. 1. The 2 5ym pinhole, together with

the optics indicated, resulted in an approximately 50ym diameter

(20 dB width) spot size at the launch point, which closely matched

the 50ym diameter core of the test fibers. A typical set of farend (FE) and near-end (NE) patterns for different launch NA's,

together with simultaneously measured spectral losses, are

shown in Fig. 2a. While the FFP 1 s widen and norrow for under- and

over-excitation, respectively, they remain essentially constant

for steady-state (SS) excitation conditions, corresponding to a launch NA of 0.23 in this particular case. The associated SS losses of 5.12 dB/km for the 3.5-km-long fiber were 0.27 dB/km higher

than the small-angle losses of 4.85 dB/km. The SS losses were confirmed for 1-km-long sections cut from above fiber within the

measurement accuracy of ±0.1 dB. (While the normalized loss in

terms of dB/km is meaningful only in conjunction with the EMD, we

continue to use it also for non-equilibrium conditions - bearing

in mind its limited meaning)

The removal of the pinhole spatial filter (Fig. 1) resulted

in an essentially uniform power distribution across the core of

the fiber, with ensuing over-excitation and excess losses even for

11
the smallest launch NA (Fig. 2b) . Inspite of the fact that the 50%

width of the NE pattern for lowest-angle excitation was still

narrow and comparable to that of Fig. 2a, its 20dB width significantly exceeded the width of the corresponding FE pattern, and

excess losses amounted to 0.33 dB/km (above 5.12 dB/km) for the

0.23 launch NA. Since most of the transient loss associated with

over-excitation already occurs within the first km, the error incurred is about 3.5-times higher for a 1-km sample length of this

fiber, or on the order of 1 dB/km. An error of such magnitude was

also obtained with a different fiber measured under these conditions

as depicted in Fig. 3 (Fiber #2, uppermost curve).

The identification and establishment of the FFP belonging to

the EMD is time-consuming and difficult, and the question arises,

to what degree this is necessary for obtaining a desired loss

measurement accuracy. As a general rule, it is important to avoid

launching highest-order and leaky modes, while the relative power

distribution of lower modes is typically not critical. Since a

long 'tail' in the NE pattern is indicative of the presence of

such highest modes, it is essential that the 20 dB width of this

pattern does not exceed the width of the corresponding radiation

pattern at the FE point. From Fig. 2b it becomes apparent that

an agreement of the widths at the 10 dB point is not sufficient/

and an agreement at the 13 dB (or 5%) point may be marginal, depending on the shape of the NE pattern near the baseline.

In order to evaluate how critical it is to launch the EMD exactly,

let us consider the loss-versus-launch-NA representation of several

nominally identical fibers drawn from different preforms (Fig. 3)

.

While the loss steadily increases for fiber #1, the loss is higher

for lower-mode-excitation, passes through a minimum for intermediate

NA excitation, and increases again for over-excitation in case of

fiber #2. This feature is characteristic for low-mode-mixing fibers

with the excess losses of the lower modes being attributable to

dopant-related compositional-fluctuation Rayleigh scattering , or

contaminants such as OH. Within the fiber NA, a minimal NA-dependence was observed with a recent Ge-P-doped graded-index fiber

(Fig. 3, #3). Obviously, a superior measurement accuracy and reproducibility can be achieved with fibers exhibiting the loss-versus

NA dependencies of fibers #2 and #3.

MANDREL-WRAP EXCITATION

As follows from above discussion, for higher-quality fibers

the losses are insensitive with respect to the exact launch

condition, as long as over-excitation with highest-order and

leaky modes is avoided. This feature can be utilized to substantial

simplify the loss measurement by initially over-exciting the fiber,

and eliminating the excess higher modes with a mode filter. In

its simplest form, such a filter may consist of a few turns of

fiber wrapped around a mandrel, with the filter action being mainly

determined by the bending radius, but also by micro-bending

(Fig. 4). For the 50ym diameter core, 0.23 NA graded-index fibers

considered, five 1.2cm diameter turns were found to reduce the 20dB

width of the NE patterns of Fig. 2b to match those of the FE pattern

12
As shown in Fig. 3, the losses obtained with the mandrel-wrap (MW)

filter are nearly independent of the launch NA, and typically agree

within 0.2 dB/km (max) with SS losses measured with matched-beam

excitation (as identified by the SS NA of 0.21; the SS NA is typically

5 to 10% lower than the fiber NA based on An, depending on the degree

of microbending, and the core-cladding interface quality) . An

excellent agreement was achieved between losses measured with the

MW filter and 1-km-long, initially-overexcited reference fibers,

an example of which is presented in Fig. 3a. : Similarly as the

MW filter, the reference fiber effectively eliminates excess higher

modes

.

Because of chromatic aberrations of the microscope objective

lenses, the launch optics has to be readjusted for NA-dependent loss

measurements in the 1.3ym wavelength region. The loss-versus-NA dependencies of several sub-1 dB/km Ge-P-doped fibers measured at 1.3

ym are similar to those observed at 0.82ym (i.e., some with a steadily increasing, and some with a parabolic dependence) (Fig. 5)

.

MW data were typically within 0.1 to 0.2dB/km (max. difference) of

the steady-state losses determined with matched-beam launch optics.

CONCLUSIONS

A high accuracy in the loss measurement of graded-index fibers

can be achieved by combining a two-point loss measurement with scans

of the FE and NE radiation patterns, which enable the identification

of the EMD. Because of an observed nearly stationary loss dependence

of higher-quality fibers for excitation near the EMD distribution,

an exact matching of the steady-state values is not necessary:

A good agreement of the loss data (within 0.1 to 0.2 dB, max.) can

be achieved by using a mode filter such as a long reference fiber,

or, more conveniently, a properly dimensioned (using FE and NE scans

of the radiation pattern) mandrel-wrap filter for the elimination of

excess higher-order modes of an initially over-excited test fiber.

Because mode coupling and differential microbending loss may

change during the fiber jacketing and cabling operation, the EMD

and steady-state losses are not necessarily constant. Furthermore,

because of potentially other wavelength-dependent differential-modeattenuation mechanisms (such as OH penetration from the substrate

tube) , the EMD also may vary with wavelength.

ACKNOWLEDGMENTS

The test fibers were made available by S. R. Nagel, M. Saifi

(WE Engineering Research Center), and the WE Company, Norcross, GA.

N. K. Cheung and T. Baker contributed with some of the loss measurements.

REFERENCES

1. P. Kaiser, Trans. IECE Japan, 61, 225 (1978); 2. K. Kitayama

and M. Ikeda, Appl. Opt. 17, 3660 (1978); 3. M. Eve et al, Opt. and

Quant. El. 10, 253 (1978); 4. P. Kaiser and D. L. Bisbee, Opt. Fiber

Com. Conf., Wash. DC. p. TuE2 (1979); 5. M. Ikeda et al, Appl. Opt.

15, 2116 (1976); 6. M. Tokuda et al, El. Lett. 13, 146 (1977); 7.

W. F. Love, Opt. Fiber Com. Conf., Wash. DC, p. ThG2 (1979); 8. M.

Tateda et al , Appl. Opt. 16 (1979); 9. P. Kaiser et al , Appl. Opt.

14,156 (1975).

13
FIBER

NEAR END FAR END

VARIABLE APERTURE WHEEL

40X MICROSCOPE OBJECTIVE LENS

- 25pm PIN HOLE

20X MO LENS

X-Y-Z POSITIONER MODE STRIPPER TOTAL POWER DETECTOR

SCANNING PIN HOLE DETECTOR ig.l Loss Measurement Set -Up For

Matched -Beam Excitation

OVER-EXCITATION MANDREL CLADDMG NEAR-END LAUNCH CONDITION WRAP MODE

MODEFLTER STRIPPER

Fig. 4 Loss Measurement Set-Up For

Mandrel -Wrap Excitation

I I I 4

1 1 1 1—

L = 3485m

LOSS 1 aB/km, LAUNCH NA AT 0 82 pm!

, OJO 530

512 490

485

LOSS «B/Bm. LAUNCH NA: AT 0 82* ml ,0J0 556

\023 545

,0.17 5.28 -012 5 30

0 5 0 6 0 7 0 8 0 9 10 1.1 1.2 WAVELENGTH IpinlNEAR END

FAR END

LARGE SPOT SIZE LAUNCH BEAM

Fig. 2 NA-Dependent Spectral

a) Matched-Beam

Excitation

X 0 82

1(G«/B> L«1060n

EXCITATION WITH

REFERENCE FIBER

LARGE SPOT SIZE

MATCHEO BEAM

MANDREL WRAP

Losses and Radiation Patterns For

b) Large-Spot -Size Beam

Excitation

# 1 L" 1980m

EXCITATION WITH

(MATCHED BEAM

- MANDREL WRAP

ig.3 Launch-Dependent Losses of

3 Different Fibers Measured

at 820nm

Fig. 5 As Fig. 3, But For 3 Additional Fibers Measured at

1300nm

14
An Accurate Method for the Measurement of Fiber Attenuations

A.B. Sharma, E.J.R. Hubach, and S.J. Halme

Helsinki University of Technology, Communications Lab.

SF-021 50 ESPOO 15, FINLAND

1. INTRODUCTION

It is widely known that one of the most serious problems in the measurement

of multi-mode fiber attenuations is the influence of the excitation conditions

at the input of the fiber. We have developed a precision instrument that can

measure the optical power of a stable source with an error of less than

0.05 % , implying that power changes in individual modes can be detected.

This capability allows us to accurately study the effect of modal distributions upon attenuation, and for the first time makes it possible to accurately

measure the loss of even a single low-loss (e.g. fusion) splice that is excited by the steady-state modal distribution of one fiber and "pumps" another

long fiber in which steady-state is once more reached before measurement.

In this paper, we briefly outline the essential features of this instrument, and go on to describe a measurement technique that can be used to

minimize the influence of cladding as well as leaky modes. Moreover, the use

of a simple correction factor allows the determination of the attenuation of

the fiber both for selective and for complete excitation.

2. MEASUREMENT SET-UP

The main departures of our apparatus (Fig. 1) from the conventional are in

the receiver and lamp circuits. The latter consists of a 220 V over-head projector lamp driven at 110 VDC, by a current source that is feed-back control15
led by the lamp voltage. The main voltage has been A.C. stabilized before

rect i f i cat i on

.

The receiver consists of a Ge detector (at room temperature) whose output

S(t) is synchronously sampled to produce a "signal" pulse train (t) and a

"background-and-dark-current" pulse train S^{t) . The difference of S^(t) and

S^(t) is integrated and displayed on a DVM. Integration times are determined

by counting pulse pairs from (t) and S^Ct), while all other timing operations are controlled from a crystal oscillator. The minimum integration time

is set to be about 20 s, corresponding to exactly kOSG pulse pairs, and can

be doubled in each of k steps to a maximum of 65536 pulse pairs. With fiber

attenuations of up to about 10 dB, and the minimum integration time, the

random error (standard deviation to mean) of the system is better than

0.05 % , while the drift of the mean over a 2^4 hour period is about 0.1 %.

The drift over a day-time period of less than 8 hours is not separable from

the random variation.

3- MEASUREMENT METHOD

An important aspect of mode control which is rarely mentioned is the effect of mode filtration upon detection. For example, the use of butt-jointed

windowed detectors with relatively small active areas can be one source of

inadvertent mode filtration. In our method, we exploit the above possibility

by deliberately introducing mode filtration in the following way. One end of

the fiber is excited by a beam with large numerical aperture and spot size.

The near field of the opposite end is magnified, and a circular field stop of

the same diameter as the magnified core is centered at the image. The limited

image is then de-magnified and focused onto the detector. Comparison of the

outputs of a long and a short fiber then yields the attenuation A for uni16
form excitation of all guided modes.

The influence of leaky modes in the above measurement can be reduced by

using a sufficiently small aperture, as can be inferred from e.g. [1]. In

this case, the powers from the long and short fibers are also measured with

the small aperture, to yield the attenuation of the lower order modes

only. This can be related to the attenuation of all guided modes by:

A

T

= (Vno

)A

L

, (1)

where n Q

is the ratio of sma 1 1 - to- 1 a rge aperture powers from the long fiber,

and r\. is the ratio of the small aperture power to the power in all guided

modes. For uniform excitation, and a sufficiently short fiber, n. can be

obtained from the near-field or refractive index profile of the fiber [2].

With the assumption of an a profile, integration over the core region yields

the following result:

n. = (r/a)

2 [l-2(r/a)a /(a+2)]/[l-2/(a+2)] , (2)

where r is the radius of the aperture, and a is the radius of the magnified

fiber core. The behaviour of r\. as a function of a is shown in Fig. 2, from

which we can conclude that r\.^ is not very sensitive to profile shape, and

can be simply evaluated for a = 2 (Fig. 3). If the measured ref ract i ve- i ndex

profile is available, accuracy can be improved by numerically evaluating n...

Preliminary measurements on a fiber with a specified attenuation of

3 dB/km at 1.1 ym gave the following results: 4.120 dB/km with only partial

filtration of cladding modes, 3-0^9 dB/km with their complete removal

(r/a = 1), and 2.860 dB/km with a field aperture of (r/a) = 0.8. The random

error of these values is estimated to be about ± 0.004 dB/km. The last of

these measurements (r/a = 0.8) includes almost all weakly-attenuated leaky

modes, and, indeed, use of (1) and (2) yields a corrected complete-excitation

17
value of 3-050 dB/km, in excellent agreement with the measured result for

(r/a) = 1.

k. CONCLUSION

We have outlined the essentials of a new method for attenuation measurement based on a precision instrument that, to our knowledge, yields accuracies that are at least an order of magnitude better than the best values

reported in the literature. The advantages of the method a re : ( 1 ) that it

allows unambiguous specification of measurement conditions, and should there

fore be easily repeatable in other laboratories, and (2) that it avoids the

use of primary-coating solvents.

REFERENCES

1 M.J. Adams, D.N. Payne, and F.M.E. Sladen: Opt. Comm. VJ_, 20k (1976)

2 D. Gloge and E.A.J. Marcatili: Bell Syst. Tech. J. 52, 1563 (1973)

Stabilization

Sync

Chopper Monochromator () Ge -detector

Sampling

Circuit

s,(t)

s,l r]

Control Logic

PushT

start

Fig. 1. Top left.

Fig. 2. Bottom left.

Fig. 3. Bottom right,

18
A FIBER CONCATENATION EXPERIMENT USING A

STANDARDIZED LOSS MEASUREMENT METHOD

by

Allen H. Cherin and Emory D. Head

Bell Telephone Laboratories

2000 Northeast Expressway

Norcross, Georgia 30071

INTRODUCTION

The emergence of optical fibers as an important communications

medium has created the need for standardized test procedures for evaluating fiber transmission characteristics.

Because of differential mode attenuation, the loss measured in

multimode optical fibers depends on how the source launching conditions

distribute the power among the propagating modes of the fiber. To obtain

repeatable and accurate attenuation measurements of optical fibers,

standardized launch conditions must be established. Figure 1 shows a

block diagram for a two point attenuation measurement that uses a mode

filter to establish the launching conditions. The mode filter is selected

to generate a far field radiation pattern at point A at the input end of

the test fiber that is matched to the width of the far field pattern

after a long length at point B. In this paper a proposed standardized

loss measurement method using a mode filter to establish input launching

conditions is described and evaluated. An operational method for

selecting mandrel wrap mode filters is given along with results from a

concatenation experiment to provide the reader with an indication of

the accuracy (length scaling predictability) of the loss measurement method.

19 i
MODE FILTER CHARACTERIZATION

Figure 2 illustrates a two step operational method for sel

ing a mode filter. First, the test fiber is overfilled in numerical

aperture and spot size at the input end and its far field radiation

pattern measured at its output end. The output far field pattern

of a mode filter overfilled at its input end is then measured. The

widths of the patterns, at the 5% points, of the test fiber (62) and

the mode filter (6^) are obtained and their percent difference A6

calculated. AG is a measure of how well the mode filter approximates

the far field energy distribution at the output of a long length of

test fiber.

The mode filter selection procedure was applied to

determine the characteristics of five turn mandrel wrap mode filters

of different diameters. For a 1 km test fiber, (110 ym OD, 55 ym

core diameter, A * 1.3%, a * 2) Figure 3 shows how A9 varies as a

function of mandrel wrap diameter. The loss of the test fiber was

then measured with an overfilled input strap (no mode filter) and

with five different mandrel wrap mode filters ranging in diameter

from 3/8" to 1".

Figure 4 is a plot of the loss of the test fiber as a

function of A0. For a range of A9 from 0 to -7% (range specified

by Electronic Industries Association test procedure) three mode

filters (1/2", 5/8" and 3/4") could be selected for the standard

loss measurement. The choice of a specific mode filter is not

critical since the standard deviation of the loss measured with the

three mode filters in this range was q = 0.15 dB/km. This deviation

was less than the uncertainty of the loss measurements (0.3 dB/km) in

a recent Bell System interlocation round-robin study."*"

20
CONCATENATION EXPERIMENT

Using a five turn 1/2" diameter mandrel wrap mode filter, the

loss of five 1 km multimode graded index fibers of similar characteristics

(110 ]im OD, 55 Um core diameter, A -1.3% a z 2, A = .82 urn) were

measured and compared with the loss of a 5 km concatenated link formed

by these fibers. Figure 5 shows the results of this experiment. The

total concatenated loss including splice losses (22 dB) differed from

the sum of the losses of the individual section plus measured splice

loss by .5 dB. This amounts to 0.1 dB/km, which is well within the

interlocation standard deviation of 0.3 dB/km."*"

REFERENCE

1. A. H. Cherin and W. B. Gardner, "Standardization of Optical Fiber

Transmission Measurements," Laser Focus, to be published.

TWO POINT ATTENUATION MEASUREMENT

CHOPPED

SIGNAL

FROM

FILTERED SOURCE

VACUUM MODE

CHUCK FILTER

LONG LENGTH OF FIBER TO

BE MEASURED

(OUTPUT MEASUREMENT)

CHOPPED

SIGNAL

FROM

FILTERED SOURCE

VACUUM

CHUCK MODE

FILTER TO

DETECTOR

(INPUT REFERENCE MEASUREMENT)

FIGURE 1

It

TO

DETECTOR

21
FIGURE 2

FIGURE 3

FIGURE 4

22

2 "J

1®

!6

14

12

10

MEASURED CONCATENATED LOSS INCLUDING SPLICES

x PREDICTED LOSS (SUM OF LOSS OF INDIVIDUAL

SECTIONS)

+ PREDICTED LOSS + MEASURED

SPLICE LOSS

12 3 4 NUMBER OF 1 Km SECTIONS FIGURE S

22
AUTOMATED LOSS MEASUREMENT SET FOR OPTICAL CABLES

by

L. C. Hotchkiss

Bell Telephone Laboratories

2000 Northeast Expressway

Norcross, Georgia 30071

INTRODUCTION

The Bell System standard lightwave transmission system (designated FT3)

employs ribbon-based cables containing relatively large numbers of fibers.

Each ribbon contains 12 fibers and is factory terminated at both ends in

silicon chip array connectors.''" An automated optical loss measurement set

has been developed for final factory testing of these cables which employs

a non-destructive insertion loss measurement technique. The test can measure

a 144-fiber cable in one hour to an accuracy of +0.4 dB (2a), and has been

used successfully to measure cables shipped to several Bell System fiber

optic installations. In comparison, manual 2-pt measurement techniques are

unsuitable since they are destructive and require about 24 hours for a

144 fiber cable.

GENERAL DESCRIPTION

The test set is illustrated in Figure 1. The optical power source used

is a 0.82 micrometer feedback stabilized Gallium-Aluminum-Arsenide laser.

The light is launched through a single fiber "pigtail," whose end is clamped

in a stationary position. A multiple fiber array connector which terminates

two 12-fiber ribbons is mounted on a pair of X-Y micropositioner stages in

close proximity to the pigtail end. The micropositioners are controlled by

a microcomputer and are used to align the ribboned fibers, one at a time, to

the launch pigtail (essentially a 1x24 optical, servo loop switch).

23
One of the 12-fiber ribbons leads directly to a receive photodiode and

is used to monitor any drift in the laser and receive electronics. The second

ribbon passes around a mandrel-wrap equilibrium mode simulator and has been

cut and then terminated on each end with single ribbon array connectors.

Ribbons to be measured are temporarily spliced into the optical circuit here.

The test set is initially "zeroed" by measuring the power transmitted

through a one-meter section of ribbon. The power values measured for each

individual fiber are recorded and used as the input values for measurements

which follow. A ribbon whose loss is to be measured is then spliced into

the optical circuit in place of the reference ribbon, and the output of each

of its fibers is measured. The loss of each fiber is then found by subtracting

this measured value from the reference value for that path. During a measurement, the center of each fiber is found by using a computer controlled search

algorithm. The fiber is searched from side to side and from top to bottom,

moving the stages one micron at a time until the edges are found. During this

center search, a record is kept of the maximum value found. The stages then

return to this preliminary maximum point and a second, more detailed, peak

search algorithm is performed to determine the maximum output value for that

fiber

.

TEST RESULTS

The precision of the test set was checked by making repeated measurements

on the one-meter reference ribbon. The reference ribbon was measured 20 times

and the temporary array splices were disconnected, flipped over (fiber 1 to

fiber 1, then fiber 1 to fiber 12) and reconnected in between each pass.

The standard deviations of the ten measurements of each fiber path were

24
computed and the mean was only 0.04 dB. Figure 2 is a histogram showing the

distributions of these deviations.

The accuracy of the test set was checked with another test. Here, a

12-fiber ribbon approximately 1 km in length was measured several times by

the widely accepted two-point technique. The same ribbon was then measured

several times by the insertion loss technique using the automated test set.

Figure 3 is a histogram of the fiber-by-fiber differences between the two

sets of measurements. The insertion loss measurements averaged 0.14 dB greater

than the two-point loss measurements (offset). This result was anticipated

and is due to differential splice loss effects of the two array splices. The

standard deviation of the differences was 0.2 dB. Test results on over

50 reels of production cable have confirmed both the offset and deviation

values.

SUMMARY

An automated test set has been developed for final factory loss measurements on connectorized fiber optic cables. The set is presently in use on

production cables and has proven to be accurate, fast and easy to use by

production personnel.

REFERENCE

1. Miller, C. M. , "Fiber-Optic Array Splicing with Etched Silicon Chips,"

B.S.T.J., Vol. 57, No. 1, Mar., 1975.

25
SINGLE FIBER LAUNCH

0.82 fim

PIGTAIL-^

LASER

2x12 ARRAY

CONNECTOR

MANDREL WRAP

EQUILIBRIUM MODE

SIMULATOR

X-Y MICRO POSITIONERS

MICROCOMPUTER

RECEIVE ELECTRONICS rcv L DIODE i

REFERENCE-- RIBBON OR

RIBBON

UNDER TEST

INSERTION LOSS

PRINTER

INDEX MATCH

LIQUID

FIGURE 1. AUTOMATED LOSS SET FOR OPTICAL CABLES

DISTRIBUTION OF STANDARD

DEVIATIONS OF MEASUREMENTS

ON THE TWELVE INDIVIDUAL

FIBER PATHS

JJJl J 01 02 .03 04 05 .06 07 08 .09 STANDARD DEVIATION (dB)

. _l L 1 J -.20 -.10 0.0 +.10 +.20 +.30 +.40 +.50 DIFFERENCE BETWEEN INSERTION LOSS AND 2-PT LOSS MEASUREMENT (dB) FIGURE 2. TEST SET REPEATABILITY FIGURE 3. TEST SET ACCURACY

26
Comparative measurements regarding the attenuation of optical fibres

for the Eindhoven - Helmond field trial.

X XX XXX X

A. Diekema, L.H.M. Engel, G. A.M. Goltstein, P. Matthijsse,

xxxx

and J.W. Versluis.

x The .Netherlands Postal and Telecommunications Services,

Dr. Neher-laboratory (P.T.T.) xx Philips Telecommunicatie Industrie B.V. (P.T.I.) xxx N.K.F. Kabel B.V. xxxx N.V. Philips' Gloeilampenfabrieken, Product Division Glass.

Abstract

Equipment measuring the attenuation of optical fibres is compared.

Using the far field pattern of the short fibre adjustment of the steady state,

conditions can be performed. Hence, the influence of different launching

conditions is minimized. Doing so, the values measured in four laboratories,

each with their own equipment, harmonize reasonable well. Comparison was necessary to investigate the Eindhoven - Helmond field trail.

Introduction

In the spring and summer of 1980 extensive measurements were performed on a number of fibres used in the 13,5 km, 140 Mb/s field trial Eindhoven - Helmond

in the Netherlands. Besides the field measurements carried out by P.T.T.

,

measurements were also done by Philips after drawing and by N.K.F. after cabling

the fibres. In addition P.T.I., being the system designer, participated in the

measurements. To be able to compare the results extensive consultations were

necessary}^ In the end the results harmonized reasonably well. This paper gives

the precautions taken and the attained result.

Measuring equipment

The attenuation is measured applying the cut-back method (1,5 m) . Such

launching conditions are used to approximate the steady state in the fibre

under test from the beginning.

To compare the measuring methods used, a six fibre bundle on reel with fibres

typical of those in the field trial was sent round the four laboratories.

The measurements showed that the equilibrium mode distribution of the six

fibres corresponds with a numerical aperture NA (defined at 10% of the

27
maximum photometric intensity of the far field radiation pattern) of about

0,19. An additional parameter, the form factor Y= NA^/NA^., was used to

describe the far field distribution more accurately*^ ; the equilibrium values

turned out to be 0,56 approximately. The theoretical NA of the fibres is about

0,28.

The far field radiation patterns of the fibres were measured either by a far

field meridional scan or by an integrating method (P.T.T.). Here a large area detector is shifted along the axis of the radiation pattern. This method enables

low level measurements as may occur in the field. Moreover the measurements are less affected by coherence effects in short pieces of fibre. However, the

results are not comparable directly with those of the other method.

The equilibrium values as mentioned above are created by applying a mode

scrambler, a one to one lens coupler and a well-chosen diaphragm between light

source and fibre under test. Each laboratory made its own equipment using

different components. The mode scrambler, a round or rectangular type, is based

2) on the random distribution of ball-bearings in several sizes . The fibre in this

scrambler was selected independently by each participant. One participant

(Philips Glass) used a halogen lamp with filters as a light source, the others

a laser diode.

Launching conditions

Using the cut-back method it is simple to compare the far field radiation pattern

both of the long and the short fibre. It appeared to be important to observe the

differences between them as it influences the attenuation value obtained.

In table I two sets of measurements are shown.

Set A was performed by simply projecting the output distribution from the fibre

in the mode scrambler 1 to 1 ( na ^q = 0,20; ^

= 0,58) on the fibre to be measured.

At first glance this was a reasonable approximation to the steady state values.

But the values after cutting back (1,5 m) went upward instead of approaching

the steady state values (NA = 0,19; = 0,56). Even the long fibres showed

too high values.

For the second series of measurements on the same fibres (B) the launching

parameters were varied to attain the desired value of the NA and ^ after

1,5 m of the first fibre. These values were also used for the remaining 5 fibres.

28
A change of attenuation is perceivable, although the inaccuracy of the

measurement itself should be taken into account.

A repeatability of 0,05 to 0,1 dB gives an uncertainty of 0,1 to 0,2 dB between

measurements. The trend, however, is present. These measurements were performed

with the same set-up using a halogen lamp and a 10 nm broad filter at 850 nm.

Comparative measurements, results

After the importance of the launching conditions had been demonstrated, it was easy

to agree that the far field pattern after 1,5 m fibre is to approximate the

steady state distribution. For practical reasons the measurements were performed

in such a way that for one fibre the launching conditions were adjusted and then

left unaltered for the other fibres. As the first fibre was independently chosen

in each laboratory^ this may account for a part of the differences still present

in the values of the four laboratories. Other reasons may be the far field

pattern of the short fibre being inaccurate if an LD is used in combination with

a meridional scan and that the temperature of the bundle must be kept within

limits. Room temperatures in the four buildings were different, but not

measured accurately.

Yet, one value measured by P.T.I, (yellow) shows an unexplained difference.

Table II shows the results of the measurements so far. The results of two of the

participants are corrected for the wavelength difference (nearly 0,1 dB/10 nm.km).

The length of the fibre bundle was approximately 1200 m.

Conclusions

It is possible to achieve reasonable to good agreement between the attenuation

values measured at different places with different equipment.

It is mandatory, however, that the measurement conditions be equal and are measured in the right way.

The launching conditions agreed on must be kept within small margins.

A good approach is to use the far field radiation pattern of the cut-back

length to determine the launching conditions

.

References

1) G.W. Day, I.C.C. '79 Conference Record, June 10 - 14, Boston Ma 10.4

2) J.W. Versluis and J.G.J. Peelen, Philips Telecommunication Review, 37

(1979) 215 - 230.

29
Table I Changes in attenuation values due to launching conditions

Fibre Group Short fibre Long fibre Attenuation

colour

A B NA

io i

NA

io x (dB)L= 1,2km

Blue 1 0,21 0,58 0,20 0,59 5,5

1 0,19 0,52 0, 19 0,56 5,2

1 Black 2 0,22 0,58 0,21 0,62 5,3

2 0,19 0,55 0, 18 0,58 5,0

Yellow 3 0,22 0,59 0, 19 0,61 5,1

-j 0,22 0,58 0, 19 0,57 5,0

White 4 0,22 0,59 0,20 0,60 5,2

4 4 9

Red 5 0,22 0,60 0,20 0,60 5,0

5 0,20 0,51 • 0, 19 0,58 4,9

Green . 6 0,21 0,57 0,20 0,61 5,4

6 0,20 0,52 0, 18 0,56 4,9

Table II

Results of comparative measurements in the four laboratories.

Laboratory Attenuation at 850 nm (dB) L = 1 ,2 km Measuring

waveRed Blue Yellow Green Black White length (nm)

Philips A 5,0 5,5 5,2 5,4 5,3 5,2 850

Philips B 4,9 5,2 5,0 4,9 5,0 4,9 850

NKF I** 4,8 5,2 5,0 5,1 5,0 4,9 859

NKF II

XXX

4,9 5,1 4,9 4,9 4,9 4,9 859

PTI I 5,0 5,4 5,6 4,9 858

PTT I 4,8 5,0 4,8 5,0 4,8 850

x Different launching conditions.

xx Before transportation to another laboratory (80 km)

.

xxx After transportation from that laboratory.

30
PROFILE CHARACTERIZATION OF

OPTICAL FIBERS AND PREFORMS

by

H. M. Presby and D. Marcuse

Bell Laboratories

Crawford Hill, Holmdel, N. J. 07733

The light transmission properties, of optical fibers are

determined by the dielectric materials from which they are made.

In particular, it is the shape of the refractive index profile

that is responsible for the impulse response and hence the bandwidth of optical fibers. There is, therefore, a definite need for

precise techniques for measuring refractive index profiles.

Because the index profile of the preform is faithfully

reproduced in the fiber, it would suffice to measure only index

profiles of preforms. However, since most techniques are not

applicable to preforms, profile measurements are often performed

on fibers. Most preforms are made by a vapor phase method (MCVD)

that deposites glass in concentric layers. Consequently, the

index profiles of preforms are rich in structure whose transverse

dimensions become small compared to the wavelength of light and

hence unobservable once the fiber is drawn. Only in a region near

the fiber axis, where the deposition layers are thickest, is a

ripple structure usually observed in the fiber.

How accurately profile measurements should be made and

what technique is best for making them are difficult questions to

answer since they involve trade-offs of many factors. To be

included are time, cost and effort considerations, all of which

31
one would like to minimize. Then there are sensitivity , accuracy

and resolution which one would like to maximize.

A handle on the question of accuracy is provided by the

following considerations. The theoretical bandwidth that can be

realized with an optimum power-law index profile is about 8000

MHz -km for a fiber with a maximum index difference of 0.02. To

achieve this high bandwidth requires that the exponent g of the

power-law profile have a definite optimum value near g = 2. A

departure of only 0.05 from the optimum value is sufficient to

reduce the fiber bandwidth by more than one order of magnitude.

Clearly, techniques for determining g to better than 0.05 are

required if a meaningful correlation between fiber performance

and index profile is to be obtained. In order to achieve this.

accuracy, An(r) [the difference between the refractive index

profile of the fiber core and its cladding value] must be measured

4 with a precision of 1 part m 10 . Very slight local distortions of the refractive index

profile from its optimum shape also decrease the fiber bandwidth

markedly. A distortion of ten sinusoidal periods over the fiber

radius with an amplitude of 1 percent of the maximum index difference reduces the bandwidth from 8000 MHz 'km to about 200 MHz«km.

The precision of the An(r) measurement would again have to be

4 about 1 part m 10 to detect the 1 percent distortion.

As to which technique is best for making these

measurements, let us consider the trade-offs connected with one

of them - interference microscopy.

This is a very accurate method for measuring the index

distributions of fibers and preforms, that can be used as the

32
standard with which other methods are compared. There are two

different interferometric methods. In the slab method, a slab

must be cut out of the preform (generally its tip) or fiber and

polished so that the faces are flat and parallel, requiring approximately one day of sample preparation. An interference microscope

is required to observe the samples. Interference-lens-attachments

to ordinary microscopes generally require passing the light twice

through the specimen thus compounding possible errors. Best

results are obtained with a single-pass Mach-Zehnder interference

microscope

.

4 Index values accurate to about two parts in 10 can be

realized routinely and by electronically processing the image of

the microscope, measurements accurate to about one part in 10^

have been achieved. Automatic computer processing of the output

also serves to reduce analysis time. Spatial index resolution is

somewhat limited in that it is not possible to combine maximum

lateral resolution and exact phase measurements in a single

instrument.

The need for sample preparation can be eliminated by use

of the transverse interferometric method. In this technique, the

fiber is immersed in matching oil on the stage of the interference

microscope and illuminated at right angles to its axis. The

matching oil removes the influence of the outer cladding boundary.

The total optical path length of a light ray is expressed as an

integral and the index distribution is obtained from the measured

fringe shift by solving an integral equation. Unlike the slab

approach, transverse interferometry assumes circular symmetry and

33
hence geometric variations, which can affect the profile, will not

be detected unless special care is taken to make several measurements for different rotational positions of the fiber. By

automating the measurement, index profiles of a fiber can be

obtained only a few minutes after manufacture. The accuracy

of the method is about an order of magnitude less than that of the

slab approach and it is subject to a very large error in the region

near the fiber axis.

The transverse interferometric method cannot be applied

directly to preforms because the fringe shift becomes too large

to observe. However, a transverse differential method can be

utilized. In this approach, one of the two beams passing through

each arm of a Mach-Zehnder interferometer is shifted by a shearing

prism. The fringe shift now represents the differential of the

optical path-length difference and can be made appropriately small

by regulating the shearing distance. The accuracy is estimated to

be about 1 percent of the index difference.

There are many other methods for measuring refractive

index profiles of fibers. In the ray tracing method it is the

deflection suffered by rays passing transversely through the core

that is used to compute the index profile. The deflection can be

measured directly, or can be deduced from the intensity distribution of the focused light (focusing method)

,

Light scattering in forward and backward directions

transversely to its axis has been used for finding the index profiles of fibers. The dependence of the reflectivity of the fiber

end face can also be used for determining the index distribution

(reflection method)

.

34
Index profiles can be deduced from the intensity

distribution of incoherent light traveling through a short section

of fiber (near-field method) or, conversely, by the distribution

of light refracted out of the fiber (refracted near-field method)

.

Furthermore, the refractive index can be found by comparison with

index matching fluids (immersion techique) or from observation of

the etch rate (etching technique) or with the help of x-ray

microprobes

.

Table 1 summarizes the various techniques and provides

comments on each of them.

35
METHOD

Interferometry

Slab

Transverse

Focusing

Ray Tracing

Scattering

Forward

Backward

Reflection

Near-field

Refracted near field

Immersion

Etching

X-ray

Microprobe

TABLE 1

Some Profiling Methods

COMMENTS

Very accurate, a standard, timeconsuming sample preparation

Fast, nondestructive, requires

expensive microscope, assumes

symmetry

Accurate, fast, applicable to

fibers and preforms, assumes symmetry

Only applicable to preforms, high

resolution

Complex in interpretation and

implementation, fibers only

Fibers and preforms, simple

implementation, limited applicability

Simple implementation, fibers only,

surface layer problems

Simple implementation, leaky mode

corrections necessary

No leaky mode corrections, fibers

only, good resolution

Simple, limited in accuracy

Mainly qualitative observations

Sophisticated instrumentation,

limited resolution, difficult

calibration

36
LINEARITY AND RESOLUTION OF REFRACTED NEAR-FIELD SCANNING TECHNIQUE

M. Young

U. S. National Bureau of Standards

Electromagnetic Technology Division

Optical Electronic Metrology Group

Boulder, Colorado 80303, USA

Refracted near-field scanning is an attractive method for precisely determining the index profile of an optical waveguide. This method was first pro1 2 posed and demonstrated by Stewart and later refined by White. In an earlier

3

paper, I analyzed the linearity and precision of the method; this paper

additionally discusses resolution and related areas.

Lambertian and non-Lambertian sources

Refracted near-field scanning depends upon measurement of the power that

is refracted through the cladding of the fiber and beyond an opaque stop. If

the source is Lambertian, then, in principle, the power transmitted around the

2 2

stop is a precisely linear function of (n - n ) , where n is the index of the

Li fiber (core or cladding) at any point and n is the index of the fluid that

4 surrounds the fiber. If n and n are approximately equal, the transmitted

Jj power is very nearly a linear function of (n - n )

.

Many non-Lambertian radiances may be approximated by a function proportional to cosTM 6. If the radiation pattern is circularly symmetric, the power

nH"2 transmitted around the stop is proportional to 1 - cos 6; the system is not

2 linear in index to the extent that this function is not proportional to sin 6.

(The power that a Lambertian source radiates into a cone is proportional to

sin 6. ) Figure (1) shows the function 1 - cos 0 vs sin 6 for several

values of m. m = 0 is a Lambertian source, m = -1 is a uniform point source,

and m = 4 and 8 are rough approximations to sources like edge-emitting LEDs

and lasers. The function is linear over almost any small range; a Lambertian

source is not strictly required for refracted near-field scanning. However,

calculations based on the radiometry of Lambertian sources may be in error by

a large factor.^

37
Linearity and calibration

To calibrate the system and determine the linearity of the output power as

a function of refractive-index difference, I use a quartz fiber with each of

several index-matching fluids and plot transmitted power vs index difference.

Figure (2), which is taken from Reference (3), shows such a plot. Each of three

quartz-fiber samples was tested with each of four oils to produce the twelve

points in the figure. Figure (2) also shows a line of best fit and estimates

of the random and systematic errors of the system. I have shown further in

Reference (3) that relative values of index (such as core-cladding difference)

may be measured to + 0.0005 if the temperature of the fluids is held sufficiently

constant

.

Resolution limit

White has argued that the spatial resolution limit of the system is that of

2 a diffraction-limited lens with an opaque central stop. The work presented here

does not entirely support this argument; rather, the resolution limit of the

system is more nearly equal to that of the lens without the stop. This is so because the condensing lens associated with the stop is not an imaging system

but merely a collector of light.

The solid line of Figure (3) shows an exact calculation of the edge-response

width (10 - 90% points) of a diffraction-limited annular lens as a function of

the fraction £ of the diameter that is obscured by the stop. The resolution

limit RL is taken to be equal to the edge-response width; the vertical axis is

normalized to the Rayleigh limit, 0.61 A/NA. The discrete points are data obtained

by scanning across the edge of a cleaved quartz fiber immersed in mineral oil.

Figure (4) shows a typical set of scans. The value of £ varies by about 10%

depending upon whether the light is focused into the fiber or the oil; the numbers

in Figure (4) are the averages of the two values.

Data derived from scans like those in Figure (4) are shown in Figure (3).

Whereas there is some effect resulting from increasing £, the data do not agree

well with the theory and show that the resolution is diffraction limited as long

as £ is less than about 0.7. When £ exceeds 0.7, resolution suffers, probably

because of diffraction of the light outside the narrow conical shell by the sharp

edge of the fiber.

38
Comparisons with other methods

At the time of this writing, we are just beginning comparisons. Figure (5)

shows measurements on a step fiber by the refracted near-field method and by

near-field scans of the exit faces of meter- and kilometer lengths using overfilled launch conditions. The former method resolves the central index dip

and shows structure in it, and also shows the presence of a complicated barrier

layer. Core-diameter measurements agree to about 10%. Comparisons with at least one other laboratory are still at an early stage.

REFERENCES

1. W. J. Stewart, in International Conference on Integrated Optics and Optical

Fiber Communication (I00C, Tokyo, 1977), pp. 395-398.

2. K. I. White, Opt. and Quantum Elect., 11, 185-196 (1979). See also

references 3-16 therein for a bibliography of other measurement techniques.

3. M. Young, Appl. Opt. 19_, 2479-2480 (1980).

4. M. Young, NBS Technical Note, in preparation.

5. M. Young, Optics and Lasers , Springer, New York, Heidelberg, Berlin (1977),

Chap . 3

6. Eric Johnson very kindly performed the edge-response calculations; Ernest

Kim supplied the near-field scans.

39
10

r—-*—

1

J a i » ' » »

» 0.5

C

Figure 3. Normalized resolution limit as

a function of relative diameter.

RL - 1 corresponds to the Rayleigh

limit.

<-0.7

0.7 3

0.8 /

1 ^

M

0.8 9 NA-0.46

Figure A. Scans across the edge of a cleaved quartz

fiber. The fiber is to the right of the step.

Figure 5. Scans of the core of a step fiber.

Top: refracted near field scan. Center: scan of exit face of 1-m length. Bottom: scan of exit face of 1-km length.

40
A COMPARISON OF TECHNIQUES TO MEASURE

THE DIAMETER OF LIGHTGUIDE FIBER

D. H. Smithgall

C. M. Schroeder

Western Electric Company

Box 900

Princeton, New Jersey 08540

INTRODUCTION

The dimensional quality of lightguide fiber determines connector design

and splice loss in high-quality lightguide communication systems. Consequentl

the outer diameter and the ellipticity of the lightguide fiber are important

factors to be measured and controlled during the manufacturing process.^ For

compatible system components, it is imperative that calibrated diameter

measurement techniques exist.

Three methods of measuring fiber diameter are examined: a microscope

technique, a light-scattering technique, and an interferometric technique.

Microscope measurements are commonly used for static measurement of fiber

cross-section. Interferometric measurement offers an alternate static

measurement technique. Light-scattering measurement may be used as a static

measurement or as a real time diameter monitor during fiber drawing.

MEASUREMENT TECHNIQUES

The cross-section dimensions of the fiber are measured with a microscope

by preparing a short fiber sample to have a well-cleaved end, and carefully

mounting the sample such that the surface to be examined lies in the focal

plane of the microscope objective. The sample may be illuminated by reflected light or light transmitted through the fiber sample (Figure 1).

A filar eyepiece, which has been calibrated with a stage micrometer, is

41
used to locate the edges of the fiber sample, or a camera system may be

used to automatically measure the fiber dimensions.

The interferotnetric micrometer (Figure 2) measures the fiber diameter

by interferometrical ly determining the separation of two fused silica probes,

between which the fiber is located. The surfaces of the probes which contact

the fiber must be plane and parallel. Fiber diameter is measured by zeroing

3

the interferometer with the probe surfaces together, inserting the fiber

between the probes and measuring their displacement. It has been found that

the measured diameter is independent of contact force for forces less than

20 gms.

The light-scattering technique uses a laser beam to illuminate the fiber,

generating a scattering pattern containing interference fringes. The fiber

diameter is determined by measuring the angular range, in the forward-scattered

pattern, containing a fixed number of fringes.^ The fiber is located on the

axis of a spectrometer table and illuminated by a HeNe laser (Figure 3). A

detector is located on the rotating arm of the table approximately 10 cm from

the fiber. As the detector is translated through an arc of approximately 60°,

the fringe pattern is observed and the precise angular range for 120 fringes

measured.

EXPERIMENTAL RESULTS

In order to compare the relative accuracy and repeatability of the three

measurement techniques, a fiber sample was drawn from a centerl ess-ground 15 mm

diameter fused-silica rod. The rod was measured to be round within .03%.

The fiber diameter measured with the interferometer was 109.1 + .1 ym

based on over twenty measurements of the fiber sample, regardless of orientation. The diameter measured by the forward light scattering was 109.0 +

.2 ym based on three independent measurements. The measurements made by

42
microscope were less consistent. For samples which were illuminated by white

light reflected from the top of the fiber sample, the measured diameter was

110 + .5 ym. When the sample was illuminated by transmitting light through

the fiber sample, the measured diameter was 108.9 + .5 ym. The microscope

measurements represent the average and spread of more than forty independent

measurements per sample. These results are representative of measurements

made on many different fiber samples other than the specially prepared sample.

Microscope measurement yielded a result comparable to those of the other

techniques only when the sample was illuminated by transmitted light, and then

only when a large number of measurements were averaged. When reflected illumination was used, the average measured diameter was significantly higher than the

results of all other techniques. The scatter in the measured diameters results

from operator judgment, chromatic aberation in the microscope, and the

difficulty in focusing over the entire fiber end. The bias in the measurement

when using reflected light for illumination resulted from an incorrect per5 ception of the edge of the fiber due to poor edge definition .

CONCLUSION

The interferometric micrometer and forward light scattering technique

provide the most accurate measure of fiber diameter. The interferometer is

easily implemented and could be used for production control and fiber standardization measurements. The commonly used microscope measurement techniques are

inconsistent and may introduce significant bias into the diameter measurement.

REFERENCES

1. D. H. Smithgall, "Application of Optimization Theory to the Control of the

Fiber Drawing Process", Bell System Tech. Journal, Vol. 58, No. 6, July-Aug.,

1979, pp. 1425-1436.

2. D. Marcuse and H. M. Presby, "Automatic Geometric Measurements of SingleMode and Multimode Optical Fibers", Applied Optics, Vol. 18, No. 3, Feb. 1,

1979, pp. 402-408.

43
3. Hewlett Packard Manual, "Laser Measurement System 5562A, Theory and

Operation"

.

4. D. H. Smithgall, L. S. Watkins, R. E. Frazee, "High Speed Non-Contact

Fiber Diameter Measurement Using Forward Light Scattering", Applied Optics,

Vol. 16, No. 9, September, 1977, pp. 2395-2402.

5. D. Nyyssonen, "Linewidth Measurement With an Optical Microscope: The

Effect of Operating Conditions on Image Profile", Applied Optics, Vol. 16,

No. 8, August, 1977, pp. 2223-2230.

RETROREFLECTOR

FORWARD LIGHT SCATTERING

Figure 3

44
AN AUTOMATIC INSPECTION SYSTEM FOR SINGLE FIBER CONNECTOR PLUGS

N . K . Cheung and N . M . Denkin

Bell Laboratories, Holmdel, New Jersey 07733

INTRODUCTION

Transfer-molded, bi conical single-fiber connectors with a mean insertion loss of 0.21 dB without index match have been

reported . In order to achieve this low loss, both the intrinsic

fiber characteristics such as core diameter and numerical aperture

(NA) , and the external alignment parameters such as transverse

offset, tilt angle, and end separation must be accurately controlled

This paper describes an automatic inspection system which measures

the aforementioned parameters, and provides statistical information

as a rapid feedback to the molding runs,

SYSTEM CONSTRUCTION

The system is a computer- controlled microscope that measures

core diameter, core eccentricity, fiber NA and fiber deflection

angle of a connector plug in an one-step operation by scanning

both the near field of the core, and its radiation pattern. The

block diagram of the measurement set-up as shown in Fig. 1 is an

ad^p^ation of the video- scanning system described by H. Presby et

al ' . The connectorized test cable is excited by a 0.82 pm LED

via a launch jumper containing a fiber with nominal parameters.

The test plug is seated in a steel gauge sleeve anchored into a sturdy bridge-like stage plate of an inverted microscope. With

this construction, the center of the sleeve drifts by less than

0.2 yum over long periods of time. The near and far-field pattern

of the connectorized fiber is imaged onto a silicon vidicon camera

whose output is coupled to a video digitizer. There, the intensity

along any selected vertical line segment is digitized, and the data

are sent to the computer. In addition to providing data acquisition

storage, and analysis, the computer controls the LED brightness,

focuses the microscope, monitors the distance between the microscope

objective lens and the fiber end face, and records the output

current of a photo-diode used to measure the light intensity

through the jumper cable. The last function detects cracks and

discontinuities along the test jumper.

METHOD OF MEASUREMENT

( i ) Core Eccentricity and Diameter

The core eccentricity of a test plug can be obtained by a single measurement of the core center position after the exact

center location of the steel gauge sleeve has been determined at

the beginning of a series of measurements. This sleeve center

determination is made by rotating a plug with nominal parameter

inside the sleeve four times in 90° steps and measuring the core

center at each position. The four points lie on a circle whose

center is the center of the sleeve. In the subsequent measurement

45
for each test plug, the computer first locates an approximate core

center position by trial scans. Two orthogonal scans are then made

through this point and parabolas are fit to the data between 10 percent and 80 percent of the maximum signal level after correction for

background level (Fig. 2) . The axes of the parabolas determine the

coordinates (x ,y ) of the core center and the distance between (xQ

,y ) and the center°of °the gauge sleeve gives the eccentricity. The core

diameter D is defined as the distance between the points at which the

parabola crosses the background level.

Fig. 3 shows a scatter plot of the eccentricities of a group of

433 plugs in an exploratory development run. The plugs are loaded

into the gauge sleeve with a specific orientation so that the distribution of the cluster of points give important information on the

molding process. For example, the centering of the alignment hole in

the molding die can be deduced from the centroid of the points. If

the fiber O.D. is much smaller than the I .D . of the alignment hole,

the points will be spread out from the centroid instead of forming a small cluster. The points on the far left hand side of Fig. 3

indicate that the fibers of these plugs have been bent as a result of

molding defects. The 3 ;am circle on the plot sets the current

tolerance limit within which the contribution to the connector loss

by the eccentricity alone is less than 0.1 dB under steady state

excitation .

(ii) Numerical Aperture and Fiber Deflection Angle

For graded index fibers with near parabolic profile, the caustic

of the rays exiting from the fiber can be approximated by a hyperboloid . Because of accuracy and instrumental considerations, the NA is

not measured from the sine of the half apex angle of the far field,

but rather from the spot size D (20 dB points) at a distance d (5*5D

q

) from the end face. Using the hyperboloid approximation, the NA is

then given by

and is in good agreement with that computed from the index profile

data. The fiber deflection angle is obtained at the same time from

the tilt of the radiation pattern at distance d.

Both the magnitudes and azimuths of the fiber deflection angles

for the above group of 433 plugs are shown in Fig. 4. The one degree

circle on the plot sets the tilt angle limit for the plugs, which

together with the 3 pm circle in Fig. 3, helps to achieve the low

connection loss reported in Ref . 1

.

ESTIMATION OF ERRORS

The major sources of the errors of the apparatus are due to the

improper seating of the plug in the gauge sleeve and the uncertainty

in the location of the sleeve center, apart from the occasional

presence of dust on the tapered surfaces of the plug and the sleeve.

NA sin (arctan (

46
These instrumental errors are estimated by repeating the measurements

for a set of 33 plugs: The standard deviation of the differences

between the two measurements for the core eccentricity is 0.35 jjm and

for the core diameter is 0.39 jam . Those of the NA and fiber deflection

angle are 0.004 and 0.08 , respectively. No long term variation has

been observed

.

Provided the test jumper is over-excited, the core diameters

from this method agree with those measured with photo-micrographs

and index profile data by better than 2%, while the NA agrees with

that from the index profile data by better than 5%. The fiber

deflection angle for 17 plugs have been checked by actual measurements of the fiber stubs after molding with an average difference

of 0.05 and a standard deviation of the differences of 0.12 . The

core eccentricity can be checked by rotating the plug in the sleeve

as described earlier. The eccentricity is determined from the radius

of the circle of the core center to a precision of 0.1 jam.

CONCLUSION

A computerized connector inspection system for the characterization of single fiber connector plugs has been constructed. In an

automated operation of less than 1-1/2 minutes duration, the system

measures the plug parameters (eccentricity and tilt angle) and fiber

parameters (core diameter and NA) of a connector. The accumulated

plot and statistical analysis of the plug parameters identify the

quality of the fabrication process, enable the screening of outof-spec units, and also serve as a feedback on the molding runs.

The information on fiber parameters is important to assure that the

fibers used meet the specification required for low loss connectors.

ACKNOWLEDGEMENTS

The authors would like to thank H. M. Presby and H. W. Astle

for helpful discussions, and for providing information about the

computer-digitizer interface and sample programs. We would like to

acknowledge the assistance of R. Ragbir, L. C. Curtis, F. L. Porth

and A. S. Dean in the system fabrication. The support and encouragement from P. K. Runge and P. Kaiser are very much appreciated.

REFERENCES

1. W. C. Young, P. Kaiser, N. K. Cheung, L. Curtis, R. E. Wagner,

and D. M. Folks, 6th European Conference on Optical Communication,

York (1980)

2. H. M . Presby, D. Marcuse, H. W. Astle, and L. M. Boggs, Bell

System Technical Journal, 58 883 (1979)

3. D. Marcuse and H. M. Presby, Applied Optics 18 402 (1979)

4. C. M. Miller and S. C. Mettler, Bell System Technical Journal,

57 3167 (1978)

47
TtiT PLUG

test lumnt

POSITION Tnndoucm

focusing

. KNOt

INVERTED MICROSCOPE

STEPPING BOTOR

VIDEO

DIGITIZER

OOP o TV-MONITOR

»«iT«PE/OI»«

P-l-N DIODE CURRENT

LED CURRENT

POCUS

POSITION SENSOR

FOCUS CONTROL

STEPPING OTOR

CONTROL

Fig. 1 Schematic of the Automatic Measuring System
BANDWIDTH MEASUREMENT IN MULT I MODE OPTICAL FIBERS

Ikutaro KOBAYASHI

Yokosuka Electrical Communication Laboratory, N.T.T.

1-2356 Take Yokosuka Japan.

1 . Introduction

Precise measurements of the

baseband frequency response of

multimode fibers are required in

order to characterize them and to

predict their behaviour when designing optical fiber transmission

systems. Until about the middle of

the '70's, these measurements had

been performed mainly by the pulsebroadening technique (PBT) [1].

Recently, however, improvements in

laser diode characteristics, such

as in stable CW-operation and in

wide-bandwidth modulation, has

enabled us to apply effectively the

swept-frequency method (SFM) to

such fiber bandwidth measurements

[2] [3].

This paper presents a comparative study of these two methods, a review of the recent results in

fiber bandwidth measurement in

Japan, and some discussion on the

future course of these measurements.

2. Performance Comparison between

Swept-Frequency Method and Pulse

Broadening Technique

Ideally, under conditions of a completely linearly responding

system, the swept-frequency method

is equivalent to the pulse broadening technique, because a given

frequency response can be transformed into the corresponding impulse response. However, there are

real operating conditions that can affect in different ways the results

obtained in bandwidth measurements

using SFM and PBT.

For example, the beam pattern

from a laser diode is dependent upon

its operating conditions and will be

different for CW- or pulsed-operation. Consequently the different

distributions in the modes of the

fiber launched will cause different

fiber bandwidths to be measured by

the two techniques.

In favour of the SFM technique

we have the fact that nonlinear

characteristics of a photo-detector

cause larger errors in the PBT measurement which requires the larger

dynamic range. Also the SNR of SFM

is, as mentioned in the next section, much superior than that of

PBT.

On the other hand, PBT, in addition to its relatively simpler

setup, provides intuitively understandable results which correspond

directly to the sources of pulse

distortion in a fiber, especially

to the index-profile control error

of a graded fiber. Therefore, this

technique has been applied for

checking the quality of graded

fiber on mass production lines.

In a comparison without regard

to system complexity the SFM technique is the more powerful because

it provides more precise results

with its advantages in SNR. Figure

1 shows the fundamental layouts.

SFM adopts a vector-volt meter, com- paring the amplitude and phase of

input and output baseband signals

through a fiber. In PBT the input

and output waveforms are compared

and the fiber response then calculated.

49
2.1 SNR Comparison

Figure 2 shows the calculated

SNR for the two methods. SFM has

the two major advantages of narrow

receiving bandwidth and the fact

that transmitted power can be con- centrated on a line spectra. That

is, even if SNR improvements that can be achieved by averaging are

taken into account, PBT is inferior

to SFM because not only does PBT

require a receiving bandwidth as wide as the fiber bandwidth but

also each frequency component of a transmitted pulse is much smaller

than the corresponding line spectra

power of SFM.

Therefore, the SNR that can be

achieved using PBT is significantly

degraded in the higher-frequency

region, the case of a wide-bandwidth fiber. In general then, SFM

is superior in SNR to PBT.

2.2 On Phase Measurement

The accuracy in phase measurement of PBT is bounded by sampler

timing error, which is, now, around

50 psec corresponding to ^/lO at

1 GHz. On the other hand, as the

phase is measured by a vector- volt

meter in SFM, the accuracy here

depends on the stability of the

signal frequency, which ideally

fiber

v -v reference

9£

vector volt meter

synthesizer

fiber

"—< v- 5b>

CZZ-^eU

D P

EG.

digital processor

pulse generator

Fig.l Measurement Layouts for Swept-Frequency and Puis Broadening Methods

50
should be kept constant during the

time that the light propagates

through the fiber. In practice

the frequency is usually stable

enough for this purpose and it is

easy to measure the phase precisely

at several GHz, where the dominant

source of error is random noise.

2.3 Examples of Measurement

Resul ts

In the above discussion it was concluded that SFM is superior to

PBT for the precise bandwidth measurement of fibers. This section

demonstrates this conclusion by

comparing the results measured

using SFM and PBT.

Figure 3 shows the measured

baseband response of a 2 km long

graded fiber, including both amplitude and phase. These results show

that SFM provides more precise measurement and a wider measurable

range than PBT.

There is a little difference in

the bandwidths measured using the

two techniques. Fiber bandwidth

dependency on wavelength was a possible source for a difference,

because the PBT measurement used

a 0.83 urn GaAlAs laser diode with

2.0 nm spectral -width as a light

source and the SFM used a 0.87 urn diode with 1.3 nm spectral width.

3. Field Measurement

Three kinds of measuring sets

have been developed [4] for use in

NTT's field research in '78 [5]: a standard type has been designed for

far-end automatic measurement, a portable type for far-end manual

measurement with battery power

supply, and a near-end measuring

set with visual output displayed on

a CRT. Figure 4 shows the internal

response of the standard measuring

set and an example of a measurement

Baseband Frequency (GHz)

Fig. 3 Measurements by SFM and PBT

4. On Some Subjects for the Establishment of Accurate Measurement

4.1 Launching Conditions

Light launching conditions

can significantly affect the frequency response of multimode

optical fibers, because they

determine the initial power distribution among the guided modes,

and hence, through differences in

modal delay, form the most important factor in the multimode fiber

response. Although mode mixing

could in principle remove such effects from fiber response, it is

well-known that the recent lowloss optical fibers do not in

themselves create enough mode

mixing that the response even of

a 1 or 2 km long fiber is independent of the launching conditions .

1 Ir terna

respcDnse

Gri decHnden fiber V 2. 4 km

Jn

Coi

1 1 1

\%

e diarrwiei : 60 pm

1 j i i 1 i 1 1 1 1 i U i I 0 200 400 600 800 1000 Frequency (MHz)

Fig. k Measurements on the Spot

51
The measurements on bandwidth

converge monotonical ly toward a unique response as the length of 1Z* fiber measured is increased. There

are two possible ways to deal with 12

this inherent scatter in the mea- 10

sured response of multimode fiber:

one way is to measure all possible ^ 8

responses with varied launch con- fa ditions, and the other way is to

define the launching conditions in £

detail and thus define one standard o measurement for the response of a fiber.

Figure 5 shows the reproducibility of the measurement using a mode exciter, consisting of step-,

graded-, and step-index short

fibers connected in series [6].

Objective lens

Frequency (MHz)

Fig. 5 Reproducibility of Measurement

Using a Mode Exciter

In either case, there remains

the problem of how to evaluate the

effective fiber response in its

individual application. These difficulties reflect the fact that

multimode fibers cannot be regarded

simply as linear response systems

in baseband electrical signal

domain.

4.2 On Reproducibility of the

Internal Response of Measuring

System

The internal response characteristics of the measuring system

must be reproducible for reliable

measurements, especially" in the case of far-end measurements in

which calibration of this response

is difficult.

The dominant factor in such

lack of stability stems from the

laser diode. This component pos- sesses three kinds of instability:

a drift due to operating conditions

such as the ambient temperature,

intrinsic nonlinear effects such as output power saturation, negative

gain, and self-pulsation [7], and

nonlinear effects induced by external disturbances, such as optical power reflected into the active

region of the diode.

Negative feedback technique is an effective method of removing

many kinds of source instability,

but is ineffective to control negative gain, self-pulsation, and

complicated nonlinear effects.

Fortunately, progress in the laser

diode manufacture has now led to

single-mode operation and has overcome the negative-gain and selfpulsation problems.

There then remains the problem

of light reflected into a laser

diode. This problem is fundamental

to the laser diode structure and

its sensitivity to multiple reso- nant structures. Some techniques

have been proposed to prevent re- flections, such as an isolator or a coupler adopting a numerical aperture mismatch [8]. Several fundamental research papers on laser

diode characteristics have been

contributed to the solution of

these problems [9].

52
4.3 Speckle Noise Problems

Coherent light, propagating

along multimode fiber, generates

dynamic speckle patterns which can lead to enough noise as to make

impossible accurate bandwidth measurements. However, with multimode

operation of a laser diode, most of

the speckle pattern noise is suppressed, and reproducible measurements are obtained. Such a multimode operation is achieved, for

example, by superimposing a highamplitude cyclic wave on the

signal driving the diode. Figure

6 shows the spread of bandwidth

measurements taken with and without this technique to suppress

speckle pattern effects.

4.4 Discussions on Fundamental

Problems in Fiber Bandwidth

Measurement

The fiber bandwidth measurement

is based on taking the ratio of two

signal amplitudes or, more exactly,

a ratio of intensity-modulation

indices. However, no effective

method is known for measuring the

modulation index directly as a ratio

of the absolute light powers.

Rather, the bandwidth measurements in practice are based on the

indirect method of detecting the

(a) without

pulse

20 % (b)with

pulse

42

L = 8.4 km

10

160 180 200 220 160 180 200 220

6 dB down bandwidth (MHz)

Fig. 6 Speckle Noise Effect in

Bandwidth Measurement

light with a photodiode and trusting

to its small -signal linearity.

Another problem area stems from

the fundamental multimode nature of

the optical fiber guide. A fiber

bandwidth is defined in the baseband

electrical signal domain. This definition contains no parameter to

describe multimode phenomena in

detail and, consequently, includes

some ambiguities. For example, a fiber can change in frequency res- ponse, when new splicing points are introduced and mode distributions

accordingly changed.

Intrinsically, therefore, the

response of a long spliced fiber

cannot be deduced with certainty

directly from the responses of its

component short fiber pieces.

5. System Design Requirements on Fiber Bandwidth Measurement

Accuracy

Fiber transmission system designers require precise, reproducible

and accurate fiber response. Most

of their requirements are met by

measurement techniques now employed.

There are, however, some other

aspects to the bandwidth of fibers

which the designer must also know.

One of the most important of these

is how to determine the response

of a long fiber consisting of many

short pieces, if the response of

each short component is known.

The frequency response of the long

fiber is not a simple linear sum or convolution of short-fiber re- sponses. A powerful bandwidthevaluation method has been proposed

by Electrical Communication Laboratories [10]. In this method the

long-fiber response is determined

by using the bandwidth values of

each fiber piece at several wavelengths .

53
It is necessary thus to measure fiber bandwidth at different

wavelengths of the source and,

in general, a wide-bandwidth mea- surement is required in order to

check the characteristics of shortfiber pieces.

Since a laser diode is not

available at such wide-bandwidth,

typically several GHz, and especially not available at several

wavelengths, ECL has built up a measuring system consisting of a gas or solid-state laser and

electro-optical crystal modulator,

which can operate up to 18 GHz [11].

Figure 7 shows examples of the

results taken with this system.

I

1

o

\ Kr-laser

fiber-1

\YSG-laser \J ID

fiber-2 —i » , j_ i, „ . i <—i—> i

' - 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3

Wavelength ( y m ) Fig. 7 Measured Bandwidths at Several

Wave 1 engths

In a word, the difficult re- quirement from a system design

point of view is to determine precisely the fiber response under

actual operating conditions, when

no long fiber has been installed

yet and there remains some inherent ambiguities in short-fiber

measurement.

6. Conclusion

This paper has reported suc- cessful measurement results using

the swept-frequency method, some techniques for its practical application in the field, and discussions on problems involved with

the definition of multimode fiber

bandwidth and its measurement.

In order to establish a welldefined useful measuring method, it seems unavoidable to investigate

more deeply several phenomena inherent to the multimode nature of

optical fibers.

Acknowledgement

The author would like to thank

the members of Yokosuka ECL of NTT

in Japan for their co-operation in

this work and the members of CRC of

DOC in Canada for their helpful

advice.

References

[I] A.H . Cherin et al., Appl . Opt.,

13,10, p. 2359 (Oct. 1974).

[2] R. Auffret et al., First Europ.

Conf. on Optical Fiber Communications, p. 60 (Sept. 1975)

.

[3] I. Kobayashi et al., IECE of

Japan Tran., E59, 4, P. 11

(April 1976).

[4] I. Kobayashi et al . , ECL Tech.

Jour. 28, 9, p. 177 (Sept. 1979).

[5] S. Shimada et al . , ECL Tech.

Jour. 28, 9, p.l (Sept. 1979).

[6] T. Tanifuji et al., Elect. Lett.,

15, 7, p. 204 (1979).

[7] T.L. Paoli et al . , IEEE Trans.

QE-11, 7, p. 525 (1975).

[8] T. Matsumoto, IECE of Japan

Trans. 62-C, 7, p. 505 (1979)

( in Japanese)

.

[9] T. Kanada et al . , IEEE Trans

QE-15, 7 (July 1979) et al

.

[10] T. Matsumoto et al . , Tech.

Digest of 5th ECOC (Sept. 1979).

[II] M. Izutsu et al . , Papers of the

Tech. Group of IECE of Japan

OQE 80-37, 1980 (in Japanese).

54
AN INTERRELATIONSHIP BETWEEN LOSS AND DISPERSION

IN MULTIMODE FIBERS

BY

Leonard G. Cohen Sei-Joo Jang

Bell Laboratories Western Electric Co., Inc.

Crawford Hill Laboratory Engineering Research Center

Holmdel, N. J. 07733 Princeton, N. J. 08540

This paper describes an interrelationship between

loss and dispersion which can result in a high bandwidth over a much wider wavelength region than predicted for perfect power-1

profiles .^ The fibers studied were drawn from MCVD preforms

with germania phosphosilicate (Ge02 _ P205~Si02) cores and

phosphoborosilicate (P20^-B202_ Si02) barrier layers which

separate the cores from silica (Si02) claddings. Results are

compared for two fibers. Fiber #1 has a moderately high OH ion

concentration and a core profile shape that is well graded for

(2) the 0 . 8-1. 1 ym wavelength region. Fiber #2 is a low-loss

fiber containing very little water.

Figure 1 illustrates bandwidth versus wavelength

curves for fibers #1 and #2. Data points at 0.825ym and

1.2 5ym wavelengths were obtained with pulsed injection laser

sources. Remaining dispersion data, within the 1.06-1.7ym

wavelength region, were obtained with an automated fiber

characterization system which uses a near infrared fiber

i (3,4) Raman laser source.

The dotted curve, applicable to fiber #1, is the

theoretical bandwidth spectrum for a 0.19 NA fiber whose

profile shape is optimized for operation near 1-um

(5) wavelength. The maximum theoretical bandwidth is

55
17GHz«km and the measured peak fiber #1 bandwidth is more

than 1.6GHz. km. The measured fiber #1 spectrum behaves

qualitatively according to theory for A<1.2ym. However, the

bandwidth does not continue to decrease but remains higher

than lGHz-km for wavelengths between 1.2 and 1.53um. In fact,

bandwidth peaks occur at wavelengths where water absorption

causes loss peaks near A=1.24ym and A=1.38um. By comparison, the

bandwidth spectrum for fiber #2 has lower peaks than fiber #1

(due to the lower level of OH ion concentration) . It has a 20-percent larger NA=0»236 and its profile shape is not as

well graded.

It is also instructive to show loss spectra in

Figure 2 for fibers #1 and #2. Fiber #1 has significant water

absorption peaks near A=0.95ym, 1.24ym, and 1.38ym. Its

minimum loss is 1.2 dB/km near A=1.17ym. Fiber #2 has lower

losses. Only one significant water peak occurs near A=1.38ym.

Losses remain less than 1 dB/km everywhere else between 1.06ym

and 1.6ym. The minimum loss is 0.45 dB/km near A=1.55ym. In

general, the loss differences between fibers #1 and #2 are most

dramatic for wavelengths longer than A=1.2ym. That is also the

wavelength region where the shape of the bandwidth spectrum for

fiber #1 most differs from theory.

Comparisons between curves in Figures 1 and 2 highlight

a clear interrelationship between loss and dispersion. Its cause

was determined by making wavelength-dependent far-field radiation

pattern measurements with the same launching conditions that were

used to make bandwidth measurements. Generally, radiation pattern

shapes dramatically broadened in wavelength regions corresponding

56
to high water absorption. Detailed results were used to deduce

that high OH ion concentrations near the center of the core

caused differential attenuation of low-order modes relative to

high-order modes. This resulted in bandwidth peaks at the

same wavelengths where water absorption peaked.

Perhaps the most important thing to learn from this

paper is that comparisons between the shapes of loss and bandwidth spectra can enlighten interpretations of modal dispersion charactersitics of graded-index fibers.

We are grateful to M. J, Buckler (Bell Labs, Atlanta)

for supplying fiber #1 along with bandwidth data at X = 0.825 urn. and X = 1.25 ym. We also thank J. Stone (Bell Labs, Crawford

Hill) for the fiber #1 loss spectra used in Figure 2. Finally,

we would like to thank M. A. Saifi (Western Electric, Engineering

Research Center) for many useful discussions.

References

1. R. Olshansky and D. B. Keck, Appl. Opt., Vol. 15, pp. 483-491.

Feb. 1976.

2. M. J. Buckler, Conference on Precision Electromagnetic

Measurements, Braunschwieg , Fed. Rep. of Germany, June 19 80.

3. L. G. Cohen, P. Kaiser, and C. Lin, Proc. IEEE, Special

Issue on Optical Communications, Nov. 19 80.

4. L. G. Cohen and C. Lin, IEEE J.Q.E., Vol. QE-14, pp. 855-859,

Nov. 1978.

5. J. A. Arnaud and J. Fleming, Elect. Lett., Vol. 12, pp. 167-169,

April 19 76.

57
FIGURE 1 BANDWIDTH SPECTRA

58
OPTIMIZATION OF CONCATENATED FIBER

BANDWIDTH VIA DIFFERENTIAL MODE DELAY

M. J. BUCKLER

Bell Telephone Laboratories

Norcross, Georgia, USA 30071

ABSTRACT

A novel differential group delay time measurement system has been used

to analyze the detailed features of pulse broadening in concatenated

multimode graded-index optical fibers. For the first time optical

modal delay equalization has been confirmed by measuring the delays

of individual mode groups. It has been found that when concatenating

fibers with minimal excess loss using low loss splices, the differential mode delay characteristics are additive. Thus, optical fiber

routes could be tailored, if necessary, for maximum information

carrying capacity.

I. INTRODUCTION

At the present time most optical fiber routes are constructed by interconnecting lightguide cables in a random fashion. In some installations! it has been found that the total route bandwidth can be a function of the order of fiber concatenation. It has been shown that

selecting adjoining fibers to have over- and under-compensated refractive index profiles provides some modal delay equalization, thus

resulting in improved total link bandwidth. However, to arbitrarily

select alternating fibers to have compensating profiles is at best

only a good rule of thumb. For ultimate concatenated fiber bandwidth

it is necessary to know a priori the modal delay characteristics of

each individual fiber, and to then use this information to select

adjoining fibers for optimal modal delay equalization.

For the experiments reported here, fibers were judiciously concatenated and modal delays were precisely measured to show for the first

time, that modal delay equalization has indeed taken place.

II. EXPERIMENTAL APPARATUS

The experimental arrangement is depicted in Figure 1. This measurement system has been described in detail in Reference 2. The unique

features of this system, which enhance its capabilities relative to

other differential mode delay measurement systems are: (1) selective

mode excitation using a single mode fiber, (2) reduction of time

jitter to +10 ps using the Hewlett Packard 5359A digital delay

generator, (3) the use of minicomputer time averaging to increase

the signal to noise ratio, (4) large screen CRT display for increased

visual sensitivity, and (5) the elimination of beam optics.

59
After precise alignment and initial positioning of the adjustable

splice, translation in one direction is used to vary the radial (r)

alignment of the single mode fiber launch into the multimode fiber

(or concatenated fibers) under test. With high-precision micropositioners the radial resolution is better than 1 ym. For each

selection of radial position, 2n averages (2. _< n j£ 9) are taken

and the result is displayed on the large screen CRT. The time

delay through the digital delay generator can be adjusted in steps

of 10 ps until the displayed pulse peak is aligned with a CRT screen vertical graticule. The output pulses corresponding to different

r values are thus observed, and the time shift of the peak of the

pulses as a function of core radial position is measured. The

radially dependent time delay can be measured with a precision of

+10 psec.

III. RESULTS AND ANALYSIS

All of the fibers used for these experiments were of the Si02-P205~Ge02

type with a 0.23 NA. In order to minimize the amount of modal power

disturbance at the interconnection points all fiber concatenations

were built using loose tube splices (^0.05 dB each). 3 Early on in

the experiments it was found that the usefulness of the differential

mode delay data is directly influenced by the amount of fiber excess

loss due to microbending , i.e., coating induced loss. Thus, three

well controlled experiments were conducted using fibers with varying

degrees of mode coupling.

EXPERIMENT I: Nearly Complete Intermodal Coupling

For this experiment two fibers (designated A and B) were selected which

had a wavelength independent externally induced excess loss of b fyl dB/km

(optical loss = aA~4 + b + c(A)). Initially, the differential mode delay

(DMD) characteristics of each individual fiber were measured. The two

fibers were then interconnected with a loose tube splice and the modal

delay characteristics were remeasured for both directions of transmission (A into B, and B into A). Figure 2 shows a plot of the

measured DMD data for both the individual fibers and the concatenations. For comparison purposes the arithmetic summation (A + B)

of the individual A and B data is shown as a dashed curve. From this

data it is obvious that the concatenation results do not obey the rule

of reciprocity. In fact, the initial fiber in the link seems to control the final link characteristics.

EXPERIMENT II: Weak Intermodal Coupling

For this experiment fibers (designated C and D) were selected which

exhibited a low excess packaging loss, b ^0.3 dB/km. Figure 3 shows

the plots of DMD data for the individual fibers and for both directions of concatenation. For this case the concatenation results are almost identical for both directions of transmission, and are very

close to the simple sum of the individual fiber DMD curves, although

the first fiber in the link still seems to have slightly more influence.

60
EXPERIMENT III: Negligible Intermodal Coupling

Two uncoated fibers (designated E and F) , each 1 km in length, were

used for this experiment. The DMD data for this experiment is

plotted in Figure 4. Despite the very non-monotonic nature of this

data, note how well the concatenation results agree with the sum of

the individual curves. Moreover, reciprocity holds with respect to

the order of concatenation. Also note that at the radial positions

where the two fibers have modal delays on opposite sides of zero,

modal delay equalization does take place, as expected. Thus, for

this case of negligible externally induced loss and low loss splices

(<0.1 dB) , it is possible to choose the fibers to be connected so as to minimize the magnitude of the DMD and thus improve cumulative

bandwidth relative to a random choice of fibers.

IV. CONCLUSIONS

In the past it has been assumed that selecting adjoining fibers in

a concatenated link to have over- and under-compensated refractive

index profiles would result in some form of modal delay equalizationimproved bandwidth performance. Optical modal delay equalization has

been confirmed directly for the first time by measuring the delays

of individual mode groups. It has been found that for fibers with

low b values (b. <0.3 dB/km) and with low loss splices (<0.1 dB) , the DMD characteristics are simply additive for concatenations.

Thus, optical fiber routes could in this case be tailored, if

necessary, for maximum information carrying capacity. Moreover,

as previously reported,^ the precise DMD data measured for each

fiber can also be used to optimize the preform processing parameters to enhance the individual fiber bandwidths.

REFERENCES

1. M. Eve, "Multipath Time Dispersion Theory of an Optical Network,"

Optical and Quantum Electronics, 3-0, No. 1, pp. 41-51 (January 1978

2. M. J. Buckler, "Differential Group Delay Measurement Using Single

Mode Fiber Selective Excitation," Conference on Precision Electromagnetic Measurements, Braunschweig, Germany, June 24, 1980.

3. C. M. Miller, "Loose Tube Splices for Optical Fibers," BSTJ, 54,

No. 7, pp. 1215-1225 (September 1975).

4. M. J. Buckler, J. W. Shiever, and F. P. Partus, "Optimization of

Multimode Fiber Bandwidth Via Differential Group Delay Analysis,"

Proceedings of the Sixth European Conference on Optical Communication, York, September 17, 1980.

61
LASER VACUUM

SINGLE MODE CHUCKS MULTIMOOE

FIBER UUHJ

PULSER|

LOOSE

TUBE

SPLICE

CHE -H-B--BH oSf &-B-^ 1 1 VACUUM

ADJUSTABLE CHUCK

SPLICE SPLICE

DIGITAL DELAY GENERATOR

>- —^ MINI [ LARGE |"M • COMPUTER S«E

TEN

FIGURE 1 DIFFERENTIAL GROUP DELAY MEASUREMENT SYSTEM

ooooooooo g -4t» A

FIGURE 2. DIFFERENTIAL MODE DELAY DATA FOR FIBERS WITH NEARLY COMPLETE MODE COUPLING.

800 —

1 *400 —

/

8~8 8 8~8 8

_ «'_D "* C /8 ^-c-»

c + D

D » C

C -»D

a/1

> /

. 7.

it ..

x-x-x-x—x-x-x-x D

.'8 8

L j L. i I .1

0.2 0 4 0 6 o.a 1 0 RADIAL POSITION (t/ol FIGURE 3 DIFFERENTIAL MODE DELAY DATA FOR FIBERS WITH WEAK MODE COUPLING

0 6 RADIAL POSITION (r/o>

FIGURE 4 DIFFERENTIAL MODE DELAY DATA FOR FIBERS WITH NEGLIGIBLE MODE COUPLING.

62
ATTENUATION AND PULSE BROADENING ALONG

CONCATENATED FIBER LINKS

F.P. Kapron, F.M.E. Sladen, P.M. Garel-Jones and D.G. Kneller

Bell-Northern Research, Ottawa, Canada

In this paper we investigate some new attenuation and pulse broadening

properties measured along several fiber links. The ICVD graded-index fibers

had a 0.17 NA and 50/125 Vm core/clad diameters.

1. Attenuation

1.1 Spectral Dependence of Mode Volume

Optical loss measured in concatenated fiber links is generally lower than

might be expected from totalizing attenuation measurements made on typically

1 km individual fiber lengths. Although several approaches simulating longlength conditions have been reported, investigations to date have not concerned

the wavelength dependence of a restricted mode volume along concatenated links.

We present here, for the first time, the wavelength-dependent loss (spectral

attenuation) of both fibers and splices along a link.

1.2 Experiments

The measurement utilized a tungsten lamp /mono chrometer combination, over

filling launch optics, and had a repeatability of 0.01 dB over the wavelength

range 750 to 1100 nm. The 10 fibers (each~l.l km long) were fusion-spliced

together and the 11 km link attenuation measured using a 5 m cutback length. The

power spectrum was measured at the output of the 10th fiber and again as the link was cut to vithin 5 m of the 9th splice; this yielded the in-situ spectral attenuation of the 10th fiber. A cutback just prior to the splice and measurement

of power out of the 9th fiber then yielded the in-situ spectral attenuation of

the 9th splice. The next cut was within 5 m of the 8th splice and so on until

the spectral attenuations of all fibers and splices were obtained., The link was then remade and the measurements repeated for propagation in the opposite

direction; mean values are used below.

During link assembly, splice loss was monitored by OTDR from the far-end.

Qualitative agreement was found with the measurements below.

1.3 Results

Figure 1 shows the spectral loss (upper curve) for the jointed link and for

the sum of the individual measurements (lower curve) . Note that the difference

is about 0.1 dB/km. Figure 2 shows the average spectral loss for the three splices

nearest the source (upper curve) and for the remaining six splices (lower curve)

.

63
The difference is substantial, and there is a strong wavelength dependence

in the vicinity of the water peak. This presumably results from the reduction

in mode volume due to the OH radical present in the cladding tube.

2 . Pulse Broadening

2.1 Experiment

The influence of mode volune upon fiber dispersion is less understood than

for attenuation. The purpose here was to attempt to fit data according to

several simple concatenation 'rules'. We used a GaAs injection laser diode at

0.9 ym and an InGaAsP diode at 1.3 Vm; lenses were used to overfill both the NA

and core of the fibers. The use of a mode scrambler consisting of graded-stepgraded fibers did not affect some of the preliminary results, so it was decided

not to use it during concatenation measurements so as to avoid the extra loss

penalty

.

All pulse widths were FWHM, with the fiber responses obtained by the usual

quadrature subtraction of the output and input pulse widths. For each fiber n of length L , the fiber FWHM broadening T was measured. The fibers were then

n n N

spliced into a link of N fibers of total length L(N) = E and the broadening

T(N) measured. The link was progressively cut back to each splice and the pulse

broadenings successively recorded. This was repeated for several links and both

wavelengths. At 0.9 ym, a chromatic dispersion correction of 0.264 ns/km (corresponding to a 3.3 nm FWHM source) was made, again by quadrature subtraction.

2. 2 Data Fitting

The measurements and fits were logarithmically computer-plotted as shown

in Figure 3 to 5. For a least-square continuous fit, the pulse broadening

T(L) is expressed as a function of link length L:

T(L) = T(l) L

P

(1)

Here T(l) is an effective 1 km broadening and 'p' is expected to range between

% and 1, with the latter indicating little mode mixing. A second least-squares

fit [J is .

, TOO =^E lTn

j

(2)

where the parameters 'q' ranges between h. (quadrature addition) and 1 (linear

addition). A third exact fit [2j is

? N-l 9 T (N) = E (T + 2T T R L_ ) (3) n=l n n n+1 n,n+l

where are derived 'joint correlation coefficients. Its magnitude

is expected to range between 0 and 1 for heavy and little mode-mixing,

respectively; it can be negative in the equalizing situation of having fibers

64
of opposite mode delay compensation on either side of the joint. Finally,

we can take R , ^ to be consi n,n+l

joint correlation coefficient

we can take R , .. to be constant and by least-^squares define an effective

n , n+1

R= (T

2 -ZT 2 )/2ETT 1 n n n n n+1 (4)

2.3 Results

Figure 3 shows data at 0.9 ym without correction for chromatic dispersion;

Figure 4 includes the correction. Note that the effect of the latter is negligible

at 1 km but amounts to almost 1 ns over an extrapolated 15 km. Figure 5 shows

data at 1.3 ym, though with the joints remade. The fits of Equations (1,2,4) are about equally good, but further data will be presented. The Table gives

joint correlation coefficients corresponding to Figures 3 to 5. The appearance

of coefficients greater than unity probably results from measurement inaccuracy

or the approximation of using the FWHM instead of FWRMS in Equation (3).

3. Acknowledgments

Thanks are due to D.S. Brombal, R.F. Hughes and G.K. Pacey for experimental

assistance, and to K. Abe for useful discussions.

4. References

[l] S. Suzuki et al, 5th ECOC, Amsterdam, (1979)

[2] M. Eve, Opt. Qu. Electron. 10, 41 (1978)

5

W

H

5.0

4.5

4.e

3.5

3.0

2.5

2.0

1.5

i.e

0.5

•.01700

Jointed Link

(including splice ' loss)

Sum of individual

in-situ measurements

900 1000 1200 WAVELENGTH (nm)

FIGURE 1 Fiber Spectral Attenuation

<*) o

J

S3c

H

w

STo

5

0.5

t.4

•.3

0.2 .

1.1

0.0

700

(

Average for 3 splices

nearest the source

Average for remaining

6 splices

5>00 WAVELENGTH (nm)

FIGURE 2 Splice Spectral Loss

1298

65
66
DIFFICULTIES ENCOUNTERED IN THE MEASUREMENT OF

OPTICAL FIBER INTERCONNECTION PERFORMANCE

K. SCOTT GORDON AND F. M . E. SLADEN

T & B OPTOELECTRONICS

THOMAS & BETTS CORPORATION

RARITAN, N.J. 08869

Introduction

Splices and demountable connectors allow for flexibility in

the installation and maintenance of optical fiber links and, as

such, are indispensable. Permanent joints (i.e. splices)

usually have less effect on system performance than rematable

joints (i.e connectors), but, in general, they only offer onetime installation. Joints can however profoundly influence

system performance and, since both attenuation and fiber bandwidth are affected, they warrant careful study.

To date there exists very little in the way of standard procedures for measuring the transmission loss occurring at an interconnection between two optical fibers. Furthermore,

although the term "insertion loss" (which is normally used to

describe this loss) is well understood, its use to describe the

loss encountered at a joint between two optical fibers can be

misleading. Thus we prefer to use the term "joint loss," and

one definition of this terra might be "the additional loss seen

by the system over that which would have occurred if the interconnection had been perfect and between identical fibers."

Note that in this definition the joint loss includes the losses

due to differences in fiber parameters such as might occur when

going from a fiber with a higher numerical aperture to one of a

lower numerical aperture, as well as those due to mechanical

misalignments. However, in a practical system the case of

jointing identical fibers does not normally arise and so

another definition of joint loss might read "the additional

loss seen by the system over that which would have occurred if

the interconnection had been perfect" (i.e. no misalignments).

Although the na mismatch loss, for example, is not included in

this definition it will nevertheless affect the observed joint

loss. This state of affairs has, in turn, led to the idea that

the joint loss can be attributed to extrinsic and intrinsic

factors, but once again considerable confusion exists. For

example, loss due to fiber parameter mismatch is referred to in

the literature as both "intrinsic" and "extrinsic." To avoid

this ambiguity we propose to describe those losses caused by

fiber misalignments as internal factors and those losses

67
resulting from fiber parameter mismatches and fiber transmission properties as external factors (note that the former

will to some degree be affected by the latter).

Referring back to the description of joint loss we believe

the first-mentioned definition to be the most useful, for

in practice it is the additional loss experienced by the

system that is of interest. This qualification is necessary

since, in general, the observed joint loss will depend on

the location of the joint within the system, the number and

location of any upstream joints, and the intitial excitation

conditions. In view of the aforementioned lack of standardization for joint-loss measurement, we have decided to devote the

written part of this contribution to outlining the difficulties

of these measurements and to bias our spoken contribution

towards a review of the experimental results obtained to date.

Review of Factors Affecting Joint Loss

The four factors affecting joint loss are mechanical misalignments, fiber parameter mismatch, the physical properties

of the medium separating the fiber ends and the excitation conditions existing in the fiber immediately prior to the joint.

The mechanical misalignments of general interest are:

(a) Offset: this describes the lateral misalignment

between the fiber axes, and is usually the dominant cause of

loss in a fiber interconnection,

(b) Tilt: this describes the angular misalignment between

fiber axes and is normally well controlled,

(c) Gap: this describes the distance between the endfaces

of the fibers being jointed and its influence on joint loss

will be mainly determined by fiber numerical aperture and

whether or not an index-matching medium fills the gap.

(d) Face angle: this describes the angle of the cleaved

fiber face w.r.t. the fiber axis.

The fiber parameters which primarily influence the joint loss

between two fibers are:

(a) Numerical aperture mismatch

(b) Index-profile mismatch

(c) Core-diameter mismatch

(d) Core ellipticity

(e) Core/cladding concentricity

These parameters (except perhaps the last (e)) give rise to

losses external to the joint.

To date many studies have been made investigating the effect

of these parameters - either in isolation from

68
one another or in combination. Measurements on the group

comprising mechanical misalignments have usually been performed by breaking a continuous length of fiber and

manipulating the ends , whereas measurements relating to the

guidance parameter mismatches have necessarily been

performed on different fibers. Both techniques have

inherent problems. In the former case rotation of the fiber

ends after the fiber has been broken must be avoided as core

ellipticity and index-profile asymmetries will influence the

results, whilst, in the latter case, isolation of any

particular parameter is usually very difficult due to fiber

manufacturing tolerances. Furthermore, as stated

previously, the configuration of the link can profoundly

affect the results of joint-loss measurements and it is

perhaps this phenomenon that causes most uncertainty in the

measurement of interconnection insertion loss. In a series

of detailed experiments Gordon (1) has shown that losses due

to offset when using a short transmitting fiber and a long

receiving fiber are greater than for the case of a long

transmitting fiber and a short receiving fiber. This effect

arises from the spatial transient (2) occuring after the

joint and is generally attributed to the launching of leaky

modes in the receiving fiber. The leaky modes have a

length-dependent attenuation (3) and the amount of power

contained in the leaky modes after a joint will be a

function of the fiber parameters (i.e. core diameter na;

index-profile) , the excitation conditions existing

immediately prior to the joint (including the source

wavelength) , and the distance downstream of the observation

point from the joint. Thus we are led to the concept that

it is perhaps the mode-mixing effects of a joint which are

important rather than the insertion loss per se. The joint

could therefore be considered as a mode-mixing component in

which the power contained in the guided-modes (and to a lesser extent the leaky-modes) of the upstream fiber is

coupled into the guided, radiation and leaky modes of the

downstream fiber. The power coupled into the

radiation-modes will be immediately lost and is responsible

for a localized loss at the joint whereas the power coupled

into the leaky modes will result in a joint-loss figure that

is dependent on the location of the observation point from

the joint, a situation which is further compounded by

differential-mode attenuation in the fiber. Since the

mode-mixing effect of the joint depends on the power

distribution amongst the modes in the upstream fiber, the

initial excitation conditions of this fiber will affect the

results and this has already been demonstrated with led and

laser excitation (1).

69
In order to overcome the problem of initial excitation

conditions joint-loss measurements have been made using

restricted excitation conditions in a similar manner to that

pioneered by workers at Corning (4) for fiber attenuation

measurements. However, although this is probably a

satisfactory technique when considering one joint in

isolation it may be difficult to reliably extrapolate the

results to a series of closely cascaded joints, as the

downstream joints will, in general, "see" a larger mode

volume and thus appear to be more lossy.

Optical Time-Domain Ref lectometry

Although the most common method of determining joint loss

has been that of measuring its insertion loss, some attempts

have also been made to estimate the loss using optical

time-domain ref lectometry (OTDR) . This technique obviously

has many advantages for measurements in the field, and some

workers claim good results, although there still seems to be

some doubts about obtaining quantitative results. This

problem arises because the fibers on either side of the

joint may possess differing scattering coefficients and

furthermore, the modal distribution set up by scattering

will, in general, differ from the distribution seen by the

fiber in the system environment. In an attempt to overcome

these problems , some workers have taken OTDR measurements

from both ends of the link and shown that greater accuracy

can be obtained. Another suitable technique for measuring

joint loss using OTDR could be the double-pulse method

originated by workers at Southampton University (5) , but as

yet no results are available for joint-loss measurements

made using the technique.

Measurement of the Influence of Splices on System

Bandwidth

The mode-mixing characteristics of the joint affect not only

the attenuation, but also the bandwidth of the link and this

aspect of fiber interconnection technology has been even

less explored. Cherin and Rich (6) have studied the effect

of gap and offset in the middle of a 1.35 Km link and found

that when strong mode mixing exists in the fiber these

misalignments have little effect on pulse width. However

when fiber mode mixing was small, a change in pulse width of

80% was produced by offset. The mode-conversion effects of

splices in step-index fibers have been investigated, and it

was found that even very low-loss splices induce significant

mode mixing. These results are thus consistent with our

concept of a joint behaving as a mode-mixing device.

Experiments in other laboratories (7) using multiple

connections over 2 Km of fiber have shown that the

70
mode-mixing effects of the joints can improve bandwidth, and

one connector manufacturer claims a design which helps to

preserve bandwidth by deliberately mixing the modes (8).

Modal-Noise Effects

One further aspect of joints has yet to be investigated in

detail; namely, their effect on the linearity of systems

using coherent sources such as semiconducter lasers.

Epworth (9) has already demonstrated that modal-noise

originating at interconnection imperfections can reduce the

system signal-to-noise ratio of an optical-fiber link, and

it is possible that the linearity of the system is also

affected. A detailed knowledge of such effects would be

very useful for the design of analog systems.

Conclusion

We have presented the concept of the joint as a mode-mixing

device and, as such, it will modify the distribution of

power amongst the modes propagating in the upstream fiber

establishing a new modal power distribution in the fiber

following the joint. Since, in general, some power will be

coupled into both radiation modes and leaky modes the joint

will exhibit both a localized "insertion loss" and a length-dependent "insertion loss." The mixing of the modes

will also affect the bandwidth of the link although the

results of this will, in general, only be observed many

meters downstream from the interconnection point.

71
References

(1) Gordon, K. S, "Impact of joints on the design of

optical-fiber systems," Topical meeting on Optical

Fiber Communication, Washington, 6-8 March 1979, pp.

108-110.

(2) Adams, M. J., Payne, D. N. and Sladen, F. M. E.

"Splicing tolerances in graded-index fibers," Applied

Physics Letters, 28 (9), May 1976, pp. 524-526.

(3) Adams, M. J., Payne, D. N. and Sladen, F. M. E.

"Length-dependent effects due to leaky modes on multimode graded-index optical fibers," Optics Communication

17 (2), May 1976, pp. 204-209.

(4) Holmes, T. G. "Propagation parameter measurements of

optical waveguides," SPIE Conference, Washington, April

1980.

(5) Conduit, A. J., Hartog, A. H. and Payne, D. N.

"Spectral- and length-dependent losses in optical fibers investigated by a two-channel backscatter

technique," Electronics Letters 1_6 (3), January 1980,

pp. 77-78.

(6) Cherin, A. H. and Rich, P. J. "Delay distortion

characteristics of optical fiber splices," Applied

Optics, 16 (2), February 1977, pp. 497-500.

(7) Gordon, K. S., Kapron, F. P. and Few, I. S.

unpublished.

(8) Holzman, M. A. Proceedings SPIE 150, 1978, p. 152.

(9) Epworth, R. E. "Phenomenon of Model Noise in Fiber

Systems," Topical Meeting on Optical Fiber Communication, Washington, March 1979.

72
LOSS CHARACTERIZATION OF BICONIC SINGLE-FIBER CONNECTORS

P. Kaiser, W. C. Young, N. K. Cheung, and L. Curtis

Bell Laboratories, Holmdel, N. J., 07733, USA

INTRODUCTION

Fiber connection losses are affected by intrinsic fiber

parameter mismatch', by extrinsic misalignment such as off -set,

tilt, and end-separation; by the end-face polishing quality, and by

the modal power distribution existing at the connection point. The

modal power distribution itself depends on the fiber transmission

characteristics, i.e. differential mode attenuation and mode

coupling, the location of the connector or splice, and the launch

conditions. Unique, steady-state losses are only obtained for an

equilibrium mode distribution (EMD) at the splice point-1- » 2 which

establishes itself, in general, after a few kilometers, or which can be

created within a much shorter distance by controlled-beam excitation,

or with mode filters!"^. Recently, the EMD losses of graded-index

fibers have been measured with high accuracy using a simple, mandrelwrap (MW) mode f ilter . ^ ' ^ . The main function of this filter, which

consists of five turns of fiber wound around a 1.2-cm diameter mandrel

(as optimized for 50um-core, 0.23-NA, graded-index fibers), is to

eliminate excess higher-order and leaky modes inadvertently launched

by typical excitation conditions. Since, in general, the MW filter

does not generate the EMD exactly, the question arises as to the

accuracy of connector loss measurements performed with this filter.

CONNECTOR DESIGN, AND LOSS MEASUREMENTS

The connector used for this study was the transfer-molded

biconic, single-fiber connector (Fig. 1), in which Fresnel reflection losses were .virtually eliminated by physical contact of the

fiber end-faces. Average losses of 0.21 dB have previously been

reported for 6 5 connector plugs when measured against a reference

plug (Fig. 2) 5 . While the fiber end faces were dry-polished in

above case, the losses of the same set of connectors decreased to

0.11 dB after wet-polishing with a 0.3um polishing compound (Fig. 3).

It should be noted that for the purpose of determining the highest

performance achievable, the loss of each plug was minimized by rotating

the plugs relative to each other. The average losses are generally

higher, and depend on a variety of factors as noted earlier, and as

also discussed further below.

The significance of the MW loss data, in addition to other

tests, was ascertained with a set of five jumper cables (1.5m long)

whose measured minimum and maximum losses and calculated average

losses for different test conditions are listed in Table I. The

specific test conditions were: a) Direct excitation with a pigtailed

LED; b) Excitation with MW filter, followed by a short reference

fiber (Fl) ; c) As under b, but with test cable sandwiched between

two (input and output) reference fibers (Fl and F2) ; and d) Test

jumper sandwiched between Fl and F2, which were preceded and followed by approximately 1-km long input and output fibers. It should

be noted that test conditions c) and d) contained two connectors.

73
Without discussing the complete information contained in Table I

in detail, the average losses of the five test cables, i. e., 0.33

and 0.30 dB for conditions c) and d) , respectively, demonstrate

that loss data obtained with the MW filter and with long input and

output reference fibers, are essentially the same. Furthermore, results

obtained with the simpler set-up of Fig. 2 (test condition b) agree

with the 'sandwich' data of test condition c) and d) within 0.1 dB,

which represents an excellent agreement.

CONCATENATION LOSSES

In order to eliminate the bias associated with using a particular reference fiber, plug, and coupling sleeve, more realistic

loss data are obtained for a concatenation of several connectorized

test cables. Yet, the 0.11 dB average loss of above 65 connector

plugs increased only to 0.16 dB for a minimized-loss concatenation

of the 32 jumper cables (Fig. 4), and to 0.23 dB for a random,

concatenation (Fig. 5) Again, about the same losses (within 0.1 dB)

were measured when the concatenated jumpers were preceded and followed

by 1-km-long reference fibers. It is worth noting, that the loss of one

specific connector was the same whether it was measured as the first or

last connection in the concatenated string (using MW excitation) . While the 1.5m- long test cables discussed thus far were sufficiently short so that fiber losses could be ignored, the determination

of the joint losses when long fiber sections are involved requires the

accurate measurement of the fiber losses. For example, "negative"

splice losses may be obtained, if the losses of the individual fibers

are measured too pessimistically, or if the connectorized, concatenated

fiber section is under-excited, while the fiber losses were obtained

for EMD conditions. The requirements on the accuracy become particularly

severe in the longer-wavelength region (say, at 1.3ym), where total

fiber losses amount to less than 1 dB/km. To illustrate this point,

the losses of five 1 to 2 km-long Ge-P-doped graded- index fibers were

measured at 1.3ym, using both the matched -beam (MB) and MW-filter

technique, with the MW method typically yielding slightly lower

(by 0.15 dB/km, max.) values. During the concatenation process, for

which the loss of the individual connectors was again minimized by

rotational adjustment of the plugs, the loss of each section added

was recorded and is listed in Table II. Depending on whether MB or

MW fiber loss data were used, the average connector losses were

either 0.14 or 0.27 dB, respectively, thus illustrating the limitations

of the accuracy of connector loss measurements because of the uncertainty of the fiber loss measurement. (Because of the higher

accuracy of the MB method, the 0.14 dB avg. connector loss is considered

the more representative value)

.

CONCLUSIONS

Using low-loss biconic connectors, insertion loss measurements

performed with a MW filter and long reference fibers agree within

0.1 dB. The accuracy of connector loss measurements depends critically

on establishing an approximate equilibrium-mode distribution at the

connection point, and knowing the EMD losses of the fibers to be

interconnected as can be ascertained through matched-beam, or MWfilter excitation. While connector losses well below 0.3 dB (without

index match) can be achieved with biconic connectors containing

tightly-toleranced fibers, it is obvious that inferior geometrical

74
and optical fiber characteristics can result in significantly

higher insertion losses.

ACKNOWLEDGEMENTS

The Ge-P-doped fibers were supplied by M. Saifi, S. R. Nagel

and F. V. DiMarcello, and WECo, Atlanta. The connectors were

molded by R. Spicer, and a preliminary evaluation with an automated

test set° was performed by R. Ragbir.

REFERENCES

1. P. Kaiser, NBS Symposium on Optical Measurements, Boulder (1980);

2. Y. Daido, et al , Opt. Fiber Com. Conf., Washington, D. C, p. ThE3

(1979); 3. L. G. Cohen, et al, Proc. IEEE, Nov. 1980; 4. P. Kaiser

and D. L. Bisbee, Opt. Fiber Com. Conf., Wash., D. C. p. TuE3 (1979);

5. W. C. Young, et al, 6th European Conf. on Opt. Com., York (1980);

6. N. K. Cheung and N. M. Denkin, NBS Symposium on Opt. Measurements

Boulder (1980)

.

TABLE I

Minimum, maximum, anc average connector losses of five, 1. 5m-long

test fibers measured under different conditions as described in text

Fiber #1 #2 #3

Loss (dB) a) b) c) d) a) b) c) d) a) b) c) d)

Minimum .28 .08 .13 .19 . 41 . 13 .14 .23 .28 . 10 .17 . 06

Maximum .58 .32 . 58 . 74 .66 . 41 . 62 .83 .63 . 35 . 79 . 74

Average* .43 .20 .24 .33 .54 .27 . 26 . 44 .46 .23 . 33 . 23

Fiber #4 #5 Average

,

#1 to #5

Minimum .25 .10 .22 .11 .27 . 11 .29 . 19 . 30 .10 .19 . 16

Maximum .68 .44 . 85 .78 . 60 . 36 . 84 .56 . 63 . 38 .74 . 73

Average* .47 .27 .38 .28 . 44 .24 . 43 . 28 .47 .24 . 33 . 30

*The average loss in case of input- and output reference

connectors (test conditions c) and d)) is calculated

from: Avg. Loss = (3xMin. Loss + Max. Loss) /4 ( X=0.82um)

TABLE II

Normalized and total loss of five fibers measured with the matchedbeam (MB) and mandrel-wrap (MW) technique at 1.3ym. Concatenated loss

per section added, and connector losses based on MB and MW fiber data:

Length

(m)

Loss

dB/km

Loss

(dB)

Loss/

Section

(dB)

Connector

(dB) Based

MB MW MB MW MB MW

Fiber #1 992 .75 75 .74 .74 .61 -.13 -.13

Fiber #2 1278 . 70 60 .89 .77 1.01 .12 . 24

Fiber #3 1160 . 85 70 .99 .81 1.28 .29 .47

Fiber #4 1983 . 80 65 1 .59 1.29 1.71 . 12 . 42

Fiber #5 1199 . 80 75 .96 .90 1.25 .29 .35

Tot. Loss (dB) 5 .17 4.51 5.86 .69 1.35

Avg . Con

.

Loss (dB) .14 . 27

75
1.27 cm DIA

V i pn /

CONNECTOR HOUSING WITH AUGMENT SIEEW

=GDD<0=> ^Cj) I POWER METER

' 5 TURN. 0 5

1 DIA COIL MANDREL WRAP MODE FILTER

CONNECTOR PLUG. REFERENCE CONNECTOR PLUG UNDER TEST

Fig.l Spring-Loaded, Biconic

Connector

15

6

7

<no 6 u

UJz

zo

o

u.o

otUJ

032:3

z

yo

2

Cu

Q

5

23

Z

0

Fig. 2 Schematic of Loss Test Set

NUMBER - 65

MEAN - 0. 113 dB

SIGMA - 0.048 dB

0. 00 0. 10 0. 20 0. 30

LOSS IN dB

'

I

'

1 '

I 0. 40 0. 50

Fig. 3 Losses of 65 Connector Plugs

Measured Against Reference Plug

And Minimized

NUMBER - 32

MEAN - 0.159 dB

SIGMA - 0. 065 dB

1

m

<:c a

i-u

UJZ

zo

u

u_o

a.Wi •as3

z

a

7

6

5

4

3

2

1 L_

NUMBER - 30

MEAN - 0. 2J7 dB

SIGMA - 0. J 02 dB

0. 00 0. 10 0. 20 0. 30 0. 40 0. 50

LOSS IN dB

Fig. 4 Minimized Losses of 32

Concatenated Connectors

m I

0.00 0.10 0.20 0.30 0.40 0.50

LOSS IN dB

Fig. 5 Losses of 30 Connectors

Concatenated Randomly

76
CONTRIBUTION TO SPLICE LOSS EVALUATION

BY THE BACKSCATTERING TECHNIQUE : A' STATISTICAL COMPARISON WITH INSERTION LOSS DATA

by A. LEBOUTET

C.N.E.T - ROC/SFO

Route de Tregastel

22301 LANNION FRANCE

INTRODUCTION

This paper deals with the possibility of field testing of splices by the

backscattering technique. Among the methods presently available, this technique

presents well known advantages, among which are its cost-effectiveness and low

time-consumption. We will try to assess on a statistical basis the pertinence of the

results for the system designer.

Many parameters have been identified as responsible for the insertion

loss of an optical fiber splice. On the first hand, extrinsic parameters (i.e

alignment), those on which the splice manufacturer can have an action, are tilt,

offset, end face preparation and separation. On the second hand, intrinsic parameters will be at the origin of unavoidable "primary" attenuation, which is the

lowest possible value if the extrinsic parameters are perfectly matched (i.e zero).

Those last parameters, of great importance, are numerical aperture. core radius

mismatches, and « - profile shape. Leaving apart the oc-parameter because of its

relatively lower importance, as stated in [l], the great difference, when comparing

backscattering to insertion loss, is chat the extrinsic parameters only are the cause

of oriented attenuation.

1 - Local transmission coefficient under the gaussian asumption

Table 1 shows the four possible combined effects of core radius and numerical

aperture mismatches. Case 1 is a splice from a large core-low NA fiber to a small

core-high NA fiber giving insertion loss due to core mismatch only and showing in

the reverse direction (case 4) that insertion loss is due to NA mismatch only.

Case t Definition Insertion Backscatt

1

LX Rl > fX* NGKtWfc

2

"~UL W > RZ

Nr 1 wf\i^KiA?_

I

3 krSI' *l < H2

4 N . >KA?,

o( T (o»

TaUe 1

77
We chose the gaussian hypothesis [1, 2, 3] to calculate the local transmission coefficient which is a function of two variables,^ - {R 1- ?.4)/R.A, the

relative core radius variation and, ^ & . /&2-&f)/A| , the relative peak index variation

The approximation was made that the backscattered light follows the same oy^R , cA)

low as the forward propagation, the splice parameters being changed in the

backward direction, of course.

2 - Statistical analysis

We considered that the splices were made from the random gaussian population

of fibers defined below, which represent the rnanufacturing variations [ll:

Mean core radius 60 urn, standard deviation 3 ^m

Mean NA 0,20 standard deviation 0,01.

The attenuations, a (INS) and a (BKS) , of 10,000 splices were computed

according to table 1. The distribution is given in figure 1 for insertion and

in figure 2 for backscattering together with their mean m and standard deviation a respectively m TIB, m , and cr Id0 The better centering, o < 0 is due to the

d 1 property of backscattering measurement to average the values in both directions,

and the bias mg>mj is related to the presence of 25% of OdB insertion loss (probability of case 3, table 1) whereas the backscattering shows no OdB splices at all

The cumulative distribution shows 90% of less than 1 dB (INS) splices but a problem

is the existence of a tail of out of range values, some of them being undetected

by the backscattering. So the probability of "undetectable bad splices" is not

zero, and that problem will be discussed later.

The distributions, a

TT and aTD , are far from gaussian but the variables are

J 1 J D correlated by a coefficient r such that :

A knowledge of ^jj. is given bycCrg, suggesting the estimation problem : find

A

the best estimator d of insertion loss when BKS-loss<*J& only has been measured.

Let us define £ as the mean square error (m.s.e) of a given estimator ^by

where IE stands for the expected value.

A

1) the most attractive estimator is to take for"^ the backscattering value

itself. The m.s.e is then : jr; o(jb^

which is simply the standard deviation of the distribution of the differences

INS-BKS, plotted figure 3, curve (A), with the computed value £±

= 0,112 dB. The

confidence interval is found by the cumulative differential loss distribution given

fig. 4. The conclusion is that 60 % of the splices are correctly measured by

backscattering within a + 0,1 dB approximation.

78
2) A better estimator is found by the linear regression : - AotJa +p

then the m.s.e Z2 - f (^Ji

^jr. " tO has a minium value of £z s -

<* 1 ) for a convenient choice of A and p , whose mathematical calculations will not

figure here for clarity. The main conclusion is that the m.s.e £2 decreases from

£l= 0,112 dB to £,2= 0,110 dB which is not significant compared to the computational difficulty.

Figure 4, curve B is the cumulative distribution which shows the previsible

slight increase of the confidence interval + 0,1 dB.

3) Classicaly, the absolute estimator is the non-linear function of <^jB defined as

which is the mean of the conditional probability density of tfjr if is known.

A At the cost of an extremely time-consuming procedure <^ can be estimated itself and

the conditional density ^Jrj^jB is given and averaged to find ^iC^tb)

— 4

For instance, 400 splices over 10 are selected to be measured at 0,1 + 0,05 dB by

the backscattering technique. Then 0,099 dB with a standard deviation of

0,084 dB. Other considerations lead to the conclusion that the confidence interval

formerly found with poorer estimators is not much increased.

CONCLUSION

We tested on a random population of splices the possible discrepancies of

insertion loss versus backscattering on the worst case basis of unsymetrical intr::

sic parameters. It is shown that if the backscattering value is taken as an estima

tion of insertion loss, 60 % of the results be in the + 0,1 dB range.

If the extrinsic ( symmetrical) parameters were taken into account, we believe

that the masking effects on attenuation would still increase that confidence inter

val, thus validating the method for field testing on a statistical basis. Nevertheless the important problem of the tail of high discrepancies remains and needs

some more investigation.

BIBLIOGRAPHY

£l Effects of fiber manufacturing variations on graded index fiber splices

C. M MILLER

l2 Presentation theorique des parametres intervenant dans la j onetion entre

deux fibres optiques - Internal CNET - Report

3 A loss model for parabolic-orofile fiber splices C. M MILLER

L and S.C METTLER

B. S.T.J 57 n° 9 (September 1973)

79
2077

| (24/6%)

"frequency

60

50-

40

30-Mf*—

r

Jr. } *; ft

10

F

'

0-!-n-r

m = 0.125 dB

<T r 0,130 dB

out of range : 1511 values

TT f 1 I I I 0 .20

.10 .30

.40 .61 .61 1.02

.51 .71 .92

dB

INS. LOSS DISTRIBUTION

FREQUENCY

m : 0.135 dB

<T-- 0,089 dB

dB

20 .41 .61 .0,1 1.02 .30 i51 .71 .92 BKS. LOSS DISTRIBUTION

m = 0.01 dB

<T -. 0. 112 dB

DIFFERENCIAL BKS. INS LOSS DISTRIBUTION CUMULATIVE DIFFERENTIAL LOSS DISTRIBUTION

80
Optical Cable Fault Location Using a Correlation Technique

*K.Okada, *I . Kobayashi , *K. Hashimoto , °T.Shibata, °T.Kosugi and °Y.Nagaki

* Yokosuka Electrical Communication Laboratory

Nippon Telegraph § Telephone Public Corporation

0 Anritsu Electric Co., Ltd.

Abstract; Development is underway to realize practical application of optical

fibers for optical communications in a low loss wavelength region (1.3 jim or

1.55 jUm) . Regarding optical cable fault location measured in the low loss

wavelength range, the conventional isolated pulse method was inadequate for

long-distance measurements because of the small light source power, lowered

sensitivity of the light receiver, and feeble backscattering light. The

technique described here using pseudo-random noise (PN) pulses has realized

a long-distance measurement of 10 km in an optical cable of 0.8 dB/km in

optical loss at 1.3jUm wavelength.

1 . Introduction

Optical cable transmission systems using optical fibers have begun to be

put to practical utilization, and tests in the field are steadily underway.

In optical cable fault location using backscattering light, measurements

over a distance of 8 km in an optical cable of 2.5 dB/km in optical loss are now possible at the 0.85jUm wavelength. This is performed in the isolated

pulse method using a pulse laser diode, which transmits isolated pulses from

one end of an optical fiber to observe the light backscattered from the optical fiber. This performance is possible because a light source capable of a fiber input of +20 dBm and over and a received optical gain of 30 dB and over have been realized in the 0.85 jUm wavelength band.

Nevertheless, the 0.85 jUm band where the optical loss is large in optical

fibers is not suitable for a long-distance communications system. For this

reason, development is in rapid progress in communication systems in the 1.3^m

and the 1.55 jLim wavelength bands where the optical loss in optical fibers is

at a minimum. In these wavelength bands, the repeater spacing may be extended

to several tens of kilometers. If a light source and light receiver having

capabilities equal to those used in the 0.85jLim are completed in the low loss

wavelength bands, it would also be possible to perform by the isolated pulse

method cable fault location up to several tens of kilometers.

81
In the present state of the art, however, compared with the level difference in the 0,85 ym wavelength band, low loss wavelength bands are lower by

30dB and more in the total value of the transmitting optical pulse power and

the received optical sensitivity. Thus, it is difficult to perform measurements

by the isolated pulse method.

The present paper describes the PN pulse method that permits long-distance

measurements using currently available light sources and light receivers in

the low loss wavelength bands.

2. Comparison of the PN Pulse Method with the Isolated Pulse Method

The simple isolated pulse method is inadequate for obtaining a sufficiently low S/N ratio. Therefore its S/N ratio is usually improved by averaging

through sweeping and adding up the received signal on the time axis. On the

other hand, in the PN pulse mehtod, the received signal is added up repeatedly

at every delay time in a correlating process. Here the delay time corresponds

to the fiber distance.

In the long wavelength band, the pulse laser diode is unable to produce a high power (a few watts) and is limited to certain repeated periods and

pulse widths. The CW laser diode used for the PN pulse method, however,

although small in power, has the advantage of setting the equivalent power,

repeated period and pulse width in a wider scope by the M-sequence and autocorrelation technologies.

The correlator comprising only an analog switch and an analog integrator,

thus far simpler in design, makes the PN pulse method superior to the isolated

pulse method requiring an averager of a more complicated construction.

3. Principles of the PN Pulse Method

The M-sequence code from among the PN pulse code system is used as the

correlating code since the auto-correlating function of the M-sequence code

is a triangular wave and data can be transmitted continuously for correlating

it with the data returned without causing errors by the omission of a portion

of a pulse train in the time domain. The M-sequence code auto-correlation can be expressed by the following formula.

(NT

h(T) CO x(t) -x(t-r) dt (1)

JO

where h(-r): impulse response

T: the period of the PN pulse sequence

82
N: the number of the PN pulse sequence (averaging count)

x(t): the PN pulse signal

y. the delay time

The shorter the bit time of the PN pulse train, the higher the resolution

of the distance but the lower the sensitivity. On the other hand, increasing

T while fixing the bit time of the PN pulse train will increase the power.

Increasing N of averaging in conjunction with increasing T will improve the

sensitivity or the S/N ratio. But this improvement is limited by the measuring

time which cannot be indefinitely prolonged.

A block diagram of the PN pulse method is shown in Figure 1. The output

pulse from the oscillator is put into the frequency divider which determines

the resolution of the measuring distance in the optical fiber.

The laser diode converts the delayed advance PN pulse signal from a electric level into an optical level. The light obtained by this conversion

is sent through the directional coupler to the optical fiber and is returned as backscattered light to the directional coupler to be put into the APD.

The output correlated between the output of APD converted from an optical

level into an electric level and the reference PN pulse signal is approximated

to a signal pulse response. The- averaging counter performs the averaging

procedure by repeating the advance PN pulse signal of the same delay time.

The correlator having a CR integrating circuit can very the gain.

4. Experiment

It is anticipated that a 20 km transmission is achieved in the 1.3 ]lm

optical transmission systems and such systems will require a fault location

capability of at least 10 km and possibly 20 km.

The feasibility of the PN pulse method was tested in an experiment carried

out using a 10 km long fiber. Experimental parameters are shown in Table 1,.

and the backscattered level from the 10 km long fiber is shown in Figure 2.

In Fig. 2, a Fresnel reflection from a fiber end-face is eliminated by using a refraction matched liquid. The results indicate that 10 km long fault location

is obtainable.

Figure 3 shows a logarithmic conversion of the result obtained in Fig. 2.

5 . Conclusion

A new optical cable fault location method is proposed. An experiment

confirmed that a broken point 10 km distant from the source can be detected using

a 1.3 jim wavelength. It is expected that the measuring distance can be

83
extended through a few improvements as this low loss wavelength band in the

optical fiber. The method is also effective for measuring fiber loss distribution.

6. Acknowl edgement

The authors wish to thank Drs. S.SHIMADA and H.ISHIO for their great

encouragement and beneficial discussion.

7. References

(1) M.K.Barnoski et al, Appl. Opt., 16, 9, pp . 2375-2379, 1977

(2) I.Kobayashi et al, Review of E.C.L., 27_, 11-12, pp . 1029-1048 , 1979

(3) K.Okada et al, Trans., IECE, 63-B , 2, pp. 159-166, '80/2, (in Japanese)

(4) R.A.Scholts IEEE Trans, on comm., COM- 25 , pp. 748-753, 1977

(a) Fault Locator Parameters

Peak power of laser diode

(within a fiber)

1 mW

iVavelength of laser diode 1.52 Un

Pulse repetition JSO kHz

A'ord length • N 255 * 64 bits

N': Nunber of averaging

( b) Fiber Parameters

Fiber loss at 1.52 m wavelength

including splice loss) 0.8 dB/km

Outer diameter 150 ua

Core diameter 00

Index profile graded index

PULSE OSCILLATOR

DIRECTIONAL

Fig. 1 Block diagram of the pseudorandom pulse method

Table 1

84
OPTICAL TIME DOMAIN REFLECTOMETRY BY PHOTON COUNTING

P. Healey and P. Hensel

Telecom Research Laboratories, Martlesham Heath, Ipswich, UK

Introduction

At the 1979 IEEE Workshop on Fiber Optic Measurements, we expressed doubt

that OTDR or "backscatter" would ever have sufficient range to penetrate a

full repeater section because of launch power limitations and the necessary

dynamic range of the apparatus. Analogue signal recovery techniques

appeared then to limit the range to about 25 dB of one-way fibre loss, A

law of rapidly diminishing returns applies to any straightforward method of

increasing the sensitivity, e.g. , averaging a larger number of samples.

However, we can now report a substantial improvement in the range of OTDR

achieved by using a photon counting technique. We have succeeded in

locating non-reflecting fibre breaks through one-way attenuations exceeding

40 dB and we believe there is scope for further progress. When there is a

reflection associated with the break, the range already exceeds 50 dB which

is sufficient for many fibre systems.

The Photon Counting Technique

When an avalanche photodiode is operated in a "Geiger tube" breakdown mode,

it is biassed above its normal operating voltage and breakdown may be

initiated by a single photon (1,2). In our case for example, an RCA

C30902E has a normal room temperature operating bias of 203 V. The

avalanche gain over the active area of the diode is not uniform and

gradually increasing the bias beyond 203 V causes some regions of the surface to approach breakdown before others. The number of regions

85
above breakdown grows with increasing bias until complete breakdown occurs

at 211 V. At intermediate voltages, bulk leakage causes local breakdown

and a compromise must be found between the ease with which an area of high

sensitivity can be located and the background count rate. The time constant made up of the device capacitance and the external load acts as a

quenching circuit by causing the bias voltage to fall temporarily below the

breakdown level after a pulse. It is this time constant, rather than the

input pulse length, which determines the output pulse length and hence the

25 m resolution of the apparatus.

Experimental Apparatus

Figure 1 shows a schematic diagram of the apparatus. Much of it is conventional - the distinctive features of the apparatus lie in the signal processing area. The digital delay generator triggers the laser and generates

two pulses which gate the input to a counting circuit. The first pulse can

be stepped through the period of the return signal thus admitting photon

counts and background (leakage) counts to the processor. The second pulse

admits background only. Photons can then be counted by computing the running difference between these two rates. When a count of sixteen is

reached, the sampling interval moves to the next point and thus automatically progresses at a rate determined by the signal strength. As the fibre

end is passed, the count rate drops sharply and the dwell time at each

point rises correspondingly. Because of statistical fluctuations in the

leakage, counts continue to be recorded after the fibre end, but provided

that the dwell time increases by a factor of two or three, the end is

readily located.

86
Experimental Results

Figure 2 shows the result of an experiment to locate the end of a fibre

with an attenuation of 33 dB at 85O nm. The core diameter was 40 urn, the

numerical aperture was 0.1 8 and the distant end was refractive index

matched to suppress any reflection. Beyond the fibre end, the dwell time

rises by a factor of more than fifty. Figure 3 shows the result of a

similar observation made on the same type of fibre but with an attenuation

of 40.5 dB. Here, the average dwell time rises by a factor of about three

which is still clearly discernible.

Conclusion

The photon counting approach greatly extends the range of OTDR. It also

leads to a considerable simplification of the necessary electronics through

the application of digital techniques.

Further improvements appear possible by cooling the avalanche photodiode

and by optimising the number of photon counts before stepping the sampling

interval. Cooling a photodiode reduces the leakage current and thus permits a higher bias voltage. This increases the detection probability and

eases optical alignment problems by increasing the useful area of the

device. It may also be possible to dispense with the wideband transimpedance amplifier used at present. The optimum number of photon counts made

at each sampling point may differ from the current sixteen which is a convenient and somewhat arbitrary number. Theoretical investigations are in

progress.

It is hoped to report on both these points at the Symposium.

87
Acknowledgement

The authors would like to thank the Director of the British Telecom Research

Department (part of the Post Office) for permission to present this paper.

References

1 P P Webb et al: "Single photon detection with avalanche photodiodes",

Bull Amer Phys Soc II, 813, June 1970

2 R J Mclntyre: "The distribution of gains in uniformly multiplying

avalanche photodiodes: theory", IEEE Trans on Electron Dev, ED-19 (6 )« 703

Cladding mode stripper

50% filter

Launch and receive optics

Figure 1: Schematic diagram of the apparatus

Count

equals 16 "mn n|

t | ini|ii ,111, i in ii, 11,11 mi i ii i ^

Figure 2: Dwell time for 16 counts with 33 dB of fibre loss

Count

equals 16

i i i r i ii i Time 30sec/di\

Figure 3: As above with a one—way fibre loss of 40.5 cLB

88
CHARACTERISATION OF SINGLE MODE FIBRES

K.I.White, B.P.Nelson, J.V.Wright, M.C. Brierley and A. Beaumont

British Telecom Research Laboratories, Martlesham Heath, Ipswich, England

INTRODUCTION:- We report various loss, dispersion and index profile

measurements on single mode fibres and show how the dispersion and mode cut-off

predicted by the profile agree with the experiment.

LOSS:- The spectral attenuation, measured by the insertion loss method, is

presented in fig. 1. While the fibre used for this set of measurements has

a minimum loss of 0.75 dB/km, more recent fibres have significantly lower

losses [ l] . The crosses on fig. 1 represent the absorption component of the

total attenuation. This was measured calorimetrically [2] using Krypton and

Yag lasers. The absorption was lowest (0.2k dB/km) at 1.06 um. The apparatus

has the sensitivity to measure less than 0.05 dB/km per Watt of guided power.

The difference between total loss and absorption gives the scatter loss -

-k

shown by the dots on fig. 1. By plotting the same data against X , fig. 2,

the basically Rayleigh nature of the scattering is revealed. There is less

than 0.1 dB/km of wavelength independent scatter beyond 1 um. Thus, the total

loss is determined and resolved into absorption^ Rayleigh scatter and wavelength

independent scatter.

MODE CUT-OFF:- The insertion loss apparatus can also be used to indicate

mode cut-offs. The transmission of a short length of fibre with a tight bend

is compared with that of the same fibre when straight [ 3] . The LP mode

cut-off peaks around 870 nm - fig. 3. The feature on the total attenuation

plot at this wavelength is thus verified as due to the mode cut-off.

89
DISPERSION:- The dispersion of a single mode arises from the material and

waveguide properties of the fibre. Also, any deviation from cylindrical

symmetry will lift the mode degeneracy giving two orthogonally polarized modes.

This intermcdal dispersion was measured using the 1.06 ym mode-locked pulse

train from a Yag laser. The spectral width of this laser line is sufficiently

small for the material and waveguide dispersion to be neglected. For a 6 km

1 . section fibre, the mean ~ input and output pulse widths were measured at

185 ps and 195.8 ps by a GalnAs photodiode. Deconvolution yields 11 ps/km as

the monochromatic impulse response of this fibre.

A pulsed tunable source for group delay measurements can be obtained by

employing the stimulated Raman effect [ h] . We have used a single pass of the

1.06 ym mode-locked, Q-switched line through 300 m of single mode fibre [ 5] •

The output from this fibre is passed through a monochromator , into the test

fibre and onto a photodiode. The variation of group delay with wavelength is

presented in fig. 5. A linearised Sellmeier equation was used to obtain the

fitted curve. The derivative of this gives the wavelength of minimum dispersion

as 1.37 ym. These two dispersion measurements give the wavelength of

minimum dispersion and the value of that minimum.

INDEX PROFILE:- The refracted near field technique [6] is ideal for single

mode fibres as it is simple and direct. The profile of this fibre is shown

in fig. 6. The diameter is 100 ym and the core FWHM is 6.7 ym. As the

profile is obtained across the full diameter, the fibre geometry can be

checked. The absolute index difference across the core is also obtained. One

limitation to the spatial resolution is the point spread function of the focus

of the, effectively, annular aperture lens used. This can be deconvolved

from the measured profile by a newly developed [ 7l two-dimensional Fourier

90
transform process.

THEORETICAL PREDICTIONS : - Only a small approximation is involved in treating

the profile as a step of diameter 6.7 ym and index difference 0.003^7. These

values predict a LP.^ mode cut-off at 870 nm in good agreement with the experiment.

The fibre dispersion can be obtained approximately from the material dispersion

of silica and the waveguide dispersion for this profile [ 8] . The circles on

fig. 5 are the theoretical points. The agreement is very good for an

approximate model. This is because including the effect of different core

and cladding material dispersion would increase the wavelength of minimum

dispersion by 7 nm while the deviations from a step profile have the opposite

effect [ 9] •

ACKNOWLEDGEMENTS : - Acknowledgement is made to B J Ainslie, K J Beales D Colthorpe

and C R Day for providing the fibre, to C J Todd for supporting discussions and to

the Director of the British Telecom Research Laboratories for permission to

publish this paper.

REFERENCES

1 B J Ainslie et al. Electron. Lett . 15 (1979) ^11.

2 K I White. Optical and Quant. Electron. 8 (1976) 73.

3 Y Katsuyama et al. Electron. Lett. 12 (1976) 669.

k L G Cohen and C Lin. Appl. Opt. 16 (1977) 3136.

5 BP Nelson, J V Wright and K I White. Digest of IEE Colloquium - Optical

Communications beyond 1 ym. I98O.

6 K I White. Optical and Quant. Electron. 11 (1979) 185.

7 M C Brierley, K I White and J V Wright. Digest of IEE Colloquium - Optical

Communications beyond 1 ym. 1980.

8 K I White and B P Nelson. Electron. Lett. 15 (1979) 396.

9 K I White and J V Wright. Digest of URSI symposium - Electromagnetic

Waves, Munich 1980.

91
Wavelength, pm

1.6 1.2 1.0 0.8 0.7 06

620 700 800 900 1000 1100 Time

Wavelength, nm

Fig. 3. Mode cut - off Fig. 4. Monochromatic impulse response

Wavelength, ym Radius urn

Fig. 5. Fibre group delay and dispersion Fig. 6. Single mode fibre profile

92
Effect of Curvature on the Cutoff

Wavelength of Single Mode Fibers

Paul D. Lazay

Bell Laboratories, Murray Hill, NJ 07974

A very simple procedure for the determination of mode

cutoff wavelengths in low mode number fibers has been developed.

This technique has been used to study the sensitivity of mode

cutoff to preform processing conditions and to various external

perturbations on the fiber.

The measurement technique involves plotting the power

transmitted through a fiber as the wavelength of the light is

increased. Mode cutoff is indicated by a rapid decrease in

detected power. Figure 1 shows how the second mode group cutoff wavelength, A , is identified with the intersection of the

slope of the power decrease with the power curve in the single

mode region. Fiber lengths from 100 cm to 1000 m have been

used for determining X . The uncertainty in X is ±.002 ym. 3 c c We have studied how A behaves when curvature is introc duced in the fiber. Figure 1 shows a family of cutoff curves

made as the radius, R, of a single loop in the fiber is decreased

from 80 mm (curve g) to 7 . 5 mm (curve i) . Increasing curvature,

R

-1

, caused A to shift rapidly to shorter wavelength. Figure 2

' c shows the dependence of A c on R

-1

. The solid curve is the best

oo -i -12

fit of the function A =A + A R +B(R ) to the data. Here

c c

A°° is the value of A in a perfectly straight fiber, and A and

c c B are two additional fitting parameters. This data shows how

shifts in A can be used to measure fiber curvature introduced

c

93
during coating, jacketing and cabling of single mode fibers. Also

it is clear that any technique that measures A^ by use of fiber

curvature will contain a significant error introduced by the

curvature

,

In this talk we will discuss our measurements of X and

c

higher mode group cutoff, the curvature sensitivity of A , the

use of A^ to test for fiber index and geometry uniformity, and

the use of A c

as a diagnostic for curvature introduced during

fiber processing.

94
TRANSMITTED POWER

1.15 1.20

WAVELENGTH fim

1.25 1.30 1.35

FIGURE 1

1.30

x

l-©2

UJ£1.20

<

=5:

u.

LuO

£ 1.10

o

1.00 JL

A C C R R

2

±

.02 .04 .0 6 .08 .10 .12

CURVATURE , R**', mm" 1 .14

FIGURE 2

95
A
Optical Time Domain Ref lectometry

on Single Mode Fibers Using a Q-Switched Nd : YAG Laser

D. L. Philen

Bell Laboratories

Norcross, Georgia 30071

In investigating loss mechanisms in optical fibers, it

is important to know how the loss is distributed along the fiber

length. Identifying a high loss region aids in the diagnosis of

fiber manufacturing problems and, in addition, may make it

possible to salvage a usable length of lower loss fiber. The

optical time domain ref lectometer (OTDR) has been used in the

past for this purpose, but only with multimode fibers.

The aim of this study is to extend optical time domain

ref lectometry to single mode fibers. Because of their lower

numerical aperture, single mode fibers capture much less Rayleigh

backscatter than multimode fibers. The half width at half maximum, 0 , for the far field pattern of a graded index multimode

fiber of maximum index difference A =(n 2 ~n

1 )/n 1 is given by

sin 9 m « */2A/2. The corresponding propagation angle inside

the core is arcsin \] A/2 , which is 4.6° for A = 0.013. The half

width at half maximum for the far field pattern of the HE-q mode

in a single mode fiber of core radius 5 ym and normalized frequency V = 2.27 is 1 0 h

= 1.29° at a wavelength of 820 nm. This

corresponds to a propagation angle of 0.89° inside the core of

the fiber, assuming the refractive index to be 1.45.

Assuming equal source-fiber coupling efficiencies for

97
the two fibers, equal Rayleigh scattering coefficients, and

isotropic backscatter, the ratio of the solid angles within 6^

and 0 m

will be the ratio of the backscattered signal powers in

single and multimode OTDRs. Since ( 0 . 89 °/4 .

6

0

) 2 = 0.037, the decrease in backscattered power from multimode to single mode

fibers will be 14 dB at 820 nm under these assumptions.

Most OTDRs using GaAlAs lasers are only able to perform

loss measurements over about 15 dB of one-way fiber loss. On the

basis of the above calculation, there is little hope of making a single mode fiber OTDR with a GaAlAs laser. In practice the

coupling efficiency into single mode fibers is less than for

multimode fibers, so that the situation becomes even more hopeless

In order to increase the amount of power launched into

a single-mode fiber, a Q-switched Nd:YAG laser was used as the

source. This laser emits up to 6 watts of power in a CW mode or

up to 12 KW of power in a Q-switched mode at 10 64 nm. The experimental arrangement is illustrated in Figure 1. At high power

levels, it is possible to produce stimulated Raman gain in both

2 3 single-mode and multimode optical fibers. It was possible to

observe this effect in the fibers studied here by a splitting of

the return pulse as the power level was increased. The onset of

stimulated Raman gain represents an upper limit on the amount of

power which may be coupled into a fiber for OTDR purposes. A

variable attenuator was used to reduce the laser intensity to a point below the onset of stimulated Raman gain. The pulse rate

was set at 500 pps and the laser was operated well above threshold

in order to have stable laser output pulses. The backscattered

98
radiation was signal averaged on a digital processing oscilloscope

and plotted on an HP-9862 plotter.

The decay in dB for a single-mode fiber is plotted in

Figure 2. This fiber shows a fairly uniform decay with no gross

imperfections. Taking half the slope yields a loss of 4.9 5 dB/km

along the length of the fiber. The standard two-point loss

measurement gave 10.3 dB/km. Two hundred meters were then removed

from the far end in Figure 2 and the two-point loss dropped to

5.2 dB/km, in reasonably good agreement with the value measured by

OTDR. Removing subsequent sections of the fiber did not change

the loss within the error of the two-point loss measurement.

While the present system is not optimized, it demonstrates that useful OTDR data can be obtained on single mode

fibers with a Q-switched Nd : YAG laser at 1064 nm. A 1.5 km

single-mode fiber with greater than 30 dB round trip loss was

studied and a high loss region identified. Thus a useful dynamic

range is achievable below the threshold for nonlinear effects.

Since splice loss is essentially wavelength independent,

this instrument may also be useful for characterizing splices.

99
100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 DISTANCE IN METERS FIGURE 2

References

1. W. A. Gambling, D. N. Payne, H. Matsumura, and R. B. Dyott,

"Determination of Core Diameter and Refractive Index

Difference of Single-mode Fibers by Observation of the

Far-field Pattern," Microwave, Optics, and Acoustics, 1,

13 (1976).

2. R. H. Stolen, E. P. Ippen, and A. R. Tynes , "Raman Oscillation in Glass Optical Waveguide," Applied Phys. Lett., 20,

62 (1972).

3. R. G. Smith, "Optical Power Handling Capacity of Low Loss

Optical Fibers as Determined by Stimulated Raman and

Brillouin Scattering," Applied Optics, 11, 2489 (1972).

100
INTERFEROMETRIC TECHNIQUE FOR THE DETERMINATION

OF DISPERSION IN A SHORT LENGTH OF

SINGLE MODE OPTICAL FIBER

W. D. Bomberger and J. J. Burke

Optical Sciences Center

University of Arizona

Tucson, Arizona 85721

The typical methods of measuring the dispersion of singl

mode optical fibers are the pulse broadening technique"'" and

2 the pulse delay technique. Both of these methods require

the use of sophisticated techniques for the generation and

detection of very short temporal duration optical pulses.

These methods also require relatively long lengths of single

mode optical fiber, typically on the order of a kilometer.

In this paper we present an interferometric technique

for measuring the disoersion of single mode optical fibers.

This method utilizes a Michelson interferometer, a broad

spectral width C.W. source and a short length of single

mode optical fiber located in one arm of the Michelson

interferometer.

101
The use of a wide spectral width semiconductor laser to

illuminate the interferometer results in a series of narrow

peaks in the envelope of the interferogram. The addition of

the dispersive fiber into only one arm of the interferometer

results in a broadening of the width of the peaks in the envelope

of the interferogram. Measuring the width of the interferogram

peaks, both for the case with the fiber in the interferometer

and for the case without the fiber, allows the total dispersion

of the optical fiber to be determined. The amount the peaks

are broadened is a function of the dispersion of the fiber

and the spectral width of the laser. The complicated spectral

content of the multimode semiconductor laser results in

difficulties in interpreting the interferograms , but a

relatively simple model of the spectra yields a reasonable

approximation to the interferograms obtained. An example of

the central interferogram peaks obtained for the laser operating

with a spectral width of 4 nm and the optical fiber of length

1 m is shown in Figure 1. Figure l.a shows the central

interferogram peak without the fiber in the system, the full

width at half maximum corresponds to a path length variation

of 80 urn. Figure l.b shows the central interferogram peak

with the fiber in the system, the full width at half maximum

corresponds to a path length variation of 200 ym.

The semiconductor laser used was multimode and emitted

at 0.8 7 ym. The spectral width was dependent upon the drive

current and was capable of being varied between approximately

102
2.5 nm and 5 nm, this range allowed the convenient use of fibers

from 0 . 2 m to 1 m in length. The fiber used had an 8 ym

diameter core and approximately a 0.8 ym cutoff wavelength

for single mode operation. By the interferometric technique

outlined, the second derivative of the longitudinal wave number

with respect to frequency was determined to be approximately

-24 2 1.22 x 10 sec /m at 0.87 ym. This corresponds to a temporal

pulse broadening of 77 psec/nm-km.

In conclusion an interferometric technique has been

developed to measure the dispersion of single mode optical

fibers. This technique is characterized by the use of a Michelson interferometer, a broad spectral width C. W. source

and a short length of single mode optical fiber.

Acknowledgment

The work is supported by the Air Force Office of Scientific

Research (AFSC) , United States Air Force, and the Army Research

Office, United States Army.

References

1. Bloom, D.M. , L. F. Mollenauer and Chinlon Lin, Technical

Digest, Topical Meeting on Fiber Optic Communication,

Washington, PD 4 (March 19 79)

2. Cohen, L. G. and Chinlon Lin, Appl. Opt. _16, p. 3136 (1977) .

103
FIG, 1a. Interferogram obtained without fiber in the system.

Upper Trace* .633um laser reference interferogram.

Lower Trace t .87um laser interferogram.

FIG. 1b. Interferogram obtained with the fiber in the system.

Upper Trace: .633um laser reference interferogram.

Lower Tracer ,87um laser interferogram.

104
INDUSTRIALIZED SYSTEM FOR THE AUTOMATED MEASUREMENTS OF THE OPTICAL

PROPERTIES OF WAVEGUIDE FIBERS.

Edward F. Murphy, Richard W. Lapierre, and Charles F. Laing

Corning Glass Works

Manufacturing & Engineering Division

Process Technology

Corning, New York 14 8 30

INTRODUCTION

This paper describes an automated Quality Assurance Measurement

System designed for production testing of optical waveguide fibers.

The system utilizes a mini computer capable of operating a number

of optical measurement stations. The measurement philosophy used

assures data reliability with built in checks of measurement

conditions and precision.

The thru-put requirement is in excess of 4 fibers per hour per

station depending on the type of fiber being characterized, i.e.,

double window graded index fiber, single window graded index fiber,

or short distance fiber.

The measurement techniques that have been implemented utilizes

Corning Glass Works preferred launch and measurement conditions^

including limited phase space launch conditions for attenuation

measurements2 and mode scrambled launch conditions for optical

bandwidth measurements 3

, °

QUALITY ASSURANCE PHILOSOPHY AND MEASUREMENT CONDITIONS

It is current policy to assure adherence to waveguide fiber specification by measuring all production fibers. This automated

measurement system was designed to perform this function with high

thru-put and no operator interaction or adjustment of the measurement equipment. Calibration checks are done every four hours with

known fibers in addition to the system's built in continuous self

checks. The results of the measurement of the known fibers are

corrected for any length dependency and compared against 9 5%

confidence limits knowing the measurement uncertainty.

ATTENUATION RATE

The attenuation rate of the fiber under test is determined by the

two point method-*- with the addition of the limited phase space

launch conditions recently developed^ /# A narrow spectral band

of light with spot size 70% of the nominal core diameter and 70%

of the nominal N.A. are launched into the long length of fiber

and the transmitted intensity measured. The measurement is

repeated for the first two (+ 0.5) meters of the same fiber using

the same launch conditions and without disturbing the input end

of the fiber. The fiber is wrapped on a suitable measurement

spool and optical interference filters with spectral width of 10

nanometers are used for this measurement.

105
OPTICAL BANDWIDTH

The optical power bandwidth of the waveguide is determined by

measuring the pulse-broadening characteristics of a full length

of fiber. An approximate 300-picosecond (FWHM) pulse of

light is launched into the fiber, and the output pulse is Fourier

transformed into the frequency domain. The Fourier spectrum of

a reference input pulse — determined from a standard two-meter

length of fiber -L - is analytically deconvolved from the Fourier

spectrum of the output pulse by dividing the output frequency

spectrum by the reference frequency spectrum. The frequency at

which the amplitude of the deconvolved spectrum drops three decibel

relative to the zero frequency component, is the bandwidth of the

fiber. The fiber is wrapped on a suitable measurement spool and

semiconductor lasers with spectral width of 2.1 nanometers (FWHM)

and center wavelength of 850 and 1300 nanometers are used. 6 In

addition, the temperature of each laser diode is controlled

to +0.1°C to ensure short term stability.

NUMERICAL APERTURE

The numerical aperture of the waveguide is calculated from the

far-field optical power distribution (watts/sr) exiting from two

(+ 0.5) meter of fiber^. Launch conditions are: constant

radiance, numerical aperture of .55, spectral width of 2 5 nanometers and center wavelength of 850 nanometers.

6

CORE DIAMETER

The core diameter of the waveguide is calculated from the nearfield optical power distribution exiting from two (+ 0.5) meters

of fiber^. Launch conditions are: constant radiance, numerical

aperture of .55, spectral width of 25 nanometers and center wavelength of 850 nanometers.

SYSTEM DESCRIPTION

The overall measurement system is shown in Figure 1. A multi-user

mini computer is used to control multiple stations. This required

some off loading of repetitive functions, two examples of which

are control of stepper motors by preset indexers and data

collection for bandwidth measurement by Tektronix Digital Processing oscilloscopes. The computer consists of a Digital Equipment

Corp. 11/34 computer with Industrial Control Subsystem Process

I/O and high speed digital and analog inputs and output. It is

possible to control up to 6 stations by a single mini computer

with the advantage of sharing the cost over multiple stations.

106
DEC PDP 11/34 COMPUTER

TICKET

PRINTER

GRAPH PRINTER VIDEO

CONSOLE

INSTRUMENTATIOh STATION 1

INSTRUMENTAT^

STATION 2

UP TO SIX STATIONS

PER COMPUTER.

OPERATOR

ENTRY

OPERATOR

ENTRY

FIGURE 1 - OWG QUALITY ASSURANCE MEASUREMENT SYSTEM The system is composed of the 11/34 computer and the following

major subsystems for each station: Source and Launch Optics,

receive and detection optics, instrumentation and detection

electronics, operator interface and fiber end preparation

machine^

.

SOURCE AND LAUNCH OPTICS, Fig. 2

Light from both an incandescent tungsten source, S^ and two pulsed

injection lasers, S2 and S3 are launched into the fiber under test.

SI Tungsten Source

Al Source Apertures

Ll Condenser Lens

A2 Condenser Aperture

A3 LPS Aperture Wheel

C Chopper

Bl Beam Splitter

Dl Source Monitor Detector

Fl Long-Pass Filter

L2 Collimator Lens

F2 Filters on Wheel

Ml Launch Selector Mirror

82 Beam Splitter

A4 Aperture Wheel

L3 Launch Lens

M2 Launch Rotating Mirror

S3 Laser Source 1300 nm VC Vacuum Chuck

S2 Laser Source 850 nm

MS Mode Stripper

WG Waveguide

Tl Visible Light Telescope

L2 Laser Condenser

Figure 2: Source and Launch Optics

107
A magnified image of a tungsten filament is constructed on the

Limited Phase Space Aperture Wheel, A3. Lens L2 collimates

the tungsten light and Filter Wheel F2 permits selection of

various single wavelengths filters.

The Launch Selector Mirror, M]_ , folds the tungsten light toward

the Launch Lens, L3, which is used to image tungsten and pulsed

laser light onto the fiber under test. The Aperture Wheel, A4

,

permits selection of the launch numerical aperture. The test

fiber is held in the Vacuum Chuck, VC, and is positioned

properly by means of XYZ adjustment. Telescope T]_ aids in

alignment

.

DETECTION AND RECEIVE OPTICS, Fig. 3

The output end of the test fiber is held in Vacuum Chuck, VC, and

is positioned with XYZ adjustments until properly aligned. The

Telescope, T2 / and Microscope, M, aid in the fiber alignment.

wc

MS VC L4 8, A

Waveguide

Mode Stripper Vacuum Chuck

Projection Lens Near Field Lens

Far Field Lens Attenuation Lens

Attenuation Detector 1820-1060) Scanner Mirror

Detector Pinhole Near-and-Far Field Detector Visible Light Microscope

Beam Splitter Attenuation Lens

D

5 - Attenuation Detector (1300) VA - Variable Attenuator

L, - Pulse Detector Lens

D

4 - Pulse Detector

BL - Back Light

M- - Folding Mirror

T_ - Visible Light Telescope

Figure 3: Detection and Receive Optics

Light from the test fiber is collimated and projected by the

Receive Lens, L4 , toward the Detector Function Slide, DFS. The

DFS moves perpendicular to the optical axis, and contains: two

attenuation detectors, a germanium APD for bandwidth measurements

a near field and a far field lens.

Far Field and Near Field Measurements are performed as previously

reported^

.

The Variable Attenuator, VA, automatically scales the amplitude

of the pulsed laser signals for bandwidth measurement.

108
DETECTION ELECTRONICS, Fig. 4

The Analog Detection Electronics is accomplished as shown in

Figure 4

.

Dl - Tungsten Source Monitor

D2 - 850 nm Attenuation Detector

D3 - Near Field/Far Field Detector

D4 - APD Bandwidth Detector

D5 - 1300 nm Attenuation Detector PA - P.A.R. Model 181 Preamplifier

5204 Lock-In Analyzer

DEC PDP 11/34 Computer

Tektronix 7M11 Delay Line Tektronix Digital Processing Oscilloscoo

B & H DC3002 Amplifier Hewlett-Packard Coax Switch

FIGURE 4 - DETECTION ELECTRONICS

The Bandwidth Signal Acquisition is done with a Tektronix Digital

Processing Oscilloscope. A coax switch, S^, is used to share one

oscilloscope for each pair of measurement stations, both located

on the same optical bench. The DPO digitizes and averages the

time domain pulse data before sending it to the DEC 11/34.

Data Acquisition software provides the control for automatically

performing the required measurement. Since multiple tables are

in operation simultaneously on the same controller, the multiprogramming executive RSX-llM was chosen as the operating system.

Application programs in Fortran IV were developed to control the

measurements

.

MEASUREMENT PROCEDURE

Fiber ends are prepared by removing any coating and are scribed

and broken with the end preparation machine . The operator is

directed through the set up of the fiber by the computer.

The input waveguide face is optically aligned and the end quality

is visually examined using telescope T]_. If any damage extends

into the core, a new end is prepared and this step is repeated.

The output waveguide face is aligned by using telescope T2 and

microscope M-j_, respectively. With this arrangement, the fiber

end can be positioned with a precision of about one micron.

The fiber length is entered into the operator panel and the

automatic sequence begins. At the proper time the operator is

directed to repeat the set up for the short length measurements.

109
ATTENUATION RATE

The transmission data at the desired wavelength is recorded by

measuring the intensity multiple times. All intensity measurements of lock-in amplifiers are divided by the output of the

source monitor detector. These measurements are averaged and

the long length intensity is stored.

The output fiber end is removed and the waveguide is broken two

meters from the input stage and a new end is prepared, mounted

in the output stage, and aligned as before without disturbing

the input end. The above steps are repeated and the short length

intensity is obtained.

The logarithm of the intensity ratio between the long length and

short length is calculated and normalized to 1000 meters.

BANDWIDTH

The detector output is displayed on the digital processing oscilloscope (DPO) for each station and the pulse amplitude is adjusted by a computer algorithm. The DPO is commanded to collect and

average a large number of samples. The average value of the first

25 points is used to establish a base reference level corresponding

to zero light intensity. The Fourier transform of this data is

determined using a Fast Fourier transform.

The optical power frequency spectrum of the fiber is calculated

by dividing the output pulse frequency spectrum by the reference

frequency spectrum. The frequency corresponding to one-half of

the magnitude of the zero frequency is the optical power bandwidth.

RESULTS AND PERFORMANCE

Qualification tests conducted in conjunction with other CGW laboratories

using waveguide fibers have shown good correlation. This measurement

system with multiple stations has met or exceeded all design parameter

and expectations. It has proven to be highly reliable, with minimum

downtime

.

ACKNOWLEDGEMENT

The authors would like to thank R. Hawk, D. Keck, W. Love, T. Cook,

K. Emig, R. Crevier, D. Knowles, D. Grey and M. Vance for helpful

assistance

.

REFERENCES

1. D. B. Keck and A. R. Tynes, Appl . Opt. 11, 1502 (1972).

2. G. T. Holmes, Propagation Parameter Measurement of Optical

Waveguides, Proceedings of SPIE Technical Symposium EastFiber Optics for Communications and Control (Washington,

April 8-9, 1980)

.

110
3. W. F. Love, Fiber Optics Measurement Workshop, (Washington,

D.C. , Nov. 30, 1979) . 4. G. T. Holmes, Launch Dependent Attenuation Measurements on a 10-Kilometer Concatenation Experiment, to be presented at the

proceedings of the sixth European Conference on Optical

Communications (University of York, United Kingdom, Sept. 16-18,

1980)

.

5. William M. Simpson et al, Semi-automated Measurements of the

Transmission Medium, Proceeding SPIE, 150, 133 (1978).

6. Further details can be found in Coming's Product Information.

7. W. R. Love, Novel Mode - Scrambler for use in Optical Fiber

Bandwidth Measurements, Proceeding Optical Society of

America for the Topical Meeting on Optical Fiber Communications, (Washington, D.C, March 6-8, 1979).

8. The Automatic End-preparation Machine was designed by

John Spiegler of Manufacturing and Engineering Division,

Corning Glass Works.

Ill

Wavelength Meters for Optical Cable Transmission Systems

K.Sano K.Okada T.Oki

Yokosuka Electrical Communication Laboratory, Ando Electric Co., Ltd.

N.T.T.

1. Introduction

With the recent progress of optical transmission technology, optical

cable transmission systems have come into practical use. In field tests

of optical transmission systems, optical measurement equipment , including a

wavelength meter and an optical spectrum analyzer, are required to be miniaturized, simply operated and portable.

Normally, peak wavelength of a light source is measured by a monochromator. In this paper, we propose two new types of wavelength meters, which

can read wavelength directly by using an interference filter or a quartz

rotator. We also describe a commector launching optical spectra analyzer

manufactured for measuring plural wavelengths and spectra of light

sources.

2. A-type Wavelength Meter

An interference filter is used in the A-type wavelength meter. The ratio

of transmitted power to reflected power depends on the wavelength of the

input light source.

Fig.l shows the scheme of the A-type wavelength meter, and Fig. 2 shows

the characteristics of the interference filter in use. In Fig.l, light from

an optical connector is collimated, and input into the interference filter,

then the ratio of transmitted power to reflected power indicates the wavelength required. However, when using the interference filter shown in Fig.

2

without linearlity compensation, measurement error is caused by the nonlinear lity of the filter. Thus a ROM is used to correct the measurement error.

Fig. 3 shows measurement error against input optical power in the A-type

wavelength meter. In the A-type wavelength meter manufactured, we have

113
realized a measurement error of +1 nm in the 810-890 nm range. Measurable

minimum input power is -40 dBm at the 850 nm wavelength.

Moreover, the A-type wavelength meter can measure the input optical

power by summing up the reflected optical power and the transmitted optical

power of the interference filter. Additionally, when this measurement

equipment is used as a power meter, it can detect optical power in the -40 ^ 0 dBm range.

3. B-type Wavelength Meter

In the A-type wavelength meter, wavelength can be measured only in a 100

nm range which is limited by the characteristics of the interference filter.

As a result, we recommend the B-type wavelength meter, where the quartz

rotator is used instead of the interference filter, for a wider wavelength

measurement. This is because the rotation characteristics of the quartz

rotator are dependent on the wavelength.

Fig. 4 shows the schema of the B-type wavelength meter. In this experiment, the B-type wavelength meter operates in the minimum level detection

mode to avoid measurement error that is caused by input power variance.

Consequently, wavelength can be measured, where the optical output power is

minimized, by rotating the analyzer angle. Fig. 5 shows the characteristics

of the quartz rotator in use. In this figure, the solid line gives the

characteristics in the 600-1200 nm range, and the broken line gives the char

acteristics enlarged in the 800-900 nm range. Since the quartz rotator has

good linearlity in the LED's spectral width range, the peak wavelength of

the LED can be measured. At this point, the B-type wavelength meter has

wide application in comparison with the A-type wavelength meter, apart from

resolution.

Fig. 6 shows some measurement examples by using the B-type wavelength

meter. This figure shows the analyzer output level against the analyzer

rotation angle, when beams of LD or LED are input into the polarizer. This

114
figure also indicates that the peak wavelength of either LD or LED is measured with good resolution.

4. Optical Spectrum Analyzer

In wavelength division multiplexing transmission systems, where more than

two different wavelength lights are launched into an optical fiber, each

wavelength must be measured. Moreover, in high speed transmission systems,

spectral bandwidth of a light source must be known in order to grasp the material dispersion. Therefore, we have studied and manufactured a curvetracer-type optical spectrum analyzer, which is adaptable to an optical

connector

Fig. 7 shows the scheme of an optical spectrum analyzer, and Fig. 8 shows

theoretical resolution value against the slit width. The longer the collimating length, the better the resolution. However, the length is limited by

the equipment size. The resolution requirement is determined by the longitudinal mode space of LD, which is 0.2 'V- 0.3 nm. For example, if the resolution is required within 0.1 nm for a graded index fiber of 50 urn diameter

core, the collimating length requirement is more than 500 mm. LD's

spectrum is shown in Fig. 9, when the collimating length is 530 mm. This

figure indicates that less than 0.1 nm spectrum can be distinguished.

5. Conclusion

We have proposed wavelength meters based on new principles and optical

spectrum analyzer, and have confirmed their realization. These measurement apparatuses are used in field test at the present time.

6. Acknowledgements

The authors are indebted to Drs. S.Shimada, H.Ishio and K.Hashimoto

with Yokosuka Electrical Communication Laboratory, N.T.T., and Mr. T.

Hanabusa, Mr. E.Tanaka and Mr. H.Yamamoto with Ando Electric Co., Ltd.

for their useful suggestions and guidance.

115
optica] fiber

lens interference

filter

log ratio

di splay

g . 1 Schema of A type Wavelength Meter

X

•

- 5 dBm

-15 dBm

-25 dBm input optical power

-35 dBm

-40 dBm

1 1 1

1 1

Fig.

3

820 840 860 WAVELENGTH (nm) Error in A-type Wavelength Meter

receptacle

fiber plug

lens polarizer quartz-rotator analyzer ?0= power meter

power meter

head

800 850 9 WAVELENGTH(nm)

'

Fig. 2 Transmission Characteristics

of Interference Filter

quartz-rotator length

1=19.491 mm

-I

800 850 WAVELENGTH! nm)

900

Fig. 5 Rotation Angle of Quart z - Rotator

against Wavelength

Fig. 4 Schema of B-type Wavelength Meter

grating \

connector

photodiode 3—1>—l^D— amplifier

(^)

potentiometer

Fig. 7 Schema of Optical Spectrum Analyzer

80um-Step Index

50um-Graded Index

lOym- Single mode

30 100 300 1000

SLIT WIDTH (ym)

Fig. 8 Resolution against Slit Width

f =Collimating Length a=Groove Spacing

-10

E

CO y< X 0 «858nm / AX«1nm o

oc1 -20 / X 0 "848nin

/ / AX-12nm

[CAL

OUTPUT

o

f / X 0 -858nm

/ / AX-3 7nm

OPT

DC

r*t

«tz -40 1 1 0 50 100 150 ANALYZER ANGLE (degree)

Fig. 6 Analyzer Optical Output Power

for Three Optical Sources

Xo- Peak Wavelength

4X- Spectral Bandwidth

116

Fig. 9 Spectrum of Laser Diode
117

Fiber Measurement Techniques

in West- Germany

J. Feldmann

Deutsche Bundespost

This talk will provide a summary of a meeting on "Measurement

Techniques in Optical Telecommunications" held in Berlin, October 2-4, 1980.

Topics for the conference were as follows: attenuation measurements to

1.8 vim, OTDR, investigation of fiber cables, measurements on test links,

laser measurements, splices, connectors, and standards. Forty-eight

papers were given and an exhibition was held.

119

The Preparation of Standards for "Optical Fibres and Cables"

Within the International Electrotechnical Commission

A summary prepared for the Symposium on Optical Fibre Measurement.

Boulder, Colorado, October 28 - 29, 1980.

Meint P. Smid - NKF KABEL B.V. Waddinxveen, The Netherlands.

Secretary IEC/SC46E-WG1

Introduction

The International Electrotechnical Commission has assigned the preparation

of standards for fibre optics to its subcommittee SC 46E with the following terms of reference:

" To prepare fibre optics international standards intended for application in

telecommunication equipment and in devices employing similar techniques.

This activity includes terminology, essential characteristics, measuring

methods, and functional requirements to ensure satisfactory performance for,

but not restricted to, the following:

single fibers and bundles of optical fibres, fibre optic cables, fibre optics

connectors, fibre optics accessories, fibre optics terminal devices, and

fibre optics transmitting and receiving assemblies or sub-systems containing

solid state devices and other components, which are specified as SUB-SYSTEMS

for purposes of trade and commerce."

During the first meeting in Florence in 1978, SC 46E decided to set up a number

of Working groups, consisting of experts, to assist its secretariat in the

preparation of standards.

SC46E-Working Group 1 - "Optical Fibres and Cables" was formed to draft

specifications concerning the materials, dimensions, construction,

characteristics (transmission, mechanical and environmental) and measuring

methods for optical fibres and cables. Other working groups are preparing

standards for connectors, safety and vocabulary.

121
Working Group 1 now consists of 23 members of 10 different industrialised

countries of North America, Japan, and Western Europe. There have been

five WG meetings accompanied by intensive correspondence. The working

group intends to present its recommendations and conclusions to the plenary

meeting of SC 46E in June 1981.

It is emphasized that the International Electrotechnical Commission adheres

to strict procedures of presentation, discussion and final decisions on proposals for standards by the National Committees which constitute the IEC.

Therefore, the working group proposals and documents must be seen as preparations for possible standards only and cannot be used as standards for

the present time.

This paper is the author's personal summary of the discussions and work of

SC 46E/WG1.

1. Procedures

In the preparation of standards for optical fibres and cables the

working group has used IEC Guide 102 as a basis. This Guide outlines

the structure of a specification system consisting of a series of

interrelated documents. Various levels of specifications can be

distinguished - basic

- generic

- sectional

- detail

For optical fibres and cables, IEC 46E/WG1 is primarily concerned with

generic and detail specifications. Basic specifications are already in

existence within IEC for a number of general requirements which are relevant also f6r many other products than optical fibres and cables.

Some other basic specifications, such as those concerned with terminology

are under preparation in other IEC quarters.

In preparing generic and detail specifications for optical fibres and

cables, IEC 46E/WG1 can thus restrict itself to those parameters which

are particularly and sometimes uniquely relevant to the fiber optics

technology in a wide or narrow sense. As a working rule, the group

tries to avoid specifying a parameter unless a test method can be described to measure the actual value of this parameter and, therefore its

compliance to a specified range of values. This method creates a degree

of flexibility in so far that new values of parameters can be standardized

within an already standardized measuring method.

122
To illustrate this method with an example:

The cross section specifications of a fibre can contain as parameters

the diameters of the core, the cladding, the primary coating and the

buffer layer and their non circularity. In addition, a reference

surface may be chosen and acceptable tolerances allocated to all

parameters. Finally the concentricities of various parameters can be

specified in relation to each other. This rather elaborate set of

requirements must be accompanied by a test method for each one.

If one would attempt to arrive at a full, academically complete, set

of values for the parameters and their measuring methods, the chances

of success in solving this problem (which started by being a relatively

straight forward one) become miniscule.

Only an approach where the practical limitations of measuring methods

are used as a starting point may succeed in solving the problem for

the sake of trade and commerce.

Parameters of optical fibres to be standardised.

The following parameters of optical fibres have been considered by

IEC 46E/WG1 for possible standardisation:

A. Dimensions

a) core diameter; core non-circularity

cladding diameter ; cladding non circularity

primary - coating diameter ; prim, coating non circularity

buffer diameter ; buffer non circularity

b) fibre length

c) reference surface allocation

d) concentricity errors

e) tolerances

B. Mechanical characteristics

Screening

Tensile strength

Bending

Torsion

Vibration

Flexing

Abrasion

Physical defects

123
C. Transmission characteristics:

Attenuation

Impulse response

Baseband frequency response

Backscattering

Numerical aperture

Refractive index profile

Microbending sensitivity

Effective core size

Effective numerical aperture

Fibre acceptance angle

* Mode volume transfer function

* Material dispersion

Modal dispersion

* Wave guide dispersion

Cut off wave-length

* provisional

D. Climatic/Environmental characteristics

(i.e. the ability of a fibre to meet environmental requirments)

Maximum temperature

Minimum temperature

Temperature cycling

Humidity

Contamination resistance

Mould growth

Nuclear radiation

Flammability

E. Packaging requirements.

In accordance with the working rules, for each of the above parameters,

standardisation can only occur when suitable measuring techniques have been

agreed upon. International agreement is normally only possible when the measu

ring methods have proven to adhere to acceptable criteria of reproducibility

and repeatability.

Some examples of measurement techniques under discussion in the working group

are given in the next sections of this paper.

124
3 . Some examples of measuring methods discussed in WG 1

a) measurements of fibre cross sectional dimensions.

As outlined in the introduction a complete set of measurements of

dimensions is very difficult to achieve.

However there is an urgent need to specify and propose standards for some basic parameters of the fibre such as nominal/basic core and

cladding diameters. On the basis of an operational definition of the core size (also used in CCITT ) and a generally accepted definition for

the cladding, some standard fibre sizes have been proposed , measured on the basis of optical power transmission,

e.g. for graded index fibres core dia 50 yum cladding dia 125 jam

for glass core plastic core dia 200 yum

clad step index fibres

while the above dimensions have been formally proposed within IEC,

other diameters, especially for "(quasi) step index all silica

fibres" are under discussion, e.g.

80-90 yum core dia)

125 pm cladding diaj

100 yum core dia)

140 jam cladding diaj

Proposals are also emerging for fibres manufactured by double

crucible processes.

In addition to these nominal values a measurement method is being

proposed to verify all the cross sectional dimensional parameters.

This method, the "four circle method", gives evidence whether

the fibre fits its specification. It is based upon a template

containing two sets of two concentric circles for core and cladding

diameters respectively. The two circles of one set define an acceptance field for tolerances, non circularity and concentricity errors.

A fibre will pass the test, in the context of trade and commerce, if

a position can be found where both core contour and cladding contour

lie completely within their two circles.

*core : the smallest cross sectional area of a fibre (excluding any "centro

dip") contained within the locus of points where the refractive index

n

3 =n2 +k(n r

n

2

) with: n^= max. refractive index

°2 = refr. index of cladding

k = 0.005 (prov.)

125
b. Mechanical tests.

For both proof testing and destructive strength testing of

fibres it is neccessary to define test set ups. These test set

ups contain some aspects which are arbitrary in nature but

should yield reproducible results. They must pay respect to

known phenomena related to strength of glass fibres and can be used for the sake of trade and commerce when international

acceptance can be gained in some considerable detail including clamping and pulling instructions and reporting

disciplines

c. Transmission and Optical tests.

In this field the working group, recognising the difficulty of

specifying transmission parameters which are absolute from a theoretical point of view, accepted two requirements for standard

proposals of transmission test methods:

- test values must be reproducible and repeatable.

(which causes some aspects of the test set up to be

arbitrary)

- the test must be such that, when applicable, the

test values can be used to predict the performance

of a system e.g. a concatinated length.

The problems involved with this are manyfold and the discussion in the working

group proved extremely time consuming.

As an example and also as the first result from these discussions a proposed

test method for the measurement of attenuation is presented in some detail in

the next section

.

126
Standards for measuring methods of transmission parameters

a. Attenuation measurements

One objective has been to find an attenuation coefficient, (i.e.

the attenuation of a fibre per unit of length) such that the

total attenuation of a (long) length of fibre can be accurately

predicted by multiplying length and attenuation coefficient.

This however requires a set of launching conditions which approximate equilibrium mode distribution, understood to exist

when the power distribution of field patterns at the output of

the fibre is independent of the length of the fibre.

Initially attempts were made to propose standards for a hardware

"equilibrium mode simulator" based upon mode mixing effects obtained in devices making use of either microbending phenomena

or of coupling effects from/into different types of fibers.

More recently an approach has been favoured which tries to verify

the existence of equilibrium mode distribution empirically for

different types of fibres.

In comparing various results obtained at different laboratories

the WG will submit the following proposal for a standard set of

launching conditions for graded index optical fibres.

- A standard set of equilibrium mode launch conditions

exists for a fibre of category A1 ("multimode graded

index fibres" ) with a typical theoretical numerical aperture of .19 to .23 and nominal core diameter of 50 micrometer as follows:

" a light spot with a full width half maximum intensivity

value : D„ _ = 26 + 2 /urn 0.5 —

is launched whereby, after the light has travelled

through the fibre over a distance of 2 meters, the far

field pattern is measured to have a full width half maximum value numerical aperture:

NA = .11 + .02 and the near field pattern still in0.5 —

dicates a spot diameter of 26 +_ 2 /am.

End of summary

127
II
CCITT studies on optical fib res cables measurements

By : Fabio Bigi (CCITT Secretariat - ITU) and Gastone Bonaventura (Chairman of the

Working Party on optical fibres of Study Group XV - ASST, Ministry of

Communication, Italy)

Abstract

This article indicates the general approach of CCITT with respect to

measuring methods and measuring equipments and gives the particular solution chosen

for optical fibres measurements.

The particular approach adopted for optical fibres T.easurements consists

in the adoption, as far as possible, for each defined characteristic of a corresponding "te3t reference method". The article, on the basis of the contributions

received, underlines the agreement reached on the definition of the various

characteristics and on the corresponding measuring methods at the end of the present

study period (1977-1980). Finally it indicates trends for the studies to be

undertaken by CCITT during the next study period.

1. Introduction

The studies undertaken by the CCITT (international Telegraph and

Telecommunication Union) are mainly aimed to allow international interconnections

of telecommunication equipments and networks. Several Study Groups have been

specifically involved with different aspects of measuring methods and measuring

equipments for telecommunication and have issued corresponding CCITT Recommendations.

In particular, the 0 series of Recommendations, mainly originated by Study Group IV

(Transmission maintenance of international lines, circuits and chains of circuits,

maintenance of automatic and semi-automatic networks), are dedicated to measuring

equipments. Study Group IV has also originated the Recommendations of the M series

dealing with maintenance for telephony, telegraphy and data transmission.

In studying the measuring methods and maintenance procedures for

telecommunication equipments and networks, Study Group IV considered three types of

measurements :

i) acceptance measurements, made by Administrations or private operating

agencies, with measuring equipments conforming to the 0 series of Recommendations,

in case a given equipment is to be put into service

ii) maintenance measurements, made in order to check the normal operation

for a given telecommunication equipment. The maintenance could be of a corrective

nature or of preventive nature;

iii) specific measurements, made to check a given characteristic or to

ascertain, from a particular point, the quality of a telecommunication network by

making a determined series of measurements.

129

I

|
Study Group IV generally has a continuous exchange of information with

the specialized Study Groups in order to prepare recommendations for suitable

measuring equipments following the evolution of the technique. In the case of

optical fibre studies are at an initial stage and consequently the relative test

methods are rapidly changing and improving in parallel with the evolution of the

technique. For this reason the optical fibre measurements have for the time being

been treated in the specialized Study Groups, which should first agree on the choice

of general test methods before involving Study Group IV with the detailed

specifications for the measuring equipments.

2. CCITT studies on optical fibres

The specialized Study Groups entrusted with the studies of optical

fibres by the VII th CCITT Plenary Assembly were Study Group XV (Transmission

systems) with its Question 33/XV (Physical characteristics of optical fibres)

and Study Group XVIII (Digital networks) with its Question 13AVIII (Characteristics for digital line sections on optical fibre cables).

The studies have made a progress in particular in Study Group XV,

which agreed on Recommendation G.651 dealing with a multiraode graded index optical

fibres having a core diameter of 50 /um, a reference surface diameter of 125 Aim

and operating in the 0.8-0. 9 yum wavelength range.

The aim of Study Group XV in standardizing test methods has been to

achieve comparative results for different measures. It was considered necessary

to reach agreement on the possibility to compare data from various Administrations,

obtained during different times under different conditions.

At present different values of fibres characteristics could be obtained

using different test methods and furthermore it is not convenient to specify

only one method for each paraaeter to be used. Study Group XV agreed on the need

to choose one Reference Test Method (RTM) to be used as Reference method for the

measure of each fibre parameters. At the same time , Study Group XV recognized

that, in practice other test methods could be used and adopted, provided that

their results are in accordance with those of the RTM.

In order to allow a wide use of the RTM it was agreed to look

for a method having the following aharacteristics :

- easy to specify and to realize

- capable of giving reproducible results which are of use

to the system designer, as fas as possible

- non destructive.

Moreover, RTMs are to be chosen as being applicable to a wide range

of optical fibres available at present; but it was not intended that the

recommendation of a particular test method should exclude the design of fibres

which, although meeting CCITT Recommendations could not, by reason of the

constructions, be measured using the RTM. Indeed, the designation of RTM is an indication that there is, at the present time, for a large number of types of

optical fibres, sufficient confidence that it can yield accurate results and

hence be used as a reference with which to compare the results of other methods.

Such other methods, considered to be suitable for measurements of the same parameter have been designated as Alternative Test Methods.

130
The main results obtained in the study of optical fibre test methods

are annexed to Recommendation G.651 and will be described in the following parts

of this article :

3. Geometrical and optical parameters

3.1 General

It is assumed that the geometrical and optical parameters would be

measured only in the factory or in the laboratories of certain Administrations

wishing to verify these parameters for system design or other purposes. Hence,

it is anticipated that the measurements will be conducted either on sample fibre

lengths or on samples extracted from cable factory lengths.

The reference surface diameter and non-circularity, which are external

fibre dimensions, can be measured readily and accurately by microscope or micrometer

methods and present no problem.

The core diameter and non-circularity and the theoretical numerical

aperture are defined using the refraction index profile as a basis. The remaining

parameter, core/reference surface non-circularity, can be derived from the above

through a simple geometrical calculation. Hence, it follows that all geometrical

and optical parameters, which are the subject of this Recommendation, and their

tolerances as appropriate, can be obtained through two basic tests* one for the

reference surface diameter and one for the determination of the refractive index

profile

.

3.2 Test method for the reference surface diameter

Two methods are generally suggested for measuring the reference surface

diameter : a conventional microscope or a micrometer. As a microscope may be more generally available for high precision measurements, it is designated as the "RTM".

The "micrometer" method is a suitable "Alternative Test Method". When all of the

material within the reference surface is optical material, interferometric methods

and the refracted near field technique of refractive index profile measurement can

also be used to measure the reference surface diameter.

As it will be necessary to determine the non-circularity, the tolerance

of the reference surface, and the concentricity error between reference surface

and core, the measurement must be performed along a minimum of two axes.

3.3 Test method for the refractive index profile

Axial interference microscopy 3eems to be the most generally recognized

method of achieving accurate results. However, sample preparation is time consuming

and requires high precision and expensive optical equipment. The near field method,

although generally recognized as being useful, suffers in that absolute values of

the index cannot be determined. Also, leaky modes can cause errors. Some other

methods have promise, but have not received the same degree of interest as the above.

In the light of the above, it has not been possible to achieve agreement

on a "RTM". However, the following test methods are considered to become the "RTM" :

a) axial interference microscopy

b) near field scanning

c) reflection measurements

131
d) transverse interferograms

e) forward scattering

f) refracted near field technique.

3'U Core diameter

It is implicit in the foregoing and from the definition of core diameter that the derivation of core diameter from refractive index profiles taken along a number of axes constitutes the "RTM" . A minimum of two axed must be explored. Microscopic observations, chemical etching and the measurements of certain energy density

useful

distributions in the output of the fibre are other methods recognized as "alternative test methods". recognized as

The core centre and core non-circularity can also be determined from

these "test methods" and the determination of the concentricity error between

the core and the reference surface can be derived from the data.

4.. Transmission characteristics

4.1 Cable factory length

4.1.1 Equilibrium mode distribution

There is general agreement that, for measurements on cable factory

lengths to be useful in predicting the loss and basehand response of elementary

cable sections (regenerator sections), a state of equilibrium mode distribution

must exist throughout the fibre under test. It is here understood that an equilibrium mode distribution exists when the powe^r distribution of the fields

pattering at the output of the fibre is independent of length. This is to ensure

that the total of the losses measured on individual cable factory lengths can

be in agreement with the cumulative loss measured at the resulting elementary

cable section. A variety of relevant techniques have been proposed by different

Administrations. Basically, these proposals concern methods of approximating

an equilibrium mode distribution throughout the fibre under test or of producing

an equivalent result. The proposed methods can be grouped into two general

categories :

i) the use of a mode scrambler at the input of the fibre under test

ii) selection of spot size and launching NA to produce a transmission

measurement equivalent to that under equilibrium mode distribution

conditions.

In both approaches, it is necessary to restrict the source spectral

width in order to remove spectral dependent loss effects and in particular to

minimize the effect of material dispersion on the measured basehand require.

A cladding mode stripper is useful to ensure that cladding modes

reaching the detector do not distort the results.

Some results have been reported on cumulative loss results by a number

of Administrations using either approach outlined above. Hence, the mode

scrambler and the selection of spot size and launching NA can be considered

practical means of achieving the desired result.

132
4.1.2 Attenuation

The most commonly used measurement methods are :

a) the cut-back method

b) the insertion los3 method

c) the backscatter or TDR method.

The cut-back method is generally recognized as yielding accurate

results but suffers from the disadvantage that it is destructive. The insertion

loss method introduces additional uncertainties due to 3plices or connectors.

The backscatter method is non-destructive but is limited in range.

Some Administrations were in favour of designating the cut-back

method as "reference test method" but the view has been expressed by other

Administrations that the choice should be delayed.

U» 1*3 Basehand response

Two basic approaches are recognized as being useful. One is based

on time domain measurements while the other is based on frequency domain

measurements. At present, there is no clear preference for one approach over

the other, although some proposals have been made to adopt one or the other

as reference.

At present, therefore, both basic approaches are considered as candidates to become the RTM.

4»2 Elementary cable section

It was considered premature to attempt a selection of test methods.

Nevertheless, the general principles involved in the measurement of cable

factory lengths are also applicable to elementary cable lengths. However, it

should be taken into account that in this case non-destructive and portable test

methods methods are mandatory while the need for absolute accuracy is not so great.

5» Future studies

Study Group XV intends to study during the next period ( 1981-1984.

)

a specific question on methods of measuring the characteristics of optical

fibre cables (Question I/XV). In this question main objective should be the

definition of a single reference test method for each of the fundamental

characteristics. The Question recognizes . that other test methods could be

used if they result more practical, simple and cost-effective for factory and

field applications, provided that the result obtained by been accurately

relates with the reference test methods. In particular, Question I/XV requests

study for the following points :

133
a) What recommendations are required for the reference, factory

and/or field measurements of optical fibre cables to ascertain for example : attenuation, baseband response, material dispersion, refractive index profile,

fibre dimensions, the presence and location of faults, cut-off wavelength.

b) How can practical measurement methods be established so that

they are suitable for use as alternatives to the reference methods.

As stated in the introduction, at some future date an important part

of the detailed work on the specification of measuring methods for optical

fibre cables should be transferred to other Study Groups (in particular Study

Group IV) in order to allow them to provide recommendations on measuring

equipments

.

134
WAVEGUIDE FIBER STANDARDS

DR. ROY E. LOVE

CORNING GLASS WORKS

Standard committees worldwide are in the process of establishing

standards for waveguide fibers. This paper will address the key

issues currently facing three working groups: namely, Working

Group I (SC46E) of IEC, Study Group XV of CCITT and Working Group

P-6.6 of EIA (Electronic Industries Association, USA). The IEC

and CCITT are international organizations. The rapid evolution

of fiber technology and commercial activity, combined with the

lack of extensive in-use performance data and experience, has

made the task of setting meaningful standards a formidable challenge. Working group members are responding to the urgent need

to establish a technical framework for the sale of commercial

products, while at the same time attempting to encourage (or at

least not inhibit) the development of improved or lower cost

products

.

The standard-setting process is straightforward in principle.

One first defines the fiber parameter(s) of interest. A test

method is then proposed, discussed and points of disagreement

resolved. Finally, a recommendation is submitted for the approval

of the full committee. In some cases, parameter specifications

and tolerances are proposed before there is final agreement on

definitions and test methods. Test method standards are key

documents. They are the means whereby product parameters are

assigned a value which in turn determines relative product quality,

cost or selling price. The reproducibility of the test method is

also very important. Ideally, the standard deviation of the

reproducibility error, a , should be about an order of magnitude

less than parameter tolerance requirements. Otherwise complex

and costly quality assurance procedures must be agreed upon

between user and supplier. Last but not least, wherever possible,

the measured value of a component attribute or parameter should

be directly useful for system design purposes.

These general considerations are fundamental to all standardsetting endeavors. To develop standards that are acceptable to

both users and suppliers without compromising sound engineering

practices and established scientific principles is a time consuming

and often frustrating task. In the case of waveguide fiber the

task is a little more difficult than usual since at this time

there is considerable pressure to act quickly. Fortunately,

support for this work is widespread throughout the industry and

both the organizations and individuals involved are to be commended for their efforts to date.

135
The fiber standards issues I intend to address are listed below:

(1) Light-launch conditions for measurement of attenuation

(2) Light-launch conditions for measurement of information

transmission capacity

(3) Information capacity representation (-3dB bandwidth (BW) vs

full-width half-maximum (FWHM) pulse broadening)

(4) Numerical aperture definition and measurement

(5) Core diameter definition, measurement, and tolerances

(6) Cladding diameter tolerances

(7) Splice loss specification

Light-Launch Conditions for Measurement of Attenuation

It is generally recognized that an attenuation test method standard must specify the wavelength of the light source, the spectral

width of the light source, a temperature range for measurement,

the mechanical means of supporting the fiber and, finally, the

angular and spatial distribution of the light launched into the

test fiber core. This last condition is a critical requirement.

The objective is to specify launch conditions such that the

measured attenuation of a fiber length, a- , is additive with

respect to predicting the attenuation of concatenated fiber

lengths. That is,

n

1 a

i = A (1)

where A is the measure attenuation of a concatenated link (~ 10

km long) formed from the n fibers. There is widespread agreement

that the launch condition necessary to accomplish this objective

is one which simulates equilibrium propagation conditions; that

is, the modal distribution that results after light has propagated

over long lengths of fiber, the order of 10 kilometers.

Attempts to define and characterize the modal distribution associated with equilibrium propagation conditions have been extremely

useful and illuminating. It has been found possible to create

launch conditions that satisfy the concatenation condition,

Equation (1), using either beam optics or a mode filter. (1) (2)

(3) (4) (5) (6) A mode filter device may consist of a mandrel

around which a fiber is wrapped with a prescribed tension, an S

shaped loop of fiber or a long length of fiber through which the

input light traverses before being launched into the test fiber.

Beam optics limit the spatial and angular distribution of the

input light relative to that defined by the core diameter and

numerical aperture of the test fiber. Not surprisingly, the

input effective mode volume (EMV), defined as the square of the

product of the FWHM near-field and FWHM far-field light distribution from the first 2 -meter length of the test fiber, determines

136
the attenuation that will be measured, independent of the means

used to create it (the EMV) . (7) (8) IEC Working Group I has

proposed that light launch conditions be established using this

idea. It can be shown that the input EMV required to satisfy the

concatenation condition is approximately the same, whether a modal filter or beam optics is used to create the initial launch

conditions. More work is needed in this area, but at the last

meeting of this Working Group it was tentatively proposed that

for 50 micron core diameter, 125 micron clad diameter, graded

index fibers, the FWHM value of the near-field should be 26±2

((jm) and the FWHM of the far-field light distribution should be

Sin 1 (0.11±02). It should be emphasized that these near- field

and far-field patterns are to be measured at a point approximately

2 meters removed from the input end of the test fiber.

Light-Launch Conditions for Information Transmission Capacity

Measurement

The information-carrying capacity of a waveguide fiber may be

measured in either the time or frequency domain. A number of

measurement methods for each approach have been proposed and

there is general agreement regarding the light-launch conditions.

The principle issue here is measurement reproducibility. It is

felt that, for graded index fibers, the input light distribution

should fill the mode volume of the test fiber no matter which

measurement technique is used. That is, all allowed propagation

modes should be more or less equally excited. A "mode scrambler"

device positioned between the light source and the test fiber

will be used for this purpose. By making the measurement less

sensitive to differences in the radiation-output distribution

between sources and small errors in optical system alignment, the

"mode scrambler" markedly decreases the standard deviation of the

reproducibility error. (9) (10) (11) (12) (13) (14) If a "mode

scrambler" is not used it is possible for individual measurements

to differ by 50% between laboratories or between quality assurance

facilities

The next question is, can the measured value of the informationcarrying capacity of individual fiber lengths be added together

to yield the information capacity of a multi-kilometer concatenated

length. The "mode scrambler" approach does not fulfill this requirement and, to the author's knowledge, no other practical test

method has been suggested. Currently, emperical relationships

based on field experience are used to predict the informationcarrying capacity of concatenated fiber lengths. Two of the most

commonly used relationships are:

and

(2)

(3)

137
where a is a constant equal to about 0.7, BW is the frequency at

which the transfer function in the frequency domain is down by

3dB and FWHM is given by

FWHM =/T 1 2 - T2

2 (4)

where T ± and T2 are the full-width half-maximum of the output and

input pulses respectively. These relationships assume the concatenated fibers are of equal length.

This brings us to a question that needs further study and discussion. Namely, what is the most useful and meaningful representation of the information-carrying capacity of a fiber. Both the

-3dB point of the transfer function response (Bandwidth) and the

FWHM pulse broadening are commonly employed. If the output pulse

shape was identical for all fibers (for a fixed input pulse

shape), this would not be an issue, since there would exist a

simple inverse relationship between these two quantities. Unfortunately, this is not the case.

System designers seem to prefer the frequency domain representation

since it is a measure of the fiber transfer function. Digital

communication equipment manufacturers and users, on the other

hand, seem to prefer the time domain or pulse broadening (FWHM)

representation because the equipment is only sensitive to pulse

distortion, from which bit error rates may be predicted. Choosing

between these two representations is a difficult question for the

fiber supplier, since two fibers with the same bandwidth may

exhibit different pulse spreading characteristics and conversely

two fibers with the same pulse spreading characteristic may

exhibit different bandwidths. See Figure 1. Depending upon what

measure of information capacity is requested, process yields and

fiber costs could be impacted. At this time the standard setting

groups have not actually addressed this issue except to acknowledge

that both representations appear to have merit. This may be a

satisfactory position, since it could be argued that this matter

could and should be resolved at the commercial level between

users and supplier.

Numerical Aperture Definition and Measurement

The principle issue here is the definition of the term, "numerical

aperture" . It now appears that it may be possible to reach

agreement on the following definition for the numerical aperture

of a graded index fiber.

where n! and n2 are the refractive indices of the center of the

core glass and the cladding glass respectively. This equation

also defines 0, which is sometimes referred to as the light

NA = Sin 0 (5)

138
acceptance angle. In practice the numerical aperture, as defined

above, may be assigned a value by determining the cone angle that

contains essentially 100% of the output light power from a short

length of fiber. However, noise limitations associated with the

measurement apparatus will dictate that the angle at which 95% to

98% of the total power is contained be accepted as the fiber NA.

This would not be a problem since the error is quite small and,

in any case, there is little practical consequence. It is critical,

however, that a specification for the percentage value be agreed

upon and that it be consistent with obtaining a reasonably small

value for the standard deviation of the reproducibility error, a

(NA). The NA of a fiber may also be determined by measuring n x and n2 directly. This requires the use of microscope inter ferometry techniques. Unfortunately, the associated apparatus is

rather expensive compared to that required for far- field light

distribution measurements

.

The above definition of numerical is the maximum theoretical

numerical aperture. This quantity is useful for input source

coupling calculations. Also, fiber manufacturers find it a

useful quantity to measure since it enters into the expression

for the refractive index profile of graded index fibers. That

is,

n

2 (r) = n

2 (0) (1-2A (|)

a

) (6)

at r = a, n( a ) = n . Also n( 0 ) = n . Therefore,

2 1

A = < NA > 2

(7)

2nz

i

Core Diameter Definition Measurement and Tolerances

The CCITT has defined the core diameter of graded index fibers to

be that value of the diameter where the core refractive index,

n3 , is equal to, n2 + k(n! - n2 ) where, k is a constant. It is

likely that k will be assigned a value between 0 and 0.05. There

appears to be agreement that this is a reasonable definition.

Unfortunately, measuring the refractive index profile of the core

glass requires expensive apparatus, as mentioned above. This is

an issue with some working group members, since it is felt the

cost of such apparatus could not be justified for outgoing or

incoming inspection of core diameter.

At present, core diameter is determined by measuring n3 directly

using transverse interferometry techniques as well as by scanning

and plotting the near-field light output distribution from the

fiber core, microscopic examination of the illuminated core and

microscopic examination of the core after etching the fiber end

in order to create a step at the core-clad interface. It is

139
apparent that the latter three test methods, although very widely

used throughout the industry, do not actually measure core diameter,

if one accepts the CCITT definition. This poses both conceptual

as well as practical difficulties for standardization purposes.

It should also be mentioned that there may be considerable disagreement between refractive index and near-field determinations of

the core diameter. (15)

The tolerance on core diameter recommended by the CCITT is ± 3

microns, EIA has recommended ± 5 microns, and the IEC has considered proposing a tolerance of ± 4 microns, based upon a four

concentric circle gauge method. Agreement must be reached on

tolerance recommendations, but is perhaps more important to

compare these tolerances with the standard deviation of the

reproducibility error, a (core), of the near- field and etchback

techniques, which is between 0.5 and 1.0 microns. (The author is

not aware of any published data on the reproducibility of the

refractive index profile method when used in a manufacturing

environment. ) For a or (core) of this magnitude, a straight

forward statistical calculation shows that single core diameter

measurements on the same fiber end at different facilities will

agree to within about 2 microns, 95% of the time - provided there

is no offset error between the two measurement facilities.

In summary, the issues facing the Working Groups relative to core

diameter standards are as follows:

1 . Core diameter measurement techniques commonly used throughout

the industry do not actually measure core diameter as defined

by the CCITT.

2. These commonly used methods are imprecise relative to the

tolerance requirements, the 95% confidence interval between

two independent measurements being approximately ^ the

proposed core diameter tolerances.

Clad Diameter Measurement and Tolerance

The issues here are in some respects the same as those for core

diameter. The measurement methods currently used depend upon

microscopic examination of the fiber end or micrometer gauges.

In both cases the reproducibility error is a large fraction of

proposed clad diameter tolerances (a (clad) = 0.5 to 1.0 microns).

The CCITT, IEC, and EIA have proposed ±3, ±5, and ±6 micron

tolerances respectively. Fortunately, it has been reported that

during the fiber drawing process, where clad diameter is controlled online by means of a feedback loop acting between the measurement means (usually an interference diameter microscope) and the

draw rate, it is possible to limit fiber diameter variations to a

few tenths of a micron. (16) Hopefully, this experience will be

duplicated in large scale manufacturing facilities, in which case

140
it may not be necessary to institute accept/reject procedures

based on fiber end measurements made at outgoing or incoming

quality assurance facilities. End measurements might be made

occasionally as a rough monitor of fiber end diameter rather than

as a means of insuring that individual fibers meet tolerance

specifications

Splice Loss Specification Proposal

It is interesting to consider the possibility of a splice loss

specification for fibers, if by doing so some of the confusion

and uncertainty associated with core diameter, clad diameter and

NA standards could be alleviated. Such a specification would, of

course, be accompanied by a suitable splice loss test method

which would have to be sensitive to core and clad diameter variations as well as to numerical aperture and alpha profile variations between fiber ends. A maximum and an average splice loss

might be specified. This would be useful quantities for system

design purposes, since the splice loss portion of the loss budget

could be calculated more easily and reliably. Also, it would

allow both users and suppliers to address a functional requirement,

namely splice loss, directly rather than indirectly by means of

tolerance specifications. Tolerance specifications might still

be required, but by addressing functional requirements it might

be possible to better optimize the manufacturing process with

respect to yield and cost. For example, it may be less costly or

easier to maintain tight tolerances on numerical aperture rather

than on core diameter. To illustrate this point let us recall

that to a first approximation, splice loss - to the extent it is

a function of fiber parameter variations - depends equally on the

percentage variation in NA and core diameter between two fiber

ends. Hence, a 10% tolerance on NA combined with a 6% tolerance

on core diameter, as proposed by the CCITT, would result in

approximately the same splice loss distribution as 10% and 6%

tolerances on core diameter and NA respectively, all other relevant

parameters being equal. One potential problem with this argument

is that two suppliers may be able to meet the same splice loss

specification, but with different tolerance specifications on

fiber parameters. In this case, jointing fibers selected at

random from one supplier with those from the other would result

in a higher splice loss than that indicated by the original

specification. Nevertheless, this approach is worth considering,

particularly if the tradeoff between cost and parameter variations

are similar for most fiber manufacturing processes. Finally,

there is a certain elegance and appeal to this concept since it

would allow fiber transmission properties to be characterized

with only three parameters: namely, attenuation, information

capacity 9-3dB BW or FWHM pulse broadening) and splice loss.

141
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142
REFERENCES

(1) R. Olshansky, M. G. Blankenship, D. B. Keck, Second

European Conference on Optical Fiber Communications,

(IV. 5), Paris (1976).

(2) F. T. Stone, Topical Meeting on Optical Fiber

Communications, (ThG4), Washington, DC (1979).

(3) P. Kaiser, "Numerical-Aperture - Dependent Spectral

Loss Measurements of Optical Fibers", IOOC 1977, B6-2

(1977).

(4) G. T. Holmes, "Launch Dependent Attenuation Measurements

on 10km Concatenation Experiment", 6th European Conference

on Optical Fiber Communication, York, UK (1980).

(5) M. Eriksrud et al, "Comparison Between Measured and

Transmission Characteristics of 12-km Spliced Graded

Index Fiber", Optical & Quantrum Elect. 11 , 517 (1979).

(6) M. Eve et al, "Transmission Studies on Three Graded

Index Fibre Cable Lengths Installed in Operational

Ducts", Optical & Quantum Elect. 10 , 253 (1978).

(7) D. B. Keck, "Optical Fiber Measurements", IEEE Fiber

Optic Measurements Workshop, Washington, DC (1979).

(8) G. T. Homes, "Propagation Parameter Measurement of

Optical Waveguides", SPIE Technical Symposium East,

Washington, DC (1980).

(9) M. Tokuda, S. Seikai, K. Yoshida, and N. Uchida,

"Measurement of Baseband Frequency Response of Multimode

Fiber by Using a New Type Mode Scrambler", Elect. Lett.

13 , 147 (1977).

(10) W. F. Love, Topical Meeting on Optical Fiber Communications,

Washington, DC (1979).

(11) M. Ikedor, A. Sugimura, and T. Kegami, Appl. Opt. 15 , 2116 (1976).

(12) M. Eve, et al, Second European Conference on Optical Fiber

Communication, Tour Oliver de Semes, Paris, 1976, pt. 2,

Comm. V3.

(13) T. Tanifuji, T. Horiguchi, M. Tokuda, Elect. Lett. 15 , 203 (1979).

(14) EIA, P6.6 Working Group Minutes, Boston 1980.

(15) M. I. Cohen and R. J. Klaiber, "Drawing of Smooth Optical

Fibers", TuB 4-1, Optical Fiber Transmission II, Williamsburg,

Virginia (1977).

143
INDEX OF AUTHORS

A. Beaumont. 89

F. Bigi 129

W. D. Bomberger . 101

G. Bonaventura 129

M. C. Brieriy 89

M. J. Buckler 59

J. J. Burke 101

A. H. Cherin 19

N. K. Cheung 45,73

L. G. Cohen 55

L. Curtis. 73

N. M. Denkin . 45

A. Diekema . 27

L. H. M. Engel 27

J. Feldman 119

P. M. Garel-Jones 63

G. A. M. Goltstein 27

K. S. Gordon... 67

S. J. Halme 15

K. Hashimoto 81

R. M. Hawk , 1 E. D. Head 19

P. Healey 85

P. Hensel 85

L. C. Hotchkiss 23

E. J. R. Hubach 15

S-J. Jang 55

P. Kaiser 11,73

F. P. Kapron 63

D. G. Kneller 63

I. Kobayashi 49,81

T. Kosugi 81

C. F. Laing 105

R. W. Lapi erre 105

P. D. Lazay 93

A. LeBoutet 77

R. E. Love 135

D. Marcuse 31

P. Matthijsse 27

E. F. Murphy 105

Y. Nagaki 81

B. P. Nelson 89

K. Okada 81,113

T. Oki 113

D. L. Philen 97

H. M. Presby 31

K. Sano 113

C. M. Schroeder 41

A. B. Sharma 15

144
T. Shibata 81

F. M. E. Sladen 63,67

Meint P. Smid 121

D. H. Smithgall 41

J. W. Versluis 27

K. I. White 89

J. V. Wright 89

M. Young 37

W. C. Young 73

145

a u. S. GOVERNMENT PRINTING OFFICE : 1980 311-1
NBS-114A (REV. 9-78)

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. NBS SP 597

2. Gov't Accession No.

4. TITLE AND SUBTITLE

Technical Digest - Symposium on Optical Fiber

Measurements, 1980

Editors

G. W. Day and D. L. Franzen

Z. fteciptettt's Accession No.

5. Publication Date October 1980

6. Performing Organization Code

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, DC 20234

16. Project/Task/Work Unit No. 7242425

11. Contract/Grant No.

12. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street, City, State, ZIP)

Same as item 9.

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14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

Library of Congress Catalog Card Number: 80-600141

| | Document describes a computer program; SF-185, FIPS Software Summary, is attached.

16. ABSTRACT (A 200-word or less factual summary of most s i gni ficant information. If document includes a significant bibliography or

literature survey, mention it here.) This volume contains summaries of 29 papers presented at the Symposium on Optical

Fiber Measurements held October 28-29, 1980, at the National Bureau of Standards in

Boulder, Colo. Subjects included are the measurement of attenuation, bandwidth/distortion, and index profile, joint/defect characterization, measurements on single

mode fibers, applied measurements, and measurement standards.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name;

separated by semicolons)

Attenuation; bandwidth; fiber optics; fiber optic joints; fiber optics-single mode;

index profile; measurements

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various subjects related to the Bureau's scientific and technical activities. Handbooks—Recommended codes of engineering and industrial practice (including safety codes) developed in cooperation with in- terested industries, professional organizations, and regulatory

bodies. Special Publications—Include proceedings of conferences sponsored by NBS, NBS annual reports, and other special publications

appropriate to this grouping such as wall charts, pocket cards, and

bibliographies.

Applied Mathematics Series—Mathematical tables, manuals, and

studies of special interest to physicists, engineers, chemists,

biologists, mathematicians, computer programmers, and others engaged in scientific and technical work.

National Standard Reference Data Series—Provides quantitative data on the physical and chemical properties of materials, compiled from the world's literature and critically evaluated. Developed under a worldwide program coordinated by NBS under

the authority of the National Standard Data Act (Public Law

90-396).

NOTE: The principal publication outlet for the foregoing data is the Journal of Physical and Chemical Reference Data (JPCRD)

published quarterly for NBS by the American Chemical Society

(ACS) and the American Institute of Physics (AIP). Subscriptions,

reprints, and supplements available from ACS, 1 155 Sixteenth St., NW, Washington, DC 20056.

Building Science Series—Disseminates technical information developed at the Bureau on building materials, components,

systems, and whole structures. The series presents research results,

test methods, and performance criteria related to the structural and

environmental functions and the durability and safety characteristics of building elements and systems.

Technical Notes—Studies or reports which are complete in themselves but restrictive in their treatment of a subject. Analogous to monographs but not so comprehensive in scope or definitive in treatment of the subject area. Often serve as a vehicle for final reports of work performed at NBS under the sponsorship of other government agencies.

Voluntary Product Standards—Developed under procedures

published by the Department of Commerce in Part 10, Title 15, of

the Code of Federal Regulations. The standards establish nationally recognized requirements for products, and provide all concerned interests with a basis for common understanding of the

characteristics of the products. NBS administers this program as a supplement to the activities of the private sector standardizing

organizations. Consumer Information Series—Practical information, based on N BS research and experience, covering areas of interest to the con- sumer. Easily understandable language and illustrations provide

useful background knowledge for shopping in today's technological marketplace. Order the above NBS publications from: Superintendent of Documents, Government Printing Office. Washington, DC 20402. Order the following NBS publications—FlPS and NBSIR's—from

the National Technical Information Services, Springfield, VA 22161

.

Federal Information Processing Standards Publications (FIPS

PUB)—Publications in this series collectively constitute the Federal Information Processing Standards Register. The Register

serves as the official source of information in the Federal Govern- ment regarding standards issued by NBS pursuant to the Federal Property and Administrative Services Act of 1949 as amended.

Public Law 89-306 (79 Stat. 1127), and as implemented by Executive Order 1 1 7 1 7 (38 FR 1 23 1 5, dated May 1 1

, 1 973) and Part 6 of Title 15 CFR (Code of Federal Regulations).

NBS Interagency Reports (NBSIR)—A special series of interim or

final reports on work performed by NBS for outside sponsors

(both government and non-government). In general, initial dis- tribution is handled by the sponsor; public distribution is by the National Technical Information Services, Springfield, VA 22161,

in paper copy or microfiche form.

BIBLIOGRAPHIC SUBSCRIPTION SERVICES

The following current-awareness and literature-survey bibliographies are issued periodically by the Bureau:

Cryogenic Data Center Current Awareness Service. A literature sur- vey issued biweekly. Annual subscription: domestic $35; foreign

$45. Liquefied Natural Gas. A literature survey issued quarterly. Annual

subscription: $30.

Superconducting Devices and Materials. A literature survej issued quarterly. Annual subscription: $45. Please send subscription or- ders and remittances for the preceding bibliographic services to the National Bureau of Standards, Cryogenic Data Center (736) Boulder, CO 80303.
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