NATL INST.OF STAND & JECHl«j

i

NBS SPECIAL PUBLICATION 641

of S NBS

'UBLICATIONS

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

Technical Digest - Symposium on Optical Fiber

Measurements, 1982

Sponsored by the National Bureau of Standards

in cooperation with the IEEE Transmission Systems

Subcommittee on Fiber Optics (COMMSOC)

and the Optical Society of America.
NATIONAL BUREAU OF STANDARDS

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Technical Digest - Symposium on Optical

Fiber Measurements, 1982

Digest of a Symposium sponsored by the

National Bureau of Standards

in cooperation with the

IEEE Transmission Systems Subcommittee

on Fiber Optics (COMMSOC)

and the Optical Society of America

October 13-14, 1982

National Bureau of Standards

Boulder, Colorado 80303

national Bureau ot Standard* €CT 13 1982

QC / oo

U61

, 41

no- 1

Edited by

D. L Franzen

G. W. Day

R. L Gallawa

Electromagnetic Technology Division

National Engineering Laboratory

National Bureau of Standards

Boulder, Colorado 80303

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

Issued October 1982
Library of Congress Catalog Card Number: 82-600594

National Bureau of Standards Special Publication 641

Nat. Bur. Stand. (U.S.), Spec. Publ. 641, 156 pages (Oct. 1982)

CODEN: XNBSAV

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1982

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

Price $6.50

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

This digest contains summaries of papers presented at the 1982 Symposium

on Optical Fiber Measurements, Boulder, Colorado. The program consists of

invited papers (7), contributed papers (25), and workshops (2). Approximately

60 percent of the papers submitted to the Symposium as contributed papers were accepted for presentation.

A breakdown of the contributed papers by country and subject matter

reveals the international interest in optical fiber measurements and indicates

the areas of most intense interest. According to country, the 25 contributed

papers have the following distribution:

U.S.A. - 9

U.K. - 6

Japan - 3

Australia - 1 Canada - 1 France - 1 Germany - 1 Italy - 1 Netherlands - 1 Norway - 1

According to subject matter, the 25 contributed papers were grouped into the

following measurement areas which also formed symposium sessions:

Multimode Bandwidth - 7 Single Mode - 6

Attenuation - 4

Joint/Defect - 3

Applied/Field - 3

Index Profile/Geometry - 2

It is interesting to contrast this distribution with that of the 1980

Symposium. In 1982, the dominant subject areas are multimode bandwidth and

single mode measurements; whereas, in 1980, the emphasis was on attenuation

and joint/defect. Apparently laboratories" are beginning to understand the

problems of multimode fiber attenuation, and emphasis is shifting to bandwidth

and, in particular, the prediction of concatenated lengths. Increased interest in single mode measurements naturally occurs as these fibers move out of

the laboratory and into installed systems.

The National Bureau of Standards is indebted to the IEEE Transmission

Systems Subcommittee on Fiber Optics (C0MMS0C), the Optical Society of

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

organizing this symposium.

D. L. Franzen

G. W. Day

R. L. Gallawa

Boulder, Colorado

October 1982

i i i
CONFERENCE COMMITTEE

L. Gallawa, NBS, General Chairman

L. Franzen, NBS, Program Co-Chairman

. W. Day, NBS, Program Co-Chairman

W. B. Gardner, Bell Laboratories

E. H. Hara, CRC, Ottawa

P. Kaiser, Bell Laboratories

F. P. Kapron, ITT

D. B. Keck, Corning Glass Works

M. Koyama, NTT, Tokyo

D. A. Pinnow, Times Fiber

C. J. Todd, British Telecom

1 v
CONTENTS

Page

Preface iii

Conference Committee iv

Bandwidth Measurements

Prediction of length performance of multimode gradedindex fiber, (invited)

P. R. Reitz; Corning Glass Works 1 Bandwidth studies of concatenated multimode fibre links

J. V. Wright, B. P. Nelson

British Telecom Research Laboratories, Ipswich 9

Determining the concatenated dispersion of multimode fibres

R. W. Blackmore, N. G. Batty

Plessey Telecommunications Research, Ltd 13

Is the -6 dB bandwidth fiber selection criterion still valid?

R. Bouillie, J. C. Bizeul; CNET, Lannion 17

Investigation on launching conditions in the bandwidth

measurement of graded-index fibers

M. Nishimura, S. Suzuki; Sumitomo Electric Industries, Ltd 21

Results of a Bell System bandwidth measurement round robin

F. T. Stone; Bell Laboratories, Norcross 25

Wavelength dispersion measuring equipment

J. Saito, T. Oki , H. Yamamoto; Ando Electric Co., Ltd 29

Measurement of bandwidth versus impulse response width

in multimode fibers

M. J. Buckler; Bell Laboratories, Norcross 33

Appl i ed/Fi el d Measurements

Field measurements of fiber optic cable systems, (invited)

0. I. Szentesi ; Siecor Corporation 37

Improved automated loss set for optical cables

L. S. Short, R. B. Kummer

Bell Laboratories, Norcross. 43

Prototype system for automated measurements of transmission

properties of graded index fibres

J. W. Versluis, H. P. de Wert

N. V. Philips, Eindhoven 47

Precise measurement of optical fiber breaking elongation

K. Matsui, S. Tanaka, M. Hoshikawa

Sumitomo Electric Industries, Ltd 51

v
Attenuation Measurements

Precise measurement of steady-state fiber attenuation

P. J. Vella, K. Abe, F. P. Kapron

Bel 1 -Northern Research, Ottawa

(F.P.K. now at ITT, Roanoke) 55

Modal behavior of various mode mixers and mode filters for

optical fiber measurements

A. K. Agarwal, H. Karstensen, U. Unrau

Technical University of Braunschweig 59

Influence of differential mode attenuation on backscattering

attenuation measurements

M. Eriksrud, A. R. Mickel son, S. Lauri tzen, N. Ryen

Norwegian Institute of Technology 63

Measurement of optical fibre absorption loss: A novel technique

R. Kashyap, P. Pantelis

British Telecom Research Laboratories, Ipswich 67

Measurements on Single Mode Fibers

Single-mode fiber measurement in Japan, (invited)

K. Nosu; Yokosuka Electrical Communications Laboratory, NTT 71

The characterization of monomode fibre links installed

in operational duct, (invited)

J. R. Stern, D. B. Payne, T. D. S. Wood, C. J. Todd

British Telecom Research Laboratories, Ipswich 79

Single mode fiber loss round robin

W. B. Gardner; Bell Telephone Laboratories, Norcross 85

Characterization of the bend sensitivity of single-mode

fibers using the basket-weave test

A. Tomita, P. F. Glodis,* D. Kalish,* P. Kaiser

Bell Laboratories, Holmdel, *Norcross 89

Calculation of equivalent step-index parameters for single-mode fibres

M. Fox; BICC Research & Engineering, Ltd 93

Comparison of cutoff wavelength measurements

for single mode waveguides

C. C. Wang, C. A. Villarruel, W. K. Burns

Naval Research Laboratory * 97

Measurements of fibre polarisation properties using

a photo-elastic modulator

A. J. Barlow, D. N. Payne; University of Southampton * 101

Measurements of strain in cabled monomode fibre

S. Hornung, M. H. Reeve

British Telecom Research Laboratories, Ipswich 105

vi
Standards

International fiber optic standards within the IEC

and CCITT, (invited)

M. LaRose, J. G. Nault, R. Fergusson

Bell -Northern Research In Digest Supplement

Joint/Defect Measurements

Field and laboratory transmission and OTDR splice loss

measurements of multimode optical fibers, (invited)

R. B. Kummer, A. F. Judy, A. H. Cherin

Bell Laboratories, Norcross 109

Optical connector measurement aspects, including

single mode connectors

P. Kaiser, W. C. Young, L. Curtis

Bell Laboratories, Holmdel 123

Effects of mode filter insertion on connection loss between

commercial single-fibre cables

C. Marchesi, SIRTI S.p.A., Milano; U. Rossi, CSELT, Torino 127

A new approach to joint loss measurement

R. D. Jeffery, J. L. Hullett; University of Western Australia 131

Index Profile and Geometric Measurements

Index profile measurements, (invited)

W. J. Stewart; Plessey Research Ltd., Caswell 135

A simple technique for high accuracy core-cladding

concentricity measurement of single mode fibers

D. N. Ridgway, L. J. Freeman; Bell Laboratories, Norcross 139

An interlaboratory measurement comparison of core diameter

on graded-index optical fibers

E. M. Kim, D. L. Franzen; NBS, Boulder 143

Index of Authors 147

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

PREDICTION OF LENGTH PERFORMANCE OF MULTIMODE

GRADED- INDEX FIBER

P. R. Reitz

Corning Glass Works

Waveguide Product Engineering Laboratory

Canada Road Facility

Corning, New York 14831

INTRODUCTION

The accurate prediction of the performance of concatenated

lengths of graded-index multimode optical fibers is a basic requirement to meet the industry's need for reliable installations

while simultaneously avoiding the over-design for which early

fiber systems were noted, At present, the technical community

is actively exploring the limits of this technology, trying to

determine where multimode fibers best fit into the existing and

expected communications system hierarchy. In general, these

limits are determined by available fiber performance and by

the ability of designers to make full use of this performance.

While there is still room for fiber improvement, it is clear

that much more work remains to be done to improve predictions

of link performance, especially with regard to information transmission capacity. This paper provides a review of the progress

toward this goal over the last several years, and highlights

present directions.

Prediction of Link Attenuation

Perhaps the most dramatic change that has taken place in recent years is the ability to predict link attenuation performance

1
from individual fiber measurements, which leads to the more

practical ability to select individual fiber to meet a specific

link budget requirement. This situation is, in part, a result

of improved fiber transmission characteristics. The tendency

for fibers to create a restricted "equilibrium" mode distribution (EMD) , as measured by differential mode attenuation, has

been considerably reduced. Table 1 compares the attenuation

difference between the fully-moded launch condition and an

approximate "equilibrium" distribution for typical past and

present fibers. The use of restricted launching conditions

has become rather universally accepted as the best means of

obtaining suitably accurate length-additive attenuation values.

Two particular methods of restricting the launched power distribution are widely used. The Limited Phase Space (LPS) method

creates a launch beam spot size and numerical aperture somewhat

smaller than fiber core diameter and NA, usually about 70% of

the fiber parameters. That launch distribution has been shown

to be capable of providing length scaling to an accuracy of

( 3—6

)

about 0.05 to 0.10 dB/km. A second method, the mandrelwrap mode filter has also been shown to provide acceptable

(7) length scaling. Both methods have been accepted by the U.S.

Electronic Industries Association, and are included by the IEC's

proposed definition of an equilibrium mode volume.

Although both methods launch an "equilibrium" or "quasiequilibrium" distribution, their distributions are not identical.

The mandrel wrap mode filter has the primary advantage of simplicity in use, and has become particularly widespread in field

2
measurements. While the mandrel wrap allows precise measurements, there are concerns that the distribution may not be truly

representative of an EMD, ' mainly because of the sharp cutoff the distribution assumes at some high relative mode group

number, • Tne LPS launch, on the other hand, couples progressively less relative power as increases from 0.5 to 1.0,

thereby creating a distribution similar to what one would expect for an equilibrium process created by power diffusion from

the core. However, the launched distribution is sensitive to

fiber end preparation and requires precision optical equipment

to achieve best results. On typical fibers, measurement differences between the techniques are usually less than 0.10 dB/km,

which is small enough that either can be used for link prediction. At 0.10 dB/km, residual differences between predicted and

actual link performance are difficult to separate from splice

loss measurement inaccuracy.

Optical Time Domain Reflectometry (OTDR) is widely used in

fiber cable installation for splice optimization. Improvements

in cabling technology have reduced excess cabling loss to a point

where the OTDR is an acceptable technique for locating occasional defects, although some claim the ability to obtain accur-

( 6

ate results. However, the OTDR is not generally used by those

attempting link performance prediction, as it sometimes produces

(2) "misleading results."

Prediction of Link Bandwidth

A subject noted for its complexity, the prediction of link

bandwidth performance from individual fiber measurement has not

3
yet been attained with any regularity. Most papers on the subject can be divided into two general categories: those which

cite average behavior of quantities of fiber, usually from a

practical viewpoint, and those which attempt a theoretical

approach

.

In the first category, an assortment of results have been

obtained. Kitayama et al showed a bandwidth length dependence of ^.73 on 4 8km of fiber, and found those measured results agreed with a theoretical calculation assuming 60% mode

• ( 1 5 11 12

)

conversion. Others have found bandwidth length dependence to be from %.54 to ^1.0; some do not speculate on

the cause for the observed behavior. Those who do generally

cite index profile error compensation. Others relate the length

dependence of bandwidth to the average relationship between systematic and random profile errors. An interesting observation

is contained in (2) that shows high performance fibers tending

toward the less favorable unity exponent, linear dependence.

Virtually all in-depth studies contain a common thread: that

to predict link bandwidth one must know how to predict the law

of addition of group delay. Several studies have dealt with

direct measurement of differential mode delays ( DMD ) .

( H ' ^

)

( 14

)

Others, such as Love, have shown that the mode delay characteristics of moderately high bandwidth fibers can be considered

in terms of an optimum wavelength X^, and the bandwidth at that

( 15

)

wavelength, BW . Matsumoto has recently expanded this concept by showing agreement between calculations and measurements

on 20 concatenated fibers by knowing optimum wavelength, peak

4
bandwidth, a spectral bandwidth shape factor k, but he also uses

factors said to represent average values of internal mode coupling and other parameters.

Conclusion

Although practical experience has shown good agreement between

predicted and actual link attenuation, a large amount of work remains. In particular, the adoption of test methods for individual fibers and the mathematical models that will, together, allow successful prediction of link bandwidth performance for a wide variety of fibers are still a challenge.

References

1) R. Bouillie et al, "Tuileries-Phillipe August: The First

Operation Junction Line in France," 7th European Conference

on Optical Communications (ECOC) 19 81, Copenhagen, Denmark

Paper 15.3.

2) C. J. Lilly and D. R. Bissell, "Some Initial Results and

Experience of Operational Optical Fibre Systems in the UK,

7th ECOC, 19 81, Copenhagen, Paper 15.4.

3) G. T. Holmes, 6th European Conference on Optical Communications, York, U.K., 1980.

4) R. M. Hawk, "Multimode Waveguide Attenuation Measurements,"

Technical Digest , Symposium of Optical Fiber Measurements,

1980.

5) P. R. Reitz, "Characterization of Concatenated Multimode

Optical Fibers with Time Domain Measurement Techniques,"

p. L-14, Digest, Conference on Precision Electromagnetic

Measurements, Boulder, Co., 19 82.

5
6) F. Cairglia et al, "Field Measurement, Testing, and Maintenance of Installed Optical Cable and Systems," p. 15.6, 7th

European Conference on Optical Communications, Copenhagen,

Denmark, 19 81.

7) A. H. Cherin and E. D. Head, "A Fiber Concatenation Experiment Using A Standardized Loss Measurement Method," Symposium

on Optical Fiber Measurements, 1980, Boulder, Co., Technical

Digest , p. 19.

8) P. Kaiser, "Loss Measurements of Graded-Index Fibers: Accuracy Versus Convenience," Symposium on Optical Fiber Measurements, 1980, Boulder, Co., Technical Digest , p. 11.

9) P. Kaiser, "Loss Characterization of Biconic Single-Fiber

Connectors," Symposium on Optical Fiber Measurements, 19 80,

Boulder, Co., Technical Digest , p. 73

10) K. Kitayama et al, "Transmission Characteristics of Long

Spliced Graded-Index Optical Fibers at 1.27um," IEE Journal

of Quantum Electronics , Vol. QE 15, No. 7, July 1979.

11) J. V. Wright and B. P. Nelson, "Bandwidth Studies of Jointed

Multimode Fibres," Paper 4.2, 7th European Conference of

Optical Communications, Copenhagen, Denmark, 19 81.

12) M. C. Hudson and L. C. Kenyon, "Installation and Measurement

of an 18.7 and 23.5km Unrepeatered Fiber-Optic Interoffice

Trunk," Technical Digest , Topical Meeting on Optical Fiber

Communication, Phoenix, Az., 1982, (Optical Society of America, Washington, D.C.)

13) M. J. Buckler, "Optimization of Concatenated Fiber Bandwidth

Via Differential Mode Delay," Technical Digest , Symposium on

Optical Fiber Measurements, Boulder, Co., 19 80. NBS Special

6
Publication 597.

14) W. F. Love, "Bandwidth Spectral Characterization of Modal

Compensation in Multimode Optical Fibers," Paper 4.4, 7th

European Conference of Optical Communications, Copenhagen,

Denmark, 19 81

15) T. Matsumoto, "Bandwidth Estimation of Multispliced GRIN

Fibers," Applied Optics , Vol. 21, No. 11, p. 1956, June,

1982.

TABLE 1

Relative Improvement in Fiber Differential

Mode Attenuation

850nm, dB/km 1300nm, dB/km

2-3 Years Ago 0.5-2.0 0.75-2.0

Present Fibers 0.1-0.4 0.2 -0.6

NOTE: The values in the table are the difference, in dB/km, of

attenuation measurements using uniformly excited and restricted launching conditions.

7

BANDWIDTH STUDIES OF CONCATENATED MULTIMODE FIBRE LINKS

J.V.WRIGHT and B.P.NELSON

BRITISH TELECOM RESEARCH LABORATORIES, MARTL ESHAM HEATH, IPSWICH, UK

I ntroduct i on: A difficult problem facing the designer of high bit-rate

multirnode systems is that of fibre bandwidth specification. For economic

reasons, he must reject only those fibres from a jointed link which would

otherwise prevent the necessary system bandwidth being achieved. The factors

which are important in determining the resultant bandwidth include mode

coupling, differential mode attenuation and the equalisation of modal delays

between adjacent fibres. The first two effects may operate throughout the

length of the fibre (due to micro-bending for instance) or be localised at the

joints. Further complications arise because the bandwidth of a fibre is

sensitive to the exact launch conditions and also to its enviroment, if this

introduces significant micro-bending. These problems have been studied at

BTRL over a number of years using a variety of dispersion and differential

mode delay measurements (DMD) [1] on a number of different fibre links. The

results of our most recent studies will be compared to our earlier work.

ExperimentalThe source for both dispersion and DMD measurements was a modelocked, Q-switched Nd:YAG laser which generated narrow pulses (130ps FWHM) at

1.06ym. After a single pass through a Raman generating monomode fibre [2,3],

pulses tunable over the wavelength range 1.06 to 1.6ym were selected by a monochromator. To maintain standard launch conditions, the output of the

monochromator was coupled into each fibre via a step-graded-step mode

scrambler using a V-groove joint.

All dispersion measurements were made in the time-domain with results

collected using a combination of analogue and digital averaging. Bandwidths

were provided by discrete Fourier transform techniques. The selective

excitation for the DMD measurements was achieved using the output of a monomode fibre imaged onto the core of the multirnode fibre. Again these

measurements were made in the time-domain because experience showed that too

much information was lost in the frequency-domain measurement.

Result s: Our latest measurements have been made on a link of high bandwidth

germanium doped fibres (NA = 0.20), made by Corning using the 0VP0 process.

The link contained 15 fibres in loose tube packaging (which alleviated drum

effects) and fusion spliced to give a total length of 16.2km. Initially the

9
bandwidth of each fibre was measured as a function of wavelength over the

range 1.06 to 1.6ym. Bandwidth spectra for 10 of the fibres are shown in Fig

1 where they have been arranged into two sub-groups of 5 fibres each. The

first sub-group was chosen, as far as possible, to contain fibres whose

bandwidth spectra peaked around 1.3ym. The other sub-group was chosen to

contain fibres whose spectra peaked alternately on either side of 1.3ym. The

first arrangement of fibres would be appropriate to links made from the fibres

of a manufacturer who was achieving a uniform product optimised for 1.3ym

operation. Whereas the second arrangement gives the greatest opportunity for

the equalisation of modal delay between adjacent fibres [4].

The resultant bandwidth spectrum of each sub-group is shown in Fig 2. A peak

bandwidth of 480MHz is achieved with the first sub-group whereas the

"alternated" sub-group achieves a peak greater than 600MHz (the average fibre

bandwidths at 1.3ym were 1380 and 1000MHz respectively). More detail is given

in Table 1, which is expressed in terms of the concatenation exponent q [5],

defi ned by

< b

)- 1/q

. z t b r

1/q

total individual

where B is the 3dB optical bandwidth.

A number of general features are apparent. The smallest values of q are observed in the sub-group of alternated fibres. Indeed, zero values of q are

recorded which indicate that the resultant bandwidth is greater than the

smallest individual bandwidth. This is a result of modal equalisation which

is most marked at 1.33ym where the adjacent fibres have bandwidths of similar

magnitude but spectral bandwidth curves of opposite slope. The value of q tends to increase with the number of sections, rising to 0.3 after 4 or 5 sections. This arises because perfect equalisation is not maintained

throughout the link. Degradation occurs when the mode delays become scrambled

(in the sense that the delay is no longer a monotonic function of modenumber). This scrambling may be introduced by "profile errors" in the fibres

or by mode coupling at the joints [6,7].

Now although the scrambling is detrimental to an alternated or equalised link,

it is beneficial to a link of similar fibres. Without any scrambling effects,

a perfect link of identical fibres would have an exponent q=l. In a practical

situation, mode coupling at the joints and profile differences reduce the

value of q towards a lower limit of 0.5 (square-root broadening). This

10
reduction will be most marked at the wavelengths where the fibres introduce

the maximum amount of scrambling. We may expect this where the bandwidth

peaks. In this region, the delay vs. mode-number curve changes slope and folds

back on itself. Slight profile differences will then introduce significant

scrambling which is evident in sub-group 1 where q reduces to a value of 0.64

after 5 sections.

Table 1, Concatenation exponents q

Sub- group 1 Sub-group 2

X

== 1.06 1.20 1.33 1.60ym X = 1.06 1.20 1.33 1.60ym

section = 2 1.04 .68 .80 .70 .77 0 0 0

3 1.01 .75 .58 .71 .76 0 0 .93

4 .97 .80 .63 .74 .79 .57 .30 .80

5 .93 .84 .64 .72 .80 .56 .29 .71

For a final series of measurements, sub-groups 1 and 2 were jointed to give an

11km link, and this was further extended to 16.2km using a mixed sub-group of

5 fibres which did not fall into either category 1 or 2. At 1.33ym,

bandwidths of 353MHz and 192MHz were measured after 10 and 15 sections,

corresponding to q-values of 0.49 and 0.64 respectively.

Discussion: Previous measurements on high bandwidth multimode links at 1.3pm

had revealed an almost linear extrapolation law [7] and this behaviour was

dominated largely by the presence of pre-pulses in the impulse response

function (IRF), caused by a dip in the refractive index profile. The present

measurements were made on fibres which had no refractive index dips or prepulses and very good extrapolation laws were obtained. This indicates the

importance of rejecting fibres containing spurious structure in the IRF from

high bandwidth systems.

Acknowledgements: We gratefully acknowledge BICC and Plessey Ltd. for helpful

discussions and the loan of Corning fibre used in our measurement programme.

References: [1] L.Jeunhomme and J .P .Pochol le ; Applied Optics 17_, 463, 1978

[2] R.K.Jain et al ; Appl . Phys. Lett. 30, 162, 1977

[3] K.I.White et al ; Radio and Electronic Eng. 51., 385, 1981

[4] M.Eve; Opt. and Quant. Electron. 1£, 41, 1978

[5] S.Suzuki et al ; Fifth EC0C 12.6, Amsterdam, 1979

[6] J.V.Wright et al ; Sixth ECOC 13.1, York, 1980

[7] J.V.Wright and B.P.Nelson; Seventh ECOC 4.2, Copenhagen, 1981

11
12
Determining the Concatenated Dispersion of Multimode Fibres

R.W BLACKMORE AND N.G. BATTY

PLESSEY TELECOMMUNICATIONS RESEARCH LTD., U.K.

Most multimode fibres have discrete defects in their profiles such as

the central dip, and such a defect introduces a small shift in the mean

group velocity of a small number of modes from the mean velocity of the

main group. This results in the modes travelling as two separate groups,

so that a small pulse is produced in the impulse response of a single

length of fibre. Experience has shown that when lengths of such fibres

are spliced together the overall impulse is of the form of figure 1, in

which there is a series of side pulses of decending order. J. Wright et

al (1) suggested that this could be explained by assuming 50% of mode

mixing at each splice, so that there is a continual interchange of energy

between the two modal groups. As it is not possible to accumulate enough

fibres to explore experimentally the effect of variations in the magnitude

and displacement of the minor group on the concatenated bandwidth, a

computer simulation based on this relatively simple concept, was established to determine the variation.

To demonstrate that the simulation produces satisfactory results,

comparison is made in figure 2 of measured and simulated accumulations

of bandwidth with fibre length. Their shapes are very similar, starting

with a slope around unity that falls to a value of 0.65 for the measured

group and 0.7 for the simulation after ten lengths. Having shown that

the model produces a sensible output, it is used to determine the effect

on the overall bandwidth of ten lengths of fibre whose impulse responses

have minor pulses of 5, 10 and 15% of the main energy separated up to 1.5ns

from the main pulse. The data produced by the simulation is presented

13
in figure 3 and that shows that there can be a variation of two to one

bandwidth. Each of the fibres used in the model would have had a bandwidth

of 1GHz, and thus it is demonstrated that bandwidth is not a suitable parameter for determining the dispersion of individual length of fibre.

The cause of variation in the concatenated bandwidth can be identified

in the impulse response, so that it would seem preferable to use a time

domain function for the evaluation of the performance of an individual

length. The impulse response itself is not a suitable function, because

its often irregular shape makes it difficult for making comparisons. A

better function to use is the autocorrelation function of the impulse

response, because it is normalised to its value at zero, and thus produces

a clear datum line from whcih to make evaluations. It has the further

advantage that

:

1) It smooths out many of the irregularities, making it easier for making

comparisons

.

2) Its Fourier transform is the power spectral density function.

Thus it can be readily derived from the deconvolved frequency response

of a measured impulse response or from a directly measured frequency response.

The autocorrelation furnction of the concatenated impulse response

smooths the series of side pulses into functions with 'tails', and these

'tails' have marked influences on the corresponding frequency response.

In order to establish a relationship between the 'tail' length and the

frequency response, functions of the type (mt) m exp(-mt) were used to

simulate time functions with 'tails'. Three such autocorrelation functions

are produced in figure 4, and are compared with the autocorrelation of

the Gaussian functions that have the same electrical bandwidth i.e. the

3dB point on the electrical response produced in the measurement. In each

14
instance the two functions intersect at approximately the 10% value. Thus

there is a strong relationship between the interval to the 10% height of

the 'tail' and the electrical bandwidth. As the 'tail' is produced by

the spurious pulses, it follows that these pulses have a marked influence

on the overall bandwidth. Figure 5 shows the relationship between the

tail length and the fibre length for a measured result, and it too has

a slope of 0.65. Providing the spurious pulses is greater than 10%, the

line intersects with the 10% value for the first length, showing this close

relationship between the spurious pulse and bandwidth. However spurious

pulses of less than 10% have significant influence on the performance,

therefore a lower limit of 5% is suggested. The broken line of figure

3 gives the bandwidth derived from the autocorrelation functions for 10%

spurious pulses. Compared with the directly derived results, the indirect

method produces pessimistic values, but a suitable correction can be applied

that produces EFFECTIVE BANDWIDTH = 0.3dBm + .454T

_1

, where Bm is the

measured bandwidth of the fibre and T is the interval to the 5% height

of the autocorrelation function. Using the appropriate law of concatenation it is then possible to derive the EFFECTIVE BANDWIDTH of concatenated

lengths. This together with measured results will be discussed.

ACKNOWLEDGEMENT

The authors wish to thank the directors of the Plessey Company for

permission to publish.

(1) Bandwidth studies of Joined Multimode Fibres - J.V. Wright and

B.P. Nelson E.C.O.C. 1981 (COPENHAGEN).

15
FIG.1 MEASURED IMPULSE RESPONSE OF CONCATENATED FIBRES

0' 1

0 65

10 LENGTH

km

FIG. 2 ACCUMULATED BANDWIDTHS

BANOWIOTH

250-. MHz

200-

V.5 DISPLACEMENT

OF SPURIOUS

PULSE (nS)

FIG. 3 EFFECT OF SPURIOUS PULSE

ON CONCATENATED BANDWIDTH

FUNCTIONS

WITH TAILS

GAUSSIAN

FUNCTIONS

LENGTHS

FIG. 4 COMPARISON OF AUTOCORRELATION FIG. 5 ACCUMULATED AUTOCORRELATION

LENGTHS
IS THE -6 DB BANDWIDTH FIBER SELECTION CRITERION STILL VALID?

R. Bouillie

J. C. Bizeul

Division R.O.C., 22301 Lannion, France

INTRODUCTION

The commercial availiability of graded index multimode fibers with a very

low attenuation (0,7 dB/km) at 1,3 urn, allows for a near feature the installation of high speed systems ( ^ 140 Mb/s on long repeater sections

(25 km, for the French experimental line LE MANS-ANGERS)

.

The optical power budget indicate that the limitative factor, for these

systems, is the baseband response of the concatenated link. The precise

knowledge of the attenuation/frequency characteristic for each fiber is

therefore, of paramount importance.

We proposed in this paper a new criterion for the selection of fibers to be

used for long repeater section systems, which is independant from the

classical -6 dB cut-off frequency.

Msasurements on individual fiber lengths

The reported results in this paper have been obtained using indifferently time-domain or frequency-domain measurements. These two techniques

give reliable and comparable results (+ 10 MHz) when using the same source,

launching conditions and detection.

From a batch of 30, 2,2 km, fibers produced by two manufacturers and

optimised for 1,3 jjow, we have recorded the baseband frequency response for

each fiber length. From the results, the fibers can be classified in two

categories :

a) - Those with a perfectly smooth curve all over the frequency range

(fig. 1)

b) - Those with a "bumpy" frequency characteristic as shown on fig. 2,

which corresponds, in time-domain to the appearance, at the fiber

output, of precursors. This classification is not directly com- parable with one done on the - 6dB cut-off frequency. Figure 3

presents a smooth and a "bumpy" curve for 2 fibers which have the same - 6 dB value.

On our batch of fibers 40 % belongs to the smooth category.

Measurements on concatenated links

From the previous results we have selected 11 fibers in each categorie

(a) and (b) and we have spliced them to obtain two links of 24,2 km. The

- 6 dB cut-off frequency for each fiber lenght is on the order of 700 MHz.

The overall attenuation of the concatenated links is 27 dB.

17
Figure 4 presents the frequency response for the two links. The one with the "smooth" fibers has a - 6 dB cut-off frequency of 140 MHz. The

one with "bumpy" fibers has 60 MHz. Obviously the use of fibers with a regular characteristic give a much wider bandwith than with "bumpy" fibers,

even though the individual fibers have the same cut-off frequency.

The absence of "bumpy" region in the fiber characteristic is a parameter of great importance to obtain large bandwith on concatenated links.

Theorical model

We tried to apply the linear four poles theory to the optical fibers, making

the summation (in dB) of all the fibers characteristics belonging to the

concatenated links.

The cable below shows for the "smooth" fibers the comparison for different

lengths between measurements and computed results.

Length (km) Measured (MHz) Calculated (MHz)

6,6 310 305

13,2 260 250

19,8 180 175

24,2 140 135

We found a very good agreement between model and measurements.

For the "bumpy" fibers the fitting was not good at all. We draw the conclusion that for fibers with a regular characteristic the linear four poles

theory can be applied.

A new criterion for the fiber selection

The previous results show that a simple classification of the fibers on their

-6 dB value is absolutly insufficient to insure large bandwith on long

lengths. A particular attention must be paid to the smoothness of the frequency characteristic. One must eleminate all the fibers with holes in their

characteristics in the range of the system frequency.

The formula below gives the maximum acceptable holes amplitude (A) in the

"bumpy" frequency characteristic for a given system frequency (F) . If (B) is

the attenuation provided at (F) by the system design :

A = jr where N is the number of the cable lengths which constituted

the link.

Conclusion

A new criterion for the selection of optical fibers is proposed. The -6 dB

characteristic is not yet considered as being an important parameter, it is

now replaced by the regularity of the frequency characteristic in the system

frequency range where the fibers are to be used. A formula for the maximum

acceptable holes amplitude for each fiber length is established. A theorical

model based on the linear four poles theory gives in the case of regular

fibers, a solution to the cancatenated link problem.

18
I

19

INVESTIGATION ON LAUNCHING CONDITIONS

IN THE BANDWIDTH MEASUREMENT OF GRADED- INDEX FIBERS

M. Nishimura and S. Suzuki

Sumitomo Electric Industries, Ltd.

1, Taya-cho, Totsuka-ku, Yokohama, Japan

Introduction

Accurate measurements of the baseband frequency response of gradedindex fibers are required both for quality assurance of fiber cables and

for design of fiber transmission systems. As is well-known, the measured

bandwidth is strongly affected by the launching conditions, that is, how

the launched optical power is distributed among the propagating modes.

This paper describes the experiments showing the dependence of the bandwidth upon the launching spot size and numerical aperture (NA) . It is also

reported that theoretical analysis can explain the experimental results

especially for MCVD fibers.

Exper iment

Two launching systems with different launching spot size and NA were

prepared. They are illustrated in Fig. 1. The output of a mode scrambler

is imaged by a couple of microscope objectives on the launch end of a test

fiber. Those objectives and an aperture placed between them were chosen in

each system so as to realize the launching conditions summarized in Table 1.

Ten graded-index fibers were measured with these two launching systems

by the swept-f requency method. Five fibers among them were MCVD fibers and

the others were VAD fibers. Figure 2 shows the values of the measured

bandwidths. Examples of the frequency response waveforms are shown in Fig.

3. From Fig. 2 it is found that the bandwidths of some of MCVD fibers tend

21
to decrease when the launching spot size is reduced to 25 ym and the

launching NA is 0.1. On the contrary, in the case of VAD fibers the measured bandwidths are unchanged or slightly increased. The averaged relative

difference of the bandwidths between the two launching conditions is -11%

for MCVD fibers, and +4% for VAD fibers.

Theoretical Analysis

Influence of the launching conditions upon the baseband frequency

response of a graded-index fiber with a central dip was investigated by

using the finite element method. Such index distribution are well simulated by the function as

r

2 n(r) 2 - nc

2 = 2An 2 {l -

<f) a

}{l - exp(--T)},

c a w

where nc , A and a denote the refractive index of the cladding, the relative

index difference and the core radius, respectively, a is the power-low

parameter and w is the dip width.

The impulse response can be calculated by summing up all the propagating modes whose group delay time is computed by FEM. The baseband

frequency response can be obtained from the impulse response using FFT

techniques. In the calculation differential mode attenuation was taken

into account. Besides modes with high principle mode number (PMN) , modes

with the lowest azimuthal mode number (v = 0) were assumed to suffer high

loss, since irregularity of the dip tends to scatter these modes whose

field amplitudes are maximized near the center.

Examples of the numerical results are shown in Figs. 4 and 5. Calculated mode group delays for a profile with a = 1.95 and (w/a) = 0.05 are

plotted as a function of PMN in Fig. 4. Solid curves in Fig. 5 show the

impulse response and the baseband frequency response computed from Fig. 4

assuming that all propagating modes are equally excited at the launch end.

Broken curves and dash-dotted curves represent the response waveforms when

22
assumed that excited modes are restricted to only modes with normalized PMN

less than 0.5 and 0.25 respectively.

By comparing these response waveforms, it is found that

(1) the relative amplitude of the fast-arriving pulse (mainly consisting

of mode with v = 1) increases, and

(2) the 6-dB electrical bandwidth decreases,

as the excited modes are restricted to modes with low PMN. Further calculations have suggested that this tendency is almost always found independent of slight variation of a as far as a dip exists, unless the modes with

V = 1 are assumed to be strongly attenuated by the dip.

Conclusion

From the experiments it has been found that in some cases the bandwidths of graded-index fibers with a dip decrease when measured with small

launching spot size and small NA. Theoretical analysis has revealed that

this phenomenon is due to the existence of the fast-arriving pulse peculiar

to such fibers.

Reference

[1] K. Okamoto, "Comparison of Calculated and Measured Impulse Responses

of Optical Fibers", Appl. Opt., Vol. 18, No. 13, P. 2199 (1979).

23
Mode Scrambler Te9t Fiber

ID x20 x20 xlO xlO t<Q>-^-C==l>-

(X-0.85u)

II xlO x20

Aperture

Fig.l. Launching systems used in

the experiment.

1200

^1000

M

5 800 o•~<3

T3c

(0 « 600

400

O MCVD fiber • VAD fiber

I j_

I II Launching System

Fig. 2. Bandwidths of 10 gradedindex fibers measured

using the launching

systems shown in Fig.l.

2 -

-4

-6

i r

oc =1.95 w/a=0.05

A =1.0Z X =0.85um

i • ! • ; i • | i | |

J_

0 0.5 1 Normalized Principle Mode Number

L_j I i 1 0 0.5 1 Normalized Radial Position (r/a)

Fig. 4. Mode group delays calculated for(X = 1.95 profile

with a dip.

Spot size NA

I 50um 0.20

II 25um 0.10

Table I Characteristics of

launch spot in the

launching systems shown

in Fig. 1

.

(MHz) 1000

MCVD fiber

Launching System I Launching System II

1000

VAD fiber

-10

Launching System I Launching System II

Fig. 3. Examples of measured baseband frequency response.

FREQUENCY CGHlJ

I

INDEX PROFILE

RLPHR - 1 .95 DELTA - 1 . 0 y. DIP/R - .05

-2-10 1 TIME Cnaao]

Fig. 5. Impulse responses and

baseband frequency responses

calculated from Fig. 4 assuming different exciting

conditions

24
RESULTS OF A BELL SYSTEM BANDWIDTH

MEASUREMENT ROUND ROBIN

F. T. Stone

Bell Laboratories

Norcross, Georgia 30071

Recently a mult imode-f iber bandwidth-measurement round

robin was conducted among several locations of Bell Laboratories

and Western Electric. Analysis of the results has clarified some

of the problems in obtaining agreement between test sets. The five

participants measured the seven fibers in the study three times

each. Fibers 1-5 had low OH content and were measured at both

short (825 nm) and long (1300 nm) wavelengths. Fibers 6 and 7 had

high OH content and were only measured at the short wavelength.

The instructions requested that the participants overfill the

fiber, use a mandrel wrap (3 turns around a 20-mm-diameter mandrel)

at the input end, and break off a piece of the input end containing

the mandrel for the short length measurement (without disturbing

the launch conditions). All test sets operated in the time

domain.

Table I summarizes the results. The long wavelength

measurements experienced a considerably lower o/p (5.6%) than the

short wavelength measurements (17.9%). Overall, a/v was 12.7%.

Participant 3 used the anti-Stokes radiation from a fiber-Raman

laser to perform the short wavelength measurements. If those

results are excluded, the overall a/y was 9.9% and the short wavelength o/m was 12.9%, which is comparable to the o/y achieved in an

earlier NBS-coordinated round robin involving short wavelength

diodes

.

1

25
TABLE I

Average ( y j_ ) /Standard Deviation (

) of Three 3-dB-Bandwid th

Measurements (in MHz). Short Wavelength Measurements are on Top.

The Last Column Gives the Average y = the Interlocat ion a =

and the Ratio a/y in Percent

.

Participant i = 1 2 3 4 5 Average

Fiber 1 906/39

1023/24

852/28

1037/56

1019/22

1065/23

849/29

1068/30

822/88

1203/24

890/78/8.8%

1079/72/6.6%

2 675/12

292/4

627/22

313/6

710/24

311/9

602/18

316/5

740/43

313/11

671/57/8.5%

309/10/3. 1%

3 296/13

505/6

291/12

563/9

290/4

614/5

272/1

1

661/17

330/16

582/1

1

296/21/7.2%

585/58/9.9%

4 274/1

135/1

279/5

144/3

234/10

125/3

247/2

143/6

266/4

137/4

260/19/7. 3%

137/8/5.6%

5 620/16

462/8

491/30

480/4

378/20

473/3

526/206

454/15

715/15

466/10

546/128/23.4%

467/10/2. 1%

6 398/16 375/14 200/5 363/21 390/1

1

345/82/23.8%

7 526/1

1

257/6 236/20 293/14 633/63 389/179/46.0%

One observation from this round robin is that fibers

exhibiting bimodal behavior can be extremely sensitive to excitation conditions and, for short wavelength measurements, source

spectral width. The three measurements of Table I with a/y > 10%

were on such fibers. Figure 1 illustrates the sensitivity of fiber

7 to launch conditions. The mode scrambler launched additional

energy in the secondary peak, which significantly affected the 3-dB

bandwidth. Participants 1 and 5, who did not use a mode scrambler,

measured high bandwidth values (Table I). Participants 3 and 4

used a mode scrambler and measured lower values. Participant 2

measured low values but did not use a mode scrambler. Possibly, as

26
discussed below, the larger spectral width of the source used by

that participant produced the low measurement values. Figure 2

indicates that the higher short-wavelength bandwidths measured on

fiber 5 by participants 1 and 5 persist even after a mode scrambler

is inserted. In addition to being sensitive to excitation

conditions, however, fibers with double-peaked pulse shapes such as

in Figure 2 can have an extreme sensitivity to source spectral

width: a small increase can lower the first dip in the transfer

function below the 3 dB level and halve the measured 3-dB

bandwidth. Participants 1 and 5 used sources with a narrow

spectral width, participants 2 and 4 used sources with a wider

spectral width, and participant 3 used a fiber-Raman laser with the

widest spectral width. The short wavelength results in Table I are

consistent with these relative widths. These results suggest that

achieving better than 10% agreement among test sets will require

more careful control over launch conditions and, for shortwavelength measurements, source spectra.

I gratefully acknowledge the efforts of Doug Head, Bill

Reed, and Laura Short of Bell Labs and Mary Potasek and Ron Smith

of Western Electric, who performed the measurements reported in

Table I.

1. D. L. Franzen et al . , Appl. Opt. 20, 2412 ( 1981 ).

27
-3dB

0

O

-6dB

-9dB

MS = MODE SCRAMBLER

MW = MANDREL WRAP

HALF POWER LEVEL

FIBER 7

WITH MS

NO MS

NO MS, WITH MW

^XQWITH MS AND MW

246 MHz

300 600 900

BANDWIDTH (MHz)

FIGURE 1. TRANSFER FUNCTION H(F) AND PULSE SHAPES OF FIBER 7

MEASURED AT 825nm UNDER A VARIETY OF LAUNCH

CONDITIONS.

FIBER 5

BANDWIDTH (MHz)

FIGURE 2. TRANSFER FUNCTION AND PULSE SHAPES OF FIBER 5

WITH AND WITHOUT A MODE SCRAMBLER.

28
Wavelength Dispersion Measuring Equipment

J. Saito T. Oki H. Yamamoto

Ando Electric Co., Ltd.

19-7 Kamata-4chome Ota-ku Tokyo Japan

1. Introduction

Baseband frequency response of graded-index multimode fibers depends

on several dispersion factors; intermodal, material and construction

dispersion. Generally, in graded-index multimode fibers, intermodel dispersion

> material dispersion > contruction dispersion. Therefore , baseband frequency

response depends mainly on intermodal dispersion. However, recently, the

characteristics of graded-index multimode fibers have been improved so that

material and construction dispersion are not negligible.

In this paper, equipment designed to measure the wavelength dependent

dispersion of multimode fibers is described. (Wavelength dependent dispersion

of multimode fibers is related to material and construction dispersion.)

Measurement method of this equipment is as follows. Intensity modulated optical

signals from various wavelength LEDs are emitted into the fiber under test.

The output end of the fiber is connected to a monochromator which uses a pair

of fibers to select two close wavelength from the incoming wavelengths.

These two optical signals from the fiber are demodulated by APDs and a Vector

Voltmeter which measure the phase difference between the signals.

The wavelength dependent dispersion is then obtained to calculate the phase

difference of these two signals.

2. Principle

A block diagram of this equipment is shown in Fig. 1. Each LED light source is intensity modulated by a sine waveform and one of the optical signals

is emitted into the fiber under test. The spectral width of the LED is rather

broad, so while the optical signal propagates through the fiber, a propagation

time difference occurs in every spectrum of the LED.

In the receiver, a monochromator is used to select two close wavelength

( ^2 )• Then it selects the spectrum of the LED which has propagated through

the fiber.

29
These two optical signals are modulated by a sine waveform and the resultant

propagation time difference is regarded as the phase difference ( ) of the. modulated waveform. Therefore the wavelength dependent dispersion D of a fiber

is described in equation (1)

.

D = (ps/km-nm) •• (1)

L. * 2tt f -| A i — X 2I

where L = Optical fiber length (km)

,

f = Modulation frequency (Hz) and

Xl, A 2 = Wavelengths (nm)

.

The wavelength dependent dispersion is obtained by measuring both phase

difference (i|>) and wavelength difference (AA= \ x ~ A 2 ) •

3. Measuring Equipment

3.1 Transmitter

The transmitter of this equipment has eight LEDs in which the center

wavelengths are 729, 798, 850, 889, 1060, 1145, 1255 and 1474 nm. Each LED is

intensity modulated by a sine waveform of 50MHz and each optical signal is

launched into a fiber to a 8x1 Optical Switch. The individual wavelengths are selected by the switch, and output through a connector.

3.2 Receiver

The receiver consists of a Monochromator , APDs, Frequency Converters and

a Vector Voltmeter. The received optical signal goes through a pigtail fiber

to the Monochromator, which has two pigtail fibers for output, enabling two

slightly different wavelengths to be separated from the input LED ' s wide

spectrum. Each electrical signal converted by the APD is then converted into an intermediate frequency by the Frequency Converter . The Vector Voltmeter measures

the phase difference between the two intermediate frequencies.

3.3 Monochromator

The Monochromator configuration is shown in Fig. 2. The input optical fiber

and the concave grating which move around the Rowland Circle are fixed on a plane

which rotates on the base corresponding to the wavelength to be set.

Output optical fibers are fixed on the base and also on the Rowland Circle.

In the concave grating, the relationship between the incident angle a and the

diffraction angle 3 is expressed in equation (2)

.

30
mA

sin 3 - sin a = (2)

d

where m = The order (±1, ±2, .....) and

d = Groove spacing (mm)

.

When a pair of output fibers with a core center spacing of 300 ym are

fixed, the wavelength difference (AX) between the two diffraction rays separated

is expressed as,

dcos B AA = . AB m

where AA = j A 2 — Ail,

Also, A£ = rcos 3 -A3 where,

A£, = Core spacing

r = Curvature radius of the concave grating

giving AA = ( 3) mr

Every item on the right of equation (3) is constant. If an aberration has no influence on this diffraction, the wavelength difference should be constant.

In this case, the concave grating has a groove spacing of 1/300 mm and

a curvature radius of 50 mm. Therefore the wavelength difference AA becomes

20 nm by equation (3). Fig. 3 shows the wavelength characteristics of the two

output type monochromator

.

4. Results

A wavelength dispersion data of 1 km graded-index multimode fiber of

50/125 urn is shown in Fig. 4. An outside view of this apparatus is shown in Fig. 5.

5. Conclusions

A wavelength dispersion measurement method is proposed and effective

wavelength dispersion measuring equipment has been manufactured.

6. Acknowledgements

The authors wish to thank Mr. M. Koyama and Mr. K. Okada of the Yokosuka

Electrical Communication Laboratory, N.T.T. , and Mr. E. Tanaka of Ando Electric

for their great encouragement and useful suggestions.

31
Transml tter Receiver

50MHz ©

^1

Optical

Switch

LED Light

Source

Optical Fiber

S1-AP P-S

P-5

Ge-APO

Frequency

CQnifixlfir

Mnnnrhrnmator

W>D 0

(Vector Voltmeter

F1g.l Block Diagram of Wavelength Olspersion Measuring Equipment

oncave Grating

(250/300um) \\

Core Center Spac1ng(300um)

Fig. 2 Monochromator Configuration

-30 -20 -10 0 10 20 Wavelength Deviation (nm)

F1g.3 Monochromator Characteristics

30

200

100

1 kmGraded -index 50/125um Multimode Fiber

700 800 900 1000 1100 1200 1300 1400 1500 Wavelength (nm)

Fig. 4 Wavelength Dispersion Data

Fig. 5 Outside view of Wavelength Dispersion Measuring Equipment

32
MEASUREMENT OF BANDWIDTH VERSUS IMPULSE

RESPONSE WIDTH IN MULTIMODE FIBERS

M. J. Buckler

Bell Telephone Laboratories

Norcross, Georgia, USA 30071

ABSTRACT

An empirically based relationship, which is simple yet highly correlated,

has been found to exist between optical 3dB bandwidth and impulse

response RMS pulse width in multimode graded index optical fibers. Experimentation has shown this relationship to be independent of the

degree of externally induced mode coupling. Propagation of error analysis was used to find the total +3G limits, and theoretical comparisons are made for three ideal pulse shapes - cosine, raised cosine and gaussian.

I. INTRODUCTION

Both the time domain and frequency domain fiber transmission characteristics are important for determining maximum and/or optimum bit rate - length systems, for evaluating the possible necessity of line equalization, for fiber parameter optimization, and for engineering fiber routes.

Using an empirical approach, we have investigated the relationship between

bandwidth and pulse width for graded index multimode optical fibers.

II. EXPERIMENTAL APPARATUS

Figure^l shows a block diagram of the impulse response measurement

system , A narrow optical pulse (450 psec FWHM) is launched into the

fiber; 512 samples are taken per oscilloscope sweep (20nsec full screen)

and 64 sweeps are accumulated and averaged. The minicomputer then calculates and stores the Fast Fourier Transform (FFT) for this pulse. A 0.5m

reference length is broken off of the launch end of the fiber and similarly measured. The baseband frequency response of the fiber is calculate

from the ratio of these two FFTs and the time domain response is found by

calculating the inverse FFT of the resultant response. With proper filtering the +30 measurement precision for this measurement set and procedures is +10 MHz optical 3dB bandwidth and +0.01 nsec RMS pulse width.

III. RESULTS AND ANALYSIS

A number of experiments were conducted with uncoated fibers (negligible

mode mixing) , production coated fibers (partial mode mixing) , and con- nectorized cabled fibers ("complete mode mixing") and their respective

concatenations. All of the fibers used in these experiments were of the

Si02~P20,--Ge02 type with a 0.23 NA. Moreover, for the uncoated fiber

experiments low-loss loose tube splices were used so as to achieve minima

modal disturbance. Table I shows the basic fiber and link characteristics

33
for each experiment.

TABLE I EXPERIMENT PARAMETERS

Fiber

Fiber Type

Uncoated

Coated

Connectorized

Cabled

Outer

Dia. (ym)

110

125

110

Core

Dia. (ym)

55

50

55

Length (km)

1.0

1.1 - 2.2

0.9

Maximum

Concatenated

Length (km)

9.0

8.8

7.4

Evaluation of the data for these three experiments indicated that bandwidth

and impulse response width were strongly correlated, regardless of pulse

shape. It was found that the best curve fit was a power law of the form

y = ax (1)

where y represents optical 3 dB bandwidth (MHz) and x represents RMS impulse

response width (nsec). Using a least squares linear regression analysis the

best fit values of a and b are given in Table II.

TABLE II BEST FIT POWER LAW CURVE

No. of Correlation

Experiment Data Points a b Coefficient

Uncoated Fiber 15 170.6 -1.01 0.9958

Coated Fiber 25 170.

1

-1.01 0.9964

Connectorized

Cabled Fiber 123 168.6 -1.01 0.9959

Figure 2 shows a plot of the 3 dB bandwidth versus RMS impulse response

width data measured for the connectorized cabled fiber experiment.

For ease of application of these results the form of the power law curve fit

has been simplified to

y = a

± /x. (2)

This is justified in view of the fact that the magnitude of the power law

exponent was nearly unity and insensitive to the degree of mode mixing.

Refitting the connectorized cable data (since it most nearly approximates

the application environment) gives

y = (169.1)/x. (3)

Using a propagation of error analysis the total +3a uncertainty in a^ due

to measurement imprecision and curve fitting was +11.

34
IV. CONCLUSIONS

These experiments have shown that optical 3 dB bandwidth and RMS impulse

response width are highly correlated regardless of the amount of fiber

mode mixing. It has been found that for the multimode graded index optical

fibers studied here,

3 dB Bandwidth (MHz) = 169 -

11

(4)

RMS Impulse Response Width (nsec)

with a coefficient of correlation greater than 0.99.

Establishment of this relationship allows the economy of a single simple

measurement, while still having the flexibility of making calculations or projections in either the time or frequency domain. For example, using

this function, we can now specify with a high degree of confidence, the

required 1km bandwidth, for production fibers, necessary to construct

various system regenerative repeater spans.

In the presentation, we will discuss the instrumentation, experimental

results, and propagation of error analysis which led to this newly found

bandwidth versus pulse width relationship. Also, theoretical comparisons

will be made for three ideal system response pulse shapes and examples

presented to show how this relationship allows us, for the first time, to

interrelate transmission system and manufacturing requirements.

References

1. Dannwolf, J. W. , Gottfried, S., Sargent, G. A., and Strum, R. C.

"Optical-Fiber Impulse-Response Measurement System," IEEE Trans,

on Instrumentation and Measurement, 1976, IM-25, pp. 401-406.

2. Miller, C. M. : "Loose Tube Splices for Optical Fibers," BSTJ,

1975, 54, pp. 1215-1225.

35
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a.%

SAMSUNG

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DIGITAl DIIAY

Of HI IATOI

MINI COMPUlit

11riN I APO TT

PMASI

S/J

optical atiinuatoi T OPIICAl Mf ASUttAtlMI POtll

FIGURE 1. IMPULSE RESPONSE MEASUREMENT SET

• = »JJ« C ABlf 0 Fltft

I = CONCAIENATCO FllftS

T = 6X -' 0I

. f = 0 99S*

IS 10 IS HSn» OPIICAl IMS PUIS! WIDTH |NS[C)

FIGURE 2 BANDWIDTH VERSUS IMPULSE RESPONSE WIDTH DATA FOR THE CONN ECTOR I ZED CABLED FIBER EXPERIMENT

36
FIELD MEASUREMENTS OF FIBER OPTIC CABLE SYSTEMS

0. I. Szentesi

Siecor Optical Cable, Hickory, N. C. 28603

INTRODUCTION

Field testing of fiber optic cable systems is usually performed in order to

monitor the fiber spl icing process, to ensure that end-to-end transmission

specifications (attenuation and bandwidth) are met and to provide baseline data

for cable plant maintenance.

A generalized fiber optic transmission system is depicted in Figure 1. It

consists of the electro-optical (E/0) interface, the fiber cable system, and the

opt ical -el ectr ical (0/E) interface. The E/0 and 0/E interfaces and their

associated common equipment is often referred to as the Optical Line Terminating

Equipment (OLTE). The fiber cable system consists of one or more fiber cables

joined with splices and terminated at the ends with optical fiber connectors. In

many cases the OLTE and the fiber cable system are connected via the Fiber

Distribution Frame (FDF) which is an optical patch panel.

As far as the E/0 and 0/E interfaces are concerned, the minimum optical

measurements are the output power Cor rather the power coupled into the fiber)

and the minimum required received input power. The only equipment required for

these measurements is an optical power meter and a variable optical attenuator.

The efficient testing of the transmission medium itself requires additional

equipment. This paper discusses the field measurements associated with the

optical cable portion of the system. Typical results obtained on over 1200

jointed fiber strings will be discussed.

FIELD TESTS

The most corrmon field tests before, during and after cable installation are

listed in Table 1. For each test one or more methods are given. Selection of

37
the method depends on the availability of test equipment, training of craftsmen,

availability of manpower, etc. Obviously, whether one elects to do all or any of

the tests depends on the specifications and the margins involved.

Continuity

The continuity of each fiber in an optical cable, either before or after

placement may be readily checked with a simple light source, such as a

flashlight. Attenuation of optical cables is sufficiently low that this method

of checking is possible even after several kilometers have been installed.

On-the-reel Attenuation

For field measurement of attenuation of cables on the reel the optical time

domain ref lectometer (OTDR) is the most commonly used test equipment. The

advantages over insertion loss or two length measurements are obvious in that

only one end of the cable needs to be prepared, repeatability Cup to about 10 dB)

is better, and the measurement requires less time.

Splice Attenuation

The most convenient method of measuring splice attenuation is optical time

domain ref lectometry . Unfortunately, a single measurement of the loss of a

splice can be considerably in error if the backscatter ing coefficients of the two

fibers are different. As a matter of fact, the observation of apparent splice

"gains" is not uncommon. Therefore, if an accurate measurement is required the

splice loss has to be measured from both directions and the two numbers averaged.

This is not usually done. Instead, the OTDR is used to minimize splice losses.

A typical field result is shown in Figure 2. The individual numbers may be in

error, but the mean value of 0.28 dB for the 88 fiber splices is accurate. It is

interesting to note what the effect of splice loss monitoring is in terms of the

final mean loss. During the course of splicing, six splices were remade. The

38
effect of the remakes was to lower the mean loss from 0.31 dB to 0.28 dB. In

this particular instance, splice loss measurements had no significant impact on

the final result. In another installation, however, 240 fiber splices averaged

0.34 dB. Without remakes the average would have been 0.60 dB. More importantly,

11 closures would have had to be reentered and 18 fiber splices remade to meet

end-to-end requirements. This later example illustrates the benefits of monitoring of splices as they are made.

Connector/End Splice Attenuation

Connector performance can be assessed by measuring the additional insertion

loss introduced by the connector or alternatively with the OTDR. Again the

later is more convenient since it is a single-ended measurement.

If the connectors are not installed in the field, connector ized pigtails are

spliced on to the fiber cable. The OTDR is the most convenient test

equipment for this purpose, but if the pigtail is too short, it may not be able to

resolve separately the loss of the connector and the loss of the end splice.

End-to-End Attenuation

Since the final measurements are performed on connector ized systems,

non-destructive insertion loss measurements are necessary to determine end-to-end

attenuation. The repeatability of the measurements is dominated by the

repeatability of the connector losses. To simulate equilibrium mode

distribution, a mandrel-wrap EMS can be implemented by wrapping the input pigtail

4-5 times around a 25mm OD mandrel.

Table 2 compares end-to-end measured results with calculations based on

on-the-reel cable data and splice losses measured with the OTDR. The agreement

of the average values is typically within ±1 dB.

Alternatively, for low attenuation systems, the OTDR can be used to

determine end-to-end attenuation.

39
End-to-End Bandwidth

Bandwidth can be measured either in the frequency domain or in the time

domain. In the field, measurements are most often performed in the frequency

domain. Repeatability is of the order of -10%. Both step index or

step-graded-step index mode scramblers have been used. We have found that if the

end-to-end attenuation is greater than about 15 dB at short wavelength or 10 dB

at long wavelength the effect of the mode scramblers is negligible (Table 3).

At short wavelengths, the spectral width of the laser may become important

when measuring long systems. For a 12km system, 2nm spectral width at 850nm

gives a material dispersion limited bandwidth of 183 MHz.

FIELD TEST EQUIPMENT

A few years ago no satisfactory test sets were available for field use and

Siecor had to develop a family of lightweight portable test sets. These include

the OTDR (Figure 3), a hand-held digital 1

i ghtmeter/mul t imeter, an insertion

loss test set (Figure 4) and a bandwidth test set which measures the ampl itude

response up to 400 MHz in the frequency domain (Figure 5).

Today, including Siecor there are at least three manufacturers offering

a complete line of test sets. In addition, there are numerous manufacturers

offering one or more test sets suitable for field use.

CONCLUSIONS

Field test sets for fiber optics have become available recently. Siecor

has measured over 1200 concatenated fiber strings end-to-end. Agreement between

measured and predicted attenuation readings has been excellent. The repeatability of end-to-end bandwidth measurements is about ±10% but predictability

of end-to-end bandwidth has not improved over the years.

40
TABLE 1. Fiber Optic Cable Field Tests

TEST METHOD

Prior to Installation Continuity Vi sual

Attenuat i on OTDR

Spl icing

Splice Attenuation OTDR

Connector/End Spl ice

Attenuat ion

1 . OTDR

2. Insertion Loss

After Installation

End-to-End Attenuation Insertion Loss

End-to-End Bandwidth 1. Swept Frequency

2. Pulsed

TABLE 2. Comparison of Calculated

and Measured Attenuation (7.4km)

TABLE 3. Typical End-to-End

Bandwidth (11km)

Fiber Calculated Measured

No. (Cabl e £ Spl ice) (dB)

(dB)

1 26.6 26.5

2 23.8 28.1

3 25.2 25.7

4 26.3 26.3

5 25.8 25.7

6 26.2 25.5

7 24.4 24.8

8 23.7 24.

1

Average 25.3 25.8

-3dB Optical End-to-end

Fiber Bandwic th (MHz)

No. Test Set A Test Set B

@ 1321 nm @ 1283 nm

No Mode SGS Mode

Scrambl er Scrambler

-> <-

1 135 127 120 118

2 130 137 140 120

3 139 113 110 110

4 105 115 118 100

5 105 115 100 100

6 167 178 150 150

7 105 108 110 110

8 105 114 105 105

9 115 128 115 108

10 109 121 120 105

Ave rage 122 126 119 113

41
OLTE FDF FDF OLTE

U FIBER CABLE SYSTEM ^

E/O §

h.

1 — O/E

>

—X X; X X X: Xr-

><J Fiber Connector

X— Fiber Splice

FIGURE 1. Generalized fiber optic transmission system.

FIGURE 4. Attenuation Test Set. FIGURE 5. Bandwidth test set for

both short and long wavelength.

42
IMPROVED AUTOMATED LOSS SET

FOR OPTICAL CABLES

L. S. Short and R. B. Kummer

Bell Telephone Laboratories

Norcross, Georgia 30071

INTRODUCTION

An automated loss set for optical cables (ALSOC)

has been described previously. 1 A new version (ALSOC III)

has been in use in the Western Electric lightguide manufacturing plant since August 1981. The new set has several

advantages; it eliminates uncertainty due to connector loss,

creates a steady state launch distribution as would occur in

the field, measures loss at two wavelengths, and detects

crossovers in a fiber ribbon automatically. The set also

contains an optical time domain ref lectometer (OTDR) and

visible HeNe laser for diagnosing high loss or broken fibers.

The loss of a 12-fiber ribbon can be measured at 2 wavelengths in 3 minutes with an accuracy of better than 0.1 dB.

A laboratory version which includes a time-domain bandwidth

measurement will also be described. A loss concatenation

experiment indicates that the set can be used to predict

accurately system loss.

GENERAL DESCRIPTION

Figure 1 is a block diagram of the test set. The

loss measurement is designed to approximate closely the loss

of a cable in an installed system. The optical sources are a GaAlAs laser at 825 nm and InGaAsP LED at 1300 nm. An undersized (small core, small A) launch fiber serves two purposes.

43
Its refractive index and core diameter were chosen to simulate a steady state launch distribution in the cabled fibers.

Furthermore, this undersized input fiber together with a large core, large A fiber on the detector end eliminates

intrinsic splice loss. Computer controlled microposi tioner

stages determine the optimum measurement coordinates of each

fiber in the ribbon under test, and this peaking procedure

eliminates extrinsic splice loss. The OTDR and the visible

HeNe laser can be used to troubleshoot a high loss or broken

fiber. This portion of the test set has been in use at

Western Electric since August 1981.

A time-domain bandwidth measurement was added for

laboratory system studies. Using the optimum measurement

positions from the loss measurement, the fiber under test can

be excited with either a pulsed GaAlAs injection laser at 825

nm or a pulsed InGaAsP injection laser emitting at 1278 nm.

The output pulse is detected by a Si avalanche photodiode

(APD) at the short wavelength or a Ge APD at the long wavelength, then sampled, digitized and averaged by the digital

processing oscilloscope (DPO). The microcomputer calculates

the fast Fourier transform (FFT) of the output pulse. The

transfer function of the cable is obtained by repeating the

measurement on the short reference ribbon and taking the

ratio of the two FFT 1 s . Both the OTDR and the loss measurement can be useful in the interpretation of bandwidth concatenation results.

44
RESULTS

A loss concatenation study was made on a 6 km link

consisting of 12 0.5 km 12-fiber ribbons with 11 splices.

Individual ribbon loss, splice loss, and total loss were all

measured. As shown in Figure 2, the actual concatenated link

loss agrees with the sum of the ribbon losses and splice

losses to within 0.2 dB. This indicates the test set is

simulating a steady state launch distribution and thus that

factory measurements can be used to predict accurately system

loss

.

ACKNOWLEDGEMENTS

The authors would like to thank C. E. Davis for his

work in the construction of the ALSOC III loss test set.

REFERENCES

1. L. C. Hotchkiss, "Automated Loss Measurement Set for

Optical Cables," Symposium on Optical Fiber Measurements,

1980, pp. 23-26.

45
on » y OfNIIAIOI

MICtO

COMPUTE!

FIGURE 1. AUTOMATIC MEASUREMENT SET FOR OPTICAL CABLES

25

0 £ 4 6 a -10 12 NUMBER OF SECTIONS

FIGURE 2. 825 nm LOSS CONCATENATION STUDY

46
Prototype system for automated measurements of

transmission properties_of graded index fibres

J.W. Versluis and H.P. de Wert

Optical Fibre Department, Glass development laboratory

N.V. Philips Gloeilampenfabrieken, Eindhoven, The Netherlands

Introduction

The accepted philosophy to measure all manufactured fibres requires increased

measuring speed to keep in step with the expanding production.

For this reason we have developed automated equipment in which we have

combined all transmission measurements. Besides man-independent measurement

conditions have been effected.

The system measures attenuation at three and bandwidth at two wavelengths,

length and numerical aperture in 7 minutes including the fibre cut-back to

2 metres.

The measuring techniques implemented for attenuation as well as bandwidth

employ the Philips preferred launch conditions . For both types of

2) measurement a stable modescrambler , based on microbending , is applied to

3) approximate steady state distribution of the 50/125 pm graded index fibres.

Several daily checks are built in, while weekly calibration measurements

applied to standard fibres ensure quality assessment of the measurements on production fibres.

System description

A block diagram of the system is shown in fig. 1. The Philips P 851 microcomputer is employed for general control and for part of the measurements.

The Tektronix WP 1310 system averages the pulses, stores them and performs

the FFT.

§2u££2_§D<Li§un£h optics

Special parts in fig. 2 are the fibre switch SW, the modescrambler MS and the

position detector P1, P2.

A

)

The switch employs the principle of the ball connector . The position of

the outgoing fibre F1 is controlled by a stepper motor. It turned out that

the fibre switch also acts as a modescrambler, limiting the far field

distribution from fibre FP1 and increasing those from fibres FP2 end FP3.

Three additional sources are possible.

47
This is a good start for the actual modescrambler employed to obtain a fair

approximation of the equilibrium mode distributions of the tested fibres.

Measurements over a long period of time showed a stable behaviour of the far

field distribution.

The intensity distribution is projected 1 to 1 on the fibre to be measured.

This stationary distribution, accepted for attenuation measurements, applied

to bandwidth, has shown a repeatability as favourable as for overmoded

launching by a step index fibre. Moreover the values thus obtained were

fairly well in agreement.

The position detection system P1, P2, determines the axial position of the

front end of the fibre as described in the measurement procedure below.

One micron stepper motors are employed. Source 22 is applied when the nominal

NA is determined using overmoded launching.

Detection and receive optics^ fig. 3

The position of diode D1 as shown (I) is applied for laying the fibre into

the vacuum chuck VC, and when pulse sampling or far field scanning takes

place. Position II is in use for all intensity measurements, including

alignment of the fibre.

The fibre end should be positioned accurately in axial direction, and has

to coincide with the centre of rotation of the rotatable stage as well.

Again the position detector P1, P2 is applied as described below.

A rotation of 60° in either direction allows projecting the output light

power on to an APD.

Measurement procedure

1. Insertion and alignment of the fibre.

5) The cleaved ends of the fibre being tested are examined separately

on quality with a microscope.

Experiments showed that a video monitor was insufficient to assess the

quality of the cleaves effectively. Primary coatings (with a higher re- fractive index than that of the cladding) are not removed when cleaved.

The accepted fibre ends are simply laid into the vacuum chucks, which

are mounted on XYZ-stages moved by stepper motors. After the entry of

fibre identification, geometrical data and coating type the program is

started. The chucks now move in axial direction until the fibre front

ends interrupt a fine light ray. This ray is part of the axial position

detection system P1, P2 (fig. 2 and fig. 3 ).

48
the chuck positions, and are adjusted axially. Part of the launched light

is now captured by the launch end of the fibre, which part is aligned for

maximum optical output. This automatic procedure has a radial repeatability

of one /jm or better. The repeatability of the fibres in the vacuum chucks

and the eccentricity of the coating render additional radial optimation at

the detector end unnecessary. Axial movement at the launch end are made to

correct any focal dispersion of the 1 to 1 imaging system.

2. Bandwidth

Firstly pulses of the 1300 nm laser are taken in, averaged and stored after

manual search and amplitude adjustment. Then a correction for the background

level is made while the fibre is being rotated to the silicon-APD.

The fibre switch selects the 850 nm laser source and the same steps are repeated . Besides pulse delay time is measured and the fibre lenght calculated. The WP 1310 now starts calculations using reference pulses that

were stored after the daily checks (virtually no deviations in pulse form are found between checks )

.

3. Attenuation

Meanwhile intensity measurements take place at 850, 1300 and 1600 nm controlled by the P851. The cooled germanium-detector is in position II of

fig. 3 and the output of the lock-in amplifier is averaged several times.

After the cutting back of the fibre to 2 metre without disturbing the

position of the fibre at the launch end, measurements on the above wavelenghts are repeated.

4. Numercial aperture

With source S2 the 2 metre fibre is then used to measure the far field.

The 3 % intensity angle determines the nominal NA.

Results and performance

Manual actions are reduced to simple operations. The only difficulty is the

assessment of the quality of the cleave. Qualification tests in conjunction

with the use of current measuring equipment have not shown any significant

deviations. Spread in bandwidth measurements is small, attenuation showing

larger deviations. This deviation is probably due to the fairly low signal-tonoise ration of the germanium detector, and has to be improved.

The manual search during the bandwidth measuremnets will be replaced by the

setting of a programmable delay lime in conjunction with the measured delay

time.

49
Literature

1. Wire Industry, 49, no 479 (March 1982), p

16*4

2. J.W. Versluis, J.G.J. Peelen, Optical communication fibres. Manufacture

and properties, Philips Telecommunication Review, 37, no 4 (Sept. 1979),

p 215

3. A. Diekema, L.H.M. Engel, G.A.M. Goltstein, P. Mathijsse and J.W. Versluis,

Attenuation of optical fibres for the Eindhoven - Helmond field trial,

Symposium on Optical Fiber Measurements, 1980, Boulder, Colorado, p 27

4. A. Nicia and A. Tholen, High-efficient ball-lens connector and related

functional devices for single-mode fibers, 7th European Conference on Optical Communication, 1981, Copenhagen, 7.5.

5. D. Gloge, P.W. Smith, D.L. Bisbee and E.L. Chinnock, Optical fibre end

preparation for low-loss splices, B. S.T.J. 52, no 9, (Nov. 1973), p 1579.

drivers

monochrom fibre switch filter wheels stepper motofs

lock- in amplifier PHILIPS

P851

microcomputer

TEKTRONIX WP 1310 hard copy

positi on

detector operator

entry delay

generator timer counter

Fig 1 Automated quality measurement system

S1,S2 halogen lamps F fibre under test Ll-6 lenses VC vacuum chuck

A1,A2 apertures XYZQ stages

FW1 } FW2 filterwheels Dl cooled Ge-diode

M monochromator D2 Ge-APD

C1,C2 choppers DS Si-APD

FP1-3 fibres L1,L2 lenses S3,S4 laser diodes A aperture SW fibre switch P1,P2 position detection system MS mode scrambler (adjustable)

L4,LS objectives 40x0,65

BS beam splitter

vc vacuum chuck

F2 fibre under test P1,P2 position detection system

50
Precise Measurement of Optical Fiber Breaking Elongation

K. Matsui, S. Tanaka and M. Hoshikawa

Sumitomo Electric Industries, Ltd.

1, Taya-cho, Totsuka-ku, Yokohama, 244, Japan

1. Introduction

In optical fiber technology, it is important to characterize mechanical properties precisely in addition to transmission properties. Especially fiber elongation is one of the important mechanical characteristics in

evaluating the fatigue phenomenon and predicting the mechanical lifetime.

However the conventional tensile tests based on the mechanical systems are

not always sufficient ones for the precise measurement of breaking elongation, due to the following reasons;

(1) There are deviations from linearity in the dependence of glass

elongation on stress. ^

(2) It is difficult to correct the contributions of catenary in the long

fiber span during tensile test.

(3) It is difficult to correct the stress relaxation in jacketing mater ials.

In order to solve these problems we propose a utilization of modulation phase of optical signals for the direct and precise detection of the

fiber elongation during tensile tests and at fracture.

2. Measurement System

(1) Tensile testing machine

The diagram of the testing machine is shown in Fig. 1. The tensile

testing machine is designed to satisfy the following features;

1) Optical fiber is held tightly around the 8 cm diameter mandrels

covered with rubber. Optical fiber is wound around them three times

51
and fixed by adhesive tape. Fig. 2 shows the illustration of the

chucks. Slip of fiber during load is 1 mm at most. As fiber

elongation is measured directly by modulation phase method described

later, the smallest slip mentioned above does not influence the

elongation measurement. Thus error of elongation strain is extremely

small. Loss increase is not observed during these procedures.

2) Pulling speed is controlled to ensure the constant strain rate by a

variable-speed motor.

(2) Fiber elongation measurement by modulation phase method

The measuring arrangement of fiber elongation is shown is Fig. 3. The

essence of the measurement is the detection of optical path length change

by means of modulation phase change. A sinusoidally modulated optical

signal is transmitted through the test fiber, and the phase change of the

received signal, which is related to the optical path length change, is

continuously monitored by a vector volt meter during the tensile strength

test. The physical fiber elongation A& is calculated from the observed

phase change AG (deg) by the following equation,

A „ =

1 c A0 ,,, n

< 7~Z

X

Nf X

360 ' (1)

{1 -

2 [p i2 - V(Pn+ P12 ) ] }

cl n where N is the group index (= n - X~) , f the modulation frequency, c the

dA

velocity of light, p n and p 12 the strain optic tensors, and v is the

Poisson's ratio of the glass fiber. At a frequency of 800.000 MHz, which

is usually used in our measurement, equation (1) becomes 0.884 mm/deg,

yielding the high accuracy of fiber elongation measurement.

3. Experiment

Fig. 4 shows an example of tensile breaking strain measurement of

optical fibers. The experiment was conducted under the fiber gauge length

52
of 10 meter and the strain rate of 5% a minute. The modulation frequency

is 800.000 MHz, and the change in detected modulation phase was recorded

within 1 degree up to the instance of fiber fracture. Consequently the

measurement accuracy of tensile strain against the whole fiber length was

within 0.01%. Fig. 4 clarifies the fine difference in fiber breaking

strains with different coating structure.

4. Conclusion

The elongation of optical fiber at fracture was measured with high

accuracy (below 0.01%), by optical modulation phase method. Thus it

becomes possible to characterize the mechanical properties of optical

fibers precisely upon the basis of these data and to predict the lifetime

of optical fibers with high precision.

Reference

[1] J.T. Krause, L.R. Testardi, and R.N. Thurston, Physics and Chemistry

of Glasses, 20, 135 - 139 (1979).

rope0-

mandre 1

f iber

mandre 1

load cell D C

Amp recorder

mandre 1

Fig. 1 Over view of tensile

testing apparatus.

Fig. 2 Illustration of fiber

chuck

.

53
fusion splice test fiber fusion splice

LD J

Lock in synthesizer

-d [ APD

tensi le —

force

Vector vol t meter

Chart Recorder

Fig. 3 Measuring arrangement of fiber elongation

by modulation phase method.

/ = 8 0 0.0 0 0MHz

tensile breaking strain (#)

Fig. 4 Tensile breaking strain of optical fiber

These fibers were produced by the VAD method

and the diameter of the cladding, which consists of

silica, is 125 urn and the diameter of the core,

which consists of silica doped with Ge, is 50 ym.

The fibers were coated with silicone resin and

jacketed with nylon further.

54

...
PRECISE MEASUREMENT OF STEADY-STATE FIBER ATTENUATION

P.J. Vella, K. Abe and F.P. Kapron*

Bell-Northern Research, Ottawa, Ontario

Canada, K1Y 4H7

INTRODUCTION

Various light launching techniques [1] have been employed to measure

fiber attenuation accurately and reproducibly . One of the reasons behind such efforts stems from the need for accurate prediction of fiber

link attenuation (as well as fiber bandwidth). In this paper, the

'steady-state' attenuation is recognized as a unique fiber parameter and

problems associated with its exact measurement are addressed. Furthermore,

link attenuation predictability is evaluated based upon the steady-state

measurements of individual fiber sections.

STEADY-STATE ATTENUATION

It is well known [2] that, in general, the power decay, P(L) in dB, along

an optical fiber is not a linear funcion of the distance, L, from the light

source. This is illustrated in Fig. 1 where observed light power decay

curves are shown for fibers excited with a LED and an injection laser diode

(ILD) . The output power observed by cutting back the fiber every 50~100 m

was fitted to the following equation:

PfL) = P(0) - a L - P, [1 - exp (-L/L )].

S L t

where all powers are in dBs , and attenuation rates in dB/km.

This equation was derived by assuming that the net attenuation rate could be

written as the sum of the steady-state (quasi-equilibrium) attenuation rate

a

g

and a transient attenuation rate a

t exp (-L/L

t

)

, the latter contribution

decaying with a characteristic length L

t

. In the equation, P

t (=a tL t

) is a transient loss whose existence gives rise to the following problems in

attenuation measurements. (1) A conventional two point cut-back method may

give attenuations that depend on fiber lengths and light launch conditions.

Therefore the measured attenuation is not a unique fiber parameter.

(2) The multiple cutback method of measuring as is a destructive way of

determining the expected unique fiber parameters. Therefore, it is imperative

to devise light launch conditions that do not give rise to transient effects

and to ascertain that the steady-state attenuation is really a unique fiber

parameter independent of launch conditions, lengths, etc.

Experimentally, it was established that the transient loss arises from the

following two effects: incomplete cladding mode stripping (found to occur

even with fibers coated with higher index plastics) and leaky and/or lossy

higher order modes. In order to eliminate the transient loss, an approximately

15 cm long section of the fiber coating was stripped near the launch end, shaped

into a loop and immersed into oil whose refractive index was 1.514 (N^) . With

the above stripping technique, several multiple cutback measurements have shown

that transient losses of less than 0.05 dB were consistently attainable regardless

of the light sources and launch conditions employed. The steady-state

attenuation was thus precisely measurable within 0.05 dB using this simple

stripping technique and the two point cut-back method.

55
-2-

LINK PREDICTABILITY

The steady-state attenuation of nine fiber sections was measured using the above

"loop" stripping technique . Then the fibers were spliced together to form

a 17 km link. Using an ILD source, the optical power was sequentially measured

at the output and input ends of individual fiber sections. A conventional

stripping technique, which consists of removal of the coating for a 10 cm long

section followed by immersion into the oil, was used for the power measurements

at the output ends. For the input end measurements, both the conventional

and the "loop" stripping methods were used. The measurement results are summarized in Table 1 listing insitu steady state attenuations (A-^) , two point

cut back attenuations measured with the conventional stripping method (Ac) , splice

losses including the transient loss (S^) and splice losses measured with the

conventional stripping method (S c ) (See Fig. 2). The good agreement between

the steady-state attenuations measured individually (As

) and in the links (A^)

shows that the steady-state attenuation is indeed a unique fiber parameter which,

together with the average splice loss (which includes transient loss) , can

be used to accurately predict link attenuation.

CONCLUSION

In summary, a simple method for measuring the steady-state attenuation of

optical fibers has been demonstrated. The technique was found to give

attenuation values consistent with measurements within a concatenated link.

Good link predictability was achieved based on the steady-state attenuation

by including the attenuation transient loss into the net splice loss.

REFERENCES

[1] M. Tateda et al, Appl. Opt., 18, 3272 (1979); M. Eve et al,

Opt. Quant. Electr. , 10, 253 (1978); G.T. Holmes, Techn. Digest,

6th ECOC, p. 144, York 1980; T. Le Hiep and R. Th. Kersten,

Opt. Comm. 40 (2), 111 (1981).

[2] V.V. Grigor'yants et al, Sov. J. Quant. Electron., 8 (9), 1161,

(1978); M.J. Adams et al, Opt. Comm., 17 (2), 204, (1976).

^Current address, ITT, EOPD, Roanoke, Virginia, 24019.

56
Table 1 ; Measured in situ attenuations (A-p and splice losses

A A, A S. S Link s 1 c 1 c Length (km) (dB) (dB) (dB) (dB) (dB)

0

A

s AJ.l

A

(dB) (dB) (dB)

5.61 5.43 5.72

5.53 5.57 5.81

5.51 5.54 5.69

5.73 5.82 5.96

5.65 5. 77 5.84

5.51 5.42 5.52

3.42 3.40 3.42

4.25 4.97 5.06

5.31 5.49 5 . 44

0.25 0.01

0.24 0.09

2

4

6

8

10

12

13.3 0.14 0.05

4.:>5 4.9 7 5.06

15.1 0.15 0.20

17

TOTAL LOSS(dB) 47.22 47.41 48.46 1.44 0.68

0.19 0.05

0.15 0.08

0.25 0.15

0.07 0.05

57
Fig. It

o

CL

2

Power decay curves for graded - index

BNR fibers with a LED and an ILD light

sources

.

0.6

LENGTH, L (km)

a.

Schematic for power decay for a fiber

section within the 17 km link. The

subscripts 1 and c demote loop and

conventional stripping.

.jr splice

LENGTH (km)

58
MODAL BEHAVIOUR OF VARIOUS MODE MIXERS AND MODE FILTERS FOR OPTICAL

FI8ER MEASUREMENTS

A.K. Agarwal, H. Karstensen, U. Unrau

c/o Institut fur Hochf requenztechnik , Techn. Univ., P.O. Box 3329,

D-3300 Braunschweig, Germany (Fed. Rep.)

ABSTRACT

A bearing-ball-bed mode mixer is compared with mandrel wrap, 707. excitation and newly developed taper mode filters in the approximation of the

stationary mode group power distribution (SMPD) of different fibers. The

mode group power is computed from the near-field intensity pattern (NFP)

and compared with results from differential mode delay measurements.

1.0 INTRODUCTION

Because of differential mode attenuation and delay, the characteristics

of multimode fibers depend strongly on the mode power distribution (MPD).

Fiber data can be extrapolated with sufficient accuracy to arbitrary

lengths only if the fiber carries its SMPD which is the statistical mode

of lowest attenuation. Unfortunately, SMPD is reached in low loss fibers

with low microbending only after several kilometers.

Standardized measurements require SMPD excitation. Various techniques

have been proposed to approximate SMPD at the fiber input by restricted

mode volume launching l\,2l, mode filters /3,W and mode scramblers

/5,6/. SMPD check was made either by radiation pattern comparison or by

concatenation experiments. The last method gives a good feeling for the

accuracy of attenuation measurements but not necessarily for the accuracy

of SMPD launch because of extra mode mixing at splices. The radiation

pattern inspection is highly inaccurate: the main information on the

power carried by higher order mode groups is contained in the slope of

the radiation pattern at its low intensity boundaries and cannot be ex- tracted properly by simple pattern inspection.

SMPD is characterized by its insensitivity to variations in launching

conditions. MPD's of typical low loss MCVD fibers 2-3 km in length are still dependent on excitation. However, most measurements have to be

made on such or even shorter pieces of fiber. Generally, two excitations

are prevalent: a) incoherent uniform excitation for attenuation, and b)

small mode voTume excitation by laser diodes for pulse delay measurements. In both cases the MPD profile has to be shaped by mixers or filters to approximate SMPD. It is the aim of this contribution to show

the degree of SMPD approximation achieved by various devices in both

cases

.

2.0 MEASUREMENTS

We used a computerized measurement set-up II, 8/ which derives the mode

group power distribution from the derivatives of the NFP. Assuming equal

power distribution between the modes belonging to a mode group the MPD is

known. The experiments were performed on three different fibers of which

the manufacturer specs are given in Table 1. Figs. 1a, b, c show re- sults for fibers no. 1, 2, 3, respectively, in case of uniform excita59
tion by a halogen lamp filtered to 8 2 0 + / - 1 0 nm wavelength. We depict the

difference < A >MPD between the actual MPD and an ideal uniform MPD vs. the relative mode group number m/M. Three well known arrangements were

compared: the PHILIPS bearing-ball-bed mode scrambler /6/, BELL s man- drel wrap mode filter /3/ and the 70 7. mode volume launch. In addition, curves for a new taper mode filter are given. All devices were measured

with 10 m of fiber.

Fibers 1 + 2 are quite alike, and so are Figs. 1a + b. Fiber 1 was found to exhibit slight deformations; this obviously causes less power

content of the lower mode groups and a steeper roll-off of the MPD-curve

after 2.2 km in comparison to fiber 2. However, as it will be explained

later, it is felt that both curves already approach SMPO quite closely.

Compared to the quasi-SMPD curves the mandrel wrap filter shows a more

sharp cutoff in higher mode groups. Shown here are curves for a mandrel

of 12 mm O.D. with 5 turns of loosely wrapped fiber. Larger O.D. s shift the cutoff to higher m/M values but the slope remains nearly con- stant (curves at the conference). On the other hand the restricted mode

volume excitation (707. A., and spot size) causes a too gentle roll-off, as

N already noted in /9/.

Only the scrambler and the taper filter match the quasi-SMPD curves quite

closely. It was found that the scrambler always produces quasi-SMPO if

set for approx. 1.6 dB loss, independent of the inserted fiber length.

(For construction details see /G/). If integral power comparisons are made it must be reminded that the mode content increases with mode group

order. The scrambler not only converts guided to radiation modes but

also converts power from higher to lower mode groups. Thus it has a good

loss budget. The biconical taper filter was made by heating the fiber

under tension. Tens of tapers with different diameter ratio and length were produced. It was found that a ratio of 0.6 and 1 - 2 mm of length

always give quasi-SMPO in 50/125 fjm Gl-fibers. Mainly, the ratio determines the cutoff, length and smoothness have further influences on cutoff

an slope. Fig. 1c shows generally the same results with only 900 m of a dif f erent f iber

.

Fig. 2a gives similar curves for fiber 2, but now for laser-diode excitation at 900 nm. The far-end MP0 shows still the same low-order mode

group dominance as the input MPD; only the higher mode groups are more

attenuated. Obviously, intrinsic mode mixing on 2.2 km did not make up

for the SMPD. The mandrel wrap filter again provides only an even sharper cutoff. Only the mode scrambler, now set to approx. 3.0 d8 attenuation for central excitation, fills the mode volume to a certain ex- tent but not up to the quasi-SMPD of Fig. 1b.

Nevertheless, the effect is quite substantial as it can be checked by

pulse delay measurements. For that purpose LD pulses were fed to a mono- mode fiber (a = 3.0 jum, A

N

= 0.11) which excited the test fiber at various

radii. From the measured pulse broadening a mode group bandwidth distribution (MG8D) was calculated by Fourier transform. Fig. 2b depicts MGBD

vs. m/M. The bare fiber shows a large bandwidth spread over 1.2 GHzkm

which reduces to 0.7 GHzkm if it is wrapped 5 turns around a 12 mm O.D.

60
mandrel. The use of the scrambler flattens the variation to 0.3 GHzkm

around 1.2 GHzkm which exceeds the specs by 357.. If the scrambler always

produced a SMPD at its output independent of the monomode fiber position

at its input, no variation in MGBO would be noted.

3.0 DISCUSSION

SMPD in a fiber is reached if due to mode mixing arid attenuation the

average power distribution among the mode groups remains constant. At

SMPD in average all modes are attenuated and delayed uniformly (statistical mode). Because of the actual loss of higher order modes the SMPD de- creases as a function of m/M. By comparing the 3 curves in Figs. 2a and

2b it is seen that the scrambler should show a more gentle roll-off to

produce SMPD. Going back to Fig. 1b with this information one sees that

the far-end, the taper and the scrambler curves approach SMPD quite

closely. Since the scrambler is set here for low loss, SMPD is obviously

reached more easily by tailoring a uniform excitation instead of mixing

up a low mode volume.

The usefulness of the mandrel wrap filter for standardized attenuation

measurements has already been proven 131 despite of its too steep MPD

slope. It can therefore be concluded that attenuation measurement accuracy does fortunately not depend on a very precise SMPD launch. The

ball-bed scrambler and the taper should give even better comparability of

measurements; especially a prefabricated taper to be connected to the

fiber under inspection would be a very handy precision device. In pulse

delay measurements the ball-bed scrambler is so far the only candidate to

achieve comparable results.

4.0 ACKNOWLEDGEMENT

The authors wish to thank Ir. J.W. Versluis of PH I L I P S / Eindhoven for

providing the scrambler and fiber 3, Oipl.-Ing. G . Maltz of

KABELMETAL/Harinover for lending fibers 1 and 2 and, the German Research

Foundation DFG for funding this work.

5.0 REFERENCES

IM Kaiser, P., Trans. IECE Japan 6 1 (1 978), 225 229.

Ill Eve, M. et al., Opt. and Quant. Electron. 10(1978), 253.

131 Cherin, A.H. et al. .Fiber and Integr. Optics 4(1982), 49 - 65.

/4/ Le Hiep, T. and Kersten, R.Th., Opt. Comm. 40(1981), 111 - 116.

151 Kaiser, P., Symp. on Opt. Fiber Measurements, Boulder 1980, 11 - 14.

I&l Versluis, J.W. et al., Philips Telecom. Rev. 37(1978), 215 - 230.

Ill Unrau, U. et al., Conf. Messtechnik in der optischen Nachrichtentechnik, Berlin 1980, 173 - 175.

/8/ Agarwal, A.K. et al., 3rd I00C, San Francisco 1981, 74 - 75.

79/ Calzavara, M. et al., Electron. Lett. 1 7 , No . 15. 1981. 543 - 545.

61
0.5

i

Far-end —2.22 km

Fiber *1

(a)

m/M

v. x „ Mode

scrambler

Taper ^ S

NV

1

I Fiber 1 Fiber 2 Fiber 3

j CLTO, CLTO, Phil ips

,

Manutacturers Manufacture* r <=;

i Bran ce IT1 v -\ io j-i /-\ r 1 ct i i (_ c n o 1 1 a no Length (m) j 2 2 20 y uu

A

N

! n 1 1) U , u

r\ -1-7

T r, q c IHR/kllll I 9 1 9 1 Core/Clad (pm) ' 50/125 ; 50/1 25 50/1 2 5 Bandwidth (MHzkjn) 684

1 874 641

Table 1

Fig

70V. Launch

_/L _ 0.5

2a AMPD vs. m/M with LD input

excitation (fiber 2)

Bore fiber

Fiber +3

Fig. 1 Difference in mode power

distribution AMPD to equal

excitation vs. relative mode

group number m/M with uniform

input excitation

0

Fig. 2b Mode group bandwidth distribution (MGBD) vs. m/M

with LD input excitation

(fiber 2)

62
INFLUENCE OF DIFFERENTIAL MODE ATTENUATION ON BACKSCATTERING

ATTENUATION MEASUREMENTS

M. Eriksrud, A.R. Mickelson, S. Lauritzen, N. Ryen

Electronics Research Laboratory

Norwegian Institute of Technology

N-7034 TRONDHEIM-NTH, NORWAY.

Although there have been laboratory tests of the repeatability of

backscattering attenuation measurements^, there has yet been no

careful assessment of the sensitivity of the backscattering technique

(2-3

)

to launching conditions. It has been shown that a mode-filtering technique can be used to eliminate power fluctuations due to

slowly varying fiber parameters. The resulting power decay function

then represents a measure of the actual fiber loss. However, graded

index multimode fibers can also exhibit rapid parameter variations

and differential scattering and absorption effects which cause excess

loss. The present work applies some simple modelling of the differential mode attenuation process to interpret the results of backscattering and cut-back measurements of the variation of attenuation

of telecommunication grade fibers with launching conditions.

Fig. l.a illustrates a set of backscattering attenuation curves

(loss functions) obtained from measurements from either end of a

50 ym/0.21 NA graded index fiber. As can be seen, slight changes in

attenuation values are observed with different mode-filters. The

mode-filter which most heavily supresses higher order modes (in forward and backward directions) yields the highest loss value. This

is in agreement with the cut-back attenuation measurements (Fig. 2. a)

63
which indicate that lower order modes of this fiber have higher loss

than the medium order modes. The excess loss due to lossy highorder modes is not present on the backscattering curves.

Fig. l.b illustrates a similar set of curves obtained from a

50 ym/0 . 2 NA fiber of another manufacturer. This fiber has very

different differential mode attenuation characteristics than the

fiber of Fig. l.a, as is shown in Fig. 2.b. Higher and medium order

modes have substantially higher loss than lower order modes, and

therefore, by employing mode-filters of successively smaller mode

volumes, the loss, as measured by backscattering, is reduced.

As is illustrated by the above example, the measured attenuation of

different fibers can be affected in different ways by changes in the

launching conditions. Backscattering attenuation measurements performed with various mode-filters can effectively be used to evaluate

the distribution of losses within the fiber. Further discussion of

this point will be presented at the conference.

References

1. B. Costa, B. Sorda, U. Menaglia, L. Piccari and G. Grasso:

"Attenuation measurements performed by the backscattering

technique", Electron. Lett., Vol. 16, pp 352-353, 1980.

2. A.R. Mickelson and M. Eriksrud: "Theory of the backscattering process in multimode optical fibers", Appl. Opt.,

Vol. 21, pp 1898-1909, 1982.

64
M. Eriksrud and A.R. Mickelson: "Application of the backscattering technique to the determination of parameter fluctuations in multimode optical fibers", to appear in the October

issue of IEEE J. Quantum Electron.

FibeT A

/©(D

a)

Fib er B

XD

XD

(?)

b)

1.5/iS/div

Fig. 1. Backscatter power loss function (A=840 nm) obtained from the

backscattered power curves, measured from either end of the fiber

1) without a mode-filter,

2) with a 100 m long fiber pigtail with a minimum 45 ym core and 0.20 NA as a mode-filter,

3) with a 100 m long fiber pigtail with a minimum 30 ym core and 0.18 NA as a mode-filter.

The corresponding average loss values are indicated in Fig. 2.

65
6.5

6.0

I

5-5

(0o

5.0

o 80 /xm • 36/tm

14^m

4.50.05

i

i i i 0.10 0.15 0.20 Launch numerical aperture

0.25

5.0

4.5

CD^ 4.0

(0

(0o

w^3.5

3.0 -

2.50

o 80 tan • 36 fan

Fiber B

L = 1.06 km

05 0.10 0.15 0.20 0.25

Launch numerical aperture

a) b)

Fig. 2. Attenuation (A=840 nm) measured as a function of launching

conditions with a reference (cutback) length of 2 m, using

beam optics launch with spot centered on the fiber core

(50 ym diameter). Spot diameters of 80, 36 and 14 urn were

employed.

66
\ MEASUREMENT OF OPTICAL FIBRE ABSORPTION LOSS: A NOVEL TECHNIQUE

R. KASHYAP and P. PANT EL IS

BRITISH TELECOM RESEARCH LABORATORIES, MARTLESHAM HEATH, IPSWICH, IP5 7RE, UK.

INTRODUCTION: There is increasing evidence that absorption loss between 1 and

1.6 ym is not negligible in OH free single mode fibres [1]. However, the

absorption loss contribution to total attenuation in optical fibres has become

difficult to assess with the achievement of lower losses. There are a lumber

of reports [2-5] on methods of measurement, with one achieving a loss of less

than one dB/km using a calorimetric method. We present a novel calorimetric

technique, using a pyroelectric sensor material, polyvi nyl idene fluoride

(PVF ), which has the advantage of having a large pyroelectric coefficient

and can be shaped to ideally suit this application. Using this material it is

possible to measure sub-dB/km absorption losses with increased sensitivity

with a few hundred milliwatts of laser power.

SENSORThis consists of a tube of PVF^, appropriately poled to produce a large pyroelectric coefficient, which is made at BTRL (Patent applied for).

The tube used was 500mm long, with an OD of 1.409 mm and ID of 0.941 mm. It -2 -1

had a measured pyroelectric coefficient [6] of 29 yC m K . It is

coated with conductive paint and held horizontally in a close fitting copper

tube. Intimate thermal contact between the two is ensured by further filling

the space with silver loaded conductive paint. This assembly is thoroughly

lagged in a rigid insulating polyurethane foam box to isolate it from

environmental effects. Access is easily available to both ends in order to

allow the filling of the tube with water. The water performs two functions.

Firstly, heat is conducted from the fibre to the sensor by the water, and

secondly, it acts as the inner conductor for the accumulation of the charge.

Since the sensor is an electrical insulator, water has a comparatively low

enough resistance to introduce no significant errors in the measurement. The

charge from the inner surface is picked up by a 50 micron diameter wire, one end of which is inserted into the tube to make contact with the water, and the

other end fixed to a standard 50 ohm co-axial connector. The signal pickup end

is enclosed in a small aluminium box to act as the earth terminal in

conjunction with the copper tube. A low noise co-axial lead takes the signal

to a simple charge amplifier (Kistler 5007), capable of measuring charges

below one pC. The output of the amplifier is displayed on a cha rt -recorder.

Piezoelectric effects in PVF are easily filtered by a low pass filter.

67
MEASUREMENT TECHNIQUE: Since the measurement relies on the pyro-el ect ric

effect in a PVF^ tube, a large surface area is desirable. Hence, for a given

tube diameter, a longer tube will produce a larger charge per unit rise in

temperature. Since the temperature rise is the same for every point in the

short length of a low loss fibre (when the absorbed power << input power), for

a given input power there is a linear increase in sensitivity with length.

The sensor was chosen to be 0.5m long, on grounds of convenience and physical

constraints. Precautions similar to those in Ref. 4 are taken to remove cladding modes to minimise errors. The tube is filled with water prior to

insertion of the fibre. The far end is also mode stripped and terminated at a power-meter. The laser is switched on and the charge generated measured by the

charge amplifier. Since this is essentially a quasi-static charge measurement

which requires care, well established techniques are exploited [7,8],

THEORY: The absorption in the fibre is transmitted through the water to the

inside wall of the sensor. The sensor tube detects a temperature difference

across its walls. In order to ensure that the outside wall is unaffected by

scattered radiation from the fibre, it is thermally clamped in a copper tube

of thermal capacity much greater than the PVF^ tube. This would otherwise

pose a problem owing to absorption in the conductive paint. In addition, the

copper tube defines the boundary condition for the solution of the heat flow

equation. An approximate analysis of the structure shows that for the

dimensions of the sensor:

7 Absorption Loss, a = 3.384 * 10 * X / ( y * P) dB/km ...1

where, y is the charge response of the sensor in pC/K, X is the initial slope

of the generation of charge curve in pC/s, P is the measured optical power in

mW at the output of the fibre. Using a value of 52.81 nC/K for y, we get:

a = 640.8 * X / P dB/km ...2

RESULTS: To calibrate the sensor, a silica tube of 200 micron 0D with a resistance wire through an 80 micron hole was used to dissipate a known amount

of power using an ac current. Fig 1 shows the charge build-up with time after

the current is switched on. The form is exponential in agreement with theory.

Since the measurements are qua si -static, it is preferable to measure the

initial rate of charge build-up than the final value of charge. Fig 2 shows

the linear relationship of the initial slopes (as in Fig 1) vs. the power

dissipated. The slope of this graph is 11.5 (nC/s) per watt of absorbed power.

It also shows that a measurement of 0.1 pC/s is possible where the Johnson

noise for this sensor (with a 1Hz BW) is about 8%. Using the data on the

68
sensor and the calibration slope, the equivalent measured value for loss is: a = 755.2 * X / P dB/km ...3

which compares reasonably with the theory.

Optical measurements were made on a high loss fibre to investigate the

performance of the technique. A Nd:Yag laser operating at 1.06 microns with a 1.5 W CW output was launched into a 1 ow-moded high A fibre. Fig. 3 shows a typical output. The measurements were performed for various input powers to

simulate fibres of varying losses. A spurious effect was noticed at the start

of the charging curve for all the measurements. The origin of the sharp

initial rise, which is independent of input power, is currently under

investigation. Fig. 4 shows the result of the subsequent slopes plotted

against input power. The slope gives an absorption loss of 3.1 dB/Km. for this

fibre. The lowest loss measurable for an input power of 1 W, and a best

measurement of 0.1 pC/s, is 0.08 dB/km, which corresponds to a temperature

rise of about 2 yK/s. This value can be improved by increasing the length of

the sensor, by a higher pyroelectric coefficient and by reducing the tube wall

thickness.

CONCLUSION: We have demonstrated a novel technique for measuring low

absorption losses, with a present measurement capability of 0.08 dB/km for 1 W

launched power. It is simple and reasonably easy to use. Further results on single mode fibres will be presented.

REFERENCES:

1. I. GARRETT, C.J. TODD, Opt .Qua nt . El ect ron. 14 (1982)95-143

2. T.C. RICH, D.A. PINN0W, Appl. Phy. Letts 20 (1972) 264-266

3. K.I. WHITE, J.E. MIDWINTER, Opto-el ect ronics 5 (1973) 323-334.

4. K.I. WHITE, Opt. Quant. Electron. 8 (1976). 73-75.

5. K.I. WHITE et. al . , Symp. on Opt. Fibre Meas., Boulder, 1980

6. G.R. DAVIES, University of Leeds, Private communication.

7. "Electrometer Measurements", Keithley Instruments.

8. G.S. SPESCHA, E. V0LLE, "Ki stl er Piezoelectric Measuring Instruments".

69
s/Qd 'ajej dn p||nq a6jeqo

3d 'dn p|mq a6jeip Qd 'dn p|inq a6jeqo

70
Single-mode Fiber Measurement in Japan

Kiyoshi Nosu

Yokosuka Electrical Communication Labovatory, NTT

1-2356 Take Yokosuka-shi Japan

1. INTRODUCTION

In recent years, single-mode fiber manufacturing technique

has made a remarkable progress in producing low-loss fibers. The

mass production of low-loss single-mode fibers stimulated the

progress in peripheral techniques surrounding single-mode fiber

transmission. The establishment of low-loss light coupling into

a single-mode fiber, low-loss connecting, and splicing supports

the large-capacity transmission through a single-mode fiber for

the longer repeater spacing. In Japan, a 400Mb/s fiber transmission system is now on a field trial conducted since 1980 by

NTT [1] . On the system design of single-mode fiber transmission, it

is essential to establish the measurement techniques for fiber

transmission characteristics and device performances. This paper

reports the measurement techniques for single-mode fibers in

Japan

.

2. FIBER LOSS MEASUREMENT

Fiber loss is the most important factor to determine a repeater spacing in single-mode fiber transmission. For singlemode fiber loss measurement, it is unnecessary to apply an exciter

or a dummy fiber, which is popular in multimode fiber loss

measurement, as a single-mode fiber guides only one

71
fundamental mode. The measurement is raher simpler. Neverthless, in order to keep a wide dynamic range with high resolution

in loss measurement, a light source is required to output a high

power and to couple it efficiently with a single-mode fiber,

keeping power level stable. In our measurement systems, the

dynamic range is about 70 dB at 1.3 urn and 1.5 um bands by using

a laser diode light source. The temperature dependence of an

light source output level is less than 0.1 dB over the range of

10° C to 40°C. The light source adopted the confocal lens

system [2] for efficient coupling and output level stableness. A

Peltier effect device were applied to LD temperature control and

APC loop to the output power stabilization.

For wide wavelength range loss measurement, a halogen lamp

is a popular light source. Figure 1 shows an example of singlemode fiber loss spectra spreading from 1.2 um to 1.7 um, measured

by the lamp. The dynamic range is more than 15 dB over the

whole wavelength range.

Another way to measure fiber loss like OTDR will be discussed later.

3. INSERTION LOSS MEASUREMENT

Fundamental configuration for insertion loss measurement is

the same as for fiber loss measurement. There are, however,

some issues concerning measurement accuracy or reproducibility.

One is whether a measurement object is polarlization sensitive or

not. The sensitivity causes measurement errors when source light

is polarized. Figure 2 shows a principal structure of depolarizer [3]. This device was designed to get reproducible

72
measurements in the insertion loss measurement sensitive to

polarization. In this device, birefringence material gives the

path length difference to ordinary and extraordinary rays so as

to remove the coherency between the rays. As a result, the

depolarized output light is composed by combining the two rays

which are already unable to interfere with each other. This

device is effective to remove the polarization sensitivity from

the measuring systems.

Other issue appears in connecter loss measurement. Figure 3

shows connector loss measurements in two cases of LD and LED

light surces. The connectors are of FA- type [4] designed to be

applicable both to single- and multi-mode fibers. There are

large variances of loss measurements in an LD case, although an

LED case shows reproducible measurements. A main cause is a

gap, working like an etalon , formed between connector faces.

Therefore, it will be unavoidable that a variance remains when an

LD light source is used in loss measurement of devices with

connectors as well as in connector loss measurement.

4. OPTICAL TIME DOMAIN REFLECTOMETRY

An Optical Time Domain Ref lectometer (OTDR) is a powerful

tool for supporting the cable installation and system supervisary,

because the OTDR provides not only the optical cable fault

location but the distribution loss measurement.

The OTDR configuration for single-mode fiber is essentially

similar to that for multi mode fiber although the smaller level

of backscattering in single mode fiber makes the measurement

dif f icult [ 5] . The coupling efficiency of laser diode (LD) is

73
smaller into a singl-mode fiber than into a multi mode fiber.

LD is adequate to the light source of the OTDR from the view

points of easy-handling and reliability. The Burried Heterostructure (BH) LD developed at the ECL is fit to the OTDR of

its high output power and high coupling ef f iciency [ 6 ] . Figure

4 shows an example of backscattered waveform from single-mode

fiber- The BH-LD launchs +9dBm peak power into fiber under the

test. A polarization-separated-type optical directional coupler

prevents the power leak of a source light into a sensor. A high

speed digital averaging processor is effective to widen the

measurable dynamic range. This OTDR can achieve up to 17.5dB

one way dynamic range for fault location.

Figure 5 shows the backscatted waveform of a 19km long

fiber. Sharp drops and rises appear on the exponentical decay

slope. The drops and rises are caused by not only splice loss

but fiber parameter mismatch at splice points. The results

indicate that bidirectional observation is essential to evaluate

the splice loss. It is necessary to adopt a kind of scrambler

against polarization, as polarization dependence of backscattered light causes the OTDR measurment error.

The OTDR is also applied to estimate the biref ringency in a single-mode fiber[7],

5. CHROMATIC DISPERSION MEASUREMENT

The bandwidth of single-mode fiber is limited by the chromatic dispersion. So it is significant to evaluate the chromatic

dispersion in the high speed transmission systems.

Time domain pulse delay measurement have been reported not

74
to have so good resolution as to measure the dispersion accurately.

Figure 6 shows the schematic view proposed for chromatic dispersion measurement [ 8] . In Figure 6 , LDs oscillate at slightly

different wavelength, and two phase shifters are set up to

minimize the composite signal level demodulated in the receiver.

The result at 1288nm wavelength is -2 . 4 8ps/km« nm in a 16km long

single-mode fiber.

Wavelength-variable optical source is required to measure

the dispersion over the wide wavelength region. The measurement

covering 1.0-1.6um has been reported where optical parametric

oscillator and difference method is used [9].

[Acknoledgement]

The auther thanks Drs E.Iwahashi and M.Koyama for their

encouragement, and thanks Yokosuka ECL members for their cooperation .

[Reference]

(1) E.Iwahashi, IEEE J.QE., vol.QE-17, NO. 6, 1981.

(2) M.Saruwatari et al , ibid.

(3) Y.Fujii et al , presented at Nat . Conf . IECE Japan, March, 1982.

(4) K.Nawata, IEEE J.QE., vol.QE-16, NO. 6, June, 1980.

(5) K.Aoyama et al, IEEE, J.QE., vol.QE-17, NO. 6, 1981.

(6) Y.Nakano et al, Electron. Lett., vol.17, NO. 21, Oct., 1981.

(7) M.Nakazawa et al, IEEE J.QE., vol.QE-17, NO. 12, Dec, 1981.

(8) K.Aoyama et al, presented at 1981 Nat . Conf . IECE Japan,

Mar., 19 82.

(9) A.Sugimura et al, IEEE J.QE., vol.QE-16, NO . 2 , 1980.

75
25

20

10

1 TM —i 1 1 —

Measurable limit

1 j

1 \

\

Fiber length

16. 7km

« i i

LD

_ , , Birefringence Rod lens . , 3 material

Single

mode

fiber

1.2 1.3 1.4 1.5 1.6 1.7

Wavelength (urn)

Fig.l Single-mode fiber loss characteristics

with loss measuring equipment

40 ma T.n 10dB

I—1

'—' LED

3 20 -

Fig. 2 Depolarizer

7p

0 0.2 0.4 0.6 0.8

Loss (dB)

Fig. 3 Connector losses dependent on optical sources

Okn 10km

lOdB

Fiber Length (km)

I-'icj . 4 OTDR with LD

19 km

fiber 1 + fiber 2 fiber 2 * fiber 1 {up-side down)

point A point D

splice loss: 0.3dD 0.3dB

Fig. 5 OTDR measurements of spliced fibers (with LD)

76
Phase

shift ;r'

LD

f

LD

1biphase

shi f t'isr

Coupl

er

i/ector

"^/olt

net er

APD

Transmitter

Fiber APD

Receiver

Fig. 6 Chromatic dispersion measurement with

baseband phase comparison method

77
am
The Characterization of Monomode Fibre Links Installed

in Operational Duct

J R Stern, D B Payne, T D S Wood and C J Todd

British Telecom Research Laboratories, Martlesham Heath, UK.

1 Introduction During the past year a number of experimental monomode

optical fibre links have been assembled at British Telecom Research

Laboratories in order to aid transmission performance studies and to provide

test bed links for system trials. These links include a 102 km laboratory

link 1

, a 31.6 km cabled laboratory link2 (containing 15 splices), a separate

31.5 km cabled link3 (containing 30 splices) and a 61.3 km cabled linkh

(containing 37 splices). The last 2 links have been installed in operational

duct in the vicinity of the Research Laboratories to form a complex of cables

linking Martlesham to telephone exchanges at the nearby towns of Woodbridge

and Ipswich. The 31.5 km field installation, cable manufactured

by Telephone Cables Limited (TCL), has been successfully operated at a bit

rate of 650 Mbit/s 3

. Installation of the 61.3 km cabled link, manufactured

for BT by Standard Telephone Laboratories'* , has recently been completed at

the time of writing. System trials5 have now been successfully conducted

at 565 Mbit/s (1300 ran) over the full 61.3 km and at 140 Mbit/s (1550 ran)

over an unrepeatered 90 km link formed by splicing the 2 installed cable

routes together.

The planning and assembly of these links has posed interesting problems in

fibre characterization, both in the laboratory and field, and the purpose of

this paper is to outline the approaches we have adopted. Emphasis here is on data taken from the 31.5 km field installation although preliminary data

is presented for the 61.3 km installation. Fuller information and analysis

will be available at the conference. Implications for the future field

testing and specification of monomode fibre systems will be discussed.

2 Fibre Measurements The fibre used in our trials has been produced to a matched cladding design previously described 6 with the zero chromatic dispersion point in the region of 1320 ran. During the planning and assembly

of the laboratory links and the 31.5 km installed cable route, equivalent

step index (ESI) profiling7 was extensively used to characterize and select

the individual link fibres and, as we shall see below, predict splice loss

performance. In this technique, the mode spot size variation with wavelength

79
is measured for a short length of fibre, and the ESI profile which gives the

best fit to the experimental data is selected as a good approximation for the

behaviour of the true, complex profile shape. The equivalent profile is then

used to characterize key parameters for the fibre - eg. microbending (cabling)

losses and splice losses (arising due to ESI profile mismatches). ESI profiling allows monomode fibre to be specified by only 2 parameters: equivalent

core radius and equivalent index difference. The additional parameters of

fibre outside diameter, concentricity error and spectral loss enabled the

individual lengths of fibre for the trial links to be fully specified. The

ESI measurements were carried out using a simple automated apparatus8 based

on a standard spectral loss apparatus. The mean values of the ESI parameters

and the concentricity error of the individual fibre ends spliced together in

the 31.5 km trial were as follows: Core radius (as) = A. 81 urn; std.dev = 0.42

urn, index difference (Ans) = 0.0031; std.dev = 0.00053, and concentricity

error = 0.52 ym; std.dev = 0.26 ym.

In the 61.3 km trial an alternative approach to ESI profiling was adopted

by the manufacturer (STL). The refracted near field technique 9 was used to

obtain full index profile measurements. Values of peak index difference

(Anp) and core radius were read from the index plots. The mean values

obtained for the fibre batch were: Anp = 5.03 x

10"" 3

, a = 4.34 ym. ESI

(and concentricity error) data is currently being measured at BTRL for a representative batch of the fibres. The 2 approaches to fibre profile

characterization will be discussed at the conference. Currently, we believe

that the ESI approach should prove of particular importance to the fibre user

as a simple means of characterizing and specifying both cabling and splicing

performance. The alternative approach of full profile

measurement, however, is likely to continue to be of major interest to the

fibre manufacturer.

3 Cable Measurement Both spectral and point loss (1300 nm and 1550 nm)

measurement techniques have been used in the laboratory and field programmes.

Point loss measurements have proved of particular value in the field

environment where compact equipment and speed of measurement are at a premium.

The main difficulty experienced with both types of measurement has been

accuracy. Typically fibre losses have been in the range 0.2-0.3 dB/km at

1550 nm and, for measurements made on 1-2 km lengths of fibre, errors are ~0.05 dB/km even when great care is taken. A statistical approach is

80
therefore necessary to establish the magnitude of cable incremental losses to

sufficient accuracy. Only be measuring over a large batch of fibres with a range of ESI parameters can 'safe' criteria be established for a given cable

structure . Earlier work carried out by Hornung and Reeve 1

0

at this laboratory

was used to guide fibre selection for the 31.5 km field installation. The

cable for this trial, manufactured by Telephone Cables Ltd (TCL), employed

a loose tube structure and introduced negligible microbending loss in the

critical, long wavelength (1500 nm) window where cabling incremental loss is

most likely to occur. Indeed several low An§ fibres, which had shown significant microbending loss on reel showed a marked reduction in loss at 1550 nm when packaged and cabled in this structure ( see Table 1 )

.

The cable for the 61.3 km link, manufactured by STL, was of the tight secondary package design. In cables of this type the fibre can, in principle, be

subject to differential strain from other cable components introduced during

stranding and sheathing. However, with appropriate manufacturing control,

these problems can be minimized and the final cables for the 61.3 km link

showed no incremental loss in the 1550 nm range compared with the secondary

packaged fibre loss. Further it was established by direct strain measurement

that no significant remnant fibre strain was introduced during duct installation of the 61.3 km tight packaged link.

An important objective of our field measurement programmes was to establish

the magnitude of any change in cabled fibre loss on installation in duct. It was decided that "remoted ended" loss measurements in the field would be

inaccurate and the following "loop splicing" strategy was therefore adopted

for both field trials. Two splices were made at one end of each of the

installed, A-fibre cables to form looped fibre pairs. Loop loss measurements

were then performed from a test vehicle, the loop splices being cut out and

returned to the laboratory for subsequent assessment. In this way it was possible to establish that the average cable loss had not changed on installation for the 31.5 km link (see Table 1). Final conclusions for the 61.3 km

link await measurement of the loop splices but present indications are that

,

no change in average cable loss has occurred. An extremely useful feature of

the loop splicing strategy was that, after measurements on the individual

cables had been completed, all subsequent link measurements could be made

from the laboratory as the installed cables were cumulatively spliced together.

81
A second method has also been used to characterize the loss performance of

our experimental links; Optical Time Domain Reflectometry (OTDR). This

technique proved to be of key importance in allowing the rapid and convenient

assembly of spliced, monomode links, a complete loss "finger-print" being

obtained at 1300 nm. A particular advantage is that splices and fibres can

be individually measured without the need for cutback measurements. The

photon counting OTDR apparatus developed at BTRL has a range of around 20 dB

(one way fibre loss) for detecting a fibre break (cessation of Rayieigh

scatter) but can be used as an effective measurement tool over much of that

range 11

. A 0.1 dB measurement precision is possible over a range of about

10 dB. Some care in interpretation of the OTDR traces is necessary. For

example, significantly different splice losses can be measured for light

incident in opposite directions at the splice. Occasionally a negative

splice loss is observed for one direction of propagation. These effects are thought to be due to ESI profile mismatches on either side of the splice

leading to different back scattering coeffficients. Under these conditions,

an average loss for the 2 directions should yield the true splice loss,

and the loop splicing strategy described above allowed this to be con- veniently obtained.

Good agreement has been obtained between OTDR measurements of fibre loss and

splice loss in comparison with conventional cutback techniques. The splice

loss distribution as measured by OTDR (1300 nm) is shown in Fig 1 for the

31.5 km installed system and the mean value of 0.22 dB agrees extremely well

with the mean splice loss of 0.215 dB obtained by subtracting the sum of

the cable losses from the total link loss (Table 1).

Lower average splice losses have been obtained on the 61.3 km link. The

splice loss histogram for the installed route, measured by OTDR at 1300 nm,

is shown in Fig 2. Comparison with the average splice loss obtained by the

'cutback' technique awaits the loop splice loss evaluation mentioned above.

4 Splice Loss Interpretation Theoretical calculations of the intrinsic

losses due to profile mismatch and to concentricity error show that a 0.15 dB

mean loss is expected for the 31.5 km field installed link. These calculations use as input data the measured tolerance values for the fibre pair at

each splice. A computer programme accounts for the statistical nature of the

azimuthal orientation of the concentricity error vectors. Average loss due

to the splicing machine itself (core deformation) has been shown to be around

82
0.08 dB for low concentricity error fibre. Thus the sum of the mean losses

due to the splicing technique and the fibre tolerance amounts to around

0.23 dB which agrees well with the mean value actually measured. The splice

loss for this particular link is clearly significantly influenced by fibre

production tolerances and improvements should lead to a lower mean value.

Present indications are that the lower splice losses achieved on the 61.3 km

link are in fact due to better tolerances - particularly concentricity error. Firmer conclusions should be possible by the time of the conference.

5 Conclusions Techniques for characterizing the performance of monomode

fibre links have been discussed with particular reference to experience

gained on 2 installed cable routes. The link performances obtained have

significantly exceeded our original expectations and have enhanced the

prospects for early deployment of operational monomode systems in the UK

network. The 2 trials have enabled conventional multimode cable structures,

of both the loose tube and tightly jacketed type, to be successfully tested

for monomode operation. No major cabling problems have emerged even at the

more sensitive wavelength of 1550 nm. In the link characterization programmes, a loop splicing strategy was adopted in order to overcome loss

measurement problems encountered in the field. ESI profiling and OTDR

techniques have featured prominently. The former technique may provide a basis for the complete specification of monomode fibres from the point of

view of cable incremental losses and, in conjunction with concentricity

error measurements, splice losses. The latter shows promise as an important

tool for the characterization of monomode links as well as for fault

location.

Acknowledgments The authors wish to gratefully acknowledge the contributions

of their many colleagues at BTRL involved in the assembly and characterization of the monomode links. Acknowledgment is also due to staff at Standard

Telephone Laboratories for provision of data under a British Telecom

Development Contract, to Telephone Cables Ltd, GEC Optical Fibres and the

Director of Research, BTRL, for permission to publish this paper.

References

1 Malyon, D et al: Elec Lett, 18, (1982), pp 445-447.

2 Payne, D B et al: Elec Lett, 18, pp 82-84.

83
3 Stern, J R et al: The field installation of a 31.5 km monomode optical

fibre system operated at 140 Mbit/s and 650 Mbit/s, Elec Lett, 18, (1982),

pp 631-632.

4 Irvine, J et al, 8th ECOC, Cannes, Sept 1982.

5 J E Midwinter, to be published.

6 Ainslie, B J et al: Proc 7 ECOC, Copenhagen 1981, 2.5.1.

7 Millar, C A: Elec Lett, 17, (1981), pp 458-460.

8 Millar, CA: Proc OFC 82, Phoenix, Arizona 1982, ThEE3.

9 White, K I, Opt and Quant Elec, 11, (1979), PP 185-196.

10 Hornung, S and Reeve, MH: Elec Lett, 17, (1981), pp 774-775.

11 Healey, P: ICC June 1982, Philadelphia, USA.

Table 1 MEAN FIBRE LOSSES dB/km

31.5 km Installed Link 61.3 km Installed Link

1300 nm 1550 nm 1300 nm 1550 nm

Primary coated

(on reel)

0.45 0.77*

Tubed/secondary

coated

0.45 (0.02) 0.25 (0.02)

Cabled 0.47 (0.02) 0.25 (0.02) 0.44 0.29

Installed 0.46 (0.03) 0.26 (0.03)

Spliced 0.68 (0.01) 0.51 (0.01) 0.49 0.33

*this result indicates tension on the primary coated fibre on reel. Figures in

brackets are standard errors in the mean for each measurement.

-indicates data not yet available.

A = 1.3 ;um mean splice loss = 0.22 dB

26 measured splices Note:4 splices rejected by OTDR

because of excess loss. The inclusion

of these splices would have

increased the mean splice loss

Fig. I

0.2 0.4

loss, dB

Splice histogram GEC/TCL

0.6

A = 1 .3 pm

mean splice loss = 0.1 1 dB

48 measured splices Note: 6 splices rejected by OTDR

because of excess loss. The

inclusion of these splices would

have increased the mean splice

loss to 0.14 dB

0 0.2 0.4 0.6

average splice loss, dB

Fig. J2- Splice histogram STL/STC

84
SINGLE MODE FIBER LOSS ROUND ROBIN

W. B. Gardner

Bell Telephone Laboratories

2000 Northeast Expressway

Norcross, GA 30071

Obtaining good agreement between multimode fiber loss sets

has proven difficult. The standard deviation between locations (i.

e. , test sets) is limited to about 0.2 dB/km by the problem of re1 2 producing test set launch conditions. ' This particular problem

should not exist with single mode fibers, since only one mode can

propagate. Consequently, one would expect the interlocation standard deviation to be less than 0.2 dB/km when measuring single mode

fiber attenuation. Indeed, because the loss of single mode fibers

is frequently only a few tenths of a dB/km, a loss measurement

accuracy of a few hundredths of a dB/km will be necessary. We have

recently completed a loss measurement round robin involving seven

Bell System test sets. The results indicate that this level of

accuracy is indeed attainable with single mode fibers.

Three fibers were circulated among seven Bell Labs and

Western Electric locations. The fibers were wound under low tension

with multiple layers on six inch diameter plastic spools. They

3

were of the depressed cladding design , having core diameters in

the 7.5 - 9.0 /am range, and LP-q mode cutoff in the 1.26 - 1.31 jam

range. Each location measured each fiber twice; the means of the

two measurements are listed in Table I. The cutback technique was

used, with a loop approximately 40 mm diameter present to suppress

unwanted modes. Location 5 used a 1.30 jam laser diode; all other

locations used a tungsten lamp with a monochromator

.

85
- 2 -

The interlocation standard deviation a is seen in Table I

to be typically about .03 dB/km. The one exception is the measurement at 1.55 Aim wavelength of Fiber 3. The large a in this case is

the result of a steeply rising spectral loss curve. The rise occurs

at this wavelength in Fiber 3 because this fiber has a more deeply

4 depressed cladding than Fibers 1 and 2. Since the 1.55 /am loss is

very large in this case, the larger a is not of great concern.

In conclusion, an interlocation standard deviation of JD3

dB/km has been demonstrated for single mode fiber loss measurements.

This is nearly an order of magnitude better than the o's achievable

with typical multimode fibers.

I would like to thank my colleagues W. T. Anderson, J. S.

Nobles, P. D. Lazay, J. Stone, A. R. Tynes, A. Tomita, and J. Sanchez

for performing the measurements reported here.

REFERENCES

1. A. H. Cherin and W. B. Gardner, Laser Focus 16, #8, 60 (1980).

2. D. L. Franzen, G. W. Day, B. L. Danielson, G. E. Chamberlain,

and E. M. Kim, Appl . Opt. 20, 2412 (1981).

3. P. D. Lazay, A. D. Pearson, W. A. Reed, and P. J. Lemaire,

CLEO, Washington, June 10-12, 1981, paper WG6.

4. P. D. Lazay, A. D. Pearson and M. J. Saunders, Topical Meeting

on Optical Fiber Communication, Phoenix, April 13-15, 1982,

paper ThCC2.

86
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cs

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cs cs cs cs 1 cs CS cs

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in

m ITS

m o o in o E o cs m cs cs K

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FIB FIBI INT INT

87

CHARACTERIZATION OF THE BEND SENSITIVITY OF SINGLE-MODE FIBERS

USING THE BASKET-WEAVE TEST

By A. Toraita, P. F. Glodis*, D. Kalish*, and P. Kaiser

Bell Laboratories, Holmdel, N. J. 07733; *Norcross, GA, 30071

Presently, extensive research and development efforts are

under way to optimize the index profile, geometry, coating,

and cable- structure of single-mode (SM) fibers . The

optimization includes keeping the core diameter as large as possible

for ease of splicing and connectorizat ion , while still maintaining a high resistance against macro- and microbending (MB)

losses. As in case of multimode fibers, MB losses depend on

the fiber parameters, the plastic coating, and the cable

structure. In contrast to MM fibers, however, MB losses in SM

fibers are wavelength dependent and can be eliminated in the

short-wavelength region of the SM domain as has been demonstrated

7 previously . While a truly satisfactory test regarding the MB

resistance of a SM fiber can only be made with a cabled and

deployed fiber, simulated tests must be used to facilitate the

fiber optimization process. One such test, which has become

known as the 'Basket Weave" (BW) test, consists of multi-level

winding the fiber on a spool, and measuring the spectral

7 losses for different winding tensions . A high immunity against

the ensuing stresses is a measure. of a superior j acketed- fiber

structure. In addition to possibly incurring MB's due to the

transverse pressure exerted by adjacent windings, the fiber may

suffer losses due to winding crossings, and macrobends due to

the curvature of the spool. Because of the relaxation of the

rewind stresses of the plastic-coated fiber, the excess losses

generally decrease with time, and meaningful comparisons of

data can only be made if the measurements are performed a specific time after the rewind process*

89
The SM fibers used for this study were of the depressedcladding type, which permits a nearly independent optimization

1-4

of the core size and minimum dispersion wavelength

Specifically, core diameters ranged from 8.0 to 9.3]_im, and

index differences A/A-

(Fig. 1) from 0.31/0.06 to 0.49/0.191,

respectively, as obtained with the refracted-near-f ield technique.

Fiber o.d.'s were 125ym, a/b ratios approx. 6.0, and cut-off

wavelengths ~l,22um. The spectral losses of the fibers wound

under <3 to 75g tension on 15-cm-diam. spools are shown in

Figs. 2 to 4. The SM wavelength region is bounded by the cutoff band of the 1st higher (LP^) mode on the short-wavelength

side, and by a loss edge at longer wavelengths. The longwavelength edge (LWE) may be due to the IR absorption tail,

macro- or micro-bending losses, and in case of the depressedindex fibers, also due to the leakage of the fundamental mode

2-4

through the inner cladding layer . Characteristic for

fiber A with small index depression (0.31/0.061) and large

core (9.3ym), excess losses were observed throughout the SM

domain even for moderate winding tension, and the LWE rapidly

moved to shorter wavelengths with increasing tension (Fig. 2).

While the LWE of fiber B with intermediate index depression also

moved to shorter wavelengths (Fig. 3), the loss in the 1.3um

wavelength region was constant and independent of the various

bending forces exerted. The BW test thus revealed the superiority

of fiber B in regards to resisting MB stresses, and a good

correlation was found to exist between the excess losses

encountered in the BW test, and added-losses measured during

the various cabling stages of an experimental submarine cable

design ' . While the spectral loss was independent of the winding

tension for fiber C which had the smallest core and the lowest

cladding index (Fig. 4) , a LWE appeared immediately after the

90
1.385ym OH band. After rewinding onto a 35cm diameter spool,

however, the LWE moved further into the IR, and until now

unknown loss bands appeared in the long-wavelength region

(Fig. 5). We tentatively attribute these bands to the

coupling of the core-guided mode to discrete outer cladding

modes. Additional results concerning the intensity, location,

and curvature dependence of these bands will be presented

at the Symposium.

In summary, the basket-weave test provides rapid and valuable

information concerning the macro- and microbend sensitivity

of different types of SM fibers. Because of the wavelength

dependence of the bending losses, BW tests should generally

be performed in conjunction with spectral loss measurements.

A properly-designed SM fiber does not exhibit MB -induced excess

losses in the short-wavelength region of the SM domain, which

can be extended to longer wavelengths with an improved jacket

and/or cable structure.

ACKNOWLEDGEMENTS

The index profile data were provided by M. Jo Saunders

whose cooperation was very much appreciated.,

REFERENCES:

1. P. D. Lazay, A. D. Pearson, W. A. Reed, and P. J. Lamaire,

CLEO '81, WG6-1, Washington, D. C. (1981)

2. P. D. Lazay, A. D. Pearson, and M. J. Saunders, OFC '82, Phoenix,

THCC2 (1982)

3. L. G. Cohen, D. Marcuse, and W. L. Mammel, Ibid., THCC1

4. B„ J. Ainsley, K. J. Beales, C. R. Day, H. P. Girdlestone,

and J. D. Rush, Ibid., THCC 3

5. V. A. Bhagavatula, D. B. Keck, R. A. Westwig, and P. E. Blaszyk,

Ibid. , THCC4 '

6. Y. Katsuyama, S. Mochizuki, K. Ishihara, and T. Miyashita,

Appl. Opt. 18, 2232 (1979)

7. P. Kaiser, W„ G. French, D. L. Bisbee, and J. W. Shiever,

IOOC '79, 7.4-1, Amsterdam (1979)

8 0 R 0 E. Wagner, S. M. Abbott, R. F. Gleason, R. M. Paski, A. G.

Richardson, D. G. Ross, and R. D. Tuminaro, ICC, Philadelphia '82

9. R. D. Tuminaro, private communication

91
&0

OUTER INNER CLADDING CORE

Fig . 1

LOSS<cSB/Km> LOSS(dB/KM>

1188 1288 1388 1488 1588 1688 1788 1188 1288 1388 1488 1388 1688 1788

WAVELENGTH < tin > WrtUELENGTH<nn>

Fis. 2

92
CALCULATION OF EQUIVALENT STEP-INDEX PARAMETERS

FOR SINGLE-MODE FIBRES

M. FOX

BICC Research S Engineering Ltd, London, England

Introduction It is desirable to represent the propagation behaviour of a

single-mode fibre by a few key parameters. A method currently in favour is

to define an "equivalent-step-index" fibre, whose cere radius a and relative

refractive index difference A should lead to reasonably accurate calculations

of cut-off wavelength, spot size (and hence of losses at joints or due to

microbending) and waveguide dispersion, using standard formulae. The

approaches used are either to calculate suitable average values of a and A

from the refractive index profile, or to find the values from the cut-off

wavelength and the relation between spot size and wavelength in the singlemode region. The data used would usually be experimental. In the present

paper, however, they are calculated from an assumed parabolic profile, for

1 which a spot size formula has been published by Marcuse.

Calculations from' the refractive index profile This method is given by

2 3 Stewart- and Bhagavatula. For a parabolic-index fibre, of core radius a and

relative refractive index difference A, with maximum refractive index n, the

equivalent step-index values are

a£S = 3a/4, A

ES

= ^/9 ' X g(cut-off) = 2 %naA/z /2 .4048

The last result may be compared with the correct value

y \ (cut-off ) = 2 xnaA 2 /2.4877

Calculations from curves of spot size V wavelength An experimental plot of

4

spot size against wavelength shows a sharp fall due to cut-off of the second

mode and then a slow increase; both sections of the curve are extrapolated to

4 give the cut-off values. Millar uses two procedures: either (a) the

93
calculations are based on the cut-off values of wavelength and spot size

alone; or (b) they also use the slope of the curve in the single-mode region.

5 6 A third method ,( c ) (Alard et al, Millar ) involves the fitting by least

squares of an empirical formula due to Marcuse

1 to the single-mode region of

the curve. The values of 2a and A that give the best fit are accepted.

Marcuse

1 quotes his results in terms of the field radius w^, defined in

terms of the Gaussian approximation to the field in the fibre:

2 2 E = E

Q

exp(-r /w_p ). He derives formulae for w^/a in terms of V, for both step

7

and parabolic index profiles. Snyder gives a less accurate formula for stepindex fibres.

The various procedures then involve the following calculations :

(a) X and w,, are known at cut-off, viz X and w : f oo

Assuming the maximum refractive index to be 1.45,

2a„_ = 1.8198w and A = 0.13935 (X /2a__)

2 ES o ES o ES

(b) X, w„ and dw„/d X are known at cut-off, viz X , w and w ':

4

f f ooo

Millar shows that 2ar,„ = 3.3039(w - 2Xw '/3). A„n is found as above.

ES ooo ES

(c)' w_p is known for a range of values of X in the single-mode regions:

Marcuse' s step-index formula may be written in terms of wooand X , and of XBy fitting the result to the data using least squares, "best values" of w

q and \ and hence of 2a^„ andA^, are found,

o ES ES

Simulated results For a parabolic index fibre with 2a = 11.0134 \im, n = 1.45

and 100 A = 0.3388, the cut-off wavelength is 1.174 p.m. Spot sizes have been

calculated from these parameters and Marcuse 's parabolic-index formula, and

the ESI values were calculated by the various methods. The results are:

94
No. of Wavelength Cut-off Diameter % RI diff

data pairs range (jam) A. lum)

o ^ 2a

ES (^) ES

From profile - - 1 .214 8.2601 0.3012

From spot size curve:

method (a) 1 1 .174 1 . 174 8.0000 0.3000

method (b) 1 + slope 1 .174 1 . 174 8.6260 0. 2582

method (c) 11 1 .174-1 . 761 1 .326 8.6003 0.3311

11 21 1 . 174-2 .348 1 .271 8.2686 0.3295

M 31 1 .174-2.935 1 .226 7.8112 0.3431

The simulation is inexact in that experimental data at cut-off are open to

doubt. The extrapolations are to some extent subjective, even using

g polynomial fits to both sections of the curve. There may also be a

g systematic error in the cut-off wavelength; though Millar finds good

reproducibility. Millar shows that the least squares method is sensitive to

the formula used for fitting.

The closeness of the least squares fit was tested; Using the ESI values

from 21 data pairs, the estimated values of spot size agreed with the

simulated data to within 1.8%. This may be contrasted with the use of the

parameters from the profile method, which gave deviations up to 9.8%.

Marcuse

1 shows that the power transmitted through a joint with offset d is

2 2 2 2 proportional to exp(-d /w^. ). The loss in dB is thus 4.34d /w^, ; hence the

percentage error in the calculated loss is twice the percentage error in the

spot size. The profile method thus underestimates the joint loss by up to

20%.

Comments It is evident that there are discrepancies between the ESI values

of the parameters calculated by various methods. The dependence of the

results on the range of wavelengths used in the least squares method is

particularly disturbing. Admittedly the parabolic profile is quite

95
different from the step profile, but so are practical profiles.

The least squares method, (c), does, however, provide an excellent

representation of the curve of spot size v wavelength. Methods (a) and (b)

represent the experimental value of the cut-off wavelength as well as

possible. The question of whether any of the derived values give useful values

3 9 of dispersion remains open.

'

Conclusions The best method of deriving ESI values of parameters depends on

the use to which they are to be put.

It is strongly recommended that, if ESI values of parameters are quoted

as results or in specification, the method of calculation should be stated

in detail.

Acknowledgement The author thanks the directors of BICC Research and

Engineering Ltd for permission to publish.

References

1. Marcuse,D., Bell Syst.-Tech. J., 5_6, 703-718, 1977

2. Stewart , W.J . , Electron. Lett., _16, 380-382, 1980

3. Bhagavatula,V.A. , Electron. Lett., 1_8, 319-320, 1982

4. Millar, C. A., Electron. Lett., 17, 458-460, 1981

5. Alard,F., Jeunhomme , L . , and Sansonetti ,P ., Electron. Lett., 1

7

,

958-960,1981

6. Millar, C. A., paper ThEE3 at 0FC '82, Phoenix, Ariz., April 1982

7. Snyder, A. W. , Proc . IEEE, 6_9, 6-13, 1981

8. Millar, C. A., Electron. Lett., J_8, 395-396, 1982

9. Nelson, B. P., and Hornung.S., Electron. Lett., 18_, 270-272, 1982

96
COMPARISON OF CUTOFF WAVELENGTH MEASUREMENTS FOR SINGLE MODE WAVEGUIDES

C. C. Wang, C. A. Villarruel and W. K. Burns

Naval Research Laboratory

Washington, DC 20375

The determination of a single mode fiber's cutoff wavelength,

characteristic of the fiber's index profile, is an important consideration

both to verify fiber design and to avoid modal noise^ in applications such as

communication systems and sensors. The use of an effective cutoff wavelength

which is characteristic of a particular fiber length, or condition of

curvature, can be in error by hundreds of nanometers and can lead to system

design errors. After considerable research and experimentation, three types

of experiments have evolved to measure the cutoff wavelength of the LP]]

mode on a single mode fiber: the far-field measurement of Gambling, et al.,2

the near-field transmission measurement of Murakami, et al.,3 and the

refracted power measurement of Bhagavatula, et al.^ The far-field measurement

of Ref. 2 provides the normalized frequency V from experimental

characterization of the LPqi far-field diffraction pattern, based on the

assumption of a step-index profile. The cutoff wavelength can then be

obtained by extrapolating from the experimental wavelength to the wavelength

which corresponds to the cutoff frequency of a step-index profile. The

transmission experiment in Ref. 3 points out the fiber length dependence of

the effective cutoff wavelength due to increasingly large propagation losses

as cutoff is approached. However, the authors conclude that a transmission

measurement made on a short (10-20 mm) length of fiber gave a result within +

5 nm of theory. Ref. 4 reported a refracted power technique in which they

show that a measurement of refracted power, while the LPn mode passes through

cutoff, avoids the complication of cladding modes and fiber length which are

present in any transmission experiment, and is thus suitable for a measurement

97
on a much shorter fiber length (few mm). This approach was shown to yield

discrepancies of up to 100 nm for the cutoff wavelength compared to

transmission experiments on 5 cm long fibers. To resolve the apparent

contradiction between Refs. 3 and 4 we carefully conducted measurements using

each of the three methods on the same ITT single mode fiber. We conclude that

the refracted power technique is the best direct experimental method.

Our experimental arrangement required for the far- field and near-field

transmission approaches are similar to those described in Refs. 2 and 3,

respectively. In Fig. 1, we show the setup which allows both near-field

observations of the transmitted mode pattern and refracted power measurements

at the input end of the fiber. We adapted the approach of Ref. 4 by placing

the stripped fiber in an etched Si groove and coupling out both cladding modes

and the cutoff LPn m °de through index-matching liquid and a prism. The fiber

radiation length was adjusted by coating the remainder of the fiber with black

paint.

The data from the far-field measurement, taken with a source wavelength

of 0.845 urn, yielded a normalized frequency V = 1.88 + 0.09, 2a = 4.92 + 0.14

ym and, NA = 0.103 +_ 0.002, implying a cutoff wavelength of 660 + 33 nm with

the assumption of a step index profile. The results of the transmission

measurements are shown in Fig. 2. In these measurements we demonstrated the

effective cutoff wavelength dependence on fiber length and condition of

curvature by constraining the fiber in decreasing diameter tubes. We could

not make this measurement on a fiber shorter than 2 cm because of interference

from cladding modes which could not be stripped in shorter lengths. The

observed cutoff wavelength for all fiber curvatures converges near 690 nm at

the 2 cm length. This is probably due to intrinsic fiber stiffness. The

typical data from the refracted power technique with a fitted curve are shown

98
in Fig. 3 for radiation length of 0.25 mm. Fig. 4 shows the curves for four

radiation lengths. The normalized refracted power is defined as the ratio of

the radiated power when the core is excited preferentially with the LP]] mode

to the radiated power when only cladding modes are excited and is plotted as a

function of wavelength. As was noted in Ref. 4, we observed no sudden change

in normalized refracted power at the cutoff wavelength, but rather a broad

transition region of 60-70 nm or more for each radiation length. We

associate the long wavelength boundary of the transition region with

the cutoff wavelength, assuming that the boundary is the point at which the

LP]] mode is no longer guided. The cutoff wavelengths show no marked

dependence on radiation length, with an average value near 830 nm. The large

discrepancy among the results of three independent measurements leads us to

believe that the transmission measurement on a 2 cm fiber is not

representative of a much shorter fiber. This is likely due to the large

propagation losses the LP]] mode experiences near cutoff. We conclude that an

experiment of the refracted power type is necessary to determine cutoff

wavelength with the best available experimental accuracy.

99
References

1. S. Heckman, Electron. Lett. U_» 499 C981).

2. W. A. Gambling, D. N. Payne, H. Matsumura, and R. B. Dyott, IEE J. Microwaves, Opt. and Acoust. , T_, 13 (1976).

3. Y. Murakami, A. Kawana, and H. Xsuchiya, Appl. Opt. JIB, 1101 (1979).

4. V. A. Bhagavatula, W. F. Love, D. B. Keck, and R. A. Westwig, Electron.

Lett. 16, 695 (1980).

MONO -0- CMOf*f«

5HICOH GOOOvIS

CUTOFF MEASUREMENT OfAAATuS

Fig. 1

500 700 WAVELENGTH Inml 900

Fig. 3

700

680

670

650

1 1 1 1 1

* LOOSE FIBER

• 0.5 cm DIA. TUBE O 0.05 cm DIA. TUBE

\ •

1 1

1 1 1 5- 10 20

FIBER LENGTH (cm)

50 100

Fig. 2

5 07

500 600 700 800 WAVELENTH Inml 900

Fig. 4

100
MEASUREMENTS OF FIBRE POLARISATION PROPERTIES USING A PHOTO-ELASTIC MODULATOR

A.J. Barlow and D.N. Payne

Department of Electronics, University of Southampton, Southampton, England.

INTRODUCTION

Single-mode fibres with controlled polarisation properties are required for many fibre applications. For example, high-birefringence

fibres are able to transmit linearly-polarised light, a property which is

useful in interferometric fibre sensors and in coherent transmission

systems. Low-birefringence fibres, on the other hand, have negligible

intrinsic birefringence and are suited to conventional communications,

polarisation-control devices and polarimetric sensors.

Precise measurements of fibre polarisation properties are an essential

prerequisite for the development of fibres which exhibit extremes of

birefringence. Conventionally, measurements are made using standard

polarimetric techniques (polariser and analyser) or, in the case of very high

birefringence fibres, by visual observation of the beat pattern. Neither

technique is entirely satisfactory, since for low-birefringence fibres with

retardations of <10°/m the sensitivity is low, whereas for high-birefringence

fibres, measurements are inaccurate and are limited to the visible region.

A dramatic improvement in measurement sensitivity can be obtained by

employing dynamic photoelastic modulation of the birefringence, rather

than static measurement. We report here the details of the technique and its use in the development of fibres with both the highest and the lowest

birefringence yet recorded. Furthermore, the sensitivity of the measurement

has permitted an accurate determination of the wavelength dispersion of the

stress-optic coefficient in fibres, together with its temperature dependence.

These parameters are of considerable importance in the interpretation of

birefringence measurements and in the design of fibre devices which employ

bends or twists to achieve controlled birefringence.

MEASUREMENT TECHNIQUE

The photo-elastic modulator consists of a glass plate element

vibrated at a frequency f by an acoustic transducer. The acoustic

101
vibration sets up an oscillatory photo-elastically induced retardation in

the plate. As shown in figure 1, linearly-polarised light is launched into

the fibre and the output polarisation state is analysed by the combination

of the photoelastic modulator and an analyser, which rotate as a unit. A

demodulator provides d.c. signals proportional to the amplitude of the

light-intensity modulation at frequencies f and 2f(S^ and S2 respectively).

These relate directly to the fibre birefringence properties.

In use, a Soleil compensator (Fig.l) is adjusted to null the fibre

retardation, as shown by a zero modulation component and a maximum in S£

In this way, direct birefringence measurements are obtained with a high

sensitivity. The use of the modulation permits much faster and more accurate

measurements than existing polarimetric methods, and considerably less

dependence on polariser extinction efficiency when measuring ultra-low

birefringence values. The lowest measurable birefringence is in fact

limited largely by the finite birefringence of the input lens.

RESULTS

1) Low-birefringence fibre measurements.

Ultra-low birefringence fibres can be reproducibly manufactured

using the fibre spinning technique.*' The residual birefringence present

in a spun fibre is extremely small (typically <1°) and its variation with

temperature is negligible. The modulator system has been used to perform

several measurements which confirm the properties of these fibres.

Furthermore, since the fibres are virtually polarisation transparent they

can be used to perform a number of measurements of fundamental polarisation

parameters without fear of interference from intrinsic birefringence. As an illustration of this &nd of the accuracy obtainable with the photo-elastic

modulator technique, we have performed a series of measurements of the

stress-optic coefficient and its dispersion in germanium-doped spun singlemode fibres. The stress-optic coefficient is of importance in several

forms of fibre birefringence, such as the thermal-stress birefringence

2 used to advantage in high-birefringence fibres , and the bending bire3 fringence used in the fibre isolator .

The technique used is to measure the rotation of the plane of

polarisation produced by twisting a vertically-hung fibre. When a fibre

102
is twisted at a rate £ , a stress-optic rotation a= g'£ is introduced, where

g' is the photo-elastic rotation constant and is directly proportional to

the stress-optic coefficient C. The measured values of g' as a function of

wavelength X are shown in Fig. 2, with the computed value for pure

silica shown for comparison. From the fibre curve we calculate a value of

-11 2 -1

the stress-optic coefficient C(l urn) in doped silica of -3.23 x 10 m kg

and its wavelength dispersion dC/dX = 2.4 x 10 ^"^m^kg ^"nm . The dispersion

in C is significant and can contribute substantially to polarisation

mode dispersion 10%) and to measurements of high-birefringence fibre

beat length as a function of wavelength.

The variation of g' with temperature T is shown in Fig. 3 and leads to a value for dC/dT of -4.31 x 10 "^m^kg K . This is sufficient to cause

a change in fibre bending birefringence with temperature of 0.01% K , an effect which limits the stability of fibre birefringent filters or sensor devices which use controlled bend birefringence.

2. High-birefringence fibre measurements.

The modulator technique is also useful in measurements on fibres with

very high birefringence, where, for example, it can be used to determine

very small changes due to temperature or aging. In Fig. 4, the variation of

fibre beat-length with temperature is shown for a fibre whose birefringence

resulted primarily from core ellipticity. From the slope of the curve it is

evident that about 15% of the birefringence was produced by thermal stress.

From measurements such as these , an understanding of the birefringenceproducing mechanisms in the fibre has been obtained and this has led to the

development of a fibre with a novel stress-inducing structure. Beat lengths as low as 0.6 mm at a wavelength of 0.633 vim are now regularly obtained.

Measurements of the properties of these fibres, as a function of temperature

and wavelength will be reported.

CONCLUSION

We have implemented a sensitive technique for birefringence measurement

which uses a photo-elastic modulator. A range of measurements of fibres

with extremes of birefringence have been conducted.

REFERENCES

1. A.J. Barlow, J.J. Ramskov-Hansen and D.N. Payne, Appl. Opt., 20,2962-2968,1981

2. R.H. Stolen, V . Ramaswamy , R. Kaiser and W.Pliebel, Appl. Phys. Lett .

, 33 , 699-701 , 1978

3. G.W. Day, D.N. Payne, A. J. Barlow and J . J . Ramskov-Hansen , Op t . Let ts .7, 238-240 ( 1982)

103
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104
Measurements of strain in cabled monomode fibre

S. Hornung and M.H. Reeve

British Telecom Research Laboratories, Marti e sham Heath, Ipswich, U.K.

I ntroduct i on: It is expected that, over the next decade, large quantities of

optical fibre cable will be installed into the British Telecom inland long haul

telecommunications system. Most of this cable will be installed into underground

duct. Fibre in this system is required to have a lifetime of 20 years or more. Fibre life is greatly influenced by humidity and residual strain. Moisture

ingress can be controlled by an aluminium-polyethylene laminate (APL) water

barrier in the cable, but little is known about residual fibre strain levels in

the ground. The level of residual strain is required as an input to any

realistic calculation of fibre lifetime from stress corrosion effects.

This paper reports the measurement of this residual fibre strain on 15km of

monomode fibre cable, installed into occupied duct using conventional

installation practice. Strain levels were measured on the fibres themselves by

means of pulse delay before and after installation. A wide variety of duct

conditions were encountered. However, in spite of this, residual strain levels

remained low throughout. It is felt that this result will be typical of that

found for future system installations, giving great confidence in fibre life.

Route: Measurements were performed on 10 cables (plus a spare) comprising a trial monomode system being installed between British Telecom Research

Laboratories, Martlesham Heath and the telephone exchange in Ipswich, a distance

of 15km. Each cable contains 4 fibres to provide 2 * 30km loops operating at

140MBits at 1.3ym and 1.55pm. The fibre parameters were all in the region of 9pm

core, 125ym outer diameter with peak An of order 5.3x10 . The average fibre

attenuation was 0.44 and 0.29dB/km at 1.3 and 1.55ym respectively. All LP

mode cut-offs were in the region of l.l-1.2ym. Final details of the installed

system will be available by the time of the symposium. Average joint losses of

O.lldB at 1.3ym were achieved in a laboratory pre-i nstallation trial. (1,2)

The cable was a tight -coated helical lay structure, in which fibres directly

extruded with nylon to 0.85mm were stranded around a 1.8mm coated steel wire. A polyethylene sheath with an APL water barrier gave a final outer diameter of

7mm. The secondary coated fibre had been proof tested to 0.6% strain.

Technique: The basis of the technique was to measure the length of the fibres

before and after installation by timing a laser pulse travelling the length of

105
the fibre. Pulse delay was measured for each fibre in the laboratory before

installation whilst both ends were accessible. Once installed the fibres were

loop-jointed into pairs at the far end. The combined delay was then measured for

the entire loop from a measurement vehicle. The combined delay was then compared

( after suitable corrections) with the sum of the measurements before

installation. Measurements after the installation were done once the cable had

been made fast at all intermediate manholes. The lengths of cables pulled-in

varied from 1km to 2.5km. All were installed in at least two stages, pulling-in

from a middle manhole in two directions. In some cases where high friction was encountered, the cable would be pulled-in in more stages. The installation

followed established practices. A mechanical fuse inserted between the pulling

rope and cable limited the maximum tension to 1KN (local cable strain of 0.17%).

The apparatus used for the measurements is shown in fig.l. A solid state

single heterostructure laser produces a short (2-3ns) pulse at 850nm, which is

launched into the fibre by means of lenses. The laser pulse is detected by a silicon avalanche photo-diode, and displayed on a sampling oscilloscope. The

laser is triggered by the prompt output of a digital delay generator, whilst the

oscilloscope is triggered by the delayed output of the delay generator, allowing

time for the laser pulse to reach the far end of the fibre. The laser is fired

at 10kHz rate by an oscillator connected to the external trigger port of the

delay generator. The pulse is recorded on graph paper by an XY plotter. An

accurate delay may then be obtained from the position of the pulse on graph

paper and from the delay of the generator. A pulse reflected from the launch end

of the fibre provides a t = 0 position by similar means, in order to eliminate any

delays in the equipment used. The technique may be used "double ended" if both

ends of a fibre or fibre loop are accessible, or "single ended" in which case a chemically deposited mirror is used to reflect the signal at the far end.

Correcti ons: The measurements before and after installation reveal a change of

delay. The source of this change is the residual strain experienced by the

fibres. Several corrections are required however in order to calculate the

accurate level of strain. These are:-

a_j_ Length of fibre cut off in installation and measurement preparation. This

is done by means of marks on the outer sheath. The length of steel strength

member cut off provides a further check which also reveals any shrinkage of the

outer sheath.

b. Temperature. The temperature was measured in the laboratory and in the

106
underground cable ducts, the difference was 5 to 10 degC. The fibre temperature

coefficient in this type of cable is known to be 0.103ns/km/degC. (3).

c. Strain measured by pulse delay is the result of two competing effects. The

physical extension to a fibre increases the delay by 48.91ps/km/MPa . Strain

however decreases refractive index, causing a decrease in delay of 9 . 12ps/km/MPa

(4). Therefore final strain (aL/L) is calculated by multiplying the relative

increase in delay (aT/T) by the ratio 48. 91/(48. 91-9. 12)=1. 23.

Results: Fig 2 shows the histogram of results. Each cable is represented by two

points corresponding to two loops. The distribution appears to be bi-moded. The

majority of results are around 0.01% positive strain, with a further group at

around 0.03%. The higher group suggests some snagging in the already occupied

ducts. The error is estimated to be ±0.01% per measurement.

Pi scu ssi on: The above strains are averaged over the length for each cable, and

thus some parts of the cable may experience higher strains than others. The

worse case would arise if a section of cable was held under strain between two

points where jamming occured. The upper limit is set to 0.17% by the mechanical

fuse. Strain here would then be a maximum of 0.17/2=0.085%. The highest residual

strain measured was 0.036%, which gives the length of the section under maximum

strain as 40% of cable length. However as some slack has been manually pulled

into each manhole (100-200m apart) to allow the cable to be tied to support

brackets, it is felt that any residual strain will be evenly distributed over the entire cable length.

Concl u si on: Previous work (5) has shown that fibre proof tested at 0.6% has a high probability of surviving a strain of 0.2% for 20 years.

Our measurements have shown that monomode cable can be installed in long

lengths using conventional techniques with considerably lower residual strains,

giving a large safety margin on fibre life.

Refere nces:

1. J. Leach et. al . Low Loss Splicing of a 64.2km Single Mode Fibre Link.

To be published, Electronics Letters.

2. J. Irven et al . Single Mode Optical Fibre Cables for Landline Systems.

To be presented at 8th EC0C, Cannes, September 1982.

3. J. Russell, (STL) private communication.

4. R. Kashyap, to be published.

5. W. Duncan et al . Effect of Service Environment on Proof Test of Optical

Fibres. 7th EC0C, Copenhagen 1981

107
108
FIELD AND LABORATORY TRANSMISSION AND

OTDR SPLICE LOSS MEASUREMENTS OF

MULTIMODE OPTICAL FIBERS

R. B. Kummer

A. F. Judy

A. H. Cherin

INTRODUCTION ;

All practical optical fiber systems require the use of interconnection devices such as splices and connectors. Their losses can

be a very significant factor in the design of fiber optic systems,

particularly multi-kilometer telecommunication links. This paper

describes the fundamental parameters affecting splice loss

measurements. The relationships between transmission and OTDR

(optical time domain ref lectometry ) splice loss measurements are

developed and laboratory data is presented to illustrate how

intrinsic and extrinsic splice loss parameters influence these

relationships. In addition, recent field data, obtained with an

OTDR, is included to show both the utility and limitations of OTDR

splice loss testing of installed fiber optic systems.

TRANSMISSION LOSS MEASUREMENTS ;

The measured transmission loss of a splice in a fiber optic system

can be significantly affected by the type of source used, the

source's location relative to the splice, and the characteristics

of the fibers on either side of the splice. 1-3 Let us consider

the transmission loss measurement arrangement shown in Figure 1 . If the length of the output fiber is short enough (so that the

output fiber loss can be neglected), a "short length" splice loss

can be defined as:

L s = 10 log (-2) ( 1

)

p

1

For multimode fibers, Ls may be dependent upon the proximity of

the splice to the source. If there is a long input fiber with

sufficient mode mixing between the splice and the source, Ls is

the splice loss measured under near steady state conditions; in

this case the input fiber isolates the splice from the source. If

the input fiber is short, however, Ls is strongly dependent upon

the source launching conditions. Figure 2 shows the dependence of

splice loss on source location and numerical aperture of the

launch beam. A large variation in the measured value of Ls (greater than a factor of 3 in this case) can occur if the source

launching conditions are not isolated from the splice by a long

input fiber. 1 An additional splice effect exists when a long

length of fiber follows a splice. The modal power distribution

109
along a multimode fiber is generally disturbed by a splice. This

can cause the output fiber to have a loss that is different from its

loss measured under steady state conditions. Since this difference

in fiber loss is a direct result of the splice, it can be considered

as part of the total splice loss. We may define a "long length"

splice loss, Ll as:

LL

= Ls + 6LR (2)

where 6Lr is a change from its steady state value of the output

fiber's loss caused by the splice. The total loss of the splice

plus the output fiber may be expressed either in terms of the

measured powers P-j and P3 (see Figure 1) or in terms of the

steady state fiber loss [L R

S

) and the long length splice loss (Lj- ) ; that is:

10 log (II) = L|s + L L (3)

p 1 thus

,

L L

= 10 log {^-) - L|s (4)

P

1 For a splice with extrinsic misalignment loss between identical

fibers, Ll will be greater than Lg. For a splice in which

intrinsic parameter mismatches exist between fibers, Ll can be

less than or greater than Ls. In fact, Ll can even be less than

zero. This can occur when the input fiber underfills the numerical

aperture of the receiving fiber, as for example, when a small A,

small core fiber is spliced to a large A, large core fiber. 3 Where

A is the relative refractive index difference between the core and

the cladding. In general, Lg is a well-defined localized quantity

for a given splice, whereas Ll is a function of the length, the

degree of mode mixing and the differential mode attenuation of the

receiving fiber. For evaluating the quality of splices and connectors in a laboratory or manufacturing facility, Lg is typically

measured under either steady state conditions (using a long input

fiber and identical short output fiber) or overfilled conditions

(uniform excitation conditions at input with short length of identical transmitting and receiving fibers). Lg is usually quoted as

the loss of a splice or a connector. 4

QTDR LOSS MEASUREMENTS : Consider the basic arrangement for OTDR measurements 5 consisting

of a pulsed laser and detector connected to the same end of a fiber

through an optical coupler as shown in Figure 3. A narrow optical

pulse is launched into Fiber A, after which backscatter ing and reflections are detected versus time. The signal reflected or backscattered from a given point along the fiber is detected after a round trip delay time proportional to the distance from the

110
source. Thus, the detected signal versus time represents backscattering versus length along the fiber. If we assume that both

attenuation and backscatter ing are uniform along the length of a fiber and independent of the direction of propagation, then the

power after a length l in meters is given by P(£) = Pq exp [-al] , where Pq is the initial power level and a is the fiber attenuation

coefficient in nepers/m. Some fraction, S, of this power is backscattered and captured by the fiber. It undergoes a further

exp[-ot£] attenuation before reaching the detector where its detected

power is given by

P' ( %) = P 0 S exp [-2<x£] ( 5 )

If the backscatter ing is predominantly Rayleigh, then for multimode

fibers the quantity S is given by the following equations: 7 ' 5

S = (3/4) A ar W/2 for step-index fibers

= (1/2) A ar W/2 for near-parabolic graded index

f ibers

where: a r = the component of attenuation caused by

Rayleigh scattering, a r = A/X^ where A is a wavelength independent term that is a function of the

glass compositional variations.

W = the length of the incident pulse

For a typical graded index fiber measured with a 100 ns pulse, S =

4 x 10~ 5 when the following typical parameters are assumed: A = 0.013, a r = 0.00062 nepers/m @820 nm, and W = 20 meters.

If we assume that a very narrow pulse is used to excite the two

fibers shown at the top of Figure 3, then the logarithm of the

detected backscattered power versus time will typically be given

by the plot shown at the bottom of Figure 3.

From Equation (5), the detected backscattered power from a point

immediately preceeding the splice is

= P Q SA exp[-2aA £ A ]. (6)

111
Similarly, the detected power backscattered from a point immediately

following the splice is

V = P 0 S B TAB TBA exp[-2aA *A ] (7)

where TAB and TBA are the splice transmission values in the forward

and reverse directions, respectively (e.g., the forward splice loss

in dB is LAB = 10 log TAB ) . The OTDR splice loss (lab ) is determined from the ratio of these two power levels:

LAB

~= 1 • 10 log (^2.)

= (5 log TAB + 5 log TBA ) + 5 log (8)

S A

= ( Lab + Lba

) + 5 log(^).

2 SA

From Equation (8), we see that the splice loss determined by OTDR

is equal to the average of the transmission splice loss values in

the two propagation directions, plus a term which depends on the

ratio of scattering coefficients of the two fibers. This latter

scattering term is a measurement artifact which can lead to

erroneous splice loss results if not properly accounted for. As

shown in the next sections, this term can be plus or minus several

tenths of a dB.

If access to the far end of Fiber B is available, then the scattering term can be eliminated by making an OTDR measurement from

Fiber B to Fiber A and averaging the splice loss values obtained

in the two directions. From Equation (8)

o o

( LAB + LBA j _ |- LAB + LAB

j ^ (9)

Similarly, the backscatter ing term itself may be determined by

subtracting the two unidirectional OTDR loss values:

112
o o

- L'M_) = 5 log (10)

A

EXPERIMENTS COMPARING TRANSMISSION LOSS AND

OTDR MEASUREMENTS

A series of experiments were performed using the test set shown in

Figure 4 to compare OTDR and transmission loss measurements on

fibers of either similar or mismatched intrinsic parameters (core

radius, delta, scattering coefficient) that were spliced

together

.

The first step in measuring splice loss was to determine the

steady-state loss, Lg S

r of the output fiber. This was done by

using the transmission set to measure the loss of Fiber B when

excited by a steady-state power distribution from an identical

fiber, B

1 (B and B' were actually two sections of the same fiber).

Next, a series of long length transmission and OTDR splice loss

measurements was made using either B

1 or a nonidentical fiber, A,

as the input fiber. Measurements were made at several different

transverse offsets from 0 to 15 ym. At each offset the optical

switch was used to alternately connect to the two measurement sets

without disturbing the splice. Finally, after the long length

measurements were complete, Fiber B was broken approximately 1 meter from the splice, and a series of short length transmission

splice loss measurements was made at the same offset values (see

Equation ( 1 )

)

.

Five different fibers were used in this study. Each fiber was

broken into two sections of approximately 1 km each. Two of the

fibers had reasonably large deviations from the nominal core diameter and A values but similar scattering coefficients, while

another pair had a larger scattering coefficient difference with

smaller intrinsic parameter deviations. The fifth fiber was chosen

to have nominal parameters.

These five fibers were spliced together in nine different combinations. Three were identical-fiber splices, two had primarily

intrinsic parameter mismatch, two had primarily scattering level

mismatch, and one was a "typical" splice. The splice loss results

are discussed below.

113
(a) Identical Fibers

A plot of splice loss versus transverse offset for an identical

fiber splice is shown in Figure 5. For this case the long length

transmission loss is greater than the short length loss, and the

difference increases for larger offsets. This result shows that the

changes in modal power distribution caused by an offset splice tend

to increase the loss of the output fiber. The OTDR splice loss is

in good agreement with the transmission results and is bounded by

the long and short length curves. OTDR measurements are most

accurate for identical-fiber splices since there is no scattering

coefficient mismatch and extrinsic splice loss is the same in both

directions (i.e., L^g = Lg^).

(b) Core Radius and A Mismatch

The two sets of curves in Figure 6 are for a nonidentical fiber

splice with a large core radius and A mismatch and a small scattering coefficient mismatch. The splice in Figure 6a is between a large core, large A input fiber and a small core, small A output

fiber, and Figure 6b is for the same splice with the input and

output fibers reversed (i.e., measured from the opposite end).

For the "large into small" splice (Figure 6a), there is an intrinsic short length transmission loss of 0.37 dB with zero offset,

and a long length zero offset loss of 0.61 dB. In this case, the

"large" input fiber overfills the local numerical aperture at

every point on the output fiber core causing both an immediate

(short length) splice loss and an increase in the loss of the

output fiber above the steady-state value. 3 This increase in

fiber loss is due to the increased excitation of high-order fiber

modes which have a high loss because of mode coupling into nonpropagating modes.

For the "small into large" splice (Figure 6b), the situation is

reversed. Here, because the local numerical aperture of the

"large" output fiber is underfilled for small offsets, there is

zero short length loss. Furthermore, the output fiber loss for

small offsets is less than the steady-state loss and gives rise to

an apparent negative long length splice loss. This negative loss

implies that the modal power distribution excited in the output

fiber by the "small" input fiber with no offset contains less

power in the lossy high-order modes than the steady-state distribution for that particular fiber. As the splice offset is increased, there is an increasingly large region of the output fiber

core where the local numerical aperture is overfilled, resulting

in a nonzero short length splice loss and an increase in the output fiber loss. Eventually, the offset effects dominate, and the

long length splice loss exceeds the short length value.

114
(c) Scattering Coefficient Mismatch

The splice loss results for two fibers with a large difference in

scattering coefficient, S, are shown in Figures 7a and 7b, with

the high-S fiber as the input and as the output fiber, respectively. The relatively small index profile parameter mismatch between

these fibers leads to transmission splice loss effects qualitatively similar to those discussed in the previous section, but

with a much smaller magnitude. Thus, the transmission splice loss

curves for the two directions of propagation are only slightly

different.

There is a significant difference, however, in the two OTDR splice

loss curves - the OTDR curve for the "high-S to low-S" splice

(Figure 7a) indicates a considerably higher loss than either

the long or short length transmission measurement, while the OTDR

measurement in the opposite direction (Figure 7b) is generally too

low. This difference is due to the fact that the scattering term in

Equation (8) increases the measured loss for S^ > Sg as in

Figure 7a, but decreases the measured loss for S& < Sq as in

Figure 7b. This effect may also be seen in Figure 6, where the

scattering coefficient mismatch increases the measured OTDR splice

loss in Figure 6a and decreases it in Figure 6b.

The actual OTDR signal with zero splice offset is shown in the

insert of Figure 7b. The distinct increase in received scattered

power following the splice results in a negat ive OTDR splice loss.

It should be emphasized that the negative OTDR splice loss in this

case is a measurement artifact due to a difference in scattering

coefficients, whereas the negative long length transmission splice

loss discussed in the previous section is a real quantity which

affects the end-to-end loss of a system.

From Figure 7, we see that scattering coefficient mismatch can

lead to significant errors in OTDR splice loss measurements.

Scattering mismatch error may be eliminated by averaging the OTDR

splice losses measured in the two propaqation directions. Fiqures

8a and 8b show the directional averages of the splice loss curves

from Figures 7 and 6, respectively. In both cases, the OTDR

directional average curve is in reasonable agreement with the

average of the long length transmission splice loss.

V. TYPICAL FIELD SPLICE LOSS RESULTS

Recently we measured 142 fiber splices in the field with an OTDR.

A histogram of their losses measured from only one cable end is

shown in Figure 9. It is apparent that while the mean loss is a respectable 0.12 dB, the data has a large standard deviation (a) of

115
0.24 dB as well as 41 "negative" splice losses. To determine the

amount of 51og(SB/S^) measurement error, the same splices were

measured in the reverse direction and their average difference found

as in Equation 10. A histogram of the results (Figure 10) shows

that this scattering error has essentially a zero mean but a substantial a of 0.21 dB. Subtracting the scattering error from each

splice using Eq. 9, we find in Figure 11 the true splice loss distribution has a a of 0. 1 1 dB - much less than the unidirectional

value. Note that, as statistics predicts, the a's add on a rootsum-square basis.

In the field it is difficult to measure splices in both directions

due to the OTDR's limited range. So field measured splice loss data

is usually distorted by the scattering error term. Unfortunately,

in order to design accurate transmission systems, one must determine

the true splice loss distribution with a minimum of uncertainty.

This dilemma can be moderated by the following technique. Consider

Figure 12 which shows N splices joining N+1 uniform fibers. If all

the splices are measured from the same end, then the error in

measuring their sum will equal the scattering difference between

only the first and last fibers. Assuming that the scattering error

has a standard deviation of a

1 , then the a of the sum of the N

splices will be only a' rather than the a'\/N~that would occur if no

correlation existed. Thus the uncertainty in determining the mean

splice loss will be improved by 1A/N « Note that this technique is

only valid if the fibers' backscatter ing is uniform with length.

SUMMARY

The measured transmission loss of a splice in a fiber optic system

can be significantly affected by the type of source used, the

source's location relative to the splice, and the characteristics

of the fibers on either side of the splice. We have defined the

terms short length and long length splice loss and have shown that a splice can change the loss of the output fiber following the splice.

The output fiber's loss will increase from its steady state value if

the splice loss is due to transverse offset. The output fiber loss

can either increase or decrease if the splice loss is due to intrinsic parameter mismatch. In any event, the long length splice

loss can differ from the short length value. Likewise OTDR measurements of splice loss will often differ from transmission measurements. A principal reason for this difference is that the OTDR

measures round trip rather than one way loss. In addition the OTDR

measurement is also affected by scattering error. When these

.differences are eliminated by comparing OTDR measurements made from

both ends of a transmission path with two way transmission measurements, the OTDR agrees reasonably well with transmission splice

loss

.

116
REFERENCES

1. Cherin, A. H. , Rich, P. J., Appl. Opt., Vol. 17, No. 4,

(1978), p. 642.

2. Kummer, R. B., BSTJ, Vol. 59, No. 3 (1980).

3. Kummer, R. B., Proc. 6th Eur. Conf. on Opt. Comm., York

(1980), p. 302.

4. Cherin, A. H. and Dalgleish, J. F., ITU Telecom Jour. Vol. 48,

No. XI, (1981).

5. Personick, S. D., BSTJ, Vol. 56, (1977), p. 355.

6. Kummer, R. B., Mettler, S. C. and Miller, C. M . , Proc. 5th

Eur. Conf. on Opt. Comm., Amsterdam (1979), p. 6.4-1.

7. E. G. Newmann, Elec. Comm., Vol. 34, (1980), p. 157.

117
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121

OPTICAL CONNECTOR MEASUREMENT ASPECTS, INCLUDING

SINGLE MODE CONNECTORS

P. Kaiser, W. C. Young, and L. Curtis

Bell Laboratories, Holmdel , N. J. 07733

SUMMARY

Optical and mechanical precision measurements

required to achieve insertion losses "below O.h dB

vith "biconic multi and single-mode connectors are described. Connector losses depend on the measurement procedure, and in case of multimode connectors,

may also depend on the direction of propagation and

the modal power distribution existing at the connection point.

I. INTRODUCTION

The most important performance parameter of an optical connector is its insertion loss. At first

glance, a simple loss measurement should suffice

to determine the connector loss to an accuracy which

is only limited by the stability of the measuring

apparatus used. To be meaningful, however, additional measurements are typically required in order

to fully characterize a connector and to assure a certain performance level as ve shall describe below.

Fiber connection and splice losses are affected by

a mismatch of the intrinsic waveguide parameters

such es core size and numerical aperture (NA), and

extrinsic factors of influence such as transverse

offset, angular misalignment, and end-separation.

In a less obvious but very important way, multimode

fiber connector losses are also affected by the

modal power distribution existing at the connection

point. This distribution is influenced by the launch

conditions and the fiber transmission characteristics

.

Unique, steady-state connector losses are only

obtained with a long reference fiber, or a properly

1 2 designed mode filter. ' For other conditions (such

as for over-excitation with an LED), the connector

loss has to be associated vith the corresponding

mode distribution as can be identified through

effective NA, or radiation pattern measurements.-^' A complicating factor presently is still the lack of

generally accepted definitions of some of abovementioned fiber parameters such as the effective

numerical aperture and equilibrium mode distribution

in case of multimode fibers, and the effective core

size and cut-off wavelength in case of single-mode

fibers, but extensive efforts are underway to standardize the definition of these parameters.^

In addition to the intrinsic and extrinsic factors

of influence, the connector loss may be affected by

the cable design and the mounting technique (epoxies,

crimps, etc.) used: Macro and micro-bends may

inadvertently be incorporated into the connector body

with ensuing excess losses which are unrelated to the

fiber coupling mechanism itself.

In the following, we describe some of the measurements

required in optical connector technology, using the

transfer-molded connector developed for the Bell

System FT3 (1*5 Mb/s) multimode lightwave system as an example.^ In this design, the precise alignment of

the two spring-loaded plugs is achieved in a biconic

sleeve (Figure l). Five of these connectors are used in a typical repeater span as illustrated in

Figure 2. The laser package containing a premolded

(Field Assembly-Type ) connector plug is connected to the automatic plug-in connector at the back of the

regenerator circuit board vith a short jumper cable. A standard connector is used in the cable distribution

frame. At the receiver end, the signal is routed

analogously, with the detector package having a connectorized pigtail.

II. MEASUREMENT OF PHYSICAL CONNECTOR PARAMETERS

A computer-controlled microscope test set with video

scanning system has been developed which — t.ncer observation of careful calibration procedures —

enables measurements of the core/plug eccentricity

and the fiber deflection angle to an accuracy

7 of 0.1 urn and <0.1 degree, respectively. Eccentricities below 1* inn, and deflection angles below 1 degree

have been obtained vith high yield for multimode

connector plugs in a manufacturing environment.^ Using

a modified plug fabrication process, eccentricities of

0,4 \xm, and deflection angles of ~0.26 degree

(required for connection losses on the order of 0.5 dB, see Figure 3) have been obtained vith single-mode plugs

in laboratory experiments (Figures h and 5).

By scanning both the near field of the core and its

radiation pattern, the above-described test set simultaneously yields core diameter (beam profile) and NA data. More precise data are obtained from refractive index profile measurements. The degree with

which these parameters can be controlled in the fiber

fabrication process ultimately limits the lowest loss

achievable even vith a high-precision connecter design.

Vhile fiber outer diameters can be measured vith

±0.1 ym accuracy using mechanical or optical measure- ment techniques, diameter variations along real fibers

amount to a multiple of this value even for distances

on the order of a fev centimeters. It appears, therefore, that fiber parameter matching is only

assured if the fiber is broken and reconnected at exactly the same point ("identical fiber" case).

The concentricity of the tvo cones of the biconic

alignment sleeve (Figure l) can be determined vith

123
mechanical gauges, or preferably, through connector loss measurements using tvo precise reference plugs

containing identical fibers.

The separation between the fiber end-faces is determined by the taper lengths of the plugs and

sleeves. If the manufacturing tolerances are properly chosen, the end-faces can be brought into

physical contact. This eliminates the ~0.31 dB

Fresnel reflection losses and avoids instabilities

caused by interference effects between the fiber end6 9 faces. A finite end-face seperation can be

identified through optical time domain reflectometry

(OTDR) techniques, and gap measurements vith ~0.1 JJm accuracy can be performed with an in - situ connector vith a spectral scan since the gap causes resonance peaks in the transmitted and reflected power.

In order to assure the integrity of the end-faces

under repeated-contact conditions, they have to be

polished flat within ~0.1 urn as can be determined

vith standard interferometric techniques.

III. CONNECTOR LOSS MELASUREMENTS

In its simplest form, a connector loss measuring

set-up consists of a source, a reference fiber

terminated with a nominal reference plug and sleeve,

and a large-area detector (Figure 6). The loss is determined from the reference power level and the

output of the test cable after interconnection.

Using this method, average connector losses of 0.28 dB

have been measured for 5,700 multimode connectors produced in a manufacturing environment" (Figure 7).

While the stability of the test set-up should permit measurements with ±0.01 dB precision, the accuracy

vith which the connector losses can be quoted is typically more than an order of magnitude worse because of a lack of precise reference plugs vith

ensuing loss variations for different rotational

positioning of the connector plugs and sleeves. While

this technique is suitable for the measurement of

single-mode connectors vhere the loss is independent

of the direction of propagation, in the case of multimode fibers vith different core diameters and NA's,

the loss may be negligibly small for transmission

from a small-core/NA fiber to a large-core/NA fiber,

and can be substantial in the opposite direction.

Consequently, additional fiber parameter specifications

have to be met to assure a desired system performance.

Alternately, an average value for the high- and lovloss end of the test cable can be determined by

inserting the cable vith the test plugs betveen tvo nominal reference plugs (Figure 6b), vhereby the

previously determined loss of the reference fiber

interconnection has to be added to the above combined

loss value before averaging.

The methods described thus far fail, however, to include the excess loss potentially originating in the reference plug body due to improper cable mounting

techniques. To include this loss, the two-point cut- back method has to be used (Figure 6c).

If a rotational adjustment of the plugs and sleeves

is possible, one may quote minimized, maximized,

averaged, or random connection losses. Loss data

which may be considered most meaningful for systems

applications are obtained by determining the random

loss measured during the concatenation of an ensemble

of connectorized short cables containing different

fibers satisfying specified tolerance conditions. Data obtained in this fashion for a set of 51 multimode Jumper cables are shown in Figure 8, vith the

average loss amounting to 0.33 dB (a = 0.18 dB).

Random connection losses of a set of 9 single-mode

jumpers whose eccentricities and deflection angles were presented in Figures k and 5, are shown in Figure 9. Average losses amounted to 0.39 dB

(a = 0.l8 dB), using fiber from the same supply reel.

For long fiber sections, the loss of the fiber has to be taken into account, and the accuracy with which

this loss is known directly affects the accuracy of

the connector loss measurement. The loss of connectors and splices in long concatenated fiber

sections can be determined in-situ and without knowing

12 the fiber losses by employing OTDR techniques.

However, special procedures also have to be followed

here (such as transmission in both directions) in order to obtain meaningful results.

IV. CONCLUSIONS

A large variety of optical and mechanical precision

measurements, including fiber characterization, are required to develop high-performance optical connectors as is described in this paper. In addition,

optical connectors have to be evaluated in terms

of their systems performance, since they may cause modal noise, and reflections back into the laser

cavity, vith an ensuing effect on the spectral

characteristics and laser noise, all of vhich may

result in system performance degradation. Finally,

mechanical and environmental performance tests also

have to be included in the evaluation program.

REFERENCES

1. P. Kaiser, "Symp. on Opt. Fiber Measurements,

Boulder (1980), NBS Publication 597, pp. 11

2. P. Kaiser, W. C. Young, N. K. Cheung, and

L. Curtis, Ibid. pp. 73.

3. P. Kaiser, A. C. Hart, and L. L. Blyler, Jr.,

Appl. Opt. Ik, 156 (1975).

k. Y. Daido, et al , Opt. Fiber Comm. Conf.

Washington, D.C., ThE3 (1979).

5. A. H. Cherin and W. B. Gardner, SPIE Conf.,

Washington, D.C., (1980).

6. W. C. Young, et al , Int. Wire and Cable Symposium,

Cherry Hill, 1981.

7. N. K. Cheung and N. M. Denkin, NBS Publication 597,

pp. 1+5 (see Ref. l)

8. W. C. Young and L. Curtis, to be published.

9. W. C. Young, et al , EC0C6, York (1980).

10. A. Tomita, submitted for publication in Appl.

Optics

.

11. P. Kaiser and D. L. Bisbee, Opt. Fiber Communication Conf ..Washington, D.C., TuE3 (1979).

12. P. Matthijsse and C. M. Blok, EC0C5, Amsterdam,

pp. 9-5 (1979).

124
Fig. 2. Connector hardware used in a typical FT3

installation.

4.6 cm

(1.8in)

Fig. 1. Eiconic connector design.

1.27 cm DIA

(.5in)

3.6 cm

(1.4in)

o

=>

o

ccUJ

GO2

r>z

NUMBER = 18

MEAN = 0.39 MICRONS

SIGMA = 0.17 MICRONS

A .6 .8 ECCENTRICITY (MICRONS)

Fig. k. Core/plug eccentricity data for l8 singlemode connector plugs.

CORE DIAMETER : . , ,

* OPERATIONAL M m

/8/j.m *-cUT-0FF =1.2^.m

Fig. 3. Single-aode connector losses versus offset

and deflection angle for 8 and 9 micron-core FiS- 5. Fiber deflection angles of the saae set of

fibers operated at 1.3 Um (Xcut-off :~1 .2 ym). P1 ^5 described in Figure k.

125
0) 10

REFERENCE ALIGNMENT SLEEVE

^SOURCE /

/ REFERENCE PLUG

L MODE FILTER

b)

c)

METER

TEST PLUG

REFERENCE PLUG-,

REFERENCE PLUG TEST PLUGS

REFERENCE CUT

TEST PLUGS

Fig. 6. Schematic of connector loss measuring test

sets

,

a) Single reference plug method

b) Insertion loss measurement vith reference

plugs at both ends of the test cable

c) Two-point cut-back technique

a<

6 -

5 -

3 -

2 -

NUMBER

MEAN

SIGMA

MAXIMUM

51 .33 <JB

. 18 dB

. 70 dB

I I 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 LOSS IN dB

Fig. 8. Loss data of 51 biconic connectors at an FT3 installation measured during concatenation; corrected for fiber losses.

NUMBER * 5684

AVERAGE = 0.28

to

LJ

-J

CL<

to

DO

if

MEAN

SIGMA

= 0.39 dB

= 0.18 dB

LOSS IN dB

.2 .4 .6 INSERTION LOSS (dB)

Fig. 7 Loss data of 5700 connector plugs produced

in a manufacturing environment, and measured

according to Figure 6a,

(Core diameter 50 um, o.d.=125um, NA=0.23)

Fig. 9. Loss histogram of 16 random single-mode

fiber connectors assembled vith nine test cables (core diameter 9 Um, cut-off wavelength: 1.2 urn, measurement wavelength: 1 .3 Um) (See also Figures h and 5).

126
EFFECTS OF MODE FILTER INSERTION ON CONNECTION LOSS

BETWEEN COMMERCIAL SINGLE-FIBRE CABLES

C. Marchesi - SIRTI S.p.A. - Via G.B. Pirelli 20 - 20124 Milano (I)

U. Rossi - CSELT - Via G.R. Romoli 274 - 10148 Torino (I)

Abstract

Measurements of mode power distribution on connectorized single-fibre cables

stress the strong influence of launching and propagation conditions on connection loss. The large dispersion in coupling loss values (from 0.43 to 1.13 dB

)

produced in different conditions by the same connector can be explained on the

basis of such measurements.

1. Introduction

Connectors for multimode optical fibres are widely employed in commercial tran

smission systems: their performances have a strong impact on the whole system

design. On the other hand it is well known that the loss introduced by a joint

in an optical link is not a uniquely defined variable: besides mismatches in

fibre parameters and misalignments of the fibres, the mode power distribution

(MPD) in the fibre section preceding the joint, the mode conversion introduced

by the joint and the propagation conditions in the fibre following the joint can strongly influence the overall connection loss [1].

2. Measurement set-up

IEC [2] recommends to measure connector loss either in equilibrium or in uniform mode distribution excitation: the former situation simulates reception con nectors , while the latter applies to transmission ones, if LED sources are employed. Unfortunately equilibrium MPD establishes only after propagation in a

length of fibre which, if high quality low-loss fibres are employed, may amount

to many kilometers. Several techniques have been proposed to simulate the equilibrium MPD in a short length of fibre: among the others, mandrel ^rap mode fil

ters (MW) [3] seem suitable for application to single-fibre cables, since they

result effective without extracting the fibre from the cable.

127
A preliminary set of measurements, carried out in order to define test conditions

on commercial connectors mounted on commercially available single-fibre cables,

has been performed on the basis of both optical loss and MPD measurements;

the latter were carried out following a technique whose effectivehess and relia

bility has been previously demonstrated, provided that incoherent sources are em ployed [4]. The results reported in the following refer to a single set of measu

rements : their purpose is not the connector characterization, but the definition

of the test procedure.

The near-field intensity (NFI) measurements are performed by scanning a magnified

image of the fibre with a large area Si photodetector , in front of which a pinhole (~100 urn diameter) spatially limits the image. Removing the pin-hole allows

to measure the whole power guided by the fibre: loss measurements refer to such

"integral" data. A "lock-in" amplifier transfers NFI data to a desk-top computer,

which processes them to obtain the MPD as a function of the collective mode para

meter x ( x=0 : fundamental mode; x=l : cut-off modes [5]). The source is an LED

(peak emission A~850nm) which overfills the graded-index , 50/125^um low-loss op

tical fibre. From NFI measurements the MPD ' s can be calculated.

3. Results

Two jumpers, nominally equal, called here A and B,were connected, and the result

ing measured NFI was assumed as the reference situation (case#l): this complies

with what recommended by IEC in [2], method 5, and has the purpose of leaving

launching and detection conditions fixed when inserting a further jumper. The in

sertion on jumper A of a MW, obtained by wrapping 5 turns of the single-fibre ca

ble around a smooth cylindrical mandrel 5 mm in diameter, produced a 6.1 dB loss

and the MPD shown in Fig. 1 (full line, case #2). The filtering effect of the

MW can be recognized from the drastic depletion of high order modes.

The insertion of a further MW, analogous to the previous one, on the cable B,

produced only an additional loss of 0.66 dB , and a slight narrowing of the measured MPD (Fig. 1, case #3, dashed line).

The jumper under test, nominally equal to the previous ones, called X, was now

inserted between A and B, keeping both MW ' s : the additional loss was 1 dB , and

128
the relative MPD is reported in Fig. 1 (dotted line, case #4). The relatively

high value of loss produced by the new connection can be explained by taking in

to account, besides joint loss, the additional loss introduced by the propagation in the jumper B of an MPD perturbed by the mode conversion at the connection. In fact, a comparison of the MPD 1 s relative to cases # 3 and #4 (see Fig. 1),

shows a further MPD depletion which justifies the 1 dB loss.

Removing the MW from the jumper B changed the situation completely: in this case

the loss introduced by the connector was 0.43 dB with respect to the case #2,

and the relative MPD is shown in Fig. 2 (full line, case#5). This allows to

quantify the previous additional loss induced by the propagation in the jumper B with the MW to be 0.57 dB , a value higher than the connection loss itself. A compa

rison of MPD 1 s of cases #2 and # 5 shows well the mode conversion introduced by

the joint: a transfer of optical power from low to high order modes takes place.

This mode conversion effect is shown, from a different point of view, by the MPD

of case # 6 (Fig. 2, dashed line), obtained by removing also the first MW from

jumper A. In this case, the connector produced a depletion of high order modes,

which is shown by the departure of the relative MPD from the reference uniform

one. The loss, with respect to the case#l, was 1.13 dB , stressing its critical

behaviour in nearly uniform MPD conditions [1].

Finally, the MW was inserted on the jumper B, keeping jumpers A and X undisturb

ed (case#7, Fig. 2, dotted line). The insertion of the MW behind both joints

gave rise to a loss of 5.27 dB (with respect to case #6); the power level detect

ed at the end of the link was very close to the one of case #5, but the two MPD 1 s

(Fig. 2, full and dotted lines) show noticeable differences (depletion of low or

der modes and corresponding filling of high order ones passing from # 7 to #5),

due to the different position of the MW in the two cases.

4. Conclusions

The previous results confirm that the only measurement of detected power level

is a poor characterization of connector performances; on the contrary, MPD measurements have proved to be a powerful and useful tool for the characterization

of connectors and splices for multimode optical fibres, from which remarkable in

129
formations can be deduced.

The present measurement set, carried out on unselected commercial single-fibre

cables and connectors, has confirmed the strong dependence of joint loss on laun

ching and propagation conditions: when figures concerning connector loss are pro

vided, the measurement set-up should be carefully specified in order to perform

meaningful comparisons. In this case, in fact, losses ranging from 0.43 to 1.13

dB were observed: the MPD measurement has supplied the key for understanding

such apparent discrepancies.

References

1. P. Di Vita, U. Rossi; Journal of Opt. Commun., Vol. 1, No. 1 (1980), pp. 26-

-32

2. IEC document No. 46E (Secretariat) 33

3. A.H. Cherin et al .

; Fiber and Integr. Opt., Vol. 4, No. 1 (1982), pp. 49-65

4. M. Calzavara, P. Di Vita, U. Rossi; Electron. Lett., Vol. 17, No. 15 (1981),

pp. 543-545

5. R. Olshansky; Appl. Opt., Vol. 14, No. 4 (1975), pp. 935-945

1 r

= 1 1

Mode parameter x

Fig. 1 - Measured MPD • s of cases #2, #3 Fig. 2 - Measured MPD ' s of cases #5, #6

and #4 and #7

130
A NEW APPROACH TO JOINT LOSS MEASUREMENT .

R. D. JEFFERY & J. L. HULLETT, University of Western Australia,

Department of Electrical & Electronic Engineering,

Nedlands, W.Aust. 6009.

We report here on a new display format for the accurate measurement

of joint loss made possible by two-point processing of a backscatter signal. This display is clearly superior to the more usual

log plot because it presents the results in a form that allows the

direct interpretation of joint loss and a comparison of all joint

losses within a regenerator span. Furthermore, a threshold level

can be displayed as an aid to the acceptance or rejection of joints,

a feature which simplifies the field testing of fiber splices.

The loss of fiber splices is an important consideration in the

total loss budget of optical fiber links and therefore the accurate

measurement of joint loss at distances commensurate with typical

repeater spacings is necessary. For splices between identical

fibers the backscatter method has been shown to give a good measure

of joint insertion loss provided the results obtained from both

ends of the fiber are averaged (1) . Two-point processing of

backscatter data not only provides a better format for joint loss

estimation, but it is the technique which provides a controlled

level of measurement accuracy throughout the entire fiber scan.

Thus joint assessment at long range can be performed with the

same accuracy as at the near end. The set accuracy is achieved

at each point in the fiber scan by averaging that number of samples dictated by the primary (before averaging) signal to noise

131
ratio. This ratio is estimated from the sample mean and

variance (2,3) .

With two-point processing two samples of the backscatter waveform separated by A£ are taken for each pulse launched into the

fiber, the attenuation being derived from the ratio of these

samples. Thus for samples at £^ and with A£ = ^"^i' t'ie sPat ia l

attenuation characteristic a (—^) is given by: m

J

£n[i(£ )/iU ) ] a( W = M (1)

where is the backscatter signal and I the distance to the

scattering point. The fiber is scanned by shifting the paired samples in adjoining steps along the fiber. The resultant alpha plot

reveals the fine detail spatial loss characteristic of the fiber

and its joints. Fig.l is a typical alpha scan of a fiber regenerator link which highlights even small joint losses.

For direct joint loss measurement it is better to remove the spatial

dependence A£ from eq. (1) and plot the one way loss y(i^,Z^)

between the sample points and 2,^ , where

yU ,1) = aU ,l

2

)LZ (2)

A plot of y[l ,1^) for the same link as used above is shown in Fig. 2,

with Ail being 160m and the measurement accuracy set at ±0.016dB.

Here the base line represents the relatively uniform loss of the fiber

and the peaks above the base line the excess loss caused by the joints

With joint loss measurements the sample pair is scanned in interleaved steps 40m apart and it is this choice of sample step size

that determines the spatial resolution for locating joints. Using the

132
parameters stated the scan for Fig. 2 over 6.6km was completed in

two minutes. For comparison the more usual log plot of the backscatter data is shown in Fig. 3. This plot is formed by accumulating

the loss y between samples spaced at 40m. Accuracy is maintained

at ±0.02dB for the complete scan.

The significant advantage of the type of plot shown in Fig. 2, and

not applicable to the log plot of Fig. 3, is that a threshold level

indicating a poor quality joint can be marked on the display. If

for example, the maximum acceptable splice loss is 0.3dB in a fiber

link of nominal attenuation 2dB/km then the threshold level for a

sample spacing of 160m is 0.62dB which accounts for 0.3dB joint loss

and 0.32dB fiber loss. This threshold can be drawn on the results

as shown in Fig. 2. Clearly this feature lends itself to the field

testing of fiber splices.

We have shown that two-point processing of backscatter waveforms is

an accurate method for measuring the variation in loss as a function

of length. The resulting graph of this data accurately indicates

joint loss and further allows the display of a joint rejection level

which simplifies the testing of fiber splices in the field. Although

the results presented were made at 900nm this technique is applicable

at 1300nm and results at this wavelength will be presented at a later

date.

REFERENCES

1. A. Leboutet, in NBS special publication 597.

2. J.L.Hullett & R.D.Jeffery, in technical digest, 3rd IOOC

San Francisco, 1981.

3. J.L.Hullett & R.D.Jeffery, Opt. Quant. Electron. 14, No.l, 1982.

133
run RTTEKURT10N TSLCKPNC 1 .10 08/KH

Jl-HBT-02

t* cm ii-n»T-M

1.0

3.0

Fig. 1.

Alpha Scan

.04 4.00 (.32 t.U

OfSTSHCE citnl

FIBItE OTTEHUA110N

T81ERPNCE S .010 00 ft CXX 21-HPT-02

Fig. 2.

Joint Loss Scan

S.S0 8. 60 0I3TPHCE tKn)

f J8HE LOSS 101EKBHCE 2 .020 00

Jl-HBT-02

f6 CXX 21-BBI-8?

Fig. 3.

Log Plot.

30

134

S.33 D1STBHCE IIUil
INDEX PROFILE MEASUREMENTS

W.J. Stewart

Plessey Research (Caswell) Limited

Allen Clark Research Centre

Caswell, Towcester, Northants

England.

Index profiling is an essential measurement for optical fibres

and preforms, since the index profile vitally affects system performance.

Methods for determining diameter and NA that do not involve profile

measurement are inevitably open to question, and similar difficulties

arise with such parameters as eccentricity. With the increasing

importance of monomode fibres in systems, the need for high resolution

in fibre profiling is growing. Its significance and the growing technical

needs of users are tending to keep up pressure for better and more convenient profiling methods.

Consider for the moment only those methods that actually measure

refractive index. These are ideally required to have spatial accuracy

of better than |ym, and resolution preferably better than this. Index

-5

"resolution" (relative accuracy or linearity) is required to ^ 10 , with an absolute accuracy requirement a good deal looser than this.

It might be supposed that all methods would share common ultimate limitations, since most methods essentially involve measuring differential

phase delays through adjacent parts of the fibre. There is some truth

in this. Define a ' figure-of-meri t' F as

F = dP/P . 1

" 2

dn (resolution)

Where dP/P is the fractional change in detected optical power

for an index change of dn, and l/(res) is a measure of the smallest

perceived area. Both the nearfield methods (Refracted and Bound) and

axial interference (or phase contrast) now have F's that are independent

of resolution. Values at 633nm are 39 and 24 respectively. The

superiority of the nearfield methods probably reflects their greater

use of the axial constancy of index, and might disappear for transverse

135
interference. Reflection does not fit into this scheme, so F is not

constant, but for comparison F ^ 900 at lym resolution for plausible

_2

values, or ^ 10 if signal variation as a fraction of input power is

used.

The main significance of those numbers is to underline (for

the first two) their relative similarity, and hence to make the point

that choices of technique are in fact made on grounds other than

ultimate performance. For example, very high resolution axial interference would require the preparation of very thin (micron) accurate

cross-sectional slices of fibre, or again reflection measurement is

optically very inefficient and this causes experimental problems.

It is not therefore the purpose of this paper, indeed it is

probably not practicable, to define a 'best 8 profiling technique for

all purposes, but to define the relative merits and demerits of the

various techniques. Firstly to define the problem:- optical fibres

and, generally, preforms have axial ly invariant index distributions

over macroscopic distances (that is over a measurement length). The

refractive indices do vary with wavelength but this does not normally

affect profile to a measurable degree (c.f. the variation of optimum

a with X which is different ) . The total index differences are generally less than 10 . It may be possible to assume circular symmetry.

Techniques generally divide into axial and transverse, and beyond

this they will be classified according to mechanism. This division

is indicated in Figure 1, Table 1 indicates a rough 'batting order'

for the various techniques according to various criteria, the most desirable being at the top.

In general axial methods are more useful for fibres and

transverse methods for preforms.

The individual methods will be discussed at the meeting.

136
RNF)

(emergent modes) ——— Monomode

Direct phase distortion

measurement

Transverse

Interferometry

Scattering

Deflection function

measurement

Beam deflection

Direct imaging

(spatial filtering)

Dynamic spatial filtering

Focussing

Shearing interferometry

FIGURE 1

137
Ease of Preparation Practical Resolution

Transverse methods Refracted near field

Scatteri ng

Near-field methods ) Bound N.F.

Far-field methods ) Transverse interference)

Reflection (if possible) Axial interference

Focussing

Axial interference Spatial Filtering

Data Processing Required

R.N.F.

Reflection

B.N.F.

Axial Interference

Deflection Function Methods

Far- field

Scatteri ng

TABLE 1

138
A SIMPLE TECHNIQUE FOR HIGH ACCURACY CORE-CLADDING

CONCENTRICITY MEASUREMENT OF SINGLE MODE FIBERS

D. N. Ridgway and L. J. Freeman

Bell Telephone Laboratories

2000 Northeast Expressway

Norcross, Georgia 30071

ABSTRACT Core-cladding concentricity has been measured, to an

accuracy of approximately + 0 . 1 /am using a simple technique that

incorporates a vacuum chuck and a rotational stage. A butt-joint

splice is assembled in the vacuum chuck and one fiber rotated

360°. Transmission loss from maximum to minimum is directly related to core-cladding concentricity for circularly symmetric

fibers

.

INTRODUCTION There are several contributors to splice loss in

single mode fiber splices. These include transverse offset, end

seperation, angular misalignment, unequal spot size, end face

angle and smoothness. Core non-concentricity can cause a transverse offset in an otherwise identical fiber splice. Splicing

techniques in which surface tension effects cause self alignment

of the fiber OD are subject to this offset. In large scale

manufacture, core concentricity may be difficult to maintain,

therefore a simple, accurate measurement may be needed. This

paper presents a technique for core non-concentricity measurement

that meets these requirements. An earlier technique also based

on fiber rotation was presented by Tynes and French.

MEASUREMENT PROCEDURE AND APPARATUS The fiber under test is

placed in the measurement system (Figure 1) . This system consists of a 1.3 jam LED source, the measurement apparatus (Figure 2),

an InGaAs Detector and a PAR 124 Lock-in Amplifier. The measurement procedure is simple and can be performed quickly. The test

139

i
fiber is loose-tube spliced to the launch pigtail and the other

end placed directly in the detector. This establishes a reference level for the unbroken fiber. The fiber under test, approximately four meters, is broken near the middle and the ends

prepared. These fiber ends should be smooth and have an angle

of less than 1°. The fibers are then spliced together through

a rotational stage and held in a vacuum chuck. The fiber mounted

in the rotational stage is then rotated 360° and the transmission

levels recorded every 10°. The transmission level should return

to the reference level to within + .02 dB at some point during

rotation. Maximum splice loss is calculated from the maximum

and minimum transmission level. This maximum loss is related to

transverse offset by the normalized lateral offset curve (Figure

3 4

3) . ' Core cladding concentricity is calculated from normalized

offset, d = x / s , by the simple equation e = ^ • s ; where e is

2

the eccentricity and s is the beam width of the fiber, and x / s is the normalized offset from Figure 3. This calculation assumes a round core and round fiber OD.

The two most important parts of the measurement apparatus are the vacuum chuck and rotational stage. The chuck must

be free from burrs and the groove straight. The chuck is

made of stainless steel, eliminating oxidation that might

cause problems during the measurement.

The rotational stage must be smooth and free from back

lash and front to back movement. The elimination of any excess

movement helps to provide a stable base for the rotational

measurement.

RESULTS Several fibers have been measured using this apparatus and procedure. Fiber 104D showed little if any change in

140
transmission level as can be seen in Figure 4. For this reason

it is felt there is little eccentricity present and the variation in transmission level is within the measurement or noise

level of the system. The second fiber, 104B, exhibits a small

amount of change and the eccentricity is calculated to be .31 /am.

The third fiber, 30B, in Figure 4 shows significant changes in

i transmission level and its eccentricity has been calculated to

be .71 jam. Transmission level differences were repeatable to

within + 0.01 dB and combined with the accuracy of the lateral

4 offset curve, yields a resolution and accuracy of = + 0.1 jam.

CONCLUSION A simple, accurate technique has been presented

to measure core eccentricity in single mode fibers. A buttjoint splice measurement provides transmission level information

that is used to determine core eccentricity of the fiber. This

technique could be useful for core eccentricity characterization

in large scale fiber manufacturing.

REFERENCES

1. A. R. Tynes, W. G. French, "Measurement of Single-Mode Fiber

Core-Cladding Concentricity," Electronics Letters, Vol. 14,

No. 4, February 16, 1978, pp. 113-115.

2. CM. Miller, "Loose Tube Splices for Optical Fibers," BSTJ,

54, No. 7, (September 1975), pp. 1215-1225.

3. D. Marcus, "Loss Analysis of Single-Mode Fiber Splices,"

Bell System Tech. J., (1972), 56, 703-718.

4. R. B. Kummer, S. R. Fleming, "Monomode Optical Fiber Splice

Loss: Combined Effects of Misalignment and Spot Size MisMatch," Optical Fiber Communication Tech. Digest, April 13-15,

1982, Phoenix, Arizona.

141
8P Nl SSOl I HP) 1QNVHD iAIlVl]*

c

3 <

< °r

3 <

E 5

2<

<

Q

<

ct<

a.

cn a. uj < o £

UJ

ac3

142
An Inter! aboratory Measurement Comparison of

Core Diameter on Graded- Index Optical Fibers

Ernest M. Kim

Douglas L. Franzen

U.S. National Bureau of Standards

Electromagnetic Technology Division

Optical Electronic Metrology Group

Boulder, Colorado 80303, USA

Core diameter is an important geometrical parameter used in fiber specification. Unnecessary splice loss can be reduced by joining fibers of similar

dimension. A core diameter of 50 um has been internationally accepted for

multimode graded-index fibers.

Core diameter is usually defined by measuring a distance on the fiber

refractive index profile at the points n

3

, figure 1, where the value of n

3 is

given by

n

3 = n2 + k(n

1 - n

2

) (1)

with n

: being the maximum index, n

2 the homogeneous cladding index, and k a constant generally chosen to be between 0 and 0.05 [1]. Different index profiling techniques reduced to common practice include transverse interferometry

(TI), refracted near-fiel d (RNF) , and transmitted near-field (TNF). TI and

RNF give a more direct measure of the index profile and are therefore believed

to be more accurate. TNF is easy and inexpensive to implement; thus, it is

often used by manufacturers for quality control measurements. Sladen has

shown for uniform mode excitation on short lengths of graded-index fiber, the

near-field intensity distribution across the output end face (TNF) closely

resembles the index profile [2]. Principal deviations include the presence of

tunneling leaky modes and loss of resolution near the core-cladding boundary

due to the decrease in local numerical aperture [3].

While previous interl aboratory comparisons have assessed agreement on fiber attenuation, bandwidth, and numerical aperture; none have addressed core diameter. This paper describes a TNF comparison on core diameter; participants include the National Bureau of Standards and three manufacturing members

of the Electronic Industries Association (EIA). The TNF procedures used in

the comparisons and employed at a wavelength of 850 nm are currently pending

143

) I
before the EI A and require the following conditions: a test sample length of

2.0 ± 0.2 m; a launch having a uniform intensity distribution across the core with a launch numerical aperture exceeding 0.3; a cladding mode stripper; and

a scan of the TNF image with a resolution at least consistent with theoretical

limits. Core diameter was defined using Eq (1) with a k value of 0.05. Each

reported value was the average of three measurements; one measurement included

a new output end and another a fiber rotation of 90 degrees.

Since reported averages consist of a limited number of single scans across diameters, fibers chosen for the comparison were preselected for good core circularity. NBS measured the TNF core diameter of each fiber length

supplied to participants. Seven lengths of each comparison fiber were measured with a random angular orientation; standard deviations ranged from 0.2 to

0.8 ym with an average of 0.4 urn for the six comparison fibers. Comparison

fibers exhibited diverse behavior with respect to index profile. Fiber #1 has

a step structure near the core-cladding boundary; fibers #2 and #6 have depressed index barrier layers at the core-cladding boundary; fiber #3 has a deep on-axis dip with deposition layer structure; fiber #4 has a non-circular

symmetric index dip/peak; and fiber #5 is made by an outside vapor phase deposition process and has no central index dip.

Results of the 5 percent core diameter comparisons on the six fibers are

given in table 1. Measurement standard deviations for fibers #1 to #6 are

0.3, 0.6, 0.6, 0.6, 0.6, and 0.4 urn, respectively. The average standard

deviation, 0.5 ym, is slightly larger than the average precision, 0.4 urn one standard deviation, claimed by participants for their systems. Results are therefore about as good as one could expect.

Figure 2 gives the relative performance between the participants for each

fiber compared to average measured values. Some participants are consistently

high or low with respect to the average, indicating systematic offsets. The

average of the absolute values of offsets between pairs of participants is

0.5 urn.

An NBS calibration reticle was also measured by participants to determine

the degree of systematic offset caused by dimensional calibration error. Participants measured the distance between the centers of lines 1 and 3 on a photol ithographically fabricated reticle using their usual TNF apparatus without a priori knowledge of the actual separation (figure 3). An absolute

144
calibration of the reticle was made by referencing the line separation to

multiple fringes from a Fabry-Perot interferometer using a 0.6328 urn HeNe

laser. Table 2 gives participant results compared to the actual separation of

100.1 urn. In some cases, interlaboratory calibration differences between

pairs of participants are greater than 3 percent or half of the proposed ±6

percent CCITT tolerance for core diameter. Such an offset could significantly

affect the statistics of fibers accepted between two parties. Applying calibration corrections to the data in table 1 reduces the offsets between pairs

of participants in most cases; however, for some the offsets increase indicating other sources of systematic error. Also, 5 percent core diameters for

the comparison fibers were measured at Bell Laboratories by A. H. Cherin and

M. J. Saunders using RNF and at Western Electric by C. Bice using RNF and TI;

a comparison to the TNF results will be given.

References

[1] This definition for core diameter is under consideration by the IEC,

CCITT, and EIA.

[2] F. M. E. Sladen, D. N. Payne, and M. J. Adams, A.P.L. 28, 255 (1976).

[3] J. P. Hazan, Electron. Lett. _14, 158 (1978).

Table 1.

TNF 5% Core Diameters, pm

Participant

Fiber #

1 2 3 4 5 6

25 47.5 44.7 50.7 49.6 50.3 47.5

56 46.9 44.1 49.4 48.8 49.4 46.7

83 47.6 44.9 50.2 49.4 49.7 47.5

47 47.3 43.7 49.5 48.4 50.7 47.0

Average 47.3 44.4 50.0 49.1 50.0 47.2

Standard

Deviation 0.3 0.6 0.6 0.6 0.6 0.4

145
Figure 3. Calibration pattern on NBS reticle,

all dimensions in micrometers.

Table 2. Measured 1 to 3 Distance on Test Reticle

Participant, system Measured value, % Difference ym from actual

25, TNF

56, TNF

83, TNF

47, TNF

100.0

98.9

101.9

98. 7

-0.1

-1.2

+ 1.8

-1.4

146
INDEX OF AUTHORS

K. Abe 55

A. K. Agarwal 59

A. J. Barlow........ 101

N. G. Batty. 13

J. C. Bizeul 17

R. Bouillie.... 17

R. W. Blackmore 13

M. J. Buckler 33

W. K. Burns 97

A. H. Cherin 109

L. Curtis 123

H. P. de Wert 47

M. Eriksrud 63

R. Fergusson *

M. Fox 93

D. L. Franzen 143

L. J. Freeman 13Q

W. B. Gardner 85

P. F. Glodis 89

S. Hornung 105

M. Hoshikawa 51

J. L. Hullett 131

R. D. Jeffery 131

A. F. Judy 109

P. Kaiser 89,123

D. Kalish 89

F. P. Kapron 55

H. Karstensen 59

R. Kashyap 67

E. M. Kim 143

R. B. Kummer 43,109

M. LaRose *

S. Lauritzen 63

C. Marchesi 127

K. Matsui 51

A. R. Mickelson 63

J. G. Nault *

B. P. Nelson 9

M. Nishimura 21

K. Nosu 71

T. Oki 29

P. Pantelis 67

D. B. Payne 79

D. N. Payne 101

M. H. Reeve 105

P. R. Reitz 1 D. N. Ridgway 139

U. Rossi 127

N. Ryen 63

J. Saito 29

L. S. Short 43

J. R. Stern 79

W. J. Stewart 135

F. T. Stone 25

S. Suzuki 21

0. I. Szentesi 37

S. Tanaka 51

C. J. Todd 79

A. Tomita 89

U. Unrau 59

P. J. Vella 55

J. W. Versluis 47

C. A. Villarruel 97

C. C. Wang 97

T. D. S. Wood 79

J. V. Wright 9

H. Yamamoto 29

W. C. Young 123

* = Digest Supplement

U.S. Government Printing Office 1982 - 580-048/360 Reg. 8 147
NBS-11AA [rev. 2-8C) U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET (See instructions)

1. PUBLICATION OR

REPORT NO.

NBS SP-641

2. Performing Organ. Report No. 3. Publ ication Date

October 1982

4. TITLE AND SUBTITLE

Technical Digest - Symposium on Optical Fiber Measurements, 1982

5. AUTHOR(S)

D. L. Franzen, G G W Q Day, and R c L c Gallawa

6. PERFORMING ORGANIZATION (If joint or other than NBS, see instructions)

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

7. Contract/Grant No.

8. Type of Report & Period Covered

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City, State, ZIP)

Same as item 6

10. SUPPLEMENTARY NOTES

Library of Congress Catalog Card Number: 82-600594

|] Document describes a computer program; SF-185, FlPS Software Summary, is attached.

11. ABSTRACT ( A 200-word or less factual summary of most significant information . If document includes a significant

bibliography or literature survey, mention it here)

This volume contains summaries of papers presented at the Symposium on Optical Fiber Measurements held October 13-14, 1982, at the National Bureau

of Standards in Boulder, Colorado. Subjects include the measurement of

attenuation, bandwidth, index profile/geometry, joint/defect, and single

mode fibers. Also included are applied/field measurements and standards.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolon s) Attenuation; bandwidth; fiber optics; fiber optic joints; fiber optics-single mode;

index profile; measurements

13. AVAILABILITY

Unl imited Q For Official Distribution. Do Not Release to NTIS

|~X~| Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402.

Order From National Technical Information Service (NTIS), Springfield, VA. 22161

14. NO. OF

PRINTED PAGES

156

15. Price$6.50

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Public Law 89-306 (79 Stat. 1127), and as implemented by Executive Order 11717 (38 FR 12315, dated May II, 1973) 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.

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