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NBS SPECIAL PUBLICATION 683

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

Technical Digest - Symposium on Optical Fiber

Measurements, 1984

Sponsored by the National Bureau of Standards

in cooperation with the

IEEE Optical Waveguide Communications Committee

and the Optical Society of America.
NATIONAL BUREAU OF STANDARDS

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Technical Digest - Symposium on Optical

Fiber Measurements, 1984

-

Digest of a Symposium sponsored by the

National Bureau of Standards

in cooperation with the

IEEE Optical Waveguide Communications Committee

and the Optical Society of America

October 2-3, 1984

National Bureau of Standards

Boulder, Colorado 80303

Edited by

G.W. Day

D.L. Franzen

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 1 984
Library of Congress Catalog Card Number: 84-601092

National Bureau of Standards Special Publication 683

Natl. Bur. Stand. (U.S.), Spec. Publ. 683, 148 pages (Oct. 1984)

CODEN: XNBSAV

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1984

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

The optical fiber industry continues to evolve and with it, research on the characterization of fiber and related components. The Digests of this

Symposium and its predecessors in 1980 and 1982 reflect these changes.

In 198U about 12 percent of the program dealt with singlernode fibers. In

1982 it was 25 percent and this year about 75 percent. Attenuation measurements appear to have been the major concern at the first meeting. Two years

later the problem of predicting the bandwidth of concatenated links drew a large number of contributed papers. This year the measurement of chromatic

dispersion is obviously a topic of prime concern. Several talks related to

the use of fiber in coherent communications systems and in sensing applications point to the future.

The Symposium continues to draw contributions from around the world. The

25 contributed papers accepted by the committee come from 8 countries, as

f ol 1 ows

:

U.S.A. - 10,

U.K. - 5,

Finland - 2,

France - 2,

Germany - 2,

Japan - 2,

Italy - 1, and

The Netherlands - 1.

These represent about two-thirds of the papers submitted.

The National Bureau of Standards is indebted to the IEEE Optical Waveguide Communications Committee, the Optical Society of America, and members of

the committee for their continued assistance in organizing this Symposium.

G* W. Day

D. L. Franzen

Boulder, Colorado

October 1984

i i i
Symposium Committee

U. L. Franzen, NBS, General Chairman

G. W. Day, NBS, Program Chairman

R. L. Gallawa, NBS

W. A. Gambling, Univ. of Southampton

W. B. Gardner, AT&T Bell Laboratories

M. C. Hudson, Raycom Systems

F. P. Kapron, ITT

W. F. Love, Corning Glass Works

S. Shimada, NTT

G. H. Sigel, Jr., NRL

i v
CONTENTS

Page

Preface iii

Symposium Committee iv

Single Mode Fiber Measurements, I

Single Mode Fibre Specification and System Performance (Invited)

D. 8. Payne, M. H. Reeve, C. A. Millar, C. J. Todd; British Telecom

Research Laboratory 1 Effective Cut-Off Wavelength for Single-Mode Fibers: The Combined Effect of

Curvature and Index Profile

V. S. Shah; Bell Communications Research 7 Length and Curvature Dependence of Effective Cutoff Wavelength

and LP,, -Mode Attenuation in Single-Mode Fibers

H. f. Nijnuis, K. A. H. van Leeuwen; Dr. Neher Laboratories, PTT.... 11

Cutoff Wavelength and Modal Noise in Single-Mode Fiber Systems

N. K. Cheung, P. Kaiser; Bell Communications Research lb

Single Mode Fiber Measurements, II

Issues in the Characterization of Coherent Optical Communications

Systems (Invited)

Takeshi Ito; NTT, Yokosuka Electrical Communication Laboratory 19

Bending and Microbending Loss Sensitivity of Step Index Single Mode Fibers

J. Auge, P. Dupont, L. B. Jeunhomme; Laboratoires de Marcoussis,

CGE Research Center 25

Refractive-Index Profile and Modal Dispersion Prediction for a Single-Mode

Optical Waveguide from its Far-Field Radiation Pattern

W. Freude, A. Sharma; Universitat Karlsruhe 29

A Simple Near-Field Scanning System for Refractive Index Profiles and Mode

Spot Shape

C. A. Millar; British Telecom Research Laboratories 33

Spot -Size Measurements in Single-Mode Fibres

R. Caponi, G. Coppa, P. Di Vita, U. Rossi; CSELT 37

Standards

Compatibility of National and International Standards for Optical Fiber

( Invited)

P. R. Reitz; Corning Glass Works 41

The Characterization of Fiber for Sensors

Optical Fiber Sensors (Invited)

A. Dandridge, J. H. Cole, G. H. Sigel, Jr.;

Naval Research Laboratory 49

v
The Phase Velocity and Loss Coefficient of Optical Fibers Viewed as Stiff Strings

Frank W. Cuomo; University of Rhode Island and Naval Underwater

Systems Center 55

Polarization Shuttle Pulse Technique

C. S. Brown, F. T. Stone; AT&T Bell Laboratories, Norcross 59

A New Technique for the Measurement of Axial -Stress in Optical -Fi bre Preforms

M. P. Varnham, S. B. Poole, D. N. Payne;

University of Southampton.. 63

Measurements on Multimode Fibers

Multimode Fiber Measurements — Present and Future (Invited)

A. H. Cherin; AT&T Bell Laboratories, Norcross 67

A Systematic Approach to Specifying Multimode Fiber Manufacturing Tolerances

D. W. Peckham, S. C. Mettler, R. B. Kummer; AT&T Bell

Laboratories, Norcross 73

Bandwidth Optimisation of a Multimode Fibre Installation

S. C. Hampson; BICC Telecommunication Cables 77

Automated Differential Fiber Strain Measurement System for Single

and Multimode Fiber

K. H. Hafemeister, T. A. Clarke, E. J. Buonopane; Siecor 81

The Distribution of H., Gas Along an Inland Optical Fibre Cable

S. Hornung, S. A. Cassidy, M. H. Reeve; British Telecom

Research Laboratory 85

Instrumentation and Field Measurements

Advances in Optical Time-Domain Refl ectomet ry (Invited)

A. H. Hartog; York Technology 89

1.3 |j.m Portable Ref lectometer for the Field Test of Single-Mode

Fiber Cables

J. J. Bernard, E. Depresles, L. Jeunhomme, J. L. Moncelet;

Laboratoires de Marcoussis, CGE Research Center,

M. Carratt; Compagnie Lyonnaise de Transmissions Optiques 95

Long-Term High-Stable Optical Fiber Loss Measuring Equipment

Y. Namihira, H. Wakabayashi ; KDD,

H. Yamamoto, S. Adachi; Ando Electric. 99

Accurate Determination of Optical Fibre Length from Measurements

in the Frequency Domain

D. L. Walters; Standard Telecommunication Laboratories 103

Elimination of the Influence of Q-Switched-Mode-Locked Laser

Jitter in Sampled Time-Domain Measurements

E. J. R. Hubach; Helsinki University of Technology,

A. B. Sharma; Tampere University of Technology,

S. J. Halme; Helsinki University of Technology 107

vi
Single Mode Fiber Measurements, III

Accurate Specification of Single-Mode Dispersion Measurements

Felix P. Kapron, Tom C. Olson; ITT, Roanoke Ill

Multiple-Wavelength System for Characterizing Dispersion in

Single-Mode Optical Fibers

Robert A. Modavis, Walter F. Love; Corning Glass Works 115

High Accurate Automatic Measurement Equipment for Chromatic

Dispersion Making Use of the Phase-Shift Technique with LDs

K. Tatekura, H. Nishikawa, M. Fujise, H. Wakabayashi; KDD 119

Precision Interferometric Measurement of Dispersion in Short

Single Mode Fibers

M. J. Saunders, W. B. Gardner; AT&T Bell Laboratories, Norcross 123

Interferometric Dispersion Measurement in Single-Mode Fibers with

a Numerical Method to Extract the Group Delays from the Measured

Visibility Curves

L. Oksanen, S. J. Halme; Helsinki University of Technology 127

Comparison of Chromatic Dispersion Measurements of Single-Mode

Optical Fibers by Spot-Size and Pulse Delay Method

H. Karstensen, L. Wetenkamp; Institut fuer

Hochf requenztechni k , Braunschweig 131

Field Dispersion Measurements -- A Swept Frequency Technique

R. Rao; Valtec 135

Index of Authors 139

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.

vi i

Single Mode Fibre Specification and System Performance

D B Payne, M H Reeve, C A Millar, C J Todd.

British Telecom Research Laboratories, Marti esham Heath, Ipswich, UK.

It can now be safely stated that single mode fibre is

established as a viable transmission medium for communications networks.

The fibre types being adopted around the world as 'standard'

have a nominal step index profile and are either of the matched cladding

or depressed cladding designs. During the research and development phase

of these fibres many different parameters, using a wide range of

measurement techniques, were used to characterise the behaviour and

increase our understanding of these fibre types. From this plethora of

measured characteristics and measurement techniques it was necessary to

distill the few essential characteristics and simplest measurement

techniques necessary to ensure reliable and acceptable performance of

the fibre in systems.

Two parameters are emerging that appear to be very good for

specification purposes, these are a measurement of mode width and the

cut off wavelength of the LPi t mode. These two parameters define a

plane as in figure 1. Each point in the plane defines uniquely a fibre

with nominal step index profile behaviour. As we move about the plane in

any direction, problems of one sort or another can arise that will

impair system performance. By measurement of such parameters as cabling

loss susceptibility LI, 2D, dispersion characteristics C3,4!l and fibre

loss C5,63, together with splice loss analyses C7,8] and the need to

avoid bimoded operation C93, bounds can be placed on the W, Xco plane

which will define a safe operating regime for this fibre type.

It is still a matter of some debate as to which measurement

techniques and definitions should be used for the mode width and cut off

parameters. At British Telecom we have favoured the offset splice

technique C101 as a single measurement for both parameters. This

technique has been adopted as one of two possible test methods in the

draft CCITT recommendation C113 for the mode width parameter. However in

order to obtain a measure of cut off wavelength it is necessary to use

an offset splice technique that will measure mode width as a function of

wavelength. Because it is obviously possible to set up an offset splice

measurement system to measure the width parameter at the single

wavelength required for the specification diagram' (the W, Xco plane at

1300nm) alternative techniques may be desirable for the measurement of

cut off wavelength. Because of this the CCITT recommendation proposes a method not involving the offset splice technique for cut off

measurement, this method being instead based on power transmission

techni ques.

Fundamentally it does not matter which definition or measurement

technique is used for mode width and cut off as long as they are self

consistent. For example the measurement of mode width as made by the

offset splice technique is a measure of the width of the field overlap

integral. This measurement and definition of W is a measurement of a real physical parameter, no approximation is made at this stage. If the

functional form of the field is known then this measured parameter can

1
be related to a width parameter o-f the real -field and, -for simple -fibre

structures, to models o-f the profile (when combined with the cut o-f-f measurement). As an example for -fibres with near step index profile

behaviour the fibre can be modelled via the width of the overlap

integral (as measured by the offset splice technique) and cut off

wavelength using the Marcuse relationship:

W = a (0.65 + 1.619V- 1 - 5 + 2.879V-**)

and V = 2.405A«©A = ka(Nl^ - N2^)

From this relationship an equivalent step index profile can be derived

in terms of the so called ESI parameters 'a' and 'An ' . The measured

parameters and measurement technique are a self consistent set linked

via the ESI model of the fibre profile.

Theoretically there is only one defined value for the cut off

wavelength of the second mode, that is when the mode index equals the

cladding index. Unfortunately this theoretical cut off cannot be

determined for a real fibre without recourse to some measurement

technique eg. a measurement of the refractive index profile. Because

different measurement techniques can give significantly different

results for cut off wavelength the measurement has become somewhat

controversial. However because there is only one value of cut off

wavelength for the fibre it should be possible to relate all the various

measurement methods to one reference method (regardless which one) via a

definition of cut off, appropriate to the measurement method being

employed.

The mode width and cut off wavelength parameters are being

successfully used for the specification of near step index profile

fibre. The use of the parameters could be extended to other profiles as

long as W and %ca are single valued functions of the parameters

determining the fibre profile. This requirement of single valued

functions for specification purposes could be used to define a class of

fibre. Indeed if a fibre profile was sufficiently complex that normal

manufacturing tolerances caused the general specification parameters (eg

W and \co) to become mul t

i -val ued , then the fibre could be deemed to be

unspeci f i abl e interms of these general parameters. In this case it may

be necessary to specify (and measure) the specific performance

parameters for which the fibre was designed.

So far we have considered the fibre specification in isolation

from the system specification. A fibre user is ultimately interested in

minimising system cost consistent with adequate system performance. A

notable feature of optical systems is that the parameters affecting

system performance are all random variables with variances significant

compared with means. This can mean that system performance and

probabilities of system failure can be considerably influenced by

different purchasing and planning strategies without recourse to any

fine tuning of component specifications. This will be discussed further

at the conference with examples, using a statistical model of the

system, illustrating possible effects of different purchasing stategies.

If we consider the trends in optical technology and speculate

about possible system requirements five to ten years hence one or two

features of a future fibre begin to emerge.

It is very likely in the future that single mode fibre

technology will reach into all parts of communications networks, from

2
long haul submarine and trunk systems to small networks within a single

building. It would be desirable, though not essential, if the same -fibre

was used for all these networks, simply to increase economy of scale. On

long haul systems there will be a continuing demand to increase range

and capacity. Range could be limited by the power budget available and

the system loss or dispersion problems. On short distance local'

systems, range is not important but high channel number could be. To

achieve this high channel number extensive wavelength multiplexing may

be used. Also future systems, both long and short distance, may exploit

coherent optical technology.

Dispersion effects were given a considerable amount of attention

during the design of the first generation' monomode fibres (primarily

single window fibre with nominal step index profile). It is more

questionable that it will be an important system parameter for the next

generation fibre. If the transmitted optical line width is determined by

|! the message bandwidth then several Gb/s data rates are possible over

200-300 km of fibre even in the 1550 window, using present day fibre.

For the forseeable future, dispersion is a device problem not a fibre

problem. On the time scale of 5-10 years it is not unreasonable to

postulate spectral control of sources with (if necessary) external

modulation, giving line widths determined by the modulation applied. The

necessity for exotic -fibres with a primary aim of controlling dispersion

properties, can therefore be considered unlikely. However beyond second

generation' -fibre when very high bit rate systems over very long

distances could become practical, dispersion control in fibres may again

i ntrude.

The more important parameters in a future fibre will have more

to do with loss and possible power budgets, the latter being determined

ultimately by non-linear effects limiting optical power densities in the

fibre. The loss of the link will be determined by a combination of fibre

loss, splice loss, and cabling loss. Fibre designs that reduce

simultaneously all these component losses would be of interest, for

example profiles that increase the width of the overlap integral while

constraining the field shape to minimise cabling loss would be valuable.

If the profiles required to achieve these results are complex then it

may well be that simple two parameter specifications will not be

adequate and specification of the variables of interest will have to be

made separately.

For mul ti -channel coherent systems polarisation maintaining

fibres may be necessary. It may well be that this requirement will be

the most important in determining a new fibre design if dispersion is

not important and loss can only be marginally reduced by fibre design.

This will add a whole new dimension to the fibre specification.

The near step index pro-file -fibre is now being installed in

quantity into various communications networks around the world and

unless there is a very clear overiding advantage the administrations

involved will be very unwilling to consider changing to a new fibre

design for some considerable time (except maybe for small specialist

requirements). When considering totally new fibres therefore, we are

probably thinking in terms of systems application at least five to ten

years hence. On this time scale it is unlikely that the system

requirements that determined the current fibre specification will still

apply.

3
Acknowledgements.

The authors wish to thank J V Wright and K Blow for useful

discussions and the Director o-f Research, British Telecom -for permission

to publish this work.

Re+erences.

1. S Hornung and M H Reeve Elec. Lett 17, 774 (1981)

2. AD Pearson, P D Lazay, W A Reed, M J Saunders Proceedings ECOC

Canes 1982.

3. B P Nelson and S Hornung Elec Lett 18, 270 (1982)

4. U C Peak, 6 E Peterson, A Carvale Applied Optics Vol 21, No. 19

1 Oct 1982

5. M H Reeve, C A Millar Proceedings ICC 1984, Amsterdam, Vol 3, pl056

6. P D Lazay and A D Pearson Technical Digest, OFC 1982, Phoenix, THCC2

7. D Marcuse, Bell Sys. Tech. Journal, Vol 56, No. 5, May/June 1976.

8. D B Payne and D J McCartney Proceedings ICC 1984, Amsterdam,

Vol 3, pl075

9. K Petermann Elec Lett 15, 706 (1979)

10. C A Millar "Technical Digest, OFC 1982, phoenix, THEE3

11. CCITT Recommendation G652, Study Grp xv, Geneva, May 1984.

4
Cabling

problems

Cabling problems

at longer wavelengths

Qt

OfE

i_(0Q.

a;o

2 Dispersion

problems

OPERATING

AREA

Bl-moded

operation

Increasing splice loss

and fibre loss

CO

FIGURE 1

5
I
Effective Cut-Off Wavelength for Single-Mode Fibers: The Combined Effect of

Curvature and Index Profile.

V. S. Shah

Bell Communications Research, Inc. Crawfords Corner Road

Hoimdel, New Jersey 07733

SUMMARY

In single mode transmission systems, the effective cut-off wavelength is an important

design parameter separating the single-modal and bi -modal regime in an optical fiber. Controlling

the effective cut-off wavelength instead of the theoretical cut-off wavelength makes the design less

sensitive to the various loss mechanisms which always exist in a practical system. In general, the

effective cut-off wavelength is found to be somewhat shorter than the theoretically predicted value,

[1-3] resulting in a more robust design that allows the system to operate at a higher V value and

thereby confining the fundamental mode more tightly to the core. The effective cut-off wavelength

for use in the system should be short enough to attenuate the higher-order mode sufficiently to

preclude modal noise so as not to degrade the signal-to-noise ratio of the system. [4]

One of the methods being used to determine the effective cut-off wavelength is the bend

induced loss [5] (transmitted power) method. Since in this method, the results depend upon the

length of the fiber as well as the bend diameter, CCliT study group XV recently recommended that

2 meter length of fiber with a single loop of 28 cm diameter be used.

In light of the importance of the effective cut-off wavelength in system design and the

need to standardize its range, a study program is in progress in our Laboratory to gain further

understanding of the dependence of the effective cut-off wavelength on various parameters. Initial

results of the measurement of the effective cut-off wavelength (with the bend diameter as a

parameter) on three fibers are reported here. One fiber is of matched cladding design while the

other two fibers are of depressed cladding design (Figure 1). The measurements were made using a

7
2 meter long fiber. For each fiber , the transmitted power levels for several radii of curvature R are

compared with that for the fiber with a small single loop 2 5 cm in diameter. The effective cut-off

wavelength is determined to be the wavelength above which the small loop introduces a loss of less

than .ldb in the output power.

Figure 2 shows the results of the above measurement made on the fibers. The result for

fiber #1 (matched cladding design) shows that as expected, the effective cut-off wavelength shifts

towards smaller values (due to the increasing radiation loss) as the radius of curvature R=D/2

decreases. However, for the other two fibers, (depressed cladding design) the effective cut-off

wavelength increases at first and then decreases as R decreases from an initial large value.

Using the procedure similar to that employed by Marcuse [6] to calculate the radiation

loss of LPqj mode due to a constant curvature in a depressed cladding fiber, a radiation loss

formula is being developed for LP-q mode in a depressed cladding fiber to explain the experimental

observation. We will report and discuss the results of such calculation (which is based on an infinite

outer cladding model) . Additional measurement data will also be presented

ACKNOWLEDGEMENT

The author would like to thank P. Kaiser and W. C. Young for their encouragement and

helpful discussions.

REFERENCES

[1] W. T. Anderson, "Measuring the Effective Cut-Off Wavelength of a Single-Mode Fiber - The

Effect of Fiber Length," Conf. on Precision Electromagnetics Meas. Digest, L-12 (Boulder,

June 1982)

[2] P. D. Lazay, 'Effect of Curvature on the Cut-Off Wavelength of Single Mode Fibers," Tech

Digest Symp. Opt. Fiber Meas., 93 (Boulder, Oct. 1980)

8
[3] G. W. Tasker, et al., "Low-Loss Single-Mode Fibers with Different ^O-j - Si02

Compositions," Appl. Opt., 17, 1836 (1978).

[4] N. K. Cheung and P. Kaiser, "Modal Noise in Single-Mode Fiber Transmission System," to

be published in FjCOC on Opt. Comm. '84, Stuttgart.

[5] Y. Katsuyama, et. al. "New Method for Measuring V-Value of a Single-Mode Optical

Fiber," Beet. Lett, 12, 699 (1976)

[6] D. Mareuse, 'Inference of Curvature on the Losses of Double Clad Fibers," Appl. Opt., 21,

4208 (1982).

n

1

"2

|—-2a-H

|

j

n

1

n

3

J Ln

_2

2b -

(b)

Fig.1 Fiber design (a) Matched cladding; (b) depressed cladding

9
10
LENGTH AND CURVATURE DEPENDENCE OF EFFECTIVE CUTOFF WAVELENGTH AND LP n

1

-MODE

ATTENUATION IN SINGLE-MODE FIBERS

H.T. Nijnuis, K.A.H. van Leeuwen

Dr. Neher Laboratorium, PTT

P.O. Box 421, 2260 AK Leidschendam

The Netherlands

The cutoff phenomenon of the next higher order (LP11) mode in singlemode fibers is best described by the wavelength-dependent attenuation

coefficient of the LPii-mode (aii). This coefficient also depends on the geometrical condition of the section of fiber under consideration. The simple

case of a fiber, bent to a constant radius of curvature, is sufficient to model the fiber in most practical applications. Therefore, the specification of

aii as a function of wavelength (A) and radius (R) generally provides a satisfactory operational description of the cutoff behavior.

It is currently common however, to describe the cutoff in terms of an effective cutoff wavelength (Ace ), defined as the wavelength above which there is

(practically) no light transmitted in the LPii-mode through a section of fiber.

Making this definition strict by requiring a minimum total attenuation of the

LPii-mode in the studied fiber section, it is clear that Ace will depend on the

length of the fiber section (L) as well as on its geometrical condition. It can easily be shown that with this definition a description in terms of the function Ace (L,R) is completely equivalent to the description by the function

otn(X,R)

.

The specification of a single number, e.g. the effective cutoff wavelength

of a section of fiber of specified length under specified conditions measured

according to the CCITT-recommendations 1

, usually does not provide enough information to determine e.g. whether modal noise may become a problem in a specific

situation. However, determination of the complete function Ace (L,R) (or an(A,R))

for every reel of fiber to be used is, of course, not realistic. Thus it would

be attractive to obtain some empirical "rules-of-thumb" giving the length- and

curvature dependence of a fiber with a given type of index-profile, leaving

only one cutoff wavelength to be determined for each individual reel as is common practice already.

In this contribution we report on measurements of the length and curvature

dependence of the effective cutoff wavelength in a number of single-mode fibers

11
with various types of index-profile, aimed at establishing such empirical rules.

The results indicate approximately linear relations between the cutoff wavelength and the logarithm of the fiber length (as has been found before2 ), and

between cutoff wavelength and the inverse of the radius of curvature to which

the fiber is bent.

The experimental apparatus consists of a standard set-up used for wavelength-dependent attenuation measurements. The light of a filament lamp is filtered by a monochromator and coupled into the test fiber through a microscope

objective. Care is taken to overfill both LPqi and LPi^ modes. Light emerging

from the far end of the fiber is detected with a pigtailed InGaAs photodiode.

For fibers having non-stripping primary coatings, cladding mode strippers are

used both at the input- and output end of the test fiber.

The total attenuation of the LP\i mode as a function of wavelength is determined from three wavelength scans of the monochromator. First the detector

signal is recorded with the test fiber wound a number of turns (1 to 20)

around a spool with the desired radius, second, the number of turns is reduced

to either 0 or 1 , the excess fiber removed and the signal is again recorded as

a function of wavelength and third, a few small loops (1 to 2 cm radius) are inserted in the fiber at the detector end and the last scan is made. The number

and radius of these loops are chosen so as to remove all light in the LPi| mode

in the wavelength region of interest.

Designating the three recorded detector signal vs. wavelength curves Pi (A),

P2(A) and ?3(A) respectively, the attenuation of the LP;q mode (in dB) is calculated : -10 10 log((P 1 (A)-P 3 (A))/(P 2 (A)-P 3 (A))) . Dividing the attenuation by the length of the fiber section that has been removed in the second step of the procedure, au(A) curves result. An effective cutoff

wavelength is obtained by determining the wavelength at which the attenuation

equals 3 dB (arbitrary value chosen for convenience).

Of the tested fibers, first the cutoff wavelength has been determined at

14 cm radius of curvature for a number of fiber lengths ranging from 1 to 20

meters. Afterwards, the cutoff wavelength for three meters of fiber bent to a number of radii ranging from 2.25 to 14 cm has been determined.

The results for four studied fiber samples are shown in Figs. 1-4, in which

the effective cutoff wavelength is plotted against the logarithm of fiber length

and against the inverse of the radius of curvature. Best-fit straight lines

through the data points are also shown.

12
The straight lines in Figs. 1-4 correspond to a simple three-parameter

empirical formula for the effective cutoff wavelength:

Ace (L,R) = A 0 - a log(L) - b/R

with Ao the cutoff wavelength of 1 meter of stretched fiber, and a and b co- efficients representing the length-dependence and bending-sensitivity of the

cutoff wavelength respectively.

Table 1 summarizes the best-fit parameters for the 4 studied fibers.

SAMPLE PROFILE TYPE A 0 (nm) a(nm/decade) b(nm/cm x )

1 matched cladding 1139(5) 41(5) 617(22)

2 matched cladding 1227(7) 50(12) 564(28)

3 strongly depressed cladding 1282(6) 74(8) 375(14)

4 slightly depressed cladding 1166(12) 35(14) 375(53)

TABLE 1 Best-fit parameters for studied fibers. Errors are statistical.

Both length-dependence and bending-sensitivity are seen to vary by a factor

of two between fibers. This stresses the impossibility to describe the cut-off

of the LPi]^ mode by a single parameter. It is interesting to note that the coefficients a and b as determined for the two matched-cladding type fibers (supplied by different manufacturers) are nearly equal. This suggests the possibility of estimating these parameters on the basis of the index-profile, which

possibility, if confirmed, will be clearly of considerable practical value.

It is obvious from Table 1 that the cutoff wavelength in both depressedcladding fibers does not depend as strongly on the curvature of the fiber as in

both matched-cladding fibers. This can be understood by assuming the dominant

loss mechanism for the LPi^ mode to be different for both types of index-profile

("tunneling" of light from the core to the outer cladding3 in depressed-cladding

fibers, and bending losses in matched-cladding fibers).

A consequence of the weak curvature dependence of the LPii-mode attenuation

in depressed-cladding fibers is, that insertion of a single loop of 2 cm radius

in the fiber for the removal of the LP\i mode as recommended by the CCITT 1 is

not sufficient.

References

(1) CCITT - COM XV-289-E - Draft recommendation G.652

(2) Anderson, W.T. and Lenahan, T.A.: J. of Lightwave Technology LT-2, 238 (1984)

(3) Jeunhomme, L.B.: Single-mode fiber optics (1983) (New York: Marcel Dekker)

I 13
Figure l a

: Effective cutoff wavelength vs. the logarithm of fiber length (in meters). Fiber sample 1.

.5 1. 1.5

Figure 2 a : Effective cutoff wavelength vs. the logarithm of fiber length (in meters). Fiber sample 2.

-.5 0. .5 1. 1.5 — "lo»(L) Figure 3a : Effective cutoff wavelength vs. the logarithm of fiber length (in meters). Fiber sample 3.

.5 — '"loud.) Figure 4a

: Effective cutoff wavelength vs. the logarithm of fiber length (in meters). Fiber sample A.

.2 .3 .4 .5 — 1/R <cb-') Figure 1 D

: Effective cutoff wavelength vs. the in verse of radius of curvature. Fiber sample 1.

—. l/R (ca-') Figure 2^: Effective cutoff wavelength vs. the in verse of ridius of curvature. Fiber sample 2.

.2 .3 — l/R (car 1 ) 7igure 3^: Effective cutoff wavelength vs. the i: verse of radius of curvature. Fiber sample 3.

.1 .2 .3 .4 .5 l/R (ca- 1 ) Figure 4°: Effective cutoff wavelength vs. the ii verse of radius of curvature. Fiber sample 4.

14
CUTOFF WAVELENGTH AND MODAL NOISE IN SINGLE-MODE FIBER SYSTEMS*

N. K. CHEUNG and P. KAISER

BELL COMMUNICATIONS RESEARCH, INC.,

HOLMDEL, NEW JERSEY 07733 U.S.A.

In practical single-mode fiber communication systems

one generally distinguishes between the theoretical cutoff

wavelength of the first higher order mode, and the

effective cutoff wavelength which may be substantially

shorter than ^

tlj by as much as 100 to 200 nm. The

theoretical cutoff wavelength is computed from the

refractive index distribution of the core, and its value is

obtained experimentally with short, straight fiber sections

(say, on the order of 1 cm). Near the theoretical cutoff

wavelength the LP-^ mode is highly attenuated, and since its

relative intensity depends strongly on fiber length and

curvature, single-mode fibers can actually be operated

slightly in the multimode domain [1]. The effective cutoff

wavelength is therefore defined for a fiber of a specific

length deployed with a certain bend radius. While in the

past different combinations of such parameters have been

used for this purpose [2] -- making a meaningful comparison

of cutoff wavelength data originating in different

laboratories difficult ~ most recently an agreement has

been reached to determine the effective cutoff wavelength

with a 2 meter long single-mode fiber containing a 28 cmdiameter loop [ 3 ]

.

The challenge arises to properly choose the effective

cutoff wavelength relative to the systems operating

wavelength, while considering practical cabled-fiber lengths

deployed with particular bending radii. In this regard, it

should be noted that the effective cutoff wavelength was

found to decrease logarithmically with fiber length, with a reduction of 20 to 30 nm per decade of length (in meters)

having been observed for certain types of fibers [4] (i.e.

the effective cutoff wavelength of a 1 km long fiber is

approximately 60 to 90 nm shorter than that of a 1 m fiber

section). Similarly, the effective cutoff wavelength

typically decreases with curvature for matched-cladding

fibers, but has much less dependence on curvature for

depressed-cladding fibers [5].

In order to avoid multimode and associated modal noise

and systems degradation effects [6], it appears prudent to

choose the cutoff wavelength as low as possible with respect

to the 1.3 ym operating wavelength region. However, a short

*) Fart of this work was performed while the authors were

with A.T.&T. Bell Laboratories.

15
cutoff wavelength results in weaker guidance of the

fundamental mode in the 1.55pm wavelength region, with

accompanying increased sensitivity of such f ibers ' towards

macro-bends and micro-bend-induced cabling losses. Since it

is desirable to maintain lowest losses in the 1.55 ym

wavelength region, the cutoff wavelength has to be optimized

such as to avoid modal noise effects in the 1.3 ym region,

and to minimize the sensitivity of the fundamental mode

towards macro- and micro-bend- induced excess losses in the

1 . 55 ym region.

In this paper we report on the observation of modal

noise effects for different fiber interconnection schemes

containing intentionally overmoded fiber sections [7],

Connector ized fiber sections of length L and cutoff

wavelengths X c2 were inserted between the laser pigtail and

another jumper cable with cutoff wavelengths X (Fig. 1).

X_, was measured as described above, and was chosen to be

1 /2"4 ym throughout. The laser wavelength X satisfied the

condition < ^ L

< X C2 . with ^ L= 1.26 ym, modal noise

was readily observed for 1 m long interconnection cable

sections having > 1«31 ym, and with the overmoded fiber

deployed straight (Fig. 1(a)). The noise was due to the

mode-partitioning of the multi-longitudinal-mode laser

employed resulting in the fluctuation of the coupling

between the LPg^ and LPji modes at the two joints of the

central jumper. The modal noise disappeared when a 2.5 cm

diameter loop was introduced into the central jumper (Fig.

Kb)), or when the jumper was removed (Fig. 1(c)). A

typical power penalty measurement performed for aim long

fiber with a cutoff wavelength of 1.46 ym deployed straight

is shown in Fig. 2. For a longer fiber section L of 30 m

deployed with a diameter of 35 cm or a section L of 1 km

wound on a 15 cm spool, no power penalty was observed in

spite of the high 1.46 ym cutoff wavelength (Fig. 3). It is

important to note that modal noise has also been observed

when the fiber used in the laser pigtail has a high cutoff

wavelength X relative to X , and A as shown in Fig.

1(d).

C1 L

In conclusion, we have demonstrated that single-mode

fiber systems can be operated well above the effective

cutoff wavelength provided the fibers are sufficiently long,

or are deployed with appropriate bends. Cutoff wavelengths

of 1.31 ym have been used successfully in previous singlemode fiber system experiments where the laser wavelength was

as low as 1.272 ym [8], and fibers with cutoff values as

high as 1.46 ym have been used in conjunction with 1 km long

fiber sections in the experiments described in this paper.

In order to leave some margin for laser wavelength

variations and wavelength measurement tolerances, cutoff

16
values for outside plant fibers on the order of 1.35 y m are

considered to be readily usable. In case of shorter fiber

sections such as pigtails, interconnect cables, and repair

lengths, mode filters in the form of bends or loops with

suitable radii can be introduced to sufficiently attenuate

the first higher order mode. On the other hand, mode filters

are not required if the cutoff wavelength of short fibers is

chosen lower than that of the outside plant fiber, say, by

approximately 100 nm.

Acknowledgment is given to P. Glodis of A.T.&T. Bell

Laboratories who supplied the fibers used in this

experiment

.

REFERENCES

[1] P. Kaiser et al, Topical Meeting on Optical Fiber Fiber

Transmission, Technical Digest, Williamsburg- (1977)

[2] W. T. Anderson, Conference on Precision Electromagnetic

Measurements, L-12, Boulder (1982), IEEE Cat. No.

82CH1737-6

[3] CCITT SGXV Recommendation G.652, Geneva (1984)

[4] Y.- Kitayama and S. Tanaka, OFC'84, Tun 14, New Orleans

(1984)

[5] V. Shah, Symposium on Optical Fiber Measurements,

Boulder (1984)

[6] S. Heckmann, Electron Lett., 17, p. 499 (1981)

[7] N. K. Cheung and P. Kaiser, ECOC'84, Stuttgart (1984)

[8] P. Kaiser et al, IOOC'83, 27C2-4, Tokyo (1983)

17
(a) LASER

LP01

*C1 CO *C2

p VARIABLE

L 01 ATTENUATOR 03

—

REC.

(b) LASER

*C1 *C2 =00= 00= *C1 REC

(C) LASER

*C1 X C1 =DO=4>= REC

(d)

Xc2

LASER [ REC

Fig. 1 Different interconnection schemes to

study modal noise in single-mode

fibers X C1 <

A L <

A C2

WITH 1 m 0VERM00ED

JUMPER

36 36 34 33 32 31

RECEIVED POWER (dim)

29 28

WITH 1 km 0VERM00E0

FIBER WITHOUT 1 km OVERMOOEO FIBER

-36 -35 -34 -33 -32 "31 "30 -29 "28 RECEIVED POWER (OBm)

Fig. 2 Bit-error-ratio curves with

and without overmoded jumper for

config. shown in Fig. 1(a), X T = 1.26 ym, X ni = 1.24 ym, \ = 1.46 ym,

Fig. 3 Bit-error-ratio

curves with and without

1 km of overmoded fiber, same wavelength combinations as in Fig. 2

and L = 1 m

18
ISSUES IN THE CHARACTERIZATION OF COHERENT

OPTICAL COMMUNICATIONS SYSTEMS

Takeshi Ito

Yokosuka Electrical Communication Laboratory, NTT

1-2356 Take, Yokosuka-shi , Japan

These few years, research on coherent optical transmission systems

has been prosecuted enthusiastically by a couple of laboratories. It

raises up world-widely the "revival" of an interest in coherent systems.

The present major interest is to seek ever more sensitive receivers,

ever longer repeater spans, ever greater transmission capacities and so

on. Laying stress on the sensitivity of a receiver, this paper compares

a coherent system with a direct detection system, referring to published

or reported experimental data, and describes the future research items.

As well known, theoretical shot-noise bounds for a predetermined

error rate are almost equal between coherent and direct detection

systems, if a quantum detector is used(l). One of appeals for coherent

systems seems that the sensitivity could easily reach the shot-noise

bound, compared with direct detection systems. The other is that

optical angular modulation (FSK, PSK) suppress the stimulated Brillouin

scattering so that a greater power could be launched into a fiber(2).

Receiver sensitivity and transmitting optical power that is

obtainable at a 1.5 micron wavelength are shown in Fig.l, including an

expected sensitivity and observed data for direct detection systems.

Here, photodetector sensitivity is assumed to be close to Si-detector

sensitivity at 0.8 micron wavelengths. The PSK modulation-homodyne

19
detection scheme

direct detection

will imp

scheme

rove the sensitivity more than lOdB over the IM

Features of coherent transmission experiments conducted so far are

summarized in TABLE I (3)-(8). All of fundamental modulation-detection

schemes have been examined at a bit rate from 100 to 200Mb /s, at wavelengths from 0.82 to 1.52 microns. The achieved receiver sensitivity

ranges from 70 to 1300 photons per bit. Using a He-Ne gas laser as a

signal oscillator, its sensitivity reaches nearly the theoretical

shot-noise bound. On the other hand, if a laser diode is directly

modulated, few differences are found between the results summarized in

the table and an attainable sensitivity at the same bit rate in a direct

detection system at the present time. In most those experiments, there

appears no floor phenomenon in the relationship between the received

optical power and a bit error rate. However, a bit error rate less than

-9

10 has never been measured. More stable operations of oscillators

and control systems such as an AFC and a PLL circuit could realize a bit

error rate less than the above-mentioned value.

Major research items are summarized in TABLE II. Both a high speed

direct detection system and a coherent detection system need stabilized

single-frequency light sources. However, the requirement for coherent

system light sources is significantly different from the requirement for

the light sources in a direct detection system. A lOOMb/s PSK coherent

system requires, for example, less than 1MHz light source spectrum

width. The above performance is required for a local oscillator in a

receiver as well as for a signal oscillator in a transmitter. Against

20
the above requirement, a conventional single-frequency laser diode, such

as a DFB laser diode, has a comparatively wide spectrum width of 50 to

100MHz at 1.3 to 1.55 micron wavelengths (9) . An automatic frequency

control (AFC) circuit using a Fabry-Perot etalon or moleqular absorption

has achieved frequency stability less than 1MHz per a few minutes up to

two hours(lO). However, a practical coherent system needs long term

frequency stability as well as short term stability.

To realize the above-mentioned sophisticated modulation and

detection techniques, a coherent system requires modulators, isolators,

directional couplers and other components for optical circuitry in

addition to single- frequency narrow-spectrum-width laser diodes. It is

very important to suppress thoroughly insertion losses of those

components in order to establish a repeater gain that could be obtained

by sensitivity improvement and transmitting optical power increase.

Polarization holding through transmission medium is desirable

because of polarization sensitivity in the detection. Various types of

polarization holding fiber have been developed so far(ll). On the other

hand, polarization control is inevitable if a conventional single-mode

fiber is used as a medium, because a polarization state changes due to

the variation of circumstances such as an atmospheric temperature.

Polarization change, however, is observed to be very slow. For example,

polarization axis rotated at maximum at a rate of 20 degrees per minute

for a submarine-cabled fiber of 10km layed at the bottom of the sea(12).

In fact, polarization is manually compensated to match that the local

oscillator in the experiments summarized in TABLE I.

21
In conclusion, in addition to the characterization of fiber

polarization performance and modulation and detection component

performance, it is very important, at present, to characterize a fine

structure of laser diode spectrum as well as its frequency stability, in

order to clarify its residual AM, FM and/or PM noise characteristics and

to demonstrate the feasibility of a coherent system using a laser diode.

It is because a laser diode seems to be mostly preferable from a

practical point of view. The research on these sophisticated optical

transmitters and receivers will make clear great scope for ever longer

repeater-span transmission systems and/or ever more frequency-divisiontransmission systems. This may accelerate optical technology towards

the shot-noise limit or beyond it.

Acknowledgements The author wishes to gratefully acknowledge the

sensible and occasionally shrewd and suggestive comments from Drs.

Kiyoshi Nosu, Ikutaro Kobayashi and Yoshihisa Yamamoto. Acknowledgement

is also due to Drs. Sadakuni Shimada and Masaki Koyama for their

encouragement

References

(1) Y. Yamamoto, IEEE J. Quantum Electron., vol.QE-16, no . 1 1 , Nov. 1980.

(2) D. Cotter, Electronics Letters, vol.18, no. 12, June 1982.

(3) M.Shikada et al, Electronics Letters, vol.20, no. 4, Feb. 1984.

(4) D.J.Maylon et al, ibid., vol.19, no. 4, Feb. 1983.

(5) S.Saito and Y. Yamamoto, presented at 1983 Nat. Conven. IECE Japan

(in Japanese), March 1983.

(6) K.Emura et al, ibid. no. 14, July 1983.

(7) R.Wyatt, T.G.Hodgkinson and D.W.Smith, Electronics Letters, vol.19,

(8) D.J.Maylon, ibid., vol.20, no. 7, March 1984.

(9) Y. Yamabayashi and M. Saruwatari , presented at 1984 Nat. Conven. IECE

Japan(in Japanese), March 1984. Aug. 1982.

(10) T.Okoshi, IEEE Trans. Microwave Theory Tech., vol.MTT-30, no. 8,

(11) Y.Sasaki and J.Noda, presented at 1983 Nat. Conven. IECE Japan (in

Japanese), March 1983. (in Japanese), March 1984.

(12) T.Imai and K. Yanagimoto , presented at 1984 Nat. Conven. IECE Japan

22
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23
TABLE II MAJOR RESEARCH ITEMS FOR DIRECT AND COHERENT DETECTION SYSTEMS

... Direct Detection System Coherent Detection System

Signal Light

Source

Single-frequency

oscillation,

Self optical injection.

Frequency stabilization,

Narrower spectrum-width

(Residual AM, FM, PM noise

suppression)

Self optical injection.

Modulation Frequency chirping. external

Optical loss reduction,

Higher efficient modulation,

direct

RpcinHnpl UCOluual nnTQP LIUIOC QiinrirpQ^inn o l_l u u 1_ c O O J- \-J L I .

Detection APD performance

improvement

Lower noise amplifier.

Quantum efficiency improvement

PLL performance improvement,

Local osc. frequency control,

Same problems of signal source.

Fiber Non-linear effect,

Material dispersion.

Non-linear effect,

Polarization axis rotation,

Depolarization

50 Raman Scattering Limit

for ASK

s

pq

QJ>

QJ

<u5o

Oh

<D

nj

>

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Scattering Lim _ for ASK

-50

f -i 1—

-i 1 1—i—

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^ i or Abiv j Launched ijauncnea Power r r

( observed

Direct

_ Direct ASK Het

FSK Het

Fig

100 1000

Transmission Bit Rate (Mb/s)

1 Launched Power and Receiver Sensitivity

24
BENDING AND MICROBENDING LOSS SENSITIVITY

OF STEP INDEX SINGLE MODE FIBERS

_ o _

J. AUGE , P. VUPONT, L. 6. JEUNHOKM

LABORATOIRES DE MARCOUSSIS, C6E RESEARCH CENTER

9H60 MARCOUSSIS - FRANCE

INTRODUCTION

Single-mode optical fibers with step index core and matched index or depressed index claddings are finding an increasingly large variety of applications, in long range transmission, signal processing, and sensing devices.

These applications involve very different bending and microbending conditions, and it is therefore necessary to get an in-depth knowledge of bend

and microbend losses as a function of fiber parameters. We have used the

basket-weave test [ 1 ] to experimentally determine the losses in various matched cladding (MC) and depressed cladding (DIC) fibers. The results are

found to agree reasonably well with simple loss models, and the behaviour

of both fiber types is discussed in some details.

RESULTS ON MATCHED CLADDING FIBERS

Three MC fibers with approximately the same cutoff wavelength of l.lym and

-3 -3 -3

index differences An of 3x10 , 5x10 , and 7x10 were used together with a

standard parabolic index multimode fiber (50 pm core dia.; 0.2 NA) serving

as a reference. The fibers were first wound on a drum 480 mm in diameter in

one layer and without tension, and were then rewound on a 100 mm diameter

drum in 15 superposed layers with a tension of 17 g. As can be seen in Fig.

1, the experimental results exhibit for all fibers an almost constant addi25
tional loss at relatively short wavelengths. This part agrees well with the

prediction of Petermann's theory [2] which relates the SMF microbend loss

AaSM

to t ^,e mu ^'t imo<:' e microbend loss Aa^ (here Aa^ = 1.1 -0.1 dB/km) :

where r is the area-weighted R.M.S. mode intensity diameter (in ym).

o For the comparison we used for the value measured with the VAL method

[3]. The steeply rising loss which occurs at longer wavelengths for the two

SMF with the lower index difference is attributed to pure bend loss under

a radius of R = 50 mm. It is seen in fig. 1 to agree well with the predictions of a simplified model [4] :

Aap (dB/m) * 2.5 V

2 (aR)

2 exp [- 9.84 (bAn)

J/Z

R/a] (2)

where R is the radius of curvature, a is the core radius, and b is the normalized propagation constant of the fundamental mode, for which very simple

and useful approximations exist [4].

RESULTS ON DEPRESSED CLADDING FIBERS

Similar measurements as above were carried out on depressed cladding fibers

_3 having an almost constant total index difference of about 5x10 . The depressed cladding radius d was varied between d/a = 1.7 and d/a = 8 and the

negative index depression An ranged from An_/An = 0.25 to 0.5. The general shape of the curves is very similar to that of MC fibers and we again

identify a microbend loss region and a steep rise above a given wavelength

called X^ attributed to pure bend loss.

For the microbend loss, a reasonable agreement is again found between the

measurements and the predictions of Ed,. (1) with r^ measured by the VAL me26
thod. As can be seen in fig. 2, this agreement occurs regardless of the

shape of the depressed cladding, except for one point with a wide and deep

cladding for which the microbend sensitivity seems unexpectedly small.

For the bend loss which is seen in fig. 1 to increase abruptly above a given wavelength called \. t there is no simple model available for DIC fibers.

Fig. 3 shows the measured values of X, for a set of fibers with An = 0.005

and two values of the winding radius, all fibers but one matched cladding

(MC) having An /An - 0.43. Other results not shown here indicate that the

general evolution of X^ against d/a is almost independent on An (provided

V remains the same), and that X^ increases when An_/An decreases for a given d/a. These features and some indications on an empirical model for

X^ (R, d/a, An_/An) will be discussed at the conference.

REFERENCES

[1] A. Tomita et al., Tech. digest Symp. Opt. Fiber Meas., Boulder (Co),

NBS special publication 641, pp. 89-92, 1982.

[2] K. Petermann, Arch. Electron. Uebertrag., 30, pp. 337-342, 1976.

[3] F. Alard et al., Electron. Lett., 17, pp. 958-960, 1981.

[4] L. Jeunhomme, in Single-Mode Fiber Optics, Optic. Eng. Series,

Marcel Dekker, New-York, 1983.

27
Aa (dB/km)

i 1X3 1.5 17

3.5 4.0 4.5 5.0

.43

-I151 1 -I d/a

10 15
REFRACTIVE- INDEX PROFILE AND MODAL DISPERSION PREDICTION FOR A

SINGLE-MODE OPTICAL WAVEGUIDE FROM ITS FAR-FIELD RADIATION PATTERN

W. Freude and A. Sharma*

Institut fur Hochfrequenztechnik und Quantenelektronik , Universitat Karlsruhe

(W. -Germany)

* During this work on leave from Physics Department, Indian Institute of Technology, Delhi (India) , as an Alexander von Humboldt fellow.

1 . Theory

In a scalar treatment of the single-mode waveguide fields we neglect longitudinal components. Therefore the far-field $(y,d) of a rotational symmetric near-field $(r) is specified by the Kirchhoff integral, and by inversion we find

jkQ

d oo $(r) = - jk de J $ (y , d) Jn

(k_r sin y) sin y dy , (1)

0 ~

if we detect the far-field power in dependence of the angle y as

measured from the fibre axis in a meridional plane at a distance

d»a, where a is the core radius. denotes the zeroth order Bessel function of the first kind, X is the wavelength, and k^ = 2tt/X

the free space propagation constant. The far-field

£(y,d) = sign[£(y,d) ]£ (0,d)^/P (y ) /P (0) (2)

can be computed from the corresponding measured far-field power

P(y). The sign function denotes the polarity of the far-field,

which changes sign at each zero of the power. Instead of evaluating Eq.(2) directly [2] we expand $ (y ,d) in the series

M rm $(Y,d) _

j kn

d M m-1 rm 1 *(r)= lc G

(0) (a,r), jk0e 0 [(-1) m 1 c G (— ,k sin y ) (3) L

. m m cosy/d 0 m m a 0

m= 1

1 ' m= 1 with the orthonormalized GaujS-Laguerre functions

2 2

G^ V) (a,x) =2^ y! . 2 2>v/2 -a x /2 T (v) , 2 2. . A

, - a (a x ) e

7 L '(ax). (4) (y+v) ! y-1

L are the generalized Laguerre polynomials . We minimize the sum

of the weighted squared differences between the computed and the

measured far- field data having chosen the parameter a for our

basic system such that the fundamental mode G.j ( 1 / ct ,k^sin y

)

approximates the central lobe of the far-field best. The unknown

Z9
m- 1 coefficients (-1) c are fast and accurately found -by a standard

m M

2 matrix inversion procedure. A renormalisation guarantees EJc I =1

m=1 m

If we insert the fitted near-field into the Helmholtz equation we

see that

M / M ,22., D 2 _ 2 4 2 . 2 V (0). ,/ v ^(0), , , c , k„n r) - 3 = - 2a - a r + 4a I mc G' a,r / I c G' a,r , 5 0 ^ .. m m / ^ „ m m

m-^ 1 m=1

where n(r) is the refractive-index profile, $ the propagation constant, and M the maximum number of coefficients. The resulting

near-field resolution 6 depends on the maximum measured far-field

angle and on M by

6„ ^ 0.610 A/sin y / 6„ ~ — ( 1 . 2 3/M) 0 * 472

(6) B m B a

where for the second formula the largest zero of G^^ (a,r) (as

approximated by McMahon's expansion [4]) divided by M- 1 was given

in terms of an empirical function with an error smaller than 3 %

for 3^M^100.

The dispersion properties of the waveguide may be derived adopting a formula [3] which can be safely applied to weakly guiding

single-mode structures. Whether the newly developed doubly and

quadruply clad broad-band fibers do allow the underlying assumptions has not yet been verified. Some algebra leads to the dispersion factor T(A) = T ( A ) +Tr7 (A) as a sum of material and waveguide

dispersion

dn.

T(i , M 1 --|2 +

c dA _ 2

8tt cn

1 +

A dl (A)

I (X) = a

2

f e X

f (x)dx , 1(A) dA

2

U

2 (7)

f (x) =-{ I c [ (2m - 2 - x)L (0

J(x) - (2m - 2)L {0

\ (x) ]\

xl u. m m- 1 m- 2

*-m= 1 J where n^ is the refract ive- index , n 2q

^e <? rouP index of the cladding, and c the vacuum velocity of light. 1(A) may be evaluated

exactly by a Laguerre integration [4].

2. Experiment

We tested our method by comparison to published data [2] and found

that in sharp contrast to previous methods [2] we could tolerate

random errors in P(y) of at least 20 % without changing the computed refractive-index profile significantly, Fig.1: We weight

30
all relevant data according to their largely varying absolute

values (a dynamic power range D of 60 to 90 dB is typical) , while

the direct use of Eq.(1) implies the summation of widely varying

numbers, so that the highest valued P(y) have to be measured

-4

with a relative power accuracy of about A = 10 if the sidelobe information is not to be covered by the measuring uncertainty. In fact we estimated D/dB « -20 log A, which limits the

useful angle range and therefore the near-field resolution.

We carried out own experiments with a single-mode fiber at

X= 632.8 nm and found a refractive-index profile with M = 10

coefficients as in Fig. 2. Because of the required resolution

the dynamic range amounted to D = 60 dB, so the use of a laser

source was mandatory.

In measuring the waveguide dispersion where resolution is not

as important, D may be reduced to 20 dB which allows for an incoherent light source. Fig. 3a shows the measured far-field

powers along with the calculated near-field intensities I(r),

Fig. 3b, from which the waveguide dispersion evaluates to T ( X =

- 1 — 1 = 632.8 nm) =- 3 . 5 ps km nm . A least-squares error Gaussian

fit does not show a good agreement, therefore an estimate of

T^CA) using only the wavelength dependent change of the fitted

spot size [3] is 2.3 times larger.

If in the expansion Eq.(3) unsymmetric Gauft-Laguerre modes

are included one could detect the content of the higher-order

LP^ mode thereby finding the single mode operating range of

the waveguide

.

3. Conclusion

We expanded measured far-fields in M GauB-Laguerre functions, the

coefficients of which are determined with a least relative-error

squares fit by matrix inversion. Thereby the near-field is given,

from which the refractive-index profile can be calculated. Using

wavelength dependent far-field data measured with an incoherent

source the waveguide dispersion may be deduced.

Besides the general benefits of far-field measurements (uncritical mechanical and optical setup) the advantage of our method

31
lies in its inherent insensitivity against noise, so that the practical resolution limit is only given by the amount to which cladding

modes and stray light can be reduced.

Acknowledgement

We thank Prof. Dr. techn. G. K. Grau for discussions.

References

[l] Born, M. and Wolf, E., Principles of optics. Pergamon Press, Oxford, 1980.

[2] Hotate, K. and Okoshi , T. , Measurement of refractive-index profile and

transmission characteristics of a single-mode optical fiber from its exit

radiation pattern. Appl. Optics 18 (1979), 3265 - 3271.

[3] Sansonetti, P., Modal dispersion in single-mode fibres: Simple approximation

issued from mode spot size spectral behaviour. Electron. Lett. 18 (1982),

647-648.

[4] Abramowitz , M. and Stegun, I., Handbook of mathematical functions. Dover

Publications, New York, 1970.

x10" 3

6

_-r 0

i t— \ \ \ \ V

\

5 jim 6

Fig.l. Refractive index profile,

computed by the present me- thod with M= 31 ( ) and by

direct Fourier inversion [2]

(•••), or measured by an interference technique using a Q fibre ( ) [2]

Fig . 3

a Far-field power P (y) of single-mode fiber Ll 37/11 for

different wavelengths X. b Computed near- field power

I(r) (—); fitted Gaussian

function ( )

10" 3

4

(Mi Vs

Fig. 2. Refractive- index profile of

single-mode fiber Ll 37/11

with M = lO.

X = 700 nm

650 nm

632.8 nm

600 nm

580 nm

32
A SIMPLE NEAR-FIELD SCANNING SYSTEM FOR REFRACTIVE INDEX PROFILES

AND MODE SPOT SHAPE.

C A Millar

British Telecom Research Laboratories, Martlesham Heath, Ipswich, UK.

Introduction

Measurements of refractive index profile and mode spot

intensity distribution are fundamentally important for singlemode fibre

characterization. Using the Transmitted Near Field (TNF)

technique, the two measurements are essentially the same, but with

different sources and detectors in place. This paper describes a straightforward apparatus which, when appropriate sources and detectors

are used, permits high-resolution, real-time near field scanning.

Measurement system

The apparatus may be divided into three distinct parts,

the optical system, the mechanical system and the electronic system (fig

1). The optical system comprises, for TNF profiling, a tungsten lamp,

diffuser, condenser and a X60 coupling lens. Care was taken to minimize

leaky mode excitation by restricting the launch NA to 0.05 and preparing

low cleave-angle fibre ends. For mode spot distribution measurement, the

test fibre was butt-coupled, using a diffraction alignment device , to

a pigtail fibre from a 1.32um or 1.55um wavelength laser. The other end of

this short length of test fibre is placed in a V-groove clamp and the

image of the near field is projected by a microscope objective onto a slit

placed immediately in front of a mechanical chopper blade. Alternatively,

a chopper disc with circumferential photochemi ca I ly-etched slits may be

used. Light passing through the slit and chopper arrangement is collected

by a large-area photodiode; Si or Ge as appropriate. The slit, which

selects a radial sample of the TNF along its length, is sequentially

scanned by the chopper blades. The photodiode voltage V is proportional

to the sum of all the light intensity at any chopper blarflcing position r',

le

P

Since

E

2

(r) dr

v . = dr'/dt = Rw

chop

analogue differentiation of this signal gives

V

d

(t)»< Rw d/dr' j

E

2

(r) dr*E2

(r)

r

which represents recovery of the radial variation of the intensity

distribution as a time-dependent voltage. The advantage of the method is

that it is repetitive and synchronous, allowing (a) real-time viewing of

the TNF on an oscilloscope (b) signal averaging to reduce noise and (c)

digitization of waveforms for immediate processing of the fibre

properties. Positioning the fibre in the object plane enables sequential

scans to provide complete cross-sectional near field information.

The key mechanical component is the chopper motor, which must maintain

constant angular velocity, w, to ensure temporal stability. The number of

33
chopper blades determines the repetition rate of the signal, whereas the

linear velocity at the slit (at radius R) is a compromise between the

provision of adequate signal from the differentiator (higher speed) and

the bandwidth of the detector and analogue electronics (lower speed).

Calibration \

Knowing the oscilloscope timebase speed, the chopper velocity

and the spatial magnification of the optics, it is possible to calibrate

the radial dimension as a time interval. Alternatively, a fibre of known

dimensions may be used as a reference. For mode spot intensity

distributions, the peak level is set to unity and the pattern scaled

accordingly. For refractive index profiling, however, the scaling

procedure for absolute index is more involved, and methods of doing so will be presented in detail at the conference.

Results

Figure 2(a) shows the TNF refractive index profile of a singlemode fibre, using a low magnification objective lens to enable the

cladding depression to be identified. Using a higher magnification lens

the core profile shown in figure 2(b) results. Direct comparison with the

refracted near field profile for the same fibre indicates that a spatial

resolution of «v 0.5um is achieved for both. Changing the source and

detector provides the TNF mode spot distribution shown in figure 2(c), for

the same fibre at 1.32um wavelength. The spot shane is very similar to

that predicted from a finite-element analysis. of the digitized

refracted near field index profile, with an e width of 3.72um as compared with 3.9um respectively. Transverse offset measurement of e power transmission width gave a value of 4.0um. The resolution of the

scanning system is not likely to change with wavelength, but the optical

resolution of the objective lens may result in a degradation in the

infrared, which is governed by the transfer function of the lens. By

removing the objective lens, the scanning system may be applied with equal

effect to the measurement of far field intensity distributions.

Experience of this near field, the transverse offset, and numerical

methods of fibre characterization will be discussed, with particular

reference to current CCITT deliberations and to the problem of specifying

advanced singlemode fibres.

Cone lusions

A simple scanning system has been used for TNF refractive

index profiling and mode spot intensity distribution measurement. A

resolution of 0.5um has been obtained for profiles, and good agreement

for spot shape and width by two alternative methods has been achieved.

Acknowledgements

Thanks are due to B Nelson and G Warnes, and the Director

of Research of British Telecom is gratefully acknowledged for permission

to publish this work.

References

1 D H Irving, El Letts, 19, p190, 1983.

2 Y Murakami, A Kawana, H Tsuchiya, Appl Opt, 18, p1101, 1979.

3 C A Millar, paper IV-A5, 9th ECOC, Geneva, 23-26 Oct, 1983.

4 B Nelson, private communication, BTRL.

34
35
1 8tnU

FIBRE 28DEL

TNF PROFILE

5nS»20.5u«

LENS X20

I HFH I UFH > TEXT

U2R 8

Figure 2a

FIBRE 2BDEL

TNF PROFILE

2*S=i . 66un

DEU=6.3E-3

LENS Kill

Figure 2b

9 UFH B HFH >TEXT

FIBRE 2BDEL

THF SPOT

HAUL"! • 32un

2«S=1 .66un

H<t-2>«3.72u

LENS XIBI

HI

Figure 2c
SPOT-SIZE MEASUREMENTS IN SINGLE-MODE FIBRES

R. Caponi, G. Coppa, P. Di Vita, U. Rossi

CSELT - Via G. Reiss Romoli, 274 - 10148 Torino (Italy)

It is known that the dimensional specification of single-mode fibres is

performed on the basis of the so-called mode field diameter. It has not been

possible, up to now, to agree (e. g., in the CCITT) an acceptable definition

of such a parameter, and the matter is presently strongly debated. In order to

clarify the situation, it seems convenient firstly to review some definitions

of parameters related to the mode field diameter and successively to discuss

various techniques proposed recently for its measurement. It is not our aim to

penetrate the measurement problems of each proposed method, but we want to

analyze them critically in order to understand what actually is being measured

and how useful that quantity can be. Finally we expose a novel and very

promising technique that permits a direct measurement of the mode field

diameter making use of suitable masks.

In the following we will refer to the so-called spot-size w, simply related to

the corresponding mode field diameter d by d=2V2 w. If the mode field is (as practically happens) circularly symmetric, two

parameters are of relevant physical significance. The first one is defined as:

(1) wm

={ |

E

2 r

3dr/

| E

2 rdr} 1/2

,

E(r) being the field amplitude of the fibre versus the radial coordinate r. w^

is an important quantity, since microbending losses [1] and joint losses due

to small tilts [2] depend on this parameter. The second quantity is defined

as:

(2) Wj={

|

E

2 rdr/

j

(dE/dr)

2 rdr} 1/2

it is analogously important too, since the joint losses due to small lateral

offsets [2,3] and the waveguide dispersion [3,4] depend on it. Note that these

two parameters coincide if the field distribution is Gaussian, and in general

very rarely differ by more than 10 %, for fibres with conventional profiles.

Thus, it is easy to understand the importance of specifying and characterizing

the fibre by a certain mode field dimension. A first approach consists simply

in taking the 1/e radius of the near-field intensity distribution. Such an evaluation coincides with w and w. for rigorously Gaussian E(r), but

m j

37
generally gives only a too rough estimate of such parameters [5,6]. Their

evaluation can be obtained more reliably from a transmitted near-field

scanning, by applying directly Eqs. (1) and (2), but a considerable dynamic

range (> 20 dB) is required in the measurement in order to avoid truncation

errors in the tails [5,6].

Such problems can be overcome (as proposed recently [5,6]) by a suitable

analytic extrapolation of the tails of the near-field pattern with simple

exponentially decaying function. This technique provides immediate and

accurate evaluations of both w^ and w.., directly from the defining equations

(1) and (2), and could be useful particularly for fibres with non-conventional

profiles (multicladding , segmented core, etc) at higher wavelenghts, where the

difference between w

m

and w^ could be so cospicuous to indicate, in the

future, the need of providing both parameters.

A different elaboration of near-field measurements [7] (applicable also to the

far-field) permits to deduce a further parameter, w^. It is defined as the 1/e

width of the Gaussian power distribution that maximizes the coupling

efficiency in the considered fibre. An implicit definition of w^ is the

following:

1/2

(3) w

b

={

j

E(r)exp[-r2 /(2w^)]r3dr/ | E (r) exp(-r2/(2w^) ] rdr}"

It can be demonstrated that it is always: w

j^w^w

m

(the equal being valid only

for Gaussian E(r)). This means that if a single kind of mode field diameter

should be adopted, w^ could represent a good compromise between the two

relevant parameters: c w and w.. m j

A further parameter, w , based on a completely different measurement

a procedure, is defined as the lateral offset value that reduces the coupling

efficiency between two fibre samples to 1/e of the maximum [8] . Notwithstanding its euristic definition, w provides a surprisingly good

evaluation of w , particularly around the cut-off wavelength of the LP^ mode.

Such a coincidence is better for monotonical profiles and for W-type fibres

than for dipped or segmented core fibres. The evaluation of w thus permits

often a good estimate of w^, but replacing the problems arising from the

limited dynamic range of common near-field technique with the typical problems

of an offset measurement apparatus (sub-micron displacements are involved and

particular care should be devoted to the quality of the end faces of the

fibres) . This transverse offset technique is an example of integral measurement of the

field dimensions. Different methods are now under development. Let us

38
consider, e. g., Eq. (1) for w . The denominator within brackets is simply

proportional to the whole power at the output of the single-mode fibre that can be obtained by a simple integral measurement. The numerator can also be

obtained by a single integral measurement, by using a mask [9] in the

2

near-field plane whose transmittivity is proportional to r . In this way two

integral measurements permit to derive w^. Also w^ can be measured by the mask

method since it has been recently shown that it is inversely proportional to

the far-field width, a quantity that can be written as Eq. (1) but replacing

2

the near-field pattern E (r) by the far-field one and the radial coordinate by

the angular one [10] . In this case a third integral measurement, interposing

the mask in the far-field plane, permits to derive w^ immediately. The mask

can be obtained in various ways, examples of which are: a dark zone delimited

by portions of Fermat's spiral (Fig. la) or a grey distribution (Fig. lb).

A schematic set-up for this measurement is shown in Fig. 2. A first spectral

scanning is performed without mask, a second one with the mask in the

far-field plane, F.F., in order to obtain w.., a third one with the mask in the

near-field plane, N.F., in order to obtain w^. The results of a preliminar

measurement of the quantity w^ are reported in Fig. 3. Although a certain

refinement in our preliminary experimental apparatus is needed, a substantial

agreement with the data obtained by a precise and high dynamic range

near-field measurement is obtained. Considering that such similar data have

been obtained with quite completely different techniques, that exploit

different effects, such early results are very encouraging, since this "mask"

technique is much more immediate with respect to different techniques and can be suited for factory or field measurements. Refinements are now in progress

to improve the precision of the measurement, and further results will be

presented at the Conference.

References

1. K. Petermann, Opt. Quamtum Electron, 9 (1977) 176.

2. G. Coppa, P. Di Vita, CSELT Rapp. Teen., 11 (1983) 427.

3. K. Petermann, Electron. Lett., 19 (1983) 712.

4. G. Coppa et al., Electron. Lett., 19 (1983) 731.

5. G. Coppa et al., CSELT Rapp. Teen., 12 (1984) 109.

6. G. Coppa et al., Proc. of ICC '84, Amsterdam (NL) , May 14-17, 1984.

7. W. T. Anderson, D. L. Philen, IEEE J. Lightwave technol., LT-1 (1983) 20.

8. C. A. Millar, Electron. Lett., 17 (1981) 458.

9. G. Coppa et al., Patent pending, 1983.

10. C. Pask, Electron. Lett., 20 (1984) 144.

39
b)

for spot-size measurements.

Lamp Monochromator Fibre F.F. N.F. Photodetector

Lock- in amplifier

Fig. 2 - Experimental apparatus,

4.2

9

«

3.

UJN

n

UJ

I f-o

EL

3.4

3.6 -

3.G -

A

+

A

+ o

A O

A O

o

+

9 1 1

1.2 1.25 1.3 1,35 1.4 WAVELENGTH X CjinO1.45

Fig. 3 - Experimental results of

spot-size: w (triangles), w. (circles) , w, (squares) , obtained

with the near-field technique,

compared with w. obtained with

the mask technique (crosses)

40
COMPATIBILITY OF NATIONAL AND INTERNATIONAL

STANDARDS FOR OPTICAL FIBER

P. R. Reitz, Corning Glass Works, Corning, NY 14831

INTRODUCTION

Worldwide standardization activities for telecommunications-grade optical

fibers and cables have been underway for the better part of a decade.

Because the various organizations represent diverse interests, it is to be

expected that their efforts will sometimes lead to conflicting test methods.

For companies wishing to participate fully in this new industry, it is of

paramount importance to understand the details of the various requirements

in a manner that can maximize product conformance.

NATIONAL AND INTERNATIONAL STANDARDS ORGANIZATIONS

The bulk of international optical fiber standardization effort rests in

two organizations, the International Electrotechnical Commission (IEC) and

the International Telegraph and Telephone Consultative Committee (CCITT)

.

A description of the general work of these organizations can be found in

Reference 1. In the United States, the primary source of optical fiber

standards is the Electronic Industries Association (EIA) , which works in

cooperation with the American National Standards Institute (ANSI).

To understand the difficulty involved in creating compatible standards, it

is helpful to review the goals and expectations of these separate entities.

Both IEC and EIA exist to promote trade and commerce, and as a consequence

use a particular hierarchy of generic, sectional, and detail specifications

(known as the IEC-Q system) to foster an exact means of specifying characteristics of components like optical fibers and cables. As a result, IEC

and EIA are generally interested in a broad range of product specifications and in test methods that can be effectively applied to manufacturing

a product under the IEC-Q system.

The CCITT involvement in optical fibers and cables is to promote the

establishment of standard fiber types for use in public networks, and the

test methods to characterize these fibers. It reflects the viewpoint of

41
those government telecommunication administrations and operating agencies

of which it is composed, and focuses upon developing reference test methods

(RTMs) to be used for product specification and resolution of disputes.

It also sometimes acknowledges acceptable practical factory measurements

in the form of alternative test methods (ATMs).

STANDARD MEASUREMENT METHODS FOR MULTI MODE FIBERS

Attenuation

The unanimously preferred procedure for attenuation measurement is the

two-point cutback method. It has been recognized as the most accurate and

precise method, and is the CCITT RTM. CCITT and IEC also allow insertion

loss (substitution) and backscattering methods as alternatives. Although

EIA is currently reviewing backscatter measurement methods, use of these

methods for attenuation has not generally been accepted in the U.S. due to

the uncertainties contained in the interpretation of the results. Further,

most use of the technique is made with commercial instruments that generally

have no provision for controlled launching conditions. Consequently, OTDR

measurements are most often used as a process check, for example in cable

manufacturing where cutback measurements have already been performed in

fiber manufacturing.

Insertion loss (substitution) measurements are currently under study in

the EIA because of a need for non-destructive testing of connectorized

cables, and as a basic method for measurement of installed repeater sections

There is some debate as to whether this test can fairly apply to the common practice of mandrel wrapping, since the mandrel cannot be well

applied to the cable, and the reference length of cable may often be a different structure. Neither CCITT nor IEC methods address this point.

While IEC and CCITT insertion loss measurements agree almost verbatim,

there is a small difference. While it is agreed that insertion loss measures both the (cabled) fiber and connection device attenuation, the

IEC procedure allows correction for the loss of the connection while there

is no mention of this in the CCITT document.

42
Launching Conditions For Attenuation

A very strong correlation exists between IEC and CCITT documents, so agreement between these organizations is apparently complete. However,

one should note that neither provides a particular definition for achieving

proper launch conditions. This has proven to be a difficult task, as is

evidenced by the complex procedure described in EIA FOTP-50, where launching

conditions for attenuation measurements are addressed. EIA specifies an approach to define a steady-state mandrel based upon creating a somewhat

smaller angular distribution at the input than the actual far-field pattern

of the fiber under test, and also specifies the 70/70 LPS launch. The

"dummy fiber" method, also specified in FOTP-50, is qualified by the same procedure as for the mandrel.

These launch conditions were formulated with the same general intent, to

achieve steady-state loss values, but before it was recognized that launch

conditions for attenuation generally only need to avoid exciting the

highest order guided modes. Because any of several means can be used for

this purpose, it becomes a question mainly of preference, until bandwidth

measurements are considered.

Bandwidth Measurements

Bandwidth, or baseband response measurements, have been the focus of much

attention, particularly with respect to launching conditions. Because of

the effectiveness of restricting the launch for prediction of link attenuation, some have hoped that the same launch conditions would yield consistent

system predictions of bandwidth from individual fiber measurements. Many

have therefore proposed using the same launch restriction for bandwidth as has been used for attenuation, but improvement in predictability has not

occurred. Further, those different launching conditions that produced

equivalent loss values now can be seen to produce varied bandwidth values

because the launch restrictions are sufficient to attenuate some modes

that propagate even over several kilometers. EIA, in contrast, has specified the use of uniformly overfilled launch conditions - the so-called

43
full launch - mainly for reproducibility. Since the predominant transient

loss mechanisms occur within the first one or two kilometers, the measured

power distribution for bandwidth using full launch nearly approximates

steady-state conditions. Thus, if the test fiber is sufficiently long the

bandwidth measurement with full launch can be considered to be a steadystate measurement. In this manner, the intent of IEC, CCITT and EIA may

be reconcilable on this principle.

With respect to measurement methods for bandwidth, complete agreement now

exists on the equivalence of time and frequency domain techniques. This

has allowed implementation of each technique where its specific strength

is required; time domain for interpretation of fiber measurement and

frequency domain for dynamic range in field measurements.

Geometrical Measurements

Widespread agreement exists on methods for measuring these characteristics,

although some differences are evident. All three organizations either

allow or prefer the Refracted Near-Field method for determining core diameter. The core is defined on the basis of the best-fit circle to 5%

relative index points in the case of CCITT, while EIA requires only a diameter scan, but with the profile fit by a power law function between 10

and 80% index points. Core diameter is then defined as the intersection

of this profile with the zero baseline. The relevant IEC document contains

no such details.

All organizations allow transmitted near-field measurements, but again

with some differences. CCITT defines core diameter from the best-fit

circle to a number of 5% intensity points around the core periphery,

whereas EIA allows a single diametrical scan, but with two possible interpretations. One is the distance between 2.5% relative intensity points,

the other is a power-law fit between 10-80% relative intensity points

extrapolated to the baseline. EIA has determined these relationships by

considering consistency between TNF and RNF measurements. This is an area requiring further study, especially when comparing measurements on fiber

44
lengths differing by several orders of magnitude. This is especially true

because of recent proposals in CCITT to do TNFs on very short (2-3cm)

fiber samples. The net result is that average differences between techniques and fitting prescriptions can be of the order of 1-2 micrometers,

but significantly larger errors can exist for fibers with particular kinds

of index profile details that confound some of the test methods, notably

the RNF. Thus, the manufacturer who wishes to comply with all techniques

will find that a significantly tighter distribution of parameter values

may be required to avoid the possibility of rejected ware.

Other geometrical measurements are typically performed with microscope

techniques, which by virture of the very rapid contrast changes at the

air/clad interface do not seem to be unduly sensitive to interpretation;

most test methods do not require the degree of detail that creates incompatibility.

While the main benefit of the RNF method is the convenience of one technique

that can apply to both core diameter and numerical aperture, insufficient

effort has been expended to address the details necessary to produce

reliable NA results from the technique. EIA has no procedure for determining

NA from RNF measurements; only short-length overfilled far-field results

contain a prescription based upon the half angular extent of the far-field

at the 5% relative intensity points, similar to the CCITT ATM. This

result is expected to be smaller than RNF results, in part due to the 5%

intensity definition and in part due to uncertainty as to what constitutes

the peak core and homogenous cladding indices in the RNF. The exact

relationship is hard to verify because of the problems just cited, and the

fact that RNF absolute index calibration to the accuracy required for

comparison to far-field measurements is rather difficult to achieve.

Further study by Standards Committee is required. However, at issue here

(and in the case of core diameter measurements) is a difference in underlying principle between CCITT and EIA measurements. The former relies

heavily upon RNF tests (on sample lengths of l-2mm) while the latter tends

to prefer transmission measurements through a few meters of fiber as being

more indicative of functionality.

45
TEST METHODS FOR SINGLE MODE FIBERS

It is fortuitous that standardization activities were maturing before the

widespread introduction of single mode fibers, because this had led to a much higher degree of cooperative effort in the development of consistent

and compatible standards. There is basic agreement among EIA, IEC and

CCITT for virtually all major parametric tests, including spectral attenuation, cutoff wavelength and fiber dispersion. Since the geometrical

properties of single mode fibers can generally be measured by the existing

techniques for multimode fibers, the same comparison of methods applies.

One example of the maturation of the standardization process is the decision

to allow both time delay and phase shift methods for dispersion measurements

in CCITT. In the analogous case of bandwidth measurements of multimode

fibers, it took years of discussion to reach the consensus that allowed

both time and frequency domain techniques; when this similar situation

occurred in single mode fibers it was recognized as the preferred solution

almost immediately. This has avoided lengthy debate as to which method

should become the "Reference Test Method".

The preferred measurement method of mode field diameter has yet to be

resolved in CCITT. In contrast, EIA has begun to develop what is believed

to be a set of internally-consistent measurement procedures using all the

main methods; near-field, far-field (by two means), and transverse offset.

IEC has a draft document for the transverse offset method only. Technical

issues related to Gaussion fitting or a newer proposal by Petermann are key to final resolution of this question. With the range of methods

considered so far, differences at 1300nm are expected to be on the order

of ~ 5%, but may be twice as large at 1550nm.

Conclusions

In the last several years we have witnessed the rapid growth and acceptance

of optical fiber communications; this has been accompanied by a noticeable

maturation of standards activities. In large measure the existing measure46
ment standards are very similar, particularly considering the diverse

frames of reference from which they have been created. There remains,

however, a significant number of important detail differences that must

yet be resolved. This resolution will require great consideration to

fairly weigh the requirements for simplicity and flexibility with the

desire for uniformity and technical correctness. This is the ongoing

challenge of standardization.

REFERENCES

1) R. R. Fergusson, M. Larose, J. G. Nault, Digest Supplement, Symposium

on Optical Fiber Measurements, Boulder, CO, USA, 1982.

2) CCITT Contribution No. 289, April 1984, (to be published as amended

version of Recommendation G.651 - Characteristics of 50/125(Jm Multimode

Graded Index Optical Fibre Cables, 1st Quarter 1985).

3) CCITT Temporary Document 46-E, May 1984, to be published as Recommendation G.652 - Characteristics of a Single Mode Fibre Cable, 1st

Quarter 1985).

4) Electronic Industries Association, 2001 Eye Street N.W. , Washington,

DC, 20006. Refer to RS-455 - series documents.

5) International Electrotechnical Commission Technical Committee No. 46;

Subcommittee 46E: Fibre Optics Draft for the second issue of Publication XXX, Optical Fibres, Chapters I and II. (Yellow Book), and

Supplement A to Publication XXX (Grey Book)

.

47

OPTICAL FIBER SENSORS

A. Oandridge, J. H. Cole and G. H. Sigel, Jr.

Naval Research Laboratory

Washington, OC 20375

Recently, many types of sensor using optical fiber, either as the

transduction element or as a communication link to an optical sensor, have been

developed. These devices range from simple on/off types of device to highly

sensitive interferometric designs. The wide range of these devices means there

is no one type of fiber to fill the requirements of fiber sensors. In this

paper, we will describe the various types of fiber optic sensor, however, the

emphasis will be on the properties of the fiber used in the sensor, rather than

the source, demodulation or systems aspects. Typically, the more complex

interferometric sensor designs have the most stringent requirement in terms of

the fiber properties. In this case, both optical and mechanical properties of

the composite fiber structure (i.e., fiber and jacket) are important in

determining the operation of this type of device. Another approach — the

microbend sensor also requires particular mechanical and optical properties to

function in an optimum manner. Finally, the use and application of polarization

preserving fiber in interferometric sensors will be briefly discussed.

The wide range and diversity of optical fiber sensors make them difficult

to categorize, however, shown in Figure 1, five broad categories are identified.

The first four types of device measure intensity changes induced by the

parameter to be sensed. In types A and B, the light leaves the fiber, interacts

with a sensor element (typically bulk optics) before being collected by the

return fiber. In types C and 0, the light that is sensed typically remains in

the fiber, but either optical or mechanical perturbations couple light out of

the fiber core, thus allowing measurement of the phenomena causing the

perturbation. Typically, these first four types of sensor use multimode fiber

49
rather than single mode. In the fifth type of the device, the light remains 1n

the fiber coil, but a perturbation to the fibers environment causes an optical

phase shift within the fiber; this then is referenced to another fiber unexposed

to the perturbation — using interferonstry, a very accurate measurement of the

phase shift may be made. This type of device usually employs single mode

optical fiber.

Examples of these different sensor types are shown in figures 2 a,b,c,d and

e. As already indicated types A and B require fiber similar to that required 1n

communications, obviously loss (bend and microbend) should be minimized,

however, no unique fiber characteristics are required. In type C, where the

element performing the sensing 1s short, typically the refractive index of the

fiber (possibly declad) is the important parameter.

Many different variants of the extended intensity fiber sensor (type 0)

have been demonstrated. With radiation sensors, the increased fiber loss with

radiation may be optimized with specific fiber materials. Some temperature

sensing configurations also require fibers doped with specific materials. All

intensity devices require certain optical power levels which are determined by

core size and numerical aperture, etc. The microbend sensor, for example,

requires additional mechanical and optical properties of the sensing fiber. A

small fiber diameter and a large optimum microbend periodicity (see figure 3)

increase the fiber compliance and therefore improve sensor response within the

limits set by optical constraints. A soft buffer layer tends to reduce

microbend sensitivity by reducing the coupling of the mechanical deformer to the

glass fiber. A low index, soft coating relative to the cladding results 1n

guided cladding mode which can recouple into the core, thus reducing the sensor

response. Therefore, a thin, hard, higher index coating directly on the glass

fiber is optimum for the microbend sensor.

The interferometric sensor has been used in many sensor configuration (see

50
figure 2e). We will describe two types of sensor — acoustic and magnetic.

Figure 4 depicts a typical single mode optical fiber construction for

interferometric acoustic sensors. Although the outer hard coating (E) performs

the major role 1n acoustic sensitivity, the fiber composite must be designed to

provide unique characteristics for acoustic sensor applications. Typically for

a given sensor application, one wishes to package the optical fiber in as small

a volume as practical. This requirement results in a minimum bend specification

substantially lower than for typical telecommunication applications. The

refractive index of the core (A) and cladding (B) and core size must be

optimized to minimize the excess bend loss in the fiber. The overall diameter

of the glass layers, core (A), cladding (B) and substrate (C) must be reduced

from the 125 telecommunications standard to about 70-80 to reduce the

bending stress. The smaller diameter fiber also reduces loading effects. The

soft coating (0) 1s utilized to reduce mlcrobending and local stresses on the

glass; however, to minimize the fiber volume, this layer should be relatively

thin - 50 \m. Finally, a number of hard coatings can be utilized. Figure 5

shows the normalized acoustic phase sensitivity A<j>/<|>AP plotted as a function

of total fiber diameter. Oue to the dependence of the acoustic sensitivity on

both the Young's and bulk moduli, different coatings are optimum for different

fiber diameters. Metal coatings may be employed to reduce acoustic sensitivity.

Magnetic sensors use similar technology, but the fibers may be bonded directly

onto metallic glass strips or coated with magnetostrictive materials.

Finally, specific sensors (e.g., the fiber gyro) and certain configurations

require polarization preserving fiber. Many design characteristics are

retained, however, all the components (pigtailed lasers, couplers, modulators

and sensor elements) need to be fabricated from polarization preserving fibers.

51
Types of Fiber-Opt ic Sensor

B X

C X

D X

E X -*-

on/off

intensity

intensity

phase

Sensing

Element

detector

detector —intensity

A detector F

detector F

detector F

Fiber

MM

MM

MM

SM

Sensitivity

Cost .

Fig. 1

A

on/off

detector

eg switch, keyboard, proximity, security, viscometer

VISCOMETER

measurement up to 40.000 psi

Output

liquid

LED

hollow cylindrical

pressure vessel detector Time

viscosity proportional to T

Fig. 2a

1

Intensity

detector

eg acoustic, temperature, current, security,

pressure. f luorimeters

PHOTOELASTIC

FIBER-OPTIC HYDROPHONE

O'TICAl M#UT

CL«»TlC MCMAANf -

<U**0«T COLUMN — -

OUA* Y(*. W*Vf nAM "

W>V«At OUTPUTS

.OMM-KOO UN*

»«MUMQ MAMWUTTIH

-HW-Utlt put! «TM »HOTOCLAOT< HUM

- mini mON»Tme ounuu

O* aCHAVOM

Fiq. 2b

52
intensity

A detector

eg temperature, refractive index, liquid level

REFRACTIVE INDEX

Fig. 2c

Harmer D

intensity

detector

eg temperature, radiation, acoustics, turbidity

MICROBEND

phase —5~ detector

eg pressure, acoustic, ultrasonic, acceleration

magnetic, rotation, temperature, microwave, current

INTERFEROMETRY: 1) Mach Zehnder

2) Michelson

3) Sagnac

1) and 2) act as an extremely sensitive strain

gauge, capable of measuring length changes of

10 m, ie for 100m sensor 1 part in 10

18

Features: sensitivity. linearity, versatility

common technology base

STR#*ca ocroMMEft

Davis

Fig. 2d

Fig. 2e

53
Fig.4. A typical single-mode ITT fiber: core (A), cladding (fl), strate (C), soft coating (D), and hard coating (£).

NORMALIZED FIBER ACOUSTIC RESPONSE

VERSUS

FIBER DIAMETER

NORYL 10

PFA-3W

I I i '

0.M 0.6 0.8 l'°

DIMETER (MM)

Fig. 5 54
THE PHASE VELOCITY AND LOSS COEFFICIENT OF

OPTICAL FIBERS VIEWED AS STIFF STRINGS

Frank ¥. Cuomo

Department of Physics, University of Rhode Island, Kingston, R.I. 02881

and

Naval Underwater Systems Center, Newport, R.I. 02841

ABSTRACT

The standing wave method has been widely used in the determination of the

damping properties of viscoelastic solids in the 100-10,000 Hz range and loss

coefficients have been obtained either by the experimental observation of the

decay modulus or the half-power bandwidth of each resonant peak. This paper

investigates by this method the behavior of optical fibers to mechanical

vibrations. It is found that for plastic clad fibers the phase velocities are

largely dependent on the tension and mass density while for other fibers tested

the system behaves as a stiff string whereby the modulus of elasticity takes on

a more pronounced role. Experimental data are presented to illustrate the

differences in loss factors and phase velocities for several optical fiber

configurations

.

INTRODUCTION

The recent analysis of multilayer optical fibers, identifying the coating

properties which optimize the fiber acoustic sensitivity, has shown that

pressure sensitivity is strongly influenced by the elastic coefficients of the

fiber coatings. The measurement of the elastic moduli of some fiber coatings

has provided the necessary relationship between the core strains and the

properties of the fiber layers necessary to the calculation of the sensitivity

J

Optical fiber research in underwater communication has also been extensive and

it has been found that the elastic and damping properties of optical waveguides,

55
with or without ruggedization, are very useful in system characterization. In

this paper the measurement of these properties is obtained by resonance methods

utilizing three types of optical fibers. Since a string under tension can also

have stiffness, such as the case for ruggedized fibers, the approach used

considers both parameters by treating the problem of a stiff string, as

described by Morse and Ingard. 2 This method is found advantageous in the

experimental determination of the phase velocities, damping factors and elastic

moduli of the optical fibers under test.

DATA ANALYSIS

Utilizing the mathematical treatment given in reference 2 resonance modes in

optical fiber samples were generated by driving the samples transversely. The

samples were fixed at both ends at 9.5", using several known tensions, and the

resonant peaks were determined. In addition, the half-power bandwidth points

were obtained to establish the loss coefficient, tan D, at each resonance mode.

Since the expressions developed for a stiff string contain both the tension, _T,

and the modulus of elasticity, Q, it is possible to obtain the modulus, if the

tension is known, by curve fitting. This method was effectively used to

determine the extent of stiffness for each sample. Three multimode optical

fiber samples are discussed in this paper, namely, one hytrel-buffered, one

plastic-clad and a third one hytrel-buffered and ruggedized with S-glass epoxy.

Figures 1 , 2 and 3 illustrate some of the results of this investigation. Figure

1 shows the frequency dependence of the loss tangent and Figure 2 shows the same

dependence of the phase velocity. Figure 3 provides for the hytrel-buffered

fiber the wavenumbers indicative of the expected elastic moduli based on

theoretical predictions.

56
CONCLUSIONS

The following conclusions have been derived from this study:

A. RUC-GEDIZED

1 . Damping independent of frequency & tension

2. Moduli independent of tension

3. Phase velocity dependent on stiffness

B. HYTREL-CLAD

1 . Damping dependent on frequency

2. Moduli dependent on tension

3« Phase velocity dependent on tension & stiffness

C. PLASTIC-CLAD

1 . Damping dependent on frequency & tension

2. No moduli at higher tensions

3« Phase velocity dependent on tension

REFERENCES

1. Lagakos, N. , Schnaus, E.U. , Cole, J.H. , Jarzynski, J., and Bucaro, J. A.,

"Optimizing Fiber Coatings for Interferometric Acoustic Sensors," IEEE

Journal of Quantum Electronics, Vol. QE-18, No. 4, pp. 683-689, April 1982.

2. Morse, P.M., and Ingard, K.U., "Theoretical Acoustics," McGraw-Hill Book

Co., pp. 187-191, 1968.

57
I TT (P2I 2) OPT I CAL F I BER

o RUGGED I ZED OPTICAL FIBER

A PL AST I C-CL AD OPTICAL FIBER

FIGURE 1

DAMPING FACTORS OF OPTICAL FIBERS

T=4.9E*5 (DYNES)

I TT (P2I 2) OPT ICAL FIBER RUGCED I ZED OPTICAL FIBER

PlAST i C-ClAD OPTICAL FIBER

FIGURE 2

PHASE VELOCITY OF OPTICAL FIBERS

0=5 5E+ 1 I (DYNES/SO CM)

0 = 9 5E + 1 I (DYNES/SO CM) 0

0 = 2 5E+ 1 2 (DYNES/SO CM) A 3=3 2E m 2 rD v MES'SQ CM X

fly?

FIGURE 3

TT fP2 1 2) OPT I CAL F I BER

58
POLARIZATION SHUTTLE PULSE TECHNIQUE

C. S. Brown and F. T. Stone

AT&T Bell Laboratories

Norcross, Georgia 30071

INTRODUCTION

A single-mode fiber is in reality a two-mode fiber. If the fiber

is azimuthally symmetric, the two-modes have the same group velocity. Under more realistic conditions, geometrical, material,

and stress irregularities cause the two modes to travel at different speeds. This results in both pulse spreading, a potential

bandwidth-limiting factor for wideband systems, 1 and fluctuations

of the state of polarization, which limit the operation of coherent systems. 2 To satisfy the many applications of single-mode

fibers, both high-and low-birefringence fibers are being developed, and the need for practical and repeatable measurement techniques to characterize the polarization properties of such fibers

is increasing.

Several birefringence-measurement techniques are currently in

use; however, only a few can measure low birefringence. 3 The most

popular, the cut-back method, 4 is destructive 5 and exhibits poor

repeatability. 6 In this paper we report on a new technique, the

polarization shuttle pulse (PSP) method, for measuring birefringence and related polarization effects in low birefringence (i.e.,

63 = 3.6°/m to 360°/m) single-mode fibers of short lengths (20 to

100 cm)

.

BIREFRINGENCE MEASUREMENT USING THE CUT-BACK TECHNIQUE

The phase delay induced by birefringence at an arbitrary position

z along the fiber is given by 6<}>(z) = 63z where 63 = 3 X - 3y

is the difference between the propagation constants of the two

polarizations. A closely related quantity is the beat length

defined by Lb = 2ir/ 6 3. When 63 = 3x - 3y * 0, the birefringence induces a phase delay causing the state of polarization to

evolve cyclically along the fiber, with L5 the length of one

cycle. Hence there is a 27rn (n an integer) ambiguity in determining 6ij).

Each birefringence-measurement technique accounts for that 2niT

ambiguity in different ways. The cut-back method eliminates it

by determining the phase delay for two different lengths. The

difference between the two lengths, however, must be less than

the beat length of the fiber. Unfortunately, the beat length is

not known a priori, so much trial and error is necessary. Also,

the cut-back procedure itself disturbs the birefringent state of

a low-biref r igence fiber, causing poor repeatability.

To determine the repeatability of the cut-back method, we measured the phase delay on uncabled and cabled single-mode fibers

59
under three different conditions, which simulate the errors expected from different aspects of the cut-back procedure: (A) the

fiber configuration and the fiber output position relative to the

detector were held constant, (B) the angle of the fiber output

position relative to the detector was rotated through 90° , while

everything else was held constant, and (C) the fiber layout was

randomly varied, while everything else was held constant. The

standard deviations shown in Table I give an idea of the errors

involved in the various aspects of the cut-back procedure.

Note that the standard deviation of a cabled fiber is much less

than that of the uncabled fiber. The large increases in the

standard deviation come primarily from changes in the fiber layout, i.e., condition (C). Fiber rotation, i.e., condition (B),

does not effect the repeatability as much since the low twist

rates involved induce little phase delay. Because the cut-back

method inadvertently involves both conditions (B) and (C), it

contains the errors associated with those measurement variations.

For uncabled fibers, the standard deviation is almost 2tt, suggesting difficulty in removing the 2nir ambiguity.

POLARIZATION SHUTTLE PULSE (PSP) TECHNIQUE

The precision of measurements on low-birefringence fibers can be

improved by nondestruct ively measuring the length dependence of

phase delay on a single short length (<1 meter) of cabled fiber,

thereby eliminating variations in fiber layout and position. To

do that, we evaluated a polarization shuttle pulse technique.

The PSP technique is a modification of the method used to measure

pulse spreading in multimode optical fibers. 7 To measure polarization properties, a linear polarizer, a Soleil-Babinet Compensator (SBC), and a linear analyzer are added to the conventional

shuttle-pulse arrangement. The measurement set-up is shown in

Figure 1

Output from a 1 .329 vim laser diode was coupled into 7m of polarization maintaining fiber. The light was then collimated and

passed through a linear polarizer. A stable polarized output was

always observed. The shuttle-pulse arrangement consisted of two

80% reflecting mirrors and two precision fiber holders designed

to keep the short straight section of cabled fiber 25 cm)

perpendicular to the mirrors. The output pulses were collimated,

passed through the SBC and the linear analyzer, and focused onto

a PIN diode. The electrical signals were then amplified and read

off a sampling scope. In this way the phase delays for the initial pulse and the subsequent shuttles were determined. Because

the initial pulse corresponds to a fiber length L and subsequent

shuttle pulses correspond to lengths of 3L, 5L, etc., the phase

delay as a function of length is determined nondestruct ively

Several data points representing an average of 100 readings were

taken about the nulls of the initial pulse and the first and

second shuttles. As the intensity as a function of the retardation introduced by the SBC is sinusoidal, the data about the

nulls was fitted to a parabola and the minima found that way.

60
PSP data is shown in Figure 2. The mean of the difference in

phase delay between the shuttles was <5<j> = 19.6 ± 2.2 deg. for a length 2L = 52.2 cm. The birefringence was therefore 63 = 37.5 ± 4.2 deg/m. The beat length corresponding to this birefringence

was Lb = 9.5 ± 1.1 m. SUMMARY

Because the PSP method does not require moving the fiber, errors

created by twists and rearrangements (cases B and C in Table II)

are avoided. Repeatedly measuring several other fibers using the

PSP method resulted in standard errors similar to those of the

fiber reported in Table II. In all cases, the repeatability was

far better than that of the cut-back method, because only the

errors in repeatedly measuring phase delay (with all other conditions held constant) are involved. Therefore, the PSP method

gives significantly improved precision in the measurement of

short lengths of low-biref r igence fibers.

1. D. Marcuse and C. Lin, "Low Dispersion Single-Mode Fiber

Transmission - The Question of Practical Versus Theoretical

Maximum Transmission Bandwidth," IEEE J. Quantum Electronics

QE-17 , 869, (1981).

2. T. Okoshi, "Heterodyne and Coherent Optical Fiber

Communications: Recent Progress," IEEE Trans. Microwave

Theory Tech. MTT-30 , 1138, (1982).

3. F. M. Sears, "Review of Polarization Measurement Techniques,"

to be published.

4. S. R. Norman, D. N. Payne, and M. J. Adams, "Fabrication of

Single-Mode Fibers Exhibiting Extremely Low Polarization

Birefringence," Electron. Lett. J_5, 309, (1979).

5. K. Kikuchi and T. O. Okoshi, "Wavelength-Sweeping Technique

for Measuring the Beat Length of Linearly Birefringent

Optical Fibers," Optics Lett. 8, 122, (1983).

6. W. K. Burns, R. P. Moeller, and C. L. Chen, "Depolarization

in a Single-Mode Optical Fiber," J. Lightwave Tech. LT-1 , 44, (1983).

7. L. G. Cohen, "Shuttle Pulse Measurements of Pulse Spreading

in an Optical Fiber," Applied Optics J_4, 1351, (1975).

61
Table I: Std. Dev. of the Birefringence for A) Fiber

Untouched, B) Fiber Rotated, and C) Fiber

Layout Varied

A B C

Coated

( » 6m) ± 17°/m ± 19°/m ± 330°/m

Cabled

(« 3m) ± 10°/m ± 19°/m ± 71°/m

Table II: Std. Dev. of the Birefringence for the

PSP Method

A* B C

Cabled

53 28 cm)

± 4.2°/m

Absent in

PSP

Method

Absent in

PSP

Method

*The improvement over the cut-back method is due in part to

the parabolic fit. The use of the fit could reduce the

cut-back error A comparably. However, the larger errors B

and C would remain.

LEGEND : LP. LINEAR POLARIZER

S.B.C. SOLEIL-BABINET COMPENSATOR

L.A. LINEAR ANALYZER LASER

DIODE

1329 nm

DIGITAL PROCESSING

OSCILLOSCOPE

AMPL.,

BICONIC M CONN.

AR-COATED 80% REFLECTING MIRRORS A CABLED

iox lp- iox\"Ts-m7iber*XV 10x

_LA; 10X I.

\ / FIBER HOLDERS ON

POLAR.

MAINTAINING

PIGTAIL MICROPOSITIONERS

DETECTOR

InGaAs

PIN

FIGURE 1. POLARIZATION SHUTTLE PULSE MEASUREMENT SET-UP

1 mm = 23.3 deg. 8<£ = 18 6 ±22 deg 8^ = 37.5 ± 4.2 dag. / meter

L h = 9.5 ± 1.1 meters

MAIN PULSE

mln. = 9.22 mm

let SHUTTLE

mln. = 8.40 mm

-»l0.S2 mm

-w| 0.86 mm ^m2nd SHUTTLE

mln. = 7.54 mm

8 9 SBC DIAL, mm

FIGURE 2. MINIMUM PHASE ANGLE OF MAIN PULSE AND FIRST SECOND SHUTTLES FOR 28 cm OF CABLED FIBER. MINIMA ARE READ FROM THE PARABOLIC FITS.

62
A NEW TECHNIQUE FOR THE MEASUREMENT OF AXI AL - STRESS IN OPTICAL -FIBRE

PREFORMS

M.P. Varnham, S.B. Poole and D.N. Payne

Department of Electronics, The University, Southampton, UK.

The ability to measure the axial stress in optical -fibre preforms is

essential for the development of highly -birefringent1 and other

specialist fibres, in which high levels of thermal stress are deliberately introduced to modify the fibre propagation characteristics. Conversely, it has been found that the loss of telecommunications-grade fibre can be reduced by minimising the stresses within

the core 2 . To date, axial stress profiles have been measured by the

rather cumbersome method of reconstruction from the retardation

profiles measured transversely across the preform3 ' 4 ' 5 However,

this technique has not seen widespread routine use, presumably

because the results rarely justify the complexity of the measurement.

In this paper we present a new method for measuring the axial stress

profile which should see widespread adoption as a result of its

simplicity and convenience. The method has the advantage that it uses the same hardware and software that are commonly used in

transverse refractive -index profiling 6 - 7 - 8 and it is therefore

readily incorporated into existing equipment. The technique can also

in principle be applied to two-dimensional stress profiling of

asymmetric preforms. In addition, the work provides a new insight

into how thermal stresses affect fibre refractive -index profiling

techniques

.

Transverse re tractive -index profiling of both symmetric and asymmetric preforms is based on the measurement, either directly or

indirectly, of the optical path- length difference r)(p,e) between a ray passing through the preform and an equivalent ray in the indexmatching fluid6 . (Here p is the ray offset and 6 is the azimuth of

the ray - see Figure 1). The axial stress az (r,9) in the preform

causes r)(p,&) to be dependent upon the polarisation orientation of

the incident light. Hence a retardation,

63
R(P,e) = r)z

( p,e) - 7)Q (p,e) (i)

exists across the preform. Here tj , r\ are the optical

z ©

path-length differences seen by light polarised axially and transversely to the preform axis respectively. Reconstruction of the

refractive -index profile from the r) and r> data will yield two

Z 0

different —effective refractive -index profiles, n'(r,\|/) and n' (r,\J/) z e respectively. The difference between these profiles can be shown by

the application of elementary elastic theory to be:

n'(r,vp) - n'(r,vj/) = -C a (r,\J/) (2)

*:* Z

where the stress -optic coefficient C = -3.5 x 10" 5 mm2 /kg for high

silica glasses. Thus the difference between the refractive index

profiles seen transversely by z -polarised and 0 -polarised light gives

the axial stress profile directly.

The above therefore suggests a remarkably easy method for measuring

the axial stress profile a z (r,v|/) in axi -symmetric and non axi -symmetric optical fibre preforms. Using existing transverse refractiveindex profiling techniques, we simply measure n'(r,vtO and n'(r,v|/) z 6

with light polarised axially and then transversely to the preform

axis, followed by calculation of a2 (r,v|/) from equation (2). Fig. 2

illustrates the two profiles n'(r,\|/) and n'(r,vj/) measured by the

standard spatial -filtering technique6 with the sole addition of a polariser. In this case, the preform is circularly symmetric and

consists of a silica substrate, a phosphorus/fluorine -doped outer

matched cladding, a depressed boron-doped inner cladding and a germania -doped core. The axial -stress profile reconstructed from

these measurements, Figure 3, reveals that the depressed boron -doped

inner cladding supports 15 kg/mm2 of axial tensile stress, whereas

the levels of tensile stress in the core are somewhat lower at 5

kg/mm2 . As expected, the outer matched cladding and substrate

exhibit a lower compressive stress of 2 kg/mm2 .

The existence of stress -induced multivalued refractive -index

profiles in the preform which depend on the polarisation of the

measurement light source obviously affects conventional transverse

64
refractive -index profiling measurements. A related topic has been

addressed by Scherer9

, who showed that thermal stresses cannot be

neglected in the design of graded- index fibres. We note that the

effective refract ive- index profile measured with unpolarised light is

the average of the effective profiles, (n' + n')/2. Again, elasti

city theory reveals that this is equal to (n + n )/2, where n and n x y x y

are the real real refractive indices seen by light polarised in the x

and y directions respectively (see Figure 1). Fortunately, therefore, the transverse re tractive -index profiling techniques yield the

average profile seen by unpolarised light travelling down the fibre.

This is an important result, since it quantifies for the first time

how thermal stresses affect the transverse profiling methods.

The axial stress measurement is currently being applied to non axi-symmetric profiles (e.g. Bow-Tie preforms1 ). Data concerning

resolution, accuracy and repeatability will be presented at the

Conference

.

Acknowledgements

The measurements described in this paper were made on the York

Technology P101 Preform Profiler. We would like to thank York

Technology for their co-operation. The preforms were supplied by

R.D. Birch and E.J. Tarbox.

References

1. R.D. Birch et al , Electron. Lett. 24, 1036, 1982.

2. B. Ainslie et al . , in Proc. 9th ECOC, Geneva, 1983.

3. P.L. Chu et al, Appl. Opt., 18, 707, 1982.

4. R.B. Calligaro et al, Electron. Lett. 18, 474, 1982.

5. N. Shibata et al , Appl. Opt., 3507- , 1982.

6. P.L. Francois et al , IEEE J. Quant. Electron., 18, 524, 1982.

7. P.L. Chu, Electron. Lett., 15, 295, 1979.

8. D. Marcuse et al, Appl. Optics, 18, 14, 1979.

9. G.W. Scherer et al , Appl. Optics, 19, 2000, 1980.

65
z Fig. 1: Co-ordinate system

INDEX

—.—,—,—_ i , , . , i . . . f .

- +

1

1

I-J

PROFILE

.01

"3 VvwvWVW A

1 1 I 1 I H 1 1

1

1 1 1 1 1 L. 1 1

1

1 ^Vwwvvw^ +3

.01

Radius ( mm

Fig. 2: Refractive index profiles measured for light

polarised axially (dashed) and transversely (solid line)

to the preform.

flXIHL

... I .... II

STRESS

i < i

PROFILE

i i i

( kg/mm~2

i i i r 1 1 1 j r— it i r 1 >

f ~5 Radius ( mm

Fig. 3: Axial stress profile calculated from Fig. 2.

66
MULTIMODE FIBER MEASUREMENTS - PRESENT AND FUTURE

A. H. Cherin

AT&T Bell Laboratories

2000 Northeast Expressway

Norcross, Georgia 30071

I. INTRODUCTION

Multimode graded- index fibers are commonly used as the transmission medium in the intracity trunking and loop feeder distribution

portions of modern telecommunication systems. It also appears

likely that multimode fibers will be used, for the foreseeable

future, in local area networks that offer a wide variety of integrated services. Because multimode fibers are used for both short

and long distance applications, a number of different measurement

methods have been developed to describe their transmission characteristics. This paper reviews some of the standardized multimode

measurement methods and highlights current areas of concern related to the measurement of bandwidth, numerical aperture, core

diameter and attenuation. In addition a performance related specification parameter known as the "intrinsic quality factor" (IQF)

will be briefly discussed.

II. BANDWIDTH MEASUREMENT

The pulse broadening and in turn the bandwidth observed in multimode fibers is dependent upon the fiber's differential mode delay,

attenuation and mode mixing characteristics. The measured value

obtained for bandwidth is a function of the launching conditions

and spectral width of the measurement source. Figure 1 illustrates how varying the launching condition of the source with a mode scrambler followed by a variety of mode filters affects the

measured bandwidth of a multimode fiber. Although it is widely

understood that a controlled launching condition will improve the

precision of bandwidth measurements, the choice of the particular

launching condition that gives the best prediction of the bandwidth of concatenated sections is unclear. The problem of

67
choosing the optimum launching condition is exacerbated by profile

compensation effects in adjacent cable sections and imprecise

knowledge of the length scaling coefficients of the fibers. The

Electronic Industries Association (EIA) recommends overfilling the

core and numerical aperture of a factory length of test fiber by

f 1

placing a mode scrambler between the source and the fiber.

Study group XV of the CCITT recommends that bandwidth measurements

be made with the same "equilibrium mode distribution" (EMD) that

is used for attenuation measurements. The EMD launching condition

can be approximated with a mode filter following the scrambler.

Additional reduction in the measurement variation of factory bandwidth measurements can be achieved by using the 3 dB bandwidth

from a Gaussian fit to the fiber transfer function instead of the

[21 transfer function itself. The measurement of fiber bandwidth

in the field is complicated by interconnection frames in the

transmission path that contain connectors and splices. Interconnection devices can decrease the precision of a bandwidth measurement by disturbing an EMD (underfilled) launching condition if it

is present at the input end of the interconnection frame. The

most precise field measurements can be obtained by using a mode

scrambler to produce the EIA recommended overfilled launching

conditions

III. NUMERICAL APERTURE

The numerical aperture (NA) of a graded index multimode fiber is

an important parameter that is a direct measure of a fiber's light

gathering efficiency and sensitivity to microbending loss. Two

standardized measurement methods are commonly used to obtain a fiber's numerical aperture. In the first method the NA is calculated from a measured value of delta, the fractional difference

between the peak refractive index of the core (n-|) and that of

the cladding (n2). Delta is obtained along with the fiber's

core diameter from its refractive index profile. The relationship

between a fiber's maximum theoretical numerical aperture and delta

u [3] is given by:

68
The numerical aperture can also be obtained directly from a fiber's far-field radiation pattern, since it is the sine of the

maximum half-angle (in air) of the guided radiation cone. That

is, the relationship between NA and the maximum half-angle, 8 a , is:

NA = sin 6 a

Since the EIA defines a fiber's NA between points 5% above the

baseline in the far-field pattern and the CCITT defines the NA

from a fiber's index profile, values for NA obtained from the

different definitions will not be the same. In order to resolve

these differences, a number of multimode graded-index fibers were

measured using both the refractive index profile and far-field

meaurement methods. A relationship was then developed between the

5% definition for NA obtained from the far-field measurements and

the NA obtained from the profile measurements. This relationship

requires multiplying the 5% far-field NA by 1.094 to yield a value

that is equal to the maximum theoretical NA obtained from profile

measurements

.

IV. CORE DIAMETER

The core diameter of the multimode graded-index fiber is defined

by the CCITT and the EIA, from the refractive index profile, as

that diameter passing through the core center and intersecting the

index profile (shown in Figure 2) at the points n^ / where n^ = + k(n^ - ^2^' Tne EIA nas defined the value of k = 0.025 to minimize the differences between the refractive index profile and

transmitted near-field measurement methods for obtaining core

f 4 SI diameter. l,J Unfortunately the CCITT has used the value of k = 0.05 in their definition of core diameter. A simple change in the

value of k by CCITT would result in a universal definition of core

diameter that would allow the most commonly used measurement techniques to be in agreement.

59
V. ATTENUATION MEASUREMENTS

For long distance telecommunications applications, the attenuation

of a length of fiber should be measured in such a way that the

losses can be added linearly to enable the prediction of the total

attenuation of concatenated links. EMD launching conditions that

restrict power from being launched into higher order lossy modes,

yield linearly additive attenuation values. The EIA attenuation

f 61 test procedure recommends two methods (the mode filter and 70/

70 beam optics launch) to restrict the launched optical power.

Interlaboratory measurement comparisons have shown no significant

difference between the two methods and a mean standard deviation

of 0.23 dB between the participants in the interlaboratory compar-

[1] ison. 1 1 The characterization of fibers for short distance applications

poses a number of difficult problems that are currently under

study. A typical application, such as a wide band premise network, is connector intensive and uses light emitting diodes as the

optical source. It is likely that the LED source and lossy connectors will excite high order lossy modes that will produce significant transient losses in the fiber. The excitation of high

order lossy modes can produce losses that are 1-2 dB greater than

. • r 7

fiber losses measured under steady state conditions. The accurate characterization and optimum utilization of short distance

systems will require the development of techniques for determining

the transient losses in these systems.

VI. INTRINSIC QUALITY FACTOR (IQF)

An essential part of the fiber standardization process is the

characterization of transmission parameters that optimize the

economic specification of fiber tolerances and satisfy system

performance requirements. An intrinsic quality factor (IQF) has

been developed to efficiently allow the tolerancing of fiber pa-

[81 rameters that effect splice loss. Measurements of the delta

(or NA), core diameter, core/clad eccentricity and core ovality of

70
a production fiber are used as input to a computer program containing an experimentally verified splice loss model. The computer program calculates the fiber's IQF. The IQF is a figure of

merit that is a measure of the fiber's average bidirectional

splice loss relative to a nominal fiber of the same type. By

setting a tolerance on the IQF, system performance levels can be

guaranteed without negatively effecting product yield. If each

individual parameter were toleranced independently, unnecessarily

tight tolerances would result that could reduce production yield.

REFERENCES

1. D. L. Franzen, G. W. Day, and R. L. Gallawa, "Standardizing

Test Conditions for Characterizing Fibers," Laser Focus, 8/81.

2. F. T. Stone, "Problems in Bandwidth Measurment and a Suggested

Solution," Journal of Lightwave Technology, Vol. LT-1 , No. 1,

3/83.

3. D. Marcuse, "Principles of Optical Fiber Measurement,"

Academic Press, 1981.

4. A. H. Cherin, P. J. Rich and S. C. Mettler, "Measurement of

the Core Diameter of Multimode Graded-Index Fibers: A

Comparison of Transmitted Near-Field and Index Profiling

Techniques," Journal of Lightwave Technology, Vol. LT-1, No.

2, 6/83.

5. E. M. Kim and D. L. Franzen, "Measurement of the Core Diameter

of Graded-Index Optical Fibers: An Interlaboratory

Comparison," Applied Optics, Vol. 21, No. 19, 10/82.

6. EIA Document RS-455-46, "Spectral Attenuation Measurement for

Long-Length, Graded-Index Optical Fibers."

7. A. H. Cherin, E. D. Head, C. R. Lovelace and W. B. Gardner,

"Selection of Mandrel Wrap Mode Filters for Optical Fiber Loss

Measurements," Fiber and Integrated Optics, Vol. 4, No. 1,

1982.

8. D. W. Peckham, S. C. Mettler, and R. B. Kummer, "A Systematic

Approch to Specifying Multimode Fiber Manufacturing

Tolerances," NBS Symposium on Optical Fiber Measurements,

10/84.

71
1 2

FIGURE 1

n(r)

Near-parabolic refractive-index profile used for defining core

diameter.

FIGURE 2

72
"A SYSTEMATIC APPROACH TO SPECIFYING MULTIMODE

FIBER MANUFACTURING TOLERANCES"

D. W. Peckham, S. C. Mettler, and R. B. Kummer

AT&T Bell Laboratories

2000 Northeast Expressway

Norcross, Georgia 30071

INTRODUCTION

Modern low loss multimode fibers operating in the

long wavelength windows are re-emphasizing the need for low

loss fiber splicing. Moreover, advances in fiber joining

technology increase the importance of minimizing the

effects of fiber parameter variations on splice loss, making the selection of fiber manufacturing tolerances more

important.

The fiber intrinsic parameters which have the

greatest influence on splice loss are core radius (a),

delta (A) (or numerical aperture), core eccentricity, and

ovality. The traditional approach to setting manufacturing

tolerances on these parameters is to consider the effect on

splice loss of each parameter independently. This approach, however, does not take into account the compensating or compounding effect of simultaneous parameter deviations. For example, a fiber with a +6a (i.e., a positive

core radius deviation from the nominal) and a -<5A will have

a lower splice loss (when spliced to a nominal fiber) than

a fiber with the equivalent magnitude deviations, but with

both deviations of the same sign (i.e., +6a,+6A or -6a,-6A).

Since the effect of the combined parameter deviations is

73
not determined, the individual parameter tolerances are

typically specified assumming the worst case combination of

parameter deviations even though the probability of

occurrence may be small.

An appealing alternate approach to fiber intrinsic parameter specification is to use a fiber intrinsic

quality factor (IQF) specification. The IQF is a measure

of the fiber parameter deviations weighted with respect to

splice loss. With this approach, for example, a fiber with

a large deviation of one parameter is acceptable only if

the other three parameters are near nominal or if they

produce an effect on splice loss that compensates for the

large deviation. Thus, less stringent requirements on

individual fiber parameters are needed to maintain the same

mean splice loss performance as with the traditional approach (i.e., higher product yield). Alternatively, improved system splice loss can be realized while maintaining

the same yield.

INTRINSIC QUALITY FACTOR

The IQF is the average bidirectional splice loss

(the average splice loss occurring when light is propagated

in each direction through the splice) when the fiber is

spliced to a nominal fiber with no extrinsic misalignment.

For example, a 54 pm core radius, 1.25% A fiber (with no

eccentricity or ovality) when spliced to a nominal (50 pm,

74
1.3% A) fiber produces a 0.02 dB loss when light is

launched into the splice from the nominal fiber, and 0.11

dB loss in the other direction, and has an IQF of 0.065 dB

((0.02 + 0.11)/2). The IQF can be measured or it can be

calculated using factory measurement data as input to a splice loss model. Although other models could be used,

the results presented in this paper were obtained using the

empirical Gaussian splice loss model. 1

RESULTS

Table I compares the mean and maximum IQF, and

the "production yields" (normalized to the conventional

approach yield) of the two specification approaches when

applied to a fiber data base randomly generated using typical fiber parameter statistics. The example of the

"conventional" approach limits the parameter deviations so

that, when taken individually, they result in an average

bidirectional splice loss (ABSL) of 0.1 dB or less. The

"IQF" approach, with an IQF limit of 0.14 dB, maintains the

same mean splice loss performance of the fibers (0.09 dB) , but significantly decreases the worst case splice loss

(0.15 dB versus 0.56 dB), while also increasing the normalized product yield (1.00 versus 1.11) when compared to the

"conventional" approach. Alternatively, improved splice

loss could be realized while maintaining similar yield.

75
SUMMARY

A systematic approach to fiber parameter specifi

cation which includes the effects on splice loss of parame

ter deviations has been presented. An example has been

presented which illustrates the possible improvements in

splice loss performance and yield obtainable with this

approach

REFERENCES

1. S. C. Mettler, "A General Characterization of Splice

Loss for Multimode Optical Fibers," BSTJ, Vol. 58, No.

10, December 1979 and references cited therein.

TABLE I

Conventional Recommended

Approach Approach

(ABSL < 0.10 dB) (w/IQF)

A: 1.3 ±0.17% 1 .3 ± 0.29%

a: 25 ± 2.3 ( um) 25 ± 3.4 ( wn)

ecc: < 2.4 (urn) < 3.0 (ym)

Ovality < 19.6% < 24.3%

IQF - < 0.14

Max. Loss 0.56 dB 0.14 dB

(ABSL)

Mean Loss 0.09 dB 0.09 dB

(ABSL)

Normalized 1.00 1.11

Yield

76
BANDWIDTH OPTIMISATION OF A MULTIMODE FIBRE INSTALLATION

S.C. Hampson

BICC Telecommunication Cables Limited, Prescot, England.

ABSTRACT

The Autocorrelation Function (ACF) Effective Bandwidth (Ref.l) was

used to predict the concatenated frequency transmission response of a

multimode fibre installation. The -3dB (Optical) bandwidth for each route

fibre was then measured and the figure compared with that which had been

previously predicted. A programme of cross- jointing was then devised in

order to achieve the optimum transmission response for the installation. A

specific objective of gaining a -3dB (Optical) bandwidth figure greater

than 140 MHz for all route fibres was also set and achieved.

BACKGROUND

The installation in question runs from a local telephone exchange to

a repeater station, a distance of 22.8km. This incorporates two

intermediate regenerator stations thus dividing the route into 3 subsections of lengths 7.8km, 7.8km and 7.3km respectively. Due to

difficulties experienced during cable laying, several short cable lengths

were installed, see Fig 1 for a schematic of the route.

The cable system consists of eight loosely tubed fibres jointed by

the fusion arc technique. The overall joint enclosure is sealed by the

injection weld method.

The grade for the individual fibre was a -3dB (Optical) nominal

minimum modal bandwidth of 800 MHz-km and a nominal maximum attenuation of

1.2dB/km was utilised.

77
SYSTEM PERFORMANCE

The cables were manufactured at the BICC cabling plant, Manchester.

The Autocorrelation Function Effective Bandwidth for each of the

individual fibres was derived by applying the formula :

BW(ACF) = 0.3 BW(-3dB) + 0.454T

1

where BW(-3dB) is the measured fibre bandwidth and T is the half-time

to the 5% height of the autocorrelation function of the impulse response.

This calculation does not require a separate measurement other than the

routine factory measurement of frequency response, since the ACF is

obtained from this by computation. The measurement data for the

individual fibres used for the section between the two regenerators are

given in Table 1 .

To predict the route fibre frequency responses the ACF Effective

Bandwidth figure was substituted into the formulae:

Bw (Corrected) = ( Original Length ) x BW( ACF)

(Installed Length)

and BW (Section) p BW (Corrected). 6

t ©

For the 3 sections was assumed to be 0.78, while a value of 0.75

was applied for fibre sub-division.

The frequency transmission responses were then obtained for the

twenty-four route fibres. The recorded bandwidths respresent the -3dB

(Optical) point on the frequency response. These results show an

agreement to the figure which had been previously predicted. The two sets

of data are given in Table 2.

78
CROSS JOINTING PROGRAMME

To achieve the optimum transmission performance for the installation,

a computer programme was used to calculate the frequency response of

various crossed-f ibre combinations.

It was established that a total number of 18 cross splices would be

required at 6 joint locations. Bandwidth measurements were taken after

each splice had been made. This figure was then compared to the value

which had been previously predicted for that combination of fibres. On no

occasion did the cross- jointing programme require modification on account

of the measured figure showing a large variance from the value predicted.

The final figures achieved after the programme had been completed are

given in Table 3 along with the predicted value.

w CONCLUSION

The Autocorrelation Function Effective Bandwidth, proved to be a very

useful tool when predicting the frequency transmission response of

concatenated fibres. The actual spread between measured values was

decreased and the objective of a -3dB (Optical) bandwdith figure of

greater than 140 MHz was achieved for all 24 routes fibres.

The criterion established for performance prediction was very

successful, and showed that when using the ACF Effective Bandwidth to

optimise system fibres a concatenation constant lower than 0.78 could be

used in future. This is thought to be due to the fact that the ACF

Effective Bandwidth as currently defined does tend to give a pessimistic

figure.

BICC now design all their multimode optical fibre systems on the

basis of the ACF and have been extremely successful in predicting the

frequency transmission responses for these installations.

79
REFERENCES

1. Determine the Concatenated Dispersion of Multimode Fibres,

R.W. Blackmore and N.G. Batty - Symposium in Optical Fibre

Measurements, Boulder, 1982.

FIG. 1. SCHEMATIC OF CABLE INSTALATION

LENGTHI* y( 490 y 775 1920 Itl ^ 1881 \^

1960 ^ Hi ^

SECTION 1.

TH |a I 1158 2013 \^ 2106 y/ 2075 N,^ 46S ^

LENGTH tmt\y 1602 x- -x2048 \/ 1777 x- -xFIBRE NUMBER 1 2 3 4 5 6 7 8

CABLE

POSITION MEASUREMENT

1 ew i-3dB) 811 890 910 647 947 967 861 890 BW (ACFI 615 -1GHz 998 -1GHz 934 926 760 -1GHz

2 BW (-3dB> 981 987 104 800 920 923 924 809 BW IACF) 510 938 80} 962 96 7 -1GHz -1GHz -1GHz

3 BW (-3dB) 816 850 853 893 818 829 835 869 BW (ACF) 532 -1GHz 513 -1GHz 953 550 668 -1GHz

4 BW l-3dB) 861 833 819 989 97» 907 905 817 BW (ACF) 837 612 608 -1GHz 866 -1GHz 997 760

5 BW <-3dB) 811 890 910 8*7 947 9»7 861 890 BW (AC F

615 -1GHz 998 -1GHZ 933 926 760 -1GHz

TABLE 1. INDIVCUAL BANDWIDTH DATA — FIBRES FORMING SECTION ?. (VALUES IN MHz -km)

ROUTE

FIBRE

1 2 3 4 5 6 7 8

SECTION

1

PREDICTED VALUE 196 171 128 163 120 160 131 155

MEASURED

VALUE 165 141 111 157 130 166 131 132

SECTION

2

PREDICTED VALUE 98 167 118 271 184 171 243 227

MEASURE0

VALUE 101 156 111 W0 156 139 139 247

SECTION

3

PREDICTED VALUE 131 216 166 159 186 122 177 171 MEASURED

VALUE 112 186 163 155 271 123 127 155

TABLE 2MEASURE0/PRE0IC TEO FIGURES FOR STRAIGHT J0INTE0 SECTIONS

ROUTE

FIBRE

1 2 3 4 5 6 7 8

SECTION

1

PREDICTED VALUE 152 153 160 158 160 151 148 146

MEASUREO

VALUE 175 167 165 150 202 152 152 259

SECTION

2

PREDICTED VALUE 146 152 153 140 151 146 149 162 MEASURED

VALUE 180 176 149 176 149 167 200 185

SECTION

3

PREDICTED VALUE 143 140 163 155 154 140 140 155

MEASURED

VALUE 174 232 190 210 174 163 177 196

TABLE 3. FINAL MEASUREO /PREDICTED FIGURES FOR CROSS JOINTED SECTIONS

80
AUTOMATED DIFFERENTIAL FIBER STRAIN MEASUREMENT SYSTEM

FOR SINGLE AND MULTIMODE FIBER

K. H. Hafemeister

T. A. Clarke

E. J. Buonopane

Siecor Corporation

Hickory, N.C.

Introduction:

Many optical fiber cable designs exist today which incorporate free

space around the fiber to meet the tensile or temperature operating

windows (example: loose tube, slotted core). This free space allows the

fibers to remain strain-free while the cable itself is under tensile or temperature stress. Only after a certain load will the fibers use up

all the free space available to them and begin to see the effects of

the cable loading forces. The amount of load that the cable can withstand without imparting stress to the fiber is calculated by certain

design parameters. This paper describes a fiber strain measurement

system that can, as one option, measure the load level at which the

fibers in the cable first see strain, thus ensuring the parameters of

the cable design are met. This system also determines the amount of

strain seen on a fiber at any stress situation and can evaluate tbe

amount of residual strain cabled fibers see after tensile load is

released.

Theory:

A tensile load placed on an optical fiber will cause the fiber to

elongate. The nature of glass dictates that this elongation, or strain,

is linear until the fiber breaking point is reached. The fiber strain

measurement system measures this elongation by launching light pulses

into the fiber and measuring the time delay that occurs as the pulses

travel through the elongating section. This time delay is then converted

into a value of present fiber strain. /I/

The time delay is expressed as

/in L . n. T ^ AnAL

(1) At = —An + —AL +

c c c where L = length under load

AL = change in length (elongation)

c = speed of light in vacuum

n = group refractive index

An = change in refractive index due to" strain

81
Assuming An AL is negligible, the fiber strain e can then be expressed as

C r

(2) £

F ~L " "nL * At " H

The refractive index change with strain is given as

(3) M =

c n nL

where k is a fiber related proportionality factor. 12/

Substituting (3) into (2) gives

<*> e f

- r • -a: (1 - k) at

Technique

To measure fiber strain, the cable sample under test is first fixed on a long length tensile tester, which can load a section of the cable up to

126 meters long with a stress varying from 0 (neglecting the weight of

the suspended sample) up to 36 KN. The ends of the cable are brought

to the measurement system and prepared. One fiber end is placed in a bare fiber adapter and coupled to a connectorized 50 ym core fiber

pigtail mounted on a laser diode. The other end of the fiber is coupled

into a silicon PIN diode using a SELFOC lens.

Laser 846 nm FWHM 50 ps

Laser 1200 nm FWHM 200 ps

Receiver PIN Diode FWHM 250 ps

Jitter 50 ps

Resolution 10 ps = 0.002% ^

Accuracy ±0.002% e

The laser reference trigger signal is sent through a delay time generator

which compensates the sample length to adjust the position of the signal.

The signal then travels through a trigger recognizer and into a digital

oscilloscope

The received signal from the PIN diode is sent through a sampling head

into the oscilloscope. The oscilloscope averages the signal and smooths

the curve to reduce the error in calculating delay time.

Actually measuring the delay time between the reference and the test

pulse can be accomplished several ways. The method used in this system

82
has the Tektronix 7854 oscilloscope find both FWHM (half power level)

points for the reference pulse and average them using t

f = (t^ + t^)fl

(see Fig. 2 below). As load is applied to the cable, the position of

the pulse is intermittently sampled. The same procedure is used to

calculate an average t = (t_ + t,)/2. The delay time At is

test 3 4

calculated in the computer using the formula At = t - t c . ° test ref

Special Considerations:

This technique takes into account the differential refractive index

response of the fiber under test. Using the technique may cause inaccuracies due to the material dispersion through the strained fiber

section. The procedure outlined previously has been shown to be accurate

with most pulse shapes, and also compensates for attenuation and distortion of the pulse shape that may occur during testing.

Output

:

Fig. 3 shows a typical result generated by the computer and graphed using

an X-Y Plotter. Cable strain, measured using a strain gauge attached to

the cable and interfaced with the computer using an IEEE bus, is plotted

along with the fiber strain, versus tensile force. The program also

gives the strain at maximum force and after the load is released, as well

as printing specific test criteria information directly on the results

graph. The format of the graph enhances cable design analysis in that

the onset of fiber strain, the fiber strain seen at specific load levels

and the residual strain are all easily read from the graph or printed

separately on the output page.

K for a Single Mode Fiber:

A test was set up to determine the correction factor k due to refractive

index change for a single mode fiber. This test used a tight buffered

cable with one single mode fiber central to the cable, ensuring that any

strain applied to the cable is mechanically coupled to the fiber. The

cable was loaded to 350 N and the cable strain measured using a strain

gauge attached to the cable. The fiber strain was measured at both 846

and 1200 nm. Comparing the difference between the measured fiber strain

and the measured cable strain gave a correction factor at both wavelengths of k=-0.20 for the single mode fiber tested.

83
Summary

The fiber strain system described provides an easy, accurate method of

characterizing cables with respect to their design parameters.

The computer driven system is easily used in a quality control environment,

where cable designs can be type tested to ensure product specifications are

met. The system can be used with multimode and single mode systems, once cable related proportionality constants have been determined.

HI Philen, D.L., Patel, P.D.

Measurements of strain in optical fiber cables using a commercial

distance meter

ECOC 82 communication AV II-3

HI Sutor, Norbert.

Testing the Mechanical and Thermal Characteristics of Optical Cables.

Siemens Telcom Report 6 (1983), pp. 183-187.

84
The distribution of H gas along an inland optical fibre cable

S Hornung, S A Cassidy, M H Reeve B.T.R.L. Martlesham Heath, Ipswich, Suffolk, UK

1. Introduction . Recently a good deal of interest has concentrated on the effects

of H on the optical attenuation of fibre (1). The work has mainly focused on the response of fibre to relatively high levels of H , often at elevated

temperatures. Little has been reported of actual levels of in cables in

operational environments.

In this paper we report the field measurement of levels of H^ inside an optical

fibre cable and it's distribution along the cable length. The cable chosen was a BICC "Jubilee" type, linking Guildford to Aldershot. It was selected because it

contains high phosphorus doped multimode fibre, which is particularly sensitive

to H (2). We also report two follow-up experiments, which together with a

2 simple model go some way in explaining the nature of the experimental results.

2.1 Field measurement. H was detected by a portable "Exhaled H Monitor"

2 2 (made for the medical profession), which is in essence a fuel cell.

20cc samples of gas were drawn off at a series of joint housings along the cable

route via Schroeder valves (part of the cables pressuri sati on arrangement) and

introduced into the instrument. The instrument was re-calibrated at each

measurement point. The results are plotted in Figs. 1 to 3. The pressuri sati on path of Guildford to Aldershot was interrupted on either side of a midpoint

regenerator API. This resulted in two independently pressurised systems,

Gui 1 dford-APl and Al dershot-APl . The distribution of H in these sections is

2 similar, starting at 0 at the exchange (high pressure) end, and rising to a maximum value at the dead end (API). The peak value recorded anywhere was 394ppm.

Measurements on the adjoining Al dershot-Camberl ey cable also fit this pattern.

We propose the following model. is continuously generated along the cable

length at a constant rate of R cc/sec/cm of cable, and diffuses through the cable

walls at the rate C*W cc/sec/cm of cable. Here C is the partial pressure of H , and W is an effective diffusion constant. H^ is also swept along the cable by a flow of gas C*F. The flow F is a constant in a particular part of a cable and is

assumed to be caused by some leakage further down the cable. In the two cable

sections described above, a main leak is assumed to be situated at a single point

along the length. Two values of F are then used, a higher one up to the leak and

a very small one beyond it. By equalising the H^ generation and dissipation

rates we can derive the concentration of H along cable length x, as : R 2 W Wx C-jjCl-U.-CJeiH-)] (1)

85
C is the initial partial pressure of H and V is cable volume per unit

o 2 o length. Figs. 1 to 3 also show simple fits to the experimental data points using

the above equation. The calculated values of R, W and F are. given in table 1.

2.2 Diffusion measurements . To convert the fitted values of R s W and F in

arbitrary units into real numbers given in table 1, more measurements were

needed. W was measured directly. Nitrogen containing a known percentage of was passed along two short (0.1m) and two long (2.5m) cable samples, each

surrounded by a sealed copper tube. The level of H in the copper tubes was

2 —8

monitored as function of time (Fig. 4), giving W=3 . 7*10 cc/sec/atm/cm of

o cable at 20 C. Doubling the samples and using different lengths allowed the

elimination of end effects and greatly reduced the possibility of a freak defect

in the construction.

The nature of the cable construction is that the most significant route for H

2 to escape is along the aluminium barrier overlap. (Fig. 5) The diffusion through

aluminium is much slower. The aluminium barrier is polyethylene coated and the

heat of the final polyethylene layer coating seals the overlap producing a 6mm

deep and 40ym high polyethylene channel. The diffusion rate under these

conditions would be about 6.1*10 cc/sec/atm/cm of cable. The much larger

experimental value implies that the overlap region is not perfectly sealed.

2.3 Equilibrium level test . The level of was measured in a spare cable

length. As the cable had never been pressurised, this then gave a result of

level in the absence of flow. The cable was connected at one end to a supply of

compressed N and at the other to the H detector. The level of H in the

2 2 2 spare cable was found to be uniformly 364ppm, dropping to zero at the cable ends

due to diffusion. Using the constants R, W, V and C , the build up of H

o ,, o ... 0 2 can be expressed as: r - * r {

, w ~ x ,«t . . C

W

[ 1 ( 1 " R

C o

} exP- ( V~) ] ( 2 )

7 0 -4

The spare cable is 2years 5months old (7.62*10 sec), C=3. 64*10 atm, C =0.

-8 0

Using the experimentally measured W=3.7*10 cc/sec/atm/cm of cable, we calculate that R=l .5*10 cc/sec/cm of cable. Finally the experimentally

measured R=l . 5*10 cc/sec/cm of cable and W=l . 58*10 cc/sec/atm/cm of cable

o (corrected for field temperature of 5 C ),(4,5) are substituted into equation 1. giving the values of F in the various cable sections. In this calculation we note

that the partial pressure of is 1.6/10 times the concentration measured

in ppm due to the cable's pressuri sati on to 1.6atm. It is assumed that the

generation rate is the same in the field and in the spare cable.

86
3. Discussion . Taking into account the exponential nature of H^ build up, the

equilibrium level in this type of cable will be 400ppm (at 1.6 atmospheres). The

increase in attenuation of phosphor-doped multimode fibre at this level of

is very much subject to the nature of extrapolations from experiments spanning a few months to 30 years. For low phosphorus doped fibre up to approximately

0.06dB/km at 1.3ym can be expected (3).

It is apparent that the quality of the Aluminium water barrier (APL) seal is

important. Fig. 6 shows a comparison of H^ build up for the measured value of W,

for the value of W in the case of a perfectly made APL seal and in the case of a completely sealed up cable. If the water barrier was perfectly sealed then the

level of after 2.5 years would be a factor of 3 higher. A poorer water

barrier is therefore an advantage in this respect, but must be balanced against

the risk of moisture ingress.

The presence of air flow in the cables clearly reduces the average level to

well below 400ppm. A certain level of flow could therefore be engineered into any

system, which could also sweep out any moisture.

Much work still needs to be done to identify the sources of generation. In the simple model described in this paper the generation rate was assumed to

be constant. It is not clear whether H^ in inland cables is generated by

corrosion/polymer degradation process, or whether H^ is released from material

where it was trapped. These H producing processes may have quite different

profiles with time, which would make extrapolations to system lifetime difficult.

4. References

1. Increased Attenuation of Optical Fibres Caused by Diffusion of Hydrogen. K J Beales et. al . 9th. ECOC Geneva October 1983.

2. Optical Loss Increase of Phosphor-doped Silica Fibre at High Temperature in the

Long Wavelength Region. N Uesugi et. al . Appl . Phys. Lett. 4-3(4) 15 August 1983.

3. Hydrogen Effects in Installed Cables and Under Accelerated Conditions. J S Stannard-Powel 1 et. al . IEE Colloquium, June 11-12 1984 London.

4. Polythene. A Renfrew and P Morgan, 2nd Edition, 1960.

5. Polymer Handbook. J Bandrup and E H Iminergut, (Editors) 1966.

Table 1. Gui 1 df ord-APl . Al dershot-APl . Al dershot-Camberl ey

R cc/s/cm of cable 1.5 *10" U1.5 *10" U

1.5 *10" U

-2

Gui ldf ord-APl. Al dershot-APl.

i.5 no"

11

i.5 no"

11

1.53*10" 8 1.58*10" 8

-2

8.2 no 6.5 no"

2

0.03*10"

2 1.6 no"

2

W cc/s/cm of cable (5°C) 1.58*10 " 1.58*10

"

F cc/s 8.2 *10 „ 6.5 *10

n

F cc/s

2

87
xio^ ppm

Fig.l H

2 in Gui 1 df ord-APl section. Fig. 2 H

2 in Al dershot-APl section.

Fig. 3 in Al dershot-Camberl ey section.

polyethylene coated aluminium

polyethylene sheath

Fig. 4 H

2 diffusion from cable samples.

12 19 II 21 24 27 ja

T

t months

Fig. 5 Water barrier overlap. 8,

F1g.6 H

9 vs. time W. =1.6*10 "cc/s/cm/atm,

w^e.mfl^cc/s/cM/atm.

88
ADVANCES IN OPTICAL TIME -DOMAIN REFLECTOMETRY

A.H. Hartog,

York Technology Ltd., Avenger Close, Chandler's Ford, S05 3DQ, UK.

1. Introduction

In recent years, optical time-domain ref lectometry (OTDR) 1 has

progressed from a laboratory experiment to a technigue in everyday

use by manufacturers and users of optical fibres. The principal

attraction of the technique is its ability to give, non destructively, a good indication of the fibre-loss uniformity and therefore

to reveal any localised defects in the fibre. It has also been

employed as a means of measuring accurately fibre 2 and splice

losses 3

, and of studying the longitudinal variation of fibre

parameters 4 • 5. Briefly, OTDR involves the launching of a short pulse of light

into the fibre. As the pulse travels along the fibre, its energy

is lost in part to Rayleigh scattering. Some of the scattered

light is recaptured in the backward direction and guided towards

the launching end where it may be detected. The signal obtained

takes the form:

z Ps (t) - Po W 7] (z)exp [- | 2a(z)dz] (1)

6

where the time and position variables t and z are related by

z » vg t/2 (vg is the group velocity). Po W represents the forward

pulse energy, a(z) the local attenuation and i)(z) the backscatter

factor. t)(z)depends on the fibre waveguide and scattering properties and varies from typically 300 W/J for multimode fibres 6 at

850nm to 10 W/J for single-mode fibres at 1300nm7 - 8

.

89
It is clear from (1) that a backscatter signal will not in

general give the local fibre attenuation directly since any

longitudinal change in the rate-of -decay of Ps(z) could be caused

equally by variations of ri(z) or of a(z). Nevertheless, the mere

presence of a non-uniformity normally provides most of the information required and if the measurement is repeated from the opposite

fibre end, the effects may be separated unambiguously4 - 5

. Thi3

paper will review the development of OTDR with particular emphasis

on long range single-mode measurements at 1300nm and beyond.

2. P erformance Criteria in OTDR

Dynamic range is the single most important criterion of the

performance of an OTDR. It is a measure of the fibre loss through

which the backscatter signal may be measured to a given accuracy

and is expressed in "dB one-way". Although definitions abound,

that preferred by the author, particularly for long wavelength

systems, is the range at which the signal from Rayleigh scattering

becomes equal to the r.m.s. noise.

The spatial resolution of an OTDR depends on the ability of

the instrument to respond to an abrupt change of backscatter signal

along the fibre and is determined by the convolution of the probe

pulse with the impulse response of the optical receiver and dataacquisition equipment. At high resolution, the probe pulse energy

is reduced and the noise-equivalent power of the receiver is

increased; both effects lead to a reduction in the signal-to-noise

ratio. A strong trade-off thus exists between the dynamic range

and the spatial resolution of the measurement, and equipment

specifications must be considered with this point in mind. More90
over, resolutions quoted in fact frequently refer to the sample

separation of the data acquisition electronics rather than to the

system analogue risetime.

In addition, the design of an OTDR is influenced by the

requirement for portability and ruggedness which usually dictates

fairly simple optical designs, the use of injection lasers and

precludes any significant cooling of the detector. The wavelength

of the probe pulse must be close to the fibre operating wavelength

if valid attenuation measurements are to be obtained and to avoid

spurious wavelength dependent effects such as the second-mode

cut-off loss peak or the microbending edge in single-mode fibres.

As a point of reference, commercially available OTDRs at 850nm

typically use a semiconductor laser source, a silicon avalanche

photodiode detector (APD) and an all-fibre directional coupler.

Typically the dynamic range is 25dB one-way for a resolution of

10m.

3. Operation at Long Wavelength and with Single-Mode Fibres

Pulsed sources for OTDR at 1300nm are not so readily available

as at 850nm; their output power is lower and their reliability and

temperature stability are worse. Moreover, the silicon APD, an

almost ideal detector, does not respond at 1300nm and no other

material has shown a similar performance. Germanium APD 1 s are

particularly at a disadvantage because, in the bandwidths of

interest, their leakage current severely limits the receiver

sensitivity. Presently none of the commercially-available APD's

can compete at 1300nm with PIN diodes, although some of the

laboratory quaternary devices would provide a small improvement in

91
receiver sensitivity.

Additional difficulties are encountered in the design or" single-mode r ef lectometer s . For example, the power available from

single transverse-mode injection lasers is low and efficient

launching into single-mode fibres is difficult owing to the

ellipticity and astigmatism of the laser output. Moreover, the

backscatter factor is lower in these fibres than in multimode

fibres at the same wavelength. The design of single-mode OTDR '

must also avoid any polarisation sensitivity in the return optical

path, which would lead to an unwanted modulation of the backscatter

signal in accordance with the local fibre birefringence.

4 . Long-Range Single Mode OTDR at 1300nm

Much work has been devoted recently to the development of OTDR

systems for testing singlo-rnode communications at 1300nm and

beyond. In this application, the minimum reguirement is to detect

a break within a repeater span ( 25dB of fibre loss at 140 Mb/s). A

spatial resolution of 100m is usually adopted which is sufficient

to determine with certainty the fibre section at fault. There is

an overwhelming need for the equipment to be portable and rugged

which precludes, for example, the use of high power lasers and

cryogenic detectors. In practice, only low-noise analogue direct

detection and heterodyne detections are serious contenders.

In the first approach9 - 10

, a PIN photodiode operated at room

temperature is used together with an ultra-low-noise FET- input

preamplifier; the receiver sensitivity reported is 30pW before

averaging. Multichannel digital averaging allows a further

improvement in sensitivity to 30fW in a measurement time of 20 min.

92
With a semiconductor laser, a dynamic range of 30dB one-way has

been obtained, the highest value reported to date. The sensitivity

of this receiver matches that of a photon counting receiver using a Ge APD at 77K11

.

In heterodyne detection 12 - 13

, the probe pulse and hence the

backscatter ed power is frequency-shifted with an acous to-opt ic

deflector (AOD) and mixed on the photodiode with an unshifted

portion of the source power, which is used as a local oscillator. A

backscatter signal is obtained at the intermediate frequency

(IF), equal to the frequency shift of the AOD. The detected

signal is proportional to the product of the backscatter and local

oscillator (LO) electric fields. With sufficient LO power, the

signal at the IF can be made large compared with the receiver

noise. A coherent backscatter receiver can in principle be two

orders of magnitude more sensitive than a direct detection frontend. However, this scheme requires the use of extremely narrow

linewidth sources (i.e. a spectral width of the order of the IF

bandwidth) . Apart from the practicalities of producing a 1MHz

linewidth portable laser, the output power of such sources is

usually substantially lower than that available from unstabilised

lasers. This offsets at least in part the sensitivity advantages

of the coherent detection scheme and the best results published to

date13 have failed to match the dynamic range performance of direct

detection, although a receiver sensitivity better by a factor of 4

has been demonstrated. Coherent detection suffers from its

intrinsic sensitivity to the state-of -polar isat ion of the backscatter signal and to the speckle-like noise associated with the

93
use of a highly monochromatic source14 .

5 . Conclus ions

Research into both coherent and direct-detection long-range

OTDR systems is active and it is clear that further improvements

will be achieved; these will result in part from the availability

of better devices namely, higher power, narrower linewidth lasers

for coherent detection and low-leakage current APD ' s for direct

detection. Progress is rapid in both of these areas and it i3 not

yet clear which approach will prove to be the most effective in

practical instruments.

Finally the use of OTDR has spread into new areas and recently

it has been applied to the development of distributed sensors, an

area which shows considerable scope for innovation in optical time

domain ref lectometry

The author would like to thank M.P. Gold for helpful

discuss ions

References

T~. M.K. Barnoski and S.M. Jensen, Appl. Opt., 1976, 15, pp.

2112-2115.

2. B. Costa, et al., Electron. Lett., 1980, 16, pp. 352-353.

3. M.P. Gold, et al. , Electron. Lett., 1984, TO , pp. 338-340.

4. P. DiVita and U. Rossi, Opt. & Quant. Electron., 1980, 11,

pp. 17-22.

5. M.P. Gold and A.H. Hartog, Electron. Lett., 1982, JL8, pp.

489-490.

6. A.R. Mickelson and M. Eriksrud, Appl. Opt., 1982, 21, pp.

1898-1909.

7. E. Brinkmeyer, Electron. Lett.,

8. A.H. Hartog and M.P. Gold, J. Lightwave Technol., 1983, LT2,

pp. 76-82.

9. M.P. Gold and A.H. Hartog, Electron. Lett., 1984, 20, pp.

285-287.

10. M.P. Gold, submitted to J. Lightwave Technol.

11. P. Healey, Electron. Lett., 1981, 17, pp. 751-752.

12. P. Healey and D.J. Malyon, Electron. Lett., 1982, 18, pp.

862-863.

13. S. Wright, et al., Proc. 9th ECOC, Geneva, 1983, pp. 177-180.

14. W. Eickhoff and R. Ulrich, Tech. Digest 3rd IOOC, San

Francisco, 1981, pp. 76-78.

94
1.3 M m PORTABLE REFLECTOMETER FOR THE FIELD TEST

OF SINGLE-MODE FIBER CABLES

_ o _

J.J. BERNARD , E. VEPRESLES, L. JEUblHOME , J.L. MONCE LET

LABORATOIRES DE MARCOUSSIS, CGE RESEARCH CENTER

91460 MARCOUSSIS - FRANCE

M. CARRATT

C.L.T.O - C0MPA6NIE LYONNAISE DE TRANSMISSIONS OPTIQUES

35, rue Jean-Jaures - B. P. 20

95871 BEZONS CEDEX - FRANCE

INTRODUCTION

The growth of single-mode optical fiber transmission systems at 1.3 pm requi

res test instruments for field use, specially designed for attenuation measu

rements, splices control, detection and localization of possible breaks.

We describe in this paper a new portable optical time-domain ref lectometer

using the correlation technique [1], designed for the field test of singlemode fiber cables.

This device, presently beeing developped, shows a 15 dB one-way dynamic range together with a 10 m ultimate spatial resolution.

PRINCIPLE

The instrument consists of a pseudo- random 2 ^ -1 bits NRZ sequence generator, wich modulates a GalnAsP/InP buried-heterostructure single-mode laser

(see figure 1). The optical power is injected into the test fiber through a

single-mode fiber splitter and a pigtail fiber.

95
The backscattered light is then detected by an avalanche photodiode and the

electrical signal is multiplied by the shifted emitted sequence after amplification. The RC integrator provides an analog signal, the amplitude of wich

is proportional to the optical power backscattered from the analyzed point

in the fiber. The time delay between the emitted and the shifted sequences

corresponds to the two-way propagation time up to this analyzed point.

The time delay is controlled by a minicomputer, connected to the correlator

output through an analog-to-IEEE 488 interface.

By varying the time delay, we can obtain the whole backscattering curve of

the fiber.

Two kinds of measurements are possible : first the test of a complete long

length fiber (20 km or more) with a 250 m spatial resolution, second a more

accurate analysis in a specified window with a high spatial resolution (up

to 10 m), for any abscissa.

Presently, the attenuation measurement sensitivity is 0.2 dB. It can be improved by decreasing the source and reception noises.

EXPERIMENTAL RESULTS

As an illustration, figure 2 shows the test result of a 17 km -long single

mode fiber, the end of wich was immersed in an index-matching liquid, in order to avoid any optical reflection that could trouble the reception operation.

An averaging number of 16, together with a 40 m spatial resolution, gives a

13 dB one-way dynamic range. A more than 15 dB one-way dynamic range has

been attained by improving the averaging number up to 128, with a measurement time close to 20 mn.

96
CONCLUSION

This correlation optical time-domain ref lectometer , through its high perfor

mance, its large variety of software and its very easy implementation is par

ticularly well suited to the needs of single-mode fiber systems users.

ACKNOWLEDGEMENTS

The authors would like to thank then colleagues of CGE Research Center for

there help with many aspects of this work, and G. Le Blevennec of CLTO for

his technical support.

REFERENCE

[1] P. Healey "Pulse compression coding in optical time domain reflectometry". 7th ECOC, Copenhaguen, 8-11 Sept. 1981.

97
FIGURE 1 - SCHEMATIC DIAGRAM OF THE CORRELATION REFLECTOMETER

P. R. S. GENERATOR

-aCLOCK

I 300 nm

MODULATOR

LASER MODULE

Pigtol

Arc fusion splices

SMF COUPLER

Menn power Temp

Shifted REGULATOR REGULATOR

sequence 1 *

Pigtail

f ibe

Pigtoil

SECURITIES

CORRELATOR

EMISSION CONTROL

1

ail J

'

' /' Wechanicol I connector i

Index matching | 1 iquad

Ge APO J

HI Preamplifier

Anolog - IEEE INTERFACE

HP 85 COMPUTER

I

FIGURE 2 - ANALYSIS OF A 17 KM LONG SINGLEMODE FIBER

FIGURE 3 : PHOTOGRAPH OF THE REFLECTOME

SPATIAL RESOLUTION 40 m AVERAGING NUMBER 16

20 km i

98
LONG-TERM HIGH-STABLE OPTICAL FIBER LOSS MEASURING EQUIPMENT

Y. Namihira

KDD R&D Labs

1-23, Nakameguro 2-Chome

Meguro-ku, Tokyo, Japan

H. Wakabayashi

KDD Head Office

3-2, Nishishinjuku 2-Chome

Shinjuku-ku, Tokyo, Japan

H. Yamamoto S. Adachi

Ando Electric Co., Ltd.

19-7 Kamata 4-Chome, Ota-ku, Tokyo, Japan

1. INTRODUCTION

Optical fiber submarine cable systems are promising for international transmission

lines because of the possibility of a more economical high-capacity digital lines

compared with the conventional submarine systems.^" ^^"^ Optical fiber submarine

cable development requires the precise evaluation of cable transmission characteristics over a long period under various environmental conditions such as tensile force, water

pressure and ambient temperature. For this reason, cable testing facility which can

simulate the ocean bottom conditions are used to evaluate the effects of external

conditions on loss. Physical limitations on the cable testing facility, however, limit the

cable length to no more than 100 to 200 meters, and, with lengths of cable in this order,

a loss measurement resolution of 0.001 dB is required. The authors have developed a technique making use of an LED light source and a high-stability constant-temperature

box capable of control to within 0.1°C to perform measurements with a dynamic range of

approximately 10 dB, a resolution of 0.001 dB and a stability of ±0.001 dB over a 5-hour

period.^

This paper describes a comparison measuring method which enables the measure- ment of very small variations in optical fiber loss, a capability not available with

previously used direct measuring method.

2. OUTLINE OF THE MEASURING SYSTEM

Opticol source

InGoaiP- LEO

a* = I 20nm

InGoAiP- LEO

Under Itit SUF

I

MMf Ge-»P0 rnt®

' Fuiion'

> Spiice

Reference MMF

Thermometer

Ooio Proc. i Com.

|0 OOUBf

Dlajtol

GP- 18Conetant - temperature bot(CTB) SUF : Slngle-moae liber MMF : Mulllmode liber

Fig. 1 shows the block diagram

of the measuring system. To obtain

better temperature stability than

that of a laser diode, an LED (doublehetero structure InGaAsP/InP) is used

as the light source. In the

conventional comparison measuring

method, a light from one optical source is divided into two signals

using a fiber divider, one for the

Fig I Block diagram of measuring system for minute variations of optical-fiber loss

99
reference fiber (R-side) and the other for the fiber under measurement (X-side).

However using present techniques, there are some problems in the stability of

temperature characteristics of the fiber divider. Accordingly, in this paper, as the

comparison measuring method, we use the two LED optical sources with the almost same

characteristics.

On the light-receiving end, no-bias 100 um diameter Ge-APDs are used as optical

detectors to achieve high-stability and a high signal-to-noise ratio.

The optical outputs from the reference fiber (approx. 5 m in length) and the fiber

under measurement are opto-electrically converted by the Ge-APDs individually and

then the signals are processed. The LEDs are modulated at 270 Hz (i.e., a non-integer

multiple of commercial power line frequency of 50 or 60 Hz) and the receiving circuitry

consists of an amplifier, narrow-band bandpass filter and detector circuit. The A-D con- verted X-side and R-side optical powers are calculated internally, and the difference

between these two values (X-R) is displayed in four digits with a resolution of 0.001 dB.

Some of the data including the optical fiber loss variations are entered into the

controller and stored onto floppy disk as well as being printed to a plotter. To be able to

accommodate power failures occurring during long-term measurements, an auto-start

function is designed into the system.

3. ADVANTAGES OF THE COMPARISON MEASURING METHOD

In the long-term measurement of minute optical fiber cable loss variations with

high accuracy and stability, the following are some causes of systematic variations.

a. LED Output level variation caused by ambient temperature variations.

b. LED Output level variation caused by drive current variations.

c. Receiving level variation caused by wavelength variations.

d. Ge-APD Sensitivity variation caused by ambient temperature variations.

e. Variations in the receiving circuitry (amplifier circuit, bandpass filter and detector)

caused by ambient temperature variations.

f. Signal-to-noise ratio.

Both the LED and Ge-APD are enclosed in a constant-temperature box, making the

expected stability 0.003 dB at room temperature, based on the previous system results.

In addition, since this system uses the comparison method, as long as the channel-tochannel tracking of the variations between the two LEDs is approximately 8 0 96, a

stability of 0.001 dB is easily achievable. Variations with respect to drive current and

wavelength are made negligible by enclosing both the devices and drive power supply in a constant-temperature box. Also, since the receiving system amplifier, bandpass filter

and detector are commonly used in the measurement of the R and X side signals, longterm variations are cancelled. The system noise level is approximately -80 dBm, so that

100
with a signal level of -45 dBm, a signal-to-noise ratio of 35 dB is achieved. Although this

is equivalent to 0.003 dB in level variation, averaging can be expected to improve the

signal-to-noise ratio.

The internal temperatures of the constant-temperature boxes used to house the

sending and receiving sections are A-D converted individually and this data is sent to the

controller, at which software making use of this temperature data compensates for

overall temperature stability.

4. EQUIPMENT STABILITY

Table 1 shows the design goals and

actually achieved performance. Fig. 2 shows the results of a 100-hour stability test on the system performed at room temperature. Fig. 2(a) and 2(c) show the (X-R) and

(R) stability respectively, while Fig. 2(b) and

2(d) show the temperature variations of the

light source side and receiving side con- stant-temperature boxes, from which it can

be seen that both have stability of

approximately ±0.2°C. Fig. 2(e) indicates

room temperature variations.

By having the controller perform

averaging of measured data, a resolution of

0.001 dB or better is achieved.

Table I. Design goal and actual value

of measuring system

L/C jhju <j jiji Actual value

Opticol source LED InCaAsP-LED

Wavel ength 1.30 +0.03 ( urn) 1.310 ( urn) Oynamic range >IOdB ~ 12.0 dB

Measurig system

stability

<±0,003dB/yeor

± 0.001 / I00H < ±0 00ld8/year

( Target value

)

Resolution 0.001 dB 0001 dB LED output power >-45 dBm X

; -38. 6 dBm

R ,-37.7 dBm > -40 dBm

( Target value 1 Optical detector Ge-APD Ge-APD

Temperature stability

ot CTB* < ±0.2°C/year <±0.2'C/I00H

Test Fiber SMF (~9/ 125 jrm) SMF

Ambient Temperature 23±5°C 23+5'C

* CTB : constant - temperature box

5. CONCLUSION

The comparison measuring method in place of the previously used direct measuring

method shows good results in performing measurement of minute variations in optical

fiber characteristics with both high accuracy and high resolution. The system described

is suitable for use in the development of optical submarine fiber cable systems. The

system configuration is shown in Fig. 3.

6. ACKNOWLEDGEMENT

The authors wish to thank Dr. H. Kaji, Dr. C. Ota, Y. Iwamoto and M. Nunokawa in

KDD Labs., and Y. Niiro and H. Yamamoto in KDD Head Office for their discussions and

encouragement, and T. Ueno and K. Inoue in Ando Electric Co., Ltd., for their helpful

assistance and cooperations.

101
IdB)

0.002

0

-0.002

ro

0.1

(o)

(b)

20 40 60 80 Time (hours)

40 60 Time (hours)

40 60 Time (hours)

Fig 2 Meosurmg system slobilily

(ol (X-R) stability

(b) temperature variations ol the light source side CTB

(c) (R) slobilily

(d) temperature voriotions of the recieving side CTB

(el room lemDerotuie vonolions

Fiq 3 Svslem confiauroliori

References

1. Runge P.K., Trishita P.R. "The SL Undersea Lightguide System" IEEE, vol. SAC-1,

No3, April, pp.459-466, 1983.

2. Niiro Y. "Optical Fiber Submarine Cable System Development at KDD" ibid.,

pp.467-478, 1983.

3. Amano K., Niiro Y., Furusawa K., Nunokawa M., and Wakabayashi H. "50 km, Two

Repeaters Sea Trial of Optical-Fiber Submarine Cable System", Electron Lett, vol. 18

No.22, pp.952-953, 1982.

4. Furusawa K., Nunokawa M., Ejiri Y., and Yamamoto H. "Longlength Tension and

High Water-Pressure Test Facilities for Optical Fiber Submarine Cables" Technical

Report of IECE Japan, cs-83-150, pp.61-68,1983.

5. Namihira Y., Wakabayashi H., and Yamamoto H. "High-Stability Measuring

Equipment for Very Small Variations of Optical-Fiber Loss", Electron Lett, vol.18,

Np.3, pp.124-126, 1982.

102
Accurate Determination Of Optical Fibre Length

From Measurements In The Frequency Domain

D. L. Walters

Standard Telecommunication Laboratories Limited,

London Road, Harlow, Essex, UK.

INTRODUCTION

The precise measurement of the length of an optical fibre at various stages

of processing, cabling, handling and installation yields important

information which can be used to help to predict the long-term mechanical

and optical performance of the finished cable.

The large-scale routine manufacturing of a wide variety of optical cables

which is now taking place made desirable the development of equipment which

could be used regularly to evaluate fibre strain in factory and laboratory

environments, and in the field. This paper describes the operation and

application of such a measurement system.

The following capabilities were required:

a) A dynamic range capable of measuring lengths up to 50 km of single mode

fibre

.

b) An optical length resolution of 10 ppm or 10 mm, whichever is the larger

c) An accuracy of 200 ppm or 20 mm, whichever is the larger.

d) The ability to measure the length when only one fibre end is available,

with minimal equipment modification.

e) The capability to measure either absolute optical length or length

change during monitoring, with minimal equipment modification.

f) The equipment must be capable of being used by relatively unskilled

factory personnel.

Although a number of measurement systems have in the past been used to make

accurate measurements of optical length (refs. 1-4), no existing system

satisfied all of the above requirements.

103
EQUIPMENT

Figure 1 is a block diagram of the measurement system. The combination of

the dynamic range requirement and factory safety considerations necessitated

that the source be a low-power 1300 nm laser with a single-mode fibre

tail. The laser is modulated with a radio-frequency sine wave and the

vector voltmeter measures the phase difference between the signal applied to

the laser and the signal received at the output of the PINFET detector.

Figure 1 shows the situation in which the equipment has access to both ends

of the fibre. In this case coupling to the fibre is achieved with a micro-manipulator assembly at the launch end and an elastomeric splice at

the detector. Coupling loss is much less than 1 dB. If access to only

one fibre end is possible during measurement, the inaccessible end is

prevously chemically silvered and the signal is launched and received via a single-mode y-coupler, the connection to the fibre under test being again

via an elastomeric splice.

MEASUREMENT METHOD

For an rf signal with modulation frequency f , the phase change <j> across a length £ of fibre with effective refractive index N, is:

<f> = 360 £Nf/C (C = 3 x 10

8 ms

-1

)

The length can therefore be found from the gradient of the phase/frequency

characteristic, assuming the refractive index. Since the refractive index

itself varies with temperature and strain, it is useful to define the

concept of optical length i.e. N i an assumed refractive index (1.453).

In order to obtain this optical length with 10 ppm resolution a phase change

of at least 5 x 10 degrees must be observed. As with any

phase-measuring instrument, the vector voltmeter measures a phase difference

between -180° and +180°, any multiples of 360° being ignored. To

allow for this, the system controller varies the modulation frequency by a small amount at first, and then by increasingly larger amounts obtaining at

each stage a more accurate figure for the optical length while ensuring that

at no time is the frequency change sufficiently large to "miss" a 360°

phase shift. Once the operator has aligned the system optically, the

104
measurement sequence is completely automatic. Total measurement time is

about two minutes, and at the end of the measurement sequence the controller

prints the optical length, together with the estimate of its standard error calculated from a least squares fit.

The physical length of the fibre can be deduced from the optical length,

provided that the effective refractive index of the fibre at 1300 nm is

known. In practice it is the proportional change in physical length i.e.

the strain, which is normally required. To know this it is not necessary

to know the exact refractive index, but merely how it varies with

temperature and strain.

It can be shown that:

AS. = AL - HAT(gq 4 g) a Where A! = change in physical length, AL = change in optical length,

L = optical length, fi. - datum physical length, AT = Temperature change,

a - physical length temperature coefficient, jB = refractive index

temperature coefficient, a = 1 4 t 3N

N 31

8. dN The refractive index strain coefficient /N. /3l can be measured

for a particular fibre by measuring fibre extension under load v. phase

change. This parameter is linear up to at least 2% strain, and has been

found in practice to vary very little from fibre to fibre.

If it is required to measure length change while monitoring at a constant

frequency, the same equipment can be used with different software. In this case the length change AS. which produces a phase change A<f> at constant

frequency is:

A4 = A<|)C

720 fNc

EQUIPMENT PERFORMANCE AND APPLICATIONS

The measurement system meets all the specified requirements and sets of this

equipment are currently being used successfully in both laboratory and

factory environments.

105
A typical application is shown in Fig. 2, which shows the temperature

coefficient of the length of a fibre coated with a tight jacket of a high

temperature polymer. Other phenomena investigated have included the

long-term relaxation of compression produced by nylon coating, and fibre

strain relief in a number of different cable designs. These results will

be presented and discussed at the Symposium.

REFERENCES

1 A.H. HARTOG et al: Variation Of Pulse Delay With Stress And Temperature

In Jacketed And Unjacketed Optical Fibres, Optical and Quantum

Electronics 11 (1979), 265-273

2 R. KASHYAP and M.H. REEVE: Single-Ended Fibre Strain And Length

Measurement In Frequency Domain, I.E.E. Electronics Letters, (1980),

Vol. 16, No. 18, pp.689

3 D.L. PHILEN and P.D. PATEL: Measurement Of Strain In Optical Fibre

Cables Using A Commercial Distance Meter, 8th E.C.O.C., Cannes, (1982),

Paper A VII

4 S. HORNUNG and M.A. REEVE: Measurements Of Strain In Cabled Monomode

Fibre, N.B.S. Symposium On Optical Fibre Measurements, (1982)

SCHEMATIC OF MERSUREMENT SYSTEM

Qsc i 1 1 ato L aser Dp i ve

I F i b re Vecto r Vo I tmeter

1

I Phua

+ Output Rece i ver

Contro 1 1 er DVM

Ftg. 1

5

4

3

2

c 1 a

I -1

i

-2

-3

-4

-5

CHRNGE IN CORTED FIBRE LENGTH,

WITH TEMPERRTURE *

x x

-50 e 50

Temperature (*C) 100 150

Ftg. 2

106
ELIMINATION OF THE INFLUENCE OF Q-S WITCHED-MODE-LOCKED LASER

3ITTER IN SAMPLED TIME-DOMAIN MEASUREMENTS

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

Helsinki University of Technology, Communications laboratory,

Otakaari 5 A, SF-02150 ESPOO 15, Finland

Tampere University of Technology, Laboratory of Electronics,

Box 527, SF-33101 TAMPERE 10, Finland

Introduction

A tuneable short-pulse source that is being widely used for fiber measurements

is the Q-switched mode-locked Nd-YAG laser in conjunction with a "Raman

fiber" /l/. The usual method of detection is a fast diode coupled to a sampling

oscilloscope. A major concern with such a set-up is the existence of timing

jitter which tends to be about 50 ps, and which is usually associated with

jitter in the Q-switch and mode-lock pulse trains.

We have found that the sub-harmonic content of synchronizing signals is the

main source of error, and that the "jitter" is mostly a systematic effect

that can be virtually eliminated. Here we show that through a proper choice

of the fixed ratio between the Q-switch and mode-lock frequencies, timing

jitter can be effectively eliminated. For our system, the total system jitter

is 5 ps (RMS), which is less than the specified value for our sampling

oscilloscope.

The problem of sub-harmonics

In the visual inspection of normal oscilloscope traces and due to sample

filtering, the influence of sub-harmonics is to produce a jittery trace, so that a systematic effect appears to be random jitter (Figure 1). However,

the effect can be isolated by photography under slow sweep, in which case

multiple repetitive pulses can be easily identified (Figure 2).

107
The required frequency-stability for mode-locked laser operation necessitates

the use of overtone quartz-crystals to generate the driving signal (of frequency

f ) for the acousto-optic loss-modulator in the laser cavity and the digital

m

delay generator reference clock. A third overtone crystal leads to sub-harmonic

components of 1/3 f and 2/3 f due to non-linearity of the crystal

m m

characteristic. Calculations and measurements both confirm systematic timing errors of about 100 ps (peak) with sub-harmonic levels around -35 dB in relation to the level of f . Figure 3 shows the spectral content of the

m

mode-lock signal. Simple electrical filtering is not effective as the optical

devices of the laser cavity are part of the process.

The oscilloscope trigger is derived from the mode-lock signal. A comparator

detects a preset level-crossing and a trigger is generated at every upgoing

edge of the mode-lock signal f . The lower level sub-harmonics add to the

m

mode-lock reference amplitude so that the trigger level will be reached slightly sooner or later than with a spectrally clean reference (Figure 4). Therefore,

a periodic timing offset will be generated.

For an example case with one 1/3 f sub-harmonic of amplitude Al (like

m

in Figure 4), the worst case time deviation T will be:

1 w

T = T * arcsin (A1/A3) / 2n (1)

1 w m

where Al is the amplitude of the sub-harmonic, A3 the amplitude of the

mode-lock frequency f and T = 1/f . The worst case situation occurs m m m

when the sub-harmonic amplitude is maximal at a trigger level crossing.

Whether the worst case deviation is reached, depends on the relative phase

of the spectral components. The phase relation is determined by the circuit

configuration. But as the apparent jitter is determined by the limits of largest

positive and negative time-offset of three triggering moments, the (fixed)

relative phase plays a minor role. If, for example, the positive time deviation

is moved from worst case to a smaller offset, one of the negative offsets

will grow accordingly.

With third-order overtone crystals every third trigger-pulse will have the

same systematic offset. Because of the sampling of different pulses to make

up a complete picture, a multiple pulse will be displayed if samples are related

to triggers with differing offset.

108
Solution to the problem and results

The cure lies in relating correctly the timing sequences in the measurement

system. The key point is to relate the oscilloscope trigger to pulses with

identical systematic timing offset. The Q-switch is intermittently opened to

allow the generation of high-power optical pulses. One of the pulses in the

Q-switched row is chosen for display and thus sampled with the rate of the

Q-switch frequency. A digital counter string is used to generate the Q-switch

repetition rate from the mode-lock synchronizing signal. If the division ratio

of the counter is chosen to be an integer multiple of 3 then the Q-switch

is opened at a moment related to a particular timing offset and the scope

trigger will be generated accordingly.

With this technique the systematic timing error was completely eliminated

for a single-pulse display and our total random error is now 5 ps RMS (Figure

5). The necessary circuitry was incorporated in a specially designed delay

generator based on Advanced Schottky TTL. Pulse displays have been taken

from different measurements during system development. In Figure 5 the

detector is slightly overdriven to minimize the influence of other sources

of error.

Conclusion

In this paper, we draw attention to the influence of the sub-harmonic content

of crystal oscillators upon the "jitter" performance of an important source

for fiber measurements. We show that the effect is predominantly systematic

and can be easily eliminated by judicial choice of the division ratio between

mode-lock and Q-switch frequencies. The validity of our arguments is apparent

from the jitter value of 5 ps RMS in our system, in contrast to our previous

effective value of 100 ps.

References

III. Cohen L.G., Chinlon Lin, Pulse delay measurements in the zero material

dispersion wavelength region for optical fibers, Applied Optics j_6, 3136 (1977)

109
Figure I. Typical Nd-YAG pulse display on sampling oscilloscope. Presence of maximum noise amplitude at steepest edges

of the pulse reveals time-jitter origin of

noise.

Figure 3. Spectral content of the mode-lock

reference signal. The central peak is the

f m

component. Scale 0 - 100 MHz

horizontal; vertically 10 dB / div.

Figure 2. Display of laser output showing

multiple pulses. Depending on the relative phase of the sub-harmonics, three separate

pulses can be distinguished.

Figure h. Timing offset shown for an

arbitrarily phased subharmonic at one third of the mode-lock frequency. Trigger level crossings are marked with a black dot.

Figure 5. Laser output after applying proper

division ratio. Detector is slightly overdriven. Sample filtering is normal.

110
ACCURATE SPECIFICATION OF SINGLE-MODE DISPERSION MEASUREMENTS

Felix P. Kapron and Torn C. Olson

ITT Electro-Optical Products Division

7635 Plantation Road, N.W.

Roanoke, Virginia 24019-0065

INTRODUCTION

Fiber chromatic dispersion is usually specified in terras of a maximum

absolute value over a particular wavelength range. However, a maximum

±3.5 ps/km-nm between 1280 and 1340 nm, for example, is a worst-case estimate

that ignores the better fiber performance attainable over much of the wavelength

window. High bitrate single-mode systems will require a more flexible and

precise specification, such as proposed in this paper, that does not stress

manufacturing measurement time or yield.

DELAY AND DISPERSION FORMULAS

We first rewrite, in a uniform notation, expressions for the group delay

2 per unit length t( X) that have been used in the literature. One containing X _2

and X terras[l] becomes

So X

2 2

X (X) = TQ + _ (X _ _£_ ) (1)

1 8 X

where tq i s the minimum delay at the zero-dispersion wavelength Xq . The

dispersion D( X) = dx/dX is then

So X

- D (X) = — (X - (2)

1 4 X3

where So is the value of the dispersion-slope S(X) = dD/dX at Xo

.

2

Another expression [2], in which dispersion is proportional to (X - Xo)/X ,

we rewrite as

111
2 X

0 " X X T

2

(X) = t0 + So X Q ( + In — ) X XQ

Xo

D (X) = S0 ( —) (X - Xo)

X

(3)

(4)

Finally, a Taylor-expansion [3] may be written

So 0 Co o t

q (X) = TQ + _ (x _ Xo) ^ (x . ^

(5)

Co

D,(X) = So (X - Xo) (X - Xo)

3 2

(6)

where C© is the dispersion-curvature C(X) = -dD/dX at Xo.

EXPERIMENTAL FITS

Spectral group delay measurements were made on 15 similar single-mode fibers

using the common Raman fiber pulsed laser technique [1], Figure 1 is a typical

best-fit to the group delay expression with the associated dispersion curve

For all fibers, the table gives the mean best-fit parameters to the group

delay expressions:

Fit Equation

Zero-Dispersion

Wavelength (X©, nm)

3 Dispersion-Slope x 10

(So, ps/km-nm 2

)

Dispersion-Curvature x 10

(Co, ps/km-nm )

1311 1308 1314

86.7 88.5 87.2

141

Closeness of Fit

(standard deviation, ps/km) 11.2 17.4 8.5

Note that the zero-dispersion wavelength Xq and the dispersion-slope So both

depend upon the fit used. Although gives the best fit, it requires at least

four measurement wavelengths, compared to three for T^and T ^.

112
Figure 2 shows the deviations of dispersions and D

2 from the preferred

D

3 for one fiber. Although the discrepancy is as large as 1 ps/km-nm at

1550 nm, all fits agree within 0.3 ps/km-nm between 1200 and 1480 nm for this

fiber. In fact, within ±30 nm of X0) the linear approximation to all

expressions

D^X) « (X - Xq) So (7)

is within 4% of the more accurate fits D and D_.

1 3

CONCLUSIONS

Chromatic dispersion measurement results can be more accurately described

in terms of two parameters: zero-dispersion wavelength Xq and dispersion-slope

So at that wavelength. Fitting data to is especially accurate for obtaining

these, but even the simpler forms (x^ with Co = 0, or t 2

) are sufficient within

30 nm of Xq . This specification also has the advantage that throughout this

window, FWHM pulse broadening T over fiber length L may be very accurately

written[4] in terms of the source FWHM linewidth AX centered at wavelength Xs

:

T = S0 L AX [(Xs - Xo) 2 + (0.3 AX) 2

] 1/2 (8)

REFERENCES

[1] Love, W. F.: 'Bandwidth spectral characterization of GeO -P

205~Si0

2 multimode optical waveguides', 6th ECOC, York, 1980, pp 113-116.

[2] Gloge, D., Ogawa, K. , and Cohen, L. G. : 'Baseband characteristics of

long-wavelength LED systems', Electron. Lett., 1980, _16_, (10), pp 366-367.

[3] Pearson, A. D., Lazay, P. D., and Reed, W. A.: 'Bandwidth optimization of

depressed index single-mode fibre by means of a parametric study , 8th ECOC, Cannes, 1982, pp 93-97.

[4] Kapron, F. P.: 'Dispersion-slope parameter for monomode fiber bandwidth',

OFC 1984, New Orleans, pp 90-92.

113
20

1

-10-

-20

V 10 T3 (A) - T0

1 ^""V

ns/km /^^^

1

1 1150 1250 /*\ —i——i——i

1350 1450 1550

- WAVELENGTH A (nm)

A0 = 1312 nm

S0 = 91.2 x 10 3 ps/km-nm

C0 = 208 x 10 6 ps/km-nm2 A— D3 ps/km-nm

3 0 2 18 15 3

Figure 1. Spectral Group Delay Data Fit to Equation (5) and Derived

Single-Mode Fiber Dispersion of Equation (6).

in, E ° £ c

z o

oc cc ^ LU qj w — CO

1100 1200 1300 1 400 1 500 1 600

WAVELENGTH, A (nm)

Figure 2. Difference of the Dispersions of Equations (2) and (4) from the

Best-Fit (6) of Figure 1.

114
Multiple-Wavelength System for Characterizing

Dispersion in Single-Mode Optical Fibers

Robert A. Modavis and Walter F. Love

Corning Glass Works

Research and Development Division

Corning, New York 14831

Introduction

It is important to be able to characterize chromatic dispersion in singlemode fibers for purposes of quality control and process feedback. The measurement system should be simple to use and maintain, yet contain sufficient

accuracy and precision to satisfy transmission system design considerations.

This paper discusses such a system which utilizes a 5-wavelength method and

compares the measurement results with those obtained using a fiber Raman

1 > 2

laser. ' Algorithms have been developed which extend the capability from Step-

's index profile fiber designs previously considered to newer and more complex

designs ' which shift and flatten the chromatic dispersion.

Experimental

Five semiconductor lasers operating in the short-pulse mode and longwavelength range of interest are used. Three lasers operating near 1300 nm,

viz., 1190, 1270, 1330 nm and two near 1500 nm, viz., 1490 and 1560 nm were

chosen to optimize characterization of both present and future fiber designs.

These are pigtailed to step-index profile single-mode fibers whose output ends are positioned in a multi-element connector. The position of the first pigtail is adjusted manually while the remaining ones are positioned under

computer control to image the light into the test fiber as can be seen in

Fig. 1. A 1/4-meter monochromator (A-meter) is used to monitor the center

wavelength of the semiconductor lasers. The output light from the test fiber

is collected and imaged onto a germanium avalanche photodiode. High-speed

amplifiers and a sampling oscilloscope are used to process the signal.

Typical pulse widths are -200 psec (detector-limited) and the time jitter

is -20 psec.

Time delay measurements are made as a function of wavelength t(A) to

dx

within an arbitrary additive constant. The dispersion is obtained as D - -jy.

115
Results

This measurement system has been calibrated against our fiber Raman laser

2 system and a set of comparison measurements have been made. Figure 2 shows

the chromatic dispersion obtained from the two systems where a worst case is

displayed for ease of viewing. For five comparison measurements all values of

the zero dispersion wavelength, X , agreed to within 1 nm for nominal 1300 nm- design fibers. The 3-parameter polynomial approximation to the Sellmeier

2 -2 function is used to curve-fit the time delay data, viz., t = a + bX + cX , where X is wavelength.

For nominal 1550 nm dispersion shifted fibers, an empirical 3-parameter

algorithm has been developed which fits the fiber Raman system data (~10

points) to within ~20 psec/km. The time delay data and least-squares fit are shown in Fig. 3 for both systems. Agreement to within 0.25 psec nm "*"km "'"is obtained at 1550 nm.

Segmented-core fibers^ which have a flattened dispersion over the range

1300-1550 nm have also been measured. The 4-parameter algorithm used to fit

3

the time delay data is given by, t = + a^(A-b) + a^(X-b) , where b is the

inflection point. A nonlinear least squares analysis has been developed and

is used to fit the data. Figure 4 shows the results for both measurement

systems where the quality of the fit is ~20 psec/km. Typically, agreement is

0. 35 psec nm "'"km ^ for dispersion over the 1300 to 1600 nm range.

Summary

We have developed a 5-wavelength method for measuring dispersion in

single-mode fibers. Comparisons with a fiber Raman system have yielded

excellent agreement and are summarized in Table 1. Time delay curve-fitting

algorithms with a minimum number of parameters have also been obtained and

these allow the extension of the measurement system capability to dispersion

shifted and flattened fibers.

Acknowledgements

We wish to thank Mr. T. A. Cook, Mr. D. Smith, and Mr. J. Dunning for

technical assistance and Dr. W. Kane and Dr. D. B. Keck for discussions.

References

1. L. G. Cohen, P. Kaiser, and C. Lin, Proc. IEEE 68, 1203 (1980).

2. W. F. Love, in digest of 6th European Conference on Optical Communication,

York, UK, 1980, p. 113.

116
3. C. Lin, A. R. Tynes, A. Tomita, P. L. Liu and D. L. Philen, Bell System

Technical Journal 62, 457 (1983).

4. V. A. Bhagavatula, M. S. Spotz, W. F. Love and D. B. Keck, Electronics

Letters 19, 317 (1983).

5. L. G. Cohen, W. L. Mammel, and S. J. Jang, Electronics Letters ]-8, 1023

(1982).

Table I Fiber Type Comparison Accuracy Precision

(psec nm

- lkm~l) (psec nm

-

^-km- l)

unshifted (1300 nm) 0.10 0.04

shifted (1550 nm) 0.25 0.06

flattened (1300-1600 nm) 0.35 0.09

DISPERSION MEASUREMENT SYSTEM

SAMPLING OSCILLOSCOPE ©

DIGITAL DELAY GENERATOR

COMPUTER

Fig. 1 Five-Laser Dispersion Measurement System

117
E

>jr„

40

20

0

2 -20

to

ijj a

COa -40

60

5- LASER.,

Fig. 2 Comparison of dispersion for unshifted (1300 nm)

type fiber using 5-laser and

Raman systems..

1000 1200 1400 1600

WAVELENGTH (nm)

800 1000 1200 1400 1600

WAVELENGTH (nm)

1800 800 1000 1200 1400 1600 1800

WAVELENGTH (nm)

Fig. 3 Delay time versus wavelength

for shifted (1550 nm) type fiber.

An offset was introduced between the

5-laser and Raman measurements.

Fig. 4 Delay time versus wavelength

for dispersion-flattened fiber using

5-laser and Raman results.

118
HIGH ACCURATE AUTOMATIC MEASUREMENT EQUIPMENT FOR CHROMATIC DISPERSION

MAKING USE OF THE PHASE-SHIFT TECHNIQUE WITH LDs

K.TATEKURA, H. NISHIKAWA, M.FUJISE, AND H. WAKABAYASHI

KOKUSAI DENSHIN DENWA CO., LTD. (KDD)

NISHISHINJUKU 2-CHOME, SHINJUKU-KU, TOKYO 160, JAPAN

1 . INTRODUCTION In the long haul optical fiber submarine cable system, the

accurate knowledge of chromatic dispersion in single-mode fibers is

necessary because the mode partition noise''' can be the dominant

limitation on a system error rate as well as transmission loss.

Many techniques for the chromatic dispersion measurement have been

2-4

developed; most of them could be hardly adapted to industrial stage

or in the field environment because of complex set-ups and/or delicate

operation. In this paper, a high accurate measurement equipment, that makes

use of sinusoidal ly modulated LDs, is demonstrated. It is likely to match

all the requirements in respect to the repeatability and absolute accuracy

of measurement, the dynamic range, and the easiness in operation.

2. SET-UP AND PRINCIPLE The scheme of the measuring set-up is shown in

Fig.l, and Fig. 2 is the front view of the equipment whose size is about

1570(H)x540(W)x710(D) mm. The phase-shift is adopted technique because

the accuracy is very high and the set-up is simple. There are equipped

seven LDs having different wavelengths from 1200nm to 1330nm in the present

system. Each LD is sinusoidally modulated (30MHz- 800MHz), and the output

signal are devided into the main and reference signals. Part of the

reference signal is used to measure characteristics of each LD. The main

and reference signals through O/Es are once converted into IF signals

119
(455kHz), and filtered by a narrow bandpass filer in order to improve SN

ratio. Any wavelength is sequentially selected by the 7x1 single-mode

optical switch shown in Fig.l. The phase delay difference between the

signal through the test fiber and the reference signal is measured by the

phase detector.

Utilizing the measurement data, the delay time against the wavelength

is expressed by a polinominal approximation (Fig. 3) with the aid of the

root mean square technique. Finally, the chromatic dispersion (Fig. 4) is

calculated by differentiation of the above approximation.

3

PERFORMANCE Figs. 5 and 6 show the measurement results successively

carried out every one hour. In the repeatability tests, accuracy was very

good, and the variation of the zero-dispersion wavelength was within +0.5

2 nm, and that of slope in the dispersion curve was within ±0.01ps/km nm .

Absolute accuracy has been tested as well by comparison with the results

obtained on the same fibers by the fiber Raman technique . Table 1

allows the comparison of the zero-dispersion wavelengths obtained by the

fiber Raman and present techniques. Both results are in good agreement.

The present measuring equipment has the dynamic range more than 30 dB,

which is corresponding to the possible measurement fiber-length of more

than 50 km. The large dynamic range is the most excellent feature in LDs as

the light sources. The sources are thermally controlled so as to work at

constant wavelength within the surrounding temperature of 5°C-35°C. In the

present system, all the dispersion measurement processes are automatically

controlled by a personal computer, and it takes about three minutes to

measure one fiber sample.

4. CONCLUSIONS The high accurate automatic equipment for the chromatic

120
dispersion of single-mode fibers has been successfully demonstrated. The

repeatability and absolute accuracy of measurement as well as the dynamic

range meet all the practical requirements. Moreover, the easy operation

and the short measurement time are very attractive for the measurements at

field or factories.

ACKNOWLEDGMENTS: The autors wish to express their thanks to Mr. Y.Ishikawa,

Dr. H.Kaji, Mr. Y.Niiro, and Mr. Y.Iwamoto for their encouragement. They

also wish to thank Ando Electric Co., LTD. for manufacturing the equipment.

REFERENCES

1 K.OGAWA; IEEE J. Quantum Electron., vol.QE-18, no. 5, May 1982.

2 K. KITAYAMA et. al.; APPLIED OPTICS, vol.20, no. 14, July 1981.

3 C.LIN et. al.; Proc. OFC'82, Phoenix, AZ, Apr. 1982.

4 B.COSTA et. al.; Electron. Lett., vol.19, no. 25/26, Dec. 1983.

5 M.TATEDA et. al.; APPLIED OPTICS, vol.19, no. 5, Mar. 1980.

Fig.

1

Scheme of the measureing set-up

121
1000

500

1—1—i—1

1—i—i—t

\

-

III!

1200 1300 1400

A. (nm) .

Fig.3

Delay time curve

• denotes the measured points.

1325

1320

1315

1310

i

Fig.

j

10 Measure Times

15

Variation of the zero-dispersion wavelengths

in the repeatability test, where two typicai

fibers (~6Km) were measured.

1250 1300 1350 \ (nm)

Fig.

Calculated dispersion curve

o.io E

i-. 0.05

10 Measure Times.15

Fig.6

Variation of the slope of dispersion curves

in the same test as Fig. 5.

Table 1 Comparison of the zero-dispersion wavelengths

FIBER NO. 2»o (*«•) A (X) \ 0 (n«) A.*0 (na) 6 A. (na)

F 1 9.7 0.33 1331 1331 0 F 2 9.7 0.30 1334 1336 -2 F 3 9.9 0.33 1332 1336 -4 F 4 9.5 0.30 1321 1322 -1 F 5 9.4 0.30 1331 1331 0 F 6 10.3 0.29 1330 1330 0 F 7 11.9 0.28 1292 1290 2 F 8 11.7 0-29 1296 1296 0 F 9 10.0 0.30 1305 1306 -1 F 1 0 10.9 0.33 1302 1304 -2

2w0 : the mode field diameter, A : the index difference , Ao - the zero-dispersion wavelength by the fiber Raman technique,

>fo : the zero-dispersion wavelength by the phase shift technique,

6 A. : Ko - X*o , lengths of test fibers were around 8Km.

122
PRECISION INTERFEROMETRIC MEASUREMENT OF

DISPERSION IN SHORT SINGLE MODE FIBERS

M. J. Saunders

W. B. Gardner

AT&T Bell Laboratories

2000 Northeast Expressway

Norcross, Georgia 30071

Disadvantages of the conventional pulse delay method 1

for measuring single mode fiber dispersion are the high cost of

the equipment, the requirement for long (~ km) lengths of fiber,

and the eye hazard due to the high laser power levels.

To overcome these disadvantages, interferometr ic

methods for measuring dispersion in fibers using both the

Michelson 2 and Mach-Zehnder 3 ~ 6 arrangements have been devised.

We have used a 100 watt quartz halogen source, a monochromator

,

and a Mach-Zehnder interferometer with a motor-driven delay line

to measure dispersion in fibers as short as 8.6 cm. This test

set produces high visibility interference fringes at wavelengths

up to 1.7 pm without the use of a reference fiber.

A diagram of the apparatus is shown in Figure 1 . After

alignment is effected with the laser, visible fringes are obtained with the monochromator and microscope. Mirrors C and D are

used to make the fringe separation larger than the detector diameter. The monochromator is then tuned to an infrared wavelength

and the fringe visibility curve is obtained (Figure 2) by motordriving the variable delay path. This procedure is repeated at

other infrared wavelengths. All of the visibility curves are

then repeated with the fiber removed.

123
If the path difference between the two arms produces a phase difference A<j>, the condition for maximum fringe visibility

is

fx < A *> " 0 • ( 1 ) If the delay path must be adjusted an amount A £ when

the fiber is removed, then condition (1) becomes

N ( X ) = A %/ Z + 1 (2)

where N(X) = (LPg-| group delay per unit length) • c (3)

and £ is the length of the fiber.

Figure 2 is a typical set of fringe visibility curves

obtained on a 17.5 cm long fiber. Using Eqs. (2) and (3), these

data were used to generate the delay curve shown in Figure 3. A

Sellmeier fit (the solid line in Figure 3) has its minimum at the

zero dispersion wavelength (AQ ). Eight determinations of AQ

were made on an 8.6 cm long fiber and a 17.5 cm long fiber drawn

from the same preform. The means were 1470 nm and 1443 nm, respectively, and the a's are summarized in Table I. The o's for

the delays are deviations from the Sellmeier fit curve.

Table I Measurement Precision — Standard Deviations

for 8 Measurements per Fiber

8.6 cm Fiber 17.5 cm Fiber

Zero Dispersion Wavelength 16 nm 5 nm

Group Delay .025 psec .019 psec

Group Delay per Unit Length 0.29 ns/km 0.11 ns/km

124
Because of the short lengths of fiber used, possible

errors due to the presence of LP

^

^ mode power were also

investigated, and will be reported on.

REFERENCES

1. L. G. Cohen and C. Lin, Appl. Opt. 3136 (1977).

2. W. D. Bomberger and J. J. Burke, Symp. on Optical Fiber

Measurements, Boulder, October 28-29, 1980, p. 101.

3. H. T. Shang, Electron. Lett. V7, 603 (1981).

4. J. Stone and L. G. Cohen, Electron. Lett. J_8, 716 (1982).

5. M. Tateda, N. Shibata and S. Seikai, IEEE J. Quantum

Electronics, QE-17 , 404 (1981).

6. F. M. Sears, L. G. Cohen, and J. Stone, J. Lightwave

Technology LT-2, 181 (1984).

hio

Z

o

H

o

UJ

»-

UJQ /

Am,MICROSCOPE

-TO LOCK-IN

AMPLFIER TRANSDUCER]] U

t

-A :(k<o:

He-Ne ALIGNMENT

j

j LASER

5>XSPATIAL FILTER MONOCHROMATOR

100 WATT

LAMP

VARIABLE

DELAY PATH

AND Y AXIS

OF RECORDER

MOTOR

U. TO X AXIS

OF RECORDER

FIGURE 1 SCHEMATIC DIAGRAM OF EQUIPMENT

125
FIGURE 2

VISIBILITY CURVES AT 14 WAVELENGTHS (INSERT SHOWS QUALITY OF FRINGES)

WAVELENGTH IN jjm

FIGURE 3

GROUP DELAY vs. WAVELENGTH

126
INTERFEROMETRIC DISPERSION MEASUREMENT IN SINGLE-MODE FIBERS WITH

A NUMERICAL METHOD TO EXTRACT THE GROUP DELAYS FROM THE MEASURED

VISIBILITY CURVES

L. Oksanen, S. 3. Halme

Communications Laboratory

Helsinki University of Technology

SF-02150 ESPOO

Introduction: In research and manufacture it is often desirable to be able to measure

dispersion from a short piece, say a few meters, of the fiber. This can be done

conveniently with the interferometric group delay measurement method /l/,/2/,/3/

which gives the total dispersion of the fiber. We report on a simple numerical method

to extract the group delays from the measured visibility curves. This method eliminates

human bias and error inherent in visual inspection of the curves, enhances resolution

and facilitates automation of the measurement procedure.

Measurement system: The measurement apparatus is depicted schematically in figure

1. Input light is divided in the first beamsplitter and launched into both arms of the

interferometer. The fiber to be measured is inserted into the other arm and the other

arm serves as a reference. The relative propagation delay between the arms can be

adjusted by the movable corner cube. The light beams from both arms are combined

in the second beamsplitter and the resulting interference is detected. Our setup is slightly modified in order to reduce the criticality of the alignment of the

interferometer. The light source is a Nd:YAG pumped single-mode Raman fiber and

the wavelengths are selected with a monochromator. The length of the fiber is from

3 to 4 meters.

We are interested in the visibility, or the strength, of the interference as a function

of the group delay difference between the arms of the interferometer. It is measured

by changing the air path length with a motorized manipulator and sampling the receiver

output with a digital voltmeter. A typical visibility curve on one wavelength is shown

in fig. 2. The noise in the curve is caused by amplitude instability in the Nd:YAG

laser, which pumps the Raman fiber. It is rather pronounced because the manipulator

sweep is continuous and the reveiver integration time is short compared to the time

between the laser pulses. The measured values, typically 256 for one curve, are stored

on tape. Notice that the actual path delay difference in our system is four times

the distance traveled by the manipulator.

127
Delay measurement: Because of fiber dispersion the group delay in the fiber changes

with wavelength and the visibility curves are obtained at different sweep ranges of

the manipulator. The task is to extract the group delays corresponding to the positions

of maxima of the curves. If the source spectrum is symmetric this means finding

the center of the visibility curve. It can be done approximately by visual inspection

of the curves /2/. This, however, may lead to human error or a biased evaluation

of the midpoint of the curves. We have developed a simple and accurate numerical

method to extract the delays from the corresponding visibility curves.

By using the theory of partial coherence Ikl we can derive the visibility curve for a known source spectrum. We assume that the output spectrum S( A ) of the

monochromator is Gaussian, equation 1, which is realistic, considering the output

spectrum of our monochromator. A is wavelength, A^ the center wavelength and

AAthe 1/e half width of the source spectrum. When AA « A , the visibility is a

Gaussian function of the relative delayer, eqn. 2. The 1/e halfwidth T of the visibility curve is given by eqn. 3 where c is the speed of light.

The actual method to find the midpoint of the curve is based on convolution. First

we calculate a theoretical visibility curve according to eqn. 2. The result for the

wavelength and spectral width of the case of fig. 2 is given in fig. 3a. Notice the

different scales of the abscissas. If we now take a convolution of the measured and

calculated curves we get a result, the global maximum of which indicates the midpoint

of the measured curve /5/. To enhance accuracy we actually use as reference the

time derivative of the theoretical curve fig 3b. In this case also the result, depicted

in figure k, is the time derivative of the result of convolving the original curves

/5/. Now it is easy to find the zero crossing point of the convolution curve by

interpolation and thus to determine the midpoint of the measured curve. The

convolution is calculated numerically with the help of a Fast Fourier Transform (FFT)

algorithm. To eliminate overlap of the result, a brute force approach is used whereby

the calculation for 256 measured points is done for 512 points where the extra points

have been filled with zeros. The program to find the midpoint of one curve runs

for 3 minutes on a HP-85 microcomputer.

S

(1)

(3)

(2)

128
Other reference functions such as sin(x)/x and raised cosine have been tried but the

Gaussian one gave the best results. Overall it seems that the effect of the shape

of the reference curve isn't very critical, which is a good thing considering the effect

the different dispersions of air and fiber and the second order dispersion in the fiber

may have on the shape of the measured curve. The optimum spectral width for the

calculation of the theoretical curve was found to be k nm (1/e fullwidth), which is close to the measured value of the monochromator.

When the Nd:YAG laser is properly tuned the standard deviation in the delay values on one wavelength is under 0.1 ps. This value is better than previously reported and

it is achieved with a continous sweep of the motorized manipulator instead of stepping

which speeds up the measurement process.

As usual the dispersion is arrived at by least squares fitting a polynomial to the

measured delays and differentiating it with respect to wavelength. A typical result

of the delay measurement is shown in fig. 5 along with the corresponding dispersion curve. The delay values were measured at 36 different wavelengths. The shorter

dispersion curve is the result of an independent measurement (by the interferometric

method) made by CNET, France, who also supplied the fiber. The discrepancy between

the two curves is probably mainly due to the systematic error in our monochromator.

We measured the zero dispersion point as 1383 nm and CNET as 1378 nm. We have

repeated the measurement on one fiber 9 times measuring each time the delays on

11 wavelengths (not necessarily the same ones). The standard deviation of the resulting zero dispersion wavelengths was 1.2 nm.

Conclusion: We have presented a simple and accurate numerical method, based on

convolution, to extract the group delays from the visibility curves measured in the

interferometric dispersion measurement method. It enhances resolution, eliminates

operator errors and facilitates automation of the measurement.

Acknowledgments: Financial support from the Finnish Posts and Telecommunications

is gratefully acknowledged. We are also grateful to Dr. M. Monerie of CNET for

supplying us with the fiber and the results of their dispersion measurement.

129
References

1. M. Tateda, N. Shibata, S. Seikai:"Interferometric method for chromatic dispersion

measurement in a single mode fiber", IEEE Journal of Quantum Electronics, 1981,

17, pp. W-407

2. L. G. Cohen, 3. Stone:"Interferometric measurements of minimum dispersion spectra

in short lengths of single-mode fibre", Electron. Lett., 1982, 18, pp. 564-566

3. 3. Stone, L. G. Cohen:"Minimum-dispersion spectra of single-mode fibres measured

with subpicosecond resolution by white light crosscorrelation", Electron. Lett., 1982,

18, pp. 716-718

4. M. Bom, E. Wolf:"Principles of optics", Pergamon Press, 1975

5. A. B. Carlson:"Communication systems", McGraw-Hill Kogakusha Ltd., 1975

10 1 1 12 1.3 U 1.5 x/mm

1.6 1.8 x/mm 12Q0 U00 1600

130
Comparison of chromatic dispersion measurements of single-mode optical

fibers by spot-size and pulse delay method

H. Karstensen, L. Wetenkamp

c/o Institut fuer Hochf requenztechni k

, Techn. Univ., P.O. Box 3329

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

Abstract

The chromatic dispersion of a single-mode fiber can be evaluated with good

accuracy compared to a pulse delay measurement by measuring the wavelength

dependence of the fundamental mode spot size. With the assumption of simple

Gaussian field distributions the spot size is measured using the transverse

offset method. For accurate material and profile dispersion data, the refractive index profile of the fiber and also the dopants should be known at least

appr ox i matel y

.

Introduction

There is a growing interest in methods for chromatic dispersion measurements

which are simple in use but of sufficient accuracy for practice. The most

frequently used method is the measurement of the fundamental mode transit time

at different wavelengths with a fiber Raman-laser. The dispersion is then given

by the derivative of the transit time curve with respect to wavelength. This

method has the advantage of high accuracy, but a rather expensive measurement

apparatus is required. Recently, interf erometric methods have been introduced

which, however, either also need a fiber Raman-laser, or, in the case of white

light interf erometry, reqire extremely high precision adjustments.

As shown by Sansonetti / 1 /

, it is also possible to determine the waveguide

dispersion of a single-mode fiber from the spectral behaviour of the fundamental mode spot size. This theory has already been used to evaluate the dispersion of a single-mode fiber with a step-index profile 111, However, compared

chromatic dispersion, in this case the

and the final curve showed principally

waveguide dispersion

only the material

was disto the total

onl y smal 1

,

persi on

.

Petermann IZI extended the theory and took also profile dispersion into

consideration. Moreover, his theory is valid for all field functions by defining a spot radius w (see eq. (3) of ref./3/), which in the case of Gaussian

fields equals the spot radius w0 of 1 /e2 -i ntensi ty . With the assumption of

linear profile dispersion the chromatic dispersion is given as superposition

of material d i sper si on and combined waveguide and profile disThe material dispersion is obtained by weighting the dopant dependent material dispersion parameter M with the field intensity, and the expression forx^pis given by

T' = T

} + T' M

L WP

persi on i^

p

.

WP

(2-Y)

2 TT c y

d

dX

l 1 w TT 2

• C n 1

• w

2

A dw

- dX l-y . W

(1)

with X as wavelength, n

1 as refractive index of the core, and c as velocity of

light in free space. The value of Y depends on the profile dispersion parameter

P according to Y= 2/(2-P).

Throughout this work we used Petermann's relations with the assumption of

Gaussian field functions. We made measurements on four different fibers which

all have a W-like profile obtained by a Ge02 -doped core with relative refractive-index difference A

+ = 0.6. ..0.7 '/. and concentric F-doped index depression

131
around the core with a relative index difference A" = -0.3. ..0.35 "/.. Core radii were between 2.5 and 3.0 pm, the outer radii of the index depression ring

between 5 and 7 pm. In order to determine , we took the material dispersion

data of the different fiber regions from literature, and weighted them with

measured field distributions. At 1.3 pm wavelength nearly all field intensity

is concentrated in the Germania doped fiber core, whereas at 1.8 urn approximately half of the power is guided in the core and the other half in the Fluorine

doped ring and the cladding of fused silica. With dispersion data for 6.3 m'/. Ge0 2 -doped silica /4/, for 2.0 m7. F-doped silica /5/ and for quenched fused

silica /&/, we approximated the material dispersion by the simple relation

t- =

X "1 - 3ym 125—^— (2) M A km • nm

To determine the profile dispersion we must consider the fact that the total

refractive index difference A t =A+-A~ is the essential parameter for the overall

dispersion characteristic. With the data for the profile dispersion parameters

of Be02 - and F-doped silica from Adams 111 we obtain

P =

X - 0.83 ym

(3)

6 urn as approximation for the wavelength dependent profile dispersion parameter.

Experimental results

The spot size of the fibers was measured with the transverse offset method

which has, compared to other methods, the great advantage to detect all power

transmitted through the fiber. As a drawback only Gaussian fields can be measured exactly, and it is nearly impossible to evaluate more general mode fields.

The effect on our measurements was small: actual fields are very nearly

Gaussian in shape and Gaussian approximation was used anyhow.

We made with each fiber four series of measurements within the wavelength

region of approximately 1 to 1.8 urn. The spot size was measured in intervals of

50 nm with a repeatability of about ±1 '/.. Then the w

0 -values were averaged and

fitted to a polynomial by a least squares method. In Fig. 1 the resulting

curves of the spot size w

0 versus the wavelength A are shown for fibers

A,B,C and D. From these curves the waveguide dispersion was calculated with the

help of eq.(l). In order to show the influence of the profile dispersion, we calculated the dispersion slopes for each fiber, one with P = 0 and the other

with the profile dispersion according to eq.(3).

The dispersion characteristics of the fibers A to D, derived in this way, are shown in Figs. 2 to 5. In these Figures, T

» is the material dispersion, which is

assumed to be the same for all fibers, t'^ the total chromatic dispersion including profile dispersion, and the chromatic dispersion without profile

dispersion. Moreover, for comparison, the dispersion curve t'pd , evaluated by

pulse delay measurements with a fiber Raman-laser, is shown for every fiber.

Discussion

Firstly, one can see from Figs. 2 to 5 a rather good conformity of the

dispersion curves and from the spot size measurements with the dispersion

TpD from the pulse delay measurement. However, for fibers B and C the curve matches T

JpD more closely, whereas for fibers A and D the curve for t'

2 follows

T'pD

better. To understand this behaviour one has to consider the following: Differences could occur from slight measurement errors in determining w 0

( A )

Because x^p is strongly dependent on w

q(A) itself and on the derivative of w

0 with respect to A , even small errors give rise to great deviations of the

132
dispersion curve. On the other hand there is a systematic error in our dispersion evaluation, because we approximated the mode field with a Gaussian

curve. At shorter wavelengths, the Gaussian approximation holds very well, as shown in Figs. 6 a to h, where the autocorrelation curves of the fundamental

mode field, as obtained by the transverse offset method, and the corresponding

Gaussian fits of fiber D were plotted. Up to a wavelength of 1.5 urn the mode

fields are very nearly Gaussian. At longer wavelengths, however, the mode field

has tails, decaying much more slowly than the corresponding Gaussian fit.

Therefore the actual spot size w is a little wider than the Gaussian spot size

w

0

. Moreover, the mode power decreases with longer wavelength, because the

radiance of a tungsten lamp decreases with wavelength. Thus the measurement

error increases, and in the longer wavelength region the measurements are less

accurate than at shorter wavelength.

Possibly, the most important reason for the different dispersion characteristics evaluated by the two methods lies in the fact, that the pulse delay

measurements are usually made on long length fibers. Thus, this measurement

gives a dispersion characteristic averaged over the whole fiber length. On the

other hand, in spot size measurements only few meters of fiber are needed, and

the evaluated dispersion is valid only for this short piece of fiber. Thus,

differences in the dispersion characteristics are inevitable, if the fiber is

not completely homogeneous along the length. In addition to this even with

ideal fibers differences could result from different curvature. To test the

accuracy of this method more thouroughly, one should compare the given results

withthe results of a dispersion measurement technique which uses the same short

pieces of fiber, e.g. with an interference method.

Ac knowl edgement

We would like to thank Mrs. Schweitzer for the help in fiber preparation , our collegues of the university of Kai sersl autern , Dr. Klein and Mr. Lieber,

for carrying out the pulse delay measurements, and our collegues of the technical university of Braunschweig, especially Mr. Jacob, for helpful discussions.

References

l\l P. Sansonetti , Modal dispersion in single-mode fibres: simple approximation issued from mode spot size spectral behaviour, Electr. Lett. 18

(1982) ,647-648

111 J.P.Pocholle et al .

, Deter mi nat i on of modal dispersion in monomode

optical fibres from wavelength dependence of the mode spot size, Electr.

Lett. 19 (1983) ,1093-1094

IZI K.Petermann, Constraints for fundamental mode spot size for broadband

dispersion-compensated single-mode fibres, Electr. Lett. 19 (1983), 712-

714

/4/ N.Shibata,T.Edahiro, Refractive-Index Dispersion for 6e02 -, P

2 05 - and

B 2 0

3 -Doped Silica Glasses in Optical Fibers, Trans. IECE Japan E65

(1982) ,166-172

151 J. W.Fleming, D.L.Wood, Refractive index dispersion and related properties

in fluorine doped silica, Appl. Opt. 22 ( 1983) ,3102-3104

/6/ J. W.Fleming, J. W.Shiever, Thermal History Dependence of Refractive

Index Dispersion of Fused Silica, J. Am. Ceram. Soc. 62 (1979), 526

111 M.J.Adams, An Introduction to Optical Waveguides, Intersci ence,

Chichester 1981

133
Fig. 1: Measured spot size w0 versus wavelength X for fibers

A, 6 , C and D

Fig. 2: Dispersion of fiber A versus wavelength

(t^,= material dispersion, \'

f0 = dispersion from pulse delay measurement, T,= disp. from spot size measurement, x{ = disp. from spot size measurement without profile disp.)

5

0

-20-

1

^"

1 .rpo

I'O 1.'2 I 1.4 / \ 1.6 T '*h

/// \ \

'

ti\\

I //' / //' 1 /r 1 V

I //' 1 //' 1 //'

Fig. 3: Dispersion of fiber B versus wavelength

(notation as in Fig. 2) Fig. A: Dispersion of fiber C versus wavelength

(notation as in Fig. 2)

a) X= 1 .05 um b) X= 1 .2 um

c) X= 1.3 um d) X= 1 .A um

Fig. 5: Dispersion of fiber D versus wavelength

(notation as in Fig. 2)

Fig. 6: a) - h) Measured autocorrelation curves of the field function of fiber D and corresponding Gaussian fit

at different wavelength

e) x= 1 .5 um f) x= 1 .6 u m

g) \= 1.7 pm h) X= 1 .8 um

134
Field Dispersion Measurements - A Swept Frequency Technique

R. Rao

Valtec, West Boylston, Massachusetts

Introduction

As transmission rates Increase and WDM systems become operational, 1t 1s

likely that the dispersion characteristics of long spans of single mode cable

will have to be checked after Installation. This will require dispersion

measurement equipment capable of making field measurements.

Dispersion 1n single mode fibers 1s caused by the material and waveguide

properties of the fiber. For long lengths of fiber, 1t can be determined from

measuring the relative group delay through the fiber as a function of

wavelength. Present laboratory techniques for doing this measurement are unsuitable for field use due to their complexity and equipment

size.

1

' 2 A variation of the fiber Raman system discussed 1n Reference

[1] and a single frequency phase measurement

3

' 4 have been suggested for

field use. In this paper a swept frequency measurement system 1s described

where group delay 1s determined from frequency domain data. This system has

been developed specifically for field use and uses typical multlmode frequency

domain bandwidth test equipment.

Theory

The time shift r at a wavelength X with respect to a reference

wavelength which occurs due to the wavelength dependent group delay 1s given

by the equation: r = A + BX

2 +CX~2

, where A, B and C are the fit

parameters. The dispersion and zero dispersion wavelengths are obtained from

this equation:

D(X) = dr/dX = 2BX-2CX"

3 (1)

\ = (C/B)'

/4

(2)

At least three wavelengths are required to find A, B and C.

Two lasers at different wavelengths are modulated 1n phase, combined, and

launched Into a fiber as one composite signal. The Input and output signals

may be described as

Input: S^t) = A^X-j) Cos (wt) + B^X^ Cos (tot) (3)

Output: S (t) = A (X,) Cos (ut) + B (X.) Cos (u[t+r]) (4)

O 0 1 0 c

where A^ (X^, B^ (X

2

) and A

Q

(X

1 ), BQ

(X

2

) are the peak

amplitudes of each component at the Input and output respectively,

135
\.| 1s the reference wavelength, and u> = 2irf corresponds to the

modulating frequency. In general, the peak amplitudes are also a function of

the modulating frequency but this 1s being Ignored here for the sake of

simplicity 1n the discussion. From S (t) the output spectra can be shown to

be described by

S

0

(o) = A

Q

(X

1

) B

0

(X

2

) cos (a) D (5)

The two signals at the output are 180° out of phase when the time

shift, r 1s

F = nT

n +|rn for n = 0, 1, 2, 3 etc. (6)

where T

n 1s the period of the modulating frequency.

Substituting Tn

= 1 and simplifying we find

r = 2n£L (7)

and r = 1 = 1 (8)

M1n1mas occur 1n the amplitude spectra of the output signal at

frequencies where the signals are 180° out of phase, r 1s determined by

locating consecutive frequencies at which this condition occurs. Figure 1 schematically represents the measurement system. Three lasers

with wavelengths at 1318 nm, 1265 nm and 1213 nm are used. For a measurement

the 1265 or 1213 laser 1s modulated simultaneously with the 1318 laser. The

lasers, drive electronics necessary to modulate two of the lasers 1n phase and

optical couplers required to couple the signals comprise a field transportable

transmitter. The receiver 1s a longwave analog receiver with a germanium APD.

Results

At present, data 1s taken manually by tuning the spectrum analyzer

center frequency to out of phase frequencies. These frequencies are most

easily discerned with the analyzer 1n the logarithmic mode. Typically

measurements are made between 10 MHz to 500 MHz where the system has the best

phase linearity. Each laser's phase 1s adjusted so that the maximum phase

deviation between any two lasers 1s 5 at about 500 MHz. A fixed phase

offset 1s added to the 1265 laser 1n order to bring 1318/1265 minlmas to

within 500 MHz and enable shorter lengths of cable ( - 8 km) to be measured.

A (V.) 1s variable to allow adjustment of the amplitude of S (w).

0 1 0

Figures 2a and 2b show the signals from 25 km of single mode fiber obtained by

simultaneously modulating the 1318/1213 and 1318/1264 lasers respectively.

136
The separation of successive minima are clearly defined and agree to within

1 MHz. r 1s determined by averaging the minima separation Af between

10 MHz and 500 MHz. Dispersion and zero dispersion wavelengths were computed

from the delay data generated by a fiber Raman system and the swept frequency

system for several fiber links of varying lengths. The agreement 1s better

than .2 psec/nm/km for dispersion 1n the 1.20 nm to 1.55 nm region and 2 nm

for the zero dispersion wavelengths (Table 1). The close agreement between

the systems Indicate that the error 1s small and accurate results are obtainable using three wavelengths for a measurement. Delay and dispersion

curves computed from successive minima are shown 1n Figure 3. A detailed

analysis of computed dispersion error as a function of frequency separation

error 1s being done at present.

Conclusion

In conclusion, several features of the method are noted which make 1t

especially suitable for field use.

1. Large dynamic ranges required for long lengths of cable are obtainable

(>30 dB) because of the superior SNR of frequency domain techniques.

2. The technique enables measurements over widely varying lengths of cable

without Ignoring 360° phase shifts which 1s possible with a single

frequency phase measurement. Additionally, the laser wavelengths can be

spaced far apart Increasing the relative delay, reducing the effects of

laser wavelength Instabilities and thereby enhancing measurement accuracy.

3. A trigger signal or reference 1s not required for the spectrum analyzer

as 1s required for time domain or a single frequency phase measurement.

4. All the equipment, except for the single mode laser transmitter, may be

used for field multlmode bandwidth measurements.

Acknowledgement

The author wishes to thank Dr. W. Melxner and Dr. I. Aggarwal for useful

discussions and encouragement and, Dr. R. Sierra for Raman fiber dispersion

measurements

.

References

1. L.6. Cohen and C. L1n, Appl . Opt. 16, 3136 (1977)

2. K. Dalkoku and A. Suglmura , Electron. Lett. 14, 326 (1978)

3. C. L1n, A.R. Tynes, A. Tomlta, P. L. L1u, and D.L. Phllen, Bell Syst.

Tech. J. 62, 457 (1982)

4. P.J. Vella, P.M. Garet-Jones, and R.S. Lowe, Tech. Digest, Topical

Meeting 1n Optical Fiber Communication (Optical Society of America, New

Orleans, LA (1984), Paper TUN1

5

137
2 OPI

03UPL CR

FIGURE 1. S-ept frequency iwesureovnt sylte«. *i *» woduUteg

lfnult4neously with 1? or Xj.

1 Wot freflu«fK» F 1 ber l««4A

1 .284 4.4 4.2 1.J0O 1.1 -3.0 1.3)0 -0.7 -O.t I.MO 3.1 3.7 1.400 10.4 10.1

1.450 13.0 1?1

Irro DUperl on ye.eltnnlli

1, 1.311 ua 1.331 ua

. Mion ol dHptrtlM if* i«r« dl.pers.on w**elrfKjth «dU

u.trnj lh* l>-»pt *r«au«fttr .y.tc* «M « Mbtr Iimd .j.t*

FieuK t. frequency Swtr4 from n W of fiber «!«» (<) 1JU/1211 rMyletlo*

<b) I1IV1KS MixKiUtiM

Scale: Verticil 10 dtVslr nerlaMUl M mi/«l». Center fnwcy IW ntz

l.u t.2* t.le 1.4*

FICUttC 3. (•) 0*l«y »1. » *n4t (b) elteertlw n. i <l compute* frra Meet frequency frtteei 4UU

138
INDEX OF AUTHORS

S. Adachi 99

J. Auge 25

J. J. Bernard 95

C. S. Brown 59

E. J. Buonopane 81

R. Caponi 37

M. Carratt 95

S. A. Cassidy 85

A. H. Cherin 67

N. K. Cheung 15

T. A. Clarke 81

J. H. Cole 49

G. Coppa 37

F. W. Cuomo 55

A. Dandridge 49

E. Depresles 95

P. Di Vita 37

P. Dupont 25

W. Freude 29

M. Fujise 119

W. B. Gardner 123

K. H. Hafemeister 81

S. J. Halme 107,127

S. C. Hampson 75

A. H. Hartog 89

S. Hornung 85

E. J. R. Hubach 107

T. Ito 19

L. B. Jeunhomme 25,95

P. Kaiser 15

F. P. Kapron Ill

H. Karstensen 131

R. B. Kummer 71

W. F. Love 115

S. C. Mettler 71

C. A. Millar 1,33

R. A. Modavis 115

J. L. Moncelet „ 95

Y. Namihira 99

H. T. Nijnuis 11

H. Nishikawa 119

L. Oksanen 127

T. C. Olson Ill

D. B. Payne 1 D. N. Payne 63

D. W. Peckham 73

S. B. Poole 63

R. Rao 135

M. H. Reeve 85

M. H. Reeve 1 P. R. Reitz 41

U. Rossi 37

M. J. Saunders 123

V. S. Shah 7 A. Sharma 29

A. B. Sharma 107

G. H. Si gel, Jr 49

F. T. Stone 59

K. Tatekura 119

C. J. Todd 1 K. A. H. van Leeuwen 11

M. P. Varnham 63

H. Wakabayashi 99,119

D. L. Walters 103

L. Wetenkamp 131

H. Yamamoto 99

139
NBS-H4A (REV. 2-8C) U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET (See instructions)

1. PUBLICATION OR

REPORT NO.

NBS SP-683

2. Performing Organ. Report No. 3. Publication Date

October 1984

4. TITLE AND SUBTITLE

Technical Digest - Symposium on Optical Fiber Measurements, 1984

5. AUTHOR(S)

G. W. Day and D. L. Franzen

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)

Sponsored by the National Bureau of Standards in cooperation with the

IEEE Optical Wave Guide Communications Committee and the Optical Society of America

10. SUPPLEMENTARY NOTES

Library of Congress Catalog Card Number: 84-601092

~2 Document describes a computer program; SF-185, FIPS 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 31 papers presented at the Symposium on Optical Fiber Measurements held October 2-3, 1984, at the National Bureau of

Standards, Boulder, Colorado. Subjects include measurements on singlemode

fiber, multimode fiber, fiber designed for sensing applications, instrumentation,

field measurements, and standards.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolon s) fiber optics; fiber optics-multimode ; fiber optics-singlemode ; instrumentation;

measurements; optical fiber sensors

13. AVAILABILITY

Q^Unl i mi ted

|

| For Official Distribution. Do Not Release to NTIS

[Xl 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

148

15. Price

irll.S. GOVERNMENT PRINTING OFFICE: 1984-780-574/9435 USCOMM-DC 6043-P80
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