Reference ^^5

PubliNATL INST. OF STAND & TECH atJOHS

AlllDS Tfl33bfl

\

*<"?£ AU Of ^./ NBS TECHNICAL NOTE 1 046

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

Measurement of

Optical Fiber Bandwidth

in the Frequency Donnain
NATIONAL BUREAU OF STANDARDS

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the Institute for Computer Sciences and Technology.

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For sale by the Superintendent of Documents, U.S. QoTenunent Printing Office, Washington, D.C. 20i02
•fAnOMAL 9CHBAU

or STAKDAEDB

LIBItART DEC 1 t981

Measurement of ^:^ ^ '%

Optical Fiber Bandwidth

in the Frequency Domain

G. W. Day

Electromagnetic Technology Division

Nationcd Engineering Laboratory

National Bureau of Stcindards

Boulder, Colorado 80303

\ «,<'*eAu of * \\ Ur

U.S. DEPARTMENT OF COAAMERCE, Malcolm Doldrige, Secretory

NATIONAL BUREAU OF STANDARDS, Ernesr Ambler, Direcror

Issued Seprember 1981
NATIONAL BUREAU OF STANDARDS TECHNICAL NOTE 1 046

Nor Dur. Srond. (U.S.), Tech. Nore 1046, 48 pages (Seprember 1981)

CODEN: NDTNAE

US. GOVEWMENT PRINTING OFFICE WASHINGTON: 1981

For sole by rhe Supennrendenr of Documenrs, U.S. Governmenr Pnnring Office, Woshingron, D C 20402

Price $6.50 (Add 25 percenr for orher rhon US moiling)
CONTENTS

Page

1Introduction 1

2. Time Domain and Frequency Domain Specifications:

Concepts and Termi nol ogy 2 2.1 Time Domain Concepts 3 2.2 Frequency Domain Concepts 5 2.3 Correspondence Relations 8 2.4 Specification Choices 9

3. Bandwidth Limitations in Multimode Fibers 10

3.1 Distortion Mechanisms 10

3.1.1 Intermodal Di storti on 10

3.1.2 Intramodal Dispersion 12 3.2 Separation of Chromatic and Monochromatic Distortion 13

3.3 Length Dependent Effects and Launching Conditions 14

4. Frequency Domain Techniques and Systems 16 4 .

Systems 16

4.1.1 Systems Using Wideband Detection 16 4.1.2 Systems Using Narrowband Detection 18

4.1.3 Network Analyzers. 18

4.2 Choosing a System and the Matter of Phase 18

5. Description of Measurement System 20

5 .Source 20

5.2 Mode Scrambler 23

5.3 Launching Optics 25

5.4 Specimens 28

5.5 Detection 28

5.6 Electronics 30

5 .

Computati on 33

6. System Performance 33

6 .

Measurement Procedure 33

6.2 Precision and Accuracy 36

6.3 Limitations 38

7

Summary 38

8

References , 39

m
NBS Technical Notes on Optical Fiber Measurements

Danielson, B. L. An assessment of the backscatter techniques as a means of estimating loss

in optical waveguides. Nat. Bur. Stand. (U.S.) Tech. Note 1018; 1980.

Franzen, D. L. , Day, G. W. Measurement of optical fiber bandwidth in the time domain. Nat.

Bur. Stand. (U.S.) Tech. Note 1019; 1980.

Kim, E. M., Franzen, D. L. Measurement of far-field and near-field radiation patterns from

optical fibers. Nat. Bur. Stand. (U.S.) Tech. Note 1032; 1981.

Danielson, B. L. Backscatter measurements on optical fibers. Nat. Bur. Stand. (U.S.) Tech.

Note 1034; 1981.

Young, M. Refracted-ray scanning (refracted near-field scanning for measuring index profiles of optical fibers. Nat. Bur. Stand. (U.S.) Tech. Note 1038; 1981.

Day, G. W. Measurement of optical fiber bandwidth in the frequency domain. Nat. Bur. Stand. (U.S.) Tech. Note 1046; 1981.

Day, G. W., editor. The characterization of optical fiber waveguides--A bibliography with

abstracts, 1970-1980. Nat. Bur. Stand. (U.S.) Tech. Note 1043; 1981.

Chamberlain, G. E., Day, G. W., Franzen, D. L. , Gallawa, R. L. , and Young, M., Attenuation

measurements on multimode optical fibers, Nat. Bur. Stand. (U.S.) Tech. Note in preparation.

IV
Measurement of Optical Fiber Bandwidth

in the Frequency Domain

G. W. Day*

National Bureau of Standards

Boulder, Colorado 80303

The design, evaluation, and performance of a system for determining the

magnitude of the transfer function (hence, the bandwidth) of a multimode optical

fiber are presented. The system operates to about 1450 MHz using a tracking

generator/spectrum analyzer combination for narrowband detection. It is con- structed, almost entirely, from commercially available components. The system is

less complex and easier to use than an equivalent time domain system and the mea- surement precision is comparable. Background information on time and frequency

domain specifications, fiber bandwidth limitations, and alternate frequency

domain techniques is also presented.

Key words: fiber optics; optical communications; optical fiber bandwidth; optical fiber distortion; optical fibers.

1. Introduction

In determining the parameters of optical fibers, measurement practice is evolving

rapidly as the accumulated experience of manufacturers and users and the work of standards

groups lead toward uniformly accepted techniques. This Technical Note is one of a series

(see pg. iv) intended to describe the present design and capability of fiber measurement

systems now in use at the National Bureau of Standards. These systems are perhaps representative of current practice in the industry. Many of the techniques described will also be

relevant to future systems. The topic of this particular document is the measurement of

fiber bandwidth as a means of specifying the maximum rate at which information may be

transmitted thruogh the fiber. In particular, it describes a frequency domain measurement

system generally used to characterize graded index fibers in the 820 nm spectral region.

Much of the discussion is general and can be applied to other bandwidth measurement systems.

Sections 2 and 3 are tutorial and are designed to facilitate an understanding of the

later sections. Section 2 is a compilation of the concepts and terms used in the specification of the bandwidth of any transmission medium. Both time domain and frequency domain

concepts and correspondence relations are included to accommodate those who may have a pref- erence for one or the other approach. Section 3 is a review of those characteristics of

fibers that determine bandwidth limitations. It is, by necessity, only a brief overview

though the indicated references should be sufficient to guide the reader to more complete

treatments.

Various frequency domain methods have been used for fiber characterization. Section 4 summarizes these methods and discusses some of the relative merits of each.

Section 5 describes the design of the system now in use at NBS in some detail. Section

6 describes the measurement procedure used and the quality of measurements obtained.

*Electromagnetic Technology Division, National Engineering Laboratory.
2. Time Domain and Frequency Domain Specifications: Concepts and Terminology

Most frequently, information is transmitted through an optical fiber by intensity modulating an optical carrier. The modulation may be digital, that is, in the form of discrete

states, or it may be analog. In either case, the maximum rate at which information may be

transmitted is determined by the way that the fiber, through various mechanisms, acts to modify or distort the modulation. In this section we discuss the different ways that these

limitations may be specified, before examining the specific limiting mechanisms in

section 3.

In all cases we assume that the fiber will behave linearly with respect to the intensity rather than the amplitude of the optical field. We know that thif- will be true for a sufficiently incoherent source, that is, a source having a sufficiently broa spectral

width. It will not be true for a coherent or monochromatic source. Generally the assumption of linearity in intensity is believed to be adequate when the spectral width of the source is large compared to the highest frequency components of the modulating signal [1]. As a point of reference, a 1 nm spectral width at 850 nm corresponds to a frequency range of

415 GHz.

Pi(t;

Time

h(t;

AV

Time

P2(t)

AV

Time

Figure 2-1. The output waveform of a linear system, ppCt), can be described as the convo- lution of the input waveform, Pj^(t) and the impulse response, h(t), of the

system. In a transmission medium the impulse response, that is the response

to an impulse at t=0, is characterized by a delay corresponding to the propagation time and distortion.
2.1 Time Domain Concepts

With the assumption of linearity and time invariance, we can relate the time dependence

of the modulating signal at the output to that at the input through the convolution

integral . We write

00 P2(t) =

/ P^(t) h(u-t) du = p^(t) * h{t), (2-1)

—00

where p-^it) is the modulating signal applied at the input, P2(t) is the modulation observed

at the output, and h(t) is the modulation waveform that would appear at the output when a modulation waveform that was sufficiently brief that it could be considered to be a true impulse was applied at the input (fig. 2-1). The effect of propagation through the fiber is

thus completely specified through a knowledge of h(t), which is known as the impul se response of the fiber. If h(t) could be reliably determined it would therefore be a particularly well suited parameter for measurement.

As a practical matter, it is often not feasible to provide an input waveform that is of

sufficiently brief duration to be considered an impulse, nor may it be possible to solve eq

(2-1) accurately for h(t) given P]^(t) and P2(t). Therefore one commonly resorts to approximate methods of describing h(t).

One common but in general highly approximate method is to use the concept of full -duration-half-maximum (FDHM) pulse-broadening . If the measured input waveform is a pulse with a FDHM of D]^, and the measured FDHM of the output waveform is D2, then the FDHM of h(t), D^,

may be estimated by

{'^l - <

)

1/2

Df, ~ I Do - D, ) . (2-2)

The attractiveness of this approximation results from the ease with which it may be

applied. Its usefulness depends on the shape of Pi(t) and h(t), (and therefore on P2(t))

and on their relative durations.

The approximation in equation (2-2) becomes an equality when P2^(t) and h(t) have

Gaussian time dependences. For high quality graded index fibers, h(t) frequently resembles

a Gaussian shape. Therefore, if pj^(t) also resembles a Gaussian, as it does in many cases

of interest, the approximation may be useful, improving, of course, as D^ is made smaller.

When h(t) assumes other shapes, as for example in a step index fiber, the approximation

becomes rather poor.

A better approximate method of characterizing h(t) is through an examination of its

moments. The moments of a time varying function f(t) are defined as [2]

00 M^ = / t"f(t) dt, (2-3)

— 00

where n is an integer. The zeroth moment, Mq, is a simply the area under f(t). The first

moment, when normalized to Mq, is known as the central time ,
T = M^/M^. (2-4)

The variance, which is related to the first three moments, turns out to be a particularly

useful analytic tool in characterizing the impulse response approximately with a single

number. It is given by

00 0^ = ^ / (t-T)^ f(t)dt (2-5)

-°°

The square root of the variance of h(t), o^, is known as the rms pulse broadening.

The variance possesses a very useful attribute related to the operation of convolution. It is that the variance of f(t) convolved with g(t) is exactly equal to the sum of

the variances of f(t) and g(t) [2]. That is, since (eq (2-1))

P2(t) = p^(t) * h(t)

then

On^ = °n^ + °l- (2-7)

\>2 Pi "

Thus, if we could accurately measure a„ and a we could obtain a^. The usefulness of a^

in system design has been discussed by Personick [3].

Unfortunately, for technical reasons, it is difficult to measure directly o- and

Op . The variance of the impulse response and the rms pulse broadening are thus less useful means of experimentally characterizing a fiber than one might like or suppose.

Another measure of the duration of the impulse response that may be useful is its equivalent duration defined as

00/ h (t) dt

n _ -» (2-8)

"^eq hm •

Dg is thus the duration of a rectangular waveform that has the same "height" as h(t) and

encompasses the same area as h(t). The usefulness of Dg^ arises primarily from a corre- spondence relation described in section 2.2.3 below; it is most useful when h(t) is even, that is symmetric, about its central time.

The relationship among a^, D^, and Dg for a Gaussian impulse response is shown in

figure 2-2.
Figure 2-2. The relationship between three methods of characterizing a Gaussian impulse

response. DFDHM 2.33 a: D eq

2.48

2.2 Frequency Domain Concepts

The same conditions that allow the use of the convolution integral (eq (2-1)) to relate

input and output waveforms allow us to develop equivalent relations in the frequency domain

through the use of the Fourier transformation, or if the integrals of the waveforms are un- bounded, through the Laplace transform. For this work we choose the symmetrical form of the

Fourier transform pair:

F(f) = / f(t) e^'^^'^^dt

(2-9)

f(t) = / F(f) e"''^^^V.

We use the convention that frequency domain functions are identified by upper case symbols

with the corresponding lower case symbols for the corresponding time domain function.

F(f) is, in general, a complex function.

F(f) = Re [F(f)] + ilm [F(f)], [2-10)
Since f(t) is a real function, certain things can be said about the real and imaginary parts

of F(f). In particular, F(f) is Hermitian ; that is the real part is even with respect to

f = and the imaginary part is odd with respect to f = 0. Furthermore, if f(t) is even about t = 0, Im[F(f)] = 0. It is frequently convenient to write F{f) in polar form,

F(f) = |F(f)| e

^ (2-11)

which is related to the rectangular form through

|F(f)l^ = (Re[F(f)])2 + (Im[F(f)])2 (2-12)

and

<j,p(f) = tan"^ (Im[F(f)]/Re[F(f)]). (2-13)

The usefulness of these representations comes largely from the frequency domain analog

of convolution. Specifically if eq (2-1) holds, then it is easy to show that

P2(f) = P^lf) H(f). (2-14)

H(f), the Fourier transform of h(t), is generally known as the transfer function of the

fiber. It is sometimes also called the frequency response , or to emphasize that P]^(t) and

P2(t) are modulation functions it may be called the modulation transfer function . |H(f)| is

known as the magnitude of the transfer function and represents the diminution that each

spectral component of the input waveform suffers during propagation. <ii^{f) is known as the

phase of the transfer function and represents the shift in phase angle that each spectral

component incurs. With respect to ii^if) it is useful to define the group (or envelope)

delay as

1 d()>^(f)

which represents the delay that the spectral components at frequency f of the envelope of pj

incur (fig. 2-3). This is distinguished from the delay that individual components suffer

which is known as the phase delay given by

T (f) = - ^-V— (2-16)
CO

-D

CD

^—

r

>,

(D

II O) OJ > Q •r—

-l-> CL

fO 3

r- O

QJ S- +i

Qi CD

200

Frequency, MHz

400 600 800 1000

1 r

\

-2ttto \

Figure 2-3. Characteristics of a high bandwidth fiber having an approximately Gaussian

impulse response. The magnitude, if plotted linearly, would be approximately

Gaussian, as well. The phase, if measured, would be nearly linear with a slope of -2tttq rad/Hz, where Tq is the propagation time. The group delay is

nearly independent of frequency. These measurements were taken with a developmental system similar to that shown in figure 4-2.

1 -FT The shift theorem states that if F(f) is the transform of f(t) then F(f)e' ^ is the

transform of f(t-T). Therefore, when the impulse response is characterized in part by a time delay, as in the case of a fiber or other transmission medium, the phase of the transfer function contains a term linear in frequency which accounts for the distortion-free

delay plus a term that represents phase distortion,

)^(f) = -(2tt T^f + 2tt Y(f)) (2-17)
Thus the group delay in this case consists of a constant, Tq, plus a frequency dependent

term, dY(f)/df. It is this latter term, together with |H(f)|, which represents the total

distortion and which therefore determines the highest rates at which information can be

transmitted. It may be useful to think of |H(f)| as contributing a symmetric distortion and

Y(f) as contributing an anti symmetrical distortion [4].

In the time domain representation, it was noted that h(t) was a complete representation

of the system. In the frequency domain it can be shown that since the system is causal

(output does not begin before input) that either Re[H(f)] mj^ Im[H(f)] is a complete representation and in fact one can be computed from the other through the Hilbert transform [2].

However, neither |H(f)| nor Y(f) is a complete representation of a system, except for those

systems known as minimum phase systems. Fibers are not, in general, minimum phase systems

[5].

In spite of its incompleteness, |H(f)|, typically normalized to its value at f = 0, is

often used as the principal means of specifying a fiber. To futher reduce the characterization to a single number rather than a function it is common to indicate the -3 dB bandwidth , that is the lowest frequency at which |H(f)l = 0.5 or lH(f)l(j[j = -3 dB. Alternatively, one might choose a specification of bandwidth at another level, say -6 dB, or -1.5 dB. The

latter value may be attractive since in a detector or source an electrical quantity, current

or voltage, is proportional to optical power. Electrical power is thus proportional to optical power squared and lH(f)|(j[j= -1.5 dB corresponds to a -3 dB specification in terms of

electrical power.

2.3 Correspondence Relations

In addition to the correspondence between convolution in the time domain and multiplication in the frequency domain, many other relations between time and frequency domain quantities can be derived. Some are useful for component characterization; others are not. The similarity theorem [2] states that if f(t) has the Fourier transform F(f) then

f(at) has the transform |l/a| F(f/a). Thus, when comparing two impulse responses of identical shape, the ratio of durations at any arbitrarily defined points, say the 50 percent

points, is inversely proportional to the ratio of bandwidths at an independently chosen

level, say -6 dB.

The moments of h(t) can be expressed in terms of frequency domain quantities through

[2]

= /" t"h(t)dt = ^"^°^ (2-18)

(-2TTi)"

where H"(0) is the value of the nth derivitive of H(f) evaluated f = 0. These relations are probably not directly useful in metrology because of the difficulty in reliably evaluating

h"(0). However, they do lead to what may be useful approximations.

From eq (2-18) and the reciprocal nature of the transform we can relate the equivalent

width of the impulse response to the equivalent width of the transfer function, as follows:
/ h(t)dt

/ H(f) df

— oo or somewhat more generally using the shift theorem

oo

/ h(t)dt '-^ . (2-20)

h(T) " . ^^

/ H(f)e^'"^^df

— 00 This expression is probably most useful where f(t) is symmetrical about T, for then the

argument of the integral on the right side of eq (2-20) is real and equal to the transform

of h(t-T).

Because of the additive property of the variance in convolution it would be desirable

to usefully relate the variance to frequency domain quantities. The relation that arises

from eq (2-18) is probably not useful for the reasons noted before. An alternate approach

might be to i power series

H(f) = ; h(t) (1 +1^+ Ai||LiZ_+ ...)dt.

might be to express the transform in terms of the moments of h(t) by writing e ^ as a

i2iTft ^ (i2Trft)^

Then

and

2 H(f) = Mq + i2trM^f - -^^J^ W^f^ +

|H(f)|^ = H(f) H*(f)

= MQ^(l-(2TT)^a^^f^+ ...). (2-21)

Thus one may, at least in principal, obtain a^ by fitting |H(f)r to a power series in f and

isolating the coefficient of f^. The practical usefulness of this approach has not as yet

been explored.

2.4 Specification Choices

Given the array of parameters outlined in the above sections, it is often not clear how

best to specify the information carrying capacity of a fiber. The choice undoubtedly

depends on the type of fiber, the characteristics of the system in which it will be used,

measurement considerations, and the experience and preference of the designer.
It is sometimes suggested that time domain specifications should be used for digital

systems and frequency domain specifications for analog systems. Since the larger fraction

of fiber systems will be digital, one might expect time domain specifications to dominate.

Such has not been the case in industry.

In the U.S., the majority of manufacturers use time domain measurement methods but specify their product by the -3 dB bandwidth. The reasons are several. Early in the development of fiber systems it was easier to obtain suitable sources for pulsed measurements than

for cw measurements. However, it is difficult, for technical reasons, to obtain an accurate

impulse response or an accurate value for rms pulse broadening; hence frequency domain com- putations. And, perhaps more importantly, it seems to be the preference of most designers

to use bandwidth specifications. In Japan, a different situation has evolved in that many

manufacturers use both frequency domain specifications and frequency domain measurement

techniques.

If frequency domain specifications continue to dominate, it seems likely that frequency

domain measurements will grow in popularity. With presently available components, frequency

domain systems are easier to construct, simpler to operate, and more direct in

computation. Their precision is comparable to time domain systems on short fibers; for long

fibers they offer easier signal averaging.

3. Bandwidth Limitations in Multimode Fibers

3.1 Distortion Mechanisms

Regardless of general preferences for time domain or frequency domain representations,

it turns out to be more convenient to consider the limiting mechanisms in the time domain,

specifically as sources of variation in propagation time. These variations, which lead to waveform distortion, arise in two separable categories. One comes from differences in propagation constants between individual modes or mode groups, and is called intermodal distortion. The other comes from the variation in the propagation constant of individual modes

with wavelength, called intramodal dispersion . For a non-zero source spectral width, intramodal dispersion results in additional waveform distortion.

3.1.1 Intermodal Distortion

In a step index fiber, if one assumes that the angle that a light ray makes with the

axis corresponds to a particular mode designation it is intuitive that high order modes

(high angle rays) will propagate more slowly than low order modes. Thus a propagating pulse

will be broadened. It has long been known that by properly tailoring the refractive index

distribution these intermodal differences can be minimized.

For a optical waveguide in which the maximum difference in refractive index between the

core and cladding is about a percent, Gloge and Marcatilli [6] have shown how the refractive

index profile may be optimized. They considered a class of profiles given by

10
n^ [ l-2A(r/a)9]^/^ for r<a

n (r) 5 , (3-1)

n^[l-2A]^''^ E n^ for r>a

where n(r) represents the refractive index as a function of radius, n-^ is the refractive

index on axis, a is the core radius, n2 is the refractive index in the cladding and a is a parameter related to the difference between n-|^ and n2 given by

2 2 "l " "2

The parameter g, known as the profile parameter, takes on values between 1 and <». As it

does so one may consider a broad range of profiles from triangular (g=l) to parabolic (g=2) to step (g=«>).

Analysis of these profiles shows that the duration of the impulse response is a function of g and a. In particular, under the assumption that n(r) is independent of wavelength, it was shown [6] that for a particular value of g given by

9opt

= 2-2A (3-3)

the duration of the impulse response goes to zero. Olshansky and Keck [7] have extended the work of [6] to include the variation in re- fractive index with wavelength. They give an approximate expression for the rms duration of

the impulse response due to intermodal effects as follows:

^ ^

J g^ I g-2-p , ,3 ,. "inter 2C g+1 3g+2 g+2 ' where

dn,

^i^"i-^dr- (3-5)

is known as the material group index and P, sometimes known as the profile dispersion parameter, is given by

" = -2^1^- »-«

From eq (3-4), gQ-^. is seen to be

Sopt = 2 - P (3-7)

or, from a more exact version of eq (3-4) [7],

v---'^^^^^^-

11
For P = 0, eq (3-8) is approximately equal to eq (3-3).

In any case, it is apparent from eq (3-4) and (3-8) that even though P is small, its

effect can be to make aj|^j£[^ and hence the fiber bandwidth a strong function of wavelength,

particularly when the fabrication of the fiber is such that g « 9oDt* Several authors [8]

have studied the variation in the intermodal distortion with wavelength experimentally and

have observed large variations.

3.1.2 Intramodal Dispersion

To understand the broadening that occurs within a single mode it is necessary to identify those parameters that determine the velocity of propagation of a mode. Consider the

effective phase index of a mode, n , which is the ratio of the speed of light in vacuum to the phase velocity of the mode. For each mode n varies between n^^ at short wavelengths

(far above cutoff) where the power is confined near the axis to n2 at cut-off where the

power propagates through the cladding. For a specific mode, n depends on the core radius,

the wavelength explicitly, and on nj^(x), a(x), and g(x). That is,

n^^ = f{a, X, n^, a, g). (3-9)

It should not be inferred from this relation that the dependence of n on these parameters

is separable. However, it is usual to identify the dependence on a/X, which would be present even if the waveguide were composed of dispersion-free glass, as waveguide dispersion.

Then r\-^{\) is said to give rise to material dispersion , a(x) to profile dispersion , and g(x)

to what might be called profile parameter dispersion.

For those modes farthest from cut-off

n ^ n.(x). (3-10) yv 1

We therefore expect that material dispersion will be the dominant contributor to intramodal

dispersion except perhaps near where dn^^/dx = 0.

In the limit of expression (3-10) the velocity of propagation (both phase and group)

approaches that of a plane wave in a medium of refractive index n]^(x). The propagation

constant in that case is B = 2TTn,(x)/X. The time for a pulse to propagate a distance L is

T = Ji= L^= i (n,-X -^) = ^N,, (3-11)

yv V do) C 1 dX C 1

where v is the group velocity and U-^ is the material group index as stated earlier.

The intramodal pulse broadening then becomes

12
dx . dN,

P V _

J. 1^ ''intra " "Material ''s dx °s c dx

dX

where o^aterial ^^ ^^^ "^^^ pulse broadening due to material dispersion, o^ is the rms spectral width of the source, and M is known as the material dispersion parameter.

For most glasses M is zero at a wavelength near 1.3 pm. At shorter wavelengths it is

positive and at longer wavelengths negative. M is not a strong function of glass composition. At 800 nm its value for most glasses falls in the range of 120-150 ps/nm'km and at 900 nm, 70-90 ps/nm-km.

3.2 Separation of Chromatic and Monochromatic Distortion

With regard to measurements, the most important aspect of the above discussion is the

extent to which the total distortion or bandwidth depends on the spectral characteristics of

the source. Because the material of which the fiber is composed is dispersive, both oj|^j[^y^

and oiMj£R depend on wavelength. Further, oii^jr/\ is proportional to the spectral width of

the source (og). Within the limits of the analysis omjxER ^^ independent of Og. The problem, then, is in interpreting measurement results.

The variation of intermodal pulse broadening with wavelength is not readily predictable. It is thus difficult to use measurements at one wavelength to design a system that

will operate at another wavelength. Appropriate sources are available over most of the

range at which systems are likely to operate but economic considerations generally preclude

routine spectral bandwidth measurements.

The dependence of bandwidth on source spectral width generally causes greater difficulty in measurements. It is usually not possible to choose a source for measurement that

will have the same spectral line shape as will be found in the system in which the fiber

will ultimately be used. It is also difficult to accurately separate chromatic and mono- chromatic effects in a measurement result that depends significantly on both.

Usual practice therefore is to set limits on the spectral width of the source used for

measurements so that one or the other effect dominates. The focus is usually on intermodal

effects since they vary much more between fibers and since in most systems they represent

the limitation.

If the total distortion can be written as the convolution of the "intermodal impulse

response" with the "intramodal impulse response" we can write

2 2 2

'^h ' "INTER

" "INTRA-

^^"^^^

We may decide arbitrarily that intermodal effects dominate whenever

"INTRA

-= "s' m' l2 < 0.21 a,2, (3-14)

13
Table 3-1.

Values of the parameter 0.2/lMl for a germanium-phosphorus-doped

silica fiber [10]. Gaussian impulse response shapes and Gaussian

pulse shapes were assumed to generate these data.

0.2/lMl 0.2/lMl

X(nm) (GHz km nm) x(nm) (GHz km nm)

800 1.6 1200 18. 820 1.7 1250 42.

840 1.9 1300 220.

860 2.1 1340 40.

880 2.3 1510 11.

900 2.5

that is, whenever intramodal effects account for no more than ten percent of a measured

0^. This leads to

^ 0.21 0.^

0/ <

^

;," . (3-15)

If the source line shape and the impulse response are both reasonably modelled as Gaussians,

eq (3-15) may be rewritten as

AX < ^-pirrr^, o-ie)

where ax is the full -width-half -maximum (FWHM) of the source spectrum and f_3(jB is the frequency at which the measured transfer function equals -3 dB. Equation (3-16) has been used by standards committees [9] as a necessary condition for

a valid measurement of "intermodal bandwidth". As indicated in section 3.1.2, M is a fairly

weak function of glass composition but a strong function of wavelength. Table 3-1 gives the

quantity 0.2/1 Ml for a germanium-phosphorus-doped silica fiber [10]. From this data one concludes, for example, that to measure a 1 km-long fiber having a 1 GHz bandwidth at 850 nm

a source spectral width of less than 2 nm is required. In the longer wavelength region a much greater source spectral width could be used.

3.3 Length Dependent Effects and Launching Conditions

In multimode fibers several mechanisms act to render the fiber bandwidth a nonlinear

function of length. These effects can make the interpretation of data very difficult.

Certain non-ideal characteristics of a fiber result in power being coupled between

modes, an effect known as mode coupling or mode mixing . The transfer of power from modes

with low group velocities to those with high group velocities and vice versa means that

14
fiber bandwidth will decrease more slowly with length than would otherwise be the case. The

actual functional dependence frequently falls in the range of L"-*-'^ to L" , depending on the

degree of mode coupling present [11].

Mode coupling may arise from defects introduced during manufacturing; for example,

variations in diameter or index profile with length. Mode mixing also arises from bending

of the fiber, particularly from bends that have spatial periods of the order of a millimeter. This means that the manner in which a fiber is cabled or handled can have a major

effect on a bandwidth measurement. Particular care is required in the measurement of

unbuffered fiber wound tightly on a spool.

Differential modal attenuation can also cause fiber bandwidth to be a nonlinear function of length. If high order modes of the fiber are excited and if, as is frequently the

case, these modes suffer from higher attenuation than low order modes, then the rate at which bandwidth decreases with length is likely to be greatest near the input, where the

complete range of mode groups is encountered.

The combined effects of mode coupling and differential modal attenuation may result in

a complex functional dependence of bandwidth on length. For example, when both effects are important the bandwidth has been observed to decrease as L'^ near the input, progressing

gradually to perhaps \_~^'^ after a substantial distance [12], and stabilizing at that

value. Such a condition, where the functional dependence of bandwidth on length is stable,

is sometimes known as modal equilibrium because it occurs when the distribution of power

among the modes changes no further with length. The attenuation coefficient presumably

becomes independent of length in this case, as well.

Another factor that effects the dependence of bandwidth on length is variations in index profile. If, in attempting to produce a fiber in which the group velocities of all

modes are equal, some parts of the fiber are overcompensated* and other pa?ts are undercompensated, equilization occurs. This leads to a higher bandwidth than would otherwise be expected and to a very complex

variation of bandwidth with length. Such variations in the degree of cpmpensation frequently appear between fibers joined together in a link, and make the prediction of system performance difficult.

All of the problems described above make the choice of launching conditions for bandwidth measurements very difficult. Until such time as better methods of describing the

variation of bandwidth with length are found it is probably wisest to simply choose measure- ment (launching) conditions that are well defined and easy to verify and which therefore may

be expected to yield reproducible results. Certain standards groups [9] have therefore

suggested that bandwidth measurements be made in such a way that all the modes of the fiber

are excited. This means that light should be coupled into the fiber over the full area of

the core and over a range of angles as large as the acceptance angle of the fiber. These

are the criteria generally adopted in this Technical Note.

*A properly compensated fiber is one in which the group velocities of all modes are the

same. Overcompensation is the case where low order modes propagate more slowly than higher

order modes (in the power law representation, g<gQp^). Undercompensation is the opposite

case.

15
4. Frequency Domain Techniques and Systems

Many techniques have been developed for the characterization of rf components in the

frequency domain and instruments and devices for use in these measurement systems are widely

available. With care, much of this same technology can be applied to the characterization

of optical fibers. This section provides a survey of several potentially appropriate measurement methods, one of which has been extensively evaluated and is described in detail in

subsequent sections.

4.1 Systems

It may be useful to separate frequency domain techniques into two groups: those that

use wideband rf detection and those that use narrowband rf detection. Wideband detection

generally leads to a simpler and less expensive measurement system but suffers from a lack

of harmonic rejection and may give inferior noise performance. Narrowband detection is more complex but provides freedom from distortion, an important consideration when the system

includes lasers or LEDs.

4.1.1 Systems Using Wideband Detection

One of the simplest systems that might be used is shown in the block diagram of figure

4-1. An rf sweep generator provides a controlled input signal, the frequency of which can

be varied or swept over the range of interest. This signal is applied to the system under

test, which in the case of a fiber would include the optical source and detector. The output rf signal is detected by a wideband, linear (in power) rf detector. If the signal-tonoise ratio is not an important consideration this arrangement may be sufficient to determine the magnitude of the transfer function by comparing the ratio of output-to-input signal

level as a function of frequency with the test system in place to the same ratio with it

removed. If the losses in the test system are large, the signal-to-noise ratio can be

improved by amplitude modulating the input at a low frequency and using a lock-in amplifier

as shown.

Probably the greatest difficulty in using this system for fiber measurements is that

most suitable optical sources introduce harmonic distortion to the signal. With typical sources this problem may be sufficient to result in a several percent error in the transfer

function (see section 5.1).

A variation of the system shown in figure 4-1 that allows the determination of group

delay as well as transfer function magnitude [13] is shown in figure 4-2. In this case, the

input is amplitude modulated at a relatively high frequency —perhaps 10 MHz for a sweep

range from 30 MHz to 1.5 GHz. The output of the rf detector then goes into a vector voltmeter which gives the ratio of the output (envelope) magnitude to input magnitude and the

phase shift in the envelope. The group delay (section 2.2) is related to this phase shift

by the relation [13]

^g

360 fm where (j) is the phase shift in the envelope, in degrees, and f is the modulation frequency.

16
Sweep

Generator

Test rf

System Detector

'SXTX

1

1

1 If

Lockin

Amplifier

1

Modulation

Data

Processing

Figure 4-1. Block diagram of a simplified frequency domain measurement system. When the

signal-to-noise ratio is high, the lock-in amplifier may not be necessary.

Sweep

Generator

Amp. —

Test

System

rf Mod. Detector

f —

!

Sig.

Gen. ^

Vector

Voltmeter

I Data

Processing

Figure 4-2. A modification of the system of figure 4-1 which allows both magnitude and

group delay measurements. The signal generator operates at a relatively high

frequency and the vector voltmeter determines both the magnitude and phase of

the input and output modulation envelopes.

17
This is probably the simplest method of obtaining phase data on a fiber. It has been

used by the author to evaluate several fibers and except for the problem of harmonic distortion works quite well.

4.1.2 Systems Using Narrowband Detection

The wide choice of detection bandwidth, gain, sweep rate, averaging, etc. generally

available in commercial rf spectrum analyzers make that instrument a good choice for the

detector in systems using narrowband detection (fig. 4-3). To fully exploit its capability,

however, it must be used with a signal source that is frequency locked to the local oscillator of the spectrum analyzer. Thus, the source output frequency will vary coinci dentally

with the sweep of the spectrum analyzer. Such sources, known as tracking generators, and

designed for use with specific spectrum analyzers are available from several manufacturers. The system bandwidth is generally limited by the degree to which the lock between

the tracking generator and spectrum analyzer can be maintained. The simplicity of construction and operation of this system probably make it the best choice for fiber measurements

whenever magnitude information is sufficient.

In the testing of discrete rf components, a vector voltmeter is frequently used to obtain both magnitude and phase information. It is not generally used in the testing of

systems characterized by long propagation delays because of the difficulty in separating the

phase distortion information from the large linear phase shift associated with the delay. Two techniques shown in figure 4-4 allow one to avoid these difficulties to a degree. One

approach is to use a frequency synthesizer instead of a conventional sweep generator [14].

The synthesizer provides a discretely variable source frequency that can be varied arbitrarily slowly over the range of interest allowing more accurate phase measurements to be

made. The other approach is to insert a system characterized by a similar delay but much

smaller distortion in the reference line [15]. The phase distortion is then a much larger

portion of the total phase shift. A single mode fiber would be a suitable choice for a system designed to characterize multimode fibers.

4.1.3 Network Analyzers

The four measurement systems described above are fundamentally similar in function and

form to more elaborate rf measurement systems known as network analyzers which measure the

S-parameters of microwave devices and circuits. Commercially available network analyzers

can also be grouped into wide and narrowband detection types. They will measure the magnitude of the transfer function (i.e., S^2> ^21^ ^"^' depending on type, will

provide some form of phase characterization. Generally, they are also designed to measure

other parameters important in rf circuit design (e.g., impedance, S^^j^, S22)- Thus, a net- work analyzer suitable for carrying out fiber transfer function measurements is generally an expensive alternative to the limited purpose systems described.

4.2 Choosing a System and the Matter of Phase

Performance, convenience, and cost together determine the choice of measurement system. Several of the methods described in section 4.1, above, have been used at NBS and of

18
Test

System

Tracking

Generator

1 Spectrum

Analyzer

Figure 4-3. A simplified frequency domain system using narrowband detection. This

arrangement forms the basis of the system described in detail in section 5.

Frequency

Synthesizer

Test

System

_ -I.

1 Delay ,

1 1 " r Vector

Voltmeter

1 Data

Processing

Figure 4-4. A narrowband system that can provide phase information. The system works best

if a compensating delay is inserted in the reference channel of the vector

voltmeter.

19
them, provided that only magnitude information is needed, the system shown in figure 4-3 seems to be the most appropriate choice. This brings us again to the question of whether

the incomplete characterization of the information carrying capacity by |H(f)l is

sufficient.

Most high quality graded-index fibers designed for telecommunications applications have

an impulse response that is nearly symmetrical about its central time and therefore show

^ery little variation of group delay with frequency. The phase distortion that is present

is therefore probably of little consequence as long as the problems noted in section 3.3

continue to complicate the interpretation of bandwidth measurements.

Step-index and other non-optimum profiles generally do not have symmetric impulse

responses. It is conceivable that in certain systems using such fibers or in other special

purpose systems phase distortion may be important. However, it appears that at this time

these cases arise infrequently and most of the needs of the industry can be met with magnitude data alone.

The system of figure 4-3 was therefore chosen for further evaluation and use. The

design and performance details of the version now in use at NBS are given in the following

sections.

5. Description of the Measurement System

Figure 5-1 shows a block diagram of the system. The optical design is similar to that

of a time domain system described elsewhere [16], though certain refinements have been incorporated. Almost all of the components are commercially available; their important characteristics are described below along with the basic system design.

5.1 Source

The source normally used in this system is a commercially available "1-aser transmitter". It consists of a single-transverse, multi -longitudinal mode GaAlAs laser diode

coupled directly to a 2 m length of 50/125 um core/cladding diameter, graded-index fiber.

The laser is mounted on a temperature controlled substrate. Output from the laser is

further stabilized by using the detected light from the back face of the laser to control

the bias current. Radio frequency signals may be superimposed upon the bias current using a single R-C network, thus providing wide-bandwidth amplitude modulation. The entire unit,

excluding power supply, measures about 3.5 by 7 by 4 cm (fig. 5-2).

The specified output power at the laser is 2.5 mW with an amplitude stability of 0.1

percent. Measured power at the fiber output is about 1.25 mW, where a long term drift of 1 to 2 percent is noted. This drift probably results from changes in optical feedback

(reflections from the fiber ends) with temperature.

Figure 5-3 shows the spectrum of the source as measured with a 0.5 m Ebert-type spectrometer, with a resolution indicated by the width of individual longitudinal modes. The

relative power of the modes varies somewhat with time with the result that a specificiation

of spectral linewidth based on the full -width-half -maximum (FWHM) of the envelope becomes

uncertain. However, for purposes of estimating the frequency at which chromatic effects

begin to effect a bandwidth measurement (section 3.2) we choose ax = 0.9 ± 0.15 nm. This

20
L._?_J

V-si

3 a

C o <

i OKI

O)

"5. II

3O

a

o

a

en

0)u

c

w

J3o

w o> c c

<0

«0 8

CC

•ac e

(0 8

0.

E

2

Si

a> "Oo

p

e:

O)E

(U

s-3

s

C

s.

E

o

-o

c

(U3

CT

OJ

<001

<0

o

o

I LO

_J

21
Figure 5-2. Photograph of "laser transmitter" used as the source.

uuu u u uu

Wavelength, nm

Figure 5-3. Output spectrum of source obtained with a 0.5 m Ebert-type spectrometer.

22
suggests that for a typical germanium-phosphorus doped silica fiber material dispersion does

not become a contributor to the measurement below 1.9 GHz which is outside of the operating

range of the system.

Figure 5-4 shows the variation in modulation index (optical power modulation/rf drive) versus frequency. The modulation index is essentially independent of frequency from a few

tens of hertz to 600 MHz (specified bandwidth is 500 MHz). The index decreases by about 2.5

dB (optical) between 600 and 680 MHz and then increases gradually to a value about 4 dB

(optical) above its low frequency value at 1430 MHz near the limit of the system. Independent tests on the detector used in this measurement indicate that the variations in modulation index are not measurably affected by the detector. The increase at the highest frequencies may be related to self pulsing in the laser because the noise spectrum of the laser

output shows a similar increase. The dip at 680 MHz is of unknown origin.

Harmonic distortion in the source output can, as noted in section 4, cause difficulties

when broadband detection is used. Figure 5-5 shows the distortion produced by this source.

At a drive frequency of 100 MHz and an rf signal level of dBm, the second harmonic is

about 38 dB below the fundamental (manufacturer's specification: 40 dB) . However, as the

drive frequency is increased the harmonic distortion increases until at a fundamental frequency of 500 MHz both the second and third harmonics are only about 20 dB down. This would

be unacceptable for broadband detection.

5.2 Mode Scrambler

Since the results of a fiber bandwidth measurement depend rather strongly on the spatial and angular characteristics of the input light, that is to say the input mode volume,

it becomes necessary to minimize the effects of such changes in the source^ This problem is

somewhat less severe in the frequency domain system than in the time domain system [16]

since the cw laser diodes are inherently more stable spatially than those used in time

domain systems.

The usual way of maintaining launch stability is to couple the output of the source through a specially designed spatial filter. The spatial filter has the properties that the

spatial and angular characteristics of its output are independent of those of the input. In practice, the spatial filter usually consists of a piece or pieces of suitably chosen fiber

[16,17]. For this reason it has come to be known as a mode scrambler . The mode scrambler used in this system consists of two pieces of fiber. The first is

the 2 m section of 50/125 um core/cladding diameter, graded-index fiber directly coupled to the laser by the manufacturer. The second is a similar length of 80/125 ym core/cladding

diameter, step-index fiber spliced onto the first fiber in a loose tube. The radiation

angle of each fiber is nominally 0.2 rad.

Figure 5-6 shows the characteristics of the device. The near-field profiles are obtained by imaging the output face of the fiber onto a silicon target vidicon. The video

signal is processed with commercial equipment in such a way that the intensity of the image

along a line normal to the raster is determined and plotted on the monitor screen.

23
Frequency 1500 MHz

Figure 5-4. Modulation index of source as a function of frequency. The output of the

tracking generator was applied to the source and the output of the gradedindex pigtail from the source focused onto the detector.

Figure 5-5. Distortion in source output. Drive level was approximately dBm and the

vertical scale 10 dB (electrical )/div in each case, (a) Drive: 100 MHz,

Scale: 0-1000 MHz; (b) Drive: 300 MHz, Scale: 0-1000 MHz; (c) Drive: 500 MHz,

Scale: 0-1000 MHz; (d) Drive: 500 MHz, Scale: 500-1000 MHz.

24
The near field of the output from the graded-index fiber shows a rounded profile as expected, with a great deal of variation due to speckle. At the output of the step fiber

the core is completely filled and the variations due to speckle appear somewhat reduced.

The far-field pattern from the step fiber was obtained by scanning a detector along a circular arc about 10 cm from the fiber end [18]. The far-field radiation angle is about

0.19 at the 90 percent irradiance points.

The speckle evident in all the data of figure 5-6 represents a fundamental difficulty

in controlling the launching conditions for bandwidth measurements. It results from con- structive and destructive interference in the superposition of fields from the modes of the

fiber and appears whenever the coherence time of the source is long compared to the differences in group delay among the modes. The details of the speckle pattern vary with the

relative phase of the modes. Very slight movement or distortion of the fiber will produce

such phase shifts as will a change in temperature. These effects make it difficult to precisely define launching conditions. Further, when the speckle pattern shifts with time in a system where mode sensitive devices, such as couplers, are present or where there is significant differential mode attenuation, noise, generally known as modal noise results [19].

One solution to the problem of speckle is to use a less coherent (broader spectral

linewidth) source. However, as noted earlier (section 3.2) that can result in additional

limitations to the bandwidth from the material dispersion. As a practical compromise,

source spectral widths of about 1 nm are reasonable.

Another possible solution for measurement systems might be to deliberately introduce

periodic phase shifts, through for example, mechanical distortion by a piezeoelectric transducer. This should have the effect of smearing the speckle pattern. This approach has not

been seriously pursued, as yet.

5.3 Launching Optics

The section of figure 5-1 labeled launching optics is designed to image the output of

the mode scrambler onto a fiber specimen in a well defined and reproducible fashion. The

launching parameters of interest are the size of the image (the "spot size") on the input

end of the fiber specimen and the angular extent of the bundle of rays that converge to form

the image (the "launch numerical aperture", LNA) . With this system (fig. 5-7) it is possible to control these parameters independently, though rather less conveniently than is

necessary in an attenuation measurement system [20]. However, as noted in 3.3.3, the greatest interest at present is in measurements made with full excitation of the fiber.

Two microscope objectives form the image. The first, usually a lOX, 0.25 NA lens,

collimates the output of the mode scrambler. The second, in most cases identical to the

first, focuses the collimated beam onto the specimen. The ratio of the spot size on the end

of the test specimen to the spot size at the output of the mode scrambler is equal to the

ratio of the focal length of the second objective to that of the first. This is perhaps the

easiest way to control the spot size. The launch numerical aperture is somewhat more complicated to predict. If the objectives are identical, the LNA should be the lesser of

the sine of the radiation angle of the mode scrambler or the effective NA of the objective. If the objectives are different, a more specific analysis is necessary to predict the

25
:a)

Figure 5-6. Characteristics of mode scrambler, (a) Near-field pattern, nominal diameter,

83 pm. Plot at bottom is of intensity along line through image, (b) Scan of

far field of mode scrambler.

26
Figure 5-7. Photograph of launching optics. Output of mode scrambler is on manipulator at

right, specimen input on manipulator at left. Pellicle at left of center can

be rotated 90° to direct the image of either the mode scrambler output or the

specimen input onto vidicon at top of photo (see fig. 5-1).

(a) (b)

Figure 5-8. (a) Image of stage micrometer placed where specimen input would normally be

placed, to verify size of launch spot. Line spacing is 10 urn. (b) Image of

short step-index specimen. The core appears brighter because of reflected

light from output end of fiber.

27
LNA. The LMA may be limited beyond that determined by the mode scrambler or lenses by placing an aperture in the colli mated portion of the beam.

For purposes of system alignment and evaluation it is convenient to place a pellicle

beam splitter in the collimated beam and to thereby image, with some additional magnification, the input end of the fiber specimen as illuminated by the source. Figure 5-8a shows

the image produced on a stage micrometer placed at the image point; verification of the

launched spot size is thus obtained from the 10 pm line spacing on the micrometer. Figure

5-8b shows the image of a short (~ 2m) fiber specimen similarly obtained. The core appears

brighter than the cladding due to light reflected back from the output end of the fiber.

With a long fiber or with the output end index matched there is little difference in appearance between the core and cladding.

The launch numerical aperture could be verified by scanning a small detector through

the diverging beam beyond the image point. An alternate method is to determine the farfield radiation angle from a short piece of fiber (fig. 5-9).

5.4 Specimens

In most cases, the test specimen is a known length (typically 1 km) of multimode

graded-index fiber. A separate reference specimen representative of the test specimen,

generally a piece cut from one end of the test specimen, is chosen. The length of the

reference specimen must be such that the magnitude of its transfer function is essentially

unity over the frequency range of interest. As a practical matter, it is usually sufficient

to limit the length of the reference specimen to 1 percent of the length of the test specimen. Both ends of each specimen are prepared by cleaving and are inspected with a 400 power

microscope for flatness and perpendicularity. Illumination is either col linear with the

microscope axis or from the far end of the fiber. Ends with perceptable hackle or breakout

within the core region are rejected as are ends on which the entire outer surface of the

cladding can not be brought into sharp focus at the same time. The theoretical depth of

focus of the microscope is about 3 m which means that for a 125 pm fiber diameter angles

smaller than 1 to 1.5 degree are accepted.

Cladding mode strippers are not used in this system. Unlike the case with attenuation

measurements, light that remains trapped within the cladding for the full length of the

reference fiber should not affect the measurement.

It is important that the test specimen be handled in such a way that no excess microbending is applied to the fiber. This consideration can be very important when testing

uncabled fiber that has a very thin buffer.

5.5 Detection

A suitable detector must have a sufficient frequency response, linearity of response

over the range of operating levels, and a uniformity of response over the active area. The

detector chosen for this system is a 200 m diameter Si APD. The characteristics of this

detector were described previously [15] and are briefly summarized here. The impulse

response duration is about 200 ps which gives a more than adequate frequency response for

28
Figure 5-9. Far-field radiation pattern from a short piece of large acceptance angle fiber

placed in test system to verify launch numerical aperture. Fiber specifications: 100 pm core diameter, quasi-step index profile, 0.3 NA.

(a)

(b)

Figure 5-10. (a) Photograph of Si APD detector used,

(b) Plot of spatial response uniformity.

5 percent peak to peak.

Active area is 200 um diameter.

Detector is uniform to about

29
this system (see also fig. 5-3). The uniformity of response (fig. 5-10) is about 5 percent

peak to peak.

The pulse linearity is good [16]. At the higher average currents (50-100 pA) used in this system perceptable heating occurs. This is sufficient to cause a few percent change in

detector gain but allowing the detector to stabilize before measurements eliminates most of

the difficulty. Further compensation for temperature changes by the method described below

fully eliminates the problem.

The detector is mounted in a coaxial mount similar to one described by Green [21]

(fig. 5-11) and biased with the circuit of figure 5-12, which differs somewhat from the bias

circuit used with the time domain system [16].

The detector is normally biased at a point (typically 140 V) where the gain is between

5 and 10. To compensate for any temperature induced change in gain or change in laser out- put the dc current is monitored with a 10 fi sensing resistor. The slopes of the dc and rf load lines being nearly equal (1/60 versus 1/50 a~^ when the ammeter is shorted) a change in

this reference voltage is proportional to any laser output or detector ga-in induced changes

in detected rf level

.

It is necessary that the outputs of the test and reference fibers are completely

coupled to the uniform detector to insure that all mode groups are equally detected. This

is done with two microscope objectives, as shown in figures 5-1 and 5-13. Usually the objective nearest the fiber output is a 10 X, 0.25 NA while the second is a 5 X, 0.1 NA to provide a larger spot on the detector and thus average over spatial nonuniformities.

Neutral density filters (fixed and circularly variable) are placed in the collimated

region to set the signal levels for test and reference specimens equal, minimizing the

effect of detection nonlinearities.

5.6 Electronics

A more detailed block diagram of the electronic part of the system is shown in figure

5-14. The detector output passes through a wide-band (0.1 to 1400 MHz) high gain amplifier

(+26 dB) and into the input of the spectrum analyzer. A ramp generator controls the sweep

of the spectrum analyzer and hence the output of the tracking generator which is fed to the

laser source. The vertical output of the spectrum analyzer is amplified and along with the

ramp voltage and the amplified current reference signal is processed with a data logging

apparatus. This unit includes A/D conversion, timing and control circuitry, and parallel to serial conversion. The data is then recorded on magnetic tape cassettes for subsequent

computer processing.

Typically, the frequency sweep covers 10 MHz to 1200-1300 MHz in about 2 minutes.

About 250 data points are recorded with a frequency resolution of about 5 MHz. Increased

resolution or reduced sweep range can be used when required.

Calibration of the sweep is provided by substituting a step variable precision power

supply for the ramp generator and measuring the output frequency of the tracking generator

with a counter. About 100 calibration points at equally spaced frequencies are obtained.

Some slight hysteresis (~ 0.2 MHz) is observed. The reproducibility of the calibration

curve depends on tracking generator adjustment as well as other factors but measurements

30
MODIFIED

SMA

CONNECTORS

1 .6 cm

.06 mm MYLAR

DIELECTRIC

Figure 5-11. Diagram of detector mount. n

Detector

Mount

3dB f 0.1 to 1400 MHz +26dB

<^

Curren

Ref.

_Z1- 140 V '—J< lOfi

Figure 5-12. Detector bias circuitry.

31
Figure 5-13. Photograph of output coupling optics. Specimen output is on manipulator at right. Fixed and circular-variable neutral density filters are placed between

microscope objectives. Detector is in cylindrical mount at left (see fig.

5-1).

Trac kino To Generator Source

—|

Detector

Output r\ Spectrum

Analyzer L^

0.1 to 1400 ' MHz +26dB

Vert. Sweep Out.

in

Ramp

Generator

Current iv.\ ^^

Data

Logging <1

1

lOOOX

^

0.1 s OX

Data Storage

O.i s

Figure 5-14. Block diagram of electronics.

32
over several months indicate that ±0.5 MHz is readily obtained. Frequency calibration data

are plotted in figure 5-15 along with a linear least-squares fit.

The vertical output of the spectrum analyzer is calibrated using a signal generator and

a set of precision attenuators. Calibration points are taken at 1 dB electrical (0.5 dB

optical) intervals. The attenuators were chosen for their reproducibility and frequency

independence. The specified values were verified or adjusted using a bolometer-type rfmicrowave power meter accurate to 1 percent. This power meter thus becomes the magnitude

reference standard for the system. Calibration at several frequencies indicates that single

frequency calibration is sufficient. Vertical calibration data relative to a suitably

chosen reference level are shown in figure 5-16, along with a linear least-squares fit.

5.7 Computation

For each data point there is one value related to the signal level in dB, one related

to frequency, and one related to the linear value of the current reference. The first two

of these are converted to dB and frequency, respectively, by linear interpolation using the

data described in section 5.6. The vertical signal is then adjusted for changes in the

current reference by subtracting 5 times the latter (normalized) from the former. This

procedure is followed for data from both a test and a reference specimen.

The result of these manipulations is two arrays of data, magnitude versus frequency;

one for the test specimen and one for the reference specimen (fig. 5-17). Unfortunately,

the operation of the system does not insure that frequency values in the two arrays coincide

so further interpolation is required, as shown.

Verification of the computations was obtained by inserting the precision attenuator set between the tracking generator and spectrum analyzer in place of the usual test apparatus.

The attenuators were switched at intervals as the usual speed range was covered (fig.

5-18). Deviations from calibrated values (averaging over several standing wave periods

where necessary) are all less than 0.03 dB.

6. System Performance

6.1 Measurement Procedure

Following preparation of the test and reference specimens as indicated in section 5-4,

the two fibers are placed into the system. Generally, the test specimen is evaluated

first. The alignment procedure consists of observing the fiber end on the monitor and adjusting the end position so that both the end and the spot striking it are in focus and the

spot is visually centered within the core circumference. At the fiber output the image of

the output end is located on the detector by adjusting the focus for maximum signal and

adjusting the transverse position to center the spot on the detector.

The signal level is adjusted with discrete and variable neutral density filters to a satisfactory value not exceeding the limits of the detector. (If a high-loss fiber is measured no ND is used with the test specimen.) The tracking generator is adjusted for proper

tracking, the spectrum analyzer is set to scales and conditions on which calibration data is

available, the sweep is begun, and data are recorded.

33
4 6

Voltage, Volts

10

Figure 5-15. Frequency calibration data.

Amplified Vertical Output, Volts

3.0 4.0 5.0 6.0 7.0

-

1 1 1 /

_ -5

ra

- y o

+-> Q- O

- y

^

CQ -10

: y^

'aj>

CD

- y

^

t -15

- y^

-20

- /

Figure 5-16. Vertical calibration data.

34
:3

CD

CDo

Frequency

Ref.

Specimen

Data

Test

Specimen

Data

Figure 5-17. The measurement process results in two arrays of data, magnitude versus frequency, one for the test specimen and one for the reference specimen. The

frequency points are not necessarily coincident. The reference specimen data

is therefore interpolated before the differences in magnitude are obtained.

MAGNITUDE I3F TRANSFER FUNCTION

r

f

-1 -2 -

\ i rn

-

- C^^ 1—

-1 M -A

A

GNI -6

TU

-

^ r

-

~1

_J_.

I_.

I

1

d-lQ

B :

"

-12 - -

-M —1 1 1

i 1 u 1 L 1 ... 1 1 1 u —1 1- —1 1— 1 u—1 1 1 4 5 6 7 8 FREQUENCY, MHr/100

1Q 1 1 12

CALTl/Rl

Figure 5-18. Result of replacing test system with a set of precision, step-variable atten- uators and switching at intervals during scan.

35
The procedure is repeated for the reference specimen. In this case, the average cur- rent level at the detection is set equal to that with the test specimen and care is taken to ensure that the same spectrum analyzer parameters are chosen.

6.2 Precision and Accuracy

Given the difficulty of interpreting bandwidth measurements, the most important aspect

of bandwidth measurement quality at the present time is the precision, or reproducibility.

To test the precision of this system repeated measurements were made on each of two selected

fibers, using the procedure of section 6.1. The two fibers were chosen because in each case

a substantial history of both bandwidth and attenuation measurements was available [16,22]

indicating stability of characteristics. The nominal characteristics of the fibers are summarized in table 6-1. Each measurement consisted of separate test and reference data

with a completely new alignment and resetting of levels for each specimen. The results are

shown in figure 6-1 and 6-2.

Table 6-1.

Characteristics of fiber used in precision determinations

Fiber 1308 1223

Length 1.3 km 1.1 km Size 50/125 m 50/125 urn NA 0.25 0.25

Attenuation 6.0 dB/km @ 850 6.0 dB/km @ 850

Table 6-2.

Comparison in precision between a time domain system [16] and the

frequency domain system described here. The variation in FD precision

on 1308 at the -3, -6, and -9 dB levels probably results from

the different slopes of the transfer at these points.

Fiber

1308

1308

1308

1223

1223

Level

(dB)

-3

-6

-9

-3

-6

TD-Precision

(%)

0.6

1.

1.

4.

3.

FD-Precision

{%)

1.7

0.06

0.06

3.4

3.3

36
FI306S/T FI308V/W

4 S 6 7 8 9

FREQUENCY, MHr/100

FI308Y/Z FI308AA/BB FI308CC/DD

10 11 12

Figure 6-1. Superposition of the results of five independent measurements on fiber 1308.

Arrows indicate the one standard deviation imprecision at the -3, -6, and -9

dB levels in percent and at frequencies of 200, 400, and 600 MHz in dB.

MAGNITUDE OF TRANSFER FUNCTION

d-lQ|- B

-I2h

-14-

I I I 1 1 I 1 1 r

r I I I I I I I I I I 1 I I I I I I 1 I I I 1-

1 2 3 -4 5 6 7 8 9 10 1 1 12 FREQUENCY, MHi/lOfl

FI223A/B FI223D/E FI223e/H FI223J/K FI223P/Q FI223S/T

Figure 6-2= Superposition of the results of five independent measurements on fiber 1223.

Arrows indicate the one standard deviation imprecision at the -3, and -6, dB

levels in percent and at the frequencies of 500, and 1000 MHz in dB.

37
It is useful to specify precision in two ways. One is reproducibility in determining

the frequency at which the magnitude reaches a specified level, say -3 dB, -6 dB, -9 dB, etc. The other is the reproducibility in magnitude at a specified frequency. In figures

6-1 and 6-2 a one standard deviation precision at several levels and frequencies is indicated. Note that the precision on fiber 1308 is significantly better than for fiber 1223.

This difference appears to be related to the fiber characteristics— presumably differences

in mode coupling and differential attenuation--and was noted in previous work. (1308 was identified as fiber A and 1223 as fiber B in [16].) In fact, the precision obtained in that

previous work is very similar in both cases to the precision obtained here (table 6-2).

This is not at all surprising since the optical part of the system in all probability determines the system precision and the optics in the two systems are very similar.

6.3 Limitations

The frequency range over which this system is operated, 10 to ~ 1200 MHz is sufficient

to test most commercial fibers— the highest commercial specification at this time appears to

be 1000 MHz "km. However, experimental fibers with a bandwidth of 2 to 3 GHz'km and higher

have been reported, and it would be desirable to extend the range of the system.

Several factors limit the upper frequency of the system as it now exists. The presently used wideband amplifier and the tracking generator/spectrum analyzer all have limitations

in the 1400 MHz region. Each could be upgraded somewhat, though material dispersion due to source linewidth becomes a significant factor somewhat near 2 GHz. Using a more coherent

source will decrease material dispersion contributions but will increase problems in specifying launching conditions due to speckle. Unless the speckle problem can be eliminated by

other means, 2 GHz is probably the maximum useful frequency for a system of this sort at this wavelength.

It is becoming increasingly important to be able to measure long systems with substantial loss. The dynamic range of this system has not been carefully studied, but is clearly

limited, at present, to roughly 20 dB loss as a result of direct electrical coupling from

input to output. Signal-to-noise measurements indicate that when this problem is eliminated

fibers with a loss of at least 30 to 40 dB could be measured.

7 . Summary

Based on data and experience obtained to date and reported in previous sections, the

frequency domain system described in this Technical Note seems well suited for the routine

measurement of fiber bandwidth. It is relatively simple to construct using components that

are almost all commercially available. It is easy to use and requires fewer and less complex computations than time domain systems. The precision obtained is comparable to that

obtained in the time domain and well within the uncertainties observed between laboratories

[23]. Certain improvements in the system and an extension to the 1.3 or 1.55 pm region

would allow it to perform most of the multimode fiber bandwidth measurements required in the

foreseeable future.

38
While this and other work suggest that the measurement of bandwidth on a specific

length of fiber is well in hand, a great deal of work needs to be done on the length dependence of fiber bandwidth. Until new approaches to this problem are developed it is likely

that the most appropriate measurement conditions for bandwidth measurements will continue to

be a full excitation of the fiber modes.

The author is indebted tc D. L. Franzen, R, L. Gallawa, and P. A. Hudson for helpful

consultations and to J. Skudler and R. Hunter for technical assistance. The Department of

Defense, Calibration Coordination Group and the U.S. Army Electronic Proving Ground,

Ft. Huachuca, Arizona, provided partial funding for this work.

8. References

[1] (a) Personick, S. D. , Hubbard, W. M., and Holden, W. S. Measurement of the baseband

frequency response of a 1 Km fiber. Appl . Opt. 13:266-268; 1974.

(b) Personick, S. D. Baseband linearity and equilization in fiber optic digital

communications systems. Bell Sys. Tech. J. 52:1175-1195; 1973.

(c) Vassallo, Charles. Linear power responses of an optical fiber. IEEE Trans. Microwave Theory Tech. MTT-25:572-576; 1977.

[2] Bracewell, Ron. The Fourier transform and its applications. McGraw-Hill; 1965.

[3] Personick, S. D. Design of repeaters for fiber systems, in Fundamentals of optical

fiber communications. Barnoski , Michael K., ed. Academic Press; 1976.

[4] Goldman, Stanford. Frequency analysis, modulation, and noise. McGraw-Hill; 1948.

[5] (a) Sandberg, I. W.; Kaminow, I. P.; Cohen, L. G. ; and Mammel , W. L. . On the phase

of the modulation transfer function of a multimode optical -fiber guide. Bell

Sys. Tech. J. 57:99-109; 1978.

(b) Sandberg, I. W. A note concerning optical -waveguide modulation transfer functions. Bell Sys. Tech. J. 57:3047-3057; 1978.

[6] Gloge, D. and Marcatili, E. A. J, Multimode theory of graded-core fibers. Bell Syst.

Tech. J. 52:1563-1578; 1973.

[7] Olshansky, Robert and Keck, Donald B. Pulse broadening in graded-index optical

fibers. Appl. Opt. 15:483-492; 1976.

[8] (a) Cohen, Leonard G. ; Kaminow, Ivan P.; Astle, Harry W.; and Stulz, Lawrence W. Profile dispersion effects on transmission bandwidths in graded index optical

fibers. IEEE J. Quantum Electron. QE-14:47-41; 1978.

(b) Horiguchi, M.; Ohmori , Y.; and Takata, H. Profile dispersion characteristics in

high bandwidth graded-index optical fibers. Appl. Opt. 19:3159-3167; 1980.

[9] Procedures now under consideration by EIA Committee P6. 6—Working Group on Fibers and

Materials (Electronic Industries Association, 2001 Eye St., Washington, DC 20006).

[10] Data supplied by P. Reitz, Corning Glass Works, Corning, NY.

39
[11] (a) Suzuki, S.; Kawahara, T.; Tanaka, G; Yoshida, M.; Kyoto, M. ; Nakahara, T.; Kumamaru, H.; Takada, H.; Inagaki, N.; and Kitayama, K. Characteristics of

graded-index fiber by VAD method. Proc. Optical Comm. Conf., Amsterdam, 12.6-1-

-12.6-4; 1979.

(b) Kapron, F. P.; Sladen, F. M. E.; Garel-Jones, P. M.; and Kneller, D. G. Attenuation and pulse broadening along concatenated fiber links. Tech. Digest,

Symp. Opt. Fiber Meas.-1980, Boulder, 63-66; 1980. U.S. Govt. Printing Office,

Washington, DC 20402.

[12] Cohen, L. G. Shuttle pulse measurements of pulse spreading in an optical fiber.

Appl. Opt. 14:1351-1360; 1975.

[13] Swept-frequency group delay measurements. Applications Note 77-4, Hewlett-Packard,

Palo Alto, CA 94304.

[14] Kobayashi, Ikutaro. Bandwidth measurement in multimode optical fibers. Tech. Digest,

Symp. Opt. Fiber Meas.-1980, Boulder, 49-54; 1980. U.S. Govt. Printing Office,

Washington, DC 20402.

[15] Okoshi, T.; Chang, J. C; and Saito, S. Measuring the complex frequency response of

multimode optical fibers. Appl. Opt. 20:1414-1417; 1981.

[16] Franzen, Douglas L, and Day, G. W. Measurement of optical fiber bandwidth in the time

domain. Nat. Bur. Stand. (U.S.) Tech. Note 1019; 1980.

[17] Love, W. F. Novel mode scrambler for use in optical-fiber bandwidth measurements.

Tech. Digest, Opt. Fiber Commun., Washington, 118-120; 1979. Optical Society of

America.

[18] Kim, Ernest M. and Franzen, Douglas L. Measurement of far-field and near-field radiation patterns from optical fibers. Nat. Bur. Stand. (U.S.) Tech. Note 1032; 1978.

[19] Epworth, R. E. The phenomenon of modal noise in analog and digital systems. Digest,

Fourth Eur. Conf. Opt. Comm., Genova, 492-501; 1978.

[20] Chamberlain, G. E.; Day, G. W.; Franzen, D. L.; Gallawa, R. L. ; and Young, M. Attenuation measurements on multimode optical fibers. Nat. Bur. Stand. (U.S.) Tech.

Note (in preparation).

[21] Green, S. I. 50 picosecond detector pulse monitor. Rev. Sci. Instrum. 47:1083-1085;

1976.

[22] Day, G. W. and Chamberlain, G. E. Attenuation measurements on optical fiber waveguides: an interlaboratory comparison among manufacturers. Nat. Bur. Stand. (U.S.)

NBSIR 79-1608; 1979.

[23] Franzen, Douglas L. ; Day, G. W., Danielson, Bruce L. ; Chamberlain, George E.; and

Kim, Ernest M. Interlaboratory measurement comparison to determine the attenuation

and bandwidth of graded-index optical fibers. Appl. Opt. 15:2412-2419; 1981.

40
NBS-114A (REV. 2-6C)

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET (See instructions)

1. PUBLICATION OR

REPORT NO. NBS-TN-1046

2. Performing Organ. Report No. 3. Publication Date September 1981

4. TITLE AND SUBTITLE

Measurement of Optical Fiber Bandwidth in the Frequency Domain

5. AUTHOR(S)

G. W. Day

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)

Supported in part by the Department of Defense Calibration Coordination Group and

the U.S. Army Electronic Proving Ground, Ft. Huachuca, AZ.

10. SUPPLEMENTARY NOTES

[2J Document describes a computer program; SF-185, FlPS Software Summary, is attached.

11. ABSTRACT (A 200-word or less (actual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here)

The design, evaluation, and performance of a system for determining the

magnitude of the transfer function(hence, bandwidth) of a multimode optical

fiber are presented. The system operates to about 1450 MHz using a tracking

generator/spectrum analyzer combination for narrowband detection. It is

constructed, almost entirely, from commercially available components. The

system is less complex and easier to use than an equivalent time domain system and

the measurement precision is comparable. Background information on time and

frequency domain specifications, fiber bandwidth limitations, and alternate

frequency domain techniques is also presented.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolons)

fiber optics: optical coiranunications ; optical fiber bandwidth; optical fiber

distortion; optical fibers.

13. AVAILABILITY

|7l Unlimited

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[X~1 Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C.

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PRINTED PAGES

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