*«' " *o.\

NBS TECHNICAL NOTE 1019

***CAU O*
NATIONAL BUREAU OF STANDARDS

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

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

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

the Institute for Computer Sciences and Technology.

THE NATIONAL MEASUREMENT LABORATORY provides the national system of

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

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

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

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

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

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

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

calibration services. The Laboratory consists of the following centers:

Absolute Physical Quantities 2 — Radiation Research — Thermodynamics and

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

provides engineering measurement traceability services; develops test methods and proposes

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

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

Applied Mathematics — Electronics and Electrical Engineering2 — Mechanical

Engineering and Process Technology2 — Building Technology — Fire Research —

Consumer Product Technology — Field Methods.

THE INSTITUTE FOR COMPUTER SCIENCES AND TECHNOLOGY conducts

research and provides scientific and technical services to aid Federal agencies in the selection,

acquisition, application, and use of computer technology to improve effectiveness and

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

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

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

The Institute consists of the following centers:

Programming Science and Technology — Computer Systems Engineering.

'Headquarters and Laboratories at Gaithersburg, MD, unless otherwise noted; mailing address Washington, DC 20234.

2 Some divisions within the center are located at Boulder, CO 80303.
Measurement of

Optical Fiber Bandwidth in the Time Domain

Douglas L. Franzen

G.W. Day

NATIONAL BUREAU

OF STANDARDS

LIBRARY

APR i 7 1980

Pot &

no. ion

Electromagnetic Technology Division

National Engineering Laboratory

National Bureau of Standards

Boulder, Colorado 80303

fffi OF c

k,

\ednn\tg\ nrift

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

Luther H. Hodges, Jr., Deputy Secretary

Jordan J. Baruch, Assistant Secretary for Science and Technology

|, NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

Issued February 1980

H^^^^^^^M^H
NATIONAL BUREAU OF STANDARDS TECHNICAL NOTE 1019

Nat. Bur. Stand. (U.S.), Tech. Note 1019, 72 pages (Feb. 1980)

CODEN: NBTNAE

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1980

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

Stock No. 003-003-02161-0 Price $3.75 (Add 25 percent additional for other than U.S. mailing)
CONTENTS

Page

1INTRODUCTION 1 2. BANDWIDTH CONSIDERATIONS IN OPTICAL FIBERS 1 2 .

Pulse Broadening in Optical Fibers 1

2 .The Transfer Function 3 2.3 Important Parameters Affecting Bandwidth Measurements... 5

3DESCRIPTION OF MEASUREMENT SYSTEM 6

3 .

Laser Diode Sources 6

3.1.1 Short-Pulse Performance 6

3.1.2 Near-Field Radiation Patterns 8

3.1.3 Launched Power 13

3.1.4 Spectral Emission 13

3 . 2 Detectors 16

3.2.1 Mounting Configuration and Electrical

Circuit 16

3.2.2 Time Response 22

3.2.3 Linearity 24

3.2.4 Uniformity 24

3 . 3 Launching Conditions 28

3.4 System Architecture 31

4 . DATA TRANSFORMATION AND ANALYSIS 35

4 . 1 General Description 35

4.2 Accuracy Considerations 40

5

SYSTEM PERFORMANCE 42

5 . 1 Precision 42

5 . 2 Dynamic Range 46

5 . 3 Material Dispersion Limits 47

6. RESULTS OF BANDWIDTH RELATED MEASUREMENTS 48

6 . 1 Wavelength Dependence of Bandwidth 48

6 . 2 Mode Mixing 48

6 . 3 Accuracy of Gaussian Predictions 51

6 . 4 Fiber Response Pathologies 51

6.4.1 Pulse Shapes 51

6.4.2 Frequency-Phase Characteristics 55

7CONCLUSIONS 58

8 • APPENDIX 59

9 • REFERENCES 63

in
MEASUREMENT OF OPTICAL FIBER BANDWIDTH

IN THE TIME DOMAIN

Douglas L. Franzen

and

G.W. Day

A system is described for determining optical fiber

bandwidth from time domain information. A measurement gives

the optical fiber transfer function (or frequency response)

relating the output waveform to the input. An analysis is

given of the variables affecting the measurement. This

includes a discussion of such input related topics as launching conditions, mode scramblers, and laser diode

sources; output related topics include a discussion of

optical detectors. Laser diodes are evaluated with respect

to short pulse performance, near field emission, material

dispersion limits, and other spectral behavior like

chirping; detectors are evaluated with respect to time

response, linearity, and uniformity. Overall system

architecture, precision, and dynamic range are discussed. A

number of bandwidth related topics are briefly presented and

typical experimental results given. This includes examples

of: mode mixing via microbending, profile compensation,

profile dispersion, intramodal broadening, material

dispersion constants, relative magnitude-phase behavior, and

Gaussian predictions of frequency response.

Key words: Bandwidth; laser diodes; material dispersion;

mode scramblers; optical detectors; optical fibers; transfer

function.

IV
1. INTRODUCTION

Attenuation and bandwidth are the important parameters used to describe

the propagation characteristics of optical fiber waveguides. Measurement

practices with regard to these parameters continue to evolve as accumulated

practical experience and the efforts of standards groups lead toward

uniformly accepted procedures. This Technical Note is one of a series

intended to describe the present design and capabilities of fiber measurement

systems used at the National Bureau of Standards. These systems are perhaps

representative of current practice in the industry and many of the present

techniques and methods will be relevant to future systems. In this Technical

Note, the emphasis is on the bandwidth measurement of multimode, silica,

telecommunications-type fibers over the 0.8 to 0.9 ym wavelength region.

Much of the discussion is general and can be applied to most optical fiber

bandwidth measurement systems.

2. BANDWIDTH CONSIDERATIONS IN OPTICAL FIBERS

2 . 1 Pulse Broadening in Optical Fibers

Bandwidth is related to the information carrying capacity of a fiber and

includes the various sources of signal distortion. In multimode optical

fibers, differences in mode group velocity result in pulse broadening or spreading; i.e., different modes arrive at the output at different times.

This is commonly called "intermodal" broadening, and in multimode fibers is

often the most significant bandwidth limitation [1]. Due to coupling between

modes, the intermodal broadening of multimode fibers does not necessarily

scale linearily with fiber length. In fact, a square root dependence is

observed in cases of strong mode coupling [2].

Other sources of pulse broadening affect all of the fiber modes

together. This contribution to pulse broadening is commonly called

"intramodal" broadening [1]. While different modes are affected different

amounts, it is often useful to think of an average intramodal term to

describe a given fiber. Intramodal broadening is directly proportional to source linewidth and arises because different spectral components of the

source travel with different velocities. Intramodal broadening results from

both material and waveguide dispersion. Material dispersion is related to

the optical properties of the fiber core material, in particular, to the

second derivative of the refractive index with respect to wavelength.

Waveguide dispersion is related to the physical dimensions of the core, to
the index profile, and to wavelength. Because of the dependence of

intramodal broadening on source linewidth, some authors have chosen the terms

"monochromatic" and "chromatic" to distinguish intermodal and intramodal

effects. Presently there is no standard terminology to describe the various

contributions to pulse broadening. Perhaps this is because in a strict

mathematical sense, the terms cannot be clearly separated.

The concept of rms pulse broadening is useful in describing the time

domain behavior of pulse propagation in optical fibers [1]. The rms duration a, of a waveform p(t) is given by

[:; 1 / P (t)(t-T

Q

)2 at (i)

where p(t) is normalized according to

+«

/ p(t)dt = 1 , (2)

and T is the central time given by

+00 T = / tp(t)dt . (3)

The rms pulse duration of the fiber impulse response, aT

, for a fiber with

input and output waveforms p-^(t) and P2(t) respectively is approximated by

°T

- / o\ - a\ (4)

where a., and o_ are the corresponding rms values for the input and output

waveforms. In practice, rms pulse broadening is difficult to implement

because low level values near the baseline receive a large weighting in

equation (1). The total rms broadening, oT

, for a fiber can also be

expressed in terms of the inter and intramodal contributions by

T intermodal intramodal
where a. , , and a. , . represent the rms broadenings due to the intermodal intramodal

individual components.

When intramodal broadening is dominated by material dispersion, the term

is given by

x d2n t la\ a = — 7 a L (6

)

intramodal c d\^ s

where n is the on-axis refractive index of the core, o is the rms value for

s the spectral lineshape of the source, and L the length of the fiber. In the

0.8 to 0.9 ym wavelength range, the intramodal term is dominated by material

dispersion and, for silica fibers, the waveguide dispersion does not become

important until the wavelength is longer than approximately 1.1 vim. In general the importance of the various contributions to measured fiber

bandwidth depends on the type of fiber, wavelength, and source linewidth.

For laser diode sources and multimode silica fibers in the 0.8 to

0.9 um wavelength region, the intermodal term usually dominates. As the

bandwidth approaches multi-GHz-km values, however, the material dispersion

term becomes more important.

2.2 The Transfer Function

The rms pulse broadening concept was widely used in early fiber work.

Specifications are now generally based on the transfer function. In this

approach, which is analogous to conventional linear network analysis, the

fiber is described by a transfer function, which is assumed to be linear in

optical power; i.e., if p-^(t) is the input optical power as a function of

time averaged over many optical periods, P2(t) is the output similarly

averaged, and P-^(f) and ?2(f) the corresponding Fourier transforms, then the

output is related to the input by

P

2 (f) = H(f)P1 (f) (7)

where H(f) is the transfer function or frequency response in baseband

frequency f describing the fiber. In general, the quantities in eq (7) are

complex, and the transfer function can be written in polar form as
[(f) = M(f)e i<(,(f)

(8)

with M(f) and <)>(f) being real functions of frequency f.

The validity of eq (7) has been examined by others [3], [4].

Theoretical work shows that it is valid as long as the source is sufficiently

incoherent; i.e., if frequency width of P-,(f) is much less than the linewidth

of the source. No actual experiments have been performed to check the range

of validity of eq (7) in this respect. As practical sources like laser

diodes become more coherent [5], the question becomes more than academic.

If eq (7) holds, conventional linear system analysis can be used. In

this work, the bandwidth is determined by measuring M(f), the magnitude of

the transfer function and, <f>(f) the phase response, from ratios of the fast

Fourier transform of output and input waveforms. Alternatively, the

bandwidth could be completely specified by giving the impulse response which

is just the inverse transform of H(f). Algorithms exist to accomplish this

calculation if it were desired.

It is useful to derive relationships assuming Gaussian shapes for the

input pulse and fiber frequency response. We shall see later (Section 6.3)

that this is an acceptable approximation for some fibers. Since the Gaussian

shape is invariant under Fourier transformation, simple relationships

result. If the input p-, (t) and output P2(t) are given by

-4£n2(t/T

1

) 2 Pj_(t) = e (9)

and

-4£n2(t/x ) 2 P 2 (t) = e

A (10)

which are Gaussians having a FDHM (full duration at half maximum)

of t and t„ respectively, then the fiber

with the 3 dB roll-off frequency given by

of t and t„ respectively, then the fiber frequency response is also Gaussian

f D = '-^— (ID

3dB . —

*

7j itz - TX

2 2
where t is the half width broadening of the fiber. Also, the rms pulse

duration for a Gaussian is a simple expression with

a

± = .43 t

1 (13)

and

a

2 = .43 t 2

. (14)

Using eq (13) with eq (11), an expression can be found relating the 3 dB

frequency of the transfer function, f3

t3g# to the rms pulse

broadening, aT

, for the fiber with

f 3dB °T = « 19 • (15)

2 . 3 Important Parameters Affecting Bandwidth Measurements

Precision of fiber bandwidth measurements can be quite good for repeated

measurements with the same system (e.g. section 5.1). However, certain

parameters can vary among systems to cause rather large measurement

discrepancies or offsets. These parameters include launching conditions,

receiving conditions, and source spectral properties. Other factors,

representing the limitation of commercial equipment, such as sampling

oscilloscope time base non-linearity, sampling head aberration, etc. are important, but not as significant as the former problems.

Launching conditions are particularly important, because in multimode

fibers, the modes exhibit differential delay and attenuation. Thus the

output pulse shape depends on the specific modes excited at the input (see

section 3.3). Nearly as significant as the launching conditions are the

receiving conditions which determine how the modes are differentially

detected (see section 3.2.4).

Source spectral properties influence the measured bandwidth via
intramodal broadening (material dispersion). For normally encountered laser

diode linewidths, at wavelengths of 800-900 nm, this does not become

important until the fiber bandwidth exceeds 1 GHz-km (see sections 3.1.4 and

5.3). Measurements on high bandwidth fibers will therefore reflect the

varability commonly encountered in laser diode linewidths.

3. DESCRIPTION OF MEASUREMENT SYSTEM

3.1 Laser Diode Sources

3.1.1 Short-Pulse Performance

Laser diodes are the most commonly used sources for optical fiber

bandwidth measurements. They are relatively inexpensive, compact, and

usually sufficiently incoherent to satisfy the transfer function validity

requirement. Moreover, pulses that are much shorter than the electrical

driving current pulses can be generated at repetition rates useful for

sampling oscilloscopes. This "Q-switching" behavior is still poorly

understood but is believed to be related to a saturable absorption in the

optical cavity of the laser diode.

Gloge and other early workers generated short pulses from GaAs single

hetero junction laser diodes by using mercury wetted reed relays to switch

charged transmission lines [6, 7]. In this manner, pulses of 100 ps FDHM were produced at repetition rates of a few hundred Hz. Transistors operating

in the avalanche mode can also be used as fast switches [8,9,10]. Andrews

describes a useful avalanche transistor circuit for switching a charged

transmission line to produce 110 ps FDHM pulses from a GaAs laser diode

[9]. Dannwolf, et al . , report a similar circuit with a ceramic chip

capacitor as the energy storage element that produced pulses of 190 ps FDHM

[10].

Compared to mercury wetted relays, avalanche transistors have the

advantage of high repetition rate and external trigger capability. In the

present study avalanche transistor switching circuits using both silver mica

capacitors and transmission lines as the energy storage element were

examined. Transmission lines were found to be preferable producing slightly

narrower pulses with less tendency toward multiple pulsing.

The circuit, using five miniature coaxial cables (RG-174U), is shown in

figure 3-1. When a single cable is switched across a low impedance, the

current delivered is approximately AV/R where' R is the cable characteristic

impedance and AV the voltage swing available from the avalanche transistor.
coCM :>£ + IT3

M—oc a

lo

aoo >o

c\i 6o

0>D5

(0 +;O>—

if+ aoo ao

in "V

CO

IT) 3< C

•HPoc3

•i—i oup

l-l

cr. cH

CO

CO oc

(0 <<

(0 o

enc

•H

U) iH3a

>-i om

4-> • H 02 3 0) U "C ^

•H -H

cc TTTTT 3

-H

Pn
With an R of 50 ohms, a AV of 150 volts, the current delivered is 3 amps.

Thus, five cables are necessary to supply up to 15 amps to a diode. Two

different avalanche transistors were used, a 2N3700 for high AV and a 2N3904

for lower AV. The only other circuit element requiring explanation is the

diode shunting the laser diode which is intended to protect it from excessive

reverse bias. The whole circuit, except for cables, is packaged in a 3x3x6 cm instrument box.

The repetition rate was arbitrarily limited to 1 kHz. While these

circuits may be operated at several tens of kHz, the lower repetition rate was chosen to guard against diode aging. Several diodes have been in routine

operation for over one year and no circuit failures or changes in pulse

shape, threshold, jitter, wavelength, etc., have been observed.

The main factor affecting short-pulse performance is the choice of a specific laser diode; significant differences are observed among diodes of

the same type and manufacturer. The objective in short pulsing is to drive

the diodes sufficiently far above threshold without generating multiple

pulses. Shortest pulses and least tendency toward multiple pulsing was achieved by operating near the maximum voltage for a particular transistor

and with cable lengths short enough to obtain a single pulse.

Four diodes selected from a large number of RCA C30012 (GaAlAs), Laser

Diode LA-63 (GaAlAs), and RCA SG2001 (GaAs) single hetero junction diodes [11]

were chosen to cover the 0.8 to 0.9 ;im wavelength region. These diodes had

wavelengths of 803, 824, 866, and 902 nra. A typical pulse shape is shown in figure 3-2 and consists of a nearly

Gaussian-shaped main pulse of nominally 250 ps FDHM followed by a tail at 5-

10% of the peak level. Table I gives the tabulated performance for all of

the diodes. The frequency spectrum of the pulse is of prime importance;

figure 3-3 shows a typical Fourier transform of one of the diode pulses. In

obtaining this data, a delay line was used with the sampling oscilloscope;

consequently, the detected pulses were broadened to about 330 ps FDHM. The 3 dB frequency is approximately 1 GHz. Observed pulse shapes do contain some broadening from the detector (section 3.2) and in some cases are probably

less than 200 ps in actual duration.

It should be mentioned that the above temporal behavior and the spectral

emission reported in section 3.1.4 were determined from the power exiting a short length of fiber and are therefore representative of actual system use.

3.1.2 Near-Field Radiation Patterns

The emission region in laser diodes consists of a narrow strip at the PN

junction interface. In the case of C30012 and LA-63 GaAlAs diodes the

8
(a)

(b)

Figure 3-2. Typical pulses from single hetero junction laser diodes, 200 ps

per division, (a) 824 nm GaAlAs diode, (b) 902 nm GaAs diode.
TABLE I

Laser Diode Pulse Durations

Diode Pulse Duration, ps (FDHM)

803 ran, GaAlAs 250

824 nm, GaAlAs 240

866 nm, GaAlAs 290

902 nm, GaAs 250

10
ANSFORM OF INPuI. MAGNITUDE [N OB. FPEOuENC* IN Cm?

^.00 .25 .50 .75 1.00 1.25 1.50 1.75 2.00 2.25 2.50

Figure 3-3. Frequency content for a typical laser diode pulse, 902 run GaAs,

3 dB frequency approximately 1 GHz.

11
(a)

(b)

Figure 3-4. Near-field emission characteristics: (a) 803 run GaAlAs diode

operating in short-pulse mode; quantitative curve relates to

intensity along scan bar (oriented along junction image). Note

low level spontaneous emission along entire junction length with

laser emission over a small segment; (b) Same as (a) but with

inreased drive current causing other junction segments to exceed

threshold.

12
junction dimension is 150 x 2 pm while for SG2001 GaAs diodes it is slightly

smaller (75 x 2 ym) . We find that the emission characteristics vary among

diodes and cannot be accurately predicted. In the short-pulse mode the

emission generally occurs over only a small segment of the junction. This is

because the diodes are operated close to threshold and only the lowest loss

regions of the junction oscillate. Typical behavior is given in figure 3-4

for the 803 nm diode. Figure 3-4(a) is an image of the junction showing low

level spontaneous emission along the entire 150 vim junction length. Laser

emission is restricted to a 16 pm segment of the total length. However, if

the diode is driven further above threshold, other parts of the junction will

eventually oscillate, as shown in figure 3-4(b). All of the diodes exhibit

this behavior except for the 824 nm diode which oscillates over a 40 pm length of the junction immediately from threshold.

3.1.3 Launched Power

The amount of power launched into a fiber is important since it affects

signal to noise ratio. Power launched into a fiber with the diodes operating

in the short-pulse mode was determined using two different types of

calibrated detectors. One detector responded to individual pulse energy (in

the 10 pico joule range) while the other measured total average power (10

nanowatt range). Measurements on the 825 nm diode differed by less than

10%. Peak power launched into a short section of step index fiber

(55 ym dia. core, .25 NA) for the different diodes is tabulated in Table II and is between 30 and 300 mW.

3.1.4 Spectral Emission

Spectral emission properties of the source are important because they

determine the extent of intramodal pulse broadening in the fiber. The output

of laser diode source consists of both spontaneous emission (linewidth about

30 nm) and stimulated emission (linewidth about 1 to 2 nm) . For measurement

purposes, it is important that the spectrally narrow stimulated emission

dominate. Figure 3-5 (a) shows the pulse shape which is coupled into a short

length of fiber with the diode operated just below threshold. The pulse

width is about 5 ns and the cusp represents the short pulse starting to form

from stimulated emission. Above threshold, figure 3-5 (b) the short pulse is

fully produced with the peak power exceeding the spontaneous emission by a factor of 70 for this particular diode. Contributions from spontaneous

emission were judged to be insignificant for all diodes tested.

Spectral lineshapes were determined using time resolved measurements.

This was necessary to determine the extent of "chirping," a shift of

13
TABLE II

Peak Power From Laser Diodes Exiting a Short Length

of Step Index Fiber with Mode-Scrambler

Diode Peak Power, mW

803 ran, GaAlAs 150

824 nm, GaAlAs 300

866 nm, GaAlAs 65

902 nm, GaAs 35

14
5

i 1

: "":

-

•

-i

:

i !

/

y \

i ; j/^ \_

j .v^ : <—

i

:

-

|

1 1 ] 1 H

(a)

11

^ES?5

I '9

IIItil

g

JWH IBS !

(b)

Figure 3-5. (a) Spontaneous emission coupled into fiber (laser diode below

threshold), 5 ns per division. (b) Laser emission coupled into

fiber (laser diode above threshold); 200 ps per division; peak

power in (b) is 70 times that in (a).

15
instantaneous spectral emission with time that can affect bandwidth

measurements [12]. Radiation from the diodes was coupled into a short length

of fiber containing a mode scrambler (section 3.3); after relaunching into a second fiber, the radiation was passed through a monochrometer and onto a detector (BPW-28, section 3.2). By triggering the sampling oscilloscope

externally with a signal derived from the electrical trigger to the laser

diode, a dc signal was obtained that was proportional to optical power at a selected point in time. Optical power was recorded on a strip chart recorder

while the monochrometer was scanned. Line shapes were recorded on the

leading half-height of the pulse and on the trailing half height. Only the

824 nm diode showed a significant shift of line center during the pulse,

figure 3-6. During the pulse, the peak of the spectral emission moved 0.7 nm toward longer wavelength. This rate of chirping, 0.0027 nm/ps, is not very

different from the 0.0018 nm/ps previously reported for sub-nanosecond pulses

from a particular single hetero junction laser diode [12]. The 824 nm diode

differed from the rest in that it was driven higher above threshold with more

of the junction stripe oscillating. This observation seems to be consistent

with the evidence in [12] that chirping results from filaments in the

junction stripe having different wavelengths and time delays.

The spectral shapes from the four diodes showed different behavior; some were smooth Gaussian-like while others consisted of several partially

resolved cavity modes. An example of the latter behavior was the 902 nm

diode, figure 3-7, where several modes of the diode were partially resolved

by the monochrometer. Figure 3-7 was obtained using a detector having a 5 ns FDHM impulse response and therefore represents pulse energy.

Lineshapes for a given diode with a fixed alignment showed good

reproducibility. However, fine structure changes within a nominal shape

could be observed with different alignments of the diode. Nominal behavior

for the four diodes is summarized in Table III.

3.2 Detectors

3.2.1 Mounting Configuration and Electrical Circuit

A number of different detectors are available for optical fiber

bandwidth measurements. In the 800-900 nm region these principally include

silicon PIN and APD (avalanche photo diode) devices.

A detector which performed satisfactorily was an AEG Telefunken BPW-28,

silicon avalanche photodiode [11]. This detector was mounted in a structure

similar to that described by Green [13], figure 3-8. In this structure the

photodiode is mounted directly on the end of a coaxial transmission line

16
-*- .7 nm

tr t + 250 ps

829

T

824

WAVELENGTH, nm

819

CHIRPING RATE, +.0028 £01

ps

Figure 3-6. Time resolved spectral emission showing chirping behavior of

824 nm GaAlAs diode under normal short-pulse conditions. Dashed

curve is a spectrum at time t at the front half height while

solid curve is a spectrum 250 ps later at back half height ''note wavelength increase to left).

17
909 904 899

WAVELENGTH, ntn

894

Figure 3-7. Time integrated spectral emission from 902 nm GaAs diode showing

partially resolved modes.

18
TABLE III

Diode

803 nm, GaAlAs

824 nm, GaAlAs*

866 nm, GaAlAs

902 nm, GaAs

Laser Diode Spectral Lineshapes

Linewidth, nm (FWHM)

1.2

1.6

.9

1.5

General Features

Smooth with some mode structure

Smooth, Gaussian like

Two completely

resolved cavity modes

Several partially

resolved cavity modes

* Only diode which exhibited measurable chirping.

19
-3.2 cm- 1

MODIFIED

SMA

CONNECTORS

ss

kmm/A

1"

y, 1 . 6 cm

.06 mm

MYLAR

DIELECTRIC

Figure 3-8. Mounting structure for optical detector, diode indicated by part

D.

20
DETECTOR

MOUNT SMA

CONNECTOR

17m COAXIAL

CABLE

Ghvvv

50Kfi r

3dB

COAXIAL ATTENUATOR

c

2

...=

SAMPLING

OSCILLOSCOPE

Figure 3-9. Detector electrical schematic, parts integral to mounting

structure are enclosed by dashed line.

21
which is also coaxial with a disc capacitor. The low inductance disc

capacitor, C-^, is formed by pieces A and B which are separated by a thin

dielectric (.06 mm mylar). This capacitor is charged through 17 m of RG-58AU

coaxial cable. Any reflections from the bias supply are therefore delayed by

160 ns and do not appear in the time window of interest. Figure 3-9 shows

the complete electrical schematic. The cable is charged through resistor R^

and ammeter M. R-^ is typically 50 kft and limits the current in case of an accidental short circuit. The value of Ri and the capacitance of the cable

are low enough so the cable is completely charged between pulses. Ammeter M

measures the average current being supplied to the diode and is a useful

monitor when approaching avalanche breakdown. Under typical operating bias,

however, the average current is too low to be measured by the meter.

The detector load is a 3 dB SMA attenuator followed by an SMA series

blocking capacitor. This attenuator is permanently attached to the detector

and provides a constant 50 ft load; thus, sensitive electronics can be

reconnected to the detector without danger from excessive dc voltages which

could occur under open circuit conditions. Blocking capacitor C~ assures no dc voltage appears aross the sampling oscilloscope. C2 is sufficiently large

so normally encountered pulse shapes are not distorted.

3.2.2 Time Response

Ideally, detector time response should be fast compared to source pulse

duration. Additional pulse broadening due to the detector will limit the

upper bandwidth capability of the measuring system. Since the Fourier

transform of the input pulse is the important quantity, the total pulse shape

including half-duration, tail, ringing, etc., is important. It is of little

use to obtain a small half-duration and yet have significant ringing. A

well-matched detector is better for bandwidth measurements, and also for

differential mode delay measurements where reflections could be confused with

mode delay information.

A BPW-28 detector in the previously described mount was evaluated using

a 1.06 ym, CW, modelocked Nd:YAG laser [14]. This laser produced pulses of

100 ps FDHM as measured with a fast detector based on a Shottky barrierswitching diode [13]. Although this detector is fast, it was not sensitive

enough to use for fiber measurements with laser diode sources. Response of

the BPW-28 to the modelocked pulses is shown in figure 3-10. The inferred

impulse response is less than 200 ps FDHM. Notice that there is no evidence

of a significant tail or ringing. These measurements were made at the 140-V

bias voltage used in normal operation. The value of bias was selected by

increasing the voltage until the pulse exhibited additional broadening; this

22
Figure 3-10. Response of BPW-28 detector to 100 ps FDHM pulse from CW

modelocked, 1.06 ym, Nd:YAG laser, 200 ps per division.

23
yielded an avalanche gain of less than 10. This is substantially below the

maximum available gain and is consistent with the gain-bandwidth trade-off in

APD detectors

.

3.2.3 Linearity

To obtain the frequency response of optical fibers, the output pulse

must be compared to the input pulse. This requires that all components be

linear, and in particular that the optical power levels involved not saturate

the detector. Since the power of the input pulse is substantially higher

than that of the output, a variable optical attenuator is used in front of

the detector to control peak power. Thus, the detector is not used over a very large range of powers. It is also positioned in the exit optics so its

aperture is filled; therefore, the power density (irradiance) on the detector

is minimized.

The detector was tested for linearity at the highest levels used in the

measurement system (8 ma peak diode current). This was done by varying the

optical power by a factor of more than three and comparing the detected pulse

shapes at the different powers. The whole pulse shape is checked for

saturation. Figure 3-11 shows the normalized comparison; each pulse was digitally acquired from the sampling oscilloscope with each point being the

analogue average of several hundred pulses. As indicated, there is no appreciable difference between the pulses at the two power levels. Linearity

of this particular detector was thus judged to be adequate.

3.2.4 Uniformity

The detector response should be uniform across the sensitive area. If

this is not the case, then preferential mode detection can take place.

Concern is usually expressed over launching conditions to a fiber; however,

in the absence of much mode coupling, the receiving conditions are just as important. In fact, the spatial information at the output using an overfilled input is often used to make differential mode delay measurements

[15]. Recent studies have shown that the responses of several commercial APD

detectors are not necessarily uniform [16]. Consequently, the detector

uniformity was investigated.

The BPW-28 detector has a circular sensitive area . 2 mm in diameter

surrounded by a ring electrode, figure 3-12 (a). A two-dimensional raster scan of dc responsivity was made at .63 pm, figure 3-12(b). The response was constant to ± 5% across the sensitive area. A low-level annulus was also

present and corresponds to the area outside of the ring electrode.

24
V

s-4

o

a.

o ro

Q- +

o 4 A

o

C/3 >

Q

CC

LU

O

CM

+Jc

<DM

(1) H-l m

•H

TD

5

+J

-P

(0

to

CO H

3 • a c

c

•«. H

>i+j

+j •iH •H to Vja a

CD i C u

•rl cu H a

3 u U5 p T

u

CD -P Di

(1) T s

IH £

to c to H

H CD +) >

(0 CD u r-( •H

U-l >-l H CU

(-1 s

0) > a

I CO

I-o

—Ioo CD —r

CM 4

O

0)

cn •H

uaMOd asnnd Q3znviAiaoN

25
(a)

(b)

Figure 3-12 (a) BPW-28 detector, 200 pm sensitive area inside ring

electrode. (b) Detector uniformity, two dimensional d.c

at .63 p.m wavelength.

scan

26
RESPONSE

-100 +100

-« RADIAL POSITION, ym *

Figure 3-13. Detector uniformity, detected peak pulse power as a function of

radial position.

27
Responsivity was also measured at high frequency. To accomplish this, a 300 ps FDHM pulse from a laser diode was focused to a 25 urn spot. This spot

was scanned in one dimension across the detector and the peak power was obtained as a function of radial position, figure 3-13. As indicated, the

response was flat to better than 5%. Other parts of the detector were

manually scanned and similar results obtained. Detector uniformity was judged to be adequate for the present system.

3 . 3 Launching Conditions

Launching conditions determine the initial modes excited in a fiber.

Because of differential mode attenuation and delay, the results of bandwidth

measurements depend upon launching conditions. As was discussed in section

3.1.2, the emission pattern from short pulse, single hetero junction laser

diodes cannot be accurately predicted and would give non-quantifiable

launching conditions. To circumvent this problem, a mode scrambler is used

in a 2m section of step index fiber and the radiation relaunched to provide a well-defined launching condition [17].

The mode scrambler, which is conceptually similar to previous serpentine

designs [18], consists of 7 nylon posts 1 cm in diameter, placed on 1.3 cm

centers, figure 3-14(a). Three of the posts are positioned on a translation

stage and can be moved into alignment with the rest. In this manner a fiber

can be placed in the mode scrambler without breaking since the bending radius

is never less than that of the posts.

Without the mode scrambler, various near-field patterns could be

obtained from the step fiber depending upon diode alignment. Patterns ranged

from on-axis maximums to annular shapes. With the mode scrambler, the nearfield patterns were more uniform and not as sensitive to diode alignment.

With proper alignment, the near-field emission could be made very uniform,

figure 3-14(b). The resulting emission from the step fiber (55 ym, .25 NA) was imaged 1:1 by two microscope objectives and relaunched into the fiber

under test with peak transmitted power being the alignment criterion. For

this alignment , the image of the step fiber is not necessarily exactly at the

input end of the test fiber; however, for overfilled launching conditions,

this does not represent a significant uncertainty. This launching overfills

the modes of most fibers and results in a bandwidth which is lower than that

obtained with small spot excitation. The advantages of the overfilled

launching condition are: 1) it can be reproduced in other laboratories and

2) it is the starting point for other types of launching conditions which

make use of equilibrium mode simulators on the . fiber under test.

A comparison was made using two different launching conditions on the

28
FIBER

1 .3 cm

NYLON POST

1 cm DIA.

(a)

O)

Figure 3-14. (a) Serpentine mode scrambler used in a 2 m section of step

index fiber (55 ym core, .25 NA) . (b) Near-field emission from

step fiber with mode scrambler adjusted to give most uniform

pattern—curve relates to intensity along scan bar.

29
Figure 3-15 Superimposed output pulses from 1.3 km, graded index fiber

using the same source (300 ps FDHM) but with different

launching conditions, 500 ps per division. Narrow pulse is

from directly imaging laser diode onto fiber whereas broad

pulse results from use of the mode scrambler - step fiber.

both cases peak detected power was used as the alignment

criteria.

In

30
same fiber with peak transmitted power being the alignment criteria. In one

case the laser diode output was directly imaged onto the fiber while in the

other it was first passed through the mode scrambler-step fiber filter. A

superposition of the output pulses, figure 3-15, shows that the small spot

from the laser diode excites fewer modes (narrower pulse) than the mode

scrambler-step fiber launch which fills the mode volume of the fiber. For

the two different launching conditions, the fiber half-duration broadening

differs by a factor of 1.8.

An oil filled capillary is utilized to hold fibers, figure 3-16, [19].

The capillary bore is 150 um in diameter and readily accepts standard

125 pm O.D. fibers. One end of the capillary is flared to accept the fiber

while the other end has an oil reservoir covered by a thin microscope cover

slip. This holder has the following advantages: 1) it acts as a cladding

mode stripper, 2) the oil index matches the fiber end, and 3) it aligns the

fiber stabily in the system with little readjustment required when fibers are changed. The main disadvantage is the possible accumulation of dirt and

other foreign material in the capillary. For this reason the holder is

demountable and the capillary can be cleaned by a thin wire.

3 .4 System Architecture

The previously described components are assembled in the following

manner, figure 3-17. Multiple laser diodes are mounted on a translation

stage enabling different wavelengths to be easily launched into the step

fiber-mode scrambler. The output from the step fiber is collimated, passed

through an aperture wheel, and relaunched into the fiber under test.

A bandwidth measurement on a fiber is made by aligning the fiber for

peak transmitted power and then acquiring the output waveform. Next, without

disturbing the launching, the fiber is cleaved approximately 1.5 m from the

input end and the waveform exiting the short length measured. This waveform

is used as the input to the fiber under test.

At the receiving end, the output is collimated by a X10, .25 NA

microscope objective. At this point neutral density filters are inserted to

attenuate the power until it is within the linear range of the detector.

Also, to minimize detector dynamic range requirements, the peak power from

the short length is made equal to the power detected from the full length.

Finally, the collimated output is focused on the detector by a X5 microscope

objective.

Waveforms are acquired using a sampling oscilloscope. A number of

choices, figure 3-18, are available for triggering the oscilloscope. First,

figure 3-18(a), a digital time delay generator can be used to produce an

31
RING

SEAL

OIL FILL &

^ 2^^^^^^^

^^SSSSSSSSSSSSSSSi Hfc . 1 mm

MICROSCOPE

COVER SLIP

PRECISION BORE

GLASS CAPILLARY

Figure 3-16. Oil filled, precision 150 urn bore, capillary fiber holder.

32
—'=11

5 => == i uj lu 5 ^Q E

0) +1

03>1

03

7T, O

a

Oi LU I—I oi 1 , o

Ll_ > t—

en

T ^ <LU o Z <1 \

4Jc

QJE

<Du3

03(0

0)E5

•Hcc

to

g-iocoH

+J

<C

»-i 3

tP •Hmcou

QO

LU

—i 2: 0"tt iQlj Q Q. Oi <C q. LU I —>

CQ <-J CC — I—

Ll_ oS t/1

V

Q

CMO

(1)

1-1 3

Ci •H

fa

33
_l CD 1 1 "^ Z

h- "j" Q oi s: ° ' ' uj <c A >co 1 —i<a w>

>0 1— i—i O uj ! a. ll. r^ Q t ° 11 hUJ A Q

_l CD

<C Z c_> —

«—1 _J

,i HO.

en 2:

lu <: > co

r—

LUQ

q: CD

UJ i—i CD —1 CD Q.

I— ooO

CD , C_> -CD r-1 CO

—" ^=

O

"^ CO

/<- 11

TRIGGER PULSE GENERATOR C£ LU

UJ

Q

COO

_lQ i

q: LlJ uj O

CO

O

<r i—

i CJ

2uj£ I—1 -^- I CD^LU Q

<: 2: o •—

i—i i I— D.

dc s: cj> lu <: CO > to o 1

TRIGGER PULSE GENERATOR CC LU l . l O

OOO

<t i-i _1 Q

CDE

CD cn

u ,c CD

rd o CH

rH

<u rH •

•• c 'C <H

CD •H -P CD rd &>H (0 ,c V

rH CD •H

u >1 d) rd >H

en rd c « 4J

l-l (1)

U

H CD cn CD rH >0 • rH •H >1 rH CD U i-H rd CD

«) (0H P cd

u 0) P rH •H "O •H rd

m

u

n c -p CD 0) •H u E In 0) H <u •H 0) C

Q.1-H

p cn •H e 0) CTlrH

(0

H •H

0) *»-^

m Jh

T

,Q P P •H

tn *^ •H rH c Cn <D H % •H > cn h >H TS •H

CD cn <D tn-P ."-* cn rH IT (0 Tl 0) •H H 1-1 »

u ,C M CD X

+J

C T5 (1) cn <D c rC

rH Cn rd cn

p o +J rd

M-t >i <D •H U,

rd c ^

cn

H •rH -H rH

a) 0)H j: rd o •0 X U

•H >i 0) •H

0) rd P £ E H CD a V •H a) O P >0 u

CD

a

rH

H rH

p

,o rd ft) cn •H +J Vh

U) •iH •H c 0) 03 Cnp CD «W H &£ QJ

CL- e £ U

00

rHI co

CDU3

Cn H

34
electrical trigger pulse which just precedes the arrival of the optical

output pulse. For this method to be successful, the delayed trigger signal

must have low jitter and what jitter remains should be random so it can be

averaged out. Also, the range of available delays should cover all fiber

length encountered. Second, figure 3-l8(b), an electrical delay line can be

used at the output to delay the detected electrical signal while providing an advanced trigger signal. The delay line should have sufficient bandwidth to

not appreciably distort the pulse. Third, figure 3-18(c), the optical output

can be divided to provide an advanced trigger signal and then delayed

optically in another fiber. This has the advantage of high bandwidth in the

delay line but care must be taken to prevent the loss of modes on relaunching. Fourth, figure 3-18(d), the optical input can be divided to

trigger a digital time delay generator. This configuration is useful when a digital delay is required but, the laser diode exhibits excessive trigger

jitter. In the present system we utilize figure 3-18 (b) with a Tektronix

7M11 delay line. To observe differential mode delay, however, figure 3-18 (b)

is not applicable and the method of figure 3-18 (a) is used.

To digitally record a waveform, the sampling oscilloscope time base is

swept by a ramp voltage while two four and one-half digit, digital voltmeters

read the vertical out signal and the time base ramp. Analogue averaging is

used to increase the signal to noise ratio and is accomplished with a capacitor in parallel with the vertical output to give a time constant of

.2 s. Consequently, a vertical data point is the result of several hundred

pulses averaged. A number of choices involving different levels of

automation are available for storing data points and taking fast Fourier

transforms (FFT). A description of these is beyond the intended scope of

this report [20,21]. The system, in its present state, uses a large general

purpose computer to compute the transforms.

4. DATA TRANSFORMATION AND ANALYSIS

4. 1 General Description

Data from the acquisition system appears in the form of a 2xN integer

array identified as Y(I), T(I) where Y(I) is related to the pulse amplitude

and T(I) is related to time. In most cases the array is 256 elements long, a choice resulting from several compromises. Y(I) varies from about -7200 to

+7200 and T(I) from +0000 to +9500 so that, in each case, the resolution in

sampling the waveform is about 1:10 . Two such arrays result from a single measurement, one representing the

35
IDENTIFICATION

NO. POINTS

TIME SCALE

PLOT SCALING START

1

INPUT

ARRAY

SCALE INPUT

WAVEFORM & STORE

PLOT INPUT

WAVEFORM

OUTPUT

ARRAY

SCALE OUTPUT

WAVEFORM

STORE

PLOT OUTPUT

WAVEFORM

I

X FORM INPUT

SCALE & STORE

PLOT MAGNITUDE

OF X FORM

J

X FORM OUTPUT

SCALE & STORE

PLOT MAGNITUDE

OF X FORM

\

'

X FORM OUTPUT

^X FORM INPUT

PLOT MAGNITUDE

& PHASE OF

RATIO

1

LIST MAGNITUDE

& PHASE VS FREQUENCY

Figure 4-1 Flow diagram showing various computer program outputs

36
INPUT PULSE. TIME IN N5

(a)

0.00 5.00 10.00 15.00 20.00 25.00 30.00 35.00 40.00 45.00 50.00

OUTPUT PULSE. TIME IN NS

(b)

O.OO 6.00 10.00 15. OO 20.00 25.00 SO. 00 35. OO 40. OO 45.00 50.00

Figure 4-2. Typical set of computer generated output for a fiber bandwidth

measurement: (a) input pulse (time), (b) output pulse (time),

(c) Fourier transform of input pulse, (d) Fourier transform of

output pulse, (e) magnitude of fiber transfer function, and (f)

phase of fiber transfer function.

37
TRANSFORM OF INPUT, MAGNITUDE IN D6 . FREQUENCY IN GHZ CI 16A/B

0.00 . 10 .20 .30 .50 .60 .70 .90 I .00

(c)

TRANSFORM OF OUTPUT. MAGNITUDE IN DB . FREQUENCY IN GHZ GM6A/6

0.00 .10 .20 .30 .10 .SO .60 .70 .80 .90 I . 00

(d)

38
MAGNITUDE OF FIBER TRANSFER FUNCTION IN OB. FREQUENCE IN G*Z G116A/B

-fl.5000

-6.0OO0

-9.0OO0

I0.6OOO

I2.0OO0

.20 .30 .50 .60 .70 .80 .90 I .00

(e)

WASE OF FIBER TRANSFER FUNCTION. FREQUENC* IN GhZ CM6A/B

.63

- .00

-.31

-.63

.94

1.26

O.OO .10 .20 .30 .10 .50 .60 .70 .80 .90 I.OO

(f)

39
input waveform and the other the output waveform. These arrays are stored in

the mass storage of a large, multipurpose computer, for subsequent

processing.

The analysis program provides for the scaling of both waveforms, the

Fourier transformation of each, and the computation of the complex ratio of

the transform of the output to the transform of the input (the transfer

function). Figure 4.1 indicates the order in which these functions are carried out. Typical outputs for one fiber are shown in figure 4.2.

The Fast Fourier Transform (FFT) subroutine is a general purpose routine

locally available. It is distinguished by its general versatility and by the

fact that the input array need not be an integer power of two long, although

that restriction greatly increases the efficiency.

4 . 2 Accuracy Considerations

The principal factor in assuring accurate computation of the transfer

function is the choice of appropriate sampling parameters, specifically N,

the number of samples, and T, the time interval between samples. The time

interval must be chosen so that the transform is effectively zero at

frequencies greater than f = 1/2T. Errors resulting from a failure to

satisfy this condition are known as aliasing. Resolution in the frequency

domain, that is, the frequency interval betweens points is given

by Af = l/NT. Maintaining good resolution without introducing significant

aliasing errors or requiring a particularly large number of samples thus

requires certain compromises.

Because the system in its present configuration uses slow data

aquisition equipment, the number of samples is generally limited to no more than 256. These samples are spaced across about 95% of the sweeptime of the

sampling oscilloscope. Most fibers measured with this system have a 3 dB

bandwidth below 800 MHz. In this case a 50 ns sweep is used so that

T = 0.186 ns and Af = 21 MHz. For higher bandwidth fibers, a 20 ns or 10 ns sweep is used.

The greatest problem with aliasing errors occurs when transforming the

input waveform using the longest sweep time. Figure 4.3(a) shows the

transform of a typical input pulse with the frequency range expanded to

f=l/2T or 2.7 GHz. At this frequency the magnitude is approximately 10 dB

below its value at low frequency.

To estimate the magnitude of the aliasing error in this waveform, a sampled Gaussian pulse having a similar 10 dB bandwidth was generated and

transformed. Figure 4.3(b) shows the magnitude of the transform and the

40
DIFFERENCE,

m dB

/(

^\

ap aaniiNOvw asNOdsga £fS

<r

<? <

^^1> $1 5 I a >!

a>

a

a> c 5 -p a> (0

in «-i

d •l-l 0) oID fH <U CD »c C k •p <d . IH (0 -P o u C D

in o m -H

a

S-i 'C

O E

tT> ^ S-i C CD O o

-H .H

in trp >1 D C CD £. ^

rH E CTK-I

to co <e •H rC K (0 U) U O O -rH •H

I-l <D » -P r- rfl ,c to >,

-P J-i ,-1 cm in CD (0 Cn-P C c CD IT) -P C -H E > f0 - P -H (0 U -p D -P ,C ifl fC a id E C X

0) -H (0 en cu to -P -h c H (0 ill •H <C D W CO ,-H C

O,

D 0. rt) ^ (0 E

+J £5

O (0 E-i

D ^- CO [n a, to Pm c CD -H . IW t c

E-< (0 Q)

M-l fo CO <D fc E-i s

fo 0) P E +J fo ,c o

l-i c -P X! -H 03 «H -i-l >C 0) •

co a, c u E c a> HJ c Vj

ffl VD -P 0)O ^m u *-* u m

En CM

C It! CI) co -H >-^ IW c ^HJ 1!) 4-4 (0

(0C >-i 10 -H

M

^- cfl

O (0 <C -p I

0)^1D

Cn •H

41
difference between that magnitude and an analytically computed transform. At

frequencies below about 1250 MHz the difference is less than about 0.1 dB.

At frequencies below 800 MHz where these sampling parameters are used, the

maximum difference is 0.045 dB, which is within the observed precision of the

complete system. Some caution in interpreting these numbers is necessary,

however, because the shape of the transform is different in the two cases. One approach to decreasing the aliasing error without decreasing the

resolution or requiring excessive measurement time is to artificially extend

the baseline. One might, for example, take 256 samples over about 10 ns and

artificially extend the data to include 1024 points over 40 ns . The aliasing

error would be substantially reduced while the resolution would be only about

20% worse. This aproach has not yet been used in this system because of

uncertainty in the procedure for creating the simulated data, in particular,

because the low frequency part of the transform is strongly dependent on the

data in the baseline.

Another concern is the manner in which the transforms are scaled. The zero frequency point has no meaning in this system since dc levels in the

data are arbitrary. One choice is to reference the entire transform to the

first point in the frequency domain. Thus, it is important to maintain good

frequency resolution, even for high bandwidth fibers because any variation

between zero frequency and the first point is lost. Furthermore, the

transform does not necessarily decrease monotonically with frequency, which

sometimes can be the case for the first few points in the low frequency

range; consequently, the apparent value of the transform can have values

greater than unity. Presently, this system normalizes the transforms to the

largest value and the interpretation of such effects is left to the user.

SYSTEM PERFORMANCE

5.1 Precision

Precision refers to the reproducibility of a measurement. A precision

statement, however, says nothing about "systematic" error which is the offset

of the average of a large number of measurements from the true value. In the case of optical fibers it is difficult to assign systematic errors since

launching condition have yet to be standardized.

Precision is defined here as the reproducibility of the frequency

response curve (magnitude). To determine this, measurements were repeated

five times on the same fiber. For each measurement the fiber was recleaved

and the necessary parts of the measurement system realigned. Three

42
FIBER A

CO

-aul

COzoQ.CO

LUCE

.2 .4 FREQUENCY, GHz *-

.8

Figure 5-1. System precision, 824 nm diode; error bars are ±1 standard

deviation (2 standard deviations in length) for five repeated

bandwidth measurements. Fiber A: graded index, 50 pm core,

125 pm O.D., .25 NA, plastic jacketed, 6.0 dB/km attenuation at

850 nm, 1.3 km length.

43
-O—i

-3 —

00 K -D -D

LU00zO

0LCO

LUcc

-9 —

-12 —

FIBER B

.8 1.2 1.6

FREQUENCY, GHz 1

2.0 2.4

Figure 5-2. System precision, 902 nm diode; error bars are ±1 standard

deviation (2 standard deviations in length) for five repeated

bandwidth measurements. Fiber B: graded index, 50 pm core,

125 umO.D., .25 NA plastic jacketed, 4.6 dB/km attenuation at

850 nm, 1 . 1 km length.

44
FIBER C

.09 dB

a

LUCO

Z!O

a.CO

LU

cc

-4-

.14dB

Figure 5-3. System precision, 824 nm diode; error bars are ±1 standard

deviation (2 standard deviations in length) for five repeated

bandwidth measurements. Fiber C: graded index, 62 urn core,

125 uim O.D., .25 NA, light buffer, 4.0 dB/km attenuation at 850

nm, 1 km length.

45
multimode, graded index, telecommunications type fibers with a range of

bandwidths were chosen for this set of measurements and represent fiber from

two different manufacturers. Also, measurements were made using two

different laser diodes. The results therefore represent typical system

performance.

Results are given in figures 5-1, 5-2, and 5-3 with two-dimensional

error bars representing an estimate of ± one standard deviation determined

from the five measurements. To summarize the results, error bars are displayed at the 3, 6, 9, and 12 dB points on the frequency response curve. These frequencies were determined by interpolating between discrete FFT

points and this precision is indicated by the horizontal error bars. The

closest discrete frequencies to the average 3, 6, 9, and 12 dB frequencies were used to determine the precision in the vertical axis. In all cases, a 256 point FFT was used and the time base chosen so that the separation

between frequency points was 21 MHz for fiber A and 54 MHz for fibers B and

C. In plotting the figures, a smooth curve was drawn through the points.

Some fibers gave better measurement precision than others. Fiber A with

the lowest bandwidth (200 MHz) gave the best precision with the frequency at

a given response being determined to 1% and the response at a given

frequency being determined to .1 dB. For fiber B with medium bandwidth

(720 MHz), the corresponding numbers varied from 2 to 4% and .1 to .5 dB with

the highest frequency part of the curve showing the poorest

reproducibility. Fiber C with the highest bandwidth (1.3 GHz), gave

corresponding precisions of 1 to 2% and .1 to .2 dB respectively.

5. 2 Dynamic Range

Dynamic range applies to both the maximum fiber bandwidth and

attenuation that can be accommodated by the system. If only the intermodal

contribution is to be determined from a measurement, the upper bandwidth

limit is determined by the spectral properties of the laser diodes and is in

the 1-2 GHz 'km, 3 dB frequency range depending upon the specific diode.

Although higher bandwidths can be measured, the material dispersion

contributions become too large and accurate corrections become difficult

(section 4.3). Limitations due to source pulse width (260 ps FDHM) do not

become important until the 3 dB frequency exceeds 3 GHz.

Maximum attenuation limits depend upon the amount of signal averaging

done or on the signal level available to the sampling oscilloscope trigger

for good stability. Present sources, attenuated with a 1.0 neutral density

filter and detected with a BPW-28 APD (section 3.2), produce from . 2 to 2 volts at the detector for short lengths of fiber having negligible

46
attenuation and high bandwidth.

If an advanced trigger-electrical delay line is used with the output

signal, figure 3-18(b), we find about 15 mv of signal should be available for

the oscilloscope trigger to provide good long term stability. If the trigger

level is not a problem, for example, with a digital time delay generator

(figure 3-18(a)), then the amount of signal averaging determines the

attenuation limits. For the present amount of analogue averaging (section

3.4), the noise level is less than .1 mv. Both the fiber attenuation and

bandwidth will determine the peak voltage level available at the detector

output. Signal level in the present system has not been a problem for most

of the currently available telecommunications type fibers at the 1-1.5 km

length.

5 . 3 Material Dispersion Limits

In characterizing the bandwidth of multimode fibers, it is desirable to

know the intermodal contribution. If the measurement contains significant

intramodal broadening, the result depends upon the source spectral properties

and does not represent the ultimate bandwidth potential of the fiber.

Material dispersion results in intramodal pulse broadening with the RMS

contribution to total pulse broadening given by eq (6), section 2. From the

observed diode lineshapes and occasional chirping behavior, it seems unlikely

that eq (6) could be used to calculate a significant correction term with a high degree of confidence. A more prudent approach would be to place an upper bandwidth limit on measurements made with a particular diode; i.e., for

measured fiber bandwidths below a certain maximum 3 dB bandwidth, f_, the m

contribution from intramodal effects for a given diode would be less than 6

percent of the measured bandwidth.

Material dispersion constants were measured for a number of commercial

fibers with the apparatus described in the Appendix. The results give M (-M

plotted) in figure 7-3 where

M = —j . (16)

c dX z

Since the M values at a given wavelength show little variation among fibers

(a ± 10% variation would nearly bracket the data), the value of fm determined

is, roughly a property of a given diode. Using a Gaussian approximation for

spectral, pulse and fiber frequency response' shapes along with the M values

from Figure 8-3, eq. (5) and (6) predict an f of 1.0, 2.0, and 1.4 GHz'km

47
for the 803, 866, and 902 nm diodes respectively. Chirping observed in the

824 nm diode should partially compensate the material dispersion broadening

for fibers exceeding a critical length [12]. From the linear chirping theory

of [12], a critical length of 1.1 km is predicted. For shorter lengths,

pulse compression would occur. For the pulse widths involved (260 ps ) , and

the fiber lengths measured (1 km or more), intramodal broadening effects are negligible for the 824 nm diode if fiber bandwidth is below 1.1 GHz 'km.

For a measurement at the bandwidth limit f , the six percent correction

could be handled in the following manner. Add 8% to the measured bandwidth

in the frequency domain and then take a ± 4% error. This allows for nearly a ± 40% uncertainty in determining f . Such a generous uncertainty is possible

since at f the intramodal contribution represents a small correction to the

bandwidth.

6. RESULTS OF BANDWIDTH RELATED MEASUREMENTS

The purpose of this section is to demonstrate typical fiber bandwidth

behavior in a quantitative manner. Results were selected from about 20

fibers and represent a diverse mix of properties.

6 . 1 Wavelength Dependence of Bandwidth

Differences in the refractive index dispersion of the constituents used

in making graded index optical fibers results in an index profile that is

wavelength dependent. This "profile dispersion" directly affects the optical

fiber bandwidth which depends critically on the shape of the index profile.

Cohen has shown that bandwidth may increase or decrease with wavelength

depending upon the profile characteristics [22].

Figure 6-1 shows a 1.6 km fiber with a rather large wavelength

dependence in the bandwidth. Figure 6-1 (a) gives the output at 803 nm while

figure 6-1 (b) is the output at 902 nm. The inputs were 330 ps FDHM for both

wavelengths. Measurements of the transfer functions indicated the 3 dB

bandwidth decreased from 0.90 GHz 'km at 803 nm to 0.61 GHz 'km at 902 nm.

6. 2 Mode Mixing

In a multimode optical fiber, power can be coupled between modes as a result of perturbations in the optical characteristics induced by physical

changes. If fast and slow modes are mixed, the delay times tend to be

equalized and the bandwidth increases. Figure 6.2(a) shows the output of a 1

48
OUTPUT PULSE

803 nm

[VA

won 111 H

mm m

WMr M

A II

m m

OUTPUT PULSE

902 nm

Figure 6-1. Output pulses from 1.5 km, graded index optical fiber at two

wavelengths, (a) 803 nm, (b) 902 nm for nearly identical shaped

input pulses of 330 ps FDHM..

49
(a)

(b)

Figure 6-2. Influence of microbending on bandwidth of a lightly buffered

graded index optical fiber, 1 km length, (a) output pulse with

fiber wound loose in a single layer, (b) output pulse with

fiber wound under stress in multiple layers (824 ran diode). In

(b) the output pulse shape is barely distinguishable from the

input; due to microbending bandwidth has been increased by

approximately a factor of four.

50
km lightly buffered fiber, loosely wound in a single layer on a measurement

spool. The input pulse duration was about 300 ps . Figure 6.2(b) shows the

output using the same source but now with the fiber under tension and wound

in multiple layers so as to introduce microbending. The bandwidth has now increased so that the output pulse is barely distinguishable from the input

and is largely representative of the intramodal broadening occuring in the

fiber. Fiber attenuation also increased between these two cases; an additional microbending loss of more than 10 dB was incurred to achieve the

above bandwidth enhancement

6.3. Accuracy of Gaussian Predictions

If the fiber frequency response and input pulse are assumed to be

Gaussian, the 3 dB bandwidth may be predicted by a simple deconvolution of

the input pulse duration from the output pulse duration (section 2. eq.

(11)). The accuracy of this assumption was checked using the present system

to measure the actual bandwidth. All fibers measured over a given period of

time with the system are included with no preselection. Output pulse shapes

ranged from symmetric to assymmetric (usually the latter), some with long

leading edges others with long trailing edges; however, none of the pulses

exhibited multiple peaks. The results are given Table IV. The agreement is

close enough for some purposes and is in accord with similar statements made

by Midwinter [23].

6 . 4 Fiber Response Pathologies

The impulse response - transfer function characteristics of multimode

fibers show a rich variability. The purpose of this section is to

demonstrate the range of behavior that can be observed.

6.4.1 Pulse Shapes

In some cases, output waveforms from multimode fibers exhibit multiple

pulses or peaks. Results similar to those reported by Eickhoff have been

observed where energy is carried in an on-axis index peak and for a 1 km

fiber arrives over 10 ns later than the main pulse [24]. In another

instance, we measured a fiber having 20 multiple peaks resolved to the halfwidths in its impulse response. In most situations, however, the fiber

response has a single maximum which is frequently assymmetric about the

peak. The assymmetry is characterized by either a low level leading edge or

a low level trailing edge; this results from an index profile which does not

adequately equalize mode arrival time. Figure 6-3 is typical of this

51
TABLE IV

Accuracy of Gaussian Assumption for Determining

3 dB Bandwidth from Half-Duration Pulse Broadening

Graded

Index

Fiber

Length,

km

Bandwidth,

Gaussian

Prediction,

MHz

Actual

Bandwidth,

MHz

Percentage

Difference,

%

308 1.4 256 198 + 29%

223 1.1 846 732 + 16%

116 1.6 612 673 - 9%

206 1.0 713 752 - 5%

1206 1.4 280 325 - 14%

1208 1.4 189 210 - 10%

1212 .7 467 402 + 16%

207 1.0 1048 1062 - 1%

156 1.0 164 184 - 11%

52
samHHKSS

(a)

(b)

Figure 6-3. Opposite pulse broadening behavior in graded index optical

fibers, (a) 1.1 km fiber (fiber B, 902 nm) with some modes

arriving too early, (b) 1.3 km fiber (fiber A, 824 nm) with

some modes arriving too late.

53
(a)

(b)

Figure 6-4. Characteristic impulse response for step index fibers (a)

85 liin core, .18 NA, .8 km length, 824 ran and (b) 55 ym

core, .25 NA, 1 . 2 km length, 824 nm.

54
behavior. If one assumes most of the energy is carried in low to medium

order modes, then the fiber in figure 6-3 (a) represents a profile with

insufficient doping near the cladding (high order modes arrive too early and

are overcompensated with respect to group velocity) while the opposite (too

much doping and undercompensation) would be true in figure 6-3(b).

While graded index fibers exhibit great variations in behavior,

multimode step fibers have a characteristic impulse response. The impulse

response has a rapid rise with a nearly ramp-like decay lasting for a longer

time (figure 6.4). This general behavior is roughly predicted by simple mode

propagation models [25].

6.4.2 Frequency-Phase Characteristics

Since output pulses from fibers show diverse behavior, it is not

surprising that the frequency response characteristics also exhibit a diversity. This is evident in a comparison of the magnitude and phase

responses for fibers. Fibers have been observed where the significant

distortion in the phase has occurred both before and after the frequency

where the magnitude is down by 3 dB. Examples of both kinds of behavior are given in figures 6-5 and 6-6. No distortion in the phase is represented by a straight line on these plots; i.e., linear phase shift with frequency. For

the fiber of figure 6-5, the phase distorts significantly before the 3 dB

frequency; while in figure 6-6, the phase is fairly linear out to the 5 dB

frequency.

55
CO

-«—sfviviava 'ldiHs 3SVHd

CO «tf CN

>1uc

Q) c

3

1 o

•H

<U -P

J-l U| in

-p

TO TO 3 •H

TO 'C ^

(J) > TO c

r0

•H

-p 0) o Cn c Vt 3

M-lb Cn

(1) c

U-l H . TO P »-- C •H E

(d £ C

n ;

i -H

+J X CN

;»i N _e

0) cc

v. 0) £

(UENCY,

ase

of .C -P H Cn

4-1 C

(U

a; t> rC >D E LU U4 c x cc -H

LU "O ro c fC • i« "0

<u (0 » •c U <C 3 CD

-P >-l •rH (0 Q) c ,Q

cn >-i -H

rt) O fa s U-J ^

in

1

0)

(-1 3

cn •H

fa

«—ap'aaniiNOVi/M

56
SIWICIVH 'ldlHS3SVHd

u

CD >.£

O -H C fa

Q) v_ Da

1 c

<u o

S-i -H

M-l +J

co3 -P

to co M -H

CD X> >

CDm

(0

ap'aaniiNOviAi

c

•HP

•H

•HX

Q) CD .

a) 2

in ,C O -H *t

M-l CN

Q) CD

CO X

id CD « S ^ -c •H CT TS C

C -C CD (0 CD <H

"D

Q) (d E

'O U X

D Cr> +> in •h <d • C H

cn ^

<d o « E <n a

VO

CD

S-i

•H

fa

57
7. CONCLUSIONS

Perhaps the greatest uncertainty in making bandwidth measurements on multimode fibers is the lack of standardized launching conditions. This

makes a discussion of systematic error difficult because at the present one cannot say that one launching condition is more "correct" than another. The

launching conditions presently employed with this system tend to over fill

the fiber under test. A spot size of 55 pm and a launch NA of .25 will

fill the mode volume of most fibers meeting the proposed standard core size

of 50 ym. Such a launching condition has the following merits. 1) It will

give a lower estimate to the bandwidth encountered in actual systems and

therefore could be used in "worst case" calculations. 2) It is the starting

point for the use of equilibrium mode simulators (EMS). An EMS would strip

away specific high loss modes which do not propagate very far in the fiber.

3) It can be reproduced by other laboratories and is independent of a particular diode radiation characteristic.

Some additional work needs to be done by standards groups on the

terminology used in reporting bandwidth. For multimode fibers, reporting the

intermodal contribution seems appropriate. The intramodal part could be

specified as a function of wavelength by the manufacturer based on refractive

index data and specified per unit fiber length and source linewidth. Also, a problem remains on reporting the bandwidth per unit length where the actual

measurement length is quite different from the unit length.

With improved fiber characteristics at the longer wavelengths, more

emphasis will be placed on bandwidth measurements over

the 1.1 - 1.5 ym region. Many of the points emphasied in this Technical Note

still remain valid. Material dispersion however will not be as important

since the material broadening is approximately 40 ps/km.nm at 1.1 urn and even

less for longer wavelength.

58
8. APPENDIX

This appendix describes measurements of optical fiber material

dispersion constants. Material dispersion was measured by a technique

described by Gloge et al. [26]. In this method, the dispersion is determined

by noting the differential delay between pulses from two different wavelength

laser diodes. This work utilizes the same method with the following

extensions: (1) four laser diodes with appropriate wavelengths to cover the

whole 0.8 - to 0. 9-micrometer wavelength region, (2) shorter pulse widths to

provide good resolution, (3) a mode scrambler to reduce effects of

differential launching conditions, and (4) the use of high-bandwidth fibers

to minimize intermodal contributions.

An experimental configuration for determining material dispersion is

shown in figure 8-1. By pulsing pairs of diodes, pulses at two different

wavelengths were launched down the test fiber. Diodes at 803, 824, 866, and

902 run were used to cover the 0.8- to . 9-micrometer region. Pulses were separated slightly in time before launching to avoid overlap which would

shift the positions of the peaks. A pulse pair before and after traversing a 1-km fiber length is shown in figure 8-2(a) and (b). In this case, a wavelength difference of 21 nm gives a differential delay of 2.4 ns

Measurements of differential delay between pulses were repeatable within ±

100 ps.

Material dispersion was determined for fibers labeled 2, 4, and 5,

figure 8-3. These fibers had lengths of 1.0, 1.5, and 1.4 km and exhibited

pulse FDHM broadenings of .42, .74, and 1.1 ns , respectively. Bandwidth (3

dB optical) determined in the frequency domain by the fast Fourier transform

(FFT) ratio of output to input pulses was 1.1, 0.9, and 0.4 GHz-km,

respectively. A single point, A, inferred from the data of [26] is also

shown in figure 8-3. Other data from the literature representing fibers

measured at CSELT using similar techniques is included (curve C) [27].

Overall, results do indicate significant, measureable differences between

fibers. However, at any one wavelength, a ± 10 percent variation would

nearly bracket the data. The result on fiber 2 has the least influence from

intermodal delay of the fibers measured. This fiber exhibited the highest

bandwidth and the least variation of bandwidth with wavelength. Also, the

manufacturer of fiber 2 provided sufficient, representative index data to

determine eq. (16), at 800, 850, and 900 nm. Values of M from this data are indicated by the dashed curve and are slightly above the experimental

values. While the above measurements were 'limited to the 800-900 nm wavelength region, other experimental results are available from the

literature for longer wavelengths [28], [29].

59
\W^T7P^-^ol^QJ5^^L

MODE

SCRAMBLER

FIBER UNDER

TEST

VZK FIBER HOLDER

& CLADDING MODE

STRIPPER

Figure 8-1. Apparatus for measuring material dispersion by simultaneous

propagation of pulses from pairs of laser diodes.

60
GROUP DELAY DISPERSION, 1 km FIBER

INPUT PULSE

PAIR

803 nm

^B

Bil

Ml Rki Ml!

ill I 1

m

mm

m1

P«

OUTPUT

PULSE

PAIR

824 nm 803 nm

Figure 8-2. Example of material dispersion in an optical fiber showing a pulse pair (a) before and (b) after propagating through a 1 km

length of fiber.

61
"I

Ui

DCLUQ.

C/5

LU

I-<

150

140

130

120

110

100

90

80 -

70 -

60 _l_ _L 800 820 840 860

WAVELENGTH, nm *

880 900

Figure 8-3 . Material dispersion constants for a number of commercially

available graded index fibers labeled 2, 4, and 5. Curve C and

point A are measurements from the literature while the dashed

curve is calculated from index data supplied by the

manufacturer of fiber 2.

62
9 . REFERENCES

[I] Olshansky, R. and Keck, D. B., Pulse broadening in graded-index optical

fibers, Applied Optics, 15 , No. 2, 483-491 (Feb. 1976).

[2] Cohen, L. G., Shuttle pulse measurements of pulse spreading in an optical fiber, Applied Optics, 14 , No. 6, 1351-1356 (June 1976).

[3] Cohen, L. G., Astle, H. W., and Kaminow, I. P., Frequency domain

measurements of dispersion in multimode optical fibers, A.P.L., 30 , No.

1, 17-19 (Jan. 1, 1977).

[4] Vassallo, Linear power responses of an optical fiber, IEEE Transactions

on Microwave Theory and Techniques, MTT-25, No. 7, 572-576 (July 1977).

[5] Machida, S., et al, Interference of an AlGaAs laser diode using a 4.15 km single mode fiber cable, IEEE Journal of Quantum Electronics,

QE-15 , No. 7, 535-537 (July 1979).

[6] Gloge, D., Chinnock, E. L., and Lee, T. P., Self-pulsing GaAs laser for

fiber-dispersion measurements, IEEE Journal of Quantum Electronics,

QE-8, No. 11, 844-846 (Nov. 1972).

[7] Keck, D. B. , Spatial and temporal power transfer measurements on a low-loss optical waveguide. Applied Optics, 13, 1882-1888 (Aug. 1974).

[8] Hansen, J. B. and Schmidt, W. A., A fast risetime avalanche transistor

pulse generator for driving injection lasers, Proc. IEEE, 55, No. 2,

216-217 (Feb. 1967).

[9] Andrews, J. R. , Inexpensive laser diode pulse generator for optical

waveguide studies, Rev. Sci. Instrum., 45, No. 1, 22-24 (Jan., 1974).

[10] Dannwolf, J. W. , et al , Optical-Fiber impulse-response measurement

system, IEEE Transactions on Instrumentation and Measurement, IM-25 , No. 4, 401-406 (Dec. 1976).

[II] In order to describe adequately the experimental procedure and permit

reproduction of this work, certain commercial components are mentioned

here and elsewhere; in no way does this represent an endorsement by

NBS. Other components may work equally well or better.

63
[12] Wright, J. V. and Nelson, B. D. , Pulse compression in optical fibers,

Electronics Letters, 13, No. 12, 361-363 (June 1977).

[13] Green, S. I., 50 picosecond detector pulse monitor, Rev. Sci. Instrum.

47, No. 9, 1083-1085 (Sept. 1976).

[14] The authors would like to thank Matt Young of the Time-Domain Metrology

Section of NBS for use of his mode-locked Nd:YAG laser.

[15] Costa, B. , et al , Wavelength dependence of differential group delay in

graded-index optical fibers: application to fiber-links

characterization, paper ThG5, Topical Meeting on Optical Fiber

Communications, March 6-8, 1979, Washington, D.C.

[16] Green, S. I., Wideband 1.064 micrometer detector evaluation, McDonnell

Douglas Astronautics Co. Final Report NA55-20616 (NTIS #

N77-32459/8wz)

.

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

measurements, paper ThG2 Topical Meeting on Optical Fiber

Communications, March 6-8, 1979, Washington, D.C.

[18] Tokuda, M., et al, Measurement of baseband frequency response of

multimode fibre by using a new type of mode scrambler, Electronic

Letters, JL3_, No. 5, 146-147 (March 3, 1977).

[19] Dakin, J. P., et al, Launching into glass fibre optical waveguides,

Optical Communications, 4_, No. 5, 354-357 (Jan. 1972).

[20] Andrews, J. R. , and Young, M. , Time domain measurements on optical

communications components, NBSIR - to be published.

[21] Ramirez, R. W. , Evaluating fibers digitally, Laser Focus, 15 , No. 4,

46-52 (April 1979).

[22] Cohen, L. G. , et al , Profile dispersion effects on transmission

bandwidths in graded index optical fibers, IEEE J. Quantum Electronics,

QE-14 , No. 1, 37-41 (Jan. 1978).

[23] Midwinter, J. E., Optical Fibers for Transmission, John Wiley & Sons,

New York, N.Y. (1979).

64
[24] Eickhoff, W. and Krumpholtz, 0. , Pulse response and microbending losses

of an optical glass-fibre waveguide with a bump in its index profile,

Electronics Letters, 13 , No. 9, 256-257 (April 1977).

[25] Gallawa, R. L., A User's Manual for Optical Waveguide Communications,

NTIA, Report #OTR 76-83.

[26] Gloge, D. , Chinnok, E. L. , and Lee, T. P., GaAs twin-laser setup to measure mode and material dispersion in optical fibers, Applied Optics,

13 , No. 2, 261-263 (Feb. 1974).

[27] Sordo, B. Esposto, F., and Costa, B., Proceedings of the Fourth

European Conference on Optical Communications, Genova, Italy, Sept. 12-

15, 1978.

[28] Payne, D.N., and Hartog, A.H., Determination of the wavelength of zero material dispersion in optical fibers by pulse-delay measurements,

Electron. Lett., 13, No. 21, 627-629 (Oct. 1977).

[29] Cohen, L.G., and Chinlon, L., Pulse delay measurements in the zero material dispersion wavelength region for optical fibers, Applied

Optics, 16, No. 12, 3136-3139 (Dec. 1977).

irU.S GOVERNMENT PRINTING OFFICE 1980—677-096/1232 r - ob
HBS-114A (REV. 9-78)

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO.

NBS TN-1019

4. TITLE AND SUBTITLE

Measurement of Optical Fiber Bandwidth in the

Time Domain

5. Publication Date February 1980

7. AUTHOR(S)

Douglas L. Franzen and G. W. Day

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, DC 20234

10. Project/Task/WoiK Unit No.

11. Contract/Grant No.

12. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street, City, State, ZIP)

Supported in part by the Department of Defense, Calibration

Coordination Group (CCG),

13. Type of Report & Period Covert

w&vmi&m«y Co(Je

^*

____^____

15. SUPPLEMENTARY NOTES

Q Document describes a computer program; SF-185, FIPS Software Summary, is attached.

16. ABSTRACT (A 200-word or less tactual summary of most significant information. If document includes a significant bibliography or

literature survey, mention it here.)

A system is described for determining optical fiber bandwidth from time

domain information. A measurement gives the optical fiber transfer function (or

frequency response) relating the output waveform to the input. An analysis is

given of the variables affecting the measurement. This includes a discussion of

such input related topics as launching conditions, mode scramblers, and laser

diode sources; output related topics include a discussion of optical detectors.

Laser diodes are evaluated with respect to short pulse performance, near field

emission, material dispersion limits, and other spectral behavior like chirping;

detectors are evaluated with respect to time response, linearity, and uniformity.

Overall system architecture, precision, and dynamic range are discussed. A

number of bandwidth related topics are briefly presented and typical experimental

results given. This includes examples of: mode mixing via microbending, profile

compensation, profile dispersion, intramodal broadening, materials dispersion cons relative magnitude-phase behavior, and Gaussian predictions of frequency response.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name;

separated by semicolons)

Bandwidth; laser diodes; material dispersion; mode scramblers; optical detectors;

optical fibers; transfer function.

18. AVAILABILITY [^Unlimited

| | For Official Distribution. Do Not Release to NTIS

| | Order From Sup. of Doc, U.S. Government Printing Office, Washington, DC

20402, SD Stock No. SN003-003- 2161-0

| | Order From National Technical Information Service (NTIS), Springfield,

VA. 22161

19. SECURITY CLASS

(THIS REPORT)

UNCLASSIFIED

20. SECURITY CLASS

(THIS PAGE)

UNCLASSIFIED

21. NO. OF

PRINTED PAGI

72

22. Price

$3.75

—-m^-m*
NBS TECHNICAL PUBLICATIONS

PERIODICALS

JOURNAL OF RESEARCH—The Journal of Research of the National Bureau of Standards reports NBS research and develop- ment in those disciplines of the physical and engineering sciences in which the Bureau is active. These include physics, chemistry,

engineering, mathematics, and computer sciences. Papers cover a broad range of subjects, with major emphasis on measurement

methodology and the basic technology underlying standardization. Also included from time to time are survey articles on topics

closely related to the Bureau's technical and scientific programs. As a special service to subscribers each issue contains complete

citations to all recent Bureau publications in both NBS and non- NBS media. Issued six times a year. Annual subscription: domestic

$17; foreign $21.25. Single copy, $3 domestic; $3.75 foreign. NOTE; The Journal was formerly published in two sections: Section A "Physics and Chemistry" and Section B "Mathematical

Sciences." DIMENSIONS/NBS—This monthly magazine is published to in- form scientists, engineers, business and industry leaders, teachers,

students, and consumers of the latest advances in science and

technology, with primary emphasis on work at NBS. The magazine

highlights and reviews such issues as energy research, fire protection, building technology, metric conversion, pollution abatement,

health and safety, and consumer product performance. In addition, it reports the results of Bureau programs in measurement

standards and techniques, properties of matter and materials, engineering standards and services, instrumentation, and

automatic data processing. Annual subscription: domestic $11;

foreign $13.75. NONPERIODICALS

Monographs—Major contributions to the technical literature on

various subjects related to the Bureau's scientific and technical activities. Handbooks—Recommended codes of engineering and industrial practice (including safety codes) developed in cooperation with in- terested industries, professional organizations, and regulatory

bodies. Special Publications—Include proceedings of conferences sponsored by NBS, NBS annual reports, and other special publications

appropriate to this grouping such as wall charts, pocket cards, and

bibliographies. Applied Mathematics Series—Mathematical tables, manuals, and

studies of special interest to physicists, engineers, chemists,

biologists, mathematicians, computer programmers, and others engaged in scientific and technical work.

National Standard Reference Data Series—Provides quantitative data on the physical and chemical properties of materials, compiled from the world's literature and critically evaluated. Developed under a worldwide program coordinated by NBS under

the authority of the National Standard Data Act (Public Law

90-396).

NOTE: The principal publication outlet for the foregoing data is the Journal of Physical and Chemical Reference Data (JPCRD)

published quarterly for NBS by the American Chemical Society

(ACS) and the American Institute of Physics (AIP). Subscriptions,

reprints, and supplements available from ACS, 1 155 Sixteenth St., NW, Washington, DC 20056.

Building Science Series—Disseminates technical information developed at the Bureau on building materials, components,

systems, and whole structures. The series presents research results,

test methods, and performance criteria related to the structural and

environmental functions and the durability and safety characteristics of building elements and systems.

Technical Notes—Studies or reports which are complete in themselves but restrictive in their treatment of a subject. Analogous to monographs but not so comprehensive in scope or definitive in treatment of the subject area. Often serve as a vehicle for final reports of work performed at NBS under the sponsorship of other government agencies.

Voluntary Product Standards—Developed under procedures

published by the Department of Commerce in Part 10, Title 15, of

the Code of Federal Regulations. The standards establish nationally recognized requirements for products, and provide all concerned interests with a basis for common understanding of the

characteristics of the products. NBS administers this program as a supplement to the activities of the private sector standardizing

organizations. Consumer Information Series—Practical information, based on NBS research and experience, covering areas of interest to the con- sumer. Easily understandable language and illustrations provide

useful background knowledge for shopping in today's technological marketplace. Order the above NBS publications from: Superintendent of Documents. Government Printing Office, Washington. DC 20402. Order the following NBS publications—FIPS and NBSIR 's—from

the National Technical Information Services, Springfield, VA 22161.

Federal Information Processing Standards Publications (FIPS

PUB)—Publications in this series collectively constitute the Federal Information Processing Standards Register. The Register

serves as the official source of information in the Federal Govern- ment regarding standards issued by NBS pursuant to the Federal Property and Administrative Services Act of 1949 as amended,

Public Law 89-306 (79 Stat. 1127), and as implemented by Executive Order 11717(38 FR 12315, dated May 11, 1973) and Part 6 of Title 15 CFR (Code of Federal Regulations).

NBS Interagency Reports (NBSIR)—A special series of interim or

final reports on work performed by NBS for outside sponsors

(both government and non-government). In general, initial dis- tribution is handled by the sponsor; public distribution is by the National Technical Information Services, Springfield, VA 22161,

in paper copy or microfiche form.

BIBLIOGRAPHIC SUBSCRIPTION SERVICES

The following current-awareness and literature-survey bibliographies are issued periodically by the Bureau:

Cryogenic Data Center Current Awareness Service. A literature sur- vey issued biweekly. Annual subscription: domestic $25; foreign

$30. Liquefied Natural Gas. A literature survey issued quarterly. Annual

subscription: $20.

Superconducting Devices and Materials. A literature survey issued

quarterly. Annual subscription: $30. Please send subscription or- ders and remittances for the preceding bibliographic services to the National Bureau of Standards, Cryogenic Data Center (736) Boulder, CO 80303.
U.S. DEPARTMENT OF COMMERCE

National Buraau of Standards

Washington. D.C. 20234

OFFICIAL BUSINESS

Penalty for Private Use, $300

POSTAGE AND FEES PAID

U.S. DEPARTMENT OF COMMERCE COM-215

1U.S.N

SPECIAL FOURTH-CLASS RATE BOOK