Reference NBS

Publications

NBSm 79-1620

NAri INST Ql STAND & TECH RIC

AlllDM SSlbQS

TIME DOMAIN PULSE MEASUREMENTS AND COMPUTED FREQUENCY

RESPONSES OF OPTICAL COMMUNICATIONS COMPONENTS

James R. Andrews

Matt Young

Electromagnetic Technology Division

National Engineering Laboratory

National Bureau of Standards

Boulder, Colorado 80303

September 1979

--0C-—

100

.U56

79-1620

1979
1
National Bureau of StancLard JAN 3 1 1980

NBSIR 79-1620

t • *

TIME DOMAIN PULSE MEASUREMENTS AND COMPUTED FREQUENCY

RESPONSES OF OPTICAL COMMUNICATIONS COMPONENTS

James R. Andrews

Matt Young

i^lectromagnetic Technology Division

National Engineering Laboratory

National Bureau of Standards

Boulder, Colorado 80303

September 1979

U.S. DEPARTMENT OF COMMERCE, Juanita M. Kreps, Secretary

Luther H. Hodges, Jr., Under Secretary

Jordan J. Baruch, Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler. Director

CONTENTS

Abstract 1

Introduction 1

Background 1

Lasers 2

Photodiodes 3

Automatic Pulse Measurement System 4

Experiment Confirgurations 4

Test Fibers 6

Fiber Measurement Results 7

Impulse and Frequency Response of an Avalanche Photodiode 9

Acknowledgments 9

Figures 10

APPENDIX A 26

APPENDIX B 29

REFERENCES 31

lil

TIME DOMAIN PULSE MEASUREMENTS AND COMPUTED FREQUENCY RESPONSES

OF OPTICAL COMMUNICATIONS COMPONENTS

James R. Andrews and Matt Young

The purpose of this report is to demonstrate the application of the NBS

Automatic Pulse Measurement Systdtn (APMS) to measuring the pulse responses of

optical communications components and to computing their impulse and frequency

responses. For example we describe measurements of the properties of two glass

fibers and an avalanche photodiode using both a pulsed GaAs laser diode

(X = 0.9 ym) and a mode locked, Nd:YAG laser (X = 1.06 ym) . All measurements

were performed in the time domain; frequency domain data were obtained from

the time domain data by using the Fast Fourier Transform (FFT) . The impulse

response was obtained by deconvolution.

Key Words: Avalanche photodiode; FFT; fiber optics; frequency response; impulse

response; laser; photodiode.

Introduction

The purpose of this report is to demonstrate some techniques for pulse measurements on optical communications components and the application of the NBS Automatic Pulse Measurement

System (APMS) to such measurements. The components tested included step index and graded

index fibers and an avalanche photodiode. Using short pulse laser inputs, we used the APMS

to measure the transient responses of the various components in the time domain. The APMS

is a dc-18 GHz sampling oscilloscope interfaced with and controlled by a dedicated minicomputer. From the time domain data we derived frequency domain information on the APMS

minicomputer using the Fast Fourier Transform (FFT) . Both magnitude and phase data were computed to describe the transfer function, the insertion loss ^

22^(f) and the frequency

response. We derived the impulse response by a deconvolution technique. The lasers used

included a pulsed GaAs laser diode (X = 0.9 ym) and a mode locked, Nd:YAG laser (X = 1.06 ym)

.

The laser pulse durations were approximately 130 ps and 80 ps respectively.

Background

When the field of fiber optics for telecommunications was still in its infancy, the

importance of light pulse distortion in fibers was recognized by systems designers as a major problem. Early NBS work in this field was done in cooperation with the Centre

National d' Etudes des Telecommunications (CNET) m Lannion, France. The NBS-CNET

research involved measuring the pulse distortion in the high loss optical fibers available at that time [1]. High temporal resolution (''- 100 ps) measurements were made using

GaAs and argon lasers. The measurements and results were all in the time domain. However,

it was forseen at the time [1] that much more information could be obtained from the time

domain measurements when it would be possible to use computer processing and Fast Fourier

Transforms (FFT). Since that time, the use of time domain, short pulse measurements and

- 1 -
computed FFTs has become the most commonly used technique to determine the pulse distortion

and bandwidth of optical fibers [2-6]. They are part of a Military Standard first drafted

in 1976 [7].

The early CNET-NBS work developed an avalanche transistor circuit to pulse a GaAs

laser diode and demonstrated that a short 100 ps) light pulse could be obtained from

such a laser [1,8]. In 1974 an improved circuit was reported [9]. This circuit and

variations have been widely used since then by many laboratories and companies interested

in short pulse testing of optical fibers and is reproduced here as Appendix A. This report

includes a newly modified version of the 1974 circuit.

Many of the basic techniques of time domain testing of optical fibers are now well

known, although many problems remain. This report is intended in part to demonstrate our capability in this area. In addition we document some unusual multimode waveforms encountered

in a graded index fiber when tested with the NBS, high temporal resolution ('v 100 ps)

,

1.06 pm

test apparatus.

Lasers

We used two lasers; the first was a pulsed GaAs laser diode. This laser pulse generator

is described in detail in Appendix B. It is a modification of an earlier version [9].

Laser diode A^ emits at a wavelength of 0.9 pm. It has a peak forward current rating of 10 A

and a typical threshold of 4 A. With certain selected diodes, optical pulses as narrow

as 110 ps have been observed [9]. To obtain close coupling to a fiber, we removed the protective, glass face plate of

the laser. This allowed the fiber to be placed extremely close to the output face of the

laser. The fiber was not allowed to touch the laser because this drastically detuned the

cavity and altered the laser's output characteristics.

The 1.06 pm laser was a continuously mode locked, commercial YAG laser that we have

stabilized with quartz rods. The length of the cavity is about 2 m and the corresponding

pulse repetition frequency 100 MHz (or, equivalently, the period of the pulses is 10 ns).

The average output power, measured at the output m.irror, is 1 W. We have measured the duration of the pulses using an autocorrelation technique [10].

The apparatus consists of a Michelson interferometer [11] that is modified by replacing its

plane mirrors with right angle (Porro) prisms. The modification prevents the beam reflected

by the interferometer from propagating back into the laser and thereby interfering with the

mode locking process. The beam that emerges from the interferometer is focused with careful

alignment into a second harmonic generating crystal; the 530 nm radiation is

Due to NBS policy, manufacturers' names and model numbers are omitted to avoid any

endorsement

.

- 2 -
detected with a silicon photocell and a colored glass filter suitable for blocking the 1.06 pm

radiation. We use an xy recorder connected to the output of the photocell and to the moveable

arm of the interferometer to record the autocorrelation function of the pulses.

The duration of the autocorrelation function is 0.11 ns. The duration of the pulses

themselves cannot be determined precisely from an autocorrelation measurement [10]. If the

pulses are roughly gaussian in shape, then the pulse duration is 0.707 times the duration of

the autocorrelation function; for our 'pulses, this yields durations of about 0.08 ns. The

peak pulse power is therefore approximately 125 W.

Photodiodes

We tested and used several photodiodes, including PIN and avalanche. Si and Ge, which

were either commercial or research devices.

To determine the output of the laser diode we used an experimental, epitaxial Si photodiode built by the Centre National d' Etudes des Telecommunications (CNET) . The waveform

obtained is shown in figure 1. The 50% duration (FWHM) of the first pulse is 0.14 ns. The

minor bump (100 ps after the first pulse) is attributed to an internal reflection within the

photodiode mount, whereas the second major pulse (300 ps later) is a feature of the laser.

When driven with a long pulse, lasers A produce a train of relaxation oscillations with

spacing of the order of 1/3 ns. Previous tests of the CNET photodiode at 600 nm with a 'v. 10 ps, mode locked dye laser

resulted in waveforms whose duration was about 70 ps. Although photodiode impulse responses

will vary with wavelength, the 70 ps value can be used for a rough estimate of the pulse

duration t^ of laser A. We assumed that the measured waveform was the convolution of the

laser pulse with the impulse responses of the photodiode and the sampling oscilloscope. The

squares of the durations of each add roughly to yield the square of the duration of the

observed waveform. Thus,

= (t meas

- t pd t so

( 1 )

where t is the observed 0.14 ns impulse duration, t , is the assumed 55 ps impulse dura- meas pd

tion of the photodiode, and t is the 19 ps impulse duration of sampling oscilloscope C. so With these values in eq. (1) we estimate that laser A's pulse duration is approximately

0.13 ns

.

For fiber measurements at 0.9 ym, we used a fast, commercial, avalanche photodiode

(APD, designated D) because of its greater responsivity than the CNET Si photodiode. The

response of this diode to laser A consists of a fast pulse (0.35 ns, 50% duration) followed

by a slow tail, as shown in figure 2. A slower, commercial avalanche photodiode (E) was also tested to determine its impulse

and frequency responses. This measurement is described in a later section.

We used two different photodiodes at 1.06 ym. The faster was a CNET Ge photodiode'

[1,12]. Figure 3 shows its output when irradiated by the YAG laser. The duration of the

- 3 -
main impulse is 0.12 ns, the YAG laser pulse having been degraded somewhat by transmission

through the 10 m fiber pigtail used to bring the laser pulse from the laser room into the

APMS room. We estimate the impulse response duration (FWHM) of the photodiode to be of the

order of 0.09 ns. [1].

Measurements were also made at 1.06 ymi with a commercial. Si PIN photodiode (F) whose

pulse response is shown in figure 4. The duration is 0.37 ns. It is characterized by a

fast leading edge and a slower trailing edge. In this case the diode's response is considerably slower than the YAG laser; therefore, figure 4 is a close approximation to the diode's

impulse response.

Automatic Pulse Measurement System

The time domain measurements were made with the NBS Automatic Pulse Measurement System

(APMS) [4,5,13]. The APMS is basically a wideband (dc - 18 GHz), 20 ps transition time,

sampling oscilloscope interfaced with and controlled by a dedicated minicomputer (figure 5)

.

The oscilloscope was modified by NBS to permit computer control; an analog signal averager was also added to the oscilloscope to reduce its noise level from 5 mV to 500 pV. In addition, the minicomputer performs digital signal averaging; therefore, it is possible to measure low level signals with high signal-to-noise ratio. Once a signal is acquired and

stored in the computer's memory or auxiliary floppy disc, it can be processed at any later

time. For example, when we are interested in frequency domain properties, such as the

spectrum amplitude of a signal or the attenuation of a component, then the minicomputer uses

the Fast Fourier Transform (FFT) to transform the data from the time domain to the frequency

domain. The minicomputer can also perform deconvolution to determine an unknown network's

impulse or step response.

Experiment Configurations

The experimental setups for measurements on glass fibers are shown in figures 6 and 7 for each of the two wavelengths used.

At 0.9 pm, we used laser diode A as the optical pulse generator. The light output from

the laser was collected by the 80 pm, 0.14 NA glass fiber held 0.1 mm from the laser's

emitting surface by a micropositioner. The output from the fiber was focused onto avalanche

photodiode D. The photocurrent pulse was sent to sampler C through a 60 ns delay line I. At the input to the delay line a trigger signal was picked off by a 6 dB power divider tee

for triggering the sampling oscilloscope time base. The delay line reduced the effective

oscilloscope bandwidth to 3 GHz, which is adequate for these measurements.

-4 -
An alternate technique would take the signal from the APD directly to the sampler and

trigger the oscilloscope from the pulse generator. However, with kilometer long fibers,

time delays of the order of microseconds are encountered. In these situations a long

electronic time delay network must be used between the driving pulse generator and the

trigger input to the oscilloscope. Quite stringent requirements are imposed on this delay

network, because it must provide microsecond delays with less than 20 ps of jitter. The

delay line method of figure 6 is an easy way around this problem.

We used an insertion technique to measure the response of a long fiber. First the

response waveform v^(t) of the measurement system ^^;as acquired by the APMS, with a short,

30 cm piece of fiber between the laser diode and the APD. This waveform includes the con- volved effects of laser, APD, fiber, delay line and sampler. To avoid saturation and non- linearity in the APD, a density 1.0 filter was placed between the fiber output and the

lens when the short fiber was measured. Then, using the micropositioners, we replaced the

30 cm fiber with the long fiber to be tested. The new waveform V

2 (t) was also measured by

the APMS.

The magnitude and phase of the insertion loss 82 ^,

FFT V

2 (t)

^21^^^ " FFT v^(t) ( 2 )

of the long fiber were calculated by the minicomputer. From Vj^(t) and V

2 (t) the minicomputer

also computed the impulse response of the fiber by deconvolution.

Figure 7 shows the experimental setup used for the 1.06 ym laser measurements. The YAG

laser produced a train of mode locked pulses with the repetition rate of 100 MHz (as opposed

to the 50 kHz of the laser diode) . The output of the laser was focused onto the core of a 10 m step index fiber with a 5X microscope objective. The numerical aperture of the microscope objective was specified by the manufacturer as 0 . 10 ; therefore, the spot diameter at the

focus of the objective was of the order of 10 ym [ 11 ] or significantly less than the fiber s

core diameter.

The laser and the 10 m fiber pigtail became the source for our measurements on the longer

fiber cables. We used a lOX , 0.25 NA microscope objective at approximately unit magnification

to couple the radiation from the pigtail to the fiber under test. (Whereas this objective

was designed for use with a tube length [ 11 ] of 160 mm, we used it for our application

because it was readily available in a solid mount. A simple calculation shows that it should

have been easily able to resolve the exit face of the 80 ym diameter pigtail.) Assuming

that the exit face of the fiber pigtail was filled but that mode coupling was insignificant

in a 10 m fiber, we conclude that the entrance face of the test fiber was irradiated with a bundle that had a launch numerical aperture of 0.10 (that of the 5X objective). The spot

diameter on the entrance face of the test fiber was about 80 ym, equal to the diameter of the

exit face of the pigtail.

- 5 -
At the output end of the test fiber, we focused the radiation onto the photodiode

with a single element, condensing lens whose focal length was about 17 mm. We used filters

to control the total power that fell onto the photodiode.

To measure the pulse response of the laser and the pigtail, we omitted the test fiber

and the microscope objective; that is, we used the condensing lens to focus the output of the

pigtail directly onto the photodiode.

The output of the CNET, Ge photodiode was sent directly to the input of sampler C. A

6 dB power divider split the output of the sampler into two paths. One signal passed through

a broadband amplifier and triggered the oscilloscope; 60 ns later the oscilloscope's sweep

started, so the sixth pulse after the trigger pulse was displayed on the screen. The second

output from the power divider went to a spectrum analyzer. The spectrum analyzer was, among

other things, useful as an alignment tool to help find the signal when it was too small to be

observed on the oscilloscope.

Test Fibers

Two fibers were tested in these experiments. One was a 100 m, step index fiber; the

other was a 580 m, graded index fiber. Additional specifications are listed in Table I.

Table I

Test Fiber Specifications

Parameter Fiber G Fiber H

Type Step-Index Graded- Index

Length 100 m 580 m

Core Diameter 80 pm 75 pm

Cladding Diameter 135 pm 125 pm

Cladding Material SiO^ Not specified

Jacketing Material None Plastic

Core Index of Refraction 1.5 Not specified

Numerical Aperture 0.14 0.16

Attenuation < 30 dB/km @ 0.82 pm < 10 dB/km

wavelength unspecified

Pulse Spreading 5-10 ns/km Not specified

- 6 -
Fiber Measurement Results

We report here the results of experiments on two fibers: (1) 100 m, step index fiber

at 0.9 pm and (2) 580 m, graded index fiber at 1.06 pm. The response of the 100 m, step index fiber to a laser diode pulse is shox,m in figure

8. Compare this figure with the reference pulse obtained with the short 30 cm fiber,

figure 9 (see figure 2 for an expanded time scale). The pulse has been broadened, at the 50%

level, by about 1.7 ns. The insertion loss is computed from these two waveforms using equation (2) . The

attenuation in dB versus frequency is plotted in figure 10. The APMS computes frequency

to much higher frequencies than shox^m. We have plotted only those data that we feel are valid. The low frequency attenuation is 1.6 dB/100 m which compares favorably with the

manufacturer's specification of < 30 dB/km. The -3 dB effective electrical bandwidth of

the fiber is 140 MHz.

In addition to determining the transfer function, one can also use the results of these

measurements to calculate the impulse response h(t) of the fiber. Numerical deconvolution

of noisy data is a tricky process and must be done with great care to avoid obtaining

erroneous results [14]. Using NBS developed deconvolution programs, the APMS computed the

impulse response of the 100 m fiber, figure 11. The impulse response can then be integrated

over the observation period to give the step response, figure 12. The impulse response

has a fast leading edge and a slow tail xvfith a 50% duration of 0.71 ns. The step response

has a 2.21 ns transition duration (10%-90%) with the relative 100% level being defined as

the last observed amplitude value in the time period. The true 100% level will occur much

later in time; note the non-zero slope of the step response near the end of the time period.

We did not cut these fibers to determine their bandwidth versus length characteristics.

Many other workers [15, 16] have observed that fibers often exhibit pulse dispersion that is

proportional to the length for short lengths (less than the equilibrium coupling length) and

that for greater lengths distortion is proportional to the square root of the length.

Using the YAG laser and the CNET Ge photodiode, we also tested a longer (580 m) , graded

index fiber at 1.06 pm. For this length of fiber and test conditions, the observed output

waveforms were slow compared to the input waveform and thus closely approximate the fiber's

impulse response. We noted some unusual effects X'/ith this fiber. As we scanned the input

laser beam across the core, we observed radically changing pulse responses as a function of

position. The common feature of these responses was a broad base with a duration of about 1

ns; superimposed on this base were several sharp spikes. The relative amplitudes and

positions of the spikes varied rapidly with the position of the beam. At times we saw as many as seven distinct spikes.

- 7 -
Other observers have seen somewhat similar impulse responses, although the spiking

behavior was less pronounced owing to longer excitation pulses, and they did not report

variations of the impulse response with the manner of excitation [17, 18]. The authors of

both references [17] and [18] were able to account for their observations by careful numerical calculation of the impulse response; the calculations were based on accurate measurements

of the refractive index profiles. We know of no simple argument that explains the existence

of spiking, except for a brief comment in reference [18] that relates their observations

with departures of the index profile from a precise alpha profile (irrespective of the value

of alpha)

.

We recorded several pulse response waveforms using various alignment criteria that

might be used in the field. All the waveforms are plotted on a scale of 1 ns/div; figure 13

is the reference waveform plotted on the same scale. (See figure 3 for an expanded waveform

at 200 ps/cm). The various alignment criteria were:

(1) Maximum pulse amplitude, figure 14.

(2) Maximum dc photodiode current, figure 15.

(3) Minimum amount of superimposed spiking, figure 16.

(4) Strongest harmonic at 1 GHz, as observed on the spectrum analyzer, figure 17.

(5) Strongest harmonic at 1.6 GHz (highest frequency observable on the test spectrum analyzer), figure 18.

(6) "Optimum” overall waveform based upon maximizing the amplitude while

minimizing the superimposed fine structure, figure 19.

Figures 20 and 21 show the attenuation and phase of the graded index fiber using the

"optimum" waveform, figure 19. The -3 dB frequency is 633 MHz. The phase plot shows a general upward trend that is attributed to a linear phase shift term. Depending upon the

oscilloscope triggering adjustment, the relative position of the response waveform in figure

19 will be moved around in the 10 ns observation period relative to the reference waveform,

figure 13. The only effect on the results is a change in the linear phase shift term. The

attenuation remains completely unchanged. The important phase characteristic in terms of

causing waveform distortion is the deviation from the linear phase shift. Further phase

data processing can be done by the minicomputer if desired.

Frequency responses were computed for all the waveforms. The attenuation results are summarized in figure 22. All possess a common frequency roll-off characteristic for attenuations less than 10 dB. The total variation in -3 dB bandwidths is less than 7%; major

variations occur only in the high frequency characteristics.
These results could have been estimated from a simple inspection of the time domain

waveforms. The waveform of figure 16 has essentially no spikes and may be considered as a simple 1 ns pulse. The other waveforms have spikes superimposed on the 1 ns pulse, which

carries most of the low frequency information. From Fourier analysis ^^7 e know that a large

number of high frequency components are required to generate a fast transient or "spike".

Thus the waveform with the maximum pulse amplitude, figure lA, likewise has the strongest

high frequency harmonics (-12 dB at 2.9 GHz) whereas the simple 1 ns pulse waveform has

the lowest high frequency harmonics (< -27 dB at 1.3 GHz).

Impulse and Frequency Response of an Avalanche Photodiode

A moderately fast avalanche photodiode (E) was also tested in this series of experiments

to determine its impulse and frequency responses. In the measurement setup shown in figure

6, the 30 cm fiber coupled the light from the laser diode to the APD. The reference waveform

v^(t), figure 23, was obtained with APD D and had a duration of 0.33 ns. That diode was

then replaced by APD E and the response waveform v^Ct) was measured, figure 24. From these

data we see that has a duration of 2.1 ns and closely approximates the impulse duration

of E. The curve of attenuation versus frequency was computed by the APMS minicomputer,

figure 25. The -3 dB frequency is 240 MHz.

Acknowledgments

We wish to acknowledge the support of the Centre National D' Etudes des Telecommunications,

Lannion and Issy-les-Moulineaux, France, who furnished the fast Si epitaxial and Ge photodiodes as part of a long term cooperative exchange program between NBS and CNET. We also

wish to express our thanks to Douglas Franzen, Gordon Day and Bruce Danielson of NBS, who

lent us the graded-index fiber. Finally, we greatly appreciate the continued encouragement

of our group leader N. S. Nahman.

- 9 -
Figure 1. Pulse emitted by laser diode A as observed by the CNET Si photodiode.

Figure 2. Pulse emitted by laser diode A as observed by the avalanche photodiode
Figure 3. YAG laser output as observed by a CNET Ge photodiode.

Figure 4. YAG laser output as observed by PIN photodiode F.

- 11 -
Figure 5. The NBS Automatic Pulse Measurement System.

I

-12-
1 II a Q

P-.

w < cd •H

CQ

Q CQ

0^ T3 4g w a c

fO

0)

(D <r 0 4-*^ 2 1-1 v£) hJ

•rH i: ~-

f=-«

m

W CN C I o m Di

P3 (D -o 0) O H

L I - 13-

Figure

6.

S„

test

set-up

for

0.9

pm

fiber

measurements.
1.06

pm '

APMS

Laser

Room *

Room - 14 -

Figure

7.

Test

set-up

1.06

pm

fiber

measurements

using a

Nd:YAG

laser.
Figure 8. Response of the 100 m step index fiber to the

laser diode pulse.

i

100 mV

t

2 ns

-T

Figure 9. Input reference pulse at 0.9 pm.

- 15-
Attenuation

(dB)

Figure 10. The attenuation versus frequency for the 100 m

step index fiber.

- 16-
Figure 11. Deconvolved impulse response of the 100 m step index fiber.

Figure 12. Step response of the 100 m step index fiber.

- 17 -
Figure 13. Reference waveform. YAG laser laser pulse into the

CNET Ge photodiode.

Figure 14. Pulse response of the 580 m graded index fiber.

Input launching conditions adjusted for maximum

impulse amplitude.
I

I

Figure 15. Pulse response of the 580 m graded index fiber.

Input launching conditions adjusted for maximum

DC photodiode current.

Figure 16. Pulse response of the 580 m graded index fiber.

Input launching conditions adjusted for a "square"

pulse response with tlie minimum amount of superimposed "spiking"

-19-
Figure 17 . Pulse response of the 580 m graded index fiber. Input

launching conditions adjusted for maximum 1 GHz harmonic.

Figure 18. Pulse response of the 580 m graded index fiber. Input

launching conditions adjusted for maximum 1.6 GHz harmonic.

- 20-
Figure 19. Pulse response of the 580 m graded index fiber.

Input launching conditions adjusted for "optimum"

output pulse based upon maximizing the amplitude

while minimizing the superimposed fine structure.

- 21 -
Attenuation

in

dB

Figure 20. Relative attenuation, versus frequency.

- 22-
I

I

-23-

Figure

22.

Frequency

responses

of

the

580

m

graded

index

fiber

at

1.06

ym

for

various

input

launching

conditions.
1

10 mV

5 ns <

\ .

L——

Figure 23. Test impulse from diode A.

Figure 24. Impulse response of avalanche photodiode E.

- 24 -
Attenuation

in

dB

Figure 25. Frequency response of avalanche photodiode E.

- 25-
APPENDIX A

Inexpensive laser diode pulse generator for optical

waveguide studies*

James R. Andrews

National Bureau of Standards Boulder, Colorado 80302

(Received 10 August 1973)

An inexpensive GaAs laser diode pulse generator is presented. This generator has found application in the evaluation of optical pulse dispersion in glass fiber optical waveguide studies. It is capable of producing

optical impulses as narrow as 110 psec at a wavelength of 0.9 fx and a pulse repetition rate of 50 kHz. With a slight modification, it may be used to produce optical pulses of considerably longer duration at reduced repetition rates.

Many laboratories throughout the world are currently

investigating the possibilities of using optical telecommunications. Significant results have emerged from several laboratories during the last year in the development of low-loss optical waveguides. The most promising is the

circular dielectric waveguide. The University of South- ampton has obtained a minimum attenuation of 7 dB/km

in a multimode liquid-filled glass fiber.^ Researchers at Corning have achieved 4 dB/km with a multimode silica

fiber.^ These minimum values of attenuation are achieved

in the spectral range of wavelengths from 0.9 to 1.1 /i. Thus they are ideally suited to be paired with either GaAs laser diodes (X = 0.9 m) or YAG lasers (X = 1.06/u)- A problem that does exist with multimode waveguides

is pulse dispersion due to the different phase velocities

of the various modes. This dispersion imposes definite bandwidth limitations for the use of these fibers. For

laboratory studies of optical pulse dispersion in fibers, a source of narrow optical impulses is extremely useful.^ This paper describes an inexpensive optical pulse generator

that can be used for these studies. The optical pulse generator is a GaAs laser diode driven by an avalanche transistor as shown in Fig. 1. The circuit

is an avalanche transistor adaptation of the classical mercury switch-charged transmission line pulse generator."* The charge line DLl is charged through to BVcer

of Ql. (BVcer and BVceo are the transistor breakdown

voltages between the collector and emitter with the base- emitter circuit consisting of a resistor or open circuit, respectively.) When a positive trigger pulse is coupled

through Cl and R2 into the base of Ql, avalanche break- down is initiated. Ql rapidly switches to its lower break- down voltage BVceo and allows the charge line DLl to discharge into R5 and the laser diode CRl. The current

pulse duration is twice the electrical delay time of the charge line DLl.

The laser diode is an SG-2001. This same laser has been manufactured under the earlier type numbers of 40856 and TA-7606. This laser has a peak forward current

rating of 10 A. Its lasing threshold is t}rpically 4 A. The

maximum forward current pulse duration is 0.2 ^.sec. The avalanche transistor Ql is a 2N3904, which is an

inexpensive general purpose switching and amplifier transistor. The transistor is selected for a breakdown voltage

BVcer of approximately 130 V. When avalanched this

transistor will exhibit a AF of approximately 100 V. This in turn gives a peak current of 10 A in the circuit of Fig. 1. The actual peak current may be varied somewhat by

adjusting the U+voltage.

Resistor R5 is used to limit the maximum forward current Ifm in the laser diode. The value of R5 in series with CRl is nominally 5 ff to match the characteristic impedance of the charge line. The voltage drop across CRl

is nominally 4.5 V at 10 A and thus it may be approximated as a 5 SI resistor. Two additional 50 S2 coaxial cables, PI and PV, are included for oscilloscope monitoring of the laser diode

current and voltage waveforms. The 50 S2 impedance of these cables is much higher than the circuit impedances

to which they are attached. Thus they may be considered

to be high impedance voltage probes. PV monitors the voltage across CRl. PI monitors the voltage across the

series combination of R5 and CRl. The difference voltage

[F(R7) — F(RF)] is proportional to the current through CRl.

The triggering requirements of this circuit are not

critical. A pulse of +1.2 V, 1 nsec 10%-90% transition time, and 1 nsec duration is sufficient for stable triggering.

It may also be triggered directly from TTL logic with

B+ (1 50 - 300V)

Fig. 1. GaAs laser diode pulse generator schematic.

22 Rev. Sci. Instrum., Vol. 45, No. 1, January, 1974 22

- 26 -
23 James R. Andrews: Laser pulse generator 23

Fig. 2. Photograph of GaAs laser diode

pulse generator.

a slower pulse of 10 nsec transition time. It is necessary

to limit the ma.\imum pulse repetition rate to avoid e.\- ceeding the dutj' cycle limitations of the laser diode and

the avalanche transistor. The maximum duty cycle is 0.1%

for the laser diode and 0.05% for the transistor. The actual circuit construction is shown in Fig. 2. It is constructed entirely over a rigid copper ground plane, such as a copper clad circuit board. This ground plane is bolted onto the optical bench. The 5 Q microstrip trans- mission line D/A consists of a 1.27 cm wide strip of 76

thick FEP tape and a 1.27 cm wide strip of copper tape mounted directly on the ground plane. The FEP and copper

tape assembly is held in jilace b}- a covering of 1.9 cm wide

adhesive tape. The laser diode is mounted in an 8-32 nut

that is soldered at a right angle to the ground plane. All of the other components are soldered in [ilace using their own

leads for support. .All components arc positioned ver\- close

lie. 3. Delected output from Ta-7606 Ga.As laser diode. Single Q-switched impulse operation. Vertical scale is 5 mWdiv. Horizontal

scale is 50 psec/div.

to the ground jilane. .All leads are cut to the shortest possible lengths to minimize parasitic lead inductances. These pre- cautions are necessary to obtain a fast transition time with

this very low impedance (5 Q) circuit. The shields for the 50 P

coaxial cables are also soldered directly to the ground

plane. The cost of this generator, including the laser diode,

is less than S30.00. When GaAs injection laser diodes are subjected to a fast- rising steplike injection current pulse, the\' have been observed to exhibit two phenomena in their optical outj)ut.‘ The first is a delay in the optical output of the order of

several nanoseconds after the injection current has reached

the lasing threshold /u,. I'he second is Q-switching ex- hibited as a damped oscillation of the optical output with

resonant frequency of the order of 1.5 GHz. Both of these phenomena may be observed when using this generator.

Fig. 4. Long pulse operation of Ga.\s la.scr diode. Top Iran i> diode

injection current at 4.25 A/div. Itotlom trace is detected output at 400 m\' div. Horizontal scale is 2 nscc^div.

Rev. Sci. Inslrum., Vol. 45. No. 1, January, 1974 - 27 -
24 James R. Andrews: Laser pulse generator 24

For an application requiring a single narrow optical impulse, such as pulse dispersion studies in optical waveguides, the (^-switching may be used to advantage. If the duration of the injection current pulse is successively shortened, a point will be reached in which only the first pulse in the Q-switched train is excited. The actual duration of the optical impulse will vary depending upon the

laser diode. Thus to obtain a very narrow impulse it may

be necessary to make a selection of the best diode among

a purchased lot. With this generator and a TA-7606 laser diode the

result shown in Fig. 3 was obtained. In this case the length of charge line, DIA, used was 20 cm. The maximum

pulse repetition rate was 50 kHz. The photodiode used was an experimental diode.'' A 28 psec transition time sampling oscilloscope was used for the measurement. The

observed impulse duration (50% level) is 110 psec. Some

SG-2001’s have been found to give similar results. For relatively long optical pulses, it is necessary to simply increase the length of the charge line. The results

for a length of 1 m are shown in Fig. 4. The injection current waveform 10%-90% transition time is 4 nsec. The detected optical output shows the laser delay, Q switching, and multiple filament lasing. The maximum pulse

repetition rate for this particular generator was 5 kHz.

This paper has presented an inexpensive GaAs laser diode pulse generator with application in optical waveguide studies. It is capable of producing optical impulses

as narrow as 110 psec at a wavelength of 0.9 y. and a pulse

repetition rate of 50 kHz.

*Conlribution of National Bureau of Standards. 'W. A. Gambling, Electron. Power 18, 446 (1972).

*E. A. Torrero, Electron. Des. 30 (1972). ®Remy Bouillie and James R. Andrews, Electron. Lett. 8, No. 12, 309 (1972). ‘James R. Andrews, QST 56, No. 5, 16 (1972). °M. J, .Adams, S. Griindorfer, B. Thomas, Caroline F. L. Davies, and D. Mistry, IEEE J. Quantum Electron. OE-9, No. 2, 328

(1973;. 'L’Echo des Recherches (Centre National D'Etudes des Telecommunications, Issy-les-Moulineaux, France, 1972), p. 55.

Rev. Sci. Instrum., Vol. 45, No. 1, January, 1974 - 28 -
APPENDIX B

GaAs Laser Diode Pulse Generator (modified)

The GaAs laser diode pulse generator [9] described in Appendix A has been used at NBS

for several years. However, due to variations in avalanche transistors and laser diodes the

need was felt for some time to modify the previous circuit to allow for adjustment of the

pulse duration of the pump current. This was necessary to insure excitation of a single

optical pulse. With too long a current pulse, laser A will produce a train of relaxation

oscillations at approximately a 3 GHz rate. Figure B-1 is the circuit diagram of the modified, avalanche transistor pulser. It should be compared with figure 1 in Appendix A. The major changes are (1) that the 5 ohm

delay line DLl has been replaced with capacitors (C4-C7) and (2) that emitter resistor R5

has been deleted. The use of capacitors and the elimination of the emitter resistor allows

a larger initial surge current into the laser diode CRl. Other workers have reported compact

pulsers using ceramic-chip capacitors instead of transmission lines [3, 19]. The physical

mounting configuration is similar to that used in Appendix A. All wiring leads are kept

extremely short to minimize the circuit inductance. Likewise the array of four capacitors

(C4-C7) is used instead of a single capacitor to minimize inductance. C7 is a variable

capacitor to permit minor adjustment of the pump current duration. Variation of the B+

voltage permits minor adjustment of the pump current's magnitude.

As noted in Appendix A, it is necessary to select the avalanche transistor Q1 and the

laser diode CRl to optimize performance. In particular, the higher the breakdown voltage,

BV , the greater the pump current will be. If a particular laser when pulsed produces

LhK more than one relaxation oscillation, then the value of C7 should be reduced. If there is

not enough adjustment range then the other capacitors, C4-C6, can be increased or decreased

as needed.

A 50 ohm coaxial cable is used as a probe to monitor the voltage across the laser

diode CRl. Figure B-2 shows a typical voltage waveform at this test point.

- 29-
R7 43 R6 100 kfi

Figure B-1. Modified avalanche transistor pulser for GaAs laser diode.

Figure B-2. Typical operation. Voltage waveform across the laser

diode. Vert. lOV/cm, Horiz. 500 ps/cm.

- 30-
REFERENCES

[1] Andrews, James R. , Measurement of laser pulse dispersion in glass fiber optical waveguides, NBS Memorandum for the Record, NBS, Boulder, Colo., (July 1972).

[2] Nelson, B. P. and Stern, J. R. , Pulse propagation measurements on slightly overmoded

glass fibers, 1st European Conference on Optical Fibre Communications, London,

Sept. 16-18, 1975, 13-15.

[3] Dannwolf, J. W. , et al.. Optical-fiber impulse-response measurement system, IEEE Trans,

on Inst. & Meas., IM-25 , No. 4, Dec. 1976, 401-406.

[4] Cans, W. L. , et. al. , Applications of an automated pulse measurement system to telecommunication measurements. Conference sur la Mesure dans les Telecommunications, URSI,

Lannion, France, (October 1977), 165-170.

[5] Andrews, J. R. , Automatic network measurements in the time domain, Proc. IEEE, No. 4,

(April 1978), 414-423.

[6] Ramirez, R. W. , Evaluating fibers digitally. Laser Focus, (April 1979), 46-52.

[7] Mil-Std-1678 , Military Standard - Fiber optics test methods and Instrumentation, draft

version, 17 Dec. 1976, Method 6050, Pulse Spreading, pp. 6050-1 to 7.

[8] Andrews, J. R. , Light impulses from GaAs injection diode lasers, talk presented at Fibre Optical Communication Meeting, London, March 28, 1972.

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

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

[10] Greenhow, R. C. , and Schmidt, A. J., Picosecond Light Pulses, in Advances in Quantum

Electronics, edited by D. W. Goodwin, Academic Press, New York, London, 1974.

[11] Young, M. , Optics and Lasers, An Engineering Physics Approach, Springer, New York,

Berlin, Heidelberg, 1977, Chapters 2, 5.

[12] Detection des impulsions lumineuses issues d'un laser YAG par tete de detection au germanium, L'Echo des Recherches, CNET, Issy-les-Moulineaux, France, (July 1972), 55.

[13] Gans, W. L. , Present capabilities of the NBS automatic pulse measurement system, IEEE

Trans. Inst. & Meas., IM-25 , 4, (Dec. 1976), 384-388.

[14] Guillaume, M. E. and Nahman, N. S. , Deconvolution of Time Domain Waveforms, NBSIR 79- , National Bureau of Standards, Boulder, Colorado, 1979. In progress.

[15] Kao, C. , Estimating the dispersion effects in a practical multimode waveguide cable

for fiber systems, 1st European Conference on Optical Fibre Communication, lEE, London,

(Sept. 1975), 8-10.

[16] Gambling, W. A. and D. N. Payne, Some experimental aspects of propagation in optical

fibres, 1st European Conference on Optical Fibre Communications, lEE, London,

(Sept. 1975), 197-200.

[17] Okamoto, K. , Comparison of calculated and measured impulse responses of optical fibers.

Applied Optics, 1^, No. 13, (July 1, 1979), 2199-2206.

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

Appl. Opt., 15_, (1976), 483-491.

[19] Vanderwall, J. et. al.. Subnanosecond rise time pulses from injection lasers, IEEE, J. Quantum Electronics, July 1974, pp. 570-572.

- 31 -
NBS-114A (REV. S-7B)

U.S. DEPT. OF COMM. 1. PUBLICATION OR REPORT NO. BIBLIOGRAPHIC DATA

SHEET NBSIR 79-1620

4. TITLE AND SUBTITLE

TIME DOMAIN PULSE MEASUREMENTS AND COMPUTED FREQUENCY

RESPONSES OF OPTICAL COMMUNICATIONS COMPONENTS

5. Publication Date September 1979

7. AUTHOR(S)

James R. Andrews and Matt Young

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS 10.

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, DC 20234

11. Contract/Grant No.

12. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS rs/ree/. City, state, ziP) 13. Type of Report & Period Covered

14, SDoiBotiae Aitencv CtMto

'

i Z, $ Acc^srfon No«

15, SUPPLEMENTARY NOTES

I I Document (jescribes a computer program; SF-185, FIPS Software Summary, is attached.

16. ABSTRACT (A 200-word or leas factual aunmiary of moat aigni ficant information. If document includes a significant bibliography or

The purpose of this report is to demonstrate the application of the

NBS Automatic Pulse Measurement System (APMS) to measuring the pulse

responses of optical communications components and to computing their

impulse and frequency responses. For example we describe measurements

of the properties of two glass fibers and an avalanche photodiode using

both a pulsed GaAs laser diode (X = 0.9 ym) and a mode locked, Nd:YAG

laser (X = 1.06 ym) . All measurements were performed in the time domain;

frequency domain data were obtained from the time domain data by using

the Fast Fourier Transform (FFT) . The impulse response was obtained by

deconvolution.

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

separated by semicolons)

Avalanche photodiode; FFT; fiber optics; Frequency response; impulse response;

laser; jphotodiode

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