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AD-782 440

HIGH-EFFICIENCY, SINGLE-FREQUENCY LASER AND MODULATOR STUDY

R. C. Ohlmann, et al

Lockheed Missiles and Space Comp-ny, Incorporated

Prepared for:

Office of Naval Research Advanced Research Projects Agency

31 December 1973

DISTRIBUTED BY: m

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National Technical Information Service U. S. DEPARTMENT OF COMMERCE 5285 Port Royal Road, Springfield Va. 22151

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HIGH-EFFICIENCY, SINGLE- FREQUENCY LASER AND

MODULATOR STUDY

Uth Quarterly Technical Report

N-JY-71-22 31 December 1973

R. C. Ohlmann, K. K. Chow, J. J. Younger, W. B. Leonard, D. G. Peterson, and J. Kannelaud

ARPA Order No. 306 Contract No. N00014-71-C-0049 Expiration Date: 15 June 1974 Program Code 421 Amount: $399,684 Effective Date of Contract: 1 September 1970

Scientific Officer: Director, Physics Programs Physical Sciences Division Office of Naval Research, Arlington, Va. 22217

Principal Investigator: R. C. Ohlmann (415) 493-4411, Ext. 45275

Sponsored by

Advanced Research Projects Agency

The views and conclusions contained in this document are those of the authors and should not be interpreted as necessarily repre- senting the official policies, either expressed or implied, of the Advanced Research Projects Agency of the U.S. Government.

Lockheed Palo Alto Research Laboratory LOCKHEED MISSILES &c SPACE COMPANY. INC

A Subsidiary of Lockheed Aircraft Corporttion " Palo Alto, California 94304

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Section 1

INTRODUCTION

The work described in this report is the final year's effort of a three-

year program to study wide bandwidth laser communications at 1.06-^m

wavelength. Earlier efforts of this program (during the fir^c two years)

were directed to studying the means to produce a high-efficiency, single-

frequency neodynium doped yttrium aluminum garnet (Nd-VAG) laser

operating at 1.06 imn, and to produce high-efficiency, octave-bandwidth,

microwave light modulators for this wavelength (Refs. I and 2). In

addition, a study was also made of laser communication configurations

that were suitable for very high data rates. The overall objective of this

year's program, then, is to apply the results of these earlier studies to

assemble a laboratory communication system that uses the entire modu-

lation bandwidth available. In particular, the final result of this program

is to be a laboratory demonstration of a laser-communication system

having a bandwidth of 2 GHz.

1.1 OBJECTIVES

The detailed objectives of this study program are as follows:

(1) Design a laboratory communication system, using 1.06-^m

radiation from a N^YAG laser and having a system bandwidth

of 2 to 4 GHz.

1-1

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(2) Assess the state-of-the-art of high-frequency photodetectors,

with emphasis on a cross-field photomultiplier tube (PMT), that

offer good frequency and spectral response and are suitable as the

receiver elements of this communication demonstration.

(3) Define and design any necessary microwave, digital-modulation,

and frequency-modulation subsystems for the communication

system.

(4) Assemble and operate the various subsystems.

(5) Demonstrate and evaluate the system performance in a laboratory

environment.

1.2 LASER COMMUNICATION SYSTEMS

Work performed during this quarter has led to the successful achievement of

objective (2) and partial achievement of objectives (3), (4), and (5). The trans

mission of 1 - GHz analog and 1-Gbit/sec digital data over the laser link

has been accomplished in the laboratory. However, improvement of signal-

to noise ratios of analog signals as well as reduction of intermodulation

problems between the analog signals are most desirable. Work is in progress

to achieve these goals. It is estimated that a full-fledgc demonstration, with

improved signal-to-noise ratios, will be made by March 31, 1974.

1.3 PUBLICATIONS

A paper, entitled "Recent Advances in Ultra wideband Bandpass Optical

Beam Modulators, " has been presented at the 1973 International Electron

1-2

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entitled "A 2-Gbit/sec Laboratory Laser Communication System" is under

preparation for submission to the Sixth DoD Conference on Laser Technology,

Colorado Springs, Colorado, in March 1974.

4*

1-3

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Section 2

SYSTEM DESIGN

In the Third Semiannual Technical Report (Ref. 3), detailed considerations

and descriptions of the system design have been givin. Therefore, in this

report, only a summary will be given below, reference is made to the Third

Semiannual Technical Report for further details.

Block diagrams showing various transmitter and receiver subsystems and

the quadriphase shift-keying (QPSK) modulation format used in the 2 Gbit/sec

laboratory communication system demonstration are given in Figs. 2-1 and

2-2. The system can handle either a two-gigabit-per-second rate data

stream by modulating it onto two microwave subcarriers using the QPSK

modulation format as shown in these figures, or analog signals having a band-

width of one gigahertz together with one gigabit per second digital data. For

the transmission of this combined ana log/digital data, one of the QPSK sub-

systems is substituted by an analog signal subsystem as shown in Fig. 2-3.

Detailed block diagrams showing the operation of the analog FM subsystem

are given in Figs. 2-4 and 2-5. To fill the available analog channel bandwidth,

several local TV channels and a narrowband FM music channel at 400 MHz

are used. This is shown in Figure 2-4.

2-1

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VIIF TV SIGNAL PREAMPLIFIER

AUDIO SIGNAL SOURCE

ANALOG FREQUENCY MODULATION SUBSYSTEM

41

Fig. 2-4 Diagram Illustration: the FM Subsystem and the Combination of FM Analog and QPSK Digital Signals

2-5

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2-6

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»; The bulk of the digital data transmission work was done during <-he first six

months, only a small amount of work in tha* direction was done during this

quarter. The transmission of the combined one gigahertz FM analog and

one-gigabit QPSK digital data was initiated during the first six months. Some

difficulties were encountered because the subcarrier frequency chosen for

the analog portion (3. 5 gigahertz) appeared too high (Ref. 3). As a result,

the 2. 5 gigahertz subcarrier was used for the transmission of FM analog

signals as shown in Figs. 2-3, 2-4 and 2-5. This necessitated a certain

amount of change in components and system design. Fortunately, it is not

a major task and, by the middle of this quarter, the experimental demon-

stration has begun to ttke shape.

2-7

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Section 3

CRITICAL COMPONENTS

Most of the critical components required for the system demonstration have

been discussed in some detail ai the Third Semiannual Technical Report

(Ref. 3). Therefore, we shall only present the changes that have Leen made

during this quarter. For additional information, reference is made to the

Third Semiannual Report.

3. 1 THE OPTICAL MODULATOR

The performance of the optical modulator was much improved during the first

six months. Uniform frequency response was obtained in the 2 to 4 gigahertz

band as indicated by optical sideband measurements shown in Fig. 3-1.

Actual measurement of modulation index near the peak response showed that

80% modulation index for 0. 5145 p,m was obtainable at 8. 3 W of rf drive power.

mmm

This modulator was used in the initia] tests for the 2-Gbit/sec data transmission

and performed well, as reported in the Semiannual Report (Ref. 3). Unfortunately,

in the initial experiments for the 1-GHz analog and the 1 Obit/sec digital

transmission, the electro-optical crystal inside the modulator was fractured.

A new modulator crystal was substituted into the modulator. Because of the

schedule, it was not possible to make detailed tuning and matching to duplicate

the results obtained earlier. Modulation index as a function of frequency,

3-1

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obtained for the final version of the modulator, is as shown in Fig. 3-2.

Obviously, the perforrr Ance was not as good as that obtained earlier, but

this proved adequate for the demonstration of the 1GHz analog and tho

1-Gbit/sec digital data transmission. Laboratory measurements indicate

that approximately 35% modulation index is repeatedly obtainable at 2. 0 W

of drive at 3 GHz. •

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Fig. 3-1 Relative Optical Sideband Power as a Function of Monulation Frequency at Low Drive Power Levels for the First Version of the Optical Modulator.

3-2

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Fig. 3-2 Modulation Index as a Function of Frequency at 2-W Drive Power Level for the Final Version ot the Modulator

3. 2 PHOTODETECTORS

;:

Since the cross-field photomultiplier tube (CFPMT) specially ordered from

Varian Associates did not fulfill our requirements, much effort was expended

in the selection of a suitable photodetector. Solid-state photodiodes were

investigated; these included silicon PIN diodes, germanium PIN diodes, as

well as a silicon avalanche diode. AU these diodes have rather low frequency

response in the 3- to 4-GHz range; however, both silicon and germanium

PIN diodes appeared useful. Test results are briefly described as follows.

3-3

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In these tests, the frequency response of the optical modulator was first

determined by measuring the opt'.cal sideband pov/er as a function of modu-

lation frequency in a Fabry-Perot scanning interferometer. The modulated

laser beam is then detected by the photodetector under test. The output of

the photodetector is fed into a microwave spectrum analyzer, swept at a

very low frequency. The frequency response as displayed on the cathode-

ray tube of the spectrum analyzer then gives the combined effects due to the

optical modulator and the photodetector. That is, the display gives the

apparent received microwave power as a function of frequency. Since the

received power is proportional to the square of the modulation index, the

apparent modulation index as seen by the spectrum analyzer can be obtained

by taking the square root of the power spectrum. When this apparent

modulation index is divided by the known modulation index for the optical

modulator as measured previously, frequency response of the photodetector

is obtained. For convenience, ♦nese tests were initially made at 0. 5145 M,m.

Those having favorable frequency response were later tested at 1. 06 [Jim.

Figure 3-3 is the frequency response curve for a commercial silicon

avalanche diode (TI Model XL 55). It is seen that beyond 3. 0 GHz, the

response was very poor. Since avalanche noise was also very high, the

diode was not useful even at the visible wavelength. There is a germanium

avalanche diode (Tl XL 57) available for 1. 06 \im wavelength. Its specifications

on frequency response and noise characteristic are much worse than those

of TI XL 55. Therefore, the germanium avalanche diode was not seriously

considered.

3-4

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3-5

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H. ^ Figure 3-4 is the response curve for a silicon PIN diode (Philco Model 4501).

This diode has a rather small junction capacitance (0. 8 pF typical), so that

its frequency response is reasonab y good even in the 3- to 4-GHz range.

Unfortunately, the spectral response at 1. 06 p,m was very poor, thereby

rendering the diode not very useful for this demonstration. For laboratory

use with visible wavelengths, however, this diode is probably the most

desirable one.

1

Figure 3-5 shows the frequency response of the Philco germanium PIN diode

(Model 4520). This diode has a large junction capacitance ( ~ 5pF), and

consequently worse frequency response, than the silicon PIN diode tested

above. Beyond 2.6 GHz, there is not much response. However, since the

diode is spectrally sensitive at 1. 06-|im and can withstand much light intensity,

it is still the best detector for the 1.06-v,m experiments. It is therefore

chosen for the 1. 06-^m tests.

■>» s

3-6

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i. 3 THE VOLTAGE CONTROLLED OSCILLATOR

Originally, the subband used for the transmission of 1-GHz bandwidth

analog signals was the 3- to 4-GHz band. The performance of two VCO's

operating in this band has been reported in the Third Semiannual Report

(Ref. 3). Because of the poor frequency response of the photodetectors as

well as the norxinearity problems in the VCO drivers, it was decided to use

the 2- to 3-GHz subband for the transmission of analog signals (see Ref. 3).

For this reason, a new VCO was ordered from Omni-Spectra having a center

frequency of 2. 5 GHz. This VCO was received at the end of the third quarter

and showed excellent performance characteristics as shown in Figure 3-6.

The overall linearity over the tuning range of 2. 23 to 2. 275 GHz was good.

In particular, over a tuning range of 160 MHz ( t 80 MHz) centered at

2. 5 GHz, excellent linearity is obtained. The voltage required for this

t 80 MHz deviation is only 1. 5V peak-to-peak, corresponding to a drive

power of o mW. Thus, this VCO showed much better response than any of

the previous ones and is most appropriate for the FM subsystem.

3.4 THE WIDEBAND FM DISCRIMINATOR

For the demodulation of analog signals, a wideband FM discriminator is

required. The design of wideband discriminators have been accomplished

at LMSC; it is based on the work of Kincheloe and Wilkens (Refs. 4 and 5),

who suggested the use of a microwave power divider and a 3-dB hybrid

coupler connected by constant impedance transmission lines. The arrange-

ment is a microwave analog of a one-dimensional optical interferometer;

its theory of operation has been presented in the Third Semiannual Technical

Report (Ref. 3).

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FREQUENCY (GHz)

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Fig. 3-9 Static Voltage Tuning Characteristics of the Final Voltage-Controlled Oscillator

3-8

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During the first half year, a discriminator operating in the 3- to 4-GHz

subband has been fabricated. Tests have shown good performance. However,

because of the change of the frequency modulation subband, a new FM dis-

criminator had to be fabricated. This was done this quarter,and the results

are shown in Fig. 3-7. Over a deviation of t 80 MHz, the output of the

discriminator is linear. Linearity over a greater range can be obtained by

carefully changing the lengths of the two interfering paths. Sinr.e this linearity

range is moro than adequate for the VCO, no additional work was performed.

-100 2.0 2.5

FREQUENCY (GHz)

3.0

Fig. 3-7 Performance Characteristic of the Final Wideband FM Discriminator

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Section 4

SYSTEM IMPLEMENTATION AND TESTS

The system layout is essentially a hardware copy of the block diagrams

shown in Figs. 2-2 through 2-5. Judicious choice of amplifiers and

attenuators has to be determined to ensure proper signal levels so that

signal-to-noise ratios , re maximized and cross-talks are minimized. The

digital subsystems have been thoroughly discussed in the Third Semiannual

Technical Report (Ref. 3) and will not be repeated here.

The FM subsystems have been assembled; initial tests have been carried

out. Between 20-25 dB signal-to-noise ratios (picture carrier to noise) for

the TV channels are typically obtained. These are considered insufficient

for good display oi the TV pictures (for good-quality TV pictures, typical

signal-to-noise ratios are in the order of 40 dB). Thus the system is still

undergoing continuous adjustment and optimization. For this reason, no

detailed system-implementation block diagram is presented here. AH the

details will be presented in the Final Report, when the system will be

optimized within the present laboratory constraints.

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Section 5

CONCLUJIONS AND FUTURE PLANS

'

The results of this quarter's work indicate that the combined 1-GHz FM

analog and 1-Gbit/sec digital data transmission is feasible over a 1. 06-^m

laser link in the laboratory. Although the analog subsystem performance is

still marginal, it is believed that with continuing effort during the next

quarter, system performance can be greatly improved.

During the next quarter, therefore, we shall painstakingly go vhrough the

iterative steps of optimization to assure that the best obtainable signal-to-

noise ratios are obtained consistent with minimum amount of intermodulation

and partial reflection at component interfaces.

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Section 6

REFERENCES

1. R. C. Ohlmann, W. Culshaw, K. K. Chow, H. V. Ftence, W. B, Leonard,

andJ. Kannelaud, High-Efficiency, Single-Frequency Laser and Modulator

Study, First Annual Techmctl Report, 30Sepl971, Contract No. N00014-

71-C-0049, Office of Naval Research/Advanced Research Project A.gency.

2. , High-Efficiency, Single-Frequency Laser and Modulator Study,

Second Annual Technical Report, 31 Dec 1972, Contract No. N00014-71-C-

0049, Office of Naval Research/Advanced Research Project Agency.

3. R. C. Ohlmann, K. K. Chow, J. J. Younger, W. B. Leonard,

D. G. Peterson, andJ. Kannelaud, "High-Efficiency, Single-Frequency

Laser and Modulator Study, " Third Semiannual Technical Report,

30 Sep 1973, Contract No. N00014-71-C-0049, Office of Naval Research/

Advanced Research Project Agency.

4. W. R Kincheloe and M. W. Wilkens, "Phase and Instantaneous

Frequency Discrimmators, " U. S. Patent 3, 395, 346, Jul 30, 1968.

5. M. W. Wilkens and W. R. Kincheloe, Microwave Realization of

Broadband Phase and Frequency Discriminators, Stanford Electronics

Laboratory, Tech. Rep., 1962/1966-2, Nov 1968. DDC ASTIA

Doc. AD 849,757.

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