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AD-782 189
HIGH-EFFICIENCY, SING LE-F REQUENC Y LASER AND MODULATOR STUDY
R. C. Ohlmann, etal
Lockheed Missiles and Space Company, Incorporated
r Prepared for:
Office of Naval Resenrch Advanced Research Projects Agency
15 June 197 4
DISTRIBUTED BY:
National Technical Information Service U. S. DEPARTMENT OF COMMERCE 5285 Port Royal Road, Springfield Va. 22151
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LMSC-D403175
HIC.H-EFFICIUNCY, SINGLE-FREQUENCY LASER AND MODULATOR STUDY
Third Annual Technical Report
N-JY-71-24 15 June 1974
R. C. Ohlmann, K. K. Chow, J. J. Younger, W. B. Leonard, D. G. Peterson, andJ, Kannelaud
ARPA Order No. 3()f) 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
A' ■: •
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 & SPACE COMPANY, INC.
A Subsidiary of Lockheed Aircraft Corporation Palo Alto, California 94304 -'
NAT10NA1 il':HNICAL . INFORMATION SERVICE
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CONTENTS
LMSC-D403178
Section
4
5
6
7
ILLUSTRATIONS
INTRODUCTION
1. 1 Objectives
1.2 Laser Communication Systems
1.3 List of Publications and Patent
SYSTEM DESIGN
2. 1 Modulation Formats and Bandwidth Considerations
2.2 (Aiadriphase-Shift-Keying Digital Modulation
2.3 FM Analog-Modulation
CRITICAL COMPONENTS AND SYSTEM IMPLEMENTATION
3.1
3,2
3.3
3.4
3.5
3.6
3.7
The Optical Modulator
Photodetectors
PN Signal Generator and Biphase Modulators/ Demodulators
The Voltage Controlled Oscillator
The Wideband FM Discriminator
Voltage-Controlled Oscillator/FM Discriminator Tests
System Implementation
3.7.1 The 2-Gbit/sec System
3.7.2 The l-GHz FM Analog and 1-Gbit/sec Digital System
SYSTEM TESTS
4. 1 QPSK Digital Modulation Results
4.2 FM Analog and QPSK Digital Modulation Results
LABORATORY DEMONSTRATIONS
CONCLUSIONS AND RECOMMENDATIONS
REFERENCES
Page
iii
1-1
1-1
1-2
1-2
2-1
2-1
2-2
2-7
3-1
3-1
3-5
3-9
3-9
3-9
3-11
3-15
3-15
3-20
4-1
4-1
4-5
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ILLUSTRATIONS
Page
2-3
'-1 Block Diagram Showing Various Subsystems and Modulation Formats Used in the Laboratory Communication System Demonstration
2-2 2-C.bits/sec Laboratory Laser Communication System: Optical Subsystem and Transmitter Electronics
2-3 Two C.bit/sec Laboratory Laser Communication System: Receiver Electronics
2-4 Diagram Illustration-, the FM Subsystem and the Combination of FM Analog and QPSK Digital Signals
The FM Demodulation System
2-3
2-8
2-10
3-2
3-3
Modulation Index Versus Frequency for 7. 5-mm-Length LiNb03
Crystal With rf Power in Reverse Direction
3-2 Modulation Index Versus Frequency for 7.5-mm-Length LiNb03 Crystal With rf Power in Forward Direction.
3-3 Relative Optical Sideband Power as a Function of Modulation Frequency at Low-Drive Power Levels
3-4 Modulation Index as a Function of Frequency at 2-W Drive Power Level for the Final Version of the Modulator J-4
3-5 Relative Frequency Response of the Varian Static Cross-Field Photomultiplier Tube - Test Performed at 0.5145 ßm
3-6 Relative Frequency Response of the TI Silicon Avalanche Diode - Test Performed at 0.5145 fjm
3-7 Relative Frequency Response of the Philco SUicon PIN Diode - Test Performed at 0.5145 imx
3-8 Relative Response of the Philco Germanium PIN Diode - Test Made at 0.5145 /jm
3-9 Static Voltage Tuning Characteristics of the Final Voltage- Controlled Oscillator
3-10 Performance Characteristic of the Final Wideband FM Discriminator 3-12
3-11 Experimantal Setup for the VCO/FM Discriminator Tests 3-13
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Preqnmoy ResponM of VCO/FM-OiseiimiiHitor iVst Compcmenti
Output of the I'M Diflcrimtnator When the VCO is Swept Botwoen 10 and 470 Mil/, by the Levelled Sweeper ami Driver Amplifier
Output of the I'M Discriminator When a ("ompensation Network Is Added to the VCO Driver
Transmitter of 2-Gbit/iec Laboratory optieal Communication System
Receiver of 2 Obit/sec Laboratory Optical Communication System
I'M Transmitter Klcctronics
FM Receiver Electronica
Sampling Scope Display ol Four 500-Mbit/aec Streams Transmittcil Through the Laser Link
Waveforma of Two SOO-Mbit/aec streams on 2.5-OHt Bubcarrier Frequency With Simultaneous 1-Obit/sec Digital Signal on 3, j-oii/. Subcarrier
Waveforms of Two lüÜ-Mbit/sec Streams on S.S-QHl Subcarrier Frequency With Simultaneous l-Gbit/eec Digital Signal on 2.5-OHl Subcarrier
Complex Microwave Spectrum as Seen at the Output of the Traveliim Wave amplifier
Complex Microwave Spectrum As Seen at the Heceiver (After Optical Detection anl Microwave Amplification)
Spectrum of TV Channels 4 and S After Transmission Through l.üG-A/m Beam, Detection by a Germanium PIN Diode, and 1 rocessed to be Ready for Display on TV sets
Spectrum of TV Channels 7 and B After Transmission Through l.O6-0m Beam, Detection by a Cermanium PIN Diode, and Processed to be Ready for Display on TV sets
Spectrum of TV Channels 4 and 5 As Received From a High-Cain Antenna and Amplified
Spectrum of TV Channels 7 and !) As Received From a Iligh-C.ain Antenna and Amplified
Waveforms of the Two 500-Mbit/sec Data as They Appear at Various Stages in the Laboratory Communication"System
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Section 1
INTRODUCTION
l.l OBJECTIVES
The work deecrtbed in this report is the tinal year's effort of a three-year program to
study Wide bandwidth laser communications at I.OS-fin WAVeleilgth. Earlier efforts
of this program (during the first two years) were directed to studying the means to pro-
duce a high-efficiency, single-frequency neodynium doped yttrium aluminum garnet
(Nd:YAG) laser OperattBg at LOG ^m. and to produce high-efficiency, octave-bandwidth,
microwave light modulators lor this wavelength (Refs. I and 2). In addition, a study
was also made of laser communication configurations that were suitable for very high
data rales. 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 modulation 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 I GHz.
The detailed objectives of this study program are as follows:
• Design a laboratory communication system, using 1.06-Mm radiation from a
Nd:YAG laser and having a system bandwidth of 2 to 4 GHz
• 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 communi-
cation demonstration
• Define and design any necessary microwave, digital-modulation, and frequency-
modulation subsystems for the communication system
• Assemble and operate the various subsystems
• Demonstrate and evaluate the sys.em performance in a laboratory environment
1-1
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Work performed during this year lias led to the suecessl'iil achievement of all the ob-
jeetives listed above,
1.2 LASER Cü.M.MlXICATION SYSTEMS
Two laboratory laser eoniniunieation lyitemi have been designed, implemented, and
demonstrated. The first one is a digital eommunieation system employing a quadri-
phase shift keying (QPSK) modulatio.i format on two microwave subearriers (-'.5 Gil/
and 3.5 GHz) to earn- four data streams totaling 2 Gbit/sec. The second system
uses 1 QHl of bandwidth to transmit a numl er of analog signals by frequency modulat-
ing a microwave tubcarrier (2.5 GHz), and the second GHz of bandwidth for the QPSK
transmission (subearrier frequency 3.5 GHz) of 1 Gbit/sec digital data. The detected
signals at the receiver all show good signal-to-noise ratios. For instance, for the
QPBK digital data, it is estimated that by using matched filter detection, the data should
be recoverable with only 2- to 3-dB degradation in performance from the theoretical
integrate-and-dump performance. For the analog EM transmission, signal-to-noise
ratios of 30 dB have been typically measured.
1.3 LIST OF PUBLICATIONS AND PATENT
The following is a list of the public:itions and patent resulting from the work that has
been supported, in total or in part, by this contract during the third year.
1. K. K. Chow, W. B. Leonard, and J. J. Younger, "Recent Advances in Ultrawide-
band Bandpass Optical Beam Modulators, " presented at the 1973 International
Electronic Devices Meeting in Washington, D.C.. December 1973
2. K. K. Chow, R. C. Ohlmann, R. B. Ward, and R. F. Whitmer, "A 2 Gbit/sec
Laboratory Laser Communication System. " accepted for publication in the
Proceedings of the Sixth DoD Conference on Laser Technology, Colorado Springs,
Colorado, March 1974
3. K. K. Chow, "Wide-Band Traveling-Wave Microstrip Meander Line Light Modu-
lator, " U.S. Patent 3, 791, 718, February 12, 1974; assigned to the United States
of America as represented by the Secretary of the Navy, Washington, D.C.
1-2
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Section 2
SYSTKM DESIGN
2. 1 MODULATION rOHMATS AND BANDWIDTH CONSIDKRATIONS
To transmit daUi at the high rates ciesired, microwave subcarrie.' modulation lormats
are chosen. That is, the intormation to be transmitted is first modulated onto a micro-
wave carrier irequency (the subcarrier), which, in turn, is modulated onto the optical
beam (the optical carrier). For a given modulation format, the data rate to be trans-
mitted determines the required system bandv/idth. Therefore, for this demonstration,
the data rate has to be chosen so that the entire available microwave bandwidth of the
optical modulator is used.
At the present moment, there is no one single data source that will occupy the octave
band from 2 to 4 GHz. Therefore, to demonstrate the bandwidth capacity of this labora-
tory communication system, a convenient way must be devised. This is obtained by
transmitting a number of signals filling that band; i.e. , the demonstration can be ac-
complished by using several microwave subcarriers, each with its own sidebands, so
as to fill up the entire modulation band of 2 GHz.
To carry out the demonstration in this fashion, two modulation formats have been
chosen. One uses quadriphase-shift-keying (QPSK) of a microwave subcarrier to trans-
mit digital signals; the other uses frequency modulation of a microwave subcarrier to
transmit analog signals. In this way, both digital- and analog-signal transmission will
be demonstrated. In fact, a combination of these two types of signals, filling the 2- to
4-GHz band, can be made to demonstrate simultaneous transmission of digital and
analog signals. In the digital QPSK format, two streams of digital data at a rate of
0. 5-Gbit/sec each are the most conveniently obtainable signals in a laboratory. *
♦For a brief description of QPSK modulation, see subsection 2.2.
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1 GH. ol bandwidth. For FM analog modulation, the modulation bandwidth is tvpically
not hmued bv the .ignal sources; rather, it is lirmted by the devices that impress the
modulation onto the subcarrier. One of these devices is usually a voltage-controlled
oscillator (VCO) whose instantaneous frequency is a function of the input modulation
voltage. For VCO». U, this frequency range, a 1-Gllz spectrum width is usually around the upper limit of their capabilities.
In tins demonstrate, therefore, wc shall use either (1) two microwave subcarriers
-.ch being QPSK-modulated at 1 Gl,.t/Sec to obtain a total data rate of 2 Gbit/sec- or
(2) two subcarriers. one QPSK-modulated to give 1 Gbit/sec and the other modulated
by vanous FM signals to occupy approximately a gigahertz of bandwidth. In this way
both type, of modulation signals will b. used and performance of the system will be in-
veetignted. Figure -I is a block digram showing the use of these various formats in
tins demonstration. The choice ot subcarrier frequencies for these formats will be
discussed in the following subsections.
2.2 QUADRIPHASF-SHIFT-KEYING DIGITAL MODULATION
Detailed block diagrams of this laboratory laser-communication system using QPSK
modulation format to transmit a 2-Gbit/sec data stream is shown in Figs. 2-2 and '-3
The optical subsystem, the digital modulation subsystems, and the modulator driver
are shown in Fig. 2-2. The microwave receiver, which amplifies the output of the
photodetector. and the subsequent digital demodulation subsystems are shown in Fig 2-3
The optical subsystem is self-explanatory; the laser and the modulator were both de-
veloped under this contract in an earlier phase. The photodetector. however, is a com-
mercial one, and there is yet no one photodetector that is completely satisfactory for
our demonstration. This point will be elaborated in subsection 3.2.
In such a digital QPSK modulation subsystem, a cw microwave signal is used as a sub-
carrier. Modulation of the subcarrier by the digital data results in a change of the rf
Phase of the subcarrier. These are shown in detail in the phasor diagram presented in
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•)_•> Because of the required bandwidth considerations, as discussed in subsec-
Uon 2.1. two subcarriers are chosen- one at 2. 5 GHz. and the other at 3. 5 GHz.
Each subcarrier is split into two channels: on. "in-phase" channel and one "m-
quadrature" channel. Each of the channels is then biphased-shilted at a biphase modu-
lator by an mdependent 50ü-Mbit/sec simulated data stream from a pseudo-random
(PR) signal generator as shown in the associated phasor diagram in Fig. 2-2. That is,
ft« phase of the microwave subcarrier of that channel is reversed at eac change of
state of the signal at the binary input terminal of the biphase modulator. Thus. U the
modulating signal is a binary "1." the phase of thai channel is undisturbed. If. on the
other hand, the modulator s.gnal is a binary "0," the ^M. of that channel is reversed
(switched by 180 electrical degrees as shown dotted in the phasor diarram).
Since the two channels are already in quadrature, the two biphase modulators will give
four possible quadrature phase relationships. Therefore, the combination of the two
channels results in the final phasor relationship shown in Fig. 2-2 and gives a total
data rate of 1 Gbit/sec for that subband.
At an input data rate of 1 Gbit/sec. the sideband power of QPSK modulation has first
nulls at 500 MHz above and below the subcarrier frequency. Out** the first nulls,
the power content is negligible. Therefore, for each subband, the center frequency of
the subband is chosen as the subcarrier frequency (2. B and 3. 5 GHz as mentioned
earlier), and a 1-GHz bandpass filter (2 to 3 GHz and 3 to 4 GHz) is used to reject the
sideband power outside the first nulls. Combination of these two subbands gxves a
total data rate of 2 Gbit/sec. This is amplified by a 10-W traveling-wave tube (TWT)
to drive the optical modulator.
in the photr .ector. the optical carrier is detected to recover the microwave sub-
carriers and their sidebands which are then sent to the first amplifier in the receiver
subsystem. After amplification, the microwave signal is divided into two halves as
shown in Fig 2-3. Each half is filtered to give one of the subbands which is then
further divided into two channels. The signal in each channel, together with a strong
2-6
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"in-phase" or "in-quadrature" reference, is directed to a biphase iemodulalor. In a
communication system, the reference signals required are normally derived from the
incoming signals. For the ease of this demonstration, howover, the reference signals
are taken from the transmitter directly by separate cables (hardwire references) to
demonstrate the capacitj of the laser communications system, without having to deal
with the added complications of phased-locked loops, etc. for the derivation of the
reference signals. These hardwire references are clearly indicated in Figs. 2-2
and 2-3.
The biphase demodulator performs synchronous demodulation of the received QPSK
signal by comparing its phase with that of the reference signal. The output signal of the
biphase modulator is filtered and amplified by a baseband amplifier to recover the
500 Mbit/sec digital data at each channel. This will be discussed in greater detail in
subsection 3.3. Two 500-Mbit/sec streams are recovered for each subband, i.e.,
■ total of 2 Gbit/sec data is recovered from both subbands.
It should be emphaf iztd here that, although the 500-Mbit/sec PB signals for both
channels in the subband are synchronous as used here in the demonstration, this
system will accept asynchronous aata with only minor degradation. This has been
demonstrated in an earlier experiment conducted at LMSC (Ref. 3).
2.3 FM ANALOG-MODULATION
For the transmission of analog signals, frequency modulation of a microwave sub-
carrier is chosen. The block diagram for this particular form of modulation is shown
in Fig. 2-4; the heart of the modulation subsystem is the voltage controlled oscillator
(VCO). The VCO accepts amplified baseband analog signals (e.g., from local TV
stations) and uses them to control a voltage-sensitive tuning element of the oscillator,
typically a varactor diode. Thus, the instantaneous output frequency of the oscillator
is proportional to the instantaneous voltage oi the analog signal. For this experiment,
several local TV channels, as well as an audio signal source which modulates a
narrow band FM (NBFM) 400-MHz signal generator, are chosen so that the sidebands
I* 2-7
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ANAIXX; FM j DICITAl. QPSK
1 J_ll_ll :,'n 3■r, 4-0^
TO ELCCIHOOPTIC Monui^roR
2 - TO 4 - Cllz TWT OPTICAL MOOUIATOM DIUVFH
FROM ./IC.n A I. . MODUl^TION STBSVSTKM NO. (3.0-4.0 QHS)
Ö
40. MHz NARROWRAND FM CARRIER—TV ^2.5 GHZ |
SUBCARRIERI
RF SPECTRUM AT THIS FREQ
2 T0 3-(;ilz BANDPASS FILTER
2.5 GHz VOLTAGE- CONTROLLFJ OSCILLATOR
VCO DRIVER AMPLIFIER
TV ANTENNA
V POWER RKCOMBINER
SIGNAL STRENGTH EQUALIZER
VHF TV AUDIO SIGNA L SIGNAL PREAMPLIFIER SOURCE
ANALOG FREQUENCY MODULATION SUBSYSTEM
Fig. 2-4 Diagram Illustration: the FM Subsystem and the Combination of FM Analog and QPSK Digital Signals
2-8
LOCKHEED PALO ALTO RESEARCH LABORATORY l O C « H i I 0 « 1 S J F ! , , . $.«CI COMfANY. iNC
A (UlSIDIAir Of (OCKNilD AIICIAFt COiroiATION
- ■ -^ — ■■ —■ ■ M^- — - —
LM8C-D40317S
i.. of the VCO occupy practically the entire gigahertz band. TV signals and audio FM
40()-MHz signals are combined, amplified, and used to control the frequency of the
VCO. At the beginning of this experiment, the microwave subcarrier for the FM sub-
band was chosen to be 3.5 GHz because it was believed that VCOs having 400 MHz of
linear tuning range could be obtained more easily at a S.S-QHc center-frequency.
However, expertenoe gained durn g this program showed that such was not necessarily
the case. In ..udiuon, the poor frequency response of photodetectors at the high-
frequoncy end (2- to 4-GHz band), coupled with the requirement that analog TV signals
require rather high signal-to-noise 'S/N) ratio to attain good reception and presentable
pictures, changed our earlier thoughts about the choice of this subcarrier frequency.
As a result, 2.5 GHz is now used as the subcarrier frequency for the analog signals,
and system demonstrations performed using this subcarrier frequency have given satisfactory results.
In this experiment, the TV signals and the 40ü-MHz NBFM signal are combined and
amplified to drive the VCO. From the VCO, the signal is passed through a 2- lo 3-GHz
bandpass filter to eliminate undesirable sidebands and spurious signals. The rf spec-
trum at that point (after the bandpass filter) is then as shown in Fig. 2-4. This signal
is sent to the power recombiner to replace the lower subband of the digital QPSK signals
(the 2- to 3-GHz subband) as shown in Fig. 2-1. After combination with the upper
subband containing the other digital QPSK signals (the 3- to 4-GHz subband), the com-
bined signal is sent to the traveling wave tube (as shown in Fig. 2-9) for the modulation of the optical signal.
The demodulation system for the FM subband is rather simple as shown in Fig. 2-5.
The received signal from the photodetector is again split into two halves; one oi them
goes to the 3.5-GHz subcarrier digital demodulation circuit as before, while the
other is directed to the FM demodulation subsystem as shown in Fig. 2-5. This signal
is filtered by a 2- to 3-GHz bandpass filter and sent through a wideband FM discrimina-
tor. It is then amplified by a baseband amplifier to recover the TV signals and the 400- MHz NBFM signal.
2-9
LOCKHEED PALO ALTO RESEARCH LABORATORY i o « H i i n Miisilis t tr«Ci cOMP*Nr INC
A S U I S I 0 i * I I OF lOCKMMO « I t r I » F I co*rot*iiON
L ■ —■ —^-""^
LM8G-D403175
FROM P:iOTODETECTüR
i . 2- TO 3-GHz BANDPASS FILTER
10- TO 40-dB BASEBAND AMPLIFIER
TO TV SETS AND 400-MHz NBFM RECEIVER
Fig. 2-5 Thü FM Demodulation System
2-10
LOCKHEED PALO ALTO RESEARCH LABORATORY IOCKHIIO ••••liil I Sf»cl COM»«Nr ,mt * tuiiioi«ir o, ^CKHIIO »MCtAfl COM,o,^,OH
■— ■ -I fll HMIlllt. ■■ I I ,|MM,^M||^gMm|aM-^.||M, j
LM8C-D403176
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1
Section 3
CRITICAL COMPONENTS AND SYSTEM IMPLEMENTATION
Most of the critical components required for the system demonstration have been dis-
cussed in some detail in the Third Semiannual Report (Ref. 4). Therefore, their per-
formance will be presented here only briefly. For additional information, reference
is made to the Third Semiannual Report. However, the changes that have been made
during the last six months will be discussed in some detail here.
3.1 THE OPTICAL MODULATOR
During the first six months, additional effort was spent in improving the 2- to 4-GHz
electrooptic modulator, since this modulator is the heart of this demonstration. In
particular, the "reverse-flow" mode of operation was carefully investigated. This is
the mode in which the rf drive power for the optical modulator flows through the circuit
in such a way that the electrooptic modulating crystal is at the input digit. Through
painstaking tuning and matching procedures, improved performance over the "forward-
flow" mode was obtained, as shown in Fig. 3-1. At a G-W input drive level, an aver-
age of 60-percent modulation index across a 3-dB bandwidth of 2.07 to 4.00 GHz is
obtained. This is a definite improvement over the "forward-flow" mode obtained last
year, shown in Fig. 3-2 as a comparison, for which approximately 62-percent average
modulation index was obtained at about a 10-W drive level.
It appears from Fig. 3-1 that the modulator was tuned to a higher passband than the
desired 2 to 4 GHz, because the low-frequency end showed a sharp drop while the
high-frequency end showed uniform response to the band-edge. Therefore, additional
turning effort was applied to move the passband lower. This was met with some suc-
cess: Fig. 3-3 shows the relative optical sideband power as a function of modulation
frequency at low-drive power levels. At these drive levels and using this particular
3-1
LOCKHEED PALO ALTO RESEARCH LABORATORY tOCKHltD MlftlllS 1 i t A t I COMrtNT. INC
A tutsioiiir OF IOCHMIIO * i ■ c t A r t coiroi/kiiON
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x,, o.iu&titn LiNbOa CRYSTAL, 0.3 BY 0.3 BY T.5 nun MODULATOR TEMPERATIRE IBS'C
MICUOWAM: POWER FROM OUTPin TO INPUT I'OHI
2.4 -f ■ ' 1 1 1 L -.(J 2.8 J.O :t • i i :J'0 s.a :). 1 3.8
MODULATION PltEQUEKCY (GHi) l.o
Fif. 5-1 ModulaUon Index Versus Frequency lor 7. 5-mm-Length UNbO, Crystal With rl Power in Reverse Direction
»,li ~j
lM£>RETtCAL INDEX FOB lO-WDRIVE
> y. c g y.
P
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CURVE OEDUCED IHoM LOWER i'owru MEASUREMENTS
THEORETICAL WDEX FOR B-W DIUVI
nr imr\-'. - s w(INDICATED)
Xf, 0.ftl48 ^n., LiNbO.-, CKYSTAI. 0.9 BY 0.3 HY 7 B mm kfODCLATDR TEMPEUATUUK 165 C. MICROWAVE POWBR PROM INI-IT TO OUTPUT PORT
L U* 2-H •■,.(l 3»2 8.4 3.0 3.V ~0
MOOULATKW FREQUENCY (GHi)
Fig. S-S Modulation Index Versus Frequency for 7.5-mm-Length LiNbO., Crystal
Wi h rf Power in Forward Direction. (Results were obtained la3st year and are included here lor purpose of comparison)
3-2
LOCKHEED PALO ALTO RESEARCH LABORATORY lOCKNilD MISSIilS > W*CI CO«l»»NT INC
* suitiDiAir or lOCKMiiD «iiciAfi coifOitriON
— - -■ Ml I ' ■ IIIIMI«!« I
L.MSC-D 10:5175
3 dB
2.0 3.0 4.0
MODULATION FREQUENCY (GHz)
Fig. 3-3 Relative Optical Sideband Power as a Function of Modulation Frequency at Low-Drive Power Levels
method of measurement, the modulation index m is approximately proportional to the
square of the sideband power. Thus, the 3-dB level is about half-way down from the
peak, as indicated in Fig. 3-3, and no increase in bandwidth over that shown in Fi«;-.
3-1 is observed. Actual measurement of modulation index near the peak response
showed that 80-percent modulation index for 0.5145 fill) was obtainable at 8.3 W of rf
drive power; this value also agrees well with that scaled up from Fig. 3-1. However,
the peak response is now considerably broader and more centered in the passband than
that shown in Fig. 3-1. In this respect, improvement in modular performance has
been achieved.
This modulator was used in the initial tests for the 2-Gbit/sec data transmission and
performed well, as reported in the Semiannual Report (Ref. 1). Unfortunately, in the
initial experiments for the 1-GHz analog and the 1 Gbit/sec digital transmission, the
3-3
LOCKHEED PALO A .TO RESEARCH LABORATORY lOCDHIID MllJIlli 1 S>»CI CO«*»N» INC
« $UISIO(»l» OF lOCKMIIO AltClAfI COI»OI«tlON
■ ' ■IIII^I ■ ■- - ^^
LMSC-D403175
electrooptical 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, obtained for the final version of the modulator is as
shown in Fig. 3-4. Obviously, the performance was not as good as that obtained earlier
but this proved adequate for the demonstration of the 1-GHz analog and the 1-Gbit/sec
digital data transmission. Laboratory measurements indicate that approximatelv 35-
percent modulation index is repeatedly obtainable at 2.0 W of rf drive at 3 GHz
** 40
3
y Q
y. Q
a c 0 31
10
0 2.0
X0 = 0.5145 Mm
LiNb03 CRYSTAL, 0.3 BY 0.3 BY 7.5 mm
MODULATOR TEMPERATUuE * 170'C
RF DRIVE = 2 W
MICROWAVE POWER FRUM OUTPUT TO INPUT
2.2 2.4 2^ 2.8 3.0 3.2
FREQUENCY (GHz)
3.4 3.6 3.8 4.0
Fig. 3-4 Modulation Index as a Function of Frequency at 2-W Drive Power Level for the Final Version of the Modulator
3-4
LOCKHEED PALO ALTO RESEARCH LABORATORY lOCXHIlD ■••••»II | ItACI COMrANT. INC
* »••••••••• O. IOC »HC1*»T COl^OIAt.ON
"•■ " ' ■ — ——-
LMSC-O403176
*P 3.2 PHOTODETECTORfi
POT the laboratory dcnu.nstration. an experiment .1 static cross-field photomulUpltor
tube (CFPMT) having a II,-V compound (InGaAsP) photocathodc was ordered iron,
VarUm AMOCiale.. Tins order was inUiated in the belief that good „uantu,,, efficiency
l--no.se figure, and good frequencv response at 4 GHz could all be obtained under a
beet-effort" arrangement. Unfortunately, the CPPMT received falls short of our
expectation.: quantum efficiency us less than 1 percent, frequency response above
3.6 OH« is poor and, worst of all, there is a spurious resonance at about 2 5 QH.
which is also one of the subcarrier frequencies. The frequency response of the
CFPMT is shown in Pig. 3-5. (The method used to deternnne the frequency response
Will be discussed tn the following paragraphs.) Therefore, this CFPMT was deemed
undeeirable for our experiment. Another CFPMT, which did not have good quantum
efficiency but did not appear to have thi. strong resonance, was used for the detection 01 2-Gbit/aec optical data transmission.
UJ CO ^^ y en
2 75 u
P
P K >«
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3 u X
3 S <
2.0 2.5 3-0 3.5 FREQUENCY (GHz)
4.0
Fig. 3-5 Relative Frequency Response of the Varian Static Cross-Field Photomultipher Tube - Test Performed at 0.5145 Mm
3-5
LOCKHEED PALO ALTO RESEARCH LABORATORY 'OC«H,,0 W,S!,lls , „.<, C0M(>ANY
* SU.S.D.A.r O, tOCM... ft,,,,*,, CO.^C.T.ON
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diod... .. „u .. . sil.C(m :,vn,r„,,u. „. A1| ||u,si <|i((|is im( ^ |ow
q«»cy ».„.n« m lllt, ,. t„ ,.0I1Z rwifei ^^^ ii()ti| ^^
diode, appcuvd „»cul. Tc.s, rcsulu .„ ,„,,„, düB„„)c<1 |)t,|(iw
■" *** **,. U. ...„„..„cv rospo,,^ „f u» „p.,.,, ,mM ,. „. ,,,.„, ^
.d...v- ero. sra„,„„B .„p....«....^..-. n« „„..„p,«, plse|. „„„„ „ Uii,n
•hc Phco^ccco.. l,„(Jl.1. tüst. ,,c ,lulpul „,. u,t, phütodett,clor i8 ; spectru. Maly2er, _„ „. . ^ ,o„. |rcquencv ^ -
.hc „PC,.., müdu,atür and „„ p|loU)dc,octoi, ^^ u i|i(
l-cn. ■c.cened nncrowavc. poWr „ . SaiKlwn u Il.eque,lcv. „^ ^ P
::::ü:"om"iü r •*"• ^ i'rc ••'—■■—«-«-»». —a... p i:: scc, ,„ th„ spcu,un, ana,vzt,r C!in lit, übt;iiiicd,_, ^
..ec ru.. when «. appare„t mc<luIatlon index „ „^ ^ ^ ^ ^ _ J ror the 0pllcal „K)dulalor as moasi,rod i)rc,viMis|v ^
cep... ,s „hP,^, rol. convcni0,it,C| 11K,SO icsis ^^ P -
i Lose tavta, fav„„lWe „.„„„.,.„,,. rcsponsc „„ „^ ^^^ ^ | M ^
. Mo c, XL 5.,. U ta See„ that hevond 3.„ OH., the response uas ven- poo, Sinee
:;:::: ■1rsc was ^very h'Rh'thc d"* - "o'—- - - ->,., -vc cnpd . ».„ ,s . gcrn,anium avaliinchc dlodc XL
»avetength. Speemeatiens n. tts fre(,Ue„ey .-esponse and noise eha,.aeterlslie re much worse than those of n YT re TU r scrtous., eonstdel ^ "" -™-i—.danehe dtode was not
Figure 3-7 ,s the response eueve for . silieon PIN diode (Phiieo Mode. .,501,. This dtode has . .ther sma„ juncUo„ capacila„0„ ^ ^ ^ ^ ^ ^
3-6
LOCKHEED PALO ALTO RESEARCH LABORATORY 'Ot,H,,0 M1S,,tls , $p/lc( COMtANY |NC
* lutiiD.A., or .orKM.,D MMt«n coiro.A.toN
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2.2 2.4 2.6 2.8 3.0 3.2 3.4
FREQUENCY(GHz)
3.6 3.8 4.0
. Fig. 3-6 Relative Frequency Response of the TI Silicon Avalanche Diode -
Test Performed at 0.5145 ^m
2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6 3.8 4.0
FREQUENCY (GHz)
Fig. 3-7 Relative Frequency Response of the Philco Silicon PIN Diode - Test Performed at 0.5145 \ixa.
3-7
LOCKHEED PALO ALTO RESEARCH LABORATORY IOCKHEIO Mlltllit * IfACI ( o » ► A N » INC
* >U«SIDillI Of .OCKMI1D »ilClAFI COirOIATION
• - ' ■M^MM
LM8C-D403176
ivsponse is reasonably good even in the 3- to 4-GHl range. Unfortunately, the spec-
tral response at 1. 8 pm was very poor, therein rendering the diode not very usel'ul
for tiiis demonstration. For laboratory use with visible wavelengths, however, this
diode is probably the most desirable one.
Figure 3-8 shows the frequency response of the Philoo Germanium PIN diode (Model
45:";/). This diode lias a large junction capacitance (- 5 pF), 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 LOG pm
and can withstand much light intensity, it is still the best detector lor the l.tHJ-jmi
experiments. It is therefore chosen for the I.ü6-/Lim tests.
2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6 FREQUENCY (GHz)
3.8 4.0
Fig. 3-8 Relative Response of the Philco Germanium PIN Diode Test Made at 0.5145/urn
3-8
LOCKHEED PALO ALTO RESEARCH LABORATORY IOCKHIID MISIIlfS t SfACi COMfANT. INC
« SUtSIDI*>r OF IOCKHIID A I • ( « « I 1 COIFOIATION
• --
LA1SC-D4Ü3175
3.3 PN SIGNAL GENERATOR AND BIPHASE MODULATÜRS/DEMODULATüRS
Li the digital modulation subsystem, two important subassemblies are required.
These are the B00-Mbit/wc pseudorandom (PR) signal generators and the balaneed
biphase modulators. Both device« have been discussed extensively in the Third
Semiannual Report (Ref. 4) and will not be repeated here.
3.1 THE VOLT^GE CONTROLLED OSCILLATOR
Originally, the subband used for the transmission of 1-GHz bandwidth analog signals
was the 3- to 4-GIlz band. The performance of two VCO's operating in this band has
been reported in the Third Semiannual Report (Rcf. 4). Because of the poor frequency
response of the photodetectors as well as the nonlinearity problems in the VCO drivers,
it was decided to use the 2- to 3-GIIz subband for the transmission of analog signals
(see Ref. 4). 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 Fig. 3-9. 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 (± 80 MHz) centered at 2.5 GHz, excellent linearity is ob-
tained. The voltage required for this i 80 MHz deviation is only 1.5 V peak-to-peak,
corresponding to an rr.s drive power of 6 mW. Thus, this VCO showed much better
response than any of the previous ones and is most appropriate for the FM subsystem.
3.5 THE WIDEBAND FM DISCRIMINATOR
For the demodulation of analog signals, a wideband FM discriminator is required.
The desired characteristics of the FM discriminator arc as follows:
e Good linearity over a frequency range of 2- to 3-GHz and ability to handle
modulation frequencies up to 400 MHz
e High sensitivity to input frequency variation and low sensitivity to input
amplitude variations
3-9
LOCKHEED PALO ALTO RESEARCH LABORATORY tOCKHIID MISSIlii > SPACI COMfANT INC
» SUIllD'AIr OF 1OCKHII0 «IICIAfT COIPOIATION
mm iliMMriMiailHi Wfimn—«■ ir- "-— ■■ ^—-- ■ ■■ - ■ - ■■ - ——^—m*****
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-8.2
-8.7
2.2 2-4 2.5 2.6
FREQUENCY (GHz) 2.7 2.8
Fig. 3-9 Static Voltage Tuning Characteristics of the Final Voltage- Controlled Oscillator
3-10
LOCKHEED PALO ALTO RESEARCH LABORATORY LOCKHIID MISSILES 1 S^ACI COMFANT. INC
* SUiSIDI«lr OF I O C II H I I D AIICI<'I C O t P O » A 1 I 0 N
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LMSC-B103175
• Output level high enough that the noise in the succeeding 400-MHz widc-
baseband amplifier does not appreciably degrade the signal-to-noise ratio
established in the photodetector
The design of wideband discriminators has been accomplished at LMSC; it is based on
the work of Kincheloe and Wilkcns (Refs. 5 and 6), who suggested the use of a micro-
wave power divider and a 3-dB hybrid coupler connected by constant impedance trans-
mission lines. The arrangement is a microwave analog of a one-dimensional optical
interferometer; its theory of operation has been presented in the Third Semiannual
Technical Report (Ref. 4),
During the first half year, a discriminator operating in the 3- to 4-GHz suhband has
been fabricated and tested. Linearity was good for deviations up to 250 MHz, as ic-
ported in the Third Semiannual Technical Report (Ref. 4). Because of the change of
the frequency modulation subband, a new FM discriminator had to be fabricated. This
was done in the third quarter and the results are as shown in Fig. 3-10. Over a devia-
fion of * 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.
Since this linearity range is more than adequate for the VCO, no additional work was
performed.
3.6 VOLTAGE-CONTROLLED OSCILLATOR/FM DISCRIMINATOR TESTS
To determine whether the voltage-controlled oscillator (VCO) and the FM Discriminator
will perform properly in the system tests, both were connected together and their com-
bined performance was evaluated. The test setup is as shown in Fig. 3-11. The VCO
is driven by a baseband amplifier having a flat response up to 550 MHz. The output of
the VCO is filtered by a bandpass filter and fed into the FM discriminator. The output
of the discriminator is filtered by a low-pass filter having its cutoff at 470 MHz. The
lowpass filtered output is fed to a spectrum analyzer and the resulting power spectrum
displayed on the cathode ray tube. Figure 3-12 shows the frequency response charac-
teristic of .he baseband amplifier. The lower trace is a levelled output of the swept
3-11
LOCKHEED PALO ALTO RESEARCH LABORATORY lOCKNIfD MlillllJ a $»ACI COMrANf INf
» sui»ioi*ty or IOCKMIID «IICIAFT COIPOIATION
m ^ MM
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LMSC-D1();U75
100
2.5
FREQUENCY (GHz)
Fig. 3-10 Performance Characteristic of the Final Wideband FM Discriminator
oscillator levelled between 10 MHz to 1 GHz. The upper trace shows the amplified
output of the baseband amplifier: clearly the amplification is constant within the band
of 10 MHz to 550 MHz. Figure 3-13 shows the recovered baseband signal which is
swept between 10 and 450 MHz by the levelled sweeper after FM discrimination and
lowpass filtering. It is seen that between 50 MHz and 470 MHz, the output varies by
approximately 15 dB. Since this is not most ideal for a communication system, a
compensation network was incorporated m the VCO's driver to obtain a better response
As a result, the frequency response is indeed improved as shown in Fig. 3-14. Be-
tween 50 and 470 MHz, the relative variation is only about 5 dB. This version was
therefore u^ed in the tests.
3-12
L LOCKHEED PALO ALTO RESEARCH LABORATORY lOCKMIID MI1SI1II 1 SfACi COMrANT. INC
* 1 U • '. I 0 I » » » Of lOCKHIID AIICIAFT COIPOIATION
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LMSC-D4Ü3175
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LOCKHEED PALO ALTO RESEARCH LABORATORY LOCKMIIO MISIIlit * SPACI C O M f » N Y INC
* iUtSIDIAIr Of IOCKHIID »IICI»»I C O I • O t « 1 I 0 N
■ ■ . ■
Mwm *•■•■■■ ww^mx^mw^K^m^mm 4 ' ■ ""■ ■' "
FREQUENCY (GHz)
LMSC-D4Ü3175
Pig. 3-12 Frequency Response of VCO/B'M-Discriminator Test Components. Lower Trane: levelled swept generator output. Upper Trace: VCO Driver amplifier output when swept by the levelled sweeper
0.5 FREQUENCY (GHz)
1.0
Fig. 3-13 Output of the FM Discriminator When the VCO Is Swept Between 10 and 470 MHz by the Levelled Sweeper and Driver Amplifier
3-14
LOCKHEED PALO ALTO RESEARCH LABORATORY IOCKNIID MIStfllS | iPACi COMPANr. INC
* SUtilOIAIV OF LOCKHflO ftlMIAM COIPOI1TION
■ - -■ - - -■ ■ ■■■HI
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0.5
FREQUENCY (GHz)
1.0
Fig. 3-M Output of the FAI Discriminator When a Compensation Network Is Added to the VCU Driver. Obvious Im- provement is observed
3.7 SYSTEM IMPLEMENTATION
The system layout is essentially a hardware copy of the block diagram- -l.own in
Figs. 2-2 through 2-5. Judicious choice of amplifiers and attem-ators has to be de-
termined to ensure proper signal levels so that signal-to-noise ratios are maxi-
mized and cross-talks are minimized.
3,7.1 The 2-Obit/sec System
Figure 3-15 shows the transmitter end of the laboratory system transmitting 2-Gbit/
sec digital data. The components found in the lower and upper shelves are identified
below.
3-15
LOCKHEED PALO ALTO RESEARCH LABORATORY lOCKHiiO III t Hill t S»ACi COMPANY. INC
A IVIdalAII Of I O C « H I I D AltCIAII COIPOIATION
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ün the lower shelf, the optical components in this system are shown. These consist of:
• A sinRle-frequency laser (a commercial ion laser is shown used, to facilitate
initial alignments and tests)
• A polarizer, which sets the light polarization at the desired 45-deg angle to
the optical compensator and modulator crystal axes
• A servo-motor-driven optical compensator, which automatically sets the
optical bias for the modulator
• A focusing lens, which places the optical beam waist at the center of the
modulator crystal
• The electrooptical modulator
• An optical bias control unit, which senses the static optical bias condition of
the modulator and produces an error signal to drive the compensator to
quarter-wave optical-bias
• An output polarizer which converts optical polarization modulation into
intensity modulation
An optical spectrum analyzer is shown in Fig. 3-15, but this is used only for monitor-
ing purposes.
On the upper shelf, the transmitter electronics are shown. Components for the two
subbands - one derives its subcarrier signal from a 2.5-GHz oscillator and the other
from a 3. 5-GHz oscillator - are clearly shown. Each subband includes identical
types of components consistent with the frequency band. For instance, in the 2- to
3-GHz subband, power from the 2.5-GHz oscillator passes through a 6-dB coupler,
with the main output directed to the receiver as the phase reference signal. The
-6 dB output passes through a 90-deg hybrid to provide the "in-phase" and the "in-
quadrature" channels. Each channel is then biphase-modulated at 500 Mbit/sec by
the biphase modulator. The two channels are recombined in the power recombiner,
amplified, and filtered to give a QPSK-modulated 2.5-GHz output. This is combined
with the QPSK-modulated 3. 5-GHz output to give a composite signal containing 2-Gbit/
sec total data in a dual QPSK modulation format.
3-17
LOCKHEED PALO ALTO RESEARCH LABORATORY IOCKMIIO MISIIIII t IFACI COMPANT
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An illustration of the spectrum of the QPSK-moduhited 2-Gbit/sec data, using two sub-
carriers as is done here, is shown in the center of Fig, 3-15. It should again be
emphasized here that if PR generators at higher data rates (e.g. , 1 Gbit/sec) were
readily available, the entire 2-Gbit/sec data transmission could have been achieved
using only ono subcarrier (e, g,, 3 GHz). The reasons for using two subcarriers are:
(1) To make up the desired 2-Gbit/sec data rate in the most convenient way
(2) To show that different data streams can be modulated onto different sub-
carriers which can then be multiplexed in the transmitter electronics and
demultiplexed in the receiver electronics
Figure 3-1G shows the receiver end of the laboratory system. The optical system is
again on the lower shelf, consisting simply of an optical attenuator, a focusing lens,
and a photodetector (a static cross-field multipler tube- CFPMT- is shown), A dif-
ferent CFPMT from the one purchased under this contract was used in these measure-
ments because this tube did not appear to have the objectionable resonance at 2, 5 GHz.
From the CFPMT, the detected microwave signal is directed to the receiver elec-
tronics, shown in the upper shelf. The signal is divided into two halves in the 3-dB
coupler: each half is used for the demodulation of digital data in one subband. Again,
the microwave components used for both subbands are identical in type, the only dif-
ference being in the frequency response. For instance, for the lower subband, the
signal is filtered to give the 2- to 3-GHz components and amplified. This is then again
divided into two halves, each half becoming the input signal for a biphase demodulator
(BDM), The phase reference signal for a given subband comes from the 6-dB coupler
in the transmitter, and is passed through a precision phase-shifter. At the BDM,
therefore, the reference signal has a precise phase relationship to the input signal to
effect proper demodulation. The reference signal is split by a 90-deg hybrid into an
"in-phase" and an "in-quadrature" component; each component is directed to a BDM
for the demodulation of a channel. This has been discussed in much detail in the Third
Semiannual Technical Report (Ref. 4). As a result, four streams of 500-Mbit/sec data,
identical to the outputs from the word generators, are obtained. These are shown dis-
played on the sampling oscilloscope on the upper shelf.
3-18
LOCKHEED PALO ALTO RESEARCH LABORATORY IOCKHII0 MISSIliS 4 WACI ( o w !• * N Y INC
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3.7.2 The 1-GHz FM-Analog and 1-Gbit/sec Digital System
Tbj digital portion of this system is identical to one of the digital channels shown in
Figs. 3-15 and 3-16. Specifically, the one that is used in this system has its sub-
carrier frequency at 3.5 GHz.
The FM-analog portion uses a 2.5-GHz subcarrier, onto which four local TV channels
(Channels 4, 5, 7 and 9) and a narrowband FM music at a carrier frequency of 400 MHz
are modulated. Figure 3-17 shows a detailed block diagram of the hardware and the
monitoring instrumentation used in the transmitter. Various filters, amplifiers, and
attenuations are inserted into the system to ascertain minimum intermodulation, to
maintain good signal-to-noise ratios, as well as to minimize reflections between com-
ponents. The implementation here is mainly an experimental one - by trial and error -
to determine the causes of, and consequently eliminate or minimize, various undesir-
able effects. In particular, the VCO bias and drive level have to be adjusted quite
stringently to attain good signal-to-noise ratios and minimum intermodulation. This
is achieved in the laboratory as indicated by the system test results reported in Section 4.
Figure 3-18 shows a detailed block diagram of the receiver. Again, the implementation
is an experimental one. However, since the power levels being dealt with in the re-
ceiver are rather low (milliwatt arid below), saturation or intermodulation is not a prob-
lem. The main undesirable effects to minimize are reflections between components.
To this end, isolators and attenuators have been inserted wherever required.
Results on system measurements are presented in Section 4.
3-20
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Section 4
SYSTEM TESTS
Experimental results of critical components such as the optical modulator, the voltage
controlled oscillator, etc., as well as those of subsystem tests have been discussed in
the earlier sections where appropriate. Only the system test results will be discussed
in this section.
1.1 QFSK DIGITAL MODULATION RESULTS
Work on the quadriphase-shift-keying digital modulation was performed during the first
half of the year as a preliminary check of the system performance. Results obtained
are shown in Figs. 4-1, 4-2, and 4-3. These figures show the typical results of the
2-Gbit/sec laboratory communication demonstration. Figure 4-1 is a sampling scope
display of the 2-Gbit/sec data received via the laser link. A total of four 500-Mbit/
sec channels are displayed: two of the channels are transmitted through the 2.5-GIIz
subcarrier, and two are transmitted through the 3.5-Gllz subcarrier. All channels
show good recovered signals.
Figures 4-2 and 4-3 show the degradation that takes place in the signals after being
transmitted through the laser link. This includes the degradation due to the electro-
optic modulator and the photodetector. Each figure is for the data over a particular
subcarrier: the top two traces show the sampling scope display of the two 500-Mbit/
sec data outputs that result from a direct connection (hardwire) from the traveling-
wave tube amplifier, with suitable attenuation, to the microwave receiver. The
bottom two traces show the two 5ÜO-Mbit/sec outputs that result from the transmis-
sion over the laser link. These tests were run using a commercial argon ion laser,
operating single-mode, single-frequency at A =» 0.5145/um. The visible wavelength,
rather than 1.06 ^m, was used for these initial tests because of the ease of optical
alignment and better photodetector response.
4-1
LOCKHEED PALO ALTO RESEARCH LABORATORY lOCKHIID MISII1IS * SPACI < O W P A N r INC
* lUISIOIAIr OF lOCKHIID «IICIAIT COI'OIATION
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Upper Two Traces: 500 Mbit/sec each on 2. 5-GHz subcarrier Lower Two Traces: 500 Mbit/sec each on 3.5-GHz subcarrier
Fig. 4-1 Sampling Scope Display of Four 50Ü-Mbit/sec Streams Transmitted Through the Laser Link
The traces in Figs. 4-2 and 4-3 show that there is not a one-to-one correspondence
between the data bits in the upper traces and those in the lower traces, because the
amount of delay in the laser transmission and reception causes a different portion of
the PR signal to appear on the oscilloscope, and no provision has been made in these
experiments to compensate for this difference in delay. The important aspect of these
data is to show the degree to which the character of the data waveform is degraded.
From these traces, it is clear that the rise and fall times are lengthened and the
corners rounded. However, it is also clear that the hits are still readily recognizable
as ones or zeroes even to the eye, although those in Fig. 4-3 appear to be noisier.
This noisy performance arises from the fact that the traveling-wave tube rsed has
relatively lower gain at the high-frequency end. and that the cross-field photomultiplier
tube (CFPMT), which also has a lower response at the high-frequency end, is used In
these measurements. It is estimated that by using matched-filter detection, the data
should be recovered with 2- to 3-dB degradation in performance from the theoretical integrate-and-dump performance.
4-2
LOCKHEED PALO ALTO RESEARCH LABORATORY IOCKMIID MIStlliS * l>*CI COMf»N» INC
* SUtSIDIAir Of IOCKHII0 AIICIArT COI^OiATION
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Fig. 4-2 Waveforms of Two 500-Mbit/sec Streams on 2.5-GHz Subcarrier Frequency With Simultaneous 1-Gbit/sec Digital Signal on 3. 5-GHz Subcarrier
4-3
LOCKHEED PALO ALTO RESEARCH LABORATORY lOCKHilO MISSIltS t SPACI COMPANY INC.
» suisiDiAir or IOCKHIIO «IICIAFT coifo>»rioN
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a. Direct connection between transmitter and receiver electronics
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Fig. 4-3 Waveforms of Two 500-Mbit/sec Streams on 3.5-GHz Subcarrier Frequency With Simultaneous 1-Gbit/sec Digital Signa) on 2. 5-GHz Subcarrier
4-4
LOCKHEED PALO ALTO RESEARCH LABORATORY lOCKMIID Milt I til « SPACI COMPANr, INC
A SUISIDIAIT Of lOCKMiiD AIICIA>T COIPOIAtlON
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The performance for the 2-Gbit/sec transmission shown above is considered good for
this feasibility demonstration since it shows that the system does have the designed
bandwidth of 2 GHz and will accept the desired high data rate. Not much fine tuning of
the system was done because this demonstration was made purely to ascertain the
feasibility of transmitting 2 Obit/sec data. In any event, the main thrust of this con-
tract is to obtain the 1-GHz/sec analog signal and the i-Gbit/sec digital signal demon-
stration to be discussed in subsection 4.2.
4.2 FM ANALOG AND QPSK DIGITAL MODULATION RESULTS
The main thrust of this feasibility study is the transmission of a l-GIIz analog and a
1-Gbit/sec digital data over a LOG-^m laser beam. Therefore, much "debugging"
and refinements have been made for this system. The results obtained unequivocall
demonstrate the feasibility of such a transmission in the laboratory. ■j
Initial tests on this transmission were done during the first six months using the visi-
ble laser. The 3.5-GHz subcarrier was used for the analog signals, and the 2.5-GHz
subcarrier was used for the digital signals. (The use of these subbands was different
from that shown in Fig. 2-4, as will become clear in the latter paragraphs of this sub-
section.) Some difficulty was experienced with the 1-GHz analog subsystem as detailed in the following paragraphs.
The first voltage-controlled oscillator (VCO) used for the FM transmission of the
analog signals required rather high driving voltages. To provide these high voltages
in a 50-tt system, high drive power is required. This forces the driver amphfier to
operate in saturation, thereby causing intermodulation between the various analog sig-
nals (local TV channels 4, 5, 7, and 9). In addition, because of the power limitation
of the driver amplifier, the FM deviation was rather low so that the received signal-
to-noise ratio (S/N) was high. After much effort had been expended in designing fre-
quency filters (traps) and inserting various attenuators and amplifiers to reduce
intermodulation, the results were still considered unsatisfactory.
4-5
LOCKHEED PALO ALTO RESEARCH LABORATORY lOCKHIID ■••lilt | t Sr*CI COMPANr. INC
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A second VCO was then ordered. This VCO required much less driving power for its
operation so that good linearity with reasonable deviation could be obtained. Results
showed that intermot'ulation had been greatly reduced. However, the S/N ratios for
the received signals were still rather poor, typically in the 20-dB range. (For good
TV picture reception, a S/N ratio in the range of 40 to 45 dB is required.)
The causes for such a low S/N performance are attributed to:
(1) The frequency deviation is still small (~ 35 MHz at a carrier frequency
of 3. 5 GHz) so that there is only a small amount of signal content in the FM
subsystem.
(2) The allowable optical power on the photocathode is limited (~ 5 pW) and
the high-frequency response of the CFPMT is poor, causing further reduc-
tion of the signal content with respect to system noise.
Consideration was then given to using the lower subband (2.5-GHz subcarrier) for FM
analog transmission and the upper subband (3.5-GHz subcarrier) for digital transmis-
sion as shown in Fig. 2-4. This change in direction was based on the following facts:
(1) High power (several watts), wideband (dc to 500 MHz) baseband amplifiers
are not commercially available. Thus, to minimize intermodulation,
frequency deviation has to be kept small; i. e., the signal content is kept
small.
(2) The performance of the CFPMT is poorer than expected in the high-
frequency end of the band.
(3) In the lower subband, the traveling-wave tube, the photodetectors all have
good response so that higher S/N is obtainable.
(4) For the transmission of digital signals, an S/N ratio of 20 dB gives a good
signal display, while for the transmission of analog signals, a minimum
S/N of 30 dB is required.
In the final demonstration, therefore, the 2.5-GHz subcarrier is used for the trans-
mission of analog signals; onto the subcarrier are modulated local TV channels 4, 5,
4-6
LOCKHEED PALO ALTO RESEARCH LABORATORY IOCKHIID MIStllll » W A C I COMPANT, INC
A SUtSIDIAIT OF IOCKHIID » I • c • » t I COIfOliTION
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7, and 9, as well as a narrowband FM music (FM about a 400-MHz carrier frequency).
The digital data are modulated onto the 3.5-GHz subcarrier by using a QPSK format.
At 1-Gbit/sec, the sidebands of the QPSK digital modulation occupy the entire 3- to
1-GHz subband. The analog signals used occupy most of the 2- to 3-GHz subband.
Figure 4-4 shows the complex microwave frequency spectrum of this modulation for-
mat as it is detected at the output of the traveling-wave-tube (TWT) amplifier. (Since
the TWT is the driver amplifier for the optical modulator, this spectrum represents
the input signal to the optical modulator.)
In Fig. 4-4, the lower subband, viz., the 2- to 3-GHz subband, is shown in the left
half. A subcarrier frequency of 2.5 GHz is shown in the center. The upper and lower
sidebands of that subcarrier are shown on either side. The peaks of the sidebands are
local TV channels 4 and 5, and 7 and 9, as well as the 400-MHz narrowband FM carrier.
The two adjacent TV channels are barely resolved in this picture. The upper subband,
the 3- to 4-GHz subband, is QPSK-modulated by 1-Gbit/sec stream data. It is seen
that the frequency spectrum is quite complex.
After transmission through the optical beam, the detected microwave spectrum is as
shown in Fig. 4-5. In this figure, gain of the spectrum analyzer has teen adjusted so
that the 2.5-GHz subcarrier power gives the same amount of deflection as that shown
in Fig. 4-4. By comparing these two figures, it is apparent that the analog signals
suffer relatively minor amount of attenuation upon going through the laser beam, while
digital signals do suffer a much higher relative attenuation. This higher amount of at-
tenuation in the 3- to 4-GHz subband is primarily due to the lower response of the
photodetector and the lower optical modulation index in that subband.
For the analog FM subsystem, it is necessary to obtain a signal-to-noise ratio of
greater than 30 dB before an acceptable TV picture can be obtained. If the signal-to-
noise ratio 's less than 30 dB, the TV pictures become "snowy" and begin to lose de-
tails. A good picture as obtained from a well-directed antenna normally contains a
signal-to-noise ratio of 40 to 45 dB. In this demonstration, therefore, most of the
4-7
LOCKHEED PALO ALTO RESEARCH LABORATORY lOCHHIIO Mlttlllt I tfACi COMPANT, INC
A SUISIDIAIT OF IOCKHIID AIICIAFT COI'OIATION
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2. 5-GHz SUBCARRIERI
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SPECTRUM OF QPSK MODULATION ON 3. 5-GHz 5UBCARRIER (MODULATION RATE - I Gbit/sec)
2.0 2.5 3.0 3.5 FREQUENCY (GHz)
Pig. 1-4 Complex Microwave Spectrum us Seen at the Output of the Travelinc- \.ave Amplüier. 2- to 3-üHz Subband: Analog Modulation; both upper and lower sidebands are shown. 3- to 4-GHz subband, spectrum of QPSK modulation at 1 Gbit/sec
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Fig. 1-5 Complex Microwave Spectrum As Seen at the Receiver (After Optical De ee ion and Microwave Amplification). The spectrum is idenUcal
. r u I?'1 ? Flf' 1"1- eX06P* for ***** attenuation in the 3- to 4-GnZ subl)an(l
4-6
LOCKHEED PALO ALTO RESEARCH LABORATORY IOCKMII0 Mlttllll « iF»CI COMfANT
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io«er traces show tKroUghoat the same portion ot the other 600-„bi./st.c ^
4-9
LOCKHEED PALO ALTO RESEARCH LABORATORY »•«■;••• ■••Mill . ,f.c. co«,.N,. ,NC
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FREQUENCY (3 MHz/DIV)
Fig. 4-6 Spectrum of TV Channels 4 and 5 After Transmission Through 1.06-jxm Beam, Detection by a Germanium PIN Diode, and Processed to be Ready for Display on TV Sets. P - Picture Carrier; S ■ Sound Carrier; and C - Chromatic Subcarrier
-^ FREQUENCY (3 MHz/DIV)
Fig. 1-7 Spectrum of TV Channels 7 and 9 After Transmission Through LOG-^m Beam. Detection by a Germanium PIN Diode, and Processed to be Ready for Display on TV Sets. P . Picture Carrier; S - Sound Carrier; and C - Chromatic Subcarrier
1-10
LOCKHEED PALO ALTO RESEARCH LABORATORY IOCKMIIO ■•tlllll ( 1 f A C I C O M r A N r (NC
A S U t S I 0 I A I > Of 1 O ( . M ( I D AIICIAII COIPOIATION
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FiR. 1-8 Spectrum of TV Channels 1 and 5 As Received From a High-Gain Antenna and Ampliiied. FM Broadcasting Stations are also received; these are filtered out later in the system. P a Picture Carrier: S Sound Carrier: and C ■ Chromatic Subcarrier
CHANNEL 7 CHANNEL 9
-•► FREQUENCY (3 MHzA)IV)
Fig. 4-9 Spectnun of TV Channels 7 and !) As Received From a High-Gain Antenna and Amplified. P Picture Carrier: S - Sound Carrier: and C • Chromatic Subcarrier
4-11
LOCKHEED PALO ALTO RESEARCH LABORATORY lOCHHIIO WISilllS t S>»CI COI
* SUISIDIAIr Ot IOCKM1ID AIICIAFT
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a. Outputs From the PK Word Goneratoi
o. c[- ^*,i??M! 0nt0 the QP8K M«1»1»*0' ;"ld Subsequently Demodulated by the yi SK Demodulator and Amplified. Direct cable connection between the Modula- tor and the Demodulator (U) It of RG 142/U). Slight ringing was observed
Fig. 4-10 Waveforms of the Two SOO-Mblt/ieo Data as They Appear at Various Stages in the Laboratory Communication System. Each trace in an oscillograph is a portion of a SOO-Mbit/sec stream, biphase modulated onto the 3.6-GHz BUbcarrier. Vertical scale: linear; horizontal scale -- 2 nsec/cm
4-12
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c. Same as in Fig. 4-10b Except for the Addition a 'A- to i-GHz Bandpass Filter a he Transmitter Output, and Another 3- to 4-GHz Filter at the Receiver Output. Much rounding-otf of the corners was observed.
d. Same as in Fig. -l-lOc Above, Except for the Addition of the TWT Much ringing due to phase-distortion in the TWT was observed.
Fig. l-lo (Cent.)
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one at the transmitter low-level amplifier output, and another at the receiver amplifier
output. Much rounding-off of the data bits is observed. Figure 4-10d shows the same
data streams after the driving amplifier for the optical modulator (the TWT) operating
at a 3-W drive level has been added. Additional distortions, primarily phase distor-
tions causing ripples in the data streams, have now been added and are observed. As
a result of this phase distortion, a phase compensator was added at the output of the traveling-wave tube. The result of using that phase compensator is shown in Fig.
4-10e: the cistortion has been considerably reduced.
Figure 4-10f shows the detected signal after the data streams have been transmitted
over the l.oe-^m laser link, all the electronics being the same as that for Fig. 4-10e.
Comparison between these two figures shows that additional phase distortion has oc-
curred in Fig. 4-lOf. These distortions occur in the electrooptical modulator and
the photodetector since these two are the only added elements in Fig. 4-10f. However,
by passing these waveforms through a clipping amplifier, much waveform restoration '
is obtained as shown in Fig. 4-10g. Comparing Fig. 4-10g with 4-10a (that from the
generator output) shows that there is only a small amount of distortion through the
entire process. ALaiii, by using matched filter detection techniques, it is estimated
that the results obtained in this feasibility demonstration should not be worse than 2 dB
from the theoretical intograte-and-dump performance.
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Section 5
LABORATORY DEMONSTRATIONS
The 2-Gbit/sec laboratory optical communication system was demonstrated in August
1973 to Dr. R. E. Behringer, ONR Pasadena Office, and Dr. I- . Quelle, ONR Boston
Office. The l-GIIz analog FM and 1-Gbit/sec digital QPSK laboratory system was
demonstrated in April 1974 to Dr. R. E. Behringer, ONR Pasadena Office.
A lull-scale demonstration and discussion were given in June 1974 to the following
government personnel:
R. E. Behringer
F. Quelle
M. White
J. Ivory
J. Soules
I. Rowe
A. Wood
ONR/Pasadena
ONR/Boston
ONR/Boston
ONR/Chicago
ONR/Cleveland - Pasadena
ONR/New York
ONR/Boston
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Section 6
CONCLUSIONS AND RECOMMENDATIONS
The results of this year's work clearly shows that all the program objectives have teen
met and exceeded: Optical communications in the laboratory having a bandwidth of 2 GHz
arc feasible using a l.Ofi-fUn laser beam. In particular, two laboratory systems have
been demonstrated: one is a 2-Gbit/sec digital data-transmission system employing
QPSK modulation on microwave subcarriers; the other is the simultaneous transmission
of a 1-GHz analog-FM signal and a QPSK modulation using a 1-Gbit/sec digital data
stream.
For the 2-Gbit/sec digital data transmission, good digital waveforms are recoverable;
it is estimated that the per'ormance of the system is perhaps no worse than a degrada-
tion of 2 dB from the theoretical integrate-and-dump operations. For the simultaneous
transmission of analog aid digital data, typical results show that a signal-to-noise
ratio of greater than 30 dB can be obtained for the analog signals (local TV channels).
This performance corresponds to a degradation of about 10 dB, due to the transmission
over the optical link and required electronics, from those signals received directly
from a high-gain TV antenna (the starting signal). The digital part shows the same re-
coverable results as in the 2-Gbit/sec data transmission case.
Since the feasibility of such communications has been demonstrated in the laboratory,
the next step will be to improve the component performance, both electrooptic and
electronic, so that phase and amplitude distortions can be minimized in the system.
Another pressing development is that of a wideband photodetector with flat frequency
response up to 4 GHz, high sensitivity, low noise, reasonable current-saturation
limit, and long lifetime.
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Section 7
REFERENCES
R. C Ohlmann. W. Culshaw, K. K. Chow, H. V. Hanoe, W. B. Leonard, and J- Kan^laud- Righ-Emciencv, Sint-le-Frequencn- Laser and Ktodtltotor gto^
First Annual Technical Report, 30 Sep 1Ü71, Contract No. N00014-71-C-0049
Office of Naval Research/Advanced Research Project Agency
■""' aifcggjcJSSSL Sintde-Frequnncv Uggr and Modulator Study. Second
Annual Technical Report. 31 Dec 1972, Contract No. N00014-71-C-0049. Office
of Naval Research/Advanced Research Project Agency
R. B. Ward and D. G. Peterson, Design Performance Verification T Technical
Report AFAL-TR-73-371, Nov 1973. Air Force Avionics Laboratory, Wright- Patterson Air Force Base, Ohio
R. C Ohlmann, K. K. Chow, J. J. Younger, W. B. Leonard, D. G. Peterson
und J. Kannelaud, High-Efficiency, Sinde-Freuuencv Laserand Modul.-.tnr
Study, Third Semiannual Technical Report, 30 Sep 1971, Contract No. NÜÜ014-
71-C-ÜÜ49, Office of Naval Research/Advanced Research Project Agency
W. R. Kincheloe and M. W. Wilkens, "Phase and Instantaneous Frequency
Discriminators," U.S. Patent 3,395,346, Jul 30, 19G8
M. W. Wilkens and W. R. Kincheloe, Microwave Realization of Broadband gjgg
and Frequency Discriminators, Stanford Electronics Laboratory, Tech. Rep.,
1962/1966-2, Nov 1968, DDC ASTIA Doc. AD 849,757
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