FM-CW laser radar at 10.6 microns

Collection

Survival, Water, Medical Field Manuals

Case

Military Manuals

Sub-case

Chance, Thomas Henry

Download PDF ↗ Live URL (may be dead)

Document text

“Calhoun

Institutional Archive of the Naval Postgraduate School

Calhoun: The NPS Institutional Archive
DSpace Repository

Theses and Dissertations 1. Thesis and Dissertation Collection, all items

1974-12

FM-CW laser radar at 10.6 microns

Chance, Thomas Henry

http://ndl.handle.net/10945/17025

Downloaded from NPS Archive: Calhoun

| Calhoun is the Naval Postgraduate School's public access digital repository for
. (8 D U DLEY research materials and institutional publications created by the NPS community.
«ist L Calhoun is named for Professor of Mathematics Guy K. Calhoun, NPS's first

nN i KNOX appointed — and published -- scholarly author.
| inp
, LIBRARY Dudley Knox Library / Naval Postgraduate School
411 Dyer Road / 1 University Circle
Monterey, California USA 93943

http://www.nps.edu/library

FM-CW LASER RADAR AT 10.6 MICRONS

Thomas Henry Chance

NAVAL POSTGRADUATE SCHOOL

Monterey, California

FM-CW LASER RADAR AT 10.6 MICRONS

Thomas Henry Chance

December 1974

Thesis Advisor: C. H. Rothauge {

Approved for public release; distribution unlimited.

1164026

—__-_ i —_

UNCLASSIFIED

SECURITY CLASSIFICATION OF THIS PAGE (When Dete Entered)

REPORT DOCUMENTATION PAGE

- REPORT NUMBER 2. GOVT ACCESSION NO.

4. TITLE (and Subtitie)

FM-CW Laser Radar at 10.6 Microns

READ INSTRUCTIONS
BEFORE COMPLETING FORM

3. RECIPIENT'S CATALOG NUMBER

S$. TYPE OF REPORT & PERIOO COVERED

Electrical Engineer;
December 1974

6. PERFORMING ORG. REPORT NUMBER

,7. AUTHOR(e) - CONTRACT OR GRANT NUMBER(»)

Thomas Henry Chance

10. PROGRAM ELEMENT, PROJECT, TASK
AREA & WORK UNIT NUMBERS

9. PERFORMING ORGANIZATION NAME AND AOORESS

Naval Postgraduate School
Monterey, California 93940

11. CONTROLLING OFFICE NAME AND ADORESS
Naval Postgraduate School December 1974
ena ee ran oF re

- MONITORING AGENCY NAME & ADORESS(it ditterent trom Controiling Olfice) | 1S. SECURITY CLASS. (oft thie report)

Naval Postgraduate School

Uncl ified
Monterey, California 93940 pe OO ee

1Sa. OECLASSIFICATION/ DOWNGRADING
SCHEDULE

Approved for public release; distribution unlimited

16. OISTRIBUTION STATEMENT (of thie Report)

17. OISTRIBUTION STATEMENT (of the abstract entered in Biock 20, ff different from Report)

18. SUPPLEMENTARY NOTES

19. KEY WORDS (Continue on reveree side if neceseary and identify by block number)

Optical Radar FM-CW Optical Radar
Lidar

Optical Coherent Detection

Laser Radar

20. ABSTRACT (Continue on reveree aide if necessary and identity by biock number)

The feasibility of a continuous-wave frequency-modulated
radar with a co, laser as a transmitting source was
investigated.

A developmental system was constructed and tested and
the feasibility of an optical radar utilizing coherent
detection at 10.6 microns was demonstrated. The radar had

DD , ee 1473 —s EDITION OF | NOV 65 1S OBSOLETE WNCWASSME LED

= ‘es en SS SS PS SS
(Page 1) —oeee SECURITY CLASSIFICATION OF THIS PAGE (When Dete Entered)

UNCLASSIFIED '

CECURITY CLASSIFICATION OF THIS PAGE(Whan Deta Entered:

Block #20 Continued

the capability of instantaneous range and velocity
determination.

ACK
eer ong) L473 ao UNCLASSIFIED

an (
S/N 0102-014-6601 Z SECURITY CLASSIFICATION OF THIS PAGE(When Data Entered)

A a : . .. FM-CW Laser Radar
at 10.6 Microns

Thomas Henry Chance
Lieutenant, United States Navy
BSEE, Purdue University, 1969
MSEE, Naval Postgraduate School, 1974

Submitted in partial fulfillment of the
requirements for the degree of

ELECTRICAL ENGINEER

from the

NAVAL POSTGRADUATE SCHOOL
December 1974

E ry
nN Pos Fat
Monterey, Cal:tor

ABSTRACT

The feasibility of a continuous-wave frequency-modulated
radar with a co. laser as a transmitting source was
investigated.

A developmental system was constructed and tested and
the feasibility of an optical radar utilizing coherent de-
tection at 10.6 microns was demonstrated. The radar had the

capability of instantaneous range and velocity determination.

TABLE OF CONTENTS

I. INTRODUCTION- -------------------------------------

re GENER eoVGhEM CONG DERATIONG----------------...--

A. FM-CW RADAR-------------------- rere -eee eeeee-

Jae

Analysis of a FM-CW Radar System Using
Triangular Modulation---------------------

Signal Processing in a FM-CW Radar--------
a. Receiver Bandwidth Requirements-------
b. Pare cmopCeetllemalys1S-—-—--—-—--—-—_——

(1) Spectrum of a Near Range Target--

(2) Spectrum of a Far Range Target---
c. Spectrum Spread of Acceleration-------
d. Range Resolution----------------------
e. Velocity Resolution-------------------

f. Signal to Noise Effect of Collapsing
the FM Return-------------------------

g. Range and Velocity Filtering----------

Peon mle mOrmA ew intGER BEAM--------------------

Historical Basis of Acousto-Optic
Pen anete = = = Soe ee +

The Debye-Sears Effect--------------------

Bragg Diffraction------------------------> |

d. Modulation Frequency------------------
(1) Doppler Modulation Determination-

(2) Particulate Analysis of
Modulation-----------------------

Peaillat rem Bandgwadeh----------------------

Optical Power Transfer--------------------

fad
ai
28

28
28
30
34

6. Modulator Considerations-----------------

C. DETECTION------------------------------------
1. Photo-Voltaic Detector-------------------
a. Spectral Response and Quantum
Efficiency---------------------------
b. Detector Noise-----------------------
(1) 1/f Noise------------------------
(2) Generation-Recombination (g-r)
Noise----------------------------
(3) Johnson Noise--------------------
c. Detector Characterization------------
(1) Responsivity---------------------
(2) Noise Equivalent Power-----------
(3) D*-------------------------------
Z Gonerent le eemrelen-—---------------------
D. ATMOSPHERIC ATTENUATION----------------------
1. Transmissivity---------------------------
a. Absorption--------------------------
b. Scattering---------------------------
c. Scintillation------------------------
d. Ray Bending--------------------------
e. Turbulence---------------------------
III. MAJOR SYSTEM COMPONENTS--------------------------
Pn ee ~ -- - -- - --~ -- ~~ - -
B. ACOUSTO-OPTIC MODULATOR----------------------
C. DETECTOR--------------------------------e-eee
D. OPTICAL ANTENNA------------------------------
PV. mol EMeANaoiolo AND ALIGNMENT PROCEDURES---------

PRELIMINARY DEVELOPMENT---------------------- 65

Re Aon S RE Moi = —iein oie = 67.
SE GON DSOD TT CAS eae Moo aieeisia a= << ---~=- == --- == pal
THIRD MG Ries eto Maas =---2-------+--------- 72
eT GNMENTsTeGhiMGUIES----==---SSs=-s--=--+--==5 fae
ieee iegitendWyus CMentS---=S<-s9" oS - Ses cS a= Wee
Zn Acousta-Optic Modulatorend 1 anie ma = =o = ao

4. Collimation of Visual Optics with the

CO. Transmit Beam------------------------ 76
§. Alignment of the Newtonian Optics-------- 76
6. Signal and Local Oscillator Beam

Alignment--------------- err enero 78
F, ELECTRONICS AND SIGNAL PROCESSING------------ 79
G. SYSTEM RADAR ANALYSIS------------------------ 82
V. EXPERIMENTAL PROCEDURES AND RESULTS-------------- 84
A. DETERMINATION OF RANGE----------------------- 84
B. TARGET DETERMINATION AT 305 YARDS------------ 87

1. Perea Signal from Retro-Reflector at
SY (ental te ee ee Sa ere ee 90
2. Transmit Divergence Determination-------- 94
Sioa ouey lew WMedsuremnclt———=-———————-—-—- - 96

4. Expected Return from a Diffuse Reflector

at 305 Yards----------------------------- 97
5. Calculation of Detector Quantum
Efficiency------------------------------- 98
6. Calculation of Noise Equivalent Power---- 99
C. EXTENDED RANGE CAPABILITY-------------------- 101
D. VELOCITY DETERMINATION----------------------- 103
va. CONCLUSIONS AND RECOMMENDATIONS ------------------ 109

ee ee ee) = = = = = = = = 109

B. RECOMMENDATIONS FOR FURTHER STUDY------------ 109
C. POSSIBLE SYSTEM APPLICATIONS----------------- 110
ae PEN: EQUI Lot —- -S = === sea ees =e 12
BIBLIOGRAPHY wn rr rr rr err ree reece 114
INITIAL DISTRIBUTION LIST------------------------------ 116

Vir

LIST OF TABLES
Laser Output Power as a Function of Driver
Setting------------------------------------------
Acousto-Optic Modulator Characteristics----------

Modulator Frequency as a Function of Input
Voltage------------------------------------------

Detector 3 db Bandwidth and Quantum Efficiency
vs. Bias Voltage---------------------------------

Range Frequency Determination as a Function of
Range--------------------------------------------

Velocity Determination Results-------------------

Figure
1.

10.

dt
ah:
13.
14.

nO",
ge
18.
19.
20:
Ol.

LIST OF FIGURES

Frequency vs. Time of Transmit Signal-----------
Transmit and Return Signals vs. Time------------
Range Frequency vs. Time------------------------

Return from Target with Positive Relative
Velocity----------------------------------------

Frequency vs. Time, Near Range Target-----------
Range Frequency vs. Time, Near Range Target-----
Frequency Spectrum of Range Frequency-----------

Frequency vs. Time, Range = % Unambiguous
Range---------------e e e e -----

Range Frequency vs. Time, Range = 4% Unambiguous
De ei ee a i i a Se

Frequency Spectrum of Range Frequency, Range =
Za aor ruewst Ral sem a = = Sm M

Debye-Sears Effect------------------------------
Debye-Sears Effect------------------------------
Brage Diffraction-------------------------------

Momentum Scattering for Plane Monochromatic _
Optical and Acoustical Waves--------------------

Momentum Scattering for Acoustical Waves of
Finite Width and Diffraction--------------------

Acousto-Optic Modulator-------------------------
PoeCecelye Quantum Efficiency vs. Wavelength-----
Equivalent Circuit of a Photodiode--------------
Generalized Detector Noise Spectrum-------------
Atmospheric Transmission vs. Wavelength---------

Acousto-Optic Modulator-------------------------

oo
42
42
44
49
57

Figure
Ze

Zo.
24.
ZS
26.
Ze
Zon
Zo"
30.
lt,
Sa
Sou
34.
Siok
36.
7m
58.
ee .
40.

41.
42.

43.

Newtonian Optical Antenna----------------------- 64
First Optical System---------------------------- 66
second Optical System-----------------+----+------ 68
Third Optical System----------+------------------ 69
Signal Processing System---------<--------------- 80
First Optical System----------------------ccee-- 86
Third Optical System, View 1-------------------- 86
Third Optical System, View 2-------------------- 88
View from Spanagel Hall to Ingersoll Hall------- 88
NPS Grounds Near Range Tdrget----------<-------->- 89
WelocityaGenerating Model Railroad, Wiew 1------ ae
Velocity Generating Model Railroad, View 2------ om
Zero Range Frequencies --------------e----------- a2

Unprocessed Detector Output for 305 Yard Target- 92

Processed Return for 305 Yard Target------------ oo
Far Range Target Measurement-------------------- 102
Processed Return for 2370 Yard Target, View 1--- 104
Processed Return for 2370 Yard Target, View 2--- 104

Processed Return for Negative Relative Velocity,
View l------------- 2 ee oo en ee ee ee en ee eee 106

View 2-------------- 22-2252 ene ene eee e eee eee Hue

aa ieee a oo eC ee oe KS See ee ees + ey

ee eee et eee — = See a ae a) Siete a oo ete = Uy

Sea ee Sirs « .

ACKNOWLEDGEMENTS

I desire to give recognition and express gratitude for
the support given by the Naval Electronic Laboratory Center
San Diego, Code 2500, particularly to Dr. Greg Mooradian
and Rudy Krautwald. Also I am grateful for the assistance
of Professors John Powers, T. F. Tao and C. H. Rothauge
and for the technical support provided by Ross Seeley and
Bob Moeller, all of the Naval Postgraduate School.

This research project was conducted with Lieutenant
Maurice F. Fraunfelder, Jr., a fellow student at the Naval
Postgraduate School.

Rockwell International Science Center provided a PbSnTe
heterojunction p-i-n diode, courtesy of Dr. J. Longo,
Similar to the detector loaned by NELC (which was also
manufactured by Rockwell International Science Center).
This detector had a slightly higher quantum efficiency than
the one provided by NELC but did not have as high a fre-
quency response even when back biased at .6 v. It was
operated at 28-35 MHz (vice 35-45 MHz) but the frequency
limitation still reduced the output to 5 db below that of
the detector utilized in the system.

Aerojet Electrosystems Co. also provided a PbSnTe
heterojunction p-n diode, courtesy of Dr. Peter Wang, but
although it had a good quantum efficiency, its frequency

response was much too low to be utilized in the system.

I. INTRODUCTION

A continuous-wave frequency-modulated radar provides a
means of determining both range and relative velocity of a
target. Since the radar is continuous-wave rather than
pulsed, it enables realization of a given range capability
without requiring high peak power capability such as would
be required by a pulse radar. FM-CW radars have been de-
veloped for years but with the advent of the laser, a |
transmit power source of extremely high frequency and co-
herence became available.

Initially there was not the laser stability nor the
means of frequency modulation cf the beam available to make
a system such as this practical. This capability exists
now and was utilized in the developmental system.

The laser source provides a narrow beam and enables the
use of a high-gain optical HIE which is small and light-
weight (since gain is proportional to X} * and A is 10.6 um).
This narrow beam allows excellent bearing resolution but
requires a relatively long search time for any given volume
of interest. It would also require excellent beam steering
capability for a tracking application.

Heterodyne detection of 10.6 um radiation at high IF
frequency has only recently become feasible with the de-
velopment of fast detectors with a cut-off wavelength
greater than 10.6 um. Heterodyning also provides higher

sensitivity and low vulnerability to jamming.

Operation at 10.6 um wavelength is outside the capability
of any known intercept receiver and could not be detected by

any known anti-radiation seeker.

II. GENERAL SYSTEM CONSIDERATIONS

A. FM-CW

FM-CW radars offer several distinct advantages over
standard pulse radars. In the CW radar the ratio of peak
power to average power is near or equal to one. Since the
maXimum range of a radar is proportional to the fourth root
of the average power this allows for good range capability
without requiring components and circuitry capable of pro-
viding and switching high power and voltages as in a pulse
radar [1]. Elimination of this requirement results in large
Savings in system size and expense.

P_GoA :
Raax = (gp )* (1)
max 167 an

in
where Pi. is the average transmitted power, G is the antenna
Gain, Oo 1s the target cross section, A, is the effective

receive aperture and Pai

oe is the minimum required return

Signal power [2]. Normal FM-CW radars utilize two antennas
and require feed over cancellation [3] while pulse radars
require one antenna and a duplexer. Both these approaches
are improved on by an optical radar which can use one an-
tenna with no cancellation or duplexing. Additionally an

optical antenna provides extremely high gain with small size.

io Analisis of 7 FM-CW Radar System Using Triangular

Modulation
FM-CW radars determine range and velocity by ana-

itazamonthe difference between the frequenciés of the return

and transmitted signals. If the transmitted signal is as

shown in Figure 1 where F

=} : ;
modulating ~ T is the modulating

frequency and F is the frequency of the transmitted

transmit
Signal, the return signal reflected from a stationary tar-
get will be identical to that being transmitted only delayed
il.

at = 48 (2)
where R is the target range and c is the free space velocity
of the propagation of light. The time-frequency display of
the transmit and return signals is shown in Figure 2.

If the instantaneous transmit and return signals are
mixed the difference frequency can be extracted and processed.
The time-frequency display of the difference frequency is
shown in Figure 3.

The difference frequency directly yields range in-

formation. The rate of change of transmitted frequency

(df/dt) is given by

Ciecie. -
dt ~ T/2 = 4AF he (3)

where T is the modulating period, 2AF is the total range of
frequency deviation and ES is the modulating frequency.
Range frequency and thus range information is pro-

vided from

wie
range dt ae (4)
R eweeet. Cc. Fran e (5)
ange 2 2 ara :

FOTAF
FO

FO AF t
KF IF fe — 1

Figure 1. Frequency vs. Time of Transmit Signal.

bt |

Figure 2. Transmit and Return Signals vs. Time.

range

K— +> lar |

Figure 3. Range Frequency vs. Time.

hy,

Since for a given system df/dt is constant;

Range = constant ° F ange (6)

and thus range can be directly determined from the transmit
and return difference frequency.

If there is a relative velocity between the radar
and target the returned signal will have an additional shift
in frequency due to doppler shift. Doppler shift is given
by the relationship [4]

: _ *Nelocitytelative’*o 7)
doppler Cc

where Fy is the frequency of the transmitted optical signal.
Comparison of the transmit and return signals, when the re-
turn signal has doppler information from the positive rela-
tive veiocity, yieids a time-frequency display as in Figure
4.

The difference frequencies F. and Fy can be processed

to extract both range and doppler frequency information.

s coe ay ene 7 cy - F ange 7 Fdoppler (8)

xmt GEV a

= F +

ee Sey: range Faoppler

(9)

In systems where the expected range frequency offset
is greater than the expected doppler shift, range and dop-

pler information can be extracted by processing as:

(10)
E Z ea, b _ “Prange ; Faoppler? i (Frange : F doppler?
range Z Zz
la)
FE i “hy ae a (F ange i F doppler? es (F ange 7 F doppler?
doppler 2 : 2

F transmit
- - - F receive

Figure 4. Return from Target with Positive Relative Velocity.

If the expected doppler frequency shift is greater than the
expected range frequency of equations (10) and (11) are
reversed.

It is readily seen from equation (7) that the rela-
tive velocity is the product of a constant times the doppler
shift (any variation in FA due to modulation is negligible).

2. Signal Processing in a FM-CW Radar

There are numerous interactions between bandwidth
requirements, signal processing and expected target perform-
ance. The receiver input bandwidth must be great enough to
accommodate the return signal which is both FM and possibly
doppler shifted, IF bandwidth and processing filters must
be able to process difference frequencies, doppler broaden-
ing and range broadening. These bandwidths affect range
and velocity determination and available modulation rates.

a. Receiver Bandwidth Requirements

The receiver bandwidth must be able to pass the
FM spectrum of the transmit signal and also accommodate any
doppler shift within the range of interest.

The bandwidth requirement of an FM system is
given by

BW = 2(1+8)F = 2(1+AF/F )F... (12)

For a frequency excursion of 5 MHz at a modulating rate of
1.75 kHz this yields a bandpass requirement of 10.0035 MHz
Or approximately 2 AF.

As stated, additional allowance must be made

for doppler offset. The amount of the offset is given by

equation (7). Fora CO, faceneune OpuLeal Lyequencey ad s

2.83 x 10!3 Hz. This yields

eee
a = 52.4 kHz/km/hr

F doppler Cc

95.8 kHz/knot. (13)

If the maximum relative velocity of interest was 1000 knots
this would require an allowable offset of +95.8 MHz. This
requirement for a wide receiver bandwidth yields a very wide
noise bandwidth which reduces receiver sensitivity. This
can be largely offset by narrower velocity range of interest.
b. Range Spectrum Analysis

The spectrum of the range frequency requires an
IF bandwidth sufficient to pass the spectrum. Regretably
the spectrum of the range frequency varies with the range.
This spectrum will be considered at very short range and at
a range of one half the maximum unambiguous range.

(1) Spectrum of a Near Range Target. Figures
5S and 6 show the frequency-time relationship for a near
range target. The frequency spectrum of the range frequency
can be closely approximated by assuming rectangular pulses
of duration t and period AT. This spectrum is shown in
mreure 7. It should be noted that the first sidebands exist
in the main lobe at reduced magnitudes. As the range in-
mEcdoceomule ratio of the pulse width to the period decreases
and the lobe spreads until at the point where the pulse width
is less than half the period two sidebands will exist within
MicmMdin lobe of the spectrum. Before this point is reached

the approximation of a rectangular pulse is not nearly as

Figure 5. Frequency vs. Time Near Range Target.

F range ee
Efe. |

Figure 6. Range Frequency vs. Time Near Range Target.

z = = a
meclire 7, Frequency Spectrum of Range Frequency. t=5 E =e

exact and a better approximation is that of a trapezoidal
pulse.

(2) Spectrum of a Far Range Target. Figures 8
and 9 show the frequency-time relationships for a target
whose range is one half the maximum unambiguous range of
the radar. The frequency spectrum of the range frequency
is that of a trapezoidal pulse whose maximum frequency
exists for one half the period. The spectrum envelope is

found from the product of [5,6]

sin(mt,/T/2) sin[mn(t,+t,)/T/Z]

:
mt ,/T/2 m (t,+t )/T/2

sin(mt,/T/2) sin{m3t,/T/2]
~ mnt,/1/2 : see) 2

(14)

This spectrum is shown in Figure 10. It can be seen that
the spectrum is being spread and a greater percentage of
the signal energy is in the first sidebands.
c. Spectrum Spread Acceleration
An accelerating target causes a spread in the
doppler frequency of the return signal. This is analyzed

in a straightforward manner as follows:

CaN ee
F doppler ~ c (7)
dFy BEY gc F Ds oF : oe
dt dt. ce @ ccel
2°A
AF, = feet ° ste (16)
O

At is determined by the filter bandwidth so in the limit of
the bandwidth
23

—= T __>|<—— : ee AT 225) |< ATI

Figure 8. Frequency vs. Time Range = % Unambiguous Range.

range

T
tea et, o> 4

Figure 9. Range Frequency vs. Time Range = 45 Unambiguous
Range.

sin(2nmt,/T) sin(6nmt,/T)

—tntt,/1 © onwt,/T
2nmt, 6nmt, iT
glo
\

em fer yy, 0hlhlUfLULUN CR
a“

Fo-3Frm Fo-2Fm fe-frt — Fo FotFm ForlFm For3Fm f
7 4 2
—! o
a! ZI xz

Range = i Unambiguous Range

O 2ty

4ty

Sidelobes 18.4 db down

72. ri

Figure 10. Frequency Spectrum of Range Frequency.

1 1

At = BW AE C17)
/2-A |
TO |

where AF is the maximum allowable spreading of the spectrum.
In practice the filter bandwidths are sufficient to accommo-
date very high accelerations.

d. Range Resolution

Range resolution is determined by the bandpass

of the resolution filter or the spectrum width of the range
frequency, whichever is greater. A well designed system
will have a filter width whose response time corresponds to
the modulating period and with the exception of targets ap-
proaching the maximum unambiguous range this will be suffi-
cient for resolution calculations. The maximum unambiguous

range can be determined by the following relationship

T :
_ ¢ , modulation
Runambiguous cee? 2 ; a

This limitation is determined by the propagation time for
the target signal and the period of the modulating ramp.
If the modulation rate is matched to the range filter (it
can be slower) the range resolution is a direct function

of the modulation bandwidth [6].

From equation (2) and matching the filter to aa this becomes

[7]

AR = 5 * soap (21)
which indicates that improved resolution requires a larger
frequency excursion of the FM signal.

e. Velocity Resolution
Velocity resolution is similar to range resolu-
tion in that the wider the filter bandwidth the poorer the

velocity resolution. Doppler resolution is given by the

relationship
c*AF 2
- doppler = c*Filter BW
AV 4 2F —— (22)

f. Signal to Noise Effect of Collapsing the FM
Return

| When the transmit and received signals are mixed
together the spectra of the two signals collapse and there
results a relatively narrow frequency signal which is a
function of target range and velocity. The resultant sig-
nals can be passed through bandpass filters and thus the
resultant noise bandwidth is drastically reduced while the
Signal information is passed. The overall effect of this
processing is to reduce the noise present and greatly improve
the signal to noise ratio.

g. Range and Velocity Filtering

A means of determining the range or velocity

frequencies is necessary in order to provide the inherent
information. One method for doing this is to feed the in-
formation into a parallel bank of filters and provide a

display according to which filter passes the information.

An alternate and more efficient approach is to utilize a
sliding filter which sweeps the spectrum with respect to
time in a known controlled manner. This approach allows
relatively high resolution over a broad range with rela-

tively few components and thus less bulk and expense.

B. MODULATION OF A LASER BEAM
1. Historical Basis of Acousto-Optic Modulation

The diffraction of light by accoustical waves in a
medium was predicted by Brillouin in 1922 [9]. This phe-
nomonon was observed by Debye and Sears in 1932 and is now
known as the Debye-Sears effect. There was some initial
investigation of the effect but until the advent of the
laser to provide a high intensity beam which is highly co-
herent in both time and space no important application was
seen and interest waned.

The mechanism by which acousto-optic modulation is
accomplished is known as the photoelastic effect. This ef-
fect is the change in the index of refraction due to mech-
anical strain and is present in some crystals and liquids.
Ultrasonic waves propagating through a material produce
mechanical strains which in turn cause variations in the
index of refraction. In this case the variation is known
Poe ACOUStO-Optic Cffect .

oem eiDy.e- Sears bere t

When a plane wave of light of angular frequency, wu,

enters a slab of material with an index of refraction, n,

the velocity of light in the material is reduced by the

——————
- EE

————- ;
aes 32 -

—

°® Qe. ae
. eas
= . iia ee °

factor n from its free space velocity c. The optical wave-
length A in the slab is

(23)

and the material acts as an optical delay line with a time
delay t for a given length 2&.

_ an
t= 2. (24)

The delay, $6, corresponds to a phase delay of

g=ut = On, m54

Equation (25) can also be expressed as

= - oe (26)
O

d =

where ro is the free space optical wavelength. The varia-

tion of phase delay with respect to the variation of n is

dé = ome dn. (27)

oO.
If n is varied with respect to time in a controlled manner
then the phase delay is also varied directly and a means of
phase-modulating an optical beam exists.

From standard FM analysis it is known that the phase
modulated wave consists of a carrier and sidebands which are
separated by WO where WO is the angular frequency of the
modulating mechanism, in this case the angular frequency of
the variation in the index of refraction of the slab of ma-
terial. The amplitude of the carrier and the sidebands vary
as Bessel functions where Jo corresponds to the carrier, J,

to the first sideband, etc.

When an acoustical wave is launched in a medium and
an incident optical wavefront passes through the medium as
in Figure 11, modulation will occur. The acoustical wave
launched in the modulator with angular frequency w and at
the acoustical velocity of the medium Wee propagates through
the modulator with variations of compression and rarefactions

with an acoustical wavelength ro

27V

A = = (28)
a WO

The rarefactions and compressions cause the corresponding
variations in the index of refraction. These "slabs" of
different indices ef refraction modulate the incident opti-
cal wavefront yielding the carrier and sidebands of which
the carrier and first upper order are iilustrated in Figure
er W100).

The first upper diffracted wave is indicated in
Figure 11. Higher order upper sidebands are tilted up at a
greater angle and the lower order sidebands are tilted down.
Figure 12 shows the Debye-Sears effect and the variation in
the index of refraction in the modulator.

3. Bragg Diffraction

If the wavefront Vivitetonetim the index of refraction
towclarecterized as a partially reflecting mirror and the
optical beam is incident upon the acoustical wavefront at
an angle @, as is shown in Figure 13 [9] then the diffracted
wave will be directed away with the angle of refraction 9,.

A necessary condition for diffraction in a given direction

Incident

Optical Wavefront

Compressed

ae EE SN a
Sn a ae
See en
a 2 e SLD = SLE
eR AAR ee A SST ETS oT TY
ea a en A ele Ae a oe eee
ORNS ers CSI Ts oe ee an ie aE Cae
| ers Sa A NRT EOE SL Seg SIS ne ei Pg ey
SSS ee
SN FR aS Oe a eae eam)
eS a
Dilating
me eee

Compressing
eigen ee) | nae
Sa. ae

eS
Ompressed

ie eR ae ae Se |
eS
a =]
re
ier
LT SS
= ee
| ee ees
ee
aaa eS

Acoustic Wave - ae

Figure 11.

Bete c-ceans Effect.

W FW)

Distance

/l\

MGous tical

Index of
Refraction 1
Wave

Ww
m

Figure 12. Debye-Sears Pape t .

a
Incident
Optica
Beam
x C
g Len g
r) - = r
3 | _aip
x O
Unsiffracted

Beam

Figume 1S eambrdge 1 ffraction.

is for all points on the reflecting surface to contribute
in phase. To satisfy this requirement for the two points
of diffraction B and D the difference between the paths AD
and BC must be an integral multiple of the optical wave-

length os This condition is satisfied when

x(cosé. - cos8,) = md (29)
where m= o0, #1, #2,... This can be satisfied for all
points x only when m = 0. This requirement indicates that
6. = @.. Additionally for different wavefronts of the modu-

1 Yr
lator (separated by . COMMiterme Pemeons CTUCCIVelY 1t as

required that

2A ,siné = ro (30)

where oF ge oe This is known as Bragg diffraction since
it is the same requirement for X-ray diffraction in a lattice
Since ra is much larger than the optical wavelength, ho? sing
is small and is usually approximated by 6. It is also seen
from Figure 13 that the angular separation between the dif-
fracted and undiffracted beam is twice the Bragg angle 8.
a. Modulation Frequency

Determination of the frequency shift in the dif-
fracted beam can be determined from two approaches. The
first is a somewhat simplistic consideration of geometry
and doppler shift and the second a consideration of energy
and momentum requirements through the utilization of wave
ieietoues..

(1) Dopper Modulation Determination. The

acoustical wavefronts which cause diffraction as shown in

Figure 13, are propagating through the acousto-optic modu-
lator with velocity Vo: This will yield a doppler shift in
the diffracted beam according to the relationship
2V_sin6f
_._

F doppler 7 Cc ($1)

where f is the optical frequency. From the Bragg equation
(30) the relationship between sin® and the two wavelengths

are shown. The doppler shift is given by

2V Af = V

F doppler 7 2X. a (32)

.
© |

where E. is the frequency of the acoustical wave. If the
two wavefronts are closing as is shown in Figure 13 the
doppler shift is positive and the frequency of the diffracted
wave is gee If the relative velocity between the two wave
fronts is negative the frequency of the diffracted wave is
ae

(2) Particulate Analysis of Modulation. Light
has a dual wave-particle nature and analysis can be done by
consideration of its particulate nature. Photons have an
energy hw and momentum nic Simthaviy the acoustic phonons
have an energy hw , and momentum hk. Im any interaction
between the photons and phonons both energy and momentum
must be preserved since both the incident photon and phonon
cease to exist and a new diffracted photon is created.
Figure 14 shows the wave vector relationship between the
incident photons ke the diffracted photon Kk and the acous-

->
tical phonons Ko: Conservation of momentum requires that

5.5

h(k,+k,) of the colliding phonon and photon equal hk. of

the scattered photon,

- |
See. (33)
Ww. = Ww. +W =wtw (34)

Wo =W. - WwW =wW- Ww (353)

which shows that the diffracted wave is modulated by the
acoustic frequency.
Since the acoustic frequencies are less than

10'° and the optical frequencies are greater than 10??

Ww =w tw = wo, SO ra ~ Ik, | (35)

and the magnitude of the two optical wave vectors is approxi-

mately the same. From Figure 14 it is seen that

Ik. | = 2|k, [sine (e.)
and since
[k. | = 2n/a, (38)
it follows that
2A ,sin® = ro (39)

which is the Bragg diffraction requirement.

Pitre sl eiomconepeniecetly monochromatic plane
waves. In actuality there is both wave divergence and some
igequeney Variation. <Ihtus rather than being only one exact

angle of diffraction allowed there is a small range. This

effect is illustrated in Figure 15 where only acoustical
divergence is considered [11].

4. Modulation Bandwidth

The modulator bandwidth is inversely proportional to

the transit time of the acoustic wave across the optical
beam. A means of increasing the bandwidth is to focus the
optical beam and have its waist centered in the modulating
medium. The waist diameter can be determined from the

relationship [12]

Wai (

Z2&n2 FA
jr (40)
0

1
where W, is the focused optical waist diameter, ne Site
unfocused beam waist and F is the focal length of the

focusing lens. The expression for modulator bandwidth is

given by [13]

Af = .54 Va/Wo ni Ne (41)

where Ni is the acoustical velocity.
S. Modulation Power Transfer
The amount of power transferred from the incident

beam to the modulated beam is a direct function of inter-

action length % of the beam through the modulating medium

and the amount of change of the index of refraction in the
medium. The latter is a direct function of the acoustical
power up to a saturation level in the medium. This power

transfer is also inversely related to the optical wavelength.
In the previous section it was noted that the modulating

bandwidth is inversely proportional to the optical waist

rr curve of Constant |k|

Figure 14. Momentum Scattering for Plane Menechromatic
Optical and Acoustical Waves.

Figure 15. Momentum Scattering for Acoustical Waves of
Finite Width and Diffraction.

Intensity Variation of
the Acoustic Wave

prs Curve of Constant |k|

diameter, there is a tradeoff between the waist diameter
and the length that the optical wavefront is planar so

this must be considered with respect to the diffraction ef-
ficiency. The expression for diffraction efficiency is

given in (3). With some manipulation this reduces to

mieracted 5
—— ree Oy 1)

m7XAn
a Coe?
incident O

(42).
Another approach to this is given in [14].
6. Modulator Considerations

A diagram of an acousto-optic modulator is shown in
Fectres G6. there 1s a transducer to couple the electrical
modulating signal into the crystal. To date the transducer
is often the limiting component with respect to the band-
width. At the opposite end of the modulating crystal there
is an absorbing medium to prevent reflection of the acousti-

cal waves back into the optical beam.

Cee DETECTION

Detection of optical signals encompasses many aspects
Mewever, only those pertinent to the CO, laser radar will
Demeovered.

ieee enoto-Voltare Detector

Photo-Voltaic detectors are semiconductor diodes in

which incident photons create electron-hole pairs in or with-
in one diffusion length of the depletion region of the diode.
These generated carriers are accelerated in opposite direc-
tions by the intrinsic field present within the depletion

region and depending upon biasing arrangement, give a signal

38a

Acoustical Absorbing Medium
Acousto-Optic Medium

ae imansaicer

Modulator Driver

Information
Signal

Peures!6, Acousto-Optic Modulator.

either in current flow variation or output voltage varia-
tion. In properly constructed and biased diodes the genera-
ted excess carriers move at scattering limited ve loercy and
there is virtually no recombination within the depletion
region.
a. Spectral Response and Quantum Efficiency

The absorption of photons in the semiconductor
material is a strong function of the wavelength of the in-
cident photon. To a much lesser degree there is variation
with temperature since both the semiconductor energy gap
and surface recombination velocity are effected by tempera-
ture. The maximum wavelength that will generate an excess
carrier in the semiconductor material is given by the rela-
tionship [15]

~ he = eZ aGin
lon - oa —

g &

where Lye is the cut-off wavelength and Ey is the forbidden
energy gap. Wavelengths longer than he will effectively pass
through the active region of the photodiode without absorption.
Shorter wavelength photons will be more readily
absorbed until for much shorter wavelengths the photons are
essentially all absorbed very near the semiconductor surface
and the generated excess carriers are lost to the detection
process due to high surface recombination that occurs near
the boundaries of the crystal laeence meine overall effect
of the wavelength dependency of photon absorption yields a
curve as is shown in Figure 17. This shows the effective
quantum efficiency of intrinsic silicon and germanium

photodetectors.
40

The quantum efficiency is a measure of the con-
version efficiency of the photodetector in converting photons
to excess carriers. Quantum efficiency, n, is defined as
the ratio of the number of photons that generate excess
carriers to the number of photons incident upon the detector.

The current generated in a photodiode is given by
i = nqoA (44)

where n is the quantum efficiency, q is the electronic
charge, @ is the incident photon flux density and A is the
active area of the detector.

The equivalent circuit of a photodetector is
given in Figure 18 [15]. The available power into a matched

load from this circuit is [15]

4 tal 29 1

Pe ~ gl Ipp() | w2c2R (45)
_ 1 1
[ = (nga) * weceR ° (46)

Due to thermally generated carriers known as
dark current all semiconductor detectors which operate at
10.6 um must be cooled to 77°K. Due to advances in material
processing this requirement may be relaxed somewhat.

b. Detector Noise

There are three primary categories of noise in
photodetectors, all of which predominate in a general fre-
quency range.

(1) 1/f Noise. There are several types of
noise for which the power spectrum varies as the inverse of

frequency. This noise is sometimes known as modulation

Effective #4
Quantum
Efficiency '

. -6 -8 /.0 1°22 / /*G@ Z
vy 1 Wavelength (microns)

Figure 17. Effective Quantum Efficiency vs. Wavelength for
G, and S Photodetectors.

I ppl) C

Figure 18. Equivalent Circuit of a Photodiode.

noise in semiconductors or flicker noise in vacuum tubes.
1/f noise becomes negligible with respect to other noise
sources above a few kHz.

(2) Generation-Recombination (g-r) Noise. In
semiconductors the major source of noise at intermediate
frequencies (above the range where 1/f noise predominates)
1S g-r noise which is due to fluctuations in rate of charge
Carrier generation and recombination. This variation is
due in part to the randomness of the arrival of the photons
of the incident flux. This latter effect is sometimes
categorized separately as photon noise. Generation-recom-
bination noise is relatively flat with respect to frequency
up to the value where it is approximately equal to the in-
verse of the free carrier lifetime; at this point it falis
off at about 6 db per octave.

(3) Johnson Noise. Johnson noise is thermally
generated and present in all devices in accordance with
their temperature and bandwidth. The noise is independent
of frequency up to extremely high frequencies. The expres-
Sion for thermal noise power is given by [16]

4kTB
Noise, = R (47)

where k is Boltzmans constant, T is the temperature in
degrees Kelvin, B is the device bandwidth and R is the de-
Vice resistance. A profile of noise contribution as a

function of frequency from [17] is given in Figure 19.

1/£ Noise

Log
Noise
Voltage
Log Frequency
Pores t9. Generalized Detector Noise Spectrum.

c. Detector Characterization

There are several figures of merit which are
useful in describing photodiode performance. Those which
are significant with respect to the laser radar are R, NEP
: it
and D .

(1) Responsivity. Responsivity, (R), is a
measure of a detector's output for a given input. The units

of responsivity are v/w and it is given by

7 (48)
d

where ie is the rom.s. signal voltage, H is) the r.m.s. value

of the irradiance on the detector and Ag is the detector
area in cm’,

(2) Noise Equivalent Power. A more meaningful
detector parameter is noise equivalent power, (NEP). NEP
is the radiant flux necessary to provide an output signal
such that the signal to noise ratio is equal to one. NEP

can be calculated from

HA 4 HAG”
NEP = yes an (49)
WV. ae es

where Wie is the noise voltage, Ve is the signal voltage and
H and Ag are as in (48). A good photodiode will have a NEP
of 10°... - 10 °° w. In shot noise limited heterodyne opera-

tion the NEP of a photodiode is [18]

NEP = — ; (a0)

(3) oe Since both signal and noise are a
function of detector area and noise is a function of band-
width a measure accounting for these factors is D™ (dee-
Star).

| ng

-— NED tad)

B is the detector bandwidth and Ag is the detector area.
The units of Dp” are cmHz 2w"? and the measurements are
usually taken at peak responsivity. The theoretical limit
of D” for a photodiode at 10.6 um viewing a hemispherical
surrounding at 300°K is approximately 5 x 10'°. A good
detector at 10.6 um will have a D’ of about 2 or 3 x 10?!°
cmHz 2w

2 OCie Ren bewecee, fon

Optical signal processing is available in photo-
diodes by heterodyne or coherent detection. Alignment of
the phase fronts of the signal and local oscillator beams
1s extremely critical and anything which causes phase front
distortions severely degrades detection.

In the detector the shot noise due to the local
oscillator power can be made to override all other noise
sources. Operation in this condition is known as shot
noise limited operation and this condition allows detection
of signals down to a theoretical minimum known as quantum
limited operation. Neglecting the noise contribution due
to dark current (at 77°K the amount of dark current is very
small) the power signal to noise ratio for heterodyne de-

tection in a photodiode is [19,20]

2
7 2(nq/h£) *P; Pe ef
_— a. —=- ay eT, )
[ he (Py +Po*Pg) + ROR JB

where Tp and Ta are the detector and amplifier temperatures

and PL 1s the local oscillator power, Pg is the signal
power, Pe is the power due to background radiation and B is
the system bandwidth. The numerator is the power of the
difference frequency component of the detector output signal
and the denominator consists of the shot noise power and
thermal noise power.

In shot noise limited operation the power of the
local oscillator is increased until its power is greater
than the signal and background power and the shot noise due
to the local oscillator dominates all other noise contribu-
tions. At this level the signal to noise ratio becomes

aes (53)
This is twice the SNR of a shot noise limited photoconductor
[21].

For homodyne operation the signal to noise ratio 1s

doubled and is [14]

2nP
gage 2 i (54)

Nee seit
The value of heterodyne SNR given in (53) is for
optimal phase front alignment. If misalignment is due to
angle of incidence in only one coordinate the amount of
misalignment which will degrade the heterodyning efficiency

by 10% or less is [14]

Vs | (55)

where ~ is the amount of angular misalignment, A is the
optical wavelength and d is the detector size. For a 10

mil detector at 10.6 um

y < 10.4 mrad.

D. ATMOSPHERIC PROPAGATION

The propagation medium for an optical radar is the
atmosphere which affects the transmitted and return beam in
several ways.

1, fransmissivity

Energy attenuation in the atmosphere is a strong

function of wavelength. There are several atmospheric
windows whicn are located in the following approximate

ranges: (Figure 20)

- see. S
1.5 eae. 8
ioe 2.6
5 sue. 2
Ao (

8 =

The attenuation in the atmosphere is a consequence
of both absorption and scattering. The transmittance of a

path can be expressed as [17]
t=e%* (56)

where o is the extinction coefficient and x is the path
length. The extinction coefficient is the sum of the absorp-
tion coefficient a which accounts for molecular absorption

and the scattering coefficient y which accounts for scattering

“yZSueToAeM FO UOTIOUNY e& Se odUeITWSURI] DTILOYdsoNIYy "OZ OansTy

i ae bare. 2

9TNIITOW sutqsosaqy
c

09 o°H

aor rm) YSUSTOACN

Yl 4e4

L 9

+} snp ry ad ae oon

08
00T

(Juas10ed) adue.AtTUSUPL]

by molecules, haze and fog. Both a and y vary as a function
of wavelength, climate conditions, temperature, humidity
and altitude.

a. Absorption

In the region of interest (10.6 wm) there are
two absorbing molecules which are of significant importance.
These molecular absorbers are co, and H,0. Water content
may vary from as much as 2% of a very humid atmosphere at
sea level to a very minor amount in extremely arid regions.
The water vapor content of the atmosphere decreases rapidly
with altitude until at 40,000 ft it is considered negligible.
Water vapor content is also inversely proportional to tem-
perature, thus cold climates or weather minimize the
absorption due to H,0.

Carbon dioxide constitutes approximately .032%
of the atmosphere by volume. This percentage is relatively
constant up to an altitude of about 30 miles, then decreases
by a factor of ten for every 10 miles increase in altitude.
CO, content varies little with temperature or weather and
thus is a much less variable component than water vapor.

There are several other absorbers such as
ammonia, carbon monoxide, sulfur dioxide, methane or ni-
trous oxides which are normally present in negligible
amounts. In industrial or polluted regions these gases may
be present in sufficient quantities to produce significant

attenuation.

b. Scattering
The relationship between the scattering coef-
ficient and wavelength which is often used in predicting

transmissivity is [17]

yx? (57)

where ~ is a function of the ratio of the particle size to
the wavelength. If the particles are small with respect to
the wavelength yp is equal to 4 and the scattering process

is known as Rayleigh scattering and is strongly dependent
upon wavelength. It is this wavelength dependent scattering
which causes the sky to appear blue. For larger particles
the value of ~ approaches zero, this process is called Mie
scattering and is independent of wavelength. For most fogs
YW is zero for the visible spectrum and thus the fog appears
white.

In general the atmospheric categorization of
haze consists of particles whose radii range up to .5 um
and that of fog consists of particles which range from .5 -
80 um with a distribution peak which usually ranges from
59 - 15 um. These numbers indicate that at 10.6 um haze
will have minimal effect but fog will greatly reduce trans-
missivity. Actual experience obtained by the Naval Elec-
tronics Laboratory Center at San Diego seems to indicate
that fog may not scatter as much as was expected.

Rain reduces the transmissivity in direct pro-
portion to its intensity. Raindrops range in size up to

approximately 3 mm and thus wp has a value of zero and Mie

‘eo

scattering with wavelength independence is the scattering
phonomonon. All experience to date indicates that rainfall
is the predominate attenuating factor at 10.6 um.
c. Scintillation
Scintillation is the phenomonon which causes
uncorrelated variation of intensity and apparent directional
changes in a radiant source. This phenomonon is caused by
relatively rapid discontinuous variations in the index of
refraction in the propagation path. Scintillation is the
primary effect which renders amplitude modulation of an op-
tical beam for atmospheric propagation a poor second choice
when compared to frequency modulation.
d. Ray Bending
Variations in the index of refraction of a more
continuous nature can cause beam bending. This can cause
errors in angular location of targets and other abberations
due to ducting as is experienced in more conventional radars.
e. Turbulence
Atmospheric turbulence causes pheonomona such
as scintillation and phase front distortion. Turbulence
Causes variations of intensity and phase across the beam
Mimedce wy = lirect detection [14] allows integration of the
intensity variation as the receive aperture increases, but
for coherent detection there can be a decrease in signal to
i@msemrdtloOwas aperture S1zeé 1S increased since the varia-
tions in phase front degrade the signal rather than inte-

Beate Out.

IIIT. MAJOR SYSTEM COMPONENTS

A. LASER

The laser utilized in the radar system is a 3 w, ver-
tically polarized CO, laser (10.6 um wavelength) built by
Honeywell Corporation. The laser is a closed system laser
and is water cooled. It has an internal piezzo-electric
transducer (PZT) which allows some modulation and control
for line shifting. The PZT was not utilized in the system.

The laser power output as a function of driver setting
is given in Table I. The divergence of the laser was com-
puted as 1.71 mrad. This value is somewhat lower than the
value estimated by NELC but NELC did not measure the
divergence.

A cooling water hose connection inside the laser head
came unconnected during operation of the laser. The water
caused a short from the high voltage leads (+10 KV) to
ground which resulted in the destruction of an inductor in
the laser power supply filter. This required operation of
the laser with an unfiltered power supply and as a result
the output power dropped from 3.05 w to 2.3 w and contained
a 120 Hz ripple. The result of this was a drop in peak
transmitted power from 1.2 w to .65 w and a drop in un-
Pcuscad local oscillator power from .95 w to .51 w.

The laser was microphonic (amplified low frequency
mechanical vibrations) but this was filtered out in the

Signal processing and presented no problem.

5.5

Driver Supply Setting Power Output (Watts)

hee v25
76 64
80 td
82 5
84 Ve
86 dee
88 14915
90 i LS
92 LS
94 2.45
9§ 2054
98 20S
100 2a
102 Zoo
104 Dao S
110 (Max) 3206

Table I. Laser Output Power as a Function of Driver Setting.
(Data taken 18 July 1974)

B. ACOUSTO-OPTIC MODULATOR

The acousto-optic modulator utilized in the system is
made by Isomet. Its specifications are listed in Table II
[22]. The crystal is germanium and both the modulator and
Amey are RFI hardened. A sketch of the modulator is shown
in Figure 21. Table III shows the modulation frequency as
a function of input voltage. While the laser power supply
was filtered the modulator was frequency modulated from
35 - 45 MHz and the modulated output power was 1.2 w while
the unmodulated beam power was .93 w for a modulating ef-
ficiency of 56%. During unfiltered operation the efficiency
was unchanged. This performance was completely satisfactory
considering that the Bragg angle varies with frequency and
the modulator angle was held constant at approximately 2.2°
(the Bragg angle for 40 MHz operation).

The modulator crystal is germanium which has an acousti-
cal velocity of 5.4 x 10° cm/sec [23]. This yields an
acoustical wavelength at 40 MHz of 135 um.

There are two 5 inch focal length focusing lenses with
the modulator. The first is to focus the incoming beam to
a minimum waist size to increase the modulator bandwidth
and the second lens is to recollimate the output beams.

There is a time delay between the driver output and
the interaction of the acoustical and optical beams. This
propagation delay in the crystal is due to the time delay
required to propagate from the transducer to the optical
beam and gives an additional frequency offset to the range

frequency. In earlier system alignments this time delay

Modulator

Bpexatins wavelength

Rise time

Deflection efficiency >
Contrast ratio

Static transmission efficiency

Coatings

Optical aperture

Acoustic center frequency

Nominal impedance

Cooling

Driver
Bandwidth
Input impedance

Linearity >

10.6 um
70 nsec
60% with DC input
1000:1
88%
AR at 10.6 um
1 mm
40 MHz
50 2

water cooled

35 - 45 MHz
SO ohms

5% deviation

Input voltage z= Onaziy (55 Miz) to
- 9,27v (45 MHz)

Output amplitude variation

+ | 7ap

Table II. Acousto-Optic Modulator Characteristics.

BNC Connector

Impedance Matching Coil

Aluminum Case

Electrode and Heat Sink

Transducer

Germanium Crystal

Copper

Acoustic Absorber

Figure 21. Acousto-Optic Modulator.

Input Output Input Output

Voltage Frequency Voltage Frequency
(V.) (MHz ) (V) (MHz )
6.0 are 8.1 421
6. 35.6 2 42.4
6.2 do .9 5 8 42.8
6.3 36.2 8.4 43.1
6.4 36.6 8.5 43.4
6.5 57.40 8.6 43.7
6.6 Siled 8.7 44.0
6.7 Dela6 8 44.3
6.8 Syne ~ 44.6
6.9 To. 5 9.0 44.9
7.0 38.6 9.1 45.2
ee 3 8m9 Dare 45.5
fod Soc oF 435.8
1S Soo 9.4 46.0
7.4 50a oD 46.3
hod 40.2 9.6 46.6
7.6 40.5 ied), 46.9
a, 40.8 OFS AT
7.8 7 ee 47.4
7.9 a. 10.0 47.7
8.0 41.8

Table III. Modulator Frequency as a Function of Input
Voltage.

was approximately 1.6 us which translates to a frequency
offset of 56 kHz. After realignment and repositioning of
the height of the modulator the delay was approximately

1 ws which corresponds to 35 kHz. A 1 us delay corresponds

to 5.4 mm travel through the crystal.

C. DETECTOR

The detector used in the system was a PbSnTe heterojunc-
tion p-i-n photodiode made by Rockwell International Science
Center [19]. The detector area was 4 x 10°" cm? and opera-
tion at liquid nitrogen temperatures (77°K) is required.

The zero bias quantum efficiency was listed as .14 with
a peak quantum efficiency of .32 with .2 v bias (Table IV).
The measured zero bias quantum efficiency was .034. No rea-
son is known for this change except apparent detector degra-
dation. Some of the personnel at NELC expressed the opinion
that the detector had degraded. Determination of quantum
efficiency in heterodyne operation indicates a biased
quantum efficiency of approximately .12.

Shot noise limited operation was not achieved with the
detector. An estimated local oscillator power incident upon
the detector was .1 mw. The detector dynamic impedance
varies with biasing but with zero bias the resistance is ap-
proximately 50 2. Using a quantum efficiency of .14 either
Signal or local oscillator power greater than .9 mw would
be required. For a quantum efficiency of .034 greater than
3.7 mw would be required. This exceeds the incident power

capability of the detector (3 mw).

Bias Voltage 3 db Bandwidth

(Volts)

0
ure
70S

Table IV.

(MHz )

200

Quantum Efficiency

14
20
27
350

Detector 3 db Bandwidth and Quantum Efficiency vs.

Bias Voltage [19]

The 3 db bandwidth is listed as 200 MHz. The maximum
frequency at which the detector was sparated was 55 MHz. No
roll-off due to frequency response was noted.

The detector was mounted in a stainless steel SAT dewar
with a side looking window. The window transmissivity is
unknown but a transmissivity of .8 was estimated for calcu-

lation purposes.

D. OPTICAL ANTENNA

The special antenna utilized in the radar system is a
dual transmit/receive Newtonian lens system [24]. Figure
21 is a diagram of the lens system. The antenna has the
following parameters:

Dual antenna - transmits and receives

Transmit/receive folding mirror - double flat parallel
surfaces elliptical 1" x 25"

Receive aperture mirror - 6" diameter parabolic, 60"
focal length

System tho Seen

Effective t/no (due to blockage)[17]- 10.45

The system yields the following system performance cap-
abilities for diffraction limited optics:

Airy disc diameter [25] d = 2.44A(f/o) = 259 um (58)
Due to blockage of transmit ellipse [17] d = 270 um
Depth of focus x = 4i(f/no)? = 4.24 mm (59)
Near limit of field to achieve airy disc

ce = D?/2A = 1096 m= 1199 yd (60)
Field of view for detector diameter equal to airy disc

[17]
61

8 = d/f = 170 urad x .01° (61)

Field of view for 8 mil detector [14]

2.44eA¢d
8 = § = yp EEO ee urad (62)

airy mirror

Angular target resolution

a= - Ss ilocgial hes 1) 0) See (63)

Maximum allowable incident angular wavefront misalignment

between signal and local oscillator beam for 10% heterodyning

degradation using an 8 mil diameter detector: [26]
Bee, °
Y= gz = -013 rad = .75 (64)

Receive aperture = 1(6/2)* = 28.3 in?

Folding mirror surface = 1(3/4)* = 1.77 in?
(blockage area somewhat larger)

Percent blockage - = 7%

In the above relationships i is the optical wavelength
Tomine £ 15 the focal length (60") and Dis the receive
mirror diameter.

To transmit directly ahead of the optical antenna the
elliptical flat is turned 45° with respect to the antenna
axis and the transmit beam comes in at 90° with respect to
the antenna axis (Figure 22).

To provide a visible indication of the co, beam location
a HeNe laser beam at 632.8 nm is being transmitted aligned

with the 10.6 um beam. As can be seen in photographs (Figures

. a — ee 2S

27, 28 and 29), there is a rifle scope trained on the mirror
through which the HeNe beam enters the transmit system. The
scope view follows the transmit beams and allows specific
location of the output beams. When the system is aligned
beam steering by moving the optical table will not require
any additional alignment.

Receive alignment can be accomplished by looking into
the receive folding surface of the elliptical mirror. Re-
ceive view can be determined by moving horizontally and
vertically and insuring the target is in the center of the

elliptical mirror blockage.

— es —“

ee Transmit/Receive
Elliptical Flat
Betector ae | Transmit Beam
Beam
a) Return Beam
Ltt
2
Parabolic —
ee ee. ol

Figure 22. Newtonian Optical Antenna.

IV. SYSTEM ANALYSIS AND ALIGNMENT PROCEDURES

A. PRELIMINARY DEVELOPMENT

The radar system development proceeded through several
stages. The initial step was the assembly and alignment of
a system which would modulate the laser beam, transmit the
beam, receive the return and utilizing heterodyne detection
extract the modulated signal. Figure 23 shows the initial
system utilized to accomplish this.

Alignment of the system is very crucial and is somewhat
complicated by the fact that the CO. beam at 10.6 um is
invisible. The presence of the CO. beam can be determined
by using heat sensitive graph paper or heat sensitive plastic
encapsulated liquid crystals. The graph paper is used when
the power is relatively high and the liquid crystal paper is
utilized for powers down to a few milliwatts.

The beam splitters utilized are germanium with one side
anti-reflection coated for 10.6 um wavelength. The germanium
lens and splitters are opaque to visible light. The values
listed for the beam splitters first show the reflectance
then transmittance. A 90/10 splitter means that 90% of the
co, beam is reflected and 10% is transmitted through the
splitter. Unless indicated otherwise the lenses, splitters
and mirrors are 2 inches in diameter. All mirrors are front
surfaced.

Beam splitters provide a means of separating portions of

a beam into known components or combining two beams together

‘wa3sks TeotzdoO 3saTqy °¢7 san3tTy

poieTnpowuyg

YASVT
C
ae 1o3De30q | AL | LOSET
SU9T BUTSNDO iQ eNeh
i Ri ie wf eee
Hf SATIDIY
LOLITW wedsg “O°7 Hp JIWSUPIT
$6/§ 10331 TdS% !
t Toasty =
~
we og | pu era te a
pezeTnpoy «4! TN
S/S6 SUDT SUD] | 7
19331Tds suTSnod0g uTSndo04 i oS
c ee SS
lit
0S/0S S6/S adoos
IT3d9-o3snosy | It !
1OII TW dinjwiady sl
LOLITW SUTYIOTY weog ar
A |

for collimated joint transmission. One effect of using beam
splitters is that to split and combine beams the reflecting
surfaces must be at an angle to the beam paths, for a right
angle deflection the surfaces must be oriented at a 45°
angle with respect to the transmission path. The angular
presentation of the surfaces reduces the amount of surface
area available for transmission or reflection and at 45°
the circle presents an apparent ellipse with a horizontal
minor axis of 1.4 inches. The transmittance or reflective
area is reduced by the angular presentation to .707 of the
Original, or in other words, a 1.5 db reduction. This re-
duction in size presents no problem for the transmit path
but for the return path it can reduce the collected energy
Pyoi. 5S db,

The holders for the splitters and mirrors have uncoupled
horizontal and vertical rotational mobility which allows
exact direction of reflected beams. Additionally the A-0O
modulator and detector holders have 3 degree uncoupled axis
mobility with micrometer adjustment for up to one inch
travel in each direction. The A-O modulator focusing lenses,
blocking aperture, and in systems two and three (Figures 24
and 25) the transmit beam elevator and receive/LO splitter
are mounted on holders with motion in one direction up to 4

inch with micrometer adjustment.

Boer ens! OPTICAL SYSTEM
The configuration of the first system developed to be
utilized as a laser radar is shown in Figure 23. The trans-

mit path was from the CO, laser reflected from the 95/5 beam

—_ +e

"WaySAS TBDTIdQ puodsS ‘PZ 2ANBTY

LOIITW DT Toqeieg

Lasey eNeH

[Tensta
fa ee
Con SATOIDOY
LOLITW SUTYYSTS Iusuedy]
19139915
pue
LOLI Kes 4
me
weeq OT
10399390q
~ Ss F
01/06 = ™ roqattd 71] | 56/5 @ ft
19131 Tds Te SsoratW yTusUeL] meat ae ‘Wr
asinqiady ” ae Dy
SUTYIOTG weog | a S
ene 1ol4 ty pe .eTnpowug ) adods sut ysts

"weisks [Teotido patyL “Gz ean3ty

IOLITW OTLTOqeieg

adoog 8ut.ystTs

splitter (5% power losses) to the input focusing lens of
the acousto-optic modulator, through the modulator to the
output focusing/collimation lens, through the modulation
pass aperture where the unmodulated portion of the beam is
blocked, through the 50/50 splitter where half the power

is reflected out of the system, through the 5/95 splitter
(5% power loss) where the HeNe beam is collimated with the
co, transmit beam and off of the transmit/receive mirror to
the target.

The return signal after being reflected from the target,
enters the system at the transmit/receive mirror, through
the 5/95 splitter (5% power loss), reflected off of the
50/50 splitter (50% power loss), through the 5/95 splitter
(5% power loss) where the local oscillator beam is brought
in, through the focusing lens to the detector.

The local oscillator path is from the laser through the
95/5 splitter (95% power loss), off three folding mirrors,
reflected off of the 5/95 splitter (95% power loss) where
it is combined with the return signal, through the focusing
lemomeo tie detector.

In the transmit path approximately 50% of the power into
the A-O modulator is absorbed by the modulator and approxi-
mately 45% of the output is unmodulated and blocked. Of the
remaining transmit power 50% is lost in the 50/50 splitter
and another 5% is lost in the 5/95 ae Using these
numbers, if 1 watt is transmitted from the laser approximately
124 mw is transmitted. In the LO path of 50 mw through the

95/5 splitter approximately 2.5 mw is focused on the detector.

The return signal is reduced to 45% of the original power by
the splitters and to .707 of that by the aperture blockage
mentioned in the previous section, for a net return on the
detector of approximately 32% of the incoming power. In
this manner the signal power out of the A-O modulator is
reduced by 3.2 db in the transmit path and by 4.9 db in the

return path for a net ideal optics loss of 8.1 db.

C. SECOND OPTICAL SYSTEM

The optical configuration of the second system utilized
is shown in Figure 24. The important difference is the
utilization of a Newtonian lens system as a transmit/receive
antenna and using an unfocused local oscillator beam. The
transmission, receive and local oscillator paths are shown
in Figure 24. For the same 1 watt output from the laser
approximately 235 mw is transmitted for a transmit gain of
2.8 db over system one.

The receive aperture of the second system is the 6 inch
parabolic mirror of the Newtonian optics. The aperture
area of the 6 inch mirror is nine times that of the 2 inch
mirror less the elliptical mirror blockage of approximately
7% for a net 8.37 fold increase which is a 9.2 db gain due
to aperture size alone. The loss due to one 5/95 splitter,
the 50/50 splitter and due to the elliptical presentation
of the angled splitters is eliminated for another 4.7 db
gain. This nets a total gain in signal power of 16.7 db.

The utilization of an unfocused local oscillator beam

instead of a focused beam greatly reduces the difficulty in

(al

aligning the return signal and LO beam. It also provides a
safety factor in the event the LO power should increase ay
without operator knowledge because only a small portion of
the increased power would fall on the detector. The adverse
effect of using an unfocused LO beam is that it becomes much
more difficult to provide enough local oscillator power in-
cident upon the detector to achieve shot noise limited
Operation. Shot noise limited operation was not achieved in

any of the three configurations.

D. THIRD OPTICAL SYSTEM

The third and final configuration is shown in Figure 25.
The differences between system two and three are the replace-
ment of the 90/10 splitter at the output of the laser by a
mirror and the unmodulated output of the modulator is utilized
as the LO beam. This provides a 10% increase in output power
(.46 db) and a total LO power available of 510 mw as compared

to approximately 230 mw.

E. ALIGNMENT TECHNIQUES

The alignment of the optical system is extremely critical.
Because of the strict requirements a detailed description of
the alignment procedure is presented.

1. Height Adjustments

All beams must move in a path parallel to the plane

of the optical table to provide optimum alignment. To
achieve this beam, height is measured at the output of an
optical device and measured again at a distance as far away

Smee taple as possible. If the heights are not the same

shims are introduced to adjust height or axis adjustments
are made. As an example, the height of the laser beam at
the laser output was measured. The first beam splitter was
then adjusted so the beam was the same height at the oppo-
site end of the table. The horizontal axis was adjusted so
the beam was changed in direction by 90°.
2. Acousto-Optic Modulator Alignment

The CO, beam is focused into the germanium crystal
in the modulator through a germanium focusing lens with a
five inch focal length. The height of the lens was adjusted
by shims until the CO. beam passed through the lens center.
The modulator was mounted on a holder which would be adjusted
with a micrometer setting so the angle of modulator axis
could be adjusted with respect to the plane of the optical
table. The modulator and holder were both mounted to one
of the holders which are adjustable in three axes. The
modulator was then positioned so the input beam passed
through the center of the input aperture at a distance of
five inches from the modulator center to the focusing lens
so the beam waist would be in the center of the crystal.
By using a level, the modulator was rotated until its axis
was approximately two degrees from the vertical and then
Slowly adjusted until the output beam split into two beams.
The positioning of the modulator was adjusted for maximum
power in the modulated beam.

The recollimation lens was positioned in the two
PUeptteabetiis tive inches from the modulator center at a

height where the two beams passed through near the lens

center. The recollimation lens serves a dual purpose in
that it prevents a further divergence of the beams after
their focal point and it collimates the beams so the modu-
lated beam (which was deflected down at twice the Bragg
angle) travels parallel to the plane of the table.
3. Collimation of the HeNe and co, Transmit Beams :

A visible beam from a HeNe laser was transmitted
Wien) the co, beam to provide a visual indication of the lo-
cation of the CO, beam. The HeNe beam entered the transmit
path by reflecting off of the 5/95 beam splitter just prior
to transmission from the system. Since the germanium splitter
is opaque to visible light virtually all of the HeNe beam is
transmitted.

For the HeNe beam to be collimated exactly with the
co, beam it must be positioned on the face of the 5/95
splitter at the exact position that the CO, beam passes
through the splitter. The superpositioning can be deter-
mined by pressing a piece of liquid crystal paper on the
face of the splitter. The intensity of the co, beans eee =
duced by passing it through an aperture prior to the modu-
lator. Care must be taken to position the aperture so the
beam goes through the exact center so that the HeNe is
positioned on the high intensity center of the beam. The
HeNe beam can be adjusted in height by placing shims under
the laser and horizontal adjustment is accomplished by ro-
timemeesthe laser slightly in the holder.

The HeNe transmit beam can be adjusted with the

Betomaajustments on the. holder in which the splitter is

mounted. This adjustment does not effect the direction of
the CO, beam transmission. A piece of heat sensitive graph
paper was placed in the co, beam approximately ten feet
from the output, as soon as the graph paper began to darken
the CO, beam was blocked and the HeNe beam was positioned
on the small darkened spot. In this manner the HeNe beam
can be positioned in the highest intensity portion of the
CO, beam. The alignment was checked by passing both beams
through an aperture onto a piece of liquid crystal paper
located about forty feet from the output and about thirty
feet from the aperture. If the beams are not collimated
they will not be coincident at both the aperture and liquid
crystal paper. Again care must be taken to position the
aperture so the beams pass through its exact center or the
beams on the liquid crystal paper may appear to be slightly
misaligned when they are not. This appearance can be
caused by passing parts of the co. beam with a significant
difference in power density. The difference in intensity
will show on the liquid crystal and may give an appearance
of misalignment.

A final alignment check was made approximately 305
yards from the output, on the roof of Ingersoll Hall. The
dimension and location of the co, beam was determined by
chopping the transmit beam and reading the chopped output
with a PbSnTe detector whose output was amplified 40 db and
displayed on an oscilloscope. In the final alignment ad-

justment at this range the HeNe beam was lowered two inches

and moved horizontally one half inch. This movement was

less than the divergence of either beam. fee =

4. Collimation of Visual Optics with the CO, Transmit

Beam

A visual sighting capability was incorporated into
the system to enable placement of the transmit beam on a
desired position. This is accomplished by utilizing a
rifle sighting scope and a mirror with a hole in its center.
The visual system is shown in Figures 23, 24 and 25.

The sighting mirror is positioned so the beam from
the HeNe laser passes through the hole in its center. The
rifle scope is then positioned so it looks onto the surface
of the sighting mirror. The mirror is rotated until the
rifle scope is sighting along the HeNe path. [Initial align-
ment is facilitated by placing a mirror in the HeNe trans-
mission path and adjusting it so the HeNe beam is folded
back onto itself. The sighting mirror is adjusted properly
when the HeNe beam reflects back through the rifle scope.
Visual alignment is completed by adjusting the scope cross
figs Oneene brighe return trom a retro-retlector placed in
tiesneNe = beameon the root of Ingersoll] Hall. Once alignment
1s completed the transmit beam can be positioned by sighting
through the rifle scope and adjusting the optical table
Mie eicmeLOsso-hairs dre positioned on the desired location.

5. Alignment of the Newtonian Optics

Alignment of the Newtonian transmit/receive optics
Mer mecnitical Since the theorctical field of view is only
127 prad for an eight mil diameter detector (measured field

of view was 180 urad). Once the receive optics are aligned

with the transmit beam the detector is positioned at the
focal point of the lens system. If the target is then moved
out of the field of view but is still in the beam (which is
much larger than the receive field of view) the focused

one moves off of the detector. At this point if the ‘lee
tector is repositioned in the received focused beam the |
Signal can be processed as before. This demonstrates that
there is a direct interaction between receive alignment and
detector size and positioning.

By removing the 5/95 beam splitter which is positioned
between the folding elliptical mirror and the detector (Figure
25) and placing a folding mirror in front of the detector the
view of the receive optics can be determined. Looking back
imeo the receive optics the blockage of the elliptical flat
can be seen. When the HeNe beam is positioned on a retro-
reflector located on the roof of Ingersoll Hall the red re-
flection can be faintly seen through the blockage image.

This reflection is positioned in the center of the blockage
by adjusting positioning screws on the back of the six inch
parabolic’ mirror. The positioning can also be checked by
moving horizontally and vertically and adjusting the para-
bolic mirror until the reflection bisects the blockage both
vertically and horizontally.

Once the receive and transmit optics are aligned
the return visible energy from the HeNe laser can be located
by placing a white paper at the focal length of the optics
(60 inches from the parabolic mirror). If the paper is

Reneemsiightly forward of the focal point the pattern of

the returned HeNe beam with its dark spot in the center
(caused by the elliptical mirror blockage) can be seen.
If the receive optics are properly aligned at this point
the received pattern will be distorted by moving a paper
in from the sides and top and bottom in front of the para-
bolic mirror. If the received pattern distorts as soon as
the paper moves in front of the mirror from all sides then
the Newtonian optics are aligned properly.

6. Signal and Local Oscillator Beam Alignment

The signal beam and local oscillator beam phase
fronts must be aligned upon the detector surface nearly
exactly. The local oscillator beam is unfocused to facili-
tate this alignment. While one beam was being aligned the
other beam was blocked and vice versa.

With the 5/95 beam splitter removed as in the pre-
vious section, the HeNe return can be seen and the detector
levwerwcane pe Positioned So thesdetector is very near the qwe-
turn signal. The CO, beam was then chopped by a fan and
the detector position was varied in a raster type scan until
the chopped output, amplified by 40 db, was seen on the
oscilloscope. The 5/95 beam splitter was then repositioned
ietcneesienal and local oscillator path (oriented at approxi-
mately 45° with respect to each path). The beam splitter
shifts the signal beam slightly and the detector was repo-
Sitioned in the pears After the detector was again posi-
tioned in the signal beam for maximum chopped signal
amplitude the local oscillator beam was unblocked and the

eralsmuc beam was blocked. The local oscillator beam was

Swept across the dewar aperture face in a raster pattern
until the chopped output of the detector was maximized.
After the detector was properly positioned and the rest of
the system was aligned, the alignment is maintained and is ,
correct as the table is repositioned to illuminate different

targets.

F, ELECTRONICS AND SIGNAL PROCESSING

The block diagram of the signal processing system is
shown in Figure 26. The system will be first analyzed con-
Sidering a constant frequency IF of 30 MHz.

The driver for the acousto-optic modulator consists of
a voltage controlled oscillator and a power amplifier. The
output is coupled to the modulator with a tri-axial lead
which minimizes RFI. Some of the output power was reflected
back to the input terminal at a greatly reduced level. This
reflected power provided a convenient means of obtaining the
instantaneous transmitted frequency for comparison with the
instantaneous received frequency for determination of the
iomsomindevel@city Lrequcney.. Ihe reflected reference sig-
nal is amplitude modulated by the input modulating signal
which is a DC bias with a superimposed triangular modulating
voltage. The reflected reference signal is passed through
a high pass filter which eliminated the 1.75 kHz modulating
wave form.

The transmit signal was connected to the R terminal of
a balanced mixer. A 30 MHz reference signal of 1 volt am-

Pivedom this amplitude was very critical for proper

Acousto-Optic

ye Detector Modulator Driver

35-45 MHz
Plus Range and Doppler

Information
= Bias Amplifier 23 db — |
35-45 MHz
FM

\ Jan lifier 30 db C2
2 Signal $0 Miz UX )Balanced
Generator SS Mixer

Fiiter

y Amplifier 60 db

eat Balanced Mixer
OY —_e O5- 7 S@MHE

30 MHz Plus Range and Calibrated oW&Pt
Doppler Information Variable

Attenuator
= tCryertal Filter

Veresmor

Oscilloscope

eine s26, ooipnal Processing System.

operation of the balanced mixer) was connected to the L
terminal. Both the sum and difference frequencies are pre-
sent at the output of the mixer. This output was passed
through a third order Butterworth filter with a bandpass

of 60 - 80 MHz.

The 65 - 75 MHz signal was amplified 60 db and then at-
tenuated 8 db (to obtain the one volt necessary for the
balanced mixer) and connected to the L input of the second
balanced mixer.

The output of the PbSnTe Detector is a 35 - 45 MHz signal
offset by any doppler shift which may be present. This sig-
nal is amplified by a 23 db bias amplifier with a 3 db noise
figure and then by a 30 db amplifier. This signal is then
connected to the K input of the second balanced mixer where
Heeis mixed with the 65 - 75 MHz reference signal. The out-

put of the second mixer is 30 MHz + F and

range i Faoppler

a 100 - 120 MHz spread. The range information is present

at the output as a signal at 30 MHz + F and 30 MHz -
range

Frange: The double frequency spikes are present because of
the triangular frequency modulation.

The output frequencies were observed in a spectrum
analyzer. If the 30 MHz output of the signal generator into
the first balanced mixer is varied the output range frequencies
are varied in the same way. In this way the range frequencies
were swept across a 3.5 kHz crystal filter centered at 30
MHz. As the range frequencies were swept across the filter

bandwidth the signals were connected to the vertical input

of an oscilloscope. The input to the horizontal grids of

the oscilloscope was the sweep output from the sweeping
Signal generator and in this way the horizontal sweep was
calibrated to the frequency sweeping of the return signal.
The resultant output of this processing was an A scope

range presentation on the oscilloscope. The A scope pre-
sentation was not very satisfactory because of the frequency
output of the signal generator drifted making a range dis-
placement calibration impossible. Other than a demonstra-
tion of its feasibility the A scope presentation was not
utilized. The range and velocity information was determined

directly from the spectrum analyzer.

G. SYSTEM RADAR ANALYSIS
The theoretical development for a FM-CW radar is pre-
sented in Chapter II section A. The specific system parame-

ters are as follows:

Carrier frequency Pee eeosexe 10 Hz
Wavelength | X = 10.6 um
Resolution filter bandwidth BW = 3.5 kHz centered at

30 MHz
Triangular wave modulation |e
frequency FM = 1.75 Hz —
Total frequency deviation ZAF = 10 MHz centered at

40 MHz
These parameters yield the following system performance
capabilities

df/dt Sooo’ Hz/sec
(equation 3)

Unambiguous range 42.9 km
(equation 19)

Range frequency Pye kiiz/ km
(equation 5) Pisesenz/ 100 yds

Velocity frequency
(equation 13)

Range resolution
(equation 21)

Velocity resolution
(equation 22)

Acceleration spectrum spread

tolerance
(equation 17)

$2.4 kHz/km/hr
97 kHz/knot
15 m

.036 knots

6.6 g

V. EXPERIMENTAL PROCEDURES AND RESULTS

A. DETERMINATION OF RANGE

For an initial determination of the practicability of
determining range by the expected range frequency signals
the first system (Figure 23) was used. The output power
was reduced and attenuated until only a few milliwatts were
transmitted. The output beam was folded around the room
and out along the top of the sixth floor of Spannagel Hall.
A front-surfaced mirror was placed at several known dis-
tances and the range frequency was determined and compared
with the expected frequency shift. It was during this op-
eration that the offset from zero range was first noticed
fomprained ian Chapter IIIf, Section BB). Table V contains
Ciemsesults Of this Series of experiments. It should be
noted that the results agree well within the accuracy of
the frequency readings. -Figure 27 is a photograph of the
first system configuration.

An unexpected phenomenon was the appearance of a zero
range beat frequency. This is believed to be caused by
a reflection of some of the modulated beam from the output
ficemotetne Modulator crystal back into the laser. Once
in the laser, the modulated frequency is amplified and
transmitted along with the normal unmodulated frequency and
in this manner it is introduced into the local oscillator
Heat tewas determined to be in the local oscillator path

because it remained even when the output beam was blocked.

Table V.

Range

Range.
Triangular Modulation i = 10 kHz
2AF = 10 MHz df/dt =

Range Frequency Shift =

Spectrum Analyzer Dispersion

‘|

Range Frequency Determination as a Function of

43 ft

tts

toate

ft:

fits

11 ft

Expected Range Frequency
Actual Range Frequency
Frequency Offset
Corresponding Offset Time

Expected Range Frequency
Actual Range Freqmency
Frequency Offset
Corresponding Offset Time

Expected RansemPrequeney
Actual Range Frequency
Frequency Offset
Corresponding Offset Time

Expeeted Rance requency
Actual Range Frequency
Frequency Offset
Corresponding Offset Time

erie
Delay
Delay
Delay
Delay
Delay
Delay

Delay

ZX 0 ie sec
e222 a
S50 kHz/cm

a7 Smile
345 kHz

SZ 7/7 Sone
1.64 usec

EPA N37
360 ez
Beit Ja v4

1.66 usec

37 kHz
SOE eknZ
330 kHz
12o5-usec

46.8 kHz
375 kHz
528 eZ. kz

1.64 wsec

46.
387.
341.

kHz
Kola
kHz
usec

“IDO 61 WG

eons kHz
395 kHz
Sater, (KHZ
1.64 usec
76 kHz
405 KeElz,
329 kHz
1.64 usec

85.8 kHz
415 kil z
S2omee kHz

bo 4 psec

Peo cme? 7 .

FITS: Opewea) Svs pen.

Figure 28... Thad GpibdaGd be SySieeM 5

The zero range frequency is believed to be due to internal
laser amplification rather than a reflection from the output
mirror because of the marked variation of the amplitude of
the signal at different times. The signal on the LO path
was first noticed at NELC during the experience tour in San
Diego. Prior to bringing the laser to NPS it was recharged
and for the first couple of weeks of operation at NPS the
signal was not present on the LO beam. At other times the
signal was not present when the laser was first turned on
but became present after the laser had warmed up. The sig-
nal strength fluctuated from week to week and for the last
several. weeks was significantly lower than at other times.
All of these variations can logically be reasoned to be due
to the effects of pressure and temperature line broadening
and due to the variations in line width of various laser

transmission lines.

B. TARGET DETERMINATION AT 305 YARDS

The next step in yeten development was to determine the
system capability at a greater distance. For this work, the
system configuration of the second system (Figure 24) was
initially used and finawly thaeson themenird system (Papure
25). Figures 28 and 29 are photographs of the third systen.
Figure 30 shows the view from Spannagel 704 to the top of
Ingersoll Hall and Figure 31 shows the NPS grounds.

During this and subsequent experimental stages the target

was a two-inch diameter, gold surfaced retro-reflector.

The retro-reflector is shown mounted on a model railroad car

migure 29.

ne tonite

Prrntaeeny

Third Optical System.

Figure 30. V
ie

1ew from Spanagall Hall
Celivcisorl Hal] .

a Ingersoll Hall

305 Yards

Spanagel Hall

Figure 31. Naval Postgraduate Grounds, Near Range Target.

in the photographs of Figures 32 and 33. The retro-reflec-
tor is diffraction limited at 10.6 pm and the reflected
energy from it has the same divergence as the incident
beam.

The modulator was readjusted between the experiments of
Section A and determining the target on Ingersoll. At the
new height position, the time delay was approximately 1 usec
which for the 1.75 kHz modulation rate corresponds at a
35 kHz frequency offset. Figure 34 shows the zero range
frequency signal. The small spikes offset about 35 kHz
Peewmeevecmecniter in Figure 36 and are the zero range fre-
quencies along with the signal from the retro-reflector.
In both photographs the spectrum analyzer dispersion was
50 kHz/cm.

Peencelnleotenal from Retro-Reflector at 305 Yards

The system was aimed at the retro-reflector on

Ingersoll Hall and optimized for maximum signal. Figure 35
shows the unprocessed return out of the detector and Figures
36 and 37 show the processed return. The following condi-

tions existed:

Transmit PrReaqweney Seer oe Miz
Modulating Rate ema Z
Target Range 305 yards

Analyzer Dispersion and Noise Bandwidth

SOM eerentgny ‘ ee NAS Cae ae
Sana ene Nn” AS AEM nA AIS A SON AR ROO QOON GREN 200% SORE OOO..-

Velocity Generating Model
Railroad, View 1.

Figure Pe

Figuretiim Veloeeity™Generating Model
Raviyoade View 2.

Peep ememoen 50 KHz/cm

BeegUemeyeOtfset ~ 35 kHz

oo” ae

rigure 34, Zero Range Frequencies.

Dispersion 2 MHz/cm
B/N 50 db

hotal
opectrum 10 MHz

Figure 35. Unprocessed Detector

Cire ror 3505 Yard
Target.

Dbrspersion SO kHz/cm

ier separation ZU 7 Cm
Pere Range Frequencies LoD TZ
S505 Yard Range Frequencies On: kota
Range Frequencies = OS ern Zz
Pepee red Prequencies te) 5 Serer Z

PHeure 56. Processed Return for 305

O55

Warde ikaireeet .

The following are the resultant data:

Total Spectrum S/N SS db
Range Frequency S/N 60 db
Zero Range Frequency +35 kHz
Time Offset i wse

Range Frequency (including zero range) +100 kHz

Actual Range Frequency 65 kHz
Expected Range Frequency 65.1 kHz
Transmitted Power .65 w
LO Unfocused Power .ol w
Chopped Signal Output oon nly
Chopped LO Output 1.4 mv
Revurned Signal Power .75 mw

(Prior to the 5/95 splitter)
The range frequency results agreed almost exactly with those
expected. The noise bandwidth of the processed signal was
16 db narrower than the total spectrum bandwidth. There
was a 5 db S/N gain and there was a 6 db insertion loss at
theganput of the balanced mixer. The other S$ db could be
accounted for by noise addition of the mixer.
7a i ranmemit Divergence Determination

With both the transmitted and returned power known
ald the target range known, the beam divergence can be
calculated. The returned power was readily measured because
PNemmeNe focused return was easily seen. The returned power
was measured with the low-range (<200 mw) digital power
meter while the transmit power was measured with the high-

mameemneter. IThis 18 a possible source of error but all

calculations are based upon these readings being correct.
The divergence calculations are made considering the reflect-
ing and receive aperture areas as compared to the total
area of an arc subtended by a beam of a known divergence
at the known range. Since everything is known but the di-
vergence, it can readily be determined.

The arc length of a beam of a given divergence at

a given range is given by

d = RO (65)

and its area is
d R@
m(>)° = t(> a (66)

The area of the retro-reflector compared to this area is

(>)?
R

: C675)
T ( = ye

The divergence from the return from the retro-re-
flector is the same as the incident beam. This results in
an illuminated area at the receive aperture the same size
as that at the target. The ratio of the received power to

returned power is thus

on,

ee
Se (68)

7 (=)?

The net return power is thus given by

D
2 aoe eee 2 2
ie 2 y" f 2 ) r Pe dD. Dr. (69
—, oko ., a
( > )
In this situation the return power is known and thus the

divergence can be determined from

2 2 i
9 = (r! eee OY
Ie R4
ae
(70)
Oy
65 w Danie ;
2 = \ 75 mw ° G05 yd-36 in/yd)* = en poses

3. Field of View Measurement

The field of view of the receive optics was measured
by placing the retro-reflector in the beam in a position
where the return signal was maximum. The reflector was then
moved a known amount and the signal strength was noted.
The beam width was defined as the 3 db points of the signal
decrease. The field of view was determined using equation
(65))..

The 3 db beam width was determined to be nearly
symmetrical at 5 cm. At a range of 305 yards this gives
a field of view of 179 wradian. This compares reasonably
well to the theoretical far-field field of view of 127
uradian.

There are two effects not considered in this compu-
tation. The first is that the target range is not in the
Pareerolimor the optics {305 yards vice 1199 yards). The

effect of this is an enlargement of the Airy disc diameter

Warew reawces the theoretical field of vrew. Offsetting
this is the effect of the finite reflector size (2 inches
vice a point). The separation was measured from the reflec-
tor center so the reflector extended on both sides of this
point. Since the beam intensity distribution should be
nearly Gaussian this would tend to increase the apparent

beam width.

i exmeeted Return fron aeUrenuse REtlSctor aimee s Taras

The return from a diffuse target would greatly de-
crease the return signal received at the receive aperture.
If one assumes an ideal Lambertian return [14] from a dif-
fuse reflector, the area illuminated is that subtended by a
Somrpa dilevemo. n radians. Themresttane ToLlectedscne poy

would then be reflected over an area ot
A = 1R? (71)

instead of

A= (52 )?. (72)

The difference in area and thus power density is the square
of the beam divergence. For a divergence of 1.71 mrad,
Civseis a 61.5 db reduction of signal power. With a pro-
cessed S/N of 60 db, it might be possible to see the return
fommemeaideal diffuse reflector larger than the retro-reflector.
The target that was utilized for the diffuse target

consisted of two 1-1/2" x 6" pieces of sandblasted aluminum
placed side by side. The target had been exposed to air for

several months so there was undoubetdly some surface

Of,

oxidation. Additionally, the assumption of a perfectly
diffuse reflector was an approximation. The larger area of
the diffuse reflector provided a gain in reflecting area at
the target of approximately 7.6 db. The surface of the tar-
get caused attenuation of the incident power and a 6 db
reflection loss as compared to an ideal Lambertian is not
unreasonable. Using these numbers, the expected return of
the diffuse reflector would be .3 db. No return signal was
detected from the diffuse reflector even though the target
was varied widely in angular presentation.
Se cCawenlation of Detecrom Glamcumerrt tclency

The quantum efficiency of a detector is that propor-
tion of incident photons which are converted to a utilizable
Signal as compared to the total incident flux. The quantum
efficiency of a photodiode can be calculated from the
relationship [15]

hfov
nN = qPR_ (73)

Where 1 Ismuencequiantum effresency , f Pomtie treaqmency Of
thewinerdent photon, v is the detector output voltage, q is
the electronic charge, P is the incident power and R is the
zemo bias detector resistance. The ingident power is the
pOwen ine@idemt upon theadetector. The Airy disc for a far
field scarget is not in the far-field, the actual Airy disc
1s somewhat larger. The detector size is 200 um so it is
smaller than the Airy disc. The energy density profile of
the Airy disc is shown in Figure 5.5 of [17]. Since the

Gemeeeor size is less than 744 of the Airy disc diameter,

it 1s estimated that approximately 80% of the power in the
Airy disc is incident upon the detector. The power contained
in the Airy disc is approximately 84% of the total focused
power [17]. The dewar window transmittance is estimated at
.8 which is reasonable for a good window. The total power
incident at the dewar (after passing through the 5/95 beam
splitter) was .7 mw. When all of the aforementioned effects
are considered, the estimated power incident on the detector
is .38 mw. Thus the quantum efficiency is given by

-34 1 -3
ae (6.63 x 10 JiC2. 8 See Owe Goreme lO =) 034.

(1.6 x 10°°°)(3.8 x 10°)

As mentioned in Chapter III, Section C, this value is sur-
prisingly low and tends to indicate that the detector has
degraded. There are several possible sources of error in
this calculation but the result is still nearly one order
of magnitude below the expected value of .14.

6. Calculation of Noise Equivalent Power

Noise equivalent power id defined as the amount of

power necessary for a signal-to-noise ratio of one with a
unity noise bandwidth. This can be computed if the signal
power, noise bandwidth and S/N are known. Using measured
values and the same estimations utilized in the previous

section, the computation is as follows:

Signal Power = ,38 mw
S/N = 60 db
Noise bandwidth = 50 kHz

~&
7 B _ Seioeee LO. -15
(INS Toss ioe | 8 Mele

From equation (52), it can be seen that for shot noise limited
operation
hf B

ee - (74)

For unity bandwidth and quantum efficiency

NEP = hf, = (6.63x10°°")(2.83x10.) = 1.88x10°*°w

ideal

For a noise bandwidth of 50 kHz, this becomes 9.4 x Ones

and for n= .034.

NEP = 2.76 x 10 °° w.

This value is iarger than the computed value which indicates
that either the noise bandwidth is incorrect, S/N is incor-
rect Or the cGemputed value of m is incomrect. The noise
bandwidth is determined by the dispersion of the spectrum
analyzer and the S/N ratio 60 db has been repeated several
times. These factors indicate that the computed value nis
Conversion efficiency is incorrect. For shot limited opera-
Cionmee ths would correspond to a value for yn of .12 which

is near the tabulated value of approximately .32 (for a
rewemeee Dias Of .17 Vv). “Sinee shot mwoise limited operation
was not obtained, this value is reasonable. The readings
Utilazed for the computation of n in Section IV were repro-

duced several times also.

100

C. EXTENDED RANGE CAPABILITY

After successfully detecting the retro-reflector at
305 yards, an extension of target range was completed.
There was a clear area at the edge of El Estero Lake at
an unknown range. The retro-reflector was taken to this
area and illuminated. The resultant signal had a S/N ratio
of 35 db and a frequency shift of 260 kHz including the
zero range shift of 35 kHz. The net frequency shift of
225 kHz corresponds to a range of 1055 yards. The range
of 1055 yards is- approximately 3-1/2 times as far as 305
yards. Since the signal strength decreases as a. this
should give a corresponding reduction in the S/N ratio of
21.5 db. The total range (both directions) is approximately
2 km so this would provide an additional attenuation of
approximately 2 db. Thus the expected S/N ratio was 36.5
db which agrees very closely to the experimental value of
35 db.

The next step in range was to illuminate the retro-
reflector while it was on the Coast Guard pier at a range
of approximately 2400 yards (Figure 37). This is approxi-
mately eight times the range of Ingersoll Hall and again
Gonoctmderine the Rua Signal relattonsiap, this should cause
pemeeletr1on of the S/N ratio of 36.1] db. The total range
is slightly more than 4 km which would give an expected
reduction in signal strength of 4 db due to attenuation.
These considerations indicate an expected signal-to-noise

ratio of 20 db and a frequency shirt (including the zero

rot

= =

«gj——"""—" Coast Guard Pier
i

Be ~Spanagel Hall
a

Mmeeure 3/7. rar Range Target Measurement.

102

range shift) of 547 kHz. The experimentally determined
values were:

S/N 20 db

Frequency Shift 540 kHz

This frequency shift indicates an actual range of 2370
yards. Figures 38 and 39 show the signal from the Coast

Guard pier.

D. VELOCITY DETERMINATION

If a target with relative velocity is illuminated by
the radar, the return should be shifted in frequency by
the amount of 188.7 kHz/msec ° relative VietTOG1 Cy edt nemene
Sign being the same as that of the relative velocity. To
facilitate velocity measurements, a model train was set-
mpeom the roof of Ingemeollecostiat ethos bcanson lyin
inated one side of the oval track. A meter stick was po-
Sitioned along a straight portion of the track and an
electric timer was uSsecmeue measure the amount of time the
reflector was alongside the meter stick. Figures 32 and
$5 show the experlmenigalsee- Wperor seneomic a5ut ements oome
problems were experlene@e@ wrewme-mellect1 ic time, sveren
and the train was not stable enough to obtain the desired
Meleeriy range. Addieronallys there anesinherent errors
which are unavoidable when manually timing. Doppler sii iliBee
of both senses were obtained by reversing the direction of
ticemedele train. Due to a difference in elevation there
was a 5.5° angle between the velocity motion and the radar

Peemempecause Of this, the measured velocity must be

103

bispersion

/Range

IS /N

Hero Range

Range
Frequencies

Range Return

Pugmre 5s. Pwpcessed Return fox emo
Yard Target, View 1

ROAR

Dispersion 200 kHz / cmil

Range 2370 yand 4

S/N 20 db

Zero Range Zoo KHz ‘ SR es
Range = _ eS ‘
Prequcneies £505 kHz heat Oe ee Ore serer|
Range

Return +540 kHz View 2.

104

Return for
7 Pear hay oe tee

200 kHz/cm
coy ear.
20 db

cee ee

wou KAZ
2S Ure

multiplied by cos 5.5° to obtain the velocity component
relative to the radar. Four photographs are included of
the doppler shift measurements; these are Figures 40, 41,
42 and 43. Table VI lists the results of the velocity
measurements. The measured values were well within the
tolerances of measurement error.

A totally unexpected phenomenon was noted during the
velocity measurements. In addition to the expected doppler
frequency shift, the range frequencies with no doppler
shift were present at a much reduced signal strength. There
was no return from the tracks until the train with the re-
flector moved into the beam. All parts of the train were,
of course, moving at the same speed. Later the reflector
was hand held and moved and again this phenomenon was ob-
served. This zero velocity component should not have been
present since any return reflection should have been doppler

shifted.

105

Dispersion 50 kHz/cm

Figure 40. Processed Return for Negative
Peeterve Velocity... Warew, ie

el Rie AAA mat %
A iat Wiel Os

Dispersion 50 kHz/cm

Figure 41. Processed Return for Negative

Refative Velocity, View 2.

106

Drsipers vom 50 kHz/cm

Pucures 4266 Processed Return for Positive
hemimive VelLOCl ty.) yhew is

.
See

Viswemsram 50 kHz /cm

FOUL C4 eer Geos camne turn” [Oreos i time
fo bag mee LOCItLY,. View Z.

Figure 40 Down Doppler Shift

1/v = 2.7 sec/m
V = .37 m/sec
Expected Frequency Shift 69.5 kHz
Actual Frequency Shift = 80 kHz
Figure 41 Down Doppler Shift
1/v = 1.8 sec/m
V = .56 m/sec
Expected Frequency Shift 104 kHz
Actual Frequency Shift = 110 kHz
Figure 42 Up Doppler Shift
1/v =. See7 im
Vv ~ ,63 m/sec
PxXpectcuerrequency SiiLt Ty EZ
Actual Frequency Shift ~ 120 kHz
Figure 43 Up Doppler Shift
1/v 25127 sec/m
V = ,59 m/sec
PePcetoume Toqucncy oi tt 110 kHz
= 115 kHz

Actual Frequency Shift

Table VI. Velocity Determination Results.

108

VI. CONCLUSIONS AND RECOMMENDAT IONS

A. CONCLUSIONS

The experimental results obtained closely agreed with
the theoretically expected results and prove the basic
feasibility of constructing a EM-CW optical radar and
frequency modulation and coherent detection of a laser beam.
The capabilities of the developmental system were somewhat
disappointing but drastic improvements could be obtained in
Signal processing (reduce the noise bandwidth), increased
transmission power, shot noise limited detector operation

and the reduction of noise inserted in the filtering and

The alignment requirements of this type of system are
extreme and would present problems in both fabrication and
maintenance. Additionally, actual systems would have to
be rigidly constructed ane either environmentally controlled
or fabricated with neteeiaaee having nearly the same coeffi-

cient of thermal expansion.

B. RECOMMENDATIONS FOR FURTHER STUDY

There are several areas which warrant further study.
One area would be to determine the exact mechanism causing
the zero range frequency being imposed upon the local
oscillator beam and determine the variation of this with
G@eererent laser variables such as temperature and pressure

boeeweenang and different line operation.

109

Another and more intriguing question is that of the
occurrence of the zero velocity frequency components in
the velocity measurement part of the system investigation.
Since all of the returned energy should have been doppler
shifted in frequency, this presents a perplexing question.

It would also be worthwhile to extend the system
development to determine the true limit of its capabilities.
Initially this would require improvements in signal process-
ing and perhaps eventually in the area of controls to
develop steering and tracking capability.

Since a system operating at optical frequencies has
such large doppler shifts for relatively low relative
velocities, it may be feasible to determine the vibrational
Signatures of many objects (such as a jet engine of an air-

craft) and utilize this in a target recognition scheme.

eee Oool BEE SYSTEM APPLICATIONS

There are several possible applications for an optical
radar. At the start of this project, it was hoped that
Gleareatr turbulence could be determined and that a light-
weight low-power system could be developed for utilization
Perinetwamin there 1s still a possibitity of this applica-
tTomimbeimeerealized if system development 1s continued,

Another possible application is signature recognition
i-eememonea in the previous section, Tt is quite conceivable
Piewmomeraddatrewhich could deétece, provide range and velocity
Mirinmatton and target identification could be developed

along similar lines as this developmental system.

110

Another application which is both very desirable and
conceivable would be in low flying, high closing speed
target detection. The small beam size would allow detection
of near the horizon targets with extremely fine bearing
resolution capability. If a dual mode transmission capa-
bility were incorporated, a constant frequency beam could
be transmitted and instant velocity determination of any
return could be accomplished. If a doppler shift threshold
were set (say for 400 knots relative speed) a threat
warning system would be inherent. Upon recognition of a
threat (and simultaneous velocity determination) the FM
mode could be activated and range information provided.

The same characteristic which allows near the horizon
search and high bearing resolution (small beam size) pre-
vents the rapid scanning of a large volume of space. Thus
any system for this threat recognition application would
require numerous radars with each scanning a relatively
small sector. With the advent of signal storage capability
and digital processing and computer control, a system Se

as this may become feasible in the relatively near future.

Jill

APPENDIX A: EQUIPMENT LIST

Crystal Filter

Mfg. Damon Model 6647A
f, = 30 MHz BW = 3.5 kHz

Optical Table

Mfg. UNIDEK
Stainless steel - honeycomb

CO, Laser

Mfg. Honeywell
Power 3 watt - vertically polarized

Amplifier (2)

Mfg. Hewlett Packard Model 461A

Gain 20/40 db Wideband
Oscilloscope

Mts. Jektronix Model 546

Signal Generator

Mfg. Wavetek Model 134
Variable Frequency Variable waveform

Signal Generator
Mfg. Hewlett Packard Model 606A

Signal Generator

Mfg. Wavetek Model 1001

Frequency sweeping
Ro Owe ew

Mice NELC | Bias -amp

Game 25 db NF - 3 db Wideband
Amplifier

Mepe Miteo Model Au-1A

Gain 30 db ieee db )=62- 100 Miz

Power Meter
Mfg. Coherent Radiation
Power Meter

Mfg. Jodon
< 200 mw

Spectrum Analyzer
Mfg. Tektronix
Power Supply (3)

Mfg. Hewlett Packard
DC

Frequency Counter
Mfg. CMC
Balanced Mixers (2)

Mfg. Relcom
ee ees MEH z

Acousto-Optic Modulator

Mfg. Isomet
f ~ 40 MHz

Modulator Driver

Mfg. Isomet
f - 40 MHz

Power 0 - Sw

Model

201

PM-550

491

6Z16A

904

6 db insertion loss

Model
BW -

Die ER 20

10 MHz

Model DE-IR-10/5
BW - 10 MHz

10;

ded:

14.

LIST OF REFERENCES

Skolnik, M. I., Radar Handbook, McGraw Hill, 1970.

Skolnik, M. I., Introduction to Radar Systems, McGraw

Hili, 1962. e —-

> pece / A) Cw nailer
Bonnelle, G. J., ''FM-CW Radar," Aerospace Electronics, _
p. 143-148, August 1960.

Hoisington, D. B., Multiple-Target CW FM Radar, paper
presented at Asilomar Conference on Circuits and
Systems, Sixth, Pacific Grove, California, November
Owe.

Reference Data for Radio Engineers, Sth ed., P. Howard
W. Sams 4 Co., New York, 1973.

Hymans, A. J., and Lait, J., "Analysis of a Frequency -
Modulated Continuous-Wave Ranging System,'' Proceed-
ings Pb. (British), p. 305-3/2, July 1960.

Richter, J. H., "High Resolution Tropospneric Radar

Sounding,'"' Radio Science, Vv. APeVOmeel2 gD oto E=
1268, December 1969.

Kay, L., "A Comparison Between Pulse and Frequency-

Modulation Echo-Ranging Systems," Journal British
fanee., p. 105-115, February oS

Yariv, A., Introduction to 0 tical Electronics, Pp.
305-333, Holt, Rinehart and Winston, 19/71.

Adler, Robert, "Interaction Between Light and Sound,”
IEEE Spectrum, p. 42-54, May 1967.

Gordon, E. I., "A Review of Acousto-Optical Deflection
and Modulation Devices," Proceedings REE cv ame saes
momo, p. 1391-1401, October e800.

Dixon, R. W. and Gordon, E. I., Acoustic Light Modu-
lators Using Optical Heterodyne Meecineg,” ihe wert
System Technical Journal, p. 367-389, February LOG

Fang-Shang Chen, "Modulators for Optical Communica-

tions,'' Proceedings IEEE, v. Seeeeno. 10, p. 1440-
1465, October 1970.

Pratt, W. K., Laser Communication Systems, Wiley, 1969.

114

15.

to".

Lee

19.

Z0 .

Zl.

Ze.

Zo.

24.
Zo.

ZO.

Dele:

Sze, S. M., Physics and Semiconductor Devices, p. 555-
683, Wiley, 1969.

Oliver, B. M., "Thermal and Quantum Noise," Proceedings
IEEE, p. 436-454, May 1965.

Hideciek., D.,, Jr., Infrared System Engineering, Wiley,
1969.

Peyton, B., DiNarda, A., Chiou, W., Lange, R., Arams,
F., Aita, M. and Pace, F., Coherent Infrared Re-
ceivers for Laser Communications and Radar, Inter-
nal paper of AIL, a division of Cutler-Hammer,
Melville, Long Island, New York, 1973.

Rockwell International Science Center, 10.6 Micron
Photodiodes, Miscellaneous Report, September 27,
EI] 3.

Wetange, O. E., “Optical Heterodyne Detection," JEEE
pueemum, Dp. //-85, October 1968.

Moers. W., “A 10.6 pm Optical Heterodyne Communi-

wteiesystem, Applmed Optics, v. 8, no. 3, Pp.
677-684, March 1969.

Isomet Corporation, Instruction Manual for 10.6 Micron

Acousto-Optic Modulation System, December 1973.

Hogarth, C. A., Materials Used in Semiconductor De-
vices, Interscience Publishers.

Ross, M., Laser Receivers, Wiley, 1967.

Pemday, D. and Resnick, R., Physics Parts I @ Il,
peeeril2, Wiley, 1966.

Reagemeo. and Fried, D). L., “Optical Heterodyning
with Noncritical Angular Alignment ,'' Proceedings
Mee, p. 1787, December 1963.

Paemrebaer, M. F., Jr., A Heterodyne Detection FM-CW
Laser Radar Using a 10.6 um Source, Engineers

Thesis, Naval Postgraduate School, Monterey,
California, December 1974.

delkS

INITIAL DISTRIBUTION LIST

No. Copies

Defense Documentation Center Z
Cameron Station AN
Alexandria, Virginia 22314

Library, Code 0212 2
Naval Postgraduate School
Monterey, California 93940

Department Chairman, Code 52 2
Wepavement of Electrical Engineering

Naval Postgraduate School

Monterey, California 93940

Professor C. H. Rothauge, Code 52Rt 1
Department of Electrical Engineering

Naval Postgraduate School

Monterey, California 93940

Assoc Professor T. F. Tao, Code 52 Tv Hi
Department of Electrical Engineering

Naval Postgraduate School

Monterey, California 93940

Asst Professor J. P. Powers, Code 52Po it
Deparment Of Electrical Engineering

Naval Postgraduate School

Monterey, California 93940

Dee Oo. Wane (53/6241) i
Aeroject Electrosystems Co.

PROOMW: Hollyvale Street

Azusa, California 91702

Dr. Greg Mooradian 2
Code 2500

Hevale Electronics Laboratory Center

momeatalina Blvd,

San Diego, California 92152

Diwee. Longo — . 1
Rockwell International Science Center

1049 Camino Dos Rios

Thousand Oaks, California 91360

116

10.

LT Maurice Fraunfelder, Jr.
1302 Putnam Avenue
Janesville, Wisconsin 53545

LT Thomas H. Chance, USN
Naval Destroyer School
Newport, Rhode Island 02840

Related across Profoundd

Oregon Climate Docs

Grants Pass — Sustainability and Energy Action Plan p.16
The TeamINTRODUCTION GRANTS PASS SUSTAINABILITY AND ENERGY ACTION PLAN TomJan O’Hara Committee Chair Jan is an engineer with 30 years of experience in air quality, landfills, hazardous waste site cleanup, stormwater,…

WEF Publications

EFF Atlas of Surveillance — Data Library (CSV exports) p.1
Skip to main content A project of the Electronic Frontier Foundation ☰ Home Map Search the Data Data Library Glossary About Methodology Collaborate Donate Atlas of Surveillance is a project of the Electronic Frontier Fou…

Epstein Files

EFTA00072947 — PARK DIETZ & ASSOCIATES, INC.
PARK DIETZ & ASSOCIATES, INC. Forensic Experts Administrative Offices Tel: Fax: Email: Website: www.parkdietzassociates.corn • Forensic Psychiatry • Forensic Psychology • Forensic Pathology • Forensic Neurology • Forensi…
EFTA00163398 — From: FBI News Briefing <[email protected]>
From: FBI News Briefing <[email protected]> To: "FBINewsBriefing" <[email protected]> Subject: [EXTERNAL EMAIL] - FBI Daily News Briefing - May 8, 2025 Date: Thu, 08 May 2025 10:15:09 +0000 Importan…
EFTA00073070 — ELIZABETH F. LOFTUS
ELIZABETH F. LOFTUS Distinguished Professor University of California, Irvine Tel: Fax: emai : web: http: soci eco ogy.uci.edu/faculty/eloftus/ EDUCATION B.A., with highest honors in Mathematics and Psychology, UCLA, 1966…

Federal Writers' Project

[Sailors versus Rats]
gwe co/. 7"-e.S Forms to be Filled out for Each interview FORM A Circumstances of interview STATE NEW -YORK NAME OF WORKER WILLIAM WOOD ADDRESS 7012, 67th Place, Glendale, L. I. .DATE December 19, 1938 SUBJECT SAILORS VE…
[Miss Henrietta C. Dozier]
March 1, 1959. Miss Henrietta 0. Dozier Arehiteot 41b Fokinallar Life Bldg isokSonville$ Florida. Remo Shepherd, Writer. MISS RBAHIHTTA 0. DOHA, ARCHITHOT. Miss Dozier was visited in her officio, one of a suits of two ro…
[Violin-Making and Local Politics]
f." Accession no. 1;tx „re, t,,) ctd WPA L. C. PRdiECT writers UNIT Form -3 9647 Date received 10/10/40 Consignment no. 1 Shipped from Wash. Of flee Label Amount lip. (id.forms k —D) Folklore Collection (or Type) and loc…

Recent News

Exclusive—Richard C. Lyons: ‘Everybody Must and Will Be a Soldier’: The American Tradition of Service and Sacrifice Started in 1776
There was perhaps no couple better known in colonial America in the lead up to the Revolution than John and Abigail Adams. John, through his political efforts, and his wife, Abigail, through her widespread and deeply lea…
Project Launch: Unmasking National Association of Independent Schools
You’ve probably never heard of NAIS, but you need to know about it. NAIS is the driving force behind the radical racial and gender programming that has captured most K-12 private schools, particularly the elite schools. …
The Books That Churchill ReadThe self-education of a statesman.
W inston Spencer Churchill wrote with warmth and humor about his early education — and its limits. Certain passages had a touch of the tremulous, given the discipline to which he was subjected by one or two nasty schoolm…