Document text
AM AND FM MODULATION OF A HELIUM-NEON
LASER BEAM USING ACOUSTO-OPTICAL INTERACTION
Osman Unlu
rary
/a I Postgraduate School
nterey, California 93940
Monterey, California
T
AM
and
FM
Modulation
of A
Hel:
Lum-Neon
Laser
Beam Using Acousto
-Optical
Interaction
by
-
Osman Unl
u
Thesis
Advisor
John P.
.Powers
December 1972
Approved jJo/l public h.dl<iahz; di^t/'Ubutlon antimitzd.
Library
Naval Postgraduate School
Monterey, California 93940
AM and FM Modulation of a Helium-Neon
Laser Beam Using Acousto-Optical Interaction
by
Osman Unlu
Lieutenant Junior Grade, Turkish Navy
B. S., Naval Postgraduate School, 1972
Submitted in partial fulfillment of the
requirements for the degree of
MASTER OF SCIENCE IN ELECTRICAL ENGINEERING
from the
NAVAL POSTGRADUATE SCHOOL
a #*>'
Library
Naval Pcstgradual
Monterey, Calil
ABSTRACT
Acoustical amplitude and frequency modulation techniques of a CW
helium-neon laser beam are presented. The communication possibilities
of this beam for both cases (AM and FM) are verified. Theories are
discussed for the acoustooptical modulator and the p-i-n silicon
photodetector . Designs of the elements of the systems and experimental
results are presented. Operation of both systems is discussed.
TABLE OF CONTENTS
I. INTRODUCTION 5
A. OBJECTIVE 5
B. BACKGROUND 5
II. THEORY OF ACOUSTOOPTICAL MODULATION 7
III. SYSTEMS UNDER INVESTIGATION 14
A. AMPLITUDE MODULATED ACOUSTICAL LASER COMMUNICATION SYSTEM- 14
1. TRANSMITTER 14
2. DETECTOR 16
3. DETECTOR PRE-AMPLIFIER 21
4. DETECTION PROCESS • 23
5. EXPERIMENTS 25
B. FREQUENCY MODULATED ACOUSTICAL LASER COMMUNICATION SYSTEM- 27
1. TRANSMITTER 27
2. DETECTOR AND DETECTOR PRE-AMPLIFIER — : 35
3. DETECTION PROCESS 35
4. EXPERIMENT 39
IV. CONCLUSIONS 47
LIST OF REFERENCES — : : 48
INITIAL DISTRIBUTION LIST 49
FORM DD 1473 50
ACKNOWLEDGEMENT
The author wishes to express his sincere appreciation to his wife,
Aysel, for her faith and encouragement, and to Dr. John P. Powers for
his guidance and assistance.
I. INTRODUCTION
A. OBJECTIVE
It was intended to study techniques of acoustooptical modulation of
a helium-neon (6328A) laser beam. It was also decided to investigate
the communication possibilities of this laser beam by means of intensity
and frequency modulation. The detection and demodulation of this modu-
lated light beam was another part of this investigation.
B. BACKGROUND
The first working laser was invented in 1960. The properties of
light produced by a laser attracted many engineers and scientist toward
it. These properties are: high intensity, time and spatial coherence,
directionality, and the high frequency operation. It is these properties
which are common to most laser applications and which permit modes of
operation that in many cases had never been possible before.
The primary characteristic of laser source which is most important
in optical communication is coherence. This coherence phenomenon permits
very efficient modulation and detection providing the very basis for all
laser communication techniques. The potentially usable bandwidth has
been increased significantly in communication systems because lasers can
operate in the visible and infrared region of the electromagnetic frequency
spectrum, considerably higher in frequency than competitive techniques
such as microwaves or radio waves. The communications capacity can be
increased through the directionality of the laser beams. Electromagnetic
waves at lower frequencies propagate in all directions which requires that
broadcasting points must be widely separated physically in order not to
interfere with one another. The directionality properties of a laser
beam permit their use close to one another with only a very little
interference caused by scattering due to atmospheric particles. The
laser beam has also a very narrow beamwidth, implying that power can be
efficiently transmitted to distant points [Ref. 1] .
These unique properties of the laser induced many engineers to
develop many different types of lasers. First, gas and solid-state
lasers were developed. In mid-1962, lasing action in certain semi-
conductor diodes was achieved.
Many optical communication experiments have been performed employing
CW (Continuous Wave) operation of gas lasers in the visible region. In
an experiment performed by the Bell Telephone Laboratories, baseband
information frequency-modulated (FM) a 70-MHz sub-carrier signal which
in turn modulated the optical beam by means of an acoustical modulator
[Ref. 2], A helium-neon laser was used as a signal source. Television
pictures and voice frequency-multiplexed signals were transmitted over
a one-mile path. Both amplitude and phase modulation of the optical
carrier was employed with direct and heterodyne receivers.
Other optical communication experiments using a laser television
display were performed by the Zenith Radio Corporation [Ref. 3]. In
this experiment light from a helium-neon laser was frequency-modulated
by the ultrasonic waves produced by the Brag diffraction modulator.
The modulator was driven by the frequency-modulated video power and
water was used as the interaction medium.
II. THEORY OF ACOUSTOOPTICAL MODULATION
The modulation of a light wave is controlled variation of some
property of the wave such as its amplitude, phase, frequency, polar-
ization or direction of propagation. The major problem of laser com-
munication is in the modulation techniques. Primarily it is to fill
the enormous available bandwidth with usable signals imposed on the
carrier frequency.
Light modulation can be achieved by several techniques including inter-
ferometric effects, electro-optic effects, and ultransonic diffraction
cells. Most optical modulators are dependent on a variation of optical
index of refraction under the control of the modulating signal [Ref. 4].
The index variation may be caused by
a. the electro-optic effect,
b. the magneto-optic effect,
c. acoustic (pressure) effects,
d. variation in charge carrier concentration in a semiconductor, or
e. combinations of these effects.
In the investigation the modulation of laser beam was studied on the
basis of acoustic effects and the Bragg diffraction principles. It
is well known that under the correct circumstances an optical beam passing
through a transparent material containing a travelling acoustic wave
has part of its energy diffracted by the refractive index variations
associated with the acoustic wave [Ref. 5]. This condition is known as
the Bragg diffraction phenomenon. By considering the interaction of
plane wave of sound with a plane wave of light the concept of the Bragg
diffraction can easily be understood [Ref. 6]. Because of interference
effects, when a plane wave of light intersects with a plane wave of
sound it can be shown that the diffracted light wave will have signifi-
cant amplitude only if the plane waves meet at the proper angle. The
condition for diffraction may be written from the wave vector relation
among the waves as
~k+ = ~k + K (1)
where k and k+ are the propagation constants of the incident and diffrac-
ted light, respectively, and K is the propagation constant of the sound
wave. The sign depends on the propagation direction of the sound beam.
These propagation constants are defined as:
k = w/c, k+ = k (1 +ft/uj) , and K = fi/v, in which
to = the optical angular frequency
c = the unperturbed light velocity in the medium
fi = the acoustic plane wave frequency
v = the acoustic velocity
As seen from Eq . (1) the propagation vectors of incident light and
sound yield the vector of diffracted light. This wave vector relation
can be constructed as shown in Fig. 1.
Typical sound frequencies used for communication purposes are very
much less than the light frequencies, therefore
1 1± | = 1 1 | (2)
then from Fig. 1, the angle 6B can be defined:
Sm A = I K, L 2 1 » JL (3)
B I k I 2A
where A is the wavelength of the sound wave, A is the wavelength of the
light in the diffracting medium, and 0R is the Bragg angle.
8
: THE WAVE VECTOR RELATION BETWEEN LIGHT AND
ACOUSTIC V/AVES FOR DIFFRACTION (AFTER REF. 9)
(if the direction of k and K is changed, the
vector sum (k + ■ K) no longer falls on the
circle and no diffraction can occur. )
The frequency relations between the incident and the diffracted
waves can be derived from the parametric interaction theory [Ref. 7]
u>+ = w + fi (4)
where w+ =0) + Q and w_ = to — fi are respectively called the up-shifted
and the down-shifted first order diffracted beam frequencies. (The
+ relates the frequency of the diffracted wave to the respective sign
in the vector equation.) The zero-order diffracted beam has the same,
frequency as the incident light beam. These diffracted frequency values
depend.. on whether the incident light wave is directed into the progress-
ing sound wave or behind it. An up-shifted firstorder diffracted beam
is produced in the first case and down-shifted in the second case,
as shown in Fig. 2. The frequency change is equivalent to a doppler
shift. This relation will be used later in frequency modulation of the
laser beam with the sound beam.
Light modulation can be accomplished by sound waves which are
produced in the acoustic modulator when the geometrical conditions for
Bragg diffraction are properly met. The sound wave can modulate the
light in amplitude and phase, deflect it, focus it, or shift its
frequency [Ref. 8]. The amplitudes of the diffracted waves are propor-
tional to the product of the amplitudes of the incident light and sound
plane waves
U+ a UL«US (5)
where U+ are the diffracted waves, U i is the incident light wave and
Ug is the sound wave amplitude. The proportionality constant is a
function of the interaction medium and the ratio of the acoustic wave-
length to the light wavelength (A/X) . The phase of the diffracted wave
is depended on whether the wave is up-shifted or down-shifted in frequency
10
LV
UJ+- il
A
n
(a)
uu
uo
-il
(b)
FIGURE 2: BRAGG REFLECTION (a) Up-shifted Diffracted Beam
(b) Down-shifted Diffracted Beam
(Up-shifted or down-shifted beam is selected by-
making light and sound meet under the appropriate
angle ) 0 ( AFTER REF. 5)
11
[Ref. 6]. In the first case the sum of the phases of the light and
sound plane wave is produced, and in the second case the difference.
The typical Bragg diffraction acoustooptical modulator is shown in
Fig. 3. The amplitude modulation of the sound beam causes a cor-
responding intensity modulation (IM) on each of the diffracted beams at
the acoustic frequency. The frequency modulation of the sound wave
result in frequency modulation of each of the diffracted beams from
the acoustic modulator as seen in Eq. (4). The frequency shift of the
diffracted beam is exactly equal to the acoustic frequency. Only one
of the three diffracted beams is useful for purposes of communication;
the others may be eliminated by a field stop.
12
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III. SYSTEM UNDER INVESTIGATION
The optical communication application of the laser beam was studied
by using amplitude modulation and frequency modulation techniques of that
beam. In general optical communication systems can be broken down into
five parts corresponding to parts of radio and other electrical communi-
cation systems [Ref. 9]. These components are: an oscillator to generate
the carrier wave, a modulator to impress information on the wave, a
transmission medium to convey the wave, a device to receive the wave and
convert it to an electrical signal, and a demodulator to extract the
information from the wave.
A. AMPLITUDE MODULATED ACOUSTICAL LASER COMMUNICATION SYSTEM
For this experiment a helium-neon laser operating at 6328 A was to
be employed as the light signal source. A typical amplitude modulated
acoustical laser communication system block diagram can be constructed
as shown in Fig. 4.
1. Transmitter
Signal transmission was to be accomplished by amplitude modula-
tion of the laser beam in the acoustical modulator medium at a sub-
carrier frequency in the megahertz (MHz) range. The acoustical wave is
generated by a high-frequency signal generator driving an acoustical
transducer. This acoustical wave generator can be externally amplitude
modulated (AM) with an information signal of different wave forms, such
as sine wave, square wave, and triangular wave in the audio frequency
range or higher. The amplitude modulated radio frequency (RF); signal is
then applied to a power amplifier to get the necessary power for driving
the acoustical modulator. Amplitude modulation by the RF sub-carrier
14
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(amplitude modulated) of the laser beam is achieved as explained in
section II. The transmitter block diagram is shown in Fig. 5.
2. Detector
The light detector is a key element in an optical communications
system. The optical detectors in a laser communication system convert
the intensity of the optical carrier into an electrical signal. When
the laser beam is intensity or frequency modulated by the acoustical
modulator, the beam transmitted from the modulator is collected and
focused on an optical detector. For laser communication purposes
solid-state photodiodes are very useful as detectors because of their
fast response time.
At the present time a variety of different types of solid-state
photodetectors are available. Of these, the silicon PIN photodiode
has the broadest applicability [Ref. 10]. The main advantages of the
PIN photodiodes are broad spectral response, a wide dynamic range,
high speed, and extremely low noise. Typical construction of a PIN
photodiode is shown in Fig. 6.
A PIN photodiode consists of a reverse biased semiconductor
diode. Absorbtion of incident light in or near the semiconductor
junction produces hole-electron pairs. When a large reverse-bias
is applied to the photodiode these hole-electron pairs (photo-generated
carriers) will be swept out of the I-region, with the resulting current
containing a-c components corresponding to the modulation of the incid-
ent light beam [Ref. 11]. The active region is where the high-bias
field exist, between or nearly between the N and P areas (I-region).
It is desirable to have the I-layer as thick as possible and P-layer
as thin as possible for the highest quantum conversion efficiency
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Bias
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Contact
Incidenl
Light Beam
Ohmic
Contact
N- Layer
P- Layer
I- Region(Silicon)
FIGURE 6 : THE CONSTRUCTION OF P-I-N PHOTODIODE
18
(electrons per Photon) . The equivalent circuit of the PIN photodiode
is as shown in Fig. 7. R is the series resistance which has a value
inversely proportional to the thickness of the P-layer. R and C are
the diode resistance and capacitance respectively. R has a value
bigger than several gigaohms; and C has a value between two to five
picofarads. In the presence of a signal, C is modulated by the conduct-
ivity of the I-layer. At high light levels, the I--layer may be saturated
and C may become quite significant. Thus because of these effects,
increase in the rise time of the detector and decrease in quantum
efficiency can occur. The frequency response of the diodes to modulation
are calculated from R and C according to Eq. (6).
( 1 + R«7R„ ) 1
J s p c:___ (6)
Jc
Rs Cp Rs Cp
where u is the 3-dB cutoff frequency and the approximation depends on
the assumption that Rc/R-. «1. The current resulting from the incident
fa p
illumination is i , i., is the noise current of the device and i„ is the
p' N R
dark current which has a value determined by the construction and the
dimension of the particular diode type.
The thickness of the I-layer is proportional to the magnitude of
the electric field applied to the diode. When the reverse bias voltage
is increased from zero volts, three beneficial events occur: hole-
electron transit time decreases, conversion efficiency (electrons per
photon) increases slightly, and the junction capacitance decreases.
19
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P
v
■GD-
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FIGURE 7 sPHOTODIODE EQUIVALENT CIRCUIT (AFTER REF.IO)
20
3. Detector Fre-Amplif ier
The current level produced by the photodiode is proportional to
the input light power. When dealing with low-level signals, an amplifier
is required to get a usable signal level. The speed of response of the
PIN diode is limited by the terminating pre-amplif ier . Reverse bias
applied by the amplifier circuit to the photodiode should be as high as
possible (up to 50 volts) in order to reduce the diode shunt capacitance
Cp and slightly enhance the sensitivity (microamperes per microwatt) .
The input resistance of the amplifier must be as high as possible to
satisfy high gain or low noise requirements, but as low as necessary for
high speed of response [Ref. 12],
The typical detector pre-amplif ier circuit used in this work is
shown in Fig. 8. In general the transistor T2 should have low Cob as
well as low Cie, and its gain-bandwidth product should be at least fifty
times the highest desired operating frequency. The transistor Tl is
connected as an emitter follower for ' impedance matching to the photodiode
and its emitter is connected to the base of T2. Negative feedback is
obtained by connecting the resistance Rf from collector of T2 to the
base of Tl. The use of negative feedback enables a combination of low-
noise properties and high-speed operation. The signal voltage at the
collector of T2 is the product of the feedback resistance Rf times the
photodiode current (signal current). The bias can be obtained for Tl
from either the photodiode or from the feedback resistor. Bias for T2
is obtained by connecting a high enough value of resistance between its
base and ground so that all of the emitter current of Tl does not pass
to ground .
21
PKOTODIODE
-Hr
Tl
^s
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T2
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FIGURE 8 :DETECTOR PRE-AMPLIFIER (AFTER REF.12)
22
4 . Detection Process
The principle in an optical communication system is the same as
in radio which transmits information by modulating the amplitude, phase,
or frequency of a carrier and transmits the modulated carrier to a
receiver where the signal represented by the modulation is demodulated
by a detection process. The detection process can be accomplished by
using direct photodetection or a heterodyne photodetection technique.
The direct detection technique is useful for amplitude modulated
(AM) or the intensity modulated (iM)signals. The operation of any AM
photodetector can be divided into two processes: first, the conversion of
light into photo-current whose ac component represents the detected
modulation, and second, conversion of the ac component of photo-current
into output power. The current generating process is a perfect "square-
law" process in which the output current is directly proportional to
the input power and hence to the square of the input amplitude. There-
fore the result is a direct demodulated signal at the output of the
photodetector. In this process the photodetector has no special arrival
angle requirement for detection except that the photons be intercepted
by its photo-sensitive area.
A typical direct detection set-up for the diffracted light from
the acoustical modulator, which is intensity modulated (IM) with the
envelope of the modulation sub-carrier, is shown in Fig. 9. One of the
first order (up-shifted or down-shifted) diffracted beams out of the
acoustic modulator can be selected by an adjustable iris to fall on the
photodetector. Intensity modulation on these beams produces ac voltage
at the photodetector output. The modulated and the demodulated signals
can be displayed on a dual trace oscilloscope.
23
Acoustic
Modulator
Adjustable
He-Ne Laser
Beam
Amplitude
Modulated Ultrasonic Beam
Dual-trace
Oscillascope
Amplitude Modulated
RF Sub-carrier
FIGURE y : DIRECT DETECTION 3ET-UF FOR INTENSITY
MODULATED DIFFRACTED LIGHT BEAM
24
5 . Experiment
For this experiment the light source was a helium-neon laser
(1.5 mW output power) . The amplitude-modulated 15-MHz rf sub-carrier
signal was obtained by external amplitude modulation of the high-
frequency signal generator with the information signal provided by an
audio oscillator. This 15-MHz rf sub-carrier frequency was used because
it was one of the resonant modes of the acoustical modulator. This
signal was fed to the wide-band power amplifier to get the necessary
driving power for the acoustic modulator. Distilled water was used as
the interaction medium in the acoustic modulator and the amplitude
modulated sound beam was produced by a quartz-crystal transducer of
1-MHz fundamental frequency excited with the amplitude modulated 15-MHz
(15th harmonic frequency of the transducer) rf sub-carrier signal from
the modulator driver.
The frequency response of the ultrasonic transducer was determined
by measuring the intensity of light in the first order diffracted beam
as a function of rf sub-carrier frequency with input voltage at the quartz
crystal terminals held constant. The normalized intensity versus fre-
quency curve is plotted in Fig. 10. The bandwidth of the transducer is
approximately 50 kHz.
The light modulation was achieved in the acoustic modulator as
described in Section II. One of the first order (up-shifted) intensity
modulated diffracted beam was selected by an adjustable iris from the
acoustic modulator and allowed to fall on the photodetector . An HP-4220
photodiode (with a circular active area diameter of 0.020 inch) was used
as a light detector.
25
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The detector pre-amplif ier circuit was designed as described in
section III-3. This circuit is as shown in Fig. 11. The demodulated
signals from the detector pre-amplif ier and the amplitude modulated rf
sub-carrier signals from the high-frequency signal generator were
displayed and photographed on the dual-trace oscilloscope as shown in
Figures 12, 13, and 14.
The communication possibilities of the above experiment was veri-
fied using an audio output of a radio as the information signal to
modulate the 15-MHz rf sub-carrier signal. The same light modulation
and demodulation scheme was used as described before. At the output of
the detector pre-amplif ier the demodulated signal levels were very low,
therefore an audio amplifier (which has 20-kHz bandwidth) was designed
as shown in Fig. 15. The demodulated output signals from the photo-
detector were applied to this amplifier input, then the output of the
amplifier was displayed and photographed on the dual-trace oscilloscope
as shown in Fig. 16, and it was also monitored with a loud speaker. At
the output of the speaker the sound was intelligible, but the fidelity
of the sound was not good. Possible reasons for the. poor fidelity are
feeding the output of the photodetector into the audio amplifier without
any filtering or processing, pickup noise by the audio amplifier, and
improper impedance matching between the audio amplifier and the speaker.
B. FREQUENCY-MODULATED ACOUSTICAL LASER COMMUNICATION SYSTEM
The block diagram of this system is shown in Fig. 17.
1. Transmitter
A frequency modulated rf sub-carrier signal can be provided by
external frequency modulation of a high frequency voltage-controlled
27
HP-Lj220 yV
PIN Photo-
diode
— IS V
Grouna
* R in Ohms
FIGURE 11 : THE DETECTOR FRE-AMPLTFIER CIRCUIT
28
(a)
(b)
FIGURE 12 : SINE WAVE MODULATED SIGNAL (UPPER TRACE) AND DEMODULATED
OUTPUT (LOWER TRACE)
(a) Modulating signal 6-kHz, (b) Modulating signal 20-kHz
(Note : The apparent chopping of the signals is due to the action
of the dual trace oscilloscope)
29
(a.)
(b)
FIGURE 13 : TRIANGULAR WAVE MODULATED SIGNAL (UPPER TRACE) AND DEMODULATED
OUTPUT (LOWER TRACE)
(a) Modulating signal 2 -kHz, (b) Modulating signal 7 -kHz
(Note : The apparent chopping of the signals is due to the action
x of the dual trace oscilloscope)
30
(a)
Q>)
FIGURE U4 r SQUARE WAVE MODULATED SIGNAL (UPPER TRACE) AND DEMODULATED
OUTPUT (LOWER TRACE)
(M) Modulating signal £-kH^ (b) Modulating signal ltf-kHz
(Note : The apparent chopping of the signals is due to the action
of the dual trace oscilloscope)
31
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FIGURE 16 : MODULATING SIGNAL FROM A RADIO (UPPER TRACE) AND DEMODULATED
OUTPUT FROM THE PHOTODETECTOR (LOWER TRACE)
33
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34
oscillator with an audio frequency oscillator. The rf sub-carrier signal
is then applied to a power amplifier to get the necessary rf power to
drive the acoustical modulator. Frequency modulation between the laser
beam and the frequency modulated rf sub-carrier signal can be achieved
in the acoustic modulator as described in Section II.
2. Detector and the Detector Pre-Amplif ier
With the addition of a beam of light at the unshifted frequency
the same type detector as described in Section III-A, can be used to
detect the frequency-modulated diffracted beam. The detector pre-ampli-
fier is different because of difference in signal frequency.
3. Detection Process
The heterodyne photodetection process ban be used to detect a
frequency modulated diffracted beam. This arrangement provides very
good discrimination against unwanted signals. Optical detectors measure
intensity rather than amplitude of incident electromagnetic waves.
Heterodyne detectors in optical communication system also operate in this
manner .
As shown in Fig. 18, ideally, two coplanar electromagnetic
light waves of different frequencies fall on the same area of a photo-
detector. The resultant signal on the detector is given by
U = Ui Cos wit + U2 Cosw2 t (7)
where U ^ and U2 are the amplitudes and W, and uu are t^ie angular frequen-
cies of the two incident waves. To detect the presence of the signal,
the photo-intensity must be measured, which is given by the square of
Eq. (7):
2 2 2 2 2
U = U, Cos w, t + U2 Cos oo2 t + U, U2 Cos (co, + io2) +
(8)
U, U~ Cos (w, - w2)
35
Since w, and wo are in cases of current interest at optical or near
infrared frequencies, the electrical detection of these terms of
frequency coi , W2 and the sum (W1 + w2) is impossible. It has been
shown that the electron-photon correlation time required for the
generation of electron-hole pairs in the detector is on the order of
3 x 10 seconds, therefore the detector can not respond to signals
at these optical frequencies [Ref. 13]. Under these condition only
the average of these high frequency terms can be detected by the
detector. The average value of Cos2 tot term is one-half, and the
average of Cos (w-^ + u^) is zero. The final result for the detector
output is then
f I = U1 + 1l + U1 U2 Cos (Ul - w2) t (9)
2
It can be seen that from the Eq. (9) the output of the detector rep-
resent both a constant or direct current component and the difference
or beat frequency. The modulation on this beat frequency is extracted
by a conventional radio-frequency receiver or some electrical detector
depending upon the modulation type.
By using the heterodyne photodetection ideas, discussed above,
the frequency-modulated diffracted light beams from the acoustical
modulator can be detected. A typical set-up for this process is shown
in Fig. 19. In order to obtain two coherent beams for use in optical
heterodyne mixing it has been common practice to use a beam splitter
and mirror assembly similar to that shown schematically in Fig. 19.
The configuration shown would be appropriate for heterodyne (coherent)
detection of light modulated by Bragg diffraction from an acoustic
wave. The optical configuration can be arranged, so that a portion of
the original incident light falls into the same solid angle as does
36
Signal
Field
Ul
.Z
Photodetector
Ul + U2
i Splitter
Local Oscillator
• Field --
FIGURE 18 :A TYPICAL HETERODYNE DETECTION SET-UP (AFTER REF. lL)
37
•ror
Mirror
/
/
Incident Optical
Bean
Tic an Splitter
-^
Acoustic
Modulator
ode tec tor
Beam Splitter
Undiffracted
Ligl t
B an
• v quency iiocula ;
ultrasonic Bean
Frequency Modulated
RF Power , ■
FIGURE 19 : MIRRORS AND BEAM SPLITTERS ARRENGEMENT VCR OPTICAL
HETERODYNE DETECTION OF ERAGG DIFFRACTED LIGHT.
(AFTER REF. 13)
38
the diffracted light. An optical detector placed in the path of this
composite wave will detect the frequency-modulated diffracted light beams.
The output of this optical detector will then be the frequency modulated
electrical signal at rf frequencies, and the demodulation of this signal
can be accomplished by a conventional FM radio or an FM discriminator.
Heterodyne detection process can also be accomplished using a
laser local oscillator. The local oscillator and signal should be in
phase across the whole photosurface of the detector. If they are not,
beat currents at one part of the surface will be out of phase with beat
currents at another part of the surface, resulting in signal loss. The
optical mixing of the signal and local oscillator waves produces an
intermediate-frequency (IF) signal carrying the laser carrier modulation.
This IF signal is fed to an electrical detector or a conventional radio
receiver for final demodulation. Frequency control of the local oscilla-
tor is necessary to correct for frequency drifting of the laser carrier
and local oscillator and also compensate the possible large Doppler
shift at the optical frequencies. For these purposes the laser local
oscillator must be tunable over the necessary range. But there. is
generally a great amount of complexity and difficulty in tuning the
laser local oscillator. Thus, frequency demodulation is currently
impractical for long-range communications using this scheme.
4. Experiment
For this experiment the same light source and acoustic modulator
were used as for the first experiment. A frequency modulated 5-MHz
(fifth harmonic frequency of the X-Cut quartz-crystal transducer) rf
sub-carrier signal was obtained from external frequency modulation of a
high-frequency voltage-controlled oscillator (VCO) by an audio-frequency
39
oscillator. The transfer function of the high-frequency VCO is plotted
in Fig. 20. From this figure it can be seen that the ratio of frequency
deviation to the applied voltage is 2-MHz/volt. The bandwidth of the
transducer was 0.05 MHz, therefore for the external frequency modulation
of the high-frequency VCO, a 0.025 volt modulating sine wave signal was
used. Driving power for the acoustic modulator was obtained from a wide-
band power amplifier. Frequency modulation of the helium-neon laser
beam by the frequency-modulated rf sub-carrier signal was achieved as
described in Section II, (i.e. modulation of the acoustic wave frequency
produced frequency modulation of the light beam) . One of the frequency
modulated diffracted beam (up-shifted) was selected from the acoustic
modulator output and similar heterodyne detection set-up was used for
detection of this diffracted beams as shown in Fig. 19. The HP-4220 PIN
photodiode detector was used as a light detector. The detector pre-ampli-
fier circuit was designed as shown in Fig. 21.
In the initial attempts demodulation was not achieved by the
detector. The problem was at first believed to be the failure of the
photodiode/pre-amplif ier combination. Several alternative designs were
attempted without success. Finally, measurement of the intensity of the
diffracted beam showed that the portion of the diffracted beam power
falling on the detector was too low. This problem was solved by putting
the detector at the focal point of a converging lens, thus raising the
light intensity on the detector.
Another problem in the frequency-modulated optical demodulation
technique at rf frequencies was the presence of a spurious or leakage
signal channel which appeared in addition to desired (optical) channel.
This undesired signal was due to the radiation of rf power from the signal
40
o
J2;
o
M
6-"
O
■ ;
3
O
E-i
O
co
o
►--1
o
' ■
o
o
I
w
si
en
o
w
o
41
to
I
o
a
•H
pi
*
o
M
O
0
M
Pi
Pm
Pi
o
H
O
w
H
W
o
C\J
W
pi
M
to
tti
U
rt
ch
o
o
•H
S
C
•H
O
42
generating and modulating systems ("leakage")} and the reception of that
power in the receiving system ("pick-up"). This resulting rf leakage or
interference signal in the receiving system was coherent with the desired
rf signal since both originated from the same rf signal generators (VCO's)
and acoustical modulator.
After reasonable precautions to avoid pickup the demodulated
output from the photodiode detector was displayed and photographed on
the spectrum analyzer as shown in Figures 22, 23, and 24. In these
figures, the upper figure represents the noise and pickup (no signal
present on the detector), and the lower figure represents the noise plus
the signal on the detector. As seen from the difference of these upper
and lower figures the signals are detectable. The output of the detector
pre-amplif ier was also fed into a radio receiver for final demodulation,
and presented through head phones. The audio tone was audible although
noise was also present. The comparatively large amount of noise present
made an oscilloscope display of the signal impractical.
43
(»)
(b)
* Frequency Scale : fcenter - 5 MHz, Bandvifith - ♦■8 k»
FIGURE 22 : THE PHOTODETECTOR OUTPUTS ON THE SPECTRUM ANALYZER
(a) No signal present on the photodetector ( noise)
(b) 2-KHz modulating signal plus noise present
44
(*)
(b)
* Frequency Scale : fcenter " 5 MHz, Bandwidth = +8 kHz
FIGURE 23 : THE PHOTODETECTOR OUTPUTS ON THE SPECTRUM ANALYZER
(a) No signal present on the photodetector (noise)
(b) 5-KHz modulating signal plus noise present
45
(*)
00
* Frequency Scale : fcenter « * MHz, Bandwidth - + 8 kHz
FIGURE 2k : THE PH0T0DETECT0R OUTPUTS ON THE SPECTRUM ANALYZER
(a) No signal present on the photodetector (noise)
(b) -20-KHz modulating signal plus noise present
46
IV. CONCLUSIONS
The amplitude modulated acoustical laser communication systems worked
properly - the proposed designs were valid. The injection and sub-
sequent reproduction of three test wave forms (sine, square, and tri-
angular) was successful in the first communication tests. The injection
of an audio signal as a modulating signal from a radio, proved mar-
ginally satisfactory - the wave form reproduced properly but the fidel-
ity of the sound from the speaker was poor, but intelligible - in the
second test. The poor fidelity can be attributed to the lack of filtering
at the detector pre-amplif ier stage, harmonic distortions due to the
power amplifier (which was used before the acoustical modulator) , and
improper impedance matching between the audio amplifier and the speaker.
The frequency-modulated acoustical laser communication system also
worked, but the radiation rf power from the signal-generating and
modulating systems produced greater difficulty at the receiving systems.
Because of these undesired signals, noise, and the increased sensitivity
of the FM system to alignment, the quality of the demodulated output,
although detectable, was not equal to that of the AM system.
47
LIST OF REFERENCES
1. Smith, C.V., "Wideband Laser Communication Systems" IEEE Journal
of Qantum Electronics (IEEE Conference On Laser Engineering And
Applications), Vol. QE-3, No. 7, p. 251, June 1967.
2. Delange, 0. E., "Some Optical Communication Experiments," Applied
*" 'Optics, Vol. 9, No. 5, p. 1167-1175, May 1970.
3. Korpel, A., Adler, R. , Desmores, P., and Watson, W., "A Television
Display Using Acoustic Deflection and Modulation of Coherent Light,"
Applied Optics, Vol. 9, No. 10, p. 1667-1675, October 1966.
4. Miller, S. E. , and Tillotson, L. C, "Optical Transmission Research,"
Applied Optics, Vol. 5, No. 10, p. 1538-1548, October 1966.
5. Dixon, R. W. , and Gordon E. I., "Acoustic Light Modulators, Using
Optical Heterodyne Mixing," The Bell System Technical Journal,
Vol. XLVI, No. 2, p. 367-389, February 1967.
6. Powers, J. P., "Spatial Filtering Considerations in Bragg Diffraction
Imagining," To be Published in, Acoustical Holography, Vol. 4, Glen
Wade Ed., Plenum Press, New York 1972.
7. Korpel, A., "The Interaction of Sound and Light Fields of Arbitrarily
Prescribed Cross-section," Zenith Radio Corporation Research Report,
No. 66-2, September 1966.
8. Adler, R. , "Interaction Between Light and Sound," IEEE Spectrum,
Vol. 4, No. 5, p. 42-54, May 1967.
9. Gordon E. I., "A Review of Acoustooptical Deflection and Modulation
Devices," Proceedings of the IEEE, Vol. 54, No. 10, p. 1391-1401,
October 1966.
10. Hewlett Packard Application Note 915, Threshold Detection of Visible
and Infrared Radiation with PIN Photodiodes, January 1967.
11. Caddes , D. E., and McMurtry, B. J., "Evaluating Light Demodulation,"
Electronics, Vol. 37, No. 13, p. 54-61, April 1964.
12. Hewlett Packard Application Note 917, HP PIN Photodiode.
13. Cummins, H. Z., and Knable, N. , "Single Sideband Modulation of
Coherent Light by Bragg Reflection from Acoustical Waves," Proceedings
of the IEEE (Correspondence), Vol. 51, p. 1246, September 1962.
14. Pershan, P. S., and Bloembergen, N., "Frequency Response of the Photo-
mising Process" Applied Physic Letters, Vol. 2, p. 117-119, 1963.
48
Vjs-
INITIAL DISTRIBUTION LIST
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2. Library, Code 0212 2
Naval Postgraduate School
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3. Asst. Professor J. P. Powers, Code 52Po 1
Department of Electrical Engineering
Naval Postgraduate School
Monterey, California 93940
4. Lieutenant Junior Grade Osman Unlu, TN 1
Deniz Makine Okulu
Derince, Kocaeli, Turkey
5. Deniz Harb Okulu Kutuphanesi 1
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49
Unclassified
Securi tv Classification
DOCUMENT CONTROL DATA -R&D
[Security classification of title body of abstract and indexing annotation must be entered when the overall report Is classified)
1
Originating ACTIVITY (Corporate author)
Naval Postgraduate School
Monterey, California 9 3940
2«. REPORT SECURITY CLASSIFICATION
Unclassified
26. GROUP
REPOR T TITLE
AM and FM Modulation of a Helium-Neon Laser Beam Using Acousto-Optical Interaction
DESCRIPTIVE NOTES (Type of report and.lnclusive dates)
Mater's Thesis; December 1972
au THORISI (First name, middle Initial, leal name)
Osman Unlu
REPOR T D A TC
December 1972
7«. TOTAL NO. OF PAGES
51
7b. NO. OF REFS
14
. CONTRACT OR GRANT NO.
b. PROJEC T NO.
9a. ORIGINATOR'S REPORT NUMBER(S)
9b. OTHER REPORT NO(S) (Any other number* that may be aaalfned
this report)
t. DISTRIBUTION STATEMENT
Approved for public release; distribution unlimited,
I. SUPPLEMENTARY NOTES
12. SPONSORING MILITARY ACTIVITY
Naval Postgraduate School
Monterey, California 93940
I. ABSTRAC T
Acoustical amplitude and frequency modulation techniques of a CW helium-neon
laser beam are presented. The communication possibilities of this beam for both
cases (AM and FM) are verified. Theories are discussed for the acoustooptical
modulator and the p-i-n silicon photodetector. Designs of the elements of the
systems and experimental results are presented. Operation of both systems is
discussed.
FORM
I no v
'N 0101 -807-681 1
>D ,fr..1473
(PAGE 1)
50
Unclassified
Security Classification
A-3M08
Unclassified
Security Classification
?
KEY WO R D»
LINK C
AM and FM Modulation
Acousto-Optical Interaction
Helium-Neon Laser Beam
FORM
l NOV 68
5/N 0101 -807-6821
( BACK )
51
Unclassified
Security Classification
A- 31 409
2°OCT?*e
'»***
of " a"d fa,
V0?«v
Thesis
U45
c.l
1
41843
Unlu
AM
and
FM modulation
of a
hel
um-neon laser
beam
using acousto-
optical
nte faction.
thesU45
AM and FM modulation of a helium-neon la
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