An electronically controllable apparatus is described which modulates a continuous wave laser beam so as to produce an output beam consisting of coherent pulses that are electronically controllable as to both pulse repetition rate and pulse width. The apparatus includes two acoustic devices positioned so that the laser beam passes through them in sequence, and apparatus for passing sound waves through the devices to frequency shift the laser radiation as well as to diffract it. Each acoustic device generates sound waves containing a group of frequencies which result in spaced pulses. The first acoustic device is countered by the second acoustic device to produce a collimated, coherently pulsed, laser beam

Margolis, J. S.

English

NASA Technical Reports Server

United States Patent [19] [111 4,332,441 
Jun. 1,1982 Margolis *- 
[54] COHERENTLY PULSED LASER SOURCE! 
[76] Inventor: Robert A. Frosch, Administrator of 
the National Aeronautics and Space 
Administration, with respect to an 
invention of Jack S. Margolis, 
Pasadena, Calif. 
[21] Appl. NO.: 150,040 
[22] Filed: May 15, 1980 
E511 Int. ( 3 . 3  ................................................ G02F 1/32 
[52] U.S. Cl. .................................................... 350/358 
[58] Field of Search ..................... 350/358, 169, 96.13, 
350/96.14, 358; 333/30 R, 72 
~561 References Cited 
U.S. PATENT DOCUMENTS 
3,566,303 2/1971 DeMaria ............................. 350/358 
4,124,280 11/1978 Berg et al. ........................... 350/358 
Primary Examiner-William L. Sikes 
Attorney, Agent, or Firm-Paul F. McCaul; John R. 
Manning 
[571 ABSTRACT 
An electronically controllable apparatus is described 
which modulates a continuous wave laser beam 22 so as 
to produce an output beam 42 consisting of coherent 
“pulses” that are electronically controllable as to both 
pulse repetition rate and pulse width. The apparatus 
includes two acoustic devices 24,26 positioned so that 
the laser beam passes through them in sequence, and 
apparatus 32 or 34 for passing sound waves through the 
devices to frequency shift the laser radiation as well as 
to diffract it. Each acoustic device such as 24 generates 
sound waves containing a group of frequencies which 
result in spaced pulses. The spreading of a laser beam at 
40 which emanates from the first acoustic device 24 is 
countered by the second acoustic device 26 to produce 
a collimated, coherently pulsed, laser beam 42. 
7 Claims, 7 Drawing Figures 
SIGNAL 50-1 SOUYCE I 
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FIG. 3 
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4.3 32.44 1 
COHERENTLY PULSED LASER SOURCE 
DESCRIPTION 
ORIGIN OF THE INVENTION 
The invention described herein was made in the per- 
formance of work under a NASA contract and is sub- 
ject to the provisions of Section 305 of the National 
Aeronautics and Space Act of 1958, Public Law 85-568 
(72 Stat. 435; 42 USC 2457). 
TECHNICAL FIELD 
This invention relates to the production of pulses of 
coherent optical radiation by the use of sound in opto- 
acoustic devices to modulate a continuous laser beam. 
BACKGROUNDART 
There are many applications where coherent pulses 
from a laser are required, which can be varied in width 
and/or repetition rate. For example, coherent laser 
pulses are useful in optical communications, Lidar (laser 
radar), optical gyroscopes, and physics instruments. 
One way for producing such pulses is by mode locking 
lasers, but the mode locking parameters are not easily 
changed to control repetition rate or pulse duration. 
Another technique is the use of a continuous wave laser 
and a pulsed amplifier, but this has the complication of 
requiring two lasers. An even simpler apparatus for 
producing coherent pulses from a single laser, would be 
of considerable value. 
STATEMENT OF THE INVENTION 
In accordance with one embodiment of the present 
invention, an apparatus of relatively simple design is 
provided for generating a coherently pulsed laser beam 
which can be electronically controlled. The apparatus 
includes a continuous wave laser, a pair of diffraction 
bodies which are located so the laser beam passes 
through them sequentially, and transducers coupled to 
the diffraction bodies to pass sound waves through 
them. The sound waves passing through each diffrac- 
tion body are the sum of a series of monochromatic 
waves. Each monochromatic acoustic wave separately 
diffracts and frequency translates the optical laser beam. 
The spreading of the laser beam emanating from the 
first diffraction body, is counteracted by the second 
diffraction body and transducer, to produce a substan- 
tially collimated pulsed laser beam. 
BRIEF DESCRIPTION OF THE DRAWINGS 
FIG. 1 is a simplified side elevation view of a coher- 
ently pulsed laser source constructed in accordance 
with one embodiment of the present invention. 
FIG. 2 is a partial perspective view of the apparatus 
of FIG. 1. 
FIG. 3 is a diagrammatic view of a portion of the 
apparatus of FIG. 1. 
FIG. 4 is a waveform showing an idealized graph 
having the waveform of a coherently pulsed laser beam. 
FIG. 5 is a graph indicating the truncated frequency 
spectrum of the beam of FIG. 4 (the relative intensities 
are not intended to be shown). 
FIG. 6 shows graphs of the envelopes of coherently 
pulsed laser beams produced with the apparatus of the 
present invention using a finite number of acoustic fre- 
quencies. 
FIG. 7 is a graph showing the frequency spectrum of 
the acoustic waves utilized in the production of the 
2 
waveform envelopes shown in FIG. 6 (the relative 
intensities are not intended to be shown). *- - 
BEST MODE FOR CARRYING OUT THE 
5 INVENTION 
FIG. 4 shows a waveform 10 which represents an 
idealized coherently pulsed laser beam. The beam con- 
sists of optical radiation, that is radiation in the infrared, 
visible or ultraviolet wavelengths or somewhat beyond, 
10 with the beam being in the form of trains 11-13 of sub- 
stantially sinusoidal waves of optical frequency which 
are separated by quiescent periods, each train having a 
duration S and with the beginning of the trains spaced 
apart by a time T. The frequency spectrum of the wave 
l 5  10 is shown in FIG. 5, which shows that it can be con- 
sidered to consist of numerous components, each of a 
particular single frequency, separated from one another 
by a constant frequency difference d, and with a center 
frequency of fo representing the frequency of the sinu- 
2o soidal continuous wave which would result if each train 
in FIG. 4 were continued indefinitely. It may be noted 
that the components in FIG. 5 have different ampli- 
tudes, but FIG. 5 only shows their frequencies. The 
frequency spacing d between the components in FIG. 5 
25 determines the repetition rate of the pulses in the time 
domain, while the width of the pulses is, in a simplistic 
sense, determined by the width W of the spectrum. The 
exact shape of the pulses in the time domain shown in 
3o FIG. 4, is controlled by the variation in strength of the 
components in the frequency spectrum of FIG. 5. These 
functions are determined by taking the appropriate 
fourier transforms of the desired waveform. 
FIG. 1 shows apparatus of the present invention 
35 which can be utilized to generate pulsed coherent radia- 
tion such as of the type shown in FIG. 4, using a single 
laser 20 which produces a continuous wave laser beam 
22. (The term “laser” herein refers to any device which 
produces coherent optical radiation, regardless of the 
40 technique). The apparatus also includes a pair of diffrac- 
tion devices 24, 26 which include optical elements 28, 
30 of material which is transparent to the passage of 
optical radiation from the laser, and sonic transducers 
32, 34 coupled to the optical elements. The sound trav- 
45 els in a direction which is primarily perpendicular to the 
laser beam. This type of diffraction device, which is 
known, diffracts a laser beam passing through a crystal, 
by the use of travelling acoustic waves. The angle X of 
the beam relative to the sonic wavefront must be at the 
50 Bragg angle which is a small angle such as less than a 
few degrees, to obtain such diffraction. (This is de- 
scribed in Introduction to Optical Electronics by 
Amnon Yariv, published by Holt, Rienhart, Winston, 
1971, p. 308-310). Such devices have been used with 
55 single acoustic frequencies to shift the frequency of the 
laser beam, modulate it, or deflect it. The acoustic wave 
fronts, indicated at 36, produce travelling localized 
variations in density of the optical material, to diffract 
the beam through a generally small angle as though 
60 diffracted by a grating. However, because the acoustic 
wave is travelling, the laser beam is Doppler shifted by 
a frequency equal to the acoustic wave frequency. If 
several acoustic frequencies are introduced, the incident 
laser beam will be diffracted in several slightly different 
65 directions, and also Doppler shifted by several different 
corresponding frequencies. The frequency shifts of the 
laser radiation are equal to the injected acoustic fre- 
quencies. The Bragg diffraction angle is given by the 
4,332,441 
3 4 
relationship h=2d cos X, where h=wavelength of laser damzptal frequency, so as to produce a set of coherent 
radiation in the diffracting medium, d =acoustic wave- waves. 
length in the medium, and X=Bragg diffraction angle. Thus, tiieinvention provides an apparatus for ge&r- 
The divergent beam 40 which is shown leaving the first ating a coherently pulsed laser beam, utilizing a single 
diffraction device 24 is collimated by the second diffrac- 5 continuous wave laser source and an acousto-optic 
tion device 26, SO that the different components of the modulator. The acousto-optic modulator includes a pair 
optical radiation overlap in order to recombine into the of diffraction devices which each include an element 
desired coherent pulses. formed of a medium which is transparent to the passage 
The second diffraction device 26 iS operated SO that of optical radiation, and a transducer coupled to the 
the transducer 34 transmits the Same Sonic frequencies 10 medium to pass sound waves therethrough. A signal 
through the % ~ o n d  optical 30 as is Passed source which drives the transducers can produce a 
through the first one 28. However, the second diffrac- group of signals whose sum is substantially a series of 
tion device is oriented oppositely to the first one 24. As to pulse the laser beam. The diffraction devices 
- 
a result, the divergent beam components are again di- are positioned so that the laser beam passes in sequence 
verted, but in a manner SO that they emerge as a beam 42 15 through them. The divergent laser beam components 
that is collimated and therefore that can be utilized in 
beam. It may be noted that the final beam 42 is wider 
than the initial beam 22 from the laser. However, if the 
two diffraction devices 24, 26 are close to each other 20 Apparatus for generating modulated optical radia- and the angle of diffraction is small (as can always be 
tic waves), the spatial separation of the beam compo- first and second diffraction devices, each device in- nents is small and the overlap region is large. The non- cluding an optical element which is transparent to overlapping regions of the beam can be so small as to be 25 the passage of optical radiation from said source, negligible, or can be blocked out. and each device also including a sound transducer The transducers 32, 34 of the diffraction devices, can coupled to the optical element to pass sound waves be driven by a signal source 50 which produces a spec- therethrough; and trum of acoustic waves which are chosen to produce a 
and amplitude to produce an optical pulse shape having of frequencies, said signal source coupled to said 
the desired envelope. FIG. 6 shows the envelope 52 of transducers to energize each of them with the same 
the spectrum of a resultant coherently pulsed laser signals; 
which from the of five said source and diffraction devices positioned 90 that 
ponents as indicated by the components 54 in FIG. 7. 35 radiation from said passes into said first 
The sonic components 54 are separated from one an- device and the radiation emanating from said first 
other by a frequency d/2 which is one-half the fre- device passes into said second device. 
quency separation d shown in FIG. 5. me use of 2. The apparatus described in claim 1 wherein: said 
half the frequency separation is due to the fact that two .signal Source is constructed to generate a Signal Contain- 
diffraction devices are utilized. The acoustic frequen- 40 ing a predetermined frequency, a plurality of additional 
cies producing the envelope 52 of FIG. 6, are chosen SO frequencies which are higher than said predetermined 
that the spacing T between pulses is 10 times the width frequency, and a Plurality of frequencies which are 
S of each pulse. In one example of a typical application, bwer than said Predetermined frequency, with each 
the width S of each pulse may be chosen to equal that of Pair of adjacent frequencies being separated by a Prede- 
a sine wave of 1 e H z  frequency, while the frequency of 45 termined constant frequency difference, and with the 
the laser beam may be 1014Hz, SO that a large number of signal of each rrequency having a bandwidth which is 
wavelengths of the laser beam are contained in each much less than one-tenth the constant frequency differ- 
pulse, and only the envelope is shown at 52. In FIG. 6, ence. 
another envelope 56 is shown which represents the sum 3. Apparatus for generating Pulses of optical radiation 
of a group 58 (FIG. 7) of seventeen frequency compo- 50 from a continuous coherent beam of radiation, compris- 
nents which are all spaced apart by the distance d/2. It 
may be noted that in FIG. 7, only the frequencies of the an optical dement of substantially uniform index of 
components are given, and not their relative amplitudes. refraction positioned to intercept said beam of radi- 
FIG. 3 shows how the source 50 can be constructed, ation; 
by adding together the outputs of a group of signal 55 an electrical-to-sound transducer coupled to said 
generators 62, 64, 66 etc. which each produce a funda- optical element to pass sound waves therethrough 
mental frequency fo plus or minus an integral number in a direction which is primarily perpendicular to 
times a difference frequency d/2. The sum of the signal the beam of radiation; and 
generators is added and amplified in an amplifier 70 and driver means coupled to said transducer for deliver- 
delivered to the transducers such as 32 of a diffraction 60 ing electrical signals thereto which contain fre- 
device. Of course a separate circuit can be used to drive quencies whose sum defines time-spaced pulses. 
each transducer. Each transducer 32 may include sev- 4. The apparatus described in claim 3 including: 
era1 transducer elements 320, 326, 3212 located on one a second optical element positioned to intercept the 
surface of the optical element, so as to produce a sub- beam emerging from said first named element; and 
stantially planar wavefront instead of a curved one. In 65 a second transducer coupled to said optical element 
actual practice, all of the acoustic frequencies may be to pass sound waves therethrough and coupled to 
produced by a frequency synthesizer which utilizes a said driver means to receive signals therefrom. 
single crystal controlled oscillator to produce the fun- 5. The apparatus described in claim 4 including: 
generated by the first diffraction device are therefore 
a coherently pulsed laser beam. 
most applications requiring a pulsed laser collimated by the second diffraction device, to produce 
We claim: 
tion comprising: arranged by a proper choice Of frequency for the acous* a source of continuous coherent optical radiation; 
series of monochromatic waves of a frequency spacing 30 a signal which generates signals of a plurality 
ing: 
a 
5 
4,332,M 1 
source of optical radiation positioned to pass *- 
through said fmt optical element at an angle to the 
direction of sound waves passing therethrough; 
said second optical element and the transducer device 
coupled thereto being positioned so the sound 5 
waves therein move in a direction substantially 
opposite to the direction of sound waves in said 
first optical element. 
6. A method for generating pulses of optical radia- 
directing a beam of continuous optical radiation 
through an optical element which is transparent to 
the passage of the radiation; and 
tion, comprising: 10 
15 
20 
25 
30 
35 
40 
45 
50 
6 
passing acoustic waves through said optical element 
in a direction primarily perpendicular to the beam 
of optical radiation to frequency shift the optical 
radiation, with the acoustic waves chosen so their 
sum has an envelope that forms pulses. 
7. The method described in claim 6 including: 
passing the beam of optical radiation which emerges 
from said optical element through a second optical 
element and passing second acoustic waves 
through said second optical element wherein the 
sum of said second acoustic waves has an envelope 
that forms pulses. * * * * *  
55 
60 
65 


Coherently pulsed laser source

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