Thermal stabilization for an acousto-optic device

An acousto-optic device is shown which permits the use of a temperature sensitive solid state laser. The laser is mounted in a heat sink that displaces the laser as a function of temperature in a direction which cancels the drift of the laser's blur spot due to the laser's increased wavelength as a function of temperature.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an apparatus for improved control of the 
blur spot drift in an acousto-optic device due to the thermal response of 
a solid state laser source. 
2. Description of Prior Art 
The term acousto-optics (A/O) refers to an interaction of light and sound. 
Typically an RF input signal is first transformed into an acoustic wave in 
a suitable crystal material, such as lithium niobate. Variation in index 
of refraction due to the propagation of the acoustic wave within the 
crystal can be then used to deflect a beam of light, usually 
monochromatic. This process is the equivalent of the better known Bragg 
diffraction of X-rays from the planes of a crystal lattice; for this 
reason the device is called a Bragg deflector or Bragg cell as well as an 
acousto-optic deflector or modulator. The angular deflection of the 
optical beam is proportional to the frequency of the original RF input 
signal. As the process is linear, multiple simultaneous RF input signals 
yield multiple simultaneous beam deflections corresponding to the distinct 
input frequencies with the intensity of the individual deflected beams 
being proportional to the power of the original RF input signals. 
Acousto-optics have been used for a variety of applications where light 
must be modulated or deflected. An important application is the use of 
acousto-optics for wideband receiving systems. The acousto-optic 
phenomenon occurs over a substantial bandwidth, 1 GHz with existing 
devices, so that the frequency content of an unknown signal environment 
can be resolved by measuring the angle of deflection corresponding to each 
signal in the environment. Thus, the entire signal environment may be 
viewed simultaneously by a device that acts like a channelized receiver. 
Due to its inherent temperature stability, a helium neon (HeNe) laser has 
been the laser of choice for use in an acousto-optic device or system. In 
recent years, the small size and weight, lower power consumption, and high 
efficiency of a gallium arsenide (GaAs) based solid state laser has made 
it an attractive alternative to an HeNe laser. This is particularly true 
for airborne A/O receiver applications, and other contexts where these 
advantages offer improved functional performance. 
The major disadvantage associated with the GaAs class of solid state laser 
is strong thermal dependence, because lasing action is related to band gap 
width which is a function of temperature. Thermal variation of both 
optical power and wavelength can be observed for devices of this type. 
Optical power variation can be monitored and controlled, but wavelength 
variation is a more difficult problem. 
A need continues to exist for controlling either laser wavelength or for 
forcing an acousto-optic device or system to have a small sensitivity to 
laser wavelength variation. 
The only prior art approach which successfully addresses this problem is to 
imbed the entire optical package in a refrigerator/oven, thus allowing 
regulation of temperature and minimizing temperature variation. 
SUMMARY OF THE INVENTION 
Accordingly, an important object of the present invention is to control or 
obviate frequency measurement error in an acousto-optic device using a 
sold state laser, where such frequency error is substantially caused by a 
change in laser wavelength due to a change in laser operating temperature. 
The technique used in the present invention is precise design of a laser 
heat sink so that, as the laser temperature changes, thermal expansion of 
the heat sink changes the pointing angle of the laser in such a fashion as 
to cause blur spot motion due to laser wavelength temperature drift to be 
cancelled.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
Typical operation of a prior art acousto-optic frequency-measuring device 
is shown in FIG. 1. Electromagnetic energy, in the form of a light beam 
10, from a laser source 12 is directed to an acousto-optic modulator or 
Bragg cell 14 by a laser lens 16 where a portion of that light beam 10 is 
deflected by diffraction, caused by the passage of an acoustic energy wave 
through the medium of the cell, into a deflected or first order beam 18 
with the remainder of the undeflected or zero order beam stopped by an 
optical stop, not shown. A cylinder lens pair 20 serves to compress the 
light into the Bragg cell acoustic wave and recollimate it upon exiting 
the Bragg cell. The deflected light beam 18 passes through a transform 
lens 22 and appears as a blur spot in the focal plane 24, where its 
position and intensity are sensed by photosensor means 26. 
FIG. 2 shows experimental measurements of optical output wavelength in 
Angstroms as a function of laser diode case temperature in degrees 
Centigrade for a typical laser diode. Wavelength changes of 2-3 
.ANG./.degree.C. are typical for lasers of this class. 
FIG. 3 shows experimental measurements of blur spot displacement as an 
apparent frequency shift in MHz as a function of laser case temperature in 
degrees Centigrade for the same laser of FIG. 2. A predicted line 28 
indicating blur spot drift is based upon the laser wavelength change. The 
actual data shows a greater change than the predicted or expected value. 
It was discovered that this unexpected spot shift of almost twice its 
expected value reflects an additional contribution to blur spot 
displacement due to thermal expansion of the laser's heat sink. 
After the foregoing discovery, the following Bragg cell equations were 
considered. The Bragg cell angular output is not only sensitive to changes 
in RF signal frequency, but also to changes in laser wavelength or 
frequency. For a fixed laser input angle .theta..sub.l, the deflected 
output beam is diffracted at an exit angle .theta..sub.x given by 
##EQU1## 
where .lambda. is the wavelength of laser light, V is the acoustic 
velocity of the Bragg cell medium and f is the signal frequency. In normal 
operation laser wavelength .lambda. and acoustic velocity V are 
essentially constant, and exit angle is proportional only to frequency. By 
focusing the light via a lens of focal length F into a blur spot in the 
focal plane, spot position .chi. is related to frequency, since 
EQU .chi.=F.theta..sub.x (2) 
Measurement of spot position thereby gives a direct measure of signal 
frequency. When the ambient temperature is varied, however, additional 
effects are seen which corrupt or complicate direct frequency measurement. 
A small change in position or exit angle can be caused not only by signal 
frequency change but by changes in laser wavelength .lambda. or Bragg cell 
acoustic velocity V due to the temperature change. That is: 
##EQU2## 
The second term reflects changes in the exit angle due to changes in the 
wavelength of the laser beam due to temperature shift, while the fourth 
term reflects changes due to the input angle as that angle might depend on 
temperature. From equation 3, it was confirmed that a heat sink designed 
to displace the laser in the proper direction can cancel the effect of 
increasing wavelength within the laser due to temperature. This, in turn, 
causes an increase in the exit angle from the Bragg cell of the 
wavelength. 
The mechanism which converts actual laser displacement due to the expansion 
or contraction of its heat sink into additional perceived frequency error 
or blur spot displacement is shown in FIG. 4. The laser displacement 
X.sub.1 is changed into angular measure by the laser collimating lens 16 
of focal length F.sub.1. This angular measure is reconverted by the 
transform lens 22 of focal length F.sub.2 into perceived spot displacement 
X.sub.2. The ratio of laser translation to blur spot displacement is that 
of the transform lens focal length to collimating lens focal length: 
EQU X.sub.2 =(F.sub.2 /F.sub.1)X.sub.1 
In FIG. 4 the displacement of the blur spot due to the increase of the 
wavelength of the laser beam 10 which is then diffracted at a greater exit 
angle by the Bragg cell 14 is shown at X.sub.3. It is desired to balance 
the displacement X.sub.2 by the displacement X.sub.3, both of which are 
dependent on temperature. When both are balanced, the resultant motion of 
the blur spot due to temperature change is cancelled. 
FIG. 5 shows a block diagram of the laser source 12 and Bragg cell 14 in 
this interaction. Inputs to the laser 12 and its heat sink 30, FIGS. 6a 
and 6b, are current and ambient temperature; inputs to the Bragg cell 14 
include RF signal amplitude and frequency. Internal states, which appear 
as laser and heat sink outputs and thus as inputs to the Bragg cell, 
include particularly laser optical power, laser wavelength, and laser 
position. Measured amplitude from the Bragg cell 14 is normally controlled 
by closed loop control of laser optical power by way of specification of 
laser input current. Laser current control, through heating, changes both 
wavelength and laser position. Ambient temperature variation affects all 
laser outputs: power, wavelength, and position. 
The heat sink size, method of mounting and attachment, and materials choice 
are selectable within wide limits. It is therefore possible to construct 
the heat sink in a fashion so that blur spot displacement due to laser 
wavelength change is exactly offset by a position change in the laser 
which moves the spot in the contrary direction. Perceived frequency error 
for this type of operation will then be zero. 
Referring now to FIG. 6a, a laser 12, such as a solid state GaAs laser, is 
shown mounted to frame or system ground by a heat sink 30. In FIG. 6a, the 
heat sink 30 may be a block of temperature expansion material such as 
copper or brass. In one of the preferred embodiments, a block of copper 
one cubic centimeter may be used in combination with a 20 power microscope 
objective lens 16. Here the collimating lens 16 also mounts upon the heat 
sink 30 so that the source 12 of collimated light 10 moves with the lens 
16 through the displacement X.sub.1. 
In FIG. 6b, GaAs laser 12 is shown mounted upon a bimetal heat sink 32. 
Here the bimetal strip 32 may be formed from laminated copper and iron to 
transmit a torsional motion to the laser 12 to change the pointing angle 
of the beam 10. In this embodiment, the lens 16 is not mounted to the heat 
sink 32. The precise dimensions of the heat sinks 30 and 32 may be varied 
to match the temperature dependence curve, FIG. 2, of the designed laser 
12. Clearly other variations will be possible within the teachings of the 
present invention which should be limited only by the appended claims.