Phase-conjugate resonator with a double SBS mirror

A laser using a stimulated Brillouin scattering (SBS) mirror and a moving grating eliminates frequency walkoff of the reflected beams. A laser using a double-SBS mirror prevents frequency walkoff by having the other side of the SBS mirror act as a conjugate moving grating.

BACKGROUND OF THE INVENTION 
The present invention relates generally to lasers and, more particularly, 
to a laser resonator which corrects distortion in the wavefronts and 
frequency walkoff. 
The optical elements of the laser resonator determine the spatial coherency 
of the laser beam which directly affects propagation and focusing 
capabilities. Some of the problems associated with the laser beam 
formation are vibration of reflecting surfaces, misalignment, aberrations 
in the lasing medium, index inhomogeneties, etc. 
Prior art solutions to these problems require a high degree of accuracy in 
the optical components used (typically fabrication accuracy to .lambda./10 
or better), and mechanically stable oscillator cavities with low Fresnel 
numbers (typically a.sup.2 .lambda.L.about.1). A mode-selecting aperture 
is conventionally used to select the lowest order transverse mode when 
optimum spatial coherence and beam propagation are desired. Accurate 
alignment of the focusing elements, such as the cavity mirrors, aperture, 
and the like is critical in the conventional laser resonator. A large mode 
diameter is generally desirable to achieve efficient extraction of laser 
energy using conventional plane or curved mirror laser resonators. This 
can be achieved only at the expense of even more stringent optical 
quality, alignment and lasing medium uniformity. 
Another approach for producing a large mode diameter while providing better 
performance involves the use of a spatial filter. This requires placing 
two lenses and a pinhole aperture within the laser cavity in the beam 
path. However, the disadvantages of this approach include additional 
elements which must be aligned, the same great sensitivity to optical 
aberrations of the medium or the optical elements, and the resultant loss 
of power upon the aperture. An additional difficulty is that high power 
operation is precluded by laserinduced breakdown at the aperture due to 
the presence of a tightly focused beam and high power density. 
Prior attempts to correct unavoidable aberrations of the medium or optics 
have utilized a correction device external to the laser cavity. Two 
examples are the mechanically deformable mirror described in U.S. Pat. No. 
3,731,103, and the technique of U.S. Pat. No. 4,005,935. 
More recently several attempts have succeeded in correcting phase front 
distortions in a laser cavity by using the mechanically deformable mirror 
inside a laser cavity. This technique is described in "Experimental 
Studies of Adaptive Laser Resonator Techniques", R. R. Stevens and R. C. 
Lind, with anticipated publication in Optics Letters, and "Adaptive Laser 
Resonator", R. H. Freeman et al, Opt. Lett., Vol. 2, No. 3, March 1978. 
Drawbacks of this type of system include slow response times, need for 
external beam sampling to provide a feedback loop for the mechanical 
mirror servo system, and general system complexity resulting in high 
system cost and lower reliability. 
One solution to the above problem is the use of a nonlinear phase 
conjugation device called a stimulated Brillouin scattering (SBS) device 
within the laser resonator cavity. The SBS device corrects for distortions 
in the wavefronts of the laser beam by reflecting the complex phase 
conjugate image of the distorted incident optical wavefront. When the 
reflected wave encounters the abnormality which initially caused the 
distortion, because it is the phase conjugate image of the distorted wave, 
it interacts with the abnormalities to form a plane wave. But each 
reflection off of the SBS device shifts the wavelength at the reflected 
signal as a consequence of the moving grating. This frequency shift is 
typically hundreds of megahertz and accumulates progressively with each 
reflection off of the SBS device. As a consequence, within a few 
iterations the frequency of the reflected wave "walks away" from the gain 
region. Bandwidth of the laser medium and efficient energy extraction 
comes to a halt. Thus, a long-pulse mode requiring many successive 
reflections becomes very difficult to obtain, if not impossible. 
These drawbacks have motivated a search for an improved laser resonator 
having an SBS device therein. 
SUMMARY OF THE INVENTION 
The present invention is directed toward providing an improved 
phase-conjugate resonator in which the above undesirable characteristics 
are minimized or eliminated. 
The present invention employs two moving grating devices as feedback 
mirrors at both ends of the resonator together with a spatial filter, an 
output coupler, and a laser gain medium. In particular, at least one of 
the moving gratings is a Brillouin (SBS) mirror and in some arrangements 
both gratings may be. 
It is therefore one object of the present invention to provide for an 
improved phase-conjugate resonator that eliminates doppler walkoff. 
Another object of the present invention is to provide for a resonator that 
can operate in a long-pulse mode without doppler walkoff. 
These and many other objects and advantages of the present invention will 
be readily apparent to one skilled in the pertinent art from the following 
detailed description of a preferred embodiment of the invention and 
related drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
The multipass character of laser resonators greatly enhances the distortion 
potential of intracavity perturbations. Similarly, the effects of mirror 
alignments is greatly accentuated compared to extracavity optical systems. 
Further, the combined resulting intracavity errors may sometimes have a 
very high spatial frequency content. Nonlinear phase conjugation offers 
excellent capability for correcting spatial frequency distortion as 
contrasted with conventional adaptive optics with deformable mirrors. 
Referring to FIG. 1, improved laser resonator 10 as shown has therein a 
laser gain means 12, a left-hand (LH) grating means 14, a spatial filter 
16, a start-up oscillator 18, an output coupler 20, and a right-hand (RH) 
grating means such as a stimulated Brillouin scattering (SBS) mirror 22, 
not shown in any greater detail. The support structure to hold the above 
components is not shown. 
Improved laser resonator 10 employs conventional lasing material in laser 
gain means 12. Excitation of laser gain means 12 is also by conventional 
means, not shown, such as an RF source, a flashlamp, etc. 
Output coupler 20 is shown as a beamsplitter, but other means are available 
to remove the laser energy from resonator 10. Start-up oscillator 18 
initiates a pulse of laser energy from resonator 10 and is coupled into 
laser gain means 12 by a beamsplitter 24. Spatial filter 16 is positioned 
in the optical axis near grating means 14, and allows only the fundamental 
mode of laser gain means 12 to pass through resonator 10. 
As noted above, the use of only one SBS mirror 22 in resonator 10 for 
distortion correction leaves the problem of frequency walkoff as repeated 
iterations occur within resonator 10. This, of course, leads to a lower 
efficiency and eventually causes resonator 10 to stop operating. This 
doppler shift is of about 500 MHz at 1.3 .mu.m for a gaseous medium in 
laser gain means 12. 
This frequency walkoff can be eliminated by employing a moving grating 
means 14 opposite SBS mirror 22 in resonator 10. Grating means 14 should 
have the same spacing and velocity as SBS mirror 22. If SBS mirror 22 is 
moving to the right, the frequency of a reflected laser beam 26 is 
decreased an amount, .DELTA., equal to the doppler shift, and the 
resulting frequency is equal to .omega..sub.o -.DELTA.. If SBS mirror is 
moving to the left, the frequency is .omega..sub.o +.DELTA.. Because 
lasers have a gain bandwidth normally less than 2.DELTA., the amount of 
gain would be unequal in the two directions. To achieve greater 
efficiency, resonator 10 is designed so that the reflected laser beam 26 
frequency of .omega..sub.o -.DELTA. is in the center of the gain-frequency 
profile of laser gain means 12. This selection allows only pulses to occur 
when both LH grating means 14 and SBS mirror 22 are moving to the right. 
Clearly, the opposite direction is possible if the center frequency of 
gain means 12 is changed. Further, if one can decrease the doppler shift, 
.DELTA., to be within the gain bandwidth of gain means 12 an increase in 
the bandwidth operation can occur in both directions. LH grating means 14 
can be an acousto-optic cell synchronously driven with SBS mirror 22. 
Another embodiment is shown in FIG. 2 where the LH grating means 14 is 
combined with SBS mirror 22 so that SBS mirror 22 induced grating from the 
backside acts as a surrogate left-hand grating means 14. 
In order not to form a double grating, incident power going to right onto 
double SBS mirror 28, P.sub.r, must be greater than incident power going 
to the left onto double SBS mirror 28, P.sub.L by the following factor: 
EQU P.sub.r &gt;5 P.sub.L (1) 
P.sub.r must exceed the SBS threshold and P.sub.L must fall well below it. 
If P.sub.L exceeds the SBS threshold, then a second grating is formed 
which advances away from the incident field thus producing a frequency 
shift in the wrong direction. 
Given that P.sub.r must exceed P.sub.L, a residual transmission of a 
P.sub.r beam could dominate the reflected LH beam 36, and since such a 
feedthrough does not have the appropriate frequency shift for doppler 
compensation, the transmitted P.sub.r is eliminated by polarization 
rotation means of polarizer filter 32 and .lambda./2 plate 30. 
A short pulse, about 10 nsec, is used to initiate resonator 10 on a double 
SBS mirror resonator 38. This pulse must cause the initial P.sub.r to 
exceed the SBS threshold. 
A wide-bandwidth, low-power amplifier 54 may be located in LH optical train 
34 to provide additional gain for rapid start-up with pulse operation of 
the system. 
Double SBS mirror resonator 38 is a low average power system, but higher 
power is obtainable if reflective optics and grating equivalents are used. 
Clearly, with short-focal-length pinhole systems which were employed in 
FIGS. 1 and 2, a stop 40 would not survive operation at high power levels 
and air breakdown at the focus would be a distinct possibility. The 
approach to be discussed employs two elements. First, the f/number of 
spatial filter 16 is kept high by employing small diameter optics 
(.about.0.5 cm) and long (folded) paths (.about.20 m). Stop 40 is then 
essentially in the far field of optics and diameters become about the same 
size (.about.0.3 cm) as the input/output optics, which equalizes the power 
density loading. Second, the average power density loadings on these 
optical elements may be further lowered by using a rotating pinhole system 
42. The use of a large f/number system gives a large depth of focus which 
permits a pinhole 44, FIG. 3, to be formed by the intersection of two 
annular slots 46 and 48 on two rotating wheels 50 and 52, respectively. 
Appropriate drive means, not shown, are attached to wheels 50 and 52. 
Clearly, many modifications and variations of the present invention are 
possible in light of the above teachings and it is therefore understood, 
that within the inventive scope of the inventive concept, the invention 
may be practiced otherwise than specifically claimed.