With 35 FiguresThe concept of time-reversing an optical wave is an intriguing one. The timereversed (or phase-conjugate) version of an optical wave can be generated by nonlinear mixing in an appropriate materiM [5.1-3]. Photorefractive materials have been ideal for demonstrating the possibilities of phase conjugation, such as correcting a distorted picture, tracking a moving mirror (or a shiny hamburger spatula), scanning the frequency of a dye laser, and creating images having sub-micron resolution. Because photorefractive materials have a large nonlinearity to even weak light beams, new ideas can be tested quickly and easily with low-power lasers. In addition, the very large optical nonlinearity has produced unpredieted and desirable new effects, such as self-pumped phase conjugation.Although photorefractive materials are ideal for testing ideas and demonstrating devices, they are often less than ideal for implementing a device outside of the laboratory. No one has yet used phase conjugation in a photorefractive crystal (or in anything else!) to send an undistorted picture through the atmosphere to an orbiting satellite or through the ocean to a submerged submarine. Nor have any commercial applications yet been adopted, such as high-resolution lithography, or tracking a fast-moving satellite, or improving the spatial mode profile of a high-power laser. Photorefraetive crystals can be fragile, expensive, COmmercially unavailable, temperature sensitive, difficult to grow, exceedingly Sensitive to impurities, or distressingly slow (response times of a second) when used with low-powered (milliwatt) lasers. Some photorefractive crystals are optically active requiring careful manipulation of beam polarizations for efficient coupling; others are too absorptive or have nonlinearities that are too small. At present there is no crystal that embodies all of the desirable properties of a photorefractive material without any of the drawbacks.Nevertheless, the recent demonstrations of self-pumped phase conjugation, optical processing, coherent beam amplification, and associative memories using photorefractive crystals have focused academic and commercial interest on these materials. New effects that were not predicted have been observed, and have led to new research areas. Crystal growers are producing new and higherquality photorefractive crystals, and quite a few research laboratories are studyiag the origins and properties of the charge carriers in these crystals, with a view to understanding and improving the photorefractive effect itself. This chapter will review some of the applications of photorefractive materials, and will highlight phase conjugation, self-pumped phase conjugation, 157
