Optical phase conjugation (PC) is one of the most important phenomena observed in photorefractive (PR) materials.L. Solymar, D. Webb, A. Grunnet-Jepsen, "The physics and applications of photorefractive materials" (Clarendon Press: Oxford, 1996), O. Ostroverkhova and W. E. Moerner, Chem. Rev. 104, 3267 (2004) The potential applications of PC include the transmission of undistorted images through optical fibers (or the atmosphere), lensless imaging down to submicrometer-size resolution, optical tracking of objects, phase locking of lasers, refreshing of holograms for long-term optical storage, optical interferometry, beam clean-up, and image processing.
Example of using PC for the optical beam clean-up is illustrated on the left. The signal beam which contains information about the object is combined with the reference beam in a PR material, and volume hologram of the object is recorded. If the signal beam went through an aberrator (which would correspond to the situation when the object has to be imaged in the medium with turbulence, refractive index inhomogeneities, etc.), then the image would be heavily distorted (Detector 1 in Step 1). If, however, a reading beam counterpropagating to the reference beam is introduced, it generates the PC replica of the signal beam, which, upon passing through the aberrator, creates a cleaned-up image of the object, recorded by the Detector 2 (Step 2). Thus, the distortion is corrected using PC in the PR polymer film.G. Li et al., Appl. Phys. Lett. 86, 161103 (2005)