Phase conjugation, holography, and imaging through inhomogeneous media

Holography may be carried out in a wide a variety of materials and devices, such as photographic film, photopolymers, CCD cameras and nonlinear crystals. The commonality of these is the nonlinear characteristic that results in the mixing of two light beams to produce a difference term. The bases for the holographic methods to image through inhomogeneities are varied. The basis may be the generation of a conjugate field, as in holographic phase conjugation. It may be the time averaging effect of the holographic process. It may be the gating effect of short pulse light, or equivalently, the temporal coherence function. Or, it may be the light source spatial coherence function. We offer examples for each category