Modulation transfer function of photorefractive phase conjugators for two mutually coherent input beams

We investigate theoretically and experimentally the modulation transfer function of a photorefractive phase conjugator for two mutually coherent input beams. The two coherent beams are incident on a crystal and undergo stimulated photorefractive backscattering in different regions, in which the phase conjugations are generated. Then the four waves present in the crystal interact. Due to the unreciprocal energy coupling between different spatial-frequency waves and the directional dependence of the two-wave mixing gain, the output modulation exhibits two values corresponding to one input modulation. The output modulation depends on the input beam ratio, on the backscattering coefficient of the imperfection on the surface of the crystal, and on the crossing angle of the two input beams.