Theory of multiple-beam interaction in photorefractive media

We investigate theoretically the interaction of two or more beams intersecting at a small angle in a photorefractive crystal with a local type of holographic recording. All necessary diffraction orders are taken into account. It is shown that for photorefractive ferroelectrics the number of diffracted beams can be quite large. For negative nonlinearlity the high exponential gain previously predicted for three- or four-wave mixing does not exist if all diffraction orders are considered.