Coherent optical gain of multistage two-wave mixing in photorefractive crystals

We derive the intensity and phase formulas of the two-, three-, and n-stage codirectional parallel-polarization degenerate two-wave mixing in photorefractive media. These formulas are valid for both the saturation and intermediate regime and can be used in investigating the problem of increasing the coherent optical gain by the multistage two- wave mixing technique.