Regime independent coupled wave equations in anisotropic photorefractive media

In thin photorefractive materials beam coupling can be observed in the Bragg, Raman-Nath and intermediate regimes, dependent on the geometrical properties of the incident optical beams. By varying the grating spacing, achieved by changing the angular separation of the beams which create the grating, it is possible to move from Raman-Nath to Bragg regime. Application of simple Bragg or Raman-Nath models to a system in the intermediate regime could potentially lead to the derivation of incorrect coupling coefficients.