When energy exchange takes place between two waves coupled by a photorefractive phase grating in a strong nonlinear regime the light modulation does not remain constant along wave propagation and there is a spatial nonuniform grating along sample thickness. The undepleted approximation (uniform grating) is valid for short interaction lengths and or small coupling. However, for thick samples, the interaction length of the recording light beams inside the photorefractive material already is not small and therefore the approximation of uniform grating is not valid.