Multiline coherent oscillation in photorefractive crystals with two species of movable carriers

The coherent oscillation, because of nearly degenerate four-wave mixing in photorefractive crystals with two types of movable charge carriers, occurs at two spectral lines symmetrically shifted with respect to the pump freque ncy. Consequently the output oscillation exhibits the high contrast intensi ty modulation. The frequency separation of two oscillation modes land modul ation frequency of the output intensity) depend on the incident light inten sity and spatial frequency of the developing grating. A model is presented explaining this typo of oscillation by the two-maxima shape of the gain spe ctrum in crystals with sufficiently different relaxation times of two space -charge gratings, one formed by movable electrons and the other one by mova ble holes. The experimental data for coherent oscillator with tin hypothiod iphosphate (Sn2P2S6) an in reasonable quantitative agreement with the calcu lations.