Coherent oscillation in photorefractive crystal with two types of movable charge species

Summary form only given. Tin hypothiodiphosphate (Sn/sub 2/P/sub 2/S/sub 6/) is a promising photorefractive material for red and near-infrared spectral range with the gain factor more than 10 cm/sup -1/ and the millisecond response time. We report on particular behaviour of the coherent ring-loop oscillator with this crystal pumped by He-Ne laser. With no special measures the oscillation occurs with two roughly symmetric temporal frequencies and therefore the output oscillation wave exhibits the high contrast intensity modulation. The reason of this unusual feature is in special shape of the gain spectrum itself, typical for the crystals with two out-of-phase photorefractive gratings formed by the charge carriers of different sign.