Active feedback controlled beam-coupling in tin hypothiodiphosphate

摘要

The first active feedback experiments are performed with photorefractive Sn_2P_2S_6, characterized by inhibition of the steady-state gain factor because of superposition of two out-of-phase space-charge gratings (formed by movable charge carriers of different sign). The active feedback allows to suppress the compensating grating and to keep the steady-state gain factor as large as the transient one. One way to achieve this goal is to use the second harmonic of the modulation frequency to generate an error signal and to choose the modulation frequency larger than the reciprocal relaxation time of the fast grating but smaller than that of the slow grating.