Reduction of light-induced refractive-index changes by decreased modulation of light patterns in photorefractive crystals

In holographic experiments many photorefractive crystals show refractive-in dex changes much smaller than one would expect from the known electro-optic coefficients and space-charge fields. We show that degradation of the inte rference pattern is the origin of this effect. The degree of modulation of a light grating in photorefractive crystals is measured by three methods an d compared with the modulation of the grating of the incident light. All me thods, measurement of the amplitudes of fundamental and second-order gratin gs, of the grating amplitudes in a crystal with and without another crystal in front of it, and of the drift currents through an inhomogeneously illum inated sample, yield consistent results: In lithium niobate there is almost no degradation of the light pattern. However, in our barium titanate and p otassium tantalate-niobate samples the degree of modulation is smaller, and the reduction factor is 0.55 and 0.60, respectively. Inhomogeneities of re fractive-index changes are shown to be the origin of the effect. (C) 1998 O ptical Society of America. [S0740-3224(98)01512-4]. OCIS codes: 100.2650, 1 60.2260, 160.5320, 210.4810.