Two qualitatively different modes of operation are possible when the active feedback is used to optimize the grating recording in photorefractive crystals with local response. For light-induced changes of the sample optical thickness much less than optical wavelength, the feedback introduces a linear in time phase shift into one of the recording beams, i.e., leads to a non-degeneracy of two-wave mixing. For large light-induced changes the periodic in time phase variation is superimposed on the linear growth. The transition from one mode of operation to the other is shown to be similar to phase transitions, with a critical slowing down of the periodic phase variations.

© 2001 Optical Society of America

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