The temporal evolution of the beam coupling in a nominally pure, as grown KNbO3 single crystal, at both low and high modulation depths, without any applied electric field, at 514 nm in the two wave mixing configuration was measured. A complete investigation of the exponential gain in these two regimes as a function of grating spacing also was performed. The exponential gain measured during the grating formation always was negative, occurring an amplification of the reference beam at the expense of the signal beam. Some parameters of the crystal such as the electro-optic coefficient, the sign of dominant photocarriers and the empty acceptor density were calculated from the beam coupling experiments.

© 2001 Optical Society of America

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