THE TRANSFER-FUNCTION AND IMPULSE-RESPONSE OF PHOTOREFRACTIVE 2-BEAM COUPLING

We derive the transfer function and impulse response of photorefractive two-beam coupling in the undepleted pump approximation. For sufficiently strong coupling GAMMA-L, the impulse response features a broad delayed output pulse. In the two interesting limits of negligible and strong absorption alpha-L, this coupling threshold reads GAMMA-L(thr) = 4 and GAMMA(thr) = 2-alpha, respectively. The time delay and pulse height are functions of the coupling GAMMA-L, the photorefractive time constant tau and the effective absorption alpha-L. Experiments on a BaTiO3 crystal measuring the absolute square of its transfer function and the impulse response are used to determine the coupling and time constant. The experimental impulse response is found to differ from the inverse Laplace transform of the transfer function. We speculate that the discrepancy is due to beam fanning.