Most of the conventional studies of acousto-optics (AO) are under the paraxial approximation; that is, the propagation directions of the scattered light are almost along the optic axis of the AO system, or, equivalently, the envelope of the light wave varies slowly with respect to the direction of propagation. This assumption should not, however, be carried too far in the cases in which the incident or the Bragg angles are large enough that the scattered light waves do not propagate closely along the optic axis of the AO system. We study the Bragg AO effect beyond the paraxial assumption for small- and large-Bragg-angle incidence. Starting from the wave equations, which are derived from Maxwell's equations, a set of coupled equations that depict the AO interaction are derived in the Bragg regime without the paraxial assumption; i.e., the second derivative terms of the scattered-light amplitudes with respect to the propagation direction are nonnegligible. Analytic solutions that describe the evolution of the scattered light that is due to the acousto-optic effect beyond the paraxial approximation can then be found from the coupled equations. Simulation results are provided to check the validity of our solutions.
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