Theoretical and experimental analysis of the near-Bragg acousto-optic effect

A spatial Fourier transform approach is employed to investigate the acousto-optic interaction in the near-Bragg regime with cw planar sound and a light beam of arbitrary spatial profile. Four coupled differential equations that can exactly describe the near-Bragg acoustooptic interaction are derived from Maxwell's equations. A spatial Fourier transform approach is applied to the coupled-wave equations to express the interaction in the spatial frequency domain. The spatial Fourier transform approach used here is identical to the technique for solving the paraxial wave equation to derive the transfer function of propagation and hence the Fresnel diffraction formula during free-space propagation of a light beam in the presence of diffraction. Since there are no analytic solutions for these coupled equations, a well-known numerical method, viz., the Runge-Kutta-Fehlberg, is used to solve the equations. Detailed numerical simulation is performed, which involves spatially Fourier-transforming the input light beam profile to calculate the spectra of the diffracted light beam and hence their profiles in the space, using the inverse transform. Experimental results are also presented to check the validity of our approach. (C) 1998 Society of Photo-Optical Instrumentation Engineers. [S0091-3286(98)03601-0].