Distortions of transfer functions of an acousto-optic spatial filter by a decrease in the sound frequency

We examine the features of the formation of the transfer functions of an acousto-optic spatial filter made of a uniaxial gyrotropic crystal operating in tangential diffraction geometry. It is shown that a decrease in the sound frequency is accompanied by distortions of the transfer functions on the side facing the optical axis of the crystal. The distortions are caused by the influence of the second diffraction order on the distributions of axisymmetric transfer functions. The cutoff frequency of sound below which the transfer functions become practically unsuitable for two-dimensional processing of optical images is determined. It is shown theoretically and experimentally that the cutoff frequency of sound for processing a two-dimensional image with a wavelength of 0.63 μm using a spatial frequency filter made of ТеО₂ is ∼40 MHz.