Acousto-optic collinear diffraction of a strongly divergent optical beam

Collinear diffraction of a strongly divergent optical beam by a monochromatic acoustic wave is investigated theoretically and experimentally. Three-dimensional patterns of acousto-optic (AO) phase matching are calculated and their transformation with varying acoustic frequency and ultrasound propagation direction is analysed. Two effects are discussed which appear at spectral collinear filtration of optical images, namely distortion of filtered image structure and spectral resolution deterioration. It is shown that the effects result in contradictory requirements on AO cell parameters. However, a compromise is possible which allows filtering of high-spatial-resolution images with an acceptable reduction in spectral resolution. Experimental results on diffraction of a strongly divergent He-Ne laser beam in a CaMoO4 AO cell are also presented in this paper.