Design of far-infrared acousto-optic tunable filter based on backward collinear interaction

•Backward collinear diffraction allows acousto-optic filters operating in far infrared.•Designed acousto-optic filter based on acoustic mode transformation in KRS-5 crystal.•KRS-5 backward collinear filter can provide far-infrared image processing. The paper proposes a design of acousto-optic cell applying backward collinear interaction and acoustic mode transformation in a KRS-5 crystal. This cell may serve as an acousto-optic tunable filter for far-infrared spectral range and is able to operate both with collimated optical beams and with divergent beams forming images. The problem of acoustic mode transformation by wave reflection from the crystal facet away from symmetry planes has been solved. Polarization properties of the backward collinear interaction in optically isotropic media are discussed.