Acousto-Optics Department of VNIIFTRI  
  Basic Principles

The acousto-optic tunable filter (AOTF) is a solid-state electronically tunable spectral bandpass filter, see Figure 1, which operates on the principle of acousto-optic interaction in an anisotropic medium. The AOTF has recently reached technological maturity, moving from the research laboratory to the commercial environment. The AOTF utilizes an anisotropic, birefringent AO medium for its operation. The AOTF has a relatively long acoustic interaction length, required to achieve a narrow spectral bandwidth. That one can only be achieved for a certain momentum-matching configuration, i.e., the group velocity for the extraordinary wave should be collinear with the ordinary wave. It is shown on the Figure 2, where the momentum-matching vectors represent the phase velocities of the incident light k, diffracted light kd, and acoustic waves ka. In this geometry, the tangents for the incident and diffracted light beams are parallel to each other. Note that the two optical beams do not separate until the light propogate inside the AO medium. This long coherent buildup of the diffracted beam would be only partially beneficial to the AOTF. To a first order, any momentum mismatch, due to the light arriving at some angle from this ideal condition, is compensated by a change in the birefringence for this orientation, and thus the AOTF displays its most important characteristic: a large angle of view, which is unique among AO devices. The tuning approximate dependence of AOTF:

F = C · Δ n / λ,
F - frequency of the RF signal; Δ n = ne - no ; ne , no - refraction indexes for extraordinary and ordinary rays; C - is the constant; λ - light wavelength in vacuum corresponding to the center of the spectral band transmission for AOTF.
© Acousto-Optics Department of VNIIFTRI, 2002-2023

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