Non-collinear acousto-optical tunable filter configurations in biaxial crystals

A novel formulation of acoustooptic interactions is presented for predicting the Bragg angles of noncollinear acoustooptic tunable filter (AOTF) configurations in biaxial crystals. The Bragg angles of AOTF configurations in which the interaction plane contains both optical axes are determined explicitly and their dependence with respect to the refractive indices and the acoustic-wave propagation direction are treated analytically. It is shown that, in biaxial crystals, up to four noncollinear AOTF configurations can be obtained for certain directions of propagation of the acoustic wave; for any direction of propagation of the acoustic wave, at least two noncollinear configurations are always allowed. There are major differences between AOTF features obtained in biaxial crystals and those obtained in uniaxial crystals. The Bragg angles of noncollinear AOTF configurations in Tl₃AsS₄ are determined as an illustrative example. Tl₃AsS₄ is biaxial crystal that yields a very high acoustooptic figure of merit and which is highly transparent in the 0.6-10-μm optical range. For this material the acoustooptic coefficients which may be involved in the AOTF configurations are given