Theoretical and experimental studies of the acoustically tuned optical filter show that suitable crystal orientations can be divided into two broad classes based on the acoustic properties of the propagation path used. In acoustically isotropic cases, the acoustic phase and group velocities are collinear. All 32 crystalline point groups are examined for acoustically isotropic orientations that may be useful in tunable optical filters. In acoustically anisotropic cases, the phase and group velocities are not collinear. Methods are presented with which the effects caused by acoustic divergence in these cases may be compensated by similar effects due to changes of birefringence. In particular, anisotropic propagation collinear with acoustic group velocity on a path 101.2° from the optic axis in the Y, Z plane of quartz allows operation at a lower acoustic frequency and power level than that required by isotropic orientations for equivalent performance.
J. A. Kusters, D. A. Wilson, and D. L. Hammond, "Optimum crystal orientation for acoustically tuned optical filters," J. Opt. Soc. Am. 64, 434-440 (1974)