Orthogonal shear acoustic mode suppression in gallium phosphide acousto-optic Bragg cells

Excitation of the orthogonal mode in gallium phosphide leads to unwanted acousto-optic diffraction and subsequent reduction in the spurious free dynamic range of an acousto-optic device fabricated with this material. Both transducer selection and orientation with respect to the acoustooptic crystal are critical in suppressing the unwanted slow shear mode. An optimum lithium niobate transducer cut is described, and the slow shear mode suppression is calculated as a function of transducer misorientation. An alternative optical propagation direction is examined, where the coupling to the slow shear mode vanishes at the expense of increased acoustic beam divergence. The negative phase front curvature of this mode is discussed