An analysis of BAW to STW mutual transformation arising from periodic corrugation on the surface of a reciprocal coupler

In this paper we present a theoretical analysis of BAW to STW transformation occuring in a reciprocal coupler consisting of periodic corrugations on the free surface of a singly-rotated cut of a class-32 crystal. The analysis is based on the coupling-of-modes model. We consider horizontal shear waves travelling in the cross-section of such a cut that contains Z´ direction. In the considered frequency range, SSBW wavelength along Z´ is close to the period of corrugations. Boundary conditions in the coupler´s region are projected onto the average flat plane of the free surface with means of Brekhovskikh´s expansion, and the coupled solutions are sought as series of Bloch´s partial waves. Since a previous paper of ours (1995), significant advances have been made in the treatment of the dispersion equation. We especially point out the necessity of taking at least five coupled modes to obtain a determinantal equation that actually vanishes. The problem of STWs launched by BAWs falling under normal incidence onto the corrugated surface is treated for a coupler of finite length