A New General Form of 2-D Coupling-of-Modes Equations for Analysis of Waveguiding in Surface Acoustic Wave Devices

Since surface acoustic waves (SAWs) are composed of multiple displacement components, their slowness curves sometimes exhibit a more complex shape rather than elliptical or hyperbolic. As a result, the parabolic approximation of 2-D coupling-of-modes (2-D COM) equations is invalid for analysis of waveguiding in such SAW devices. Thence, this paper develops a new general form of 2-D COM equations based on the basic 2-D wave equations of SAWs which contain two displacement components. The fundamentals of the equations were established and the procedures for determining 2-D COM parameters were presented in detail. The new equations were embedded in the partial differential equation (PDE) mode of COMSOL for the simulation of different types of SAWs, including Rayleigh and shear-horizontal (SH) SAWs on ST-quartz and 42°YX-LiTaO3 (42-LT) substrates, respectively. By comparing the calculated results with 1-D COM, parabolic 2-D COM, extended thin plate model, 3-D finite-element method (FEM) calculated results, and the measured result, the applicability of these equations was verified.