A new approach to the problem of light scattering by surface acoustic waves in thin-film optical waveguides is presented. In this analysis electromagnetic fields are found to rigorously satisfy both the polarization driven wave equation and the electromagnetic boundary conditions at the frequencies of interest. A component of the solution field is found to evolve with propagation distance into a linearly growing guided wave mode that dominates the solutions under conditions that can be defined in terms of waveguide mode parameters. Analytical formulas for the growth coefficient are obtained for both the elasto–optic and corrugation scattering mechanisms for the geometries TEm → TEm, TEm → TMm, TMm → TMm and TMm → TEm The present results are found to be consistent with previous normal mode analyses of this phenomenon for the cases in which comparison is possible. Numerical calculations for thin films of As2S3 and Corning 7059 glass on fused silica substrates indicate that the elasto-optic effect does not always dominate the scattering cross section and that the corrugation mechanism must often be taken into account.
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