Abstract

This paper reports a novel ray optic analysis of scattering and absorption in thin-film waveguides. The ray optic approach, similar to the one used for grating couplers,¹ enables us to investigate the propagation of a guided mode in a waveguide with random surface roughness, random inhomogeneity of the film permittivity, and absorbing film and cladding regions in a relatively simple way. The extinction coefficient is obtained by following a zigzagging ray in the film. Over the path between the interfaces a film absorption coefficient and a lumped volume scattering coefficient are multiplied by the distance traveled between the interfaces. At the interfaces, the power amplitude is multiplied by an exponential power reflection coefficient. This coefficient is calculated by adding perturbation contributions due to different loss mechanisms to the reflection coefficients of an unperturbed waveguide. The extinction coefficient is calculated by dividing the cumulative attenuation by the bounce distance. Scattering losses show much stronger wavelength dependence than the absorption losses. The surface scattering results in strongest attenuation. Both absorption and scattering are maximum for TM higher-order modes.

© 1986 Optical Society of America