Modal Birefringence Analysis of Strained Buried-Core Waveguides

A semianalytical study of modal birefringence in weakly strained and weakly guiding buried-core waveguides is presented. In this paper, modal birefringence is expressed as a sum of form birefringence and stress birefringence. Stress birefringence is expressed as an overlap between the modal field and the stress birefringence distribution. This distribution is expressed analytically as two constant-value areas to which a spatially variant stress distribution emanating from the waveguide core is overlaid. The analysis can be expanded to include the effect of overetching and other variations that can be defined as mechanical inclusions in the cladding layer. Expressions for modal birefringence control as a function of material parameters are given, for the case of square, rectangular, and overetched waveguides. The conditions for width-independent birefringence control are identified. Modal birefringence values obtained from the analysis are compared to those obtained by finite-element analysis for the case of square, rectangular, and overetched waveguides. The modal birefringence values are shown to be within 5% of each other for a typical silica-on-silicon waveguide.