The effect of dopant diffusion on modal birefringence in silica-on-silicon waveguides

Polarization-multiplexed optical signals require that the birefringence of optical devices be controlled. Birefringence in silica-on-silicon waveguides emerges from a combination of form birefringence and stress birefringence, both of which can be affected by the core dopant distribution in the case of rectangular-core waveguides. In this paper, we present a numerical and experimental study of the effect of dopant diffusion on waveguide modal birefringence. In the numerical study, modal birefringence is calculated with a finite-element model that includes thermal stress effects. The effect of diffusion on form birefringence and stress birefringence will be illustrated. It will be shown that the initial index step of the waveguide has an influence on the evolution of the modal birefringence as a function of diffusion. In the experimental study, measurements of the phase and group birefringence will be presented as a function of waveguide width for waveguides with thermally diffused cores. It will be shown that thermal diffusion can be used for birefringence control of rectangular waveguides.