Refractive index decrease phenomena in SiO2–Ta2O5 waveguide films by CO2 laser irradiation

SiO2-Ta2O5 waveguide film exhibits a refractive index decrease up to 2% and a simultaneous film thickness increase proportional to the index decrease by CO2 laser irradiation. A partial restoration to the pre-irradiated state is observed by annealing in air at temperatures between 700 and 780 °C. The origin of the index decrease may be due to the density change with Ta2O5 crystallization. For application to the optical circuit design, the effective refractive indices of the index-decreased area are estimated, and a TE0-TE1 mode separation experiment by a 20°angle is done successfully, in order to examine the usefulness of the effective index.