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

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Abstract
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.