Photo- and Thermo-Induced Refractive Index Change of GeO_2-B_2O_3-SiO_2 Thin Films Fabricated by PECVD - Fabrication of extremely thermally stabilized gratings -

摘要

We found out that GeO_2-B_2O_3-SiQ_2 thin films fabricated by plasma enhanced chemical vapor deposition method exhibited not only large photo-induced but also thermo-induced refractive index increases, both of which were above 10~(-3). The former was observed after irradiation with KrF excimer laser, and the latter was induced by annealing at 600°C. The thermo-induced refractive index increase was closely related to the formation of thermo-induced absorption bands during the annealing, and could be suppressed by the laser irradiation prior to the annealing. Bragg gratings were printed in the films by the laser irradiation through the phase mask without H_2 loading. The diffraction efficiency decreased rapidly by the annealing up to 500°C, but drastically increased after the annealing at 600°C. The thermo-induced gratings couldn't be erased by the repeated heat treatments between room temperature and 600°C at all. Considering the suppression of thermo-induced index increase by the laser irradiation, this grating was expected to have the reverse pattern of refractive index compared to that of the as-printed one, and might be applicable to the highly reliable optical and sensing devices.