Application of photosensitive GeO 2 -B 2 O 3 -SiO 2 thin films to optical waveguide

Abstract

GeO<SUB>2</SUB>-B<SUB>2</SUB>-O<SUB>3</SUB>-SiO<SUB>2</SUB> thin films were fabricated by plasma enhanced chemical vapor deposition method. Boron codoping into a GeO<SUB>2</SUB>-SiO<SUB>2</SUB> thin film induced large absorption in the vicinity of 240nm, and OH absorption decreased compared to GeO<SUB>2</SUB>-SiO<SUB>2</SUB> films. These films of 5 micrometers in thickness exhibited large positive refractive index change without hydrogen loading by irradiation with ArF (193nm) excimer laser pulses. Induced refractive index change was approximately 0.002 which was measured by the prism coupling method. A waveguide was written in this high photosensitive glass film by UV irradiation. The guided mode of the waveguide seems to be single and estimated refractive index change was approximately from 0.003 to 0.004. Three unique phenomena were found in 0.2micrometers thick films on Si substrate. First, these films exhibited large negative refractive index and positive thickness changes by irradiation with ArF laser pulses. Induced negative index change was larger than 0.02 and thickness change was more than 1%. Silica films doped only boron or germanium didn't exhibit such negative index changes. Second, the annealing before laser irradiation decreased the photosensitivity of these films remarkably. Third, these induce refractive index and thickness changes were decreased with time rapidly. These mechanisms were under investigation.