Self-alignment of Ge nano-particles in laser induced Bragg grating in Ge–B–SiO2 film

Thermally stabilized photoinduced channel waveguides with Bragg gratings were fabricated in Ge–B–SiO2 thin glass films using the site-selective precipitation technique of Ge nanoparticles. Refractive index of the film increased after annealing longer than 10 min at 600 °C. Such increase of refractive index seems to be responsible for the precipitation of Ge nanoparticles in the film. However, irradiation prior to annealing suppressed the increase of refractive index. Consequently, the annealing reversed the photoinduced refractive index pattern and also enhanced its thermal stability. The stabilized channel waveguide with a Bragg grating showed diffraction efficiency of 18.0 and 18.7 dB for TE- and TM-like modes, respectively. The diffraction efficiencies and wavelengths for both modes never changed after heat treatment at 500 °C, whereas the conventional photoinduced grating decayed even at 200 °C.