Refractive index changes in Ge-doped silica glass induced by a KrF excimer laser irradiation

Refractive-index changes (Δn) in Ge-doped silica glass exposed to KrF excimer laser were studied by the measurements of the diffraction efficiency from a grating formed using a phase mask. The Δn of 3.2 × 10-3 was observed for a hydrogentreated (150 atm, 2 weeks, room temperature) sample after a KrF laser irradiation (25 Hz, 0.5 J/cm2/pulse, 27 kJ/cm2), while the Δn obtained for a non-treated sample is less than 10−4. The depth profile of the Δn for the hydrogen-treated sample is limited by the distribution in the hydrogen concentration inside the sample, while it is determined by the attenuation of the ultraviolet light for the non-treated sample. Ultraviolet (UV) optical absorption measurements show that the decrease at a 5.1 eV band and the increase at ~6 eV are stronger in the hydrogen treated sample than in the non-treated one. Correlation between the observed refractive index change and UV absorption change will be discussed. Also, the role of hydrogen on the enhanced photosensitivity will be discussed.