Growth kinetics and thermal annealing of UV-induced H-bearing species in hydrogen loaded germanosilicate fibre preforms

Abstract

The cores of optical fibre preforms have been exposed to UV pulsed light at 244 nm after molecular hydrogen loading under high pressure. The UV-induced changes in the sample's infrared absorption spectra have been monitored during the UV exposure by means of Fourier transform spectroscopy. The analysed spectral range spanned from 580 to 5200 cm −1. The kinetics of creation of hydroxyl, hydride and molecular water species has been investigated. The initial rate of hydride formation is similar to those corresponding to the formation of the two other species. However, the process of hydride generation is almost saturated after exposure of the glass to 10 5 pulses of UV light, while concentrations of hydroxyl and molecular water continue to grow slightly. On the other hand, the dynamics of hydroxyl or molecular water species absorption growth resulting from exposure to UV light at a fluence per pulse of 30 mJ/cm 2 is significantly slower than those at 140 mJ/cm 2. This result differs from what is observed when recording the dynamics of infrared absorption caused by the hydride since the difference between the dynamics recorded using those two fluences are scarcely significant. Periodic exposure of samples to a burst of 10 5 light pulses followed by H 2 reloading has shown that availability of molecular hydrogen plays a significant role in the growth dynamics of hydroxyls. The orders of magnitude changes in the UV-induced species concentrations have been estimated from the UV-induced infrared spectra. Thirty-minute isochronal annealing experiments reveal that the concentration of OH-bearing species is reduced by a factor of two after the step of annealing temperature at 600°C. At this step, the hydrides are almost completely bleached, whereas the molecular water concentration dramatically decreases after 200°C. The implications of these various observations are discussed in relation with the current models of photosensitivity in H 2-loaded germanosilicate glasses.