Photoviscous flow and glass macroproperties

Abstract

The interest in glass photosensitivity has been renewed due to the importance of photosensitivity phenomena in potical fibers devices which promise a host of novel applications. Among these developments, one is the ability to imprint periodic and permanent refractive-index changes (Bragg gratings) on monomode germanosilicate optical fibers using excimer lasers. Defect formation in the structure of the glass core mainly germanium GODC (germanium-related oxygen-deficient centers) defects are supposed to be responsible for the grating formation. Here the phenomena are analyzed under another perspective that is related with high temperature gradient inside monomode optical fibers and its consequences. Because temperatures much higher than the germanium silicate glass softening point could be achieved during exposure to excimer lasers, the formation of Bragg gratings is a two step process to modify vitreous media. First, UV energy should be absorbed at the fiber core - by defects, dopants and glass components - and a second step where this energy is thermalised. There is no ablation in the glass core and energy transfer is governed by viscous flow or photoviscous flow and internal structural rearrangements. Alternatives to enhance optical fiber glass photosensitivity via glass composition and viscous behaviour are discussed.