TEM CHARACTERIZATION OF STRUCTURAL-CHANGES IN GLASS ASSOCIATED TO BRAGG GRATING INSCRIPTION IN A GERMANOSILICATE OPTICAL-FIBER PREFORM

Bragg gratings were written in a slice of an optical fibre preform using two interfering uv pulsed laser beams at 243 nm. Microstructural changes within the germanosilicate glass were studied by transmission electron microscopy (TEM) and infrared spectroscopy. TEM investigation shows that the glass is densified at the grating grooves. Infrared absorption spectra have been recorded on a germanosilicate glass sample before and after an irradiation by a fringeless uv pulsed beam. Our conclusion is that uv irradiation-induced bond breaking allows structural relaxation of the non-equilibrium glass network which tends to densify without any noticeable change in coordination.