The UV-induced refractive index grating in Ge: preforms: additional CW experiments and the macroscopic origin of the change in index

Bragg gratings have been written within MCVD germanosilicate preform slices using CW laser light at 244 nm. Afterwards, scanning light interferometry was used to perform a 3D imaging analysis of the sample surface topography. The results show that the same process (namely densification) pointed out in a previous work in which we used UV pulsed exposure of preform slices is also active here. A simplified model which assumes that the UV-induced periodic corrugations result from both permanent and elastic strains, is presented. This model accounts not only for a large part of the measured refractive index change but also for the localization of the UV-induced birefringence along the grating wavevector. It also gives a satisfactory interpretation of the observations made by Fonjallaz et al, according to whom grating inscription within a germanosilicate fibre increased the axial tensile core stress.