Local Kramers-Kronig analysis of UV induced absorption changes: Dynamics of excess loss induced by exposure to light at 248 nm or 193 nm in Ge:SiO2 preforms.

It is well known that exposure of a germanosilicate glass to light at 248 nm or 193 nm leads to a permanent change in the refractive index and in the absorption spectra at wavelength shorter than 350 nm. The changes in absorption spectra are assumed to result either from the creation of point defects (color center model)1,2 and/or from light induced structure changes in the glass (densification model)3,4. A Kramers-Kronig (KK) transformation of the changes in absorption enables one to determine the related changes in refractive index which in turn can be compared with those measured at the time of a grating inscription. However using this process is rather tricky since ideally the comparison requires that when writing a grating within a fiber, one measures the UV induced excess loss spectrum at each exposure time and at each place along the grating wavevector on the spectral range within which the absorption changes are observed. As the above mentioned procedure cannot be strictly followed, we suggested a simplified method5 for performing the KK analysis. We use this method to compare the index changes which can be ascribed to absorption changes occurring in the spectral range (220 - 350 nm) at the time of a grating inscription through exposure of a germanosilicate fiber either to a fringe pattern at 193 nm or 248 nm.