PHOTOINDUCED ABSORPTION CHANGE IN GERMANOSILICATE PREFORMS - EVIDENCE FOR THE COLOR-CENTER MODEL OF PHOTOSENSITIVITY

A new technique has enabled us to resolve the strong UV absorption spectrum in UV-exposed germanosilicate preforms. Our results show, for the first time to our knowledge, that the large photoinduced absorption changes between 165 and 300 nm (similar to 700 dB/mm at 195 nm has been observed) can account for similar to 3 x 10(-4) of index change at 1.5 mu m The thermal-annealing dynamics of the photoinduced absorption changes also resemble those of fiber gratings, evidence that the color-center model of photosensitivity plays an important role in the photoinduced index changes in UV-written fiber gratings. We also find that the photoinduced absorption changes depend linearly on germania (i.e., germanium dioxide) concentrations in the preforms, and their exposure-time dependence can be fitted with a stretched-exponential function.