Kramers-Kronig analysis of the absorption change in fiber gratings

We apply a Kramers-Kronig model to several published experimental studies of photosensitivity in Ge-doped silica to calculate the predicted index change Delta n(KK) from the measured change in the absorption spectrum (induced by UV, flame brushing and ion implantation). In most cases Delta n(KK) is close to the measured index change Delta n(exp). Most of Delta n(KK) (similar to 50%-95%) arises from absorption changes between 165 and similar to 195 nm. In the few cases where Delta n(KK) is much smaller than Delta n(exp), the absorption spectrum was not measured below 200 nm, which is probably the reason for the discrepancy. Based on several observations, we also make the hypothesis that in the majority of our study population, color centers are probably responsible for most of the observed absorption change, and thus of the index change.