Contribution of photoinduced densification to refractive-index modulation in Bragg gratings written in Ge-doped silica fibers

The photosensivity phenomenom, which a basis for the fabrication of fiber gratings, is commonly ascribed to two essential physical mechanisms: creation of color centers and structural transformations. Both effects are wll established. However, there is no general agreement about which mechanism dominates in the case of type I gratings in Ge-doped silica fibers. The analysis of the color-center contribution assumes a rather straightforward application of the Kramer-Krönig relations. The challenge is to measure changes af the extinction coefficient in the deep-UV region, which was demonstrated to account for the major part of the refractive-index perturbution. On the other hand, estimation of the contribution induced by a grating inscription.