Index structure of fiber Bragg gratings in Ge-SiO2 fibers

The behavior of the concentration of photoinduced color centers in Ge-SiO2 optical fibers was compared with that of the index modulation associated with fiber Bragg gratings (FBG's) written in the same fibers. We find that the fluence dependence of the photoinduced Ge E' center, its thermal annealing behavior, and its reaction with H-2 are similar to that of the index modulation generated in both H-2-loaded and unloaded Ge-SiO2 fibers. The much higher photosensitivity of H-2-loaded Ge-SiO2 fibers is attributed to the much higher formation efficiency of Ge E' centers, with an additional contribution from GeH. A diamagnetic structure, possibly densification, is also found to contribute to the index modulation of FBG's. (C) 1997 Optical Society of America.