On one hand, the stability of fiber Bragg gratings (FBG) written either in ultraviolet (UV) hypersensitized, OH flooded, H2 loaded or in pristine fibers has been investigated through isochronal annealing experiments. The stability of the modulation proved to be higher in the hypersensitized and OH-flooded fibers. In contrast the annealing-induced shift in the Bragg wavelength (λ<sub>B</sub>) was lower in the H2-loaded and pristine counterparts. On the other hand, germanosilicate preform plates have been sensitized and then exposed in similar conditions to those used for writing the gratings. Vacuum-ultraviolet (VUV) and infrared (IR) absorption spectroscopy has been carried out with a view of monitoring the formation (or bleaching) of defect-associated bands. These observations are discussed within the framework of a defect model.
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