The threshold for the fabrication of fiber Bragg gratings with ultrafast 800-nm radiation and a phase mask was studied in SMF-28 and all-silica core fiber by use of 125-fs pulses. High-pressure molecular hydrogen loading ( loading) was observed to significantly lower the grating writing threshold in standard Ge-doped telecommunication fiber. No reduction was observed with all-silica core fiber. The index change appeared to be confined to the Ge-doped core region of the fiber. Gratings in -loaded SMF-28 had thermal annealing behavior similar to UV-induced gratings. Unlike UV-induced -loaded gratings, no absorption associated with Ge–OH defect formation was observed.
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