Photosensitization of standard fibers with deep UV laser radiation

A strong and permanent ultraviolet photosensitivity enhancement was locked into standard optical fiber pre-treated with above-bandgap 157-nm F₂-laser radiation. Below-bandgap 248-nm irradiation of the pre-treated fiber yielded fiber Bragg gratings (FBGs) with an index contrast of 6.5 × 10^-4, a value four-fold larger than that available in 16% Ge-doped photosensitive fiber. The F₂-laser photosensitivity locking was 300 times more effective than with KrF-laser pre-treatment in terms of the accumulated fluence required to lock in a similar refractive index change. Accelerated FBG aging tests equivalent to 25 years at 70°C revealed a less than 2% reflectivity decay in both 157- and 248-nm pre-treated fibers, which is sufficient to eliminate the postannealing step typically required in hydrogen-loaded fibers. The large photosensitization and the superior thermal stability of both 248- and 157-nm pre-treated fibers are associated with a new Si-H bond absorption feature at 1387 nm, which appears only during the FBG formation step.