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Strong photosensitivity responses with the use of above-bandgap 157-nm F 2-laser radiation are reported for standard germanosilicate fiber. A large 1.3 10 3 effective index change at 1.55 m was inferred by trimming strong and weak Bragg gratings in hydrogen-free fiber. The F 2-laser fluence-processing window of 50 mJ cm 2 is much lower than with traditional ultraviolet (UV) lasers. For hydrogen-soaked fiber, highly asymmetric refractive-index profiles were noted by atomic force microscopy and microreflection microscopy, yielding a peak index change of 0 01 across a small 1-m penetration depth at the fiber core. The index asymmetry appears to underlie the large 5 10 5 value of laser-induced birefringence.