Vacuum-ultraviolet laser-induced refractive-index change and birefringence in standard optical fibers

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 x 10(-3) effective index change at 1.55 mum 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-mum penetration depth at the fiber core. The index asymmetry appears to underlie the large > 5 x 10(-5) value of laser-induced birefringence.