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

Strong photosensitivity responses with the use of above-bandgap 157-nm F₂-laser radiation are reported for standard germanosilicate fiber. A large 1.3×10^-3 effective index change at 1.55 μm was inferred by trimming strong and weak Bragg gratings in hydrogen-free fiber. The F₂-laser fluence-processing window of <50 mJ/cm² 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-μm penetration depth at the fiber core. The index asymmetry appears to underlie the large >5×10^-5 value of laser-induced birefringence.