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 1.3×10/sup -3/ effective index change at 1.55 μm 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-μm penetration depth at the fiber core. The index asymmetry appears to underlie the large >5×10/sup -5/ value of laser-induced birefringence.