Atomic force microscopy study of uv-induced anisotropy in hydrogen-loaded germanosilicate fibers

Thermally treated and uv-exposed hydrogen-loaded germanosilicate fibers are profiled with an atomic force microscope after cleaving the fiber in the exposed region and etching the cleaved endface in hydrofluoric acid. Thermally treated fibers etch symmetrically throughout the core region, but the uv-exposed fibers etch differently. In the uv-exposed fibers, both the etch depth and the refractive index are asymmetric. They are highest at the core-cladding interface on the side exposed to the uv radiation. We propose that a photolytic process increases the refractive index across the entire core, but the asymmetry is the result of transient heating due to uv absorption. Furthermore, we demonstrate that uniform circumferential exposure symmetrizes the etch depth and refractive index change and reduces the induced birefringence.