Stability of the UV-Induced Index Change in Hydrogen-Loaded Fibers

Abstract

Exposure of H2-loaded germanosilicate fibers with UV light creates Si–OH, Ge–OH, and molecular water. The concentrations of these hydroxyl-related species grow in linear correlation with the index change. We show that at low temperatures (below 300 oC), molecular water plays a crucial role in the stability of gratings written in H2-loaded fibers. We suggest that at these low temperatures water destroys an index-changing species (probably GeE’ centers) to create OH bonds. This reaction decreases the refractive index of the glass, and thereby erases the grating. Therefore, the long-term stability of gratings written in the presence of hydrogen can be improved by annealing the glass at 500°C to remove any water from the fiber.