Role of hydrogen loading and glass composition on the defects generated by the femtosecond laser writing process of fiber Bragg gratings

The creation of fiber Bragg gratings (FBGs) in optical fibers by laser irradiation causes the formation of defects in the modified glass. We have used confocal fluorescence spectroscopy to identify the location and types of defects formed after writing FBGs with the femtosecond laser phase mask technique. Our results show that non-bridging oxygen hole centers (NBOHCs) and self-trapped excitons (Eδ’) are formed throughout all-silica core Sumitomo Z-fiber. Similar defects are observed for Ge-doped silica fiber, Corning SMF-28, but in this case the relative concentrations of NBOHC and Eδ’ vary from the core to the cladding. In both fibers, hydrogen loading prior to irradiation appears to passivate the defects except in the Ge-doped core where the NBOHC defects persist.