Fluorescence spectroscopy of fiber gratings written with an ultrafast infrared laser and a phase mask

Confocal fluorescence microscopy was performed on gratings generated in all-silica core optical fiber with an ultrafast infrared laser and a phase mask. Upon excitation with 488 nm light a broad fluorescence centered at 650 nm is observed. The fluorescence intensity varies periodically with a 4.5 μm pitch, in reasonable agreement with the 4.284 μm grating pitch. The fluorescence is due to non-bridging oxygen hole center defects which are created by the ultrafast laser pulses during grating formation. The fluorescence from these defects photobleach within minutes upon continued excitation with low power cw 488 nm light. After photobleaching the grating structures are still visible with white light microscopy, indicating that an additional -non-bleachable-type of structural modification occurs during grating formation.