H-Bearing species migration during UV writing of Bragg gratings in germanosilicate optical fibers

Bragg gratings (BG) have been written in H2-loaded Ge-doped preform plates by means of CW 244 nm-laser light. H-bearing absorbance profiles have been performed along the BG longitudinal axis by means of FTIR absorption micro-spectroscopy. It has been observed that the higher the UV writing time or power, the higher the OH species average concentration but the lower the OH species modulation amplitude along the Bragg gratings axis. It is suggested that OH species migrates from the bright UV fringes towards the dark ones during BG writing. Thus, these diffusing species contribute mainly to the mean index and not to the modulation index.