Influence of H2 loading on the kinetics of Type IIA fibre Bragg grating photoinscription

Type IIA Bragg gratings have been UV written within a highly germania doped silica fibre, previously loaded with hydrogen gas under different pressures (193, 150, 100, 20, 5 atm.). The kinetics of Type IIA grating formation is strongly influenced by both the loading pressure and the time of H₂ outgassing from the fibre. For pressures within 195-20 atmospheres, Type IIA grating formation is inhibited when the UV exposure is carried out soon after the fibre removal from the pressure chamber. This means that only Type I gratings could be observed. What the inscriptions are performed at increasing times of H₂ outgassing, we observe that the exposure time required to get a Type IIA grating dramatically decreases, reaching a minimum value t_min, which depends upon the loading pressure (typically, a few minutes of UV exposure are necessary to write a Type IIA grating after ten days of H₂ outgassing from a fibre previously loaded at 20 atm.). A similar behaviour is observed with the 5 atm. loaded sample, with however a specific change, since the Type IIA grating formation is not inhibited at all, and is observed when UV irradiation is performed immediately after fibre removal from the H₂ pressure vessel. The thermal behaviour of gratings written within both the loaded and unloaded fibres has also been investigated