Fiber Bragg gratings in hydrogen-loaded photosensitive fiber with two regeneration regimes

In a highly Ge-doped photosensitive fiber co-doped with phosphorus and fluorine and with a doped inner cladding, we observed, during annealing experiments, an anomalous variation of the FBG strength as a function of temperature and time. This characteristic only occurred when the fiber had been loaded with hydrogen. During a stepwise temperature increase from room temperature to 1200 °C with 15 min dwell times, the gratings showed, after an initial thermal decay, in two distinct temperature ranges a significant increase in reflectivity with time. One range with an increase in reflectivity was found between 850 °C and 1000 °C. The behavior of the FBGs in this range was very similar to that known from chemical composition or regenerated gratings. Another temperature range with increasing reflectivity was found between 500 °C and 700 °C, followed by a strong decrease in reflectivity at temperatures between 750 °C and 800 °C. Both regimes of regeneration might be associated with changes in the core and the doped inner cladding of this type of fiber. At different temperatures the drift behavior of λBλB and ΔnACΔnAC was investigated. In specific temperature regions a positive wavelength drift combined with a decrease in ΔnACΔnAC has been observed. This is in contradiction to the standard thermal decay model and seems to be a characteristic of regenerated gratings.