Annealing of High-Temperature Stable Hydrogen Loaded Fiber Bragg Gratings

We compare the isothermal and isochronal annealing response of ultrafast laser-induced first-order gratings to that of the third-order gratings up to 1000 °C. Both devices are written in H
₂
-loaded SMF-28 fiber with low-energy femtosecond laser pulses. With the first-order gratings, we observed a regeneration process that allows for a final peak index change of roughly 4% of the original grating. A high-temperature stable peak reflectivity of 39% is obtained (-2.14-dB transmission loss). The third-order gratings do not anneal out completely (they do not regenerate) and roughly 26% of the device persists at 1000 °C corresponding to a final peak reflectivity of 24% (-1.17-dB transmission loss). Due to their similar fabrication conditions, these findings suggest a strong link between the regenerated first-order gratings and the stable third-order devices. We estimate that both first- and third-order gratings of ~4.5 mm in length are good candidates for the development of high-temperature stable fiber sensors.