Annealing of Bragg gratings in hydrogen-loaded optical fiber

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Abstract
We have conducted a detailed study of the thermal stability of Bragg gratings written in hydrogen‐loaded and unloaded germanium‐doped optical fiber. Interference of either continuous‐wave or pulsed ultraviolet light was used to induce the index modulation gratings. Some gratings were kept at room temperature and others were annealed at fixed temperatures for 10–20 h. For temperatures between room temperature and 350 °C, gratings in the hydrogen‐loaded fiber showed significantly greater decay than those in the unloaded counterpart. The ultraviolet‐induced index modulation in hydrogen‐loaded fiber was reduced by 40% after 10 h at 176 °C, whereas it was reduced by only 5% in unloaded fiber under the same conditions. The annealing behavior of gratings written using the pulsed source was identical to that of gratings written with the continuous‐wave source, and the thermal stability of gratings in hydrogen‐loaded fiber did not depend on the magnitude of the index modulation. We also observed that the annealing of ultraviolet‐induced OH absorption in the hydrogen‐loaded fiber was not correlated with the grating decay. Our annealing results show that the species responsible for the index change in the hydrogen‐loaded fiber are less stable than those in the unloaded fiber.