Effect of enclosed space pressure on thermal regeneration of hydrogen-loaded fiber Bragg gratings

The type-I fiber Bragg grating (FBG) was annealed at high temperature to obtain a thermally regenerated fiber Bragg gratings, which can operate reliably in the environments above 1100 °C without any decay of the grating spectrum. However, the underlying thermal regeneration mechanism of FBG has remained unclear until now. Here the thermal regeneration mechanism of FBG was discussed by studying the phenomenon of high-temperature decay and regeneration of FBG in an enclosed space. The thermal decay and regeneration processes of FBGs encapsulated in different volumes were investigated experimentally. The experimental results confirm that hydrogen stored in the hydrogen-loaded fiber diffuses out and then increasing pressure of the enclosed space during the high-temperature regeneration and annealing process of packaged FBG. Once this pressure reaches 0.9 MPa, FBG cannot be regenerated successfully. Comparative analysis with existing regeneration mechanisms reveals that the fundamental cause of thermal regeneration for FBG tends to follow a stress-induced model. However, external pressure affects the periodic distribution of refractive index associated with stress-induced regeneration. When pressure within the confined space reaches a certain critical value, the periodic distribution of stress for FBG will be affected, thus inhibiting the regeneration of FBG.