Wavelength evolution of fiber Bragg gratings fabricated from hydrogen-loaded optical fiber during annealing

The shift in the central wavelength of fiber Bragg gratings (FBGs) during annealing of hydrogen-loaded optical fiber exposed to ultraviolet (UV) laser irradiation through a phase mask is studied and shown to be caused by thermal diffusion of hydrogen out of the fiber for both short-period and long-period FBGs. Reloading FBGs with hydrogen followed by a second annealing cycle without exposing the fiber to the UV laser reproduces the details of the wavelength evolution observed during first annealing following UV irradiation. This shows that the wavelength shift of the grating during annealing is determined by diffusion of hydrogen gas out of the optical fiber for both short-period and long-period FBGs.