Wavelength evolution of long-period fiber gratings in a water environment

In a water environment, wavelength evolution behavior of long-period fiber gratings (LPFGs) written in H2-loaded fibers after annealing is studied. The phenomena that wavelength shifts in the longer wavelength direction and then in the shorter wavelength direction is observed. A shift of the grating resonance peak (LP05) of as much as 2.5 nm is found. A water-mediated model that water molecules induce the second diffusion of the remaining H2 in the fiber and a diffusion-reaction mechanism that water molecules penetrate into fiber internal structures are proposed and are combined to explain the wavelength evolution process. Both the calculated balance point time according to the model, and the qualitative analysis according to the mechanism, correspond well with the experimental results. This research indicates that wavelength variation has to be considered or prevented when H2-loaded LPFGs are used in a water environment.