Dynamics of Hydrogen Diffusion as a Key Component of the Photosensitivity Response of Hydrogen-Loaded Optical Fibers

We present a new model predicting the photosensitivity response of hydrogen/deuterium-loaded optical fibers used for the fabrication of fiber Bragg gratings (FBGs). This model considers space- and time-dependent hydrogen concentration as a key phenomenon to explain the growth rates of strong index changes ( > 10^-3). The model outputs are compared to several photosensitivity experiments made with three different fibers and different hydrogen-loading concentrations. The experimental results presented in this paper clearly show that the time interval between each step of the grating inscription has an impact on the overall photosensitivity response. Moreover, the proposed model confirms that the variations observed in the measured photosensitivity come from changes in hydrogen concentration resulting from its consumption by the photosensitivity reaction and its diffusion into the glass. This model will find application in the precise fabrication of strong FBGs by helping to predict the optimum writing conditions.