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The image shows the lengths and the spatial periods of the gratings, as well as the Bragg wavelengths at three different RH values. The RH variation induces the volumetric expansion (when RH increases) or contraction (when RH decreases) of the hygroscopic material, resulting in the increase of both its length (L2) and of the spatial period of the grating (Λ2) when RH increases, or in the decrease of both its length (L1) and of the spatial period of the grating (Λ1) when RH decreases.
