Refractive index modulation in media with saturable photosensitivity

International audience; Permanent change of the refractive index of photo-thermo-refractive glass can be achieved by exposure to ultraviolet light followed by thermal development. The dependence of the change on the dosage is nonlinear because the photochemical process of photosensitivity involves saturation. With a holographic recording of Bragg gratings inside glass, a sinusoidally modulated dosage is imprinted into distorted modulation of the refractive index. We investigate this distorted modulation and derive analytical expressions for the amplitudes of its spatial harmonics.