Experimental and modeled dependencies of the induced refractive index on dosage of UV exposure in photo-thermo-refractive glass for different thermal treatment regimes are presented. Resulting spatial profiles of refractive index modulation in a reflecting Bragg grating recorded by a holographic technique are computed, and corresponding diffraction efficiencies are modeled. It is shown that nonlinearity of the photosensitivity response is responsible for spatial distortions of a recorded grating that result in a decrease of the diffraction efficiency.