Specific intensity profiles for designed light-induced index distributions in photorefractive crystals

The anisotropic features of the refractive index changes induced by a Gaussian beam and a top-flat square beam with different orientations are investigated both theoretically and experimentally. After that, an approach to obtain designed index distributions in photorefractive crystals by illuminations of light beams with specific intensity profiles is presented. By solving the reversed problem of the photorefractive process, the intensity distributions of the writing beams for obtaining a preassigned index profiles can be derived. Employing the writing beams with specific intensity profiles, various designed index distributions could be obtained. Gaussian type index distributions with circular symmetry are experimentally demonstrated in a slice of LiNbO₃:Fe crystal by employing specific writing beams formed by the aid of a projector.