Optical EngineeringSpecially shaped negative lens produced in a lithium niobate crystal
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The photorefractivity of iron-doped lithium niobate crystal is utilized for creating a negative lens-like structure. The refractive index inhomogeneity acting as a lens is formed due to illumination of the crystal by an optical field with Gaussian spatial distribution of intensity modified by a proper cylindrical lens. Blue line (488 nm) of an argon ion laser is used as a source of light for inducing the inhomogeneity within the crystal. Imaging properties of the photorefractive inhomogeneity are discussed in terms of wave and ray optics approaches and they are practically demonstrated by means of coherent light coming from a He–Ne laser (633 nm). Finally, the focal lengths of the “lens” are calculated using both the lens equation and the formula resulting from analysis of the phase curvature of the wave of light, which passed through the refractive index inhomogeneity.