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We have measured refractive indices and absorption coefficients for Mg:LiNbO 3 single crystals of different Mg contents ͑0-7 mol %͒ in the visible and IR ranges up to the phonon absorption edge. The obtained dependencies of optical characteristics on Mg concentration in an IR region of 4 -5 m indicate the four-step character of structural changes under subsequent Mg doping. Spectral dependencies of optical characteristics reveal the existence of two polaron resonance bands, centered at 1.3 and 3.2 m, which are not observed in the pure LiNbO 3 . The theory approximation of the band at 1.3 m, enhanced after chemical reduction, enabled us to determine the main polaron parameters for the 1.3-m band, and to make a possible assignment of the 3.2 m band. We assign this to direct transitions between energy levels of polarons trapped at Nb Nb sites and those of polarons trapped at Nb Li antisites in a lattice. A number of peculiarities are revealed in the absorption behavior at the low-energy edge from 1000 to 3000 cm Ϫ1 . The values of the uninduced part of absorption are sufficiently less than those predicted by a phonon oscillator model. Absorption spectral dependencies are modulated at a rather stable period, being of the order of frequencies of the lowest-energy longitudinal optical phonons, excited at a room temperature.Bulk crystals of doped LiNbO 3 are important for a variety of applications using nonlinear optical, 1 ferroelectric, 2 and photorefractive 3 effects. Different doping atoms enter into the defect structure of nonstochiometric crystals and change it in different ways. As a result, some dopants ͑like Fe or Cu͒ are used to increase the photorefractive sensitivity to light. The others ͑Mg, Sc, Zn͒ are necessary to improve the resistance to optical damage in nonlinear optical devices. 4 At the same time, the incorporation of doping agents, or the changing of the nonstochiometric composition, essentially modify the optical characteristics of LiNbO 3 crystals in its transparency region. 5,6 Accurate data on refractive indices are required for a variety of applications. Refractive indices were measured for concrete type of LiNbO 3 crystal: for undoped LiNbO 3 crystals of different nonstochiometric composition, 7-11 for differently Mg-doped crystals, 12-18 and others. Zelmon, Small, and Jundt 16 obtained Sellmeier equations for the largest spectral interval for congruent LiNbO 3 and for Mg:LiNbO 3 , grown from the melt of 5 mol % MgO. Their equations fit refractive indices dispersions in the total region 0.4 -5 m with the best overall absolute accuracy within Ϯ0.0002.Betzler and Schlarb 14,15 proposed a more general approach. They tried to predict the dependence of the refractive index dispersion law on the crystal composition. They obtained generalized Sellmeier equations for the wavelength range from 400 to 1200 nm with an account for the defect structure of Li-deficient LiNbO 3 crystals. 14 Later, 15 they deduced the generalized Sellmeier equations for all Mg-doped Li-deficient crystals. To obtain the best ...

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