Optical Damage Resistance in Lithium Niobate

Abstract

Lithium niobate is a universal material for optical applications (optical frequency conversion, optical and acoustooptical light modulation, lasing, photorefractive holography, etc.). This enormous versatility of optical properties is due to a pronounced dependence of its properties on composition, namely, on the crystal composition and the doping type. In spite of widespread potentials of LiNbO3, the optical frequency conversion still remains one of the most important. Although LiNbO3 is lower than other materials in rank, for example, potassium titanium oxide phosphate (KTP) with larger values of the nonlinear optical coefficients, nevertheless, the possibility to obtain a noncritical (90◦) phase matching and a rather broad angular width of the phase synchronism makes LiNbO3 one of the most attractive materials for nonlinear optics. The use of LiNbO3 for optical frequency conversion on a regular pattern of 180◦ ferroelectric domains in the quasiphase-matching (QPM) mode of operation is within the bounds of possibility. Detailed descriptions of the properties of LiNbO3 and its optical applications may be found in numerous reviews and monographs [1, 2, 3]. In nonlinear optics the performance of LiNbO3 is limited by three optically induced effects. The first of them, optical (or photorefractive) damage, is a reversible photoinduced change of the refractive indices that appears at relatively low light intensities, for example in LiNbO3:Fe even at power densities as low as a tenth of a mW/cm2, e.g., [1]). In undoped (congruent) LiNbO3 a saturated photoinduced change δn of the refractive indices is as high as 2 · 10−5–10−4. The consequences of the optical damage are, for example, a distortion of the wave front of the transmitted light, and a loss of lasing when using the material as a laser medium. The second effect limiting the use of many crystals in optics is the dark (or gray) trace effect, which is a light-induced coloration occurring in