Study of the Influence of Ionic Conductivity on Optical Absorption Coefficient of Lithium Triborate Crystals Exposed to
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Study of the Influence of Ionic Conductivity on Optical Absorption Coefficient of Lithium Triborate Crystals Exposed to High-Intensity Continuous Laser Radiation I. V. Grishchenkoa,*, Yu. S. Stirmanovb, A. V. Konyashkinb, and O. A. Ryabushkinb a
Moscow Institute of Physics and Technology (National Research University), Dolgoprudnyi, Moscow oblast, 141700 Russia b Kotel’nikov Institute of Radio Engineering and Electronics of Russian Academy of Sciences, Fryazino branch, Fryazino, Moscow oblast, 141190 Russia *e-mail: [email protected] Received March 15, 2020; revised March 15, 2020; accepted May 20, 2020
The results of study of the influence of ionic conductivity on the optical absorption coefficient of lithium triborate crystals exposed to continuous wave high-power laser radiation at 1070 nm wavelength are presented. Key words: lithium triborate, piezoelectric resonance laser calorimetry, optical absorption, laser radiation, ionic conductivity, equivalent temperature DOI: 10.1134/S0030400X20090106
INTRODUCTION The efficiency of frequency conversion of laser radiation in nonlinear optical crystals depends on the phase matching of the interacting waves, which is determined by the equality of the refraction indices in the case of generation of a second harmonic using the Type I phase matching. Its violation occurs, in particular, with a change of the average temperature and the appearance of the longitudinal and transverse temperature gradients in the crystal. As a rule, nonlinear optical crystals have low absorption coefficients in the operating range of laser wavelengths. However, even small optical absorption of the crystal in the case of conversion of high-power laser radiation can lead to the significant non-uniform heating and, as a result, to a decrease of the conversion efficiency. Currently, the output power of continuous wave single-mode Ytterbium-doped fiber lasers reaches 20 kW [1]. Exposure of the crystals to laser radiation of a kilowatt power level can result in local defects formation, degradation and even optical damage of the crystals. Therefore, detection of the formation of micro defects allows determination of the threshold intensity values at which the crystal is damaged. Nonlinear optical crystals of borate group β-BaB2O4 (BBO) and LiB3O5 (LBO) synthesized in 1980s [2, 3] are now widely used for conversion of the high-intensity laser radiation. Despite the relatively small coefficients of second order nonlinearity (d22 = 2.2 × 10–12 m/V for BBO and d32 = 0.85 × 10‒12 m/V
for LBO), they have a very high optical damage threshold [4]. Moreover, the fundamental absorption edge of these crystals is strongly shifted to the UV region (LBO and BBO crystals are transparent in the ranges from 0.16 to 2.3 μm and from 0.2 to 2.6 μm, respectively) [4]. The object of study in this paper was LBO crystal, which is widely used for generation of high-power laser radiation harmonics in visible and UV parts of the spectrum. These crystals have low coefficient of linear o
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