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2 June 1994 Coupled thermal and nonlinear effects for beam propagation in anisotropic crystals
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The self-induced phase mismatches caused by nonuniform heating and heat dissipation and overall temperature rise in a nonlinear crystal reduce the efficiency and the beam qualities of nonlinear optical processes routinely used in uniaxial and biaxial crystals. As the crystal is heated, the phase-matching angle changes because of the dependence of refractive indices on temperature. A ray-optic measure of this effect is given by the acceptance temperature of the crystal. The induced thermal gradient leads to nonuniform, nonlinear conversion over the beams and to a nonuniform wavefront (phase) in the output beam. We have developed a model of the linear and nonlinear processes and a computational procedure for assessing the impact of these effects, including diffraction and depletion. The analytic method consists of the coupling of an optical beam propagation analysis with a themal analysis of the temperature gradients induced in the crystals. We examine three application examples: second- haromic generation in beta-barium-borate (BBO), three-wave mixing in lithium triborate (LBO), and SHG in LBO. Results have significance for experiment design in the development of high- average-power, frequency-agile laser systems. In laser beacon and illuminator applications, degradation in beam quality and conversion efficiency limit system scaling to increased average power. Our model can be used to quantify the design parameters for the input laser beam and the cooled crystal package for SHG or three-wave mixing.
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Mark A. Dreger, John H. Erkkila, and David Stone "Coupled thermal and nonlinear effects for beam propagation in anisotropic crystals", Proc. SPIE 2145, Nonlinear Optics for High-Speed Electronics and Optical Frequency Conversion, (2 June 1994);

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