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Alexandrite crystal is a broadband tunable gain medium with good performance in near infrared band. At room temperature, the wavelength tuning range of the alexandrite laser is about 700~818nm, UV or deep-UV (DUV) lasers can be obtained by single or multiple optical nonlinear frequency conversions. The laser oscillations can be generated when alexandrite crystals absorb pumping light energy, and a considerable part of pumping energy will be converted into thermal energy of crystals which will lead to a thermal effect. It will affect the output laser efficiency, the stability of resonator and the quality of output laser beam. In this paper, by establishing the thermal conduction model of the crystal, the stable temperature field distribution, deformation field distribution, and thermal stress field distribution in the crystal can be obtained by solving the corresponding equations. Then the thermal effects caused by these three fields are analyzed respectively, and the corresponding focal lengths of thermal are calculated which shows an inverse relationship between the focal length of the thermal and the absorption pumping power when other conditions remain unchanged. The stable parameter range of the laser cavity can be obtained according to the stable conditions of the resonator, and it will play a guiding role in solving the thermal effect of the crystal and improving the performance of the laser.
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