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Frequency-conversion techniques such as SHG, THG and FOHG with nonlinear optical crystals are indispensable for the applications of all solid-state lasers to precision microfabrication. It is well known that the conversion efficiency is very sensitive to the temperature rise of the crystal, which is induced by laser absorption. In this study, temperature dependence of SHG efficiency has been investigated theoretically, supposing Nd:YAG laser of uniform intensity and KDP (KH 2PO 4) crystal. The coupling problem composed of complex wave amplitude equations and one-dimensional heat conduction equation was analyzed. The complex wave amplitude equations were derived considering the absorption of laser in the crystal. The main results obtained are summarized as follows: (1) Decreasing of SHG efficiency due to laser absorption for the single pulse irradiation to a crystal of 10 mm long, is less than five percent. (2) When SHG efficiency is higher, temperature-rise of the crystal is smaller. Because absorption coefficient for the fundamental wave is larger than that of the second harmonic. (3) When the temperature of crystal rises due to repeated irradiation of pulse laser, SHG efficiency fluctuates and decreases gradually. Remarkable inverse-conversion of SHG appears during laser irradiation with high power density. (4) As either pulse width or irradiation time becomes longer, analytical SHG efficiency agrees with the approximated results obtained under the simple phase-mismatching conditions.

Influence of Laser Absorption by Nonlinear Optical Crystal on SHG Efficiency.