Abstract
In the harmonic generation using nonlinear optical crystals, phase-matching technique is indispensable for obtaining high conversion efficiency. As the first step of our study, we have investigated crystal temperature variation induced by laser absorption and its influence on conversion efficiency and beam profile in second harmonic generation (SHG) by solving the coupling problem composed of heat conduction equation and complex amplitude equations. In this study, we focused on the declination of irradiation angle of laser from the phase matching angle, which is equivalent to the declination of cut angle of crystal. The influences of angle declination and temperature rise of the crystal on SHG characteristics were examined by supposing KTiOPO<sub>4</sub> (KTP) crystal. Main results obtained are as follows: When the angle declination is only 0.1 deg, conversion efficiency easily decreases to half that under perfect phase-matching condition. Significant distortion of output beam profile of second harmonic is also caused by the angle declination. This essential problem on output beam profile should be considered in precision microfabrication. Influences of angle declination and temperature change on phase mismatching are superposed. Our analysis is useful to elucidate such compounded and complicated phenomena in frequency conversion. A possibility of cancellation of phase mismatching due to angle declination by keeping crystal temperature higher than phase matching temperature was suggested through numerical examples.
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