Analysis on conversion efficiency and beam profile of second harmonic generated by KTP

Temperature dependence of second harmonic generation (SHG) was examined theoretically by solving the coupling problem composed of heat conduction equation and complex amplitude equations. The latter includes effect of laser absorption in crystal. KTiOPO₄ (KTP) was supposed as a nonlinear optical crystal, and its SHG characteristics were compared with KH₂PO₄ (KDP). Power density distribution in crystal, time variation of conversion efficiency and output-beam profile were analyzed quantitatively during repetitive irradiation of pulse laser in Gaussian mode. Polarization-direction dependence of conversion efficiency and beam profile was also investigated. As a result, it was revealed that there are various modes in output-beam profile, such as ring mode, single mode with one- or double-rings in the surrounding, depending on incident power density or crystal length. These distortions of transverse mode are caused by inverse conversion of SHG. It was also concluded that there is an optimum length of crystal to the irradiation conditions, and it can be estimated by the present study.