为了实现2μm激光输出, 采用周期性极化铌酸锂(PPLN)晶体用于光参变振荡(OPO)产生2μm激光的方法, 从PPLN晶体的Sellmeier方程出发, 考虑三波相互作用需满足的动量守恒和能量守恒条件, 进行了理论分析和数值计算, 得到在120℃时, PPLN晶体的极化周期为31.35μm的理论值; 绘制出了输出波长与极化周期、输出波长与入射角的关系曲线; 并从相位同步因子sinc2(ΔKl/2)出发, 分别计算了允许角、允许周期、允许温度。结果表明, 此方法不仅克服了利用双折射特性和色散特性方法带来的走离效应, 而且还充分利用了晶体的最大有效非线性系数, 从而理论上提高了转换效率。该理论分析为PPLN-准相位匹配-OPO输出2μm激光的下一步实验奠定了基础。

In order to generate 2μm laser, a novel method was adopted with periodically poled lithium niobate(PPLN) as a nonlinear crystal in optical parametric oscillator(OPO). From the Sellmeier function of PPLN, given the momentum conservation and energy conservation, series of theoretical analysis and numerical calculation are conducted. Firstly, it was obtained that the polarization period of PPLN was 31.35μm at 120℃ Then the curves of the relations between the wavelength and polarization period, between the wavelength and the incident angle were drawn. Finally, starting from the phase synchronization factor sinc2(ΔKl/2), the allowance angle, allowance polarization period and allowance temperature were calculated. This method not only can overcome walk-off effect of birefringence and dispersion, but also make full use of the maximum effective nonlinear coefficient of the crystals, so the efficiency of the conversion can be increased. These theoretical results provide the foundation for the future experimental study about PPLN-quasi-phase matching-OPOs.