理论探讨了高斯光束在非线性单轴晶体中的倍频效应。基于麦克斯韦方程组并考虑晶体的各向异性推导出负单轴I型晶体相位匹配下的倍频耦合波方程组。建立了具体的关于偏硼酸钡（BBO）单晶以及孪晶的物理模型并做数值求解。对求解所得到的单晶以及孪晶的谐波能流分布、晶体出射面探测点处谐波电场时域分布、总输出能量、输出光束频谱和输入光束角谱分别进行分析讨论，从束腰宽度、晶体结构以及材料性质对倍频的影响等方面做出具体合理的解释。研究结果对于单轴晶体的非线性光学性质的研究以及相应的光学晶体器件的研发等具有一定的参考价值。

The coupled wave equations for frequency doubling of Gaussian beam propagating in birefringent uniaxial anisotropic medium are given. Based on them, specific physical models of single and twin crystal of barium metaborate (BBO) material have been established, and numerically computed results show the distribution of harmonic power flow, the electric field temporal distribution of harmonic at probe point on exit surface of the crystal, the output total energy, the output spectra of light and also the input angular spectra of beam. We make detailed analysis of various influences mainly from beam waist, medium structure and properties of material on second harmonic generation and give some reasonable explanations. The research results will benifit the study on nonlinear optical properties of uniaxial medium and optical crystal devices.