平顶光束在BBO单晶和孪晶中的倍频效应

理论上建立了平顶光束在单晶和孪晶中传播的物理模型，并给出了平顶光束在各向异性介质中传播时的倍频耦合波方程,进而对其进行数值求解；最后对求解所得到的谐波能流分布、总输出能量和入射光角谱进行了分析讨论，并从束腰大小、晶体结构对倍频的影响等方面给出了合理解释。研究结果表明：当束腰相同时，孪晶倍频能流强度要高于单晶；同一种晶体，窄束腰对应倍频强度更高，然而窄束腰光束在单晶中传播时容易发生光束分裂，但孪晶可以改善这种光束分裂情况。

Second harmonic generation of flat-topped beam in single and twin BBO crystal

The coupled wave equations for second harmonic generation of flat-topped beams propagating in birefringent uniaxial anisotropic medium are presented. Based on them, specific physical models of single and twin BBO have been established and numerical computation has been carried out. The influences of beam waist, medium structure on second harmonic generation are analyzed. The calculation results indicate that a wider waist can make a better phase matching, so the second harmonic beam will maintain the flat-topped property better but with a lower intensity and twin crystal can eliminate beam walk-off in some way.