建立了超短激光脉冲在准相位匹配晶体中传输的物理模型，并对修正后的非线性薛定谔方程进行了数值求解，获得了飞秒激光脉冲在周期极化铌酸锂晶体中传输的时空演化过程。模拟结果发现当入射基频光功率超过晶体自聚焦的临界功率时，发生自聚焦过程。自聚焦效应使激光脉冲在时域上产生分裂和在空间上发生聚焦。脉冲宽度随着传输距离的增加而逐渐减小，焦点处脉冲宽度最小；脉冲在空间上被聚焦，聚焦半径随着传输距离的增加而逐渐减小，焦点处半径最小，经过焦点后发散；脉冲峰值强度在焦点处最强，经过焦点后峰值强度逐渐降低。超短脉冲倍频过程中，自聚焦效应限制了准相位匹配晶体的长度。

The physical model of ultra-short laser pulse propagation during quasi-phase matched crystal is built and modified nonlinear Schrdinger equation is solved with step Fourier method. The temporal and spatial characteristics of femto-second laser pulse propagation in periodically poled lithium niobate (PPLN) crystal are achieved numerically. It is found that when the fundamental power is beyond the critical power, temporal splitting and spatial focusing of pulse are induced by self-focusing effect. Temporal splitting results in pulse width decrease with the increment of propagation distance, and minimal width is obtained at the focal point. On the other hand, spatial focusing brings decrement of focused diameter with the increment of propagation distance, and minimal diameter is obtained at the focal point and then diffused outward. Finally, the peak intensity of the pulse reaches maximal value at the focal point, and then reduces gradually. The length of quasi-phase matched crystal is restricted by self-focusing effect during ultra-short laser frequency doubling process.