采用MgO掺杂的周期性极化铌酸锂晶体作为非线性晶体，搭建了线性平-凹腔结构的外腔式倍频系统，分析了增益介质和输入耦合镜之间的距离对二次谐波转换效率的影响。实验中，使用光纤耦合二极管泵浦Nd:YVO4激光器作为基频光源，通过改变增益介质和输入耦合镜的间距，测量了激光器的纵模数量、锁模脉冲稳定性、基频光线宽以及光-光转换效率。实验结果显示，增加增益介质和输入耦合镜之间的距离，可以在一定程度上提升自锁模脉冲的稳定性并有效减小基频光线宽。当泵浦功率较高时，适当增加该间距可以有效提升二次谐波转换效率。

The effect of the gain medium position in the resonator on the external second harmonic generation (SHG) conversion efficiency was studied. A MgO-doped periodically poled LiNbO3 crystal (MgO:PPLN) was utilized to achieve a 532-nm green-light laser outside the resonator of a mode-locked diode-pumped Nd:YVO4 1064-nm laser. We theoretically and experimentally verified that increasing the mirror/crystal separation could reduce the longitudinal mode number, narrow the linewidth and stabilize the mode locked pulse owing to the spatial burning hole effect. These phenomena would further affect the SHG conversion efficiency.The experimental results show that, increasing the mirror/crystal separation could promote the SHG conversion efficiency with the fixed focus length, especially at the higher pump power. The theoretical analysis and experimental results have reference significance for improving the SHG efficiency of the external end-pumped mode-locked laser.