本文首先从二次谐波产生（ＳＨＧ）过程中的一对耦合波方程出发，在理论上得到了当基波光以接近相位匹配的角度穿过倍频晶体时会产生自聚焦效应，并用类比方法得到了ＢＢＯ晶体相应的非线性折射率系数．在实验上，据我们所知首次将ＢＢＯ倍频晶体作为独立自聚焦元件用到了钛宝石激光器中，获得了一台较腔内不加晶体时的锁模阈值明显减小、锁模稳定性明显提高且具有自启动能力的自锁模钛宝石激光器． Starting from a couple of coupled wave equations in the process of second harmonic generation (SHG), we obtain theoretically that when the fundamental wave passes through the frequency doubling crystal at a nearly phase-matching angle, the self-focusing effect is generated. The corresponding nonlinear refractive index coefficients of BBO crystals are obtained. Experimentally, as far as we know, the BBO frequency doubling crystal was used as an independent self-focusing element for the first time in a Ti: sapphire laser, and the threshold value of a mode-locked lens without any crystal in the cavity was significantly reduced, and the mode-locking stability was obvious Improved and self-starting self-locking Ti-sapphire laser.