激光振荡器内光束质量的演变机理对获得高光束质量激光至关重要。综合考虑激光晶体的热效应和增益导引效应，建立了激光振荡器的理论模型，仿真计算并实验验证了激光振荡器内光束质量的演变。基于理论模型，研究了光束质量演变的机理。指出带负球差激光通过激光晶体时，其负球差可以被晶体正球差补偿，从而改善光束质量。这一机理对激光振荡器和放大器实现高光束质量激光输出具有重要意义。

The mechanism investigation of beam quality evolution within laser oscillator is of crucial importance for high beam quality laser output. Combining the thermal effect and gain guiding effect, a theoretical model of laser oscillator is presented. The beam quality evolution inside a laser oscillator is numerically simulated and experimentally verified. The mechanism of beam quality evolution is investigated based on the theoretical model. When the laser beam with negative spherical aberration passes through the laser crystal, the negative spherical aberration can be compensated by the positive spherical aberration of the crystal, resulting in beam quality improvement. This mechanism is meaningful for high beam quality output of laser oscillator and amplifier.