采用非稳腔结构的激光器，当系统的菲涅耳数较大时，球面腔镜尺寸随之增大，尽管可以获得高输出功率，但由于不满足傍轴条件，球面腔镜的球差对输出模式的影响变大。采用Fox-Li迭代算法，分析了腔镜类型不同时谐振腔有效菲涅耳数为675的2 kW 射频板条CO₂激光器的输出模式，并进行了实验研究。结果表明，当腔镜采用双球面镜时球差的影响显著，输出光束近似为球面波，输出平面上光束质量因子M² =14.48，光束质量差，聚焦后光束偏离光轴，难以实现高功率、高光束质量的激光输出；当腔镜均采用抛物面镜时球差的影响得以消除，输出光束近似为平面波，此时输出平面上光束质量得到改善，M² =3.96，实验结果与数值模拟结果一致。

For lasers with unstable resonator structure, the dimension of the spherical cavity mirrors increases dramatically when the Fresnel number of the system is large. Although it is possible to obtain high output power, the aberration of the resonator mirror has great influence on the output beam quality as the paraxial condition is not satisfied. The output mode of a 2 kW radio frequency slab CO₂ laser with an effective cavity Fresnel number of 675 and different types of cavity mirrors is analyzed by the Fox-Li iteration algorithm. Furthermore, experimental investigations are conducted. The results show that the spherical aberration affects the laser output mode seriously when two spherical mirrors are used, the output beam wavefront is approximately spherical, the M² factor of the beam is 14.48 at the output plane, the beam quality is poor, and the beam is off the optical axis through the focusing lens, which makes it difficult to achieve a high power and high beam quality laser output. When two parabolic mirrors are used, the output beam wavefront is approximately planar, the beam quality is improved with an M² factor of 3.96, and the simulation results are in good agreement with the experimental results.