为了解决条状负支离轴非稳腔输出线宽增宽的问题, 分析非稳腔的主要两个参量——有效菲涅耳数Neff和几何放大率M与输出模式的过量噪声因子K取值的函数关系,从负支腔的具体结构出发, 采用经典波动方程的理论唯像的将过量噪声因子表示成输出平面横模分布的泛函, 利用边界衍射波的方法将输出横模近似地解析表述成Neff和M的函数。利用该关系数值计算了K随Neff和M变化的情况。结果表明,K随Neff呈现准周期性变化, 在Neff≈s+0.85处（s为正整数）呈现周期性峰值, 输出线宽将被增宽到自然线宽的2000倍左右。该研究对分析非稳腔的输出损耗是有帮助的, 并为实际激光器的设计提供了参考。

In order to reduce the broadening of output beam line width in negative-branch off-axis unstable resonator laser, the function relationship between the two main parameters of effective Fresnel number Neff and geometry magnification M and excess noise factor K of output mode was analyzed. From negative branch structure, excess noise factor was firstly presented as functional distribution of transverse mode in output plane based on classical wave equation theory. By using the edge wave method, output transverse mode was approximated as a function of Neff and M. And then, the dependence of K on Neff and M were numerical calculated. The results indicate that K changes quasi-periodically with Neff. The cyclical peak is presented at Neff≈s+0.85 (s is a positive integer). Output line width is broadened nearly 2000 times of the natural line width. The study is useful for the research of output loss of unstable resonator and provides reference for practical laser designs.