利用耦合非线性微分方程组求解饱和吸收体光栅（SAG）的损耗谱，得到了在无谐振腔时SAG参数与光栅带宽、中心频率损耗、边模抑制比等光栅滤波特性的关系。考虑谐振腔的存在，对SAG纵模边模抑制比进行修正，数值分析了优化区范围及优化程度。结果表明，当满足饱和吸收体长度约为腔内光纤长度1/3时，可实现平坦的边模损耗谱，且在小于由注入功率决定的掺铒光纤长度内，纵模的边模抑制比较无谐振腔时提高了0.3~0.6 dB。采用扫频法初步验证了达到平坦边模损耗谱的条件。

The coupled nonlinear differential equations are utilized to solve the loss spectrum of saturable absorber grating (SAG). Without laser cavity, the influences of the parameters of SAG on the filtering behavior of the gratings such as grating bandwidth, loss of the central frequency, sidemode suppression ratio (SMSR), etc. are investigated. Furthermore, the longitudinal mode SMSR in the presence of resonator cavity and the parameter regions for the improvement of the SMSR are discussed. The flat sidemode loss spectrum can be achieved when the length of the saturable absorber is almost one third of the fiber length in the cavity, and SMSR is 0.3~0.6 dB higher than that without a cavity when the length of the SAG is under a certain value due to the power of writing beam. Finally, the condition for the flat sidemode loss spectrum is validated elementally by a frequency-sweep method.