CO2泵浦源是光泵气体太赫兹激光器的核心器件之一，其性能直接关系到太赫兹激光器的工作稳定性。CO2泵浦源一般由闪耀光栅作为全反射尾镜，构成特殊的光栅腔实现波长的选支输出。采用传输矩阵的特征向量法对光栅谐振腔的腔内光场模式进行了理论分析与数值模拟，计算了光栅腔的一系列本征模式及相应的衍射损耗。结果表明光栅腔的腔内模式特性等效于一个平凹腔，而对于大菲涅尔数的光栅腔， Littrow波长的附近支线也可能具有衍射损耗较低的低阶模，可能优先于Littrow波长的高阶模起振，导致光栅腔的波长选择性降低。

CO2 laser pump source is one of the most important core components of optically pumped THz gas laser, and its performance will directly affect the stability of the THz laser. A blazed grating is generally employed as a total reflecting tail mirror to constitute a grating resonator. The intra-cavity beam modes in the grating resonator are numerically studied with the transit matrix eigenvector method in this paper. A series of eigenmodes and the corresponding diffraction losses are calculated. It is shown that the characteristics of the intra-cavity eigenmode in a grating resonator are equivalent to those of a plane-concave resonator. For the grating resonator with a large Fresnel number, the diffraction loss of the lowest order mode in the vicinity of the Littrow wavelength may be lower than that of the higher order modes at the Littrow wavelength. In that case, the discrimination of the output longitudinal mode becomes weak.