Owing to damage, thermal issues, and nonlinear optical effects, the output power of fiber laser has been proven to be limited. Beam combining techniques are the attractive solutions in order to achieve high-power high-brightness fiber laser output. Designing such a high-power laser system relies on coherent and incoherent combination of radiation from multiple laser channels into a single beam with enhanced brightness. Spectral beam combination is a promising alternative way that allows each array to be overlapped in near-and far-field without spatial interference, thus relaxing the requirements for linewidth controlling and phase locking of individual array and practically allowing power and brightness to be scaled with the potential to combine a large number of channels. Spectral beam combination implementations can be divided into two subsets: serial and parallel, based on the combining elements. For scaling high power, we pursue spectral beam combining with parallel subsets as an alternative to other beam combination implementation. In the spectral beam combining system based on multi-layer dielectric grating, the combined beam suffers the degradation in beam quality, which is caused by the optical dispersion, and also by the random error due to the misalignment of arrays or the thermal-optic effect of grating in the experimental system. In this paper, we strictly derive the equation of M 2 variation caused by the optical dispersion in both single-grating structure and dual-grating structure. And also, we discuss how the laser linewidth, beam size, spectral separation of two adjacent channels, distance between two adjacent channels and the period of grating influence the desired beam quality in detail, separately, in the single-grating structure and the dual-grating structure. The results show that with the value of M 2 fixed, the finite beam size gives rise to a laser bandwidth decreasing in single-grating structure combination, whereas the beam size induces a laser bandwidth to increase in dual-grating structure combination. If M 2 ≤ 1.2, the laser bandwidth of dual-grating system can be over several sub-nanometers, rather than several tens of pm as in the single grating design.%基于电介质光栅的光谱合成是实现高功率高光束质量激光的重要途径.在电介质光栅的光谱合成系统中，光栅色散效应是影响合成激光光束质量的重要因素.本文推导了单光栅和双光栅光谱合成系统中由于光栅色散引起M2因子的变化公式；详细讨论了这两种合成系统中单路激光线宽、单路激光光斑半径、相邻两路激光波长差、相邻两路激光间距以及光栅周期对光束质量的影响.研究表明对于单光栅合成系统，在合成过程中若保持光束质量M2因子的大小不变，则单路激光带宽随光斑半径的增加而减小；在双光栅光谱合成系统中，在保持光束质量的前提下，单路激光带宽可随光斑半径的增大而相应增加.数值计算表明，若要满足合成光束的光束质量M 2≤1.2的要求，在单光栅系统中激光带宽需窄于亚纳米量级，在双光栅系统中激光带宽可为亚纳米.本文为高功率、高光束质量的光纤激光光谱合成系统的搭建提供了理论指导.
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