依据少模掺铒光纤放大器(FM-EDFA)理论模型，在分析均匀掺铒光纤模式增益特性的基础之上，设计了一种分层掺铒的少模光纤结构。通过遗传算法优化分层掺铒光纤中的铒离子分布，实现了FM-EDFA 信号光四模式群组的增益均衡。仿真结果表明，利用980 nm 单模抽运模式，可以使1550 nm 信号光的四模式群组平均增益达到22 dB，模间增益差值小于0.5 dB，且在C 波段各模式光谱平坦度为2 dB。简化抽运模式结构，降低掺铒光纤制作复杂性，为进一步开展四模式群组增益均衡的实验研究奠定了基础。

Based on the theoretical model of few mode erbium doped fiber amplifier (FM-EDFA), the modal gain characteristics of uniform erbium-doped fiber are analyzed and a layered Er3+ doped fiber structure is proposed. By optimizing the Er3+ doped fiber with genetic algorithm, the equalization of four mode groups is realized. The simulation results show that,with 980 nm single-mode pump mode, the average gain of four mode groups can reach 22 dB and different modal gain is less than 0.5 dB at the wavelength of 1550 nm. The flatness of modal spectra is about 2 dB. This not only simplifies both the structure of pump mode and complexity of erbium-doped fiber manufacturing process, but also provides theoretical basis for the further research of realizing the equalization of four mode groups in experiment.