阶梯式掺杂和多环式掺杂设计的掺铒光纤(EDFs)结构复杂，采用改进化学气相沉积法(MCVD)制备困难。为此，将两段铒离子分别为中心掺杂和环形掺杂的掺铒光纤级联构成四线偏振模信号光放大器。该结构放大器使中心模式信号光(LP01 模和LP02 模)和离心模式信号光(LP11 模和LP21 模)分别在中心掺杂铒光纤和环形掺杂铒光纤中获得主要增益。基于一种改进遗传算法(GA)，对该放大器两段掺铒光纤的长度和铒离子掺杂半径进行优化。结果显示，仅用270 mW LP11模抽运光对该级联结构放大器进行抽运，四线偏振模信号光可获得20 dB 的平均增益，且模式相关增益(DMG)不超过3.5 dB。

Erbium-doped fibers(EDFs) with multi-step ormulti-annular and multi-level doping profiles are difficult to be fabricated by modified chemical vapor deposition (MCVD). A cascaded amplifier, based on two EDFs with a central doping profile and an annular doping profile, is proposed to equalize modal gain among four linearly polarized modes. In the amplifier, the centered- and off-centered modes are mainly amplified in the central- and annulardoped EDFs, respectively. An improved genetic algorithm (GA) is employed to optimize the optimal fiber length and doping radius in each EDF. Results show that an average modal gain around 20 dB with differential modal gain (DMG) between signal modes being less than 3.5 dB is achieved for a pure LP11 pump beam.