文章首先分析了长波段(L波段)放大器的关键问题，然后对L波段放大器的特性和几种光路结构进行了模拟分析，并在此基础上提出了一种L波段增益可调的三级级联的放大器光路结构，给出了测试结果。测试结果表明，采用合理的光路结构可以实现具有高增益、增益平坦和较低噪声指数的L波段增益可调放大器。

This paper first analyzes the key issues of the L-band EDFA and simulates and analyzes its characteristics and several light-path structures. And then, on this basis, it presents a 3-stage cascade gain tunable light-path structure and gives the test results, which shows that we can achieve L-band gain tunable EDFA with high and flattened gain and low noise index by employing rational light-path structures.