All-fiber few-mode erbium-doped fiber amplifiers (FM-EDFAs) with isolation and wavelength division multiplexers (IWDMs) have been developed to enable flexible pumping in different directions. The FM-EDFA can achieve >30 dB modal gain with <0.3 dB differential modal gain (DMG). We experimentally simulate the DMG performance of a cascade FM-EDFA system using the equivalent spectrum method. The overall DMG reaches 1.84 dB after 10-stage amplification. We also build a recirculating loop to simulate the system, and the developed FM-EDFA can support transmission up to 3270 km within a 2 dB overall DMG by optimizing the few-mode fiber length in the loop.

The dynamic gain of a few-mode erbium-doped fiber amplifier (FM-EDFA) is vital for the long-haul mode division multiplexing (MDM) transmission. Here, we investigate the mode-dependent dynamic gain of an FM-EDFA under various manipulations of the pump mode. First, we numerically calculate the gain variation with respect to the input signal power, where a mode-dependent saturation input power occurs under different pump modes. Even under the fixed intensity profile of the pump laser, the saturation input power of each spatial mode is different. Moreover, high-order mode pumping leads to a compression of the linear amplification region, even though it is beneficial for the mitigation of the differential modal gain (DMG) arising in all guided modes. Then, we develop an all-fiber 3-mode EDFA, where the fundamental mode of the pump laser can be efficiently converted to the LP₁₁ mode using the all-fiber mode-selective coupler (MSC). In comparison with the traditional LP₀₁ pumping scheme, the DMG at 1550 nm can be mitigated from 1.61 dB to 0.97 dB under the LP₁₁ mode pumping, while both an average gain of 19.93 dB and a DMG of less than 1 dB can be achieved from 1530 nm to 1560 nm. However, the corresponding signal input saturation powers are reduced by 0.3 dB for the LP₀₁ mode and 1.6 dB for the LP₁₁ mode, respectively. Both theoretical and experimental results indicate that a trade-off occurs between the DMG mitigation and the extension of the linear amplification range when the intensity profile of pump laser is manipulated.

基于少模和多芯光纤的空分复用技术被认为是未来大幅提高单根光纤数据传输容量最重要的技术之一。采用少模光纤的空分复用技术要实现长距离传输，少模掺铒光纤放大器（Few?mode erbium?doped fiber amplifier，FM?EDFA）是补偿光纤传输损耗的关键器件，而少模掺铒光纤决定了FM?EDFA的性能。相对于单模掺铒光纤而言，少模掺铒光纤除了有增益、带宽、噪声等基本指标之外，还有一个独特的指标——差分模态增益。高差分模态增益会导致系统中断概率的提升，因此在少模掺铒光纤放大器中最小化差分模态增益以保持信号质量至关重要。本文系统阐述了差分模态增益的产生机理及改善策略，总结了少模掺铒光纤的不同设计，对比了采用不同设计及基于不同泵浦方式的少模掺铒放大光纤的性能特点，并对少模掺铒光纤的放大性能研究做出了展望。