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.

基于产生3 μm中红外激光的Er³⁺能量转换过程与能级体系，建立了掺铒氟化物光纤激光器的速率方程和传输方程，系统地研究了泵浦结构和光纤长度对输出功率、斜率效率以及腔内泵浦和激光功率分布的影响，为后续的实验提供理论依据。仿真确定了小泵浦功率的光纤激光器理想的光纤长度为4~8 m，最终实验选择4.7 m光纤完成了中红外掺铒氟化物光纤激光器连续激光输出光路的搭建， 在泵浦功率为5 W、中心波长为2.797 μm处实现了最大功率1.038 W、斜率效率20.4％的连续激光输出。

基于全保偏掺铒光纤激光器、锁相环系统和太赫兹测距光路搭建了一套太赫兹双光梳测距系统。所采用的全保偏掺铒光纤激光器重复频率为79.261 MHz。利用压电陶瓷(piezoelectric ceramics, PZT)和步进电机(stepper motor, SM)双级反馈控制的方案，实现了重复频率锁定和重复频率1.54 MHz可调。使用频率计数器对双光梳重复频率锁定效果进行监测，重复频率锁定的峰峰值抖动为±1.5 mHz，抖动标准差为0.4 mHz。将双光梳重频差设置在10 Hz，10 min内重复频率差最大抖动为3 mHz，标准差为0.6 mHz。进一步将异步采样双光梳系统应用于太赫兹测距，测量移动距离的误差为3 μm。该系统具有锁定精度高，稳定性强等优势，有望应用于生物无损检测和工业精密加工中。

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.