双端输出光纤激光振荡器可以通过一个单谐振腔结构实现两路激光输出，能够减少高功率光纤激光系统的体积和成本，在工业领域有着很好的应用前景。基于双端泵浦谐振腔结构，采用稳波长981 nm光纤耦合半导体激光器（LD）泵浦纤芯/包层直径为30/400 µm的双包层掺镱光纤，首次实现了总功率大于8 kW的双端输出光纤激光振荡器。在总最高泵浦功率为10.951 kW时，A端输出功率为3769 W，B端输出功率为4400 W，总功率为8169 W，激光器光-光转换效率74.6%，A、B端激光光束质量M²因子分别约2.13和2.36。在最高输出功率时，两端输出激光中均未观察到动态模式不稳定效应（TMI）和受激拉曼散射（SRS），通过进一步增加泵浦功率，有望实现更高功率的激光输出。

Achieving an all-fiber ultra-fast system with above kW average power and mJ pulse energy is extremely challenging. This paper demonstrated a picosecond monolithic master oscillator power amplifier system at a 25 MHz repetition frequency with an average power of approximately 1.2 kW, a pulse energy of approximately 48 μJ and a peak power of approximately 0.45 MW. The nonlinear effects were suppressed by adopting a dispersion stretched seed pulse (with a narrow linewidth of 0.052 nm) and a multi-mode master amplifier with an extra-large mode area; then an ultimate narrow bandwidth of 1.32 nm and a moderately broadened pulse of approximately 107 ps were achieved. Meanwhile, the great spatio-temporal stability was verified experimentally, and no sign of transverse mode instability appeared even at the maximum output power. The system has shown great power and energy capability with a sacrificed beam propagation product of 5.28 mm $\cdot$ mrad. In addition, further scaling of the peak power and pulse energy can be achieved by employing a lower repetition and a conventional compressor.

In this work, a confined-doped fiber with the core/inner-cladding diameter of 40/250 μm and a relative doping ratio of 0.75 is fabricated through a modified chemical vapor deposition method combined with the chelate gas deposition technique, and subsequently applied in a tandem-pumped fiber amplifier for high-power operation and transverse mode instability (TMI) mitigation. Notably, the impacts of the seed laser power and mode purity are preliminarily investigated through comparative experiments. It is found that the TMI threshold could be significantly affected by the seed laser mode purity. The possible mechanism behind this phenomenon is proposed and revealed through comprehensive comparative experiments and theoretical analysis. Finally, a maximum output power of 7.49 kW is obtained with the beam quality factor of approximately 1.83, which is the highest output power ever reported in a forward tandem-pumped confined-doped fiber amplifier. This work could provide a good reference and practical solution to improve the TMI threshold and realize high-power high-brightness fiber lasers.

一般认为，光纤激光器模式不稳定效应主要来自于泵浦源量子亏损和增益光纤泵浦吸收所产生的热效应。理论分析了光纤中的热源，发现诱发模式不稳定的热效应主要来源于泵浦吸收、其次是量子亏损；利用课题组开发的仿真软件SeeFiberLaser对该结论进行仿真。仿真结果表明：泵浦吸收系数越低，光纤中的最高温度和温度梯度越低，越有利于抑制热致折射率光栅的形成，提高模式不稳定阈值。搭建了纤芯/包层直径为30/400 μm的前向泵浦掺镱光纤激光振荡器，对比研究了中心波长为976 nm 、915 nm和940 nm的泵浦源泵浦时激光器的模式不稳定阈值特性。结果表明，分别采用中心波长为976 nm 、915 nm和940 nm 的半导体激光器作为泵浦源时，激光器模式不稳定阈值分别为279 W、502 W和697 W，光光转化效率分别为67.7%、61%和63%。由此可以发现，泵浦吸收系数对模式不稳定阈值的影响大于量子亏损对模式不稳定阈值的影响，通过改变泵浦波长降低泵浦吸收系数可以有效提升模式不稳定阈值。优化泵浦波长，兼顾量子效率和泵浦吸收系数，是光纤激光器实现高光束质量高模式不稳定阈值的重要技术路线之一。