A low-numerical-aperture (NA) concept enables large-mode-area fiber with better single-mode operation ability, which is beneficial for transverse mode instability and nonlinear effects suppression. In this contribution, we reported a high-power fiber amplifier based on a piece of self-developed large-mode-area low-NA fiber with a core NA of 0.049 and a core/inner cladding diameter of 25/400 µm. The influence of the pump wavelength and fiber length on the power scaling potential of the fiber amplifier is systematically investigated. As a result, an output of 4.80 kW and a beam quality factor of ∼1.33 were finally obtained, which is the highest output power ever reported in a fiber amplifier exploiting the low-NA fiber. The results reveal that low-NA fibers have superiority in power scaling and beam quality maintenance at high power levels.

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。

LD泵浦掺镱光纤激光器具有低成本、高效率、高光束质量等优点，在工业、科研、**等领域有着广泛的应用。在大部分实际应用中，由功率和光束质量决定的亮度是影响光纤激光器实际作用性能的核心指标。受到非线性效应（尤其是受激拉曼散射）和模式不稳定效应的限制，当前高亮度掺镱光纤激光器输出功率提升遭遇了明显的技术瓶颈。为了抑制非线性效应和模式不稳定效应，在传统方法的基础上，提出了变纤芯直径光纤和优化泵浦波长等成体系的方法以提升光纤激光器的输出功率；为了有效提高对光纤激光器的设计研发能力，提出并开发了具有自主知识产权的光纤激光仿真软件SeeFiberLaser。首先，介绍了影响宽谱高功率掺镱光纤激光器亮度提升的主要限制因素，给出了各个限制因素的抑制方法；其次，利用自研光纤激光仿真软件SeeFiberLaser对提升光纤激光器功率的方法进行优化设计，并对工业常用的振荡器和高亮度光纤激光放大器进行仿真优化；然后，介绍课题组采用后向泵浦、变纤芯直径光纤和优化泵浦波长等方法提升激光功率，实现的6~10 kW高亮度功率光纤激光器；最后，对更高亮度光纤激光器的技术方案进行讨论和展望，提出了无源器件集成化、增益传能光纤一体化等思路，提出了基于变纤芯直径增益传能一体化光纤和集成化无源器件的新型高功率近单模光纤激光器技术方案。

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.

光纤耦合半导体激光器（LD）泵浦的光纤激光放大器具有体积小、功质比高、稳定性好等优点，在工业加工和****等诸多领域都有着广泛且重要的应用。然而，受限于器件制作工艺水平及光纤中的受激拉曼效应和模式不稳定效应，LD泵浦的光纤激光放大器难以同时实现高功率及高亮度激光输出。为实现更高功率、更高亮度的光纤激光输出，需要结合现有的器件工艺水平并同时实现对放大器中的受激拉曼散射效应和模式不稳定效应的有效抑制。报道了基于单位自研大模场增益光纤成功实现13 kW功率、高光束质量激光输出。激光器采用主振荡功率放大结构，放大级采用单后向981 nm泵浦自研大模场增益光纤，在总泵浦功率为15 kW时，输出功率达到12.94 kW，光束质量M²因子约为2.85。通过进一步优化器件性能及光纤模式控制，有望实现更高功率、更高亮度的光纤激光输出。