An all-fiberized and narrow-bandwidth master oscillator power amplification (MOPA) system with record output power of 4 kW level and slope efficiency of 78% is demonstrated. Tandem pumping strategy is tentatively introduced into the narrow-bandwidth MOPA system for thermally induced mode instability (TMI) suppression. The stimulated Brillouin scattering (SBS) effect is balanced by simply using one-stage phase modulation technique. With different phase modulation signals, SBS limited output powers of 336 W, 1.2 kW and 3.94 kW are respectively achieved with spectral bandwidths accounting for 90% power of ${\sim}$0.025, 0.17 and ${\sim}$0.89 nm. Compared with our previous 976 nm pumping system, TMI threshold is overall boosted to be ${>}$5 times in which tandem pumping increases the TMI threshold of ${>}$3 times. The beam quality ($M^{2}$ factor) of the output laser is well within 1.5 below the TMI threshold while it is ultimately saturated to be 1.86 with the influence of TMI at maximal output power. Except for SBS and TMI, stimulated Raman scattering (SRS) effect will be another challenge for further power scaling. In such a high power MOPA system, multi-detrimental effects (SBS, SRS and TMI) will coexist and may be mutual-coupled, which could provide a well platform for further comprehensively investigating and optimizing the high power, narrow-bandwidth fiber amplifiers.

中文翻译：

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串联泵浦策略的高功率全光纤窄带宽MOPA系统用于热致模不稳定性抑制

展示了一种全光纤和窄带宽主振荡器功率放大 (MOPA) 系统，其输出功率达到创纪录的 4 kW 水平，斜率效率为 78%。串联泵浦策略被初步引入窄带宽 MOPA 系统，用于热致模式不稳定性 (TMI) 抑制。通过简单地使用一级相位调制技术来平衡受激布里渊散射 (SBS) 效应。在不同的调相信号下，SBS 限制输出功率分别为 336 W、1.2 kW 和 3.94 kW，频谱带宽占功率的 90%。${\sim}$0.025、0.17 和${\sim}$0.89 纳米。与我们之前的 976 nm 泵浦系统相比，TMI 阈值整体提升至${>}$5 倍，其中串联泵增加 TMI 阈值${>}$3次。光束质量（$M^{2}$输出激光的因子）在 TMI 阈值以下 1.5 以内，而在最大输出功率下，在 TMI 的影响下最终饱和到 1.86。除了 SBS 和 TMI，受激拉曼散射 (SRS) 效应将是进一步功率缩放的另一个挑战。在这样的高功率MOPA系统中，多种不利影响（SBS、SRS和TMI）将并存并可能相互耦合，为进一步全面研究和优化高功率、窄带宽光纤放大器提供了良好的平台。