2 μm high-power thulium- and holmium-doped fiber lasers: progress and future perspectives

2 μm fiber lasers, owing to their unique physical and spectral properties, hold significant potential for applications in fields such as lidar, medical surgery, environmental monitoring, and nonlinear frequency conversion. This paper presents a comprehensive review of continuous-wave and pulsed 2 μm fiber lasers. First, it elucidates the spectral characteristics and energy-level transition mechanisms of the primary gain media, including thulium-doped, holmium-doped, and thulium-holmium co-doped fibers. It systematically summarizes technical approaches, efficiency optimization strategies, and specialized fiber structures for high-power operation using mainstream pumping schemes such as 790 nm, 1.55 μm, and 1.9 μm pumping. Second, it discusses key technological methods and recent research progress for two typical operational modes: continuous-wave (multi-longitudinal mode, narrow linewidth) and pulsed (Q-switched, mode-locked, gain-switched). Finally, the primary bottlenecks limiting the high-power output and beam quality optimization of 2 μm fiber lasers are analyzed. Future development directions are proposed from five perspectives: system integration and miniaturization, ultra-wide spectral tuning, artificial intelligence-based control, advancements in high-power technology, and the development of novel gain materials.