The 2 μm laser operates in the human eye-safe wavelength range, with excellent atmospheric transmittance and non-metallic material absorption characteristics. Combined with the high power, high beam quality, high conversion efficiency, and high integratin capacity of the fiber lasers, the 2 μm laser is widely used in LIDAR, industrial processing, national defense and security, and biomedical applications. Recently, 2 μm high-power thulium-doped fiber lasers (TDFLs) are being developed rapidly with output power exceeding the order of kilowatts, for applications in medical care, military security, space communications, atmospheric pollution detection, and materials processing. However, the thermal effect, mode instability, and nonlinear effect severely limit the improvement in the output power of thulium-doped fiber lasers.

In 1988, Hanna et al. from the University of Southampton, UK, reported thulium-doped quartz fiber lasers for the first time. Limited by laser pumping technology and the preparation process of thulium-doped fibers, the average output power was 100 μW, and the slope efficiency was only 13%. The technological breakthroughs in the field of high-brightness 793‒795 nm semiconductor lasers, have also ushered in a golden stage of rapid development for TDFLs. By improving the absorption coefficient, changing the fiber structure, optimizing the pumping mode, improving the laser conversion efficiency, suppressing parasitic lasing, and optimizing the mode instability, researchers have increased the output power of TDFL from milliwatts to kilowatts (Fig. 1). In recent years, kilowatt-class narrow linewidth laser and mJ-class high pulse energy output have been achieved and used for combining laser beams through optimization, laying a solid foundation for the 10 kW-class high-power TDFL laser output.

In 2010, Ehrenreich et al. from Nufern, USA, reported the utilization of a two-stage master oscillator power amplifier (MOPA) to achieve a laser output power of >1 kW with a conversion efficiency of 53.2% in 20/400 μm TDFs. In 2018, Ramírez-Martínez et al. at the University of Southampton, UK, prepared TDFs with a high concentration of Tm/Al co-doping by improved modified chemical vapor deposition (MCVD). This was followed by a combination of solution methods, with a doping concentration of 5.6% (mass fraction) for thulium, and a maximum laser conversion efficiency of 72.4%, which is the highest conversion efficiency of TDFs utilizing the cross-relaxation process. In 2022, Heuermann et al. at the University of Jena, Germany, achieved a single-pulse energy output of 1.65 mJ with an average power of 167 W and a repetition frequency of 101 kHz in a 2 μm region, using a low-pass filter (LPF) with a core diameter of 80 μm or a rod-shaped thulium-doped fiber (rod-type fiber) in conjunction with a four-channel laser coherent combining beam (Fig. 12). This result not only demonstrates the importance of TDF in the field of ultrashort pulsed lasers but also verifies the potential of coherent beam combining technology in expanding the laser output of the 2 μm band. In 2023, the research group optimized the optical path to increase the single-pulse energy output to 1.86 mJ at the 2 μm band.

Improving the laser conversion efficiency and reducing the effect of thermally induced mode instability of the laser output performance are still the focus of future research on 2 μm high-power laser output. As the 2 μm laser output breaks through the kilowatt barrier, nonlinear effects such as stimulated Brillouin source scattering will limit the laser output power, especially for the narrow linewidth high-power lasers. In conclusion, with aluminum or germanosilicate glass material as the main line, the fiber laser conversion efficiency is being continuously improved from the perspectives of rare-earth ion-doped glass spectral properties and glass acoustic/optical field coordination, combined with the fiber structure design. The principles of mode selection in step-type fibers, numerical aperture matching in step-type fibers, higher-order mode gain suppression in ring-type fibers, and mode filtering in photonic crystal fibers, are used along with mode instability and excited Brillouin scattering threshold, essentially breaking through the TDF laser output power limit. With the development of kW-class narrow linewidth lasers and laser beam combining technology at 2 μm region, 10 kW-class TDFL will also emerge and become the focus of research and development in high-energy lasers.