The 2-5 μm mid-infrared wavelength range is a crucial region in an electromagnetic spectrum. Lasers that operate within this range play a critical role in various fields, such as defense, medical, environmental monitoring, and materials science. The generation of 2-5 μm lasers mainly includes solid-state lasers, quantum cascade lasers (QCL), inter-band cascade lasers, optical parametric oscillators (OPO), and fiber lasers. Fiber lasers have unique advantages, such as good beam quality, excellent thermal management capabilities, and robustness, which make them irreplaceable for various mid-infrared laser applications. Three methods are mainly used to generate 2-5 μm fiber lasers: 1) rare-earth-doped fiber lasers, which are the simplest and fundamental; 2) nonlinear fiber lasers based on nonlinear effects, which are effective for extending the laser wavelength, filling the spectral gaps not covered by rare-earth-doped fiber lasers owing to transition-level limitations; 3) gas-filled fiber lasers, which utilize energy-level transitions in gas molecules (N₂O, HBr, and CO₂) to achieve mid-infrared laser outputs.

This study comprehensively reviews the research and power-scaling progress in mid-infrared fiber lasers based on all-solid-state fibers. It covers three main types of mid-infrared fiber lasers: rare-earth-doped, Raman, and mid-infrared super-continuum fiber lasers. Table 1 in the main text presents representative achievements of rare-earth-doped fiber lasers in the 2-5 μm wavelength range. The continuous-wave laser output power within this range has been significantly improved, from milliwatt to watt/kilowatt levels. The highest output power values obtained using fiber lasers doped with Tm³⁺, Er³⁺, Ho³⁺, and Dy³⁺ ions are 1100, 41.6, 7.2, and 10.1 W, respectively. In particular, the longest wavelength tunability of the rare-earth-doped fiber lasers is 700 nm. Tables 2 and 3 present representative results for mid-infrared Raman fiber lasers and tunable mid-infrared Raman soliton lasers, respectively. Currently, by using tellurite, fluoride, or chalcogenide glass fibers as the Raman gain media, a second-order-cascaded Raman fiber laser operating at 3.77 μm and a tunable Raman soliton fiber laser covering the range of 2.8-4.8 μm, with an average watt-level power output in the 3-3.8 μm region, have been developed. Tables 4 and 5 list the representative research progress on germania fiber- and soft glass fiber-based supercontinuum lasers, respectively. The output power of the supercontinuum laser using germania fiber as a nonlinear medium exceeds 41.9 W, and the spectral width is 1.9-3.5 μm. The maximum output power values of the fluorotellurite fiber- and fluoride fiber-based supercontinuum laser are 50.2 W and 11.8 W, respectively, and the spectral widths are 1.22-3.74 μm and 1.9-4.9 μm, respectively.

Since the beginning of 21 century, continuous improvements in semiconductor laser technology, mid-infrared glass-fiber drawing techniques, and pumping schemes have propelled the rapid development of mid-infrared fiber laser sources. In the field of high-power mid-infrared fiber lasers operating within the range of 2.5-5.0 μm, research groups worldwide have achieved significant milestones in the past decade. Nevertheless, compared with the advanced near-infrared waveband, a significant gap still exists in the output power of mid-infrared fiber lasers. The primary challenge lies in the development of mid-infrared fibers with high damage thresholds, broad transmission windows, and advanced ion-doping capabilities. The lack of high-quality mid-infrared fiber functional devices also hinders an effective increase in the output power of mid-infrared fiber lasers. The solution lies in the development of mid-infrared fiber functional devices with high damage thresholds and broad operating bandwidths. The heat load is another critical factor limiting the enhancement of laser power, and damage to laser systems is mostly related to excessive heat loads. Therefore, new methods for suppressing heat generation and regulating heat loads are required to achieve high-power mid-infrared fiber lasers.