Mid-infrared fiber sources, emitting between 2.5 µm and 5.0 µm, are interesting for their great potential in several application fields such as material processing, biomedicine, remote sensing and infrared countermeasures due to their high-power, their diffraction-limited beam quality as well as their robust monolithic architecture. In this review, we will focus on the recent progress in continuous wave and pulsed mid-infrared fiber lasers and the components that bring these laser sources closer to a field deployment as well as in industrial systems. Accordingly, we will briefly illustrate the potential of such mid-infrared fiber lasers through a few selected applications.

We report a dual-wavelength-pumped all-fiber continuous-wave (CW) laser operating at 3.55 µm that reached an output power of 14.9 W, which is, to the best of our knowledge, a record. The laser cavity, made of an erbium-doped fluoride fiber and bounded by two fiber Bragg gratings (FBGs), operates at an overall optical efficiency of 17.2% and a slope efficiency of 51.3% with respect to the 1976 nm launched pump power. The all-fiber design of the cavity not only allows for significant power scaling of the laser output, but also improves its long-term stability at high output power. The cavity design was set according to a numerical optimization that showed very good agreement with the experimental results.

We report a simply designed gain-switched all-fiber laser emitting a maximum average output power of 11.2 W at 2.826 µm. The corresponding extracted pulse energy is 80 µJ at a pulse duration of 170 ns. These performances significantly surpass previous gain-switched demonstrations and are close to the state-of-the-art $Q$-switched laser performances near 2.8 µm, but with a much simpler and robust all-fiber design. The spliceless laser cavity is made of a heavily erbium-doped fluoride glass fiber and is bounded by fiber Bragg gratings written directly in the gain fiber through the protective polymer coating.

A monolithic fiber laser emitting 1.7 W at 3920 nm is experimentally demonstrated in a Ho³⁺:InF₃ fiber. The cavity comprises a pair of highly reflective fiber Bragg gratings written in the active fiber with the femtosecond phase-mask scanning technique and is spliced to the pump diode with a robust silica-to-fluoride fiber splice. This work is an important step toward high-power all-fiber laser operating in the vicinity of 4 µm.

In this Letter, we report, to the best of our knowledge, the first demonstration of an in-band pumped gain-switched D y 3 + -doped fiber laser operating at 3.24 µm. The monolithic cavity bounded by two fiber Bragg gratings was pumped by a gain-switched E r 3 + -doped fiber system. It produced stable nanosecond pulses in a single-pulse regime on its entire operating range from 20 kHz to 120 kHz. A record average power of 1.43 W was achieved for a repetition rate of 120 kHz, and a record pulse energy of 19.2 µJ was achieved at 60 kHz. These results represent a significant improvement in D y 3 + -doped pulsed fiber laser performances and open the way to applications in the fields of remote sensing and material processing.

We report on an ytterbium-free, erbium-doped single-mode all-fiber laser reaching a record output power of 107 W at 1598 nm, with a slope efficiency of 38.6% according to the absorbed pump power at 981 nm. The erbium-doped gain fiber, co-doped with cerium, aluminum, and phosphorus, was fabricated in-house with adjusted doping concentrations to reduce erbium ions clustering, thereby increasing efficiency while keeping the numerical aperture low to ensure a single-mode laser operation. The addition of cerium co-dopant in the core glass of an erbium system is used for the first time, to the best of our knowledge, in order to adjust the fiber’s numerical aperture without increasing the erbium concentration. Numerical modeling, validated by the experimental results, demonstrates that adding aluminum and phosphorus at high concentration mitigates erbium ions clustering, with an estimated erbium paired ions of only 5.0% in the reported gain fiber.