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 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.

We report a tunable all-fiber laser emitting a maximum output power of 2.55 W around 3240 nm. The fiber laser cavity based on a fluoride fiber doped with dysprosium ions yields an efficiency of 42% according to the in-band launched pump power at 2825 nm. Due to a custom piezoelectric fiber Bragg grating (FBG) package, mechanical strains applied to the narrowband FBG used as the input cavity coupler allowed for fast tuning of the emission wavelength over a spectral range of 1.5 nm. This laser was deployed in the field in northern Québec (Canada) to assess its performances for remote sensing of methane in the presence of a significant amount of water vapor, i.e., over a hydroelectric reservoir. The preliminary results acquired during this field campaign confirm the great potential of the proposed approach for the development of a real-time active imaging system of greenhouse gases.

The limited availability of powerful, tunable, and reliable mid-infrared sources has historically prevented their widespread adoption in spectroscopy applications, even if most greenhouse gases’ fundamental absorption lines are found in this region of the electromagnetic spectrum. However, both mid-infrared fiber lasers and ICLs have enjoyed substantial growth in available output powers in recent years. Since the two technologies have complementary benefits, combining them could prove to be an interesting avenue to explore toward the development of a powerful, easily tunable, and narrow linewidth mid-infrared source. We report what we believe to be the first demonstration of fiber amplification of a distributed feedback interband cascade laser (DFB-ICL) operating in the mid-infrared. The system, based on an in-band pumped dysprosium-doped fluoride fiber amplifier, yields 10 dB of gain and up to 30 mW of output power at 3240 nm. We believe this is an important milestone toward power scaling of single-mode, single-frequency, and rapidly tunable mid-infrared laser sources suitable for advanced gas spectroscopy.

We report what we believe to be the highest power emitted by an erbium-dysprosium (Er ³⁺ /Dy ³⁺ ) co-doped fiber laser and the first monolithic (all-fiber) design of such a co-doped system, taking advantage of two fiber Bragg gratings directly inscribed in the gain fiber. This laser emits up to 4.42 W of continuous power at 3425 nm, a wavelength very far from the gain peak of dysprosium, with a slope efficiency of 10%, according to the launched pump power. This represents a tenfold increase compared to the previously reported output power in this wavelength range from such a co-doped fiber laser. The laser is cladding-pumped by a standard 976 nm laser diode, which is a great advantage for power scaling, stability, and industrialization compared to dual-wavelength pumped erbium-doped or singly dysprosium-doped fiber lasers, which both use more complex core-pumping schemes. The emission wavelength was chosen to coincide with the absorption peak of high-density polyethylene, a common industrial polymer notoriously refractory to laser machining, opening new avenues for resonant polymer processing.

We report the highest power achieved to our knowledge for an ytterbium-free alumino-phosphosilicate co-doped erbium fiber laser with 51W of output power. We will also discuss on the 100W-level power scaling potential of such laser.