Significance   High-power mid-infrared fiber laser sources have important applications in molecular spectroscopy, optical communications, biomedical, remote sensing, environment monitoring, and national defense security. Currently, mid-infrared laser sources mainly include rare ion doped fiber lasers, Raman fiber lasers and broadband supercontinuum light sources. At present, 3-4 µm fiber lasers have been demonstrated based on rare ions (such as holmium ions, erbium ions, dysprosium ions and so on) doped fluoride glass fiber. However, limited by the inherent energy levels of rare earth ions and large quantum defects, rare earth ion-doped fiber lasers are difficult to achieve lasing at any wavelength in mid-infrared band, and the laser output power decreases significantly with the increase of wavelength. Raman fiber lasers based on the stimulated Raman scattering (SRS) effects have the characteristic of low quantum loss and flexible output wavelength. SRS is an important nonlinear optical process in optical fibers, and it is an inelastic scattering with stimulated radiation properties. Raman fiber laser has a wide gain spectral bandwidth and can realize the cascade operation. So, with an appropriate pump source and a low loss gain fiber, Raman fiber lasers operating at any wavelength within the transmission window of the fiber glass matrix can be achieved, which is inaccessible for rare earth ions doped fiber laser. In addition, the Raman soliton lasers achieved by using soliton self-frequency-shift effect is also one important way to obtain widely tunable mid-infrared laser sources. Researchers are focus on developing fiber materials with wide mid-infrared transmission window, high laser damage threshold, big Raman shift, large Raman gain coefficients, and corresponding high power mid-infrared Raman laser sources.

  Progress  This paper introduces the progress of several mid-infrared glass optical fiber materials and the corresponding Raman laser sources. At present, the nonlinear medium used in the development of mid-infrared Raman laser source is mainly based on glass fibers with low loss in the mid-infrared region, including fluoride, chalcogenide and tellurite glass fibers. Fluoride glass fibers have a low transmission loss. By using fluoride glass fiber as Raman gain media, researchers have reported a 3.7 W Raman fiber laser at 2231 nm and a Raman soliton laser source with a tunable wavelength rang covering 2-4.3 µm. Chalcogenide glasses have the widest mid-infrared transmission window and the largest Raman gain coefficients among mid-infrared glasses. By using chalcogenide glass fiber as Raman gain media, researchers reported a second-order cascaded Raman laser operating at 3.77 µm, which is the longest wavelength for the Raman fiber lasers obtained in mid-infrared glass fibers. However, its output power is quite low (several milliwatts). Compared with the fluoride and chalcogenide glass, tellurite glasses have a larger Raman frequency shift and stronger laser damage resistance. Theoretical studies show that using tellurite glass fibers as Raman gain media, a Raman fiber laser with an average output power of tens of watts and a Raman soliton laser source with a tunable wavelength range covering 2.8-4.8 µm could be achieved. Very recently, to further improve the performances of tellurite fiber-based laser sources, fluorotellurite fibers with a broadband transmission window (0.4-6.0 µm), high laser damage threshold, big Raman shift (~785 cm⁻¹), and large Raman gain coefficient (1.28×10⁻¹² m/W@2 µm) have been developed by the authors. By using them as Raman gain medium, the authors achieved fifth-order cascaded Raman shift at ~3.75 µm and build cascaded Raman amplifiers. Besides, the authors also obtained Raman soliton laser sources with wavelength tuning rang covering 1.98-2.82 µm, and dispersive wave at ~4 µm.

  Conclusions and Prospects  As one of the important technologies to obtain mid-infrared laser sources, Raman fiber lasers have received extensive attention. At present, by using fluoride, chalcogenide or tellurite glass fibers as gain media, the Raman fiber laser operating at 3.77 µm and Raman soliton laser source with a tunable wavelength range of 2-4.3 µm have been developed. The authors developed fluorotellurite fibers with good stabilities and high laser damage threshold, and preliminarily verified their potential for constructing high power mid-infrared Raman laser sources. It is believed that, in the near future, by further improving the quality of fluorotellurite glass fibers, mid-infrared Raman fiber lasers with output power up to tens of Watts or even hundreds of Watts and the mid-infrared Raman soliton laser source with a tunable wavelength range covering 2-5 µm can be realized.