Stimulated Raman scattering offers both advantages and disadvantages to high-power fiber lasers. Although it limits the power scaling of rare-earth-doped fiber lasers, it is currently the only mechanism that can generate high-power lasers at new wavelengths, thus rendering it a “promising asset.” Owing to the rapid development of fiber and semiconductor technologies, Raman fiber lasers based on stimulated Raman scattering (SRS) have enabled new achievements. They have the output power of Raman fiber lasers at a single wavelength exceeded 10 kW, and the power across various new bands has increased significantly, thus fully demonstrating their significant potential for wavelength extension. This study summarizes the research progress of continuous-wave near-infrared Raman fiber lasers from two aspects: power scaling and wavelength extension. It outlines the unique gain, mode, and nonlinear integrated control technology pathways, as well as provides references and insights for the further development of high-power fiber lasers.

In recent years, owing to the rapid development of rare-earth-doped fiber lasers, the power level of Raman fiber lasers using high-power, high-brightness ytterbium-doped fiber lasers as pump sources has exponentially increased annually. Combined with methods such as cladding pumping and hybrid gain, Raman fiber lasers have achieved a power breakthrough at the 10-kW level, i.e., approaching the highest power level of rare-earth-doped fiber lasers. Raman fiber lasers based on pure Raman gain only, despite issues such as spectral broadening and insufficient pump conversion, have achieved the highest output powers of 4 kW and 1.8 kW for amplifiers and oscillators, respectively. Meanwhile, wavelength extension based on Raman fiber lasers has progressed significantly, with the output wavelength extending from the conventional 1130 nm and 1120 nm to the shortest visible light and the longest to the mid-infrared wavelengths; the spectral range with output powers of several hundred watts encompasses more than 700 nm.

Currently, domestic and international review literatures pertaining to Raman fiber lasers, including the main research topics, are abundant. Owing to space limitations, this study focuses on summarizing the representative research achievements of near-infrared Raman fiber continuous lasers in terms of power enhancement and wavelength expansion, based on the aforementioned results. The types of fibers are primarily silicon-based fibers, and the pump sources include both fiber and semiconductor lasers, as well as different laser structures such as oscillators, amplifiers, and random cavities. Among these, power enhancement primarily includes two types of gain: ytterbium-Raman hybrid gain and single-stage pure Raman gain, whose main results are summarized in Table 1/Fig. 2 and Table 2/Fig. 4, respectively. Wavelength expansion primarily involves two implementation approaches: cascaded Raman and specific wavelength pumping (primarily direct pumping by semiconductor lasers), whose main results are summarized in Table 3/Fig. 11 and Table 4, respectively.

The rapid development of Raman fiber lasers clearly indicate significant achievements in terms of power enhancement and wavelength expansion. Currently, single-stage Raman fiber lasers pumped by fiber lasers and laser diodes (LDs) have reached power levels of 10 kW and the kilowatt range, respectively, thus rendering them an effective method for expanding the high-power laser spectrum in the vicinity of the 1-μm spectral band. Additionally, cascaded Raman fiber lasers are an important scheme for achieving high-power laser outputs over a larger spectral range, although the power level remains at approximately hundreds of watts. Although the mode and target-wavelength laser gain must be further regulated, one can assume that Raman fiber-laser technology primarily entails the suppression of higher-order Raman scattering. Therefore, the development of Raman fiber lasers must account for the characteristics of different Raman conversion processes. Additionally, the design of Raman gain fibers, the pumping methods, and the temporal, spatial, and spectral domain characteristics of the pump and seed lasers must be optimized comprehensively to achieve comprehensive control of the laser gain, nonlinear effects, and beam quality.