Significance   Lasers with special wavelengths, high power, and high beam quality have significant applications in the fields such as sodium guide star, laser ranging, and free-space communication. One of the effective approaches to extend the spectral range of lasers is based on stimulated Raman scattering (SRS), which can amplify Stokes beam with a desired wavelength using conventional pump sources. This method can produce high-power and high-quality lasers with special wavelengths, and has advantages such as flexible wavelength selection, simple structure, and strong power scalability. In recent years, SRS-based amplifiers have been applied to generate sodium guide star laser sources, and have potential for further development in other areas. This article reviews the main principles, characteristics, and research progress of high-power free-space Raman amplification technology, and discusses its future trends and application prospects.

  Progress  Currently, the commonly used gain media for Raman amplifiers include gases and crystals. Gas Raman media have advantages such as a large Raman frequency shift, low self-focusing threshold, low optical coupling wave loss, and almost unlimited size. However, they also have disadvantages such as low gain, large volume, and susceptibility to optical breakdown. Compared to gas Raman media, crystal Raman media have advantages such as high Raman gain coefficient, good thermal conductivity, stable performance, and easy miniaturization. However, there are still bottlenecks in the output power and energy of crystalline Raman amplifiers due to factors such as crystal size and damage threshold. Beam combination based on Raman amplification is also an important way to break through the power bottleneck of a single beam and achieve power scaling. This method has advantages such as simple structure, flexible design, and high expandability, and is expected to be further developed and applied in the field of high-power special wavelength lasers. The parameters of gas Raman amplifiers with free-space structures are summarized (Tab.1). At present, the peak laser power output has reached the megawatt level, and the single pulse energy has reached the joule level. The experimental parameters of some crystal Raman amplifiers are summarized (Tab.2). The pulse width of crystal Raman amplifiers is mainly in the nanosecond, picosecond, and femtosecond levels, with peak power reaching the gigawatt level and single pulse energy reaching the millijoule level.

  Conclusions and Prospects  In recent decades, Raman amplifiers in free space have made many outstanding achievements in the field of high-power special wavelength lasers. However, the output power of Raman amplifiers is still limited by factors such as the Raman medium and amplifier structure. To overcome these limitations, future developments in Raman amplification technology will focus on developing new Raman media, optimizing the preparation technology of large-size Raman crystals, improving the conversion efficiency of Raman amplifiers, and expanding the beam combination structure of high-power Raman lasers. In the future, Raman amplification technology is expected to achieve even greater results in the field of high-power special wavelength lasers.