Significance   Multi-wavelength lasers that can simultaneously or alternately output different wavelengths have various applications in optoelectronic countermeasures, LiDAR, and medical treatment. However, achieving controllable and efficient multi-wavelength laser radiation is challenging due to the limitations of the emission spectrum and intensity of the laser materials. Nonlinear optical frequency conversion technology, especially stimulated Raman scattering (SRS), is an effective way to expand the laser wavelength range and enhance the laser power. SRS is a third-order nonlinear optical effect that shifts the frequency of the pump through molecular or lattice vibrations in the medium. Raman lasers can obtain high-power, high-beam-quality, and multi-wavelength laser output by utilizing the characteristics of phase conjugation, amplification, and cascade conversion of SRS. This paper introduces the basic principles of SRS and cascaded Raman conversion, summarizes the classification and structure of typical crystal Raman lasers, and reviews the current status, challenges, and opportunities of multi-wavelength laser technology based on crystal Raman conversion.

  Progress  The working principle of the stimulated Raman scattering (Fig.2) and the excitation principle of cascaded Raman scattering (Fig.3) are first outlined in this article. Then the basic structure of Raman lasers was discussed (Fig.4), which can be classified into intracavity and external cavity based on the location of the Raman gain medium relative to the laser working material. A special case of intracavity Raman lasers is self-Raman lasers, where the laser working material and the Raman gain medium are the same. Next, the characteristics of different types of Raman gain media, including gas, liquid, and solid are analyzed. Among them, Raman crystals are regarded as a promising medium for multi-wavelength lasers due to their advantages such as high gain, compact structure, and good stability. Typical crystal Raman gain media were compared and their parameters are summarized (Tab.1). Finally, the current research status of multi-wavelength crystalline Raman lasers as well as their features are summarized. Based on the above research status, it is not difficult to find that linear cavities are still the most commonly used resonant cavity structure for generating multi-wavelength Raman lasers, and pulse lasers account for the highest proportion of the research. In addition, compared to intracavity Raman oscillators, external cavity Raman oscillators exhibit higher average and peak power, demonstrating stronger power scalability. Although microcavity Raman lasers currently have low output power and conversion efficiency, they have the characteristics such as high repetition rate and miniaturization.

  Conclusions and Prospects   In conclusion, research on multi-wavelength lasers based on crystalline Raman conversion has made significant progress in the past decade, with the discovery of new crystals, structures, and wavelengths. The use of new crystal materials such as diamond has led to a remarkable performance in power enhancement, wavelength expansion, and miniaturization of multi-wavelength Raman lasers. Future research should focus on optimizing pump parameters and oscillator design to improve conversion efficiency, expand multi-wavelength lasers' output spectral range, and improve thermal management under high-power operation to enhance system stability and beam quality. With these advancements, we can expect that multi-wavelength solid-state lasers based on crystalline Raman conversion will play a major role in future applications.