Significance   High-power continuous wave (CW) single-frequency ultraviolet (UV) lasers have the advantages of narrow linewidth and concentrated energy distribution, and have shown promising applications in scientific research, industrial production and manufacturing, medical diagnosis and treatment, and civil life, including semiconductor lithography, fine material processing, and high-precision spectral analysis. Compared with traditional excimer lasers, ion lasers and free-electron lasers that produce ultraviolet lasers, all-solid-state ultraviolet lasers have more compact structure, lower cost, higher long-term stability and better beam quality. These advantages make people pay more attention to all-solid-state ultraviolet lasers, and all-solid-state continuous-wave single-frequency ultraviolet lasers will continue to develop towards high power and high reliability.

  Progress  The basic properties of nonlinear optical crystals used to produce high-power 266 nm ultraviolet laser are comprehensively compared (Tab.1). In the design and manufacturing of ultraviolet laser, the selection of nonlinear optical crystals and their optical properties and quality will directly affect the output power and beam quality of ultraviolet laser. In order to obtain higher-performance UV laser output, β-BaB₂O₄ (BBO) crystal, CsLiB₆O₁₀ (CLBO) and other borate crystals with wide optical band, excellent nonlinear optical effect, stable physical and chemical properties, and high laser damage threshold have been discovered successively, greatly promoting the development of high-power ultraviolet lasers. At present, the method to obtain 266 nm CW single-frequency ultraviolet laser is mainly obtained by external cavity frequency doubling based on nonlinear optical frequency conversion. Among them, the external cavity resonance enhanced frequency doubling technology based on continuous wave single-frequency 1064 nm all-solid-state laser to generate fourth harmonic has become an important method to obtain continuous wave 266 nm single-frequency laser output under low power conditions. For the resonance enhanced external cavity frequency doubling technology, because the resonance enhanced cavity length of the laser system will change under the interference of external environment such as external temperature, air humidity and mechanical vibration, the resonance state of the frequency doubling cavity will be damaged, resulting in poor laser output stability and even lower laser output power, so it is very important to use the electrical feedback control system to achieve accurate, stable and real-time control of the cavity length. At present, the frequency stabilization methods such as Hänsch-Couillaud (H-C) frequency locking, Pound-Driver-Hall (PDH) frequency locking and side-mode bias frequency locking are used for electrical feedback and control of the laser resonator. According to different laser frequency locking methods, this study mainly summarizes the development status of continuous wave single-frequency 266 nm laser using H-C frequency locking and PDH frequency locking methods at home and abroad. Compared with H-C frequency locking method with simple optical path, PDH frequency locking method is easier to obtain error signals with high signal-to-noise ratio, which is conducive to more stable UV laser output. Through comprehensive investigation, the development trend of all-solid-state ultraviolet laser is prospected at the end of this paper, aiming to provide reference for the development and research of all-solid-state ultraviolet laser technology. And this study introduces the latest research result of our research group, which is the stable output of a 1.1 W single-frequency continuous wave 266 nm ultraviolet laser based on the H-C frequency locking method.

  Conclusions and Prospects  The all-solid-state CW single-frequency ultraviolet laser is developing rapidly with the efforts of researchers. The all-solid-state single-frequency CW 266 nm laser has achieved a certain degree of productization, but there are still some problems to be solved for its development towards high power, mainly focusing on the poor anti-damage ability of frequency doubling crystal and the low frequency doubling efficiency caused by the intrinsic characteristics of crystal, and it is difficult to achieve higher power laser output. This study aims to provide some references for the design and optimization of all-solid-state ultraviolet lasers in the future. In order to meet higher production requirements and fully realize the commercialization of ultraviolet lasers, all-solid-state ultraviolet lasers will eventually develop towards a more stable and higher power direction.