Deep ultraviolet (DUV) lasers are increasingly used in laser cooling, ultraviolet lithography, and optical storage. For atomic manipulation experiments such as laser cooling, with the continuous enrichment of quantum manipulation methods, deep ultraviolet lasers have been more and more used. In the study of mercury atomic optical lattice clocks, the ¹S₀→³P₁ transition wavelength for laser cooling is 253.7 nm. The cooling light needs to provide a laser power of around 100 mW. In addition, in the clock transition frequency probe, the intensity noise of the detection light is introduced into the clock frequency detection, resulting in the reduction of the signal-to-noise ratio of the clock frequency detection fluorescence signal. Therefore, a stable cooling light source plays a crucial role in the preparation of cold atoms and the detection of clock frequency.

Nonlinear frequency conversion is one of the main ways to achieve the transformation of visible light to deep ultraviolet light, and it is currently a practical and reliable way to obtain deep ultraviolet lasers. In the high-power environment, the damage of nonlinear crystals has become a key factor affecting the lifetime of all-solid-state deep ultraviolet lasers. Compared with commonly used nonlinear crystals such as BBO (β-BaB₂O₄), the CLBO (CsLiB₆O₁₀) crystal takes the advantages of high damage threshold, small walk-off angle, and low absorption, which can effectively enhance the stability in long-term operation of deep ultraviolet lasers. However, due to the defects of the Cs atoms in the crystal, the CLBO crystal is hygroscopic at room temperature, so it is necessary to control the temperature of the crystal to 150 ℃ or even higher to reduce the influence of water molecules around the crystal. Therefore, placing the CLBO crystals in oxygen can extend their lifetime. In the experiment, the frequency of the 1014.8 nm laser is stabilized by using a transfer cavity and an optical-phase-locked loop. The resonant frequency doubling of the external cavity based on nonlinear crystals is one of the ways to achieve stable and reliable deep ultraviolet lasers. The specific process is to quadruple the output light of the high-power infrared laser to obtain the required wavelength. The first stage of frequency doubling can be achieved by single-pass frequency doubling with a periodically polarized crystal, and the second stage of frequency doubling can be achieved by resonant frequency doubling. Finally, the 253.7 nm laser is obtained by these technical methods.

Results and discussions The frequency drift of the seed light in 10 h is within 20 MHz (Fig. 4). Considering the frequency drift of the wavelength meter, the seed light exhibits very good frequency stability. On the other hand, the stable seed light also plays a positive role in improving the locking effect of the frequency doubling cavity. The 253.7 nm ultraviolet (UV) laser power for a 507.4 nm laser with a single pass through the CLBO crystal is measured. The nonlinear coefficient E_nl=1.15×10^-4 /W of the CLBO crystal is obtained by fitting (Fig. 5). Under a 1.1 W input power of fundamental light, up to 198 mW power of deep UV laser output at 253.7 nm is demonstrated by cavity-enhanced frequency doubling, and the frequency doubling conversion efficiency is 18% (Fig. 6). The fineness of the bow-tie cavity is 220, and the total loss of the cavity is about 2.8%. When the power of fundamental input light is 1.1 W, the light-field pattern matching rate is 91%. Through spatial filtering, the beam quality factors of the deep ultraviolet laser reach Mx2=1.59 and My2=1.15. The relative intensity noise of the 253.7 nm laser is -90 dBc/Hz at 1 Hz. It is lower than -100 dBc/Hz at above 10 Hz (Fig. 7), mainly due to the high stability of the seed laser. The total operation time is more than 150 h. When the DUV laser continuously operates under an output power of 125 mW for 48 h, the peak-to-peak power fluctuation is 9.8%, and the root mean square (RMS) is 3.1% (Fig. 8). This highly stable deep ultraviolet laser has great significance for the preparation, cooling and fluorescence detection of mercury atom optical lattice clock.

In this paper, a 253.7 nm deep ultraviolet laser system based on CLBO crystal frequency doubling is implemented for laser cooling of mercury atoms. The output of the 253.7 nm laser based on CLBO crystal frequency doubling comes true. Through the power stabilization of green light, the heating of CLBO crystals, and the sealing and inflation of the cavity, the long-term continuous and stable output of the deep ultraviolet laser is realized. In addition, the transfer cavity frequency stabilization and the optical phase-locked loop are used to reduce the frequency drift and the noise of the seed laser. The PDH (Pound-Drever-Hall) technology is used to improve the stability of the frequency doubling cavity operation. During the 48 h operation, the UV power shows good long-term stability. Due to the damage of the output coupling mirror, the frequency doubling efficiency is reduced. Therefore, the low-damage output coupling mirror can be replaced to improve its continuous operation capability. Through spatial filtering, we also get Gaussian spots with better beam quality. This kind of deep ultraviolet laser system with high power, low noise and high stability can better meet the needs of mercury atom cooling experiments and improve the continuous operation ability of cold atom experiments.