The features of deep-ultraviolet lasers are high single-photon energy, short wavelength, and easy absorption by materials. They are widely used in high-density optical data storage, high-resolution optical microscopy, material processing, spectral analysis, scientific research, and medical sterilization and diagnostic equipment. Currently, most deep-ultraviolet lasers are obtained by two or more nonlinear frequency conversions of near-infrared lasers; however, the efficiency of this method is generally low. In recent years, rare-earth ions (Pr³⁺) that can emit a visible laser directly at room temperature have attracted considerable attention. Its emission wavelengths span over the blue (485 nm), green (523 nm), orange (604 and 607 nm), and red (640, 698, and 721 nm) regions. The appearance of Pr³⁺ also makes it possible to obtain a deep-ultraviolet laser through a single nonlinear frequency conversion. Polarized emission spectra of the Pr³⁺∶LiYF₄ crystal were measured at room temperature. In addition to the standard transition wavelengths, a weak fluorescence spectrum of the ³P₀→³H₅ transition was observed in the tested fluorescence lines at 546 nm. Recently, in our experimental group, we used a double-end-pumping Pr³⁺∶LiYF₄ crystal for frequency-doubling of the weak spectral line with a β-BaB₂O₄ (BBO) crystal and obtained a continuous deep-ultraviolet laser at 273 nm with a power of 128 mW. Compared to ordinary solid-state lasers, single-frequency lasers have the advantages of excellent stability, narrow spectral lines, and good coherence. This study added a mode selection element to explore the 273 nm deep-ultraviolet laser further. A single longitudinal mode deep-ultraviolet laser with a center wavelength of 272.93515 nm was successfully obtained, and the maximum output power was 32 mW. This study is essential for measuring the content of the antidepressant sertraline hydrochloride.

The absorption properties of polarized Pr³⁺∶LiYF₄ crystals were studied. The absorption efficiency of the Pr³⁺∶LiYF₄ crystal at 444 nm wavelength for π polarization was measured (~94%), and the absorption efficiencies at two wavelengths for σ polarization were compared. The absorption efficiency at 441 nm (~79.5%) was higher than that at 444 nm (~53%). The absorption of laser by the Pr³⁺∶LiYF₄ crystal has polarization characteristics; thus, two laser diodes (LDs) of different wavelengths were combined by polarization as the pumping source. As a result, the entire pump power can be improved, and the polarization characteristics of the pump can be retained such that the absorption efficiency of the crystal correspondingly improves. Therefore, two LDs with an output power of 3.5 W, 444 nm in π-polarization direction and 441 nm in σ-polarization direction were used as the pump source; the length of Pr³⁺∶LiYF₄ crystal was 7 mm and that of the BBO crystal was 5 mm for intracavity frequency doubling. A V-shaped folded cavity was designed (Fig. 4). The beam waist radius (69 um) in Pr³⁺∶LiYF₄ crystal was designed to be small to ensure absorption efficiency. In contrast, the beam waist radius (102 μm) in BBO crystal was designed to be relatively large to reduce the power density of deep-ultraviolet laser and damage to the crystal (Fig. 5). Simultaneously, two different Fabry-Perot (F-P) etalon combinations were used to select the longitudinal mode. Two F-P etalons, with thickness of 0.5 mm and 1.2 mm and reflectivity of 60% and 70%, respectively, were selected with an incident angle of 0.25°. According to the cavity length, the longitudinal mode interval was calculated to be 2.34 GHz (0.00233 nm). The transmittance curves of the etalon sets were simulated for different longitudinal modes when the beam was incident at a small angle. When one of the longitudinal modes had the maximum transmittance (T=100%), the adjacent longitudinal mode exhibited a single-transmission loss of approximately 20% (Fig. 6). Under the condition that the longitudinal mode in the resonant cavity oscillates and is lost multiple times, only a single longitudinal mode at T=100% can initiate the oscillation, thereby ensuring a single longitudinal mode output of the laser.

Without any mode selector in the cavity, a deep-ultraviolet laser at 273 nm with an output power of 85 mW was obtained, and the measured results were multiple longitudinal modes. After adding two etalons, the single longitudinal mode 273 nm laser spectrum was measured using a wavelength meter (High Finesse WS7). The wavelength was single, and there was no adjacent longitudinal mode. The center wavelength was 272.93515 nm, the spectral linewidth was less than 80 fm (Fig. 7), and the wavelength stability was measured for two hours at a wavelength variation of 4.5 pm(Fig. 8). The output power of a single longitudinal mode deep-ultraviolet laser at 273 nm was measured using a power meter (Coherent FieldMaxII-TO). The maximum output power of a single longitudinal mode deep-ultraviolet laser at 273 nm (32 mW) was obtained when the combined output power of the two LDs at 441 and 444 nm was 6240 mW. The curve of the laser output characteristics was fitted to the experimental results. The output power of the single longitudinal mode deep-ultraviolet laser at 273 nm increases with increasing pump power. The slope also tends to increase potentially owing to the gradual adjustment of the LD wavelength to the absorption peak of the Pr³⁺∶LiYF₄ crystal. As the pump power continues to increase, the LD wavelength gradually deviates from the absorption peak of the crystal, the thermal lens effect of the crystal intensifies, and the slope of the curve gradually flattens (Fig. 9). We used a coherent power meter to test the stability of the maximum power of a single longitudinal mode 273 nm deep-ultraviolet laser. The root-mean-square (RMS) of the power stability was 0.717% after 1 h of continuous testing (Fig. 9). We measured the far-field beam shape using a beam profile analyzer (Spiricon BM-USB-SP928-IOS), which was a long strip owing to the walk-off effect of the BBO crystal. The beam quality (M² factor) was measured as 2.29 in the X- and 2.21 in the Y- direction using a beam quality analyzer (Thorlabs BP209-VIS/M) (Fig. 10).

In this study, a simple and effective V-shaped folded cavity was designed using a Pr³⁺∶LiYF₄ crystal made in China as the laser gain medium, and a π-polarized laser with a center wavelength of 444 nm and a σ-polarized laser with a center wavelength of 441 nm were used as the pump sources. Two different F-P etalons were used to select the longitudinal mode in the cavity, and the BBO crystal doubled the fundamental frequency of 546 nm to realize the stable operation of a 273 nm single longitudinal mode deep-ultraviolet laser. The measured center wavelength is 272.93515 nm, the maximum output power is 32 mW, and the RMS power stability is 0.717% in a 1 h continuous measurement. The pump source selected in this experiment matches the absorption peak of the crystal well, maximizes the absorption efficiency, and improves the laser output power. In future, we plan to continue to optimize the resonator, increase the power of the injected pump light, and further improve the output power of the 273 nm single longitudinal mode deep-ultraviolet laser.