Fig. 1. Some spectroscopy properties of the Pr³⁺:YLF crystal. (a) Major deep red laser transitions of the Pr³⁺:YLF crystal from ³P_0,1,2 to ³F₄, ³F₃^[38]. (b) Emission cross-sections of the Pr³⁺:YLF crystal in the deep red region.

Fig. 2. (a) Experimental scheme for CW UV lasers. (b) Transmittance curves of the M3 and M4 mirrors.

Fig. 3. Output powers, laser spectrum and M² factors of the high-power CW single-wavelength UV laser at 348.7 nm. (a) Output powers with respect to absorbed pump powers and the laser spectrum. (b) M² factors of the 348.7-nm laser beam in the x and y directions.

Fig. 4. Simulation results of the 348.7-nm laser output powers under different effective thermal focal lengths. Here, f is the value of the effective thermal focal length and and are the laser beam sizes in the gain medium and nonlinear crystal, respectively.

Fig. 5. Measured results for the CW discrete tunable UV lasers. (a) Laser output powers at different wavelengths. (b) Laser spectra corresponding to (a). (c) Output powers with respect to the absorbed pump powers of the two lasers with relatively high output powers. (d) M² factors of the 347.9-nm laser beam in the x and y directions. (e) The transmittance of the -BBO crystal (normal incidence) in the deep red region.

Fig. 6. Simulation results to further understand the wavelength tuning. (a) Round trip TM mode transmittances comparison of using only the 1-mm thick quartz plate and both the plate and the -BBO crystal at the same time. (b) Relative phase-matching angles at different wavelengths of the -BBO crystal.