Fig. 1. Energy levels in Pr³⁺:YLF crystals for the transitions from ¹I₆ to ³F₄ at 692 nm, 701 nm, 705.0 nm, and 709 nm.

Fig. 2. Emission cross sections of Pr³⁺:YLF crystal under 444 nm excitation. The emission cross sections from ¹I₆ transitions at 691.7 nm, 701.4 nm, 705.0 nm, and 708.7 nm are 0.15 × 10⁻¹⁹ cm², 0.15 × 10⁻¹⁹ cm², 0.13 × 10⁻¹⁹ cm², and 0.15 × 10⁻¹⁹ cm², respectively. The emission cross sections from ³P₀ transitions at 698 nm and 721 nm are 1.07 × 10⁻¹⁹ cm² and 1.78 × 10⁻¹⁹ cm², respectively.

Fig. 3. Schematic diagram of the laser experiment for the transition from ¹I₆ to ³F₄. LD array, with 24 W output power at 444 nm. Convex lens, with a focal length of 75 mm. M1, with a high transmittance (>98.3%) at 444 nm. M1 and M2, with a high reflectance (>99.8%) at 691.7 nm, 701.4 nm, 705.0 nm, and 708.7 nm. BF (2 mm thick). Filter, with a high reflectance in the blue light region and a high transmittance at 691.7 nm, 701.4 nm, 705.0 nm, and 708.7 nm.

Fig. 4. (a) Normalized intensity at 691.7 nm, 701.4 nm, 705.0 nm, and 708.7 nm with 0.05 nm full width at half-maximum. (b) Output powers of 2.44 W, 2.10 W, 2.01 W, and 2.42 W at 691.7 nm, 701.4 nm, 705.0 nm, and 708.7 nm, respectively, with slope efficiencies of 19.8%, 16.5%, 15.8%, and 19.4% and corresponding threshold powers of 3.40 W, 3.40 W, 3.40 W, and 3.78 W.

Fig. 5. Peak-to-peak power fluctuations of 2.47 ± 0.06 W, 2.14 ± 0.06 W, 2.05 ± 0.06 W, and 2.42 ± 0.03 W at 691.7 nm, 701.4 nm, 705.0 nm, and 708.7 nm, respectively.

Fig. 6. (a) Beam quality M_x², M_y² factors of 2.29, 2.03 at 691.7 nm. (b) M_x², M_y² factors of 2.23, 1.86 at 701.4 nm. (c) M_x², M_y² factors of 2.31, 2.08 at 705.0 nm. (d) M_x², M_y² factors of 2.41, 2.04 at 708.7 nm.