Direct oscillation at 640-nm in single longitudinal mode with a diode-pumped Pr:YLF solid-state laser

A diode-pumped solid-state laser directly emitting continuous-wave (cw), single-longitudinal mode (SLM) output at 640-nm with a c-cut praseodymium-doped yttrium lithium fluoride (Pr:YLF) crystal was demonstrated for the first time, to the best of our knowledge. Combining two quarter-wave plates (QWPs) and one Brewster plate to form a twisted-mode cavity, this SLM solid-state laser achieved a maximum cw output power of 403 mW with a threshold of 600 mW and a slope efficiency of 26.8%. The emission spectrum had a linewidth of 150 MHz. The single-mode beam quality (M-2) was 1.10 and 1.07 along the x and y direction, respectively.