Development of an ultra-compact low pulse repetition Nd:YVO₄ passively mode-locked multipass resonator

We report the development of a high-efficiency, compact, and stable diode end-pumped low-frequency SESAM mode-locked <i>N</i><i>d</i>:<i>Y</i><i>V</i><i>O</i>₄ laser. By employing a multipass cavity, the laser delivers near-diffraction-limited mode-locked pulses with a repetition rate of 20.53 MHz, an average output power of 7.5 W, a pulse duration of 28.8 ps, and a peak power of 12.7 kW, corresponding to an optical-to-optical efficiency of 48.1% and a slope efficiency of 58.6%. The influence of curvature deviations of concave mirrors in the ultra-compact multipass cavity on mode-locking stability is analyzed. A theoretical model is established to elucidate the mechanism leading to multiple-pulsing, and by solving the rate equations, a numerical solution for suppressing higher-order harmonic mode-locking is obtained, which shows good agreement with experimental observations.