Power scale-up of high-pulse-energy passively Q-switched Nd:YLF laser: Influence of negative thermal lens enhanced by upconversion

We develop a practical method to extend the power scale-up for a laser in a concave-plano cavity to be influenced by a large negative thermal lens. With the developed method, we successfully scale up the output power of a compact highpulse-energy passively Q-switched Nd:YLF laser at 1053 nm with the Cr ⁴⁺:YAG crystal as a saturable absorber. At an incident pump power of 12.6 W, the maximum output power under the optimum operation at 1053 nm reaches 2.61 W with a pulse width of 6 ns and a pulse repetition rate of 4.6 kHz. More importantly, we experimentally verify that the energy-transfer upconversion significantly enhances the negative focal length of thermal lens in the passively Q-switched Nd:YLF laser.