Thermal lensing-induced dynamic beam evolution and damage behaviors in a 1 kHz, 521 mJ nanosecond Ho:YLF MOPA system

A high-power, high-energy 2-µm Ho:YLF MOPA system delivering 521 mJ pulses with a pulse width of 20.3 ns at a repetition rate of 1 kHz was demonstrated using a polarization beam combining technique. To the best of our knowledge, this represents the highest pulse energy reported for a 2-µm laser operating at a high repetition rate. The thermal-lensing-induced beam evolution behavior and potential pulse-induced damage risk during the power-scaling process of the Ho:YLF amplifiers were systematically investigated. Experimental results show that the gradually established negative thermal lens continuously modifies the beam profile, beam size, and laser fluence during power scaling, whereas the pronounced anisotropy of the Ho:YLF further introduces asymmetric beam evolution in multistage amplifiers. Meanwhile, the complex, thermal-lensing-induced evolution of laser fluence may increase the risk of pulse-induced damage, particularly under the relatively low pump intensity used in the multistage amplifier, thereby compromising the safe and stable operation of the system. This study provides useful guidance for the design and optimization of multistage amplifier systems with anisotropic and negative thermal lensing effects.