1500 W nanosecond slab laser with up to 150 mJ pulse energy for extreme ultraviolet light sources

Nd:YAG solid-state lasers are considered a promising candidate to replace CO₂ lasers as the driver laser for extreme ultraviolet (EUV) light sources. Laser-produced plasma (LPP) EUV driver lasers must simultaneously achieve higher than 10 kHz high repetition rate, hundred-millijoule pulse energy, ten-nanosecond pulse width, good beam quality, and long-term operational stability. Achieving these specifications imposes significant technical challenges, particularly in managing waste heat and suppressing thermally induced distortion at elevated temperatures. In response to the requirements of LPP EUV driver lasers, a master oscillator power amplifier (MOPA) architecture is adopted, employing an end-pumped Nd:YAG slab gain module with a zig-zag propagation path as the power amplifier. Short-pulse extraction is achieved through triple-pass amplification in a single slab combined with temporal pulse sequencing. Thermal distortion is mitigated using microchannel coolers laterally bonded to the slab surfaces. A 4f imaging relay system is implemented to image the beam profile, minimizing diffraction-induced intensity modulation during propagation. Based on a laser kinetic model, numerical simulations are carried out to compare the amplification behavior of ∼10 ns short pulses with that of long-pulse or continuous-wave (CW) signals. Output power at various repetition rates is also calculated. A laser system is subsequently constructed, producing laser pulses with a duration of 7.8 ns and an average output power exceeding 1500 W across repetition rates from 10 to 50 kHz. At 10 kHz, an average output power of 1507 W is achieved, corresponding to a peak power of 19.28 MW, and the beam quality factors in the slab thickness and width directions are 1.98 and 3.23, respectively. The single-slab configuration results in a compact overall laser structure, making it suitable as the main-pulse source for benchtop EUV systems. Using this laser as the driver to irradiate liquid metal droplets, the spreading and propulsion velocities of the droplets under various single-pulse energies are experimentally compared.