In a study published in Optics Express, Prof. FU Yuxi's team from the Xi’an Institute of Optics and Precision Mechanics (XIOPM) of the Chinese Academy of Sciences developed a cryogenically cooled Ho: YLF composite thin-disk laser oscillator with output power at the hundred-watt level. 

Laser systems emitting at 2 µm have special spectral features important for high-order harmonic generation, terahertz wave production, ultrafast optical amplification, and infrared defense. However, at room temperature, the Ho: YLF crystal works as a quasi-three-level system, which leads to a high lasing threshold and serious heat problems, making it hard to increase the power for large-scale use.

In this study, researchers changed the lasing process of the Ho: YLF crystal from a quasi-three-level to a quasi-four-level system by cooling it to 80 K with liquid nitrogen, which lowers the lasing threshold and reduces heat distortion. They then built a compact and efficient composite thin-disk laser system using a multi-pass pumping setup. 

This system used a 2 at. % doped Ho: YLF crystal closely bonded to a pure YLF layer, which helps improve structural strength. With a 12-pass pumping design, the pump absorption efficiency reached 63.7%. Together with a plane-concave resonator, the system produced 128 W of continuous-wave output with a slope efficiency of 63.2% and an optical efficiency of over 62.5%.

Experiments showed clear performance improvements of the Ho: YLF crystal at low temperatures. The fluorescence lifetime increased from 10.7 ms at room temperature to 15.4 ms at 80 K, showing better energy storage ability. The laser exhibited good stability with power changes staying within 1.4% during one hour of full-power operation. The beam stayed close to the diffraction limit, proving that the cryogenic composite thin-disk design helps reduce thermal effects and supports stable, high-power output.

"This laser serves as a powerful driving source for ultrafast science and attosecond pulse generation, while providing new ways for high-energy physics and engineering applications in the 2 µm spectral region," said Prof. FU.