In a study published in Optics Express, Prof. Fu Yuxi and his team from the Xi’an Institute of Optics and Precision Mechanics (XIOPM) developed a cryogenically cooled Ho: YLF composite thin-disk laser oscillator with output power at the hundred-watt level. The 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.
By cooling the Ho: YLF crystal to 80 K with liquid nitrogen, the research team changed its lasing process from a quasi-three-level to a quasi-four-level system, which lowered the lasing threshold and reduced heat distortion. They then built a compact and efficient composite thin-disk laser system using a multi-pass pumping setup. The system uses a 2 at. % doped Ho: YLF crystal closely bonded to a pure YLF layer, which help 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%.
Moreover, the experiment 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 also 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 simultaneously opening new pathways for high-energy physics and engineering applications in the 2 µm spectral region," said Prof. FU Yuxi from XIOPM.
(Published May7, 2025)
Fig. Schematic of the cryogenic Ho: YLF thin-disk laser based on the 12-pass pumping module. (Image by XIOPM)
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