Chinese researchers have developed for the first time a room temperature HoYLF thin film laser

In a study published in Optics Express, the research team led by Professor Fu Yuxi of the Xi'an Institute of Optics and Precision Mechanics (XIOPM) of the Chinese Academy of Sciences developed the room temperature holmium doped lithium yttrium fluoride (Ho: YLF) composite thin slice laser for the first time, which can achieve high efficiency and high-quality CW laser output.

Laser devices operating in the 2 µ m spectral range are highly regarded for their safety for the eyes, high absorption rate by water, and low atmospheric attenuation. Traditional 2 µ m lasers typically require low-temperature cooling to control thermal effects, which increases system complexity and cost, and limits their application in compact, space limited, and mobile platforms. Therefore, developing high-power room temperature 2 µ m lasers has become an important research direction.

Table 1: Overview of 2 μ m Region Thin Film Laser. Source: Bingying Lei, Liyi Zhang, Sen Yang et al, 《Near diffraction-limited in-band pumped Ho:YLF composite thin disk laser at 2 μm》，《Optics Express》（2025）.

In this study, researchers developed a novel composite thin film structure based on Ho: YLF. By combining 2 at.% doped Ho: YLF crystal with undoped YLF coating, the mechanical robustness of the crystal is significantly improved, while effectively suppressing the amplification effect of spontaneous emission, thereby enhancing the stability of laser output.

Figure 1: Schematic diagram of Ho: YLF composite thin film crystal. (a) 3D schematic diagram of Ho: YLF composite thin film. (b) Photo of Ho: YLF composite sheet welded onto SiC heat sink. (c) Cross sectional view of Ho: YLF composite thin film along the direction of pump light propagation. Source: Bingying Lei, Liyi Zhang, Sen Yang et al, 《Near diffraction-limited in-band pumped Ho:YLF composite thin disk laser at 2 μm》，《Optics Express》（2025）.

In addition, the researchers also optimized the optical pumping system, adopting a multi-channel configuration with 12 pump cycles and combining efficient thermal management strategies. This method not only ensures high power output, but also minimizes the thermal lens effect, resulting in excellent beam quality.

Figure 2: Schematic diagram of Ho: YLF thin film laser. (a) 3D schematic diagram of thin film laser based on 12 pump modules. (b) Experimental setup diagram showing a composite thin film laser head with a water-cooled radiator. Source: Bingying Lei, Liyi Zhang, Sen Yang et al, 《Near diffraction-limited in-band pumped Ho:YLF composite thin disk laser at 2 μm》，《Optics Express》（2025）.

The experimental results show that when the laser is pumped by a 1940nm thulium doped fiber laser with a diameter of 1.8mm, the peak output power reaches 26.5W, the optical efficiency is 38.1%, and the slope efficiency is 42.0%. The beam quality has almost reached the diffraction limit, and the relative standard deviation of power stability is only 0.35%.

Figure 3: Absorption and emission cross-sections of 2 at.% doped Ho: YLF crystals at room temperature. Source: Bingying Lei, Liyi Zhang, Sen Yang et al, 《Near diffraction-limited in-band pumped Ho:YLF composite thin disk laser at 2 μm》，《Optics Express》（2025）.

Figure 4 Output power of Ho: YLF thin film laser measured using 3% transmittance OC. Source: Bingying Lei, Liyi Zhang, Sen Yang et al, 《Near diffraction-limited in-band pumped Ho:YLF composite thin disk laser at 2 μm》，《Optics Express》（2025）.

Figure 5: (a) Room temperature spectra of thulium doped fiber laser pumping and (b) CW emission. The dashed line represents the absorption cross-section and emission cross-section of Ho: YLF crystal. Each spectrum represents the average of five measurements. Source: Bingying Lei, Liyi Zhang, Sen Yang et al, 《Near diffraction-limited in-band pumped Ho:YLF composite thin disk laser at 2 μm》，《Optics Express》（2025）.

Figure 6: Output beam quality. (a) M2 measurement of 26W output beam quality. (b) Beam profiles at different distances (L=-200mm, 0mm, 100mm, and 300mm). Source: Bingying Lei, Liyi Zhang, Sen Yang et al, 《Near diffraction-limited in-band pumped Ho:YLF composite thin disk laser at 2 μm》，《Optics Express》（2025）.

Professor Fu said, "This work paves the way for the development of compact and economically efficient high-power 2 µ m lasers, which may reach the level of 100W and promote the development of ultrafast laser science. It also provides a new method for developing high-power and portable infrared laser systems.

Source: Yangtze River Delta Laser Alliance