High-power lasers operating in the 2 μm region have been used extensively in scientific research, national defense, laser medicine, and environmental monitoring. Currently, 2 μm lasers are primarily generated via two technological approaches. One method involves the indirect generation of 2 μm laser through optical parametric oscillation (OPO). However, this method is associated with complex systems and inferior beam quality, thus rendering it difficult to achieve a fundamental-mode output. By contrast, the other approach involves an oscillator emitting a 2 μm region directly based on a gain medium doped with Ho3?. Owing to its simplicity and efficiency, researchers are focusing on its potential application. In thin-disk lasers (TDLs), the thickness of the gain medium is only a few hundred micrometers. Compared with fiber lasers, TDLs allow for a larger radial-longitudinal ratio and have fewer nonlinear effects. Additionally, compared with rod lasers, the large diameter-to-thickness ratio of the Ho∶YAG thin disk ensures an almost one-dimensional heat flow through the disk, thus minimizing thermal effects. In recent years, researchers worldwide have focused on multipass-pumped Ho∶YAG TDLs. In 2021, Tomilov et al. achieved an output power of 112 W with an optical-to-optical efficiency of 54.6% using a 72-pass pump module. Owing to constraints in key technologies such as multipass module design, ultrathin crystal processing, and heat-removal design, studies regarding 2 μm TDLs can not be conducted in China.

We construct a Ho∶YAG TDL using our custom-developed 72-pass module (Fig. 5). First, we analyze the energy levels and fluorescence spectrum of the Ho∶YAG laser. Subsequently, based on the absorption-efficiency curve of the crystal for the pump, we select a Ho∶YAG thin disk, which enables the reflectivity of the crystal to reach 99.98%, with a surface figure of 0.072λ (λ=632.8 nm). Subsequently, a V-shaped cavity is constructed, and the pump spot is rescaled for a high-power continuous wave (CW)output. Finally, the beam quality factor is obtained using the knife-edge method and the laser spectrum is analyzed using an infrared spectrometer.

Based on a pump spot diameter of 3.6 mm, the output power and optical-to-optical efficiency curves as a function of the pump power at different output coupling ratios are shown in Fig. 10. When the pump power is 300 W and the output coupling ratio is 3%, the output power is 129.8 W, the optical-to-optical efficiency is 43.27%, the slope efficiency is 46.98%, and the corresponding pump power density is 2.9 kW/cm². Figure 11 shows the curves of the output power and optical-to-optical efficiency versus the pump power based on a pump beam diameter of 4.5 mm. The results indicate that a maximum output power of 150.2 W is achieved through beam-spot scaling, which corresponds to a pump power density of 2.0 kW/cm², and no saturation effects. The optical-to-optical efficiency is 44.18%, which is comparable to that obtained under D_pump=3.6 mm (corresponding to a pump power density of 2.9 kW/cm²), and the slope efficiency is 48.29%. However, because of the limitations of the pump source, the output power cannot be further increased. By fitting the beam-propagation equation to the data shown in Fig. 12, the beam quality factors are measured to be 2.64 and 2.42 in the x- and y-directions, respectively, using the 90/10 knife-edge method. Figure 13 shows that the output spectrum of the Ho∶YAG TDL exhibits two peaks at 2091 nm and 2097 nm. Subsequently, wavelength selection will be performed using a Fabry-Perot (F-P) etalon.

In this study, a custom-developed 72-pass pump module pumped with a Tm-doped fiber laser at 1908 nm is utilized. A 186-μm-thick Ho∶YAG thin disk with a doping mass fraction of 2.5% and a diameter of 10 mm is used. By integrating certain coating techniques, an absorption efficiency of 97.09% is achieved. The surface figure of the Ho∶YAG thin-disk crystal with a diamond heat sink is 0.072λ. In conclusion, a maximum output power of 150.2 W, a slope efficiency of 48.29%, and an optical-to-optical efficiency of 44.18% are achieved. The beam quality factors are 2.64 and 2.42 in the x- and y-directions, respectively. To the best of our knowledge, this is the highest CW power achieved using a Ho∶YAG TDL reported thus far. By increasing the pump power and scaling the pump spot size, a higher output power can be achieved. This study establishes a crucial technological foundation for the development of 2.09 μm pulsed lasers with high average-power levels.