With recent developments, the power of lasers has continuously improved, which has caused the inevitable increase in demand for laser output modules and heat dissipation, increasing the burden in terms of volume and weight as well as the serious problems of laser thermal effect on laser beam quality. Compared with traditional lasers, direct liquid-cooled solid-state lasers use a gain medium to directly contact the cooling fluid for efficient heat exchange, which greatly reduces the laser’s cooling system weight. Scaling and amplification of the output power can be achieved by stacking the slices in the gain module, and researchers favor its excellent characteristics. Therefore, a comprehensive optimization of the design of direct liquid-cooled solid-state lasers is required to discharge as much waste heat as possible and improve the output beam quality.

For the direct liquid-cooled thin-disk selected in this study, the gain module was composed of multiple thin-disk arrays, and the pumping mode of double-side pumping was used. The pump beam enters the pump sheet array along the side of the gain sheet. The distance between them was approximately 0.3 mm, forming a typical microchannel structure, in which the specially treated laser cooling fluid flows at a high speed and removes waste heat in the thin disk. The laser was incident on the gain medium along the Brewster angle to reduce interface loss. The gain medium material was Nd∶YAG crystal, and the cooling fluid was heavy water. As the working conditions of each gain sheet were the same, the optical path difference generated by the gain medium and half-flow field of the microchannel on both sides was selected as the research object. The temperature of the sheet was calculated by Fourier’s law, the flow field of the microchannel was calculated using the Navier-Stokes equation, the SST (Shear Stress Transport) model was selected as the solution model, and the stress and deformation field of the gain medium were calculated using related technology. Finally, the obtained data was introduced to the established optical path difference model, and the wavefront aberration calculation of the gain medium was performed under the corresponding working conditions.

The results show that with the increase of the microchannel height and gain medium thickness, the optical path difference (OPD) increases (Figs. 4 and 5); however, with the increase of the Reynolds number of the cooling fluid, the OPD decreases (Fig. 6), but the optimization efficiency also decreases. The design range of the single-medium thermal power density Q is determined by two aspects: (1) when the total power remains unchanged, the larger the Q, the fewer the gain medium sheets quantity required, which benefits beam quality control; (2) a larger Q is likely to cause breakage of the gain medium and cavitation in the cooling liquid. The OPD increases with the increase of Q (Fig. 7). As the computational complexity increases exponentially when the exhaustive method is used to optimize multiple variables, meanwhile, the dispersion of variables by the exhaustive method can also lead to losing the optimal solution, this study uses genetic algorithm to optimize the design of the OPD. Compared with the OPD root-mean-square (RMS) value of 3.73 μm and peak-valley (PV) value of 7.24 μm under the pre-optimized design (Fig. 3), the optimized design OPD RMS value is 3.27 μm and the PV value is 6.11 μm (Fig. 9), which are optimized by 12.3% and 15.6%, respectively (Fig. 8).

In this study, using a larger microchannel height did not improve the heat transfer effect; on the contrary, the OPD caused by the cooling liquid accounted for the main part. Therefore, the height of the microchannel should be minimized while designing the laser. Increasing the thickness of the gain medium reduces its surface area, deteriorates the heat transfer, and produces a more inhomogeneous optical path difference distribution. Therefore, the thickness of the gain medium should be minimized in the actual design. When the Reynolds number of the cooling fluid increases to a certain extent, improving only the convective heat transfer thermal resistance between the microchannel and gain medium has a minimal effect on the overall thermal resistance. Therefore, as the Reynolds number of the cooling fluid increases, the heat transfer effect improves and the OPD decreases, but the optimization efficiency also decreases. A higher thermal power results in a more uneven optical path difference distribution. However, as the thermal power is affected by the pump light, optimizing this variable in the actual design must still consider the output power. As a smaller Q typically requires more gain slices, the beam passes through too many complicated paths, which is detrimental to beam quality. Therefore, the actual optimization of Q should be designed according to actual needs.