Diode-pumped solid-state lasers (DPSSLs) are widely used in many applications owing to their high energy, high repetition rate, and high efficiency. The gain medium is one of the core components of the DPSSL system; however, when the gain medium is subjected to a high-power pumping source, an uneven heat source is formed in it, resulting in an uneven temperature distribution. Furthermore, the cooling device can only dissipate heat to its surface, which in turn generates temperature gradients in different directions. The thermal deformation and stress caused by the temperature gradient in the gain medium eventually degrade the laser output power and beam quality. In this study, the finite element analysis (FEA) is used to optimize the design of a microchannel heat sink for laser amplifier cooling, and the effects of parameters such as microchannel bottom-plate thickness, channel height, channel width, channel wall thickness, and inlet velocity on the maximum temperature of the gain medium surface are investigated. The results are expected to provide accurate guidance for practical experiments.

To study the ability of the microchannel heat sink to cool the laser amplifier, a full-size model containing a slab-type gain medium and a microchannel heat sink is established (Fig. 1). The uppermost layer is the gain medium, the middle layer is the microchannel heat sink, and the bottom layer is the cover plate. The flow and convection-diffusion phenomena occurring in the inhomogeneous heat and microchannel heat sink within the slab-type gain medium are then studied by a flow-heat-solid multiphysical field coupling analysis. Finally, the coupling of heat and fluid under full-size model conditions is directly simulated using the ANSYS FLUENT module, in which the heat source within the gain medium is loaded through the UDF command. The pressure-velocity coupling is achieved using the SIMPLE algorithm, the flow parameter interpolation method is second-order windward, the heat sink material is purple copper, and the cooling medium is deionized water.

Pulsed pumping can be approximated as continuous pumping when the repetition rate is high (Fig. 3). When cooling the gain medium using a microchannel heat sink, the entire system can be simulated using the ANSYS FLUENT module under the set initial parameters (Fig. 4), and the distribution of thermal deposition in the gain medium can also be determined at this time (Fig. 5). When the microchannel bottom-plate thickness increases from 1 mm to 5 mm, the maximum temperature of the gain medium surface also increases (Fig. 6). The bottom-plate thickness cannot be too small, given that microchannel thermal deposition will be deformed by heat and must withstand a certain water pressure. Therefore, the optimal value of the bottom-plate thickness is set as 2 mm. When the channel height increases from 2 mm to 4 mm, the maximum temperature decreases significantly; when the height increases from 4 mm, the maximum temperature decreases, but the decrease is not significant (Fig. 7). Therefore, a channel height of 4 mm is selected. In the process of increasing the width of the channel from 0.3 mm to 1.2 mm, the maximum temperature and thermal resistance increase gradually. The pressure loss decreases significantly when the channel width increases from 0.3 mm to 0.6 mm, and the pressure loss decreases slowly when the channel width continues to increase from 0.6 mm. However, the pressure loss increases sharply when the width of the microchannel is too small. Therefore, the channel width is set at 0.4 mm (Fig. 8). The lowest temperature is observed on the surface of the gain medium when the channel-wall thickness is 0.3 mm (Fig. 9). The inlet velocity also affects the temperature of the gain medium surface (Fig. 10). When the flow rate increases from 0.5 m/s to 3 m/s, the temperature of the gain medium surface decreases, but the pressure loss increases. Therefore, the flow rate of 2.5 m/s is optimal. The equivalent heat transfer coefficient of the microchannel system is also calculated under the premise that the temperature of the system is known (Fig. 11). This value can be used to measure the cooling capacity of the microchannel cooling system under different parameters (Fig. 12). The analysis shows that the equivalent heat transfer coefficient of the microchannel system is up to 5000 W/(m²·K).

In this study, the temperature distribution characteristics of a high-repetition-rate and high-energy conduction cooling laser amplifier are numerically simulated by finite element analysis. The effect of each parameter within the heat sink of the microchannel on the maximum temperature of the gain medium surface is systematically discussed and analyzed, and the values of each parameter are optimized from the perspective of practical application and safety. The maximum temperature of the gain medium surface is the lowest when the bottom-plate thickness is 2 mm, channel height is 4 mm, channel width is 0.4 mm, and channel wall thickness is 0.3 mm. On this basis, the effect of the inlet velocity of the cooling fluid is further analyzed. The results show that the inlet velocity is not as high as possible, but its value is suitable; an extremely high inlet velocity is not conducive to a significant reduction in the surface temperature of the gain medium and will also cause a large pressure loss. Finally, the maximum temperature of the gain medium surface for specific microchannel parameters is determined. The equivalent heat transfer coefficient obtained can accurately and clearly reflect the cooling capacity of the microchannel system. The calculation results of this study can provide a strong numerical basis and theoretical foundation for practical fabrication and experiments on microchannel heat sink structures for slab laser amplifiers.