## 2 Micro-Channel Heat Sink Structure Design  
The heat source in the microchannel heat sink system is the bottom-emitting VCSEL array, and the structure is schematically shown in Figure 1. It generates a large amount of waste heat in the DBR and active areas. The design requirements of heat sink for the high-power VCSEL array are shown in Table 1.  
3D printing technology has high manufacturing flexibility of complex structures, so based on the idea of increasing the coolant heat exchange area to strengthen heat transfer, in this study, we designed microchannel heat sinks with three structures: Double-layer reflow type, honeycomb type, and needle-fin type.  
[PERSON] pointed out that one disadvantage of monolayer microchannels was the high and uneven temperature distribution along the channel direction. It is because the flow rate  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Type** & **Calculation Conditions** \\ \hline Chip Heat Source & 100 W \\ Photovoltaic conversion efficiency & 50\% \\ Chip Size & \(5\times 5\times 0.5\) mm \\ Chip Materials & GaAs \\ Cooling mass & Deionized water \\ Heat sink size & \(10\times 10\) mm \\ Heat sink materials & Oxygen-free copper \\ \hline \hline \end{tabular}
\end{table}
Table 1: Heat dissipation requirements of VCSEL arrays.  
Figure 1: Schematic diagram of the high heat flux bottom emitting VCSEL structure.  
of the coolant is relatively low, and large amount of heat generated by the chip cannot be dissipated, resulting in a temperature gradient that causes thermal stress inside the VCSEL chip. High local thermal stress is a major cause of chip failure. One way to reduce the temperature gradient is to increase the flow of coolant. Therefore, in the study, we designed a double-layer reflow microchannel with dual inlets. As shown in Figure 2, the liquid flows in the two adjacent layers of the heat sink were in opposite directions, as shown by the arrow in Figure 2b, and the upper channel is dissipated through the lower channel for secondary heat dissipation, thereby achieving uniform heat dissipation to the chip.  
The honeycomb structure is a bionic design of a natural beehive, which consists of a series of hollow cell arrays and has the advantages of structural stability and large contact area of internal channels. As shown in Figure 3, the honeycomb microchannel heat sink was designed with staggered arrangement of microchannels in a hexagonal imitation honeycomb structure, and the multi-layer channels were stacked to significantly increase the heat transfer area. The side length and wall thickness of the hexagonal microchannels were both 0.3 mm.  
Pin-fin microchannel heat sink increases the effective heat transfer area and turbulence intensity by setting up spoiler columns to achieve stronger heat dissipation than conventional long straight channels. As shown in Figure 4, the cross-section of the pin-fin spoiler column distributed in the channel was a square with a side length of 0.5 mm. In order to reduce the resistance to water, the diagonal of the pin-fin section was set parallel to the direction of water flow.  
Figure 3: (**a**) Honeycomb microchannel cooling system model. (**b**) Schematic diagram of the cross-sectional structure of the honeycomb microchannel front view.  
Figure 2: (**a**) Model of the double-layer reflow microchannel heat dissipation system. (**b**) Schematic diagram of the side-view cross-sectional structure of the double-layer reflow microchannel.  
In order to distribute the cooling mass flow and flow rate evenly, the inlets of the above three models adopt a tapering structure with the center gradually expanding to both sides, and the slow flow area of the outlet adopts a tapering structure with the center gradually narrowing to both sides, so that the cooling masses converge together.