## 4 Experiment Results and Discussion  
Experiments in room and high temperature environments were conducted to verify the actual cooling capacity of the thermal control system and the performance of the mid-infrared OPO laser. The output power of OPO laser was measured using the laser power meter in 25 \({}^{\circ}\)C and 45 \({}^{\circ}\)C temperature environments. The laser controlling temperature was set to 20 \({}^{\circ}\)C. The near-field pattern of the laser was analyzed through the laser beam quality analyzer (PY3) in room temperature. The picture of the mid-infrared OPO laser is shown in Figure 10. The black case on the left side is the electrical control unit, and the case with fans on the right side is the optical unit. Figure 10: Picture of the mid-infrared OPO laser. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \begin{tabular}{c} **Ambient** \\ **Environment** \\ **Temperature (T\({}_{a}\))** \\ \end{tabular} & \(T_{1}\) & **TEC Driving Current** & **Position of TEMs** & \(R_{3}\) & \
\begin{tabular}{c} **TEM Cooling** \\ **Output (Q\({}_{c}\))** \\ \end{tabular} &
\begin{tabular}{c} **System Cooling** \\ **Capacity (Q\({}_{c}\))** \\ \end{tabular} \\ \hline
268.15 K (25 \({}^{\circ}\)C) & 293.15 K (20 \({}^{\circ}\)C) & 5 A & TEMs near the edge & 0.04 K/W & 39.5 W & 636 W \\ \hline
318.15 K (45 \({}^{\circ}\)C) & 293.15 K (20 \({}^{\circ}\)C) & 5 A & TEMs near the edge & 0.04 K/W & 24 K/W & 387.2 W \\ \hline \hline \end{tabular}
\end{table} 
Table 3: The estimated cooling capacity of the system in different ambient environments. Figure 9: Finite-element analysis results: (**a**) in 25 degrees and (**b**) in 45 degrees. In the room temperature environment, when the electrical control unit was powered on, the thermal control system began to control the refrigeration of the TEMs to reduce the temperature of the laser. About 4 min later, the upper surface temperature of heat sink was reduced and stabilized at about 20.1 \({}^{\circ}\)C. Then the pump source was turned on and the laser began to output light. Since the pump source generated heat and transferred it to the heat sink, the temperature rose to about 20.6 \({}^{\circ}\)C. The temperature control circuit acquired rose in temperature and calculated the PWM control variable based on the PID algorithm. Then the output voltage exerted on TEMs was increased appropriately and the cooling output was amplified. About 1 min later, the temperature was reduced and stabilized at about 20.1 \({}^{\circ}\)C. The temperature variation of the pump laser over time is shown in Figure 11.  
In the high temperature experiment, the mid-infrared OPO laser was put in the high and low temperature test boxes, as shown in Figure 12, and the environment temperature was set to 45 \({}^{\circ}\)C. About 10 min after the electrical control unit was powered on, the temperature was reduced and stabilized at about 20.1 \({}^{\circ}\)C. Then, the pump source was turned on and the laser began to output light, and the temperature rose to about 20.7 \({}^{\circ}\)C. About 1 min later, the temperature was reduced and stabilized at about 20.1 \({}^{\circ}\)C, as shown in Figure 11.  
Figure 11: Temperature variation of the pump laser. Figure 12: Experiment in the high temperature environment. The output power variation of the mid-infrared OPO laser with 70% maximum pump power in room and high temperature environments is shown in Figure 13. The near-field pattern of the OPO laser at room temperature was also measured and is shown in Figure 13. The following formula was used to calculate the output power stability (\(S\)) of the OPO laser:  
\[S=\pm\frac{P_{\mathrm{max}}-P_{\mathrm{min}}}{2\times P_{\mathrm{a}r}} \tag{13}\]  
\(P_{\mathrm{max}}\), \(P_{\mathrm{min}}\) and \(P_{\mathrm{a}\mathrm{v}r}\) are the maximum, minimum and average output powers of the OPO laser with the measured values. The calculated output power stability values of the laser in room and high temperature environment were \(\pm\)0.7% and \(\pm\)1%, respectively.