## 3 Experiment and Results  
In this experiment, we pumped our OPO module with a CW, linearly polarized, single-frequency Yb-doped fiber laser (IPG Photonics). The laser can deliver a maximum power of 50 W at 1064 nm with a beam diameter of 2 mm. Its spectral linewidth is only 70 kHz, which is one of the keys to achieving a narrowband MIR output. A free space optical isolator was used to protect the pump laser from unwanted back scattering, followed by a half-wave plate (HWP) to recover the polarization of 45\({}^{\circ}\) to a vertical polarization, and matched the \(Z\)-axis of the MgO:PPLN. A focal lens L with a focal length of 150 mm beam was used to focus the pump light to the center of the MgO:PPLN, with a beam waist radius of 58 um. Considering the different diffraction properties at pump and signal wavelengths, we made the radius of signal light slightly larger than that of the pump light to optimize the overlap mode between them.  
Figure 1: The configuration of the module (**a**) and the optical schematic of the experiment (**b**). HWP: half wavelength plate; ISO: isolator; MgO:PPLN: MgO-doped periodically poled lithium niobate crystal; CW: continuous-wave.  
We measured the temperature tuning from the OPO module with both channels by varying the MgO:PPLN temperature from 20 to 170 \({}^{\circ}\)C. The MIR wavelength can be tuned from 2416.17 to 2932.25 nm or from 3142.18 to 3452.15 nm in 31.59-\(\upmu\)m- and 30.49-\(\upmu\)m-poled channels, as shown by the black and red curves in Figure 2, respectively.  
There are two wavelengths of special interest in the tuning range: 2.7 and 3.3 \(\upmu\)m, which correspond to the absorption peaks of H-O and C-H bonds, respectively. The strong molecular rovibrational transitions that can be accessed with mid-IR laser sources allowed trace-gas sensing down to the parts-per-quadrillion level. We measured the output power from the MIR module at these two wavelengths. Figure 3 shows the output power as functions of the pump power when the MgO:PPLN temperature is set at 105.6 \({}^{\circ}\)C. The MIR wavelength was 2.7 \(\upmu\)m for the 31.59-\(\upmu\)m-poled channel, corresponding to a signal wavelength of 1.75 \(\upmu\)m. For the 30.49 \(\upmu\)m channel, the MIR and signal wavelength were 3.3 and 1.56 \(\upmu\)m, respectively. The thresholds of the two above processes were 5.9 and 7.4 W, respectively. The output power for both wavelengths rose steadily as the pump power increased. The MIR output was limited by the maximum pump power of 50 W and exceeded 10.4 and 9.1 W at 2.7 and 3.3 \(\upmu\)m, respectively. The maximum conversion efficiencies were 24% for 2.7 \(\upmu\)m and 21% for 3.3 \(\upmu\)m, respectively. The conversion efficiencies of the idler output were not linear because of the saturation of the pump depletion.  
Figure 3: Output power of the module at 2.7 and 3.3 \(\upmu\)m as a function of incident pump power.  
Figure 2: The output wavelength of the module as a function of the crystal temperature.  
The power stability of the OPO module was measured. The test lasted 10 h in a room without special temperature stabilization. The pump power was maintained at 35 W throughout the entire testing process. The MIR power was about 6.2 W at this pump power. As shown in Figure 4, the RMS of the MIR output power in 10 h was 2.13%. We measured the spectra characteristic using an optical spectra analyzer (YOKOGAWA AQ6375, YOKOGAWA, Tokyo, Japan), where the measured linewidth was limited by the instruction spectral resolution of 6 GHz. However, we could infer that our OPO was oscillating in a single longitudinal mode.