## 3 Experiment Results and Discussion
Clearly, the power of the 1064 nm SF laser was too low to build its own independent parametric oscillation, since the OPO  
Figure 3: (**a**) Simulated tuning curves at different periodically poled lithium niobate (PPLN) temperatures when the pump wavelength is 1018 nm and 1064 nm separately. (**b**) The calculated signal and idler wavelengths corresponding to 1018 nm and 1064 nm pump beams at 20 \({}^{\circ}\)C.  
Figure 2: Measured (**a**) output dual-wavelength spectrum and (**b**) interference result of the SF pump source using the Fabry–[PERSON] interferometer. threshold was about 13 W. Therefore, it could be concluded that the 3.2 \(\upmu\)m beam was generated by the parametric oscillation of 1018 nm beam, while the 3.7 \(\upmu\)m beam was generated by intracavity DFG between the low-power 1064 nm beam and the high-power 1492 nm intracavity signal beam. The generated wavelengths agreed well with the simulation in Figure 3b, considering the thermal effect of the crystal [16] and slight possible deviation of the temperature control system. Meanwhile, the FWHM of 3.2 \(\upmu\)m and 3.7 \(\upmu\)m idler beams were 777.98 pm (22.7 GHz) and 270.74 pm (5.9 GHz) separately. The narrow linewidth of 271 pm (5.9 GHz) corresponded to the frequency down-conversion of SF fiber laser, which proved that characteristics of the SF pump source were successfully transferred to the mid-infrared region. Then the PPLN temperature was adjusted from 20 to 60 \({}^{\circ}\)C in steps of 5 \({}^{\circ}\)C to investigate the frequency conversion of the 1064 nm SF pump wave under different phase-matching conditions. The obtained 3.7 \(\upmu\)m mid-infrared output power as well as the conversion efficiency was shown in Figure 5a. As can be seen from the figure, the idler power firstly increased rapidly from 16.15 mW to 73.5 mW and then decreased to 3.35 mW, and the corresponding pump-to-idler conversion efficiency increased from 4.75% to 21.6% and then decreased to 0.99%. The maximum output idler power and conversion efficiency were achieved when the temperature was 20 \({}^{\circ}\)C. The experiment results clearly shown that the deviation in temperature could cause an obvious decrease in generated power and conversion efficiency. Taking the results under 15 \({}^{\circ}\)C and 20 \({}^{\circ}\)C as an example, the temperature only changed by 5 \({}^{\circ}\)C, but the 3.7\(\upmu\)m idler power and conversion efficiency decreased from 73.5 to 18.8 mW and 21.6% to 5.5%, respectively. The results also proved that the importance of meeting the phase-matching conditions was highlighted and the selected temperature in this experiment was suitable. Then we used a dichroic mirror (anti-reflection coated for 3.47-4 \(\upmu\)m, T \(>\) 95%, high reflectivity for 2.5-3.47\(\upmu\)m, T \(<\) 1%) to separate the dual-wavelength mid-infrared beam. Under the phase-matching conditions, the output idler power and conversion efficiency at different 1064 nm pump powers is shown in Figure 5b. The 3.7 \(\upmu\)m idler power monotonically increased from 8.7 mW to 73.5 mW when the 1064 nm pump power increased from 73 mW to 340 mW. The corresponding conversion efficiency increased from 12% to 21.6%, and the slope efficiency of the whole parametric process reached 20.186%. Benefiting from the high intracavity signal power, the record conversion efficiency far exceeded the efficiency of other traditional single-pass DFG processes and was almost comparable to that of OPO processes. Some other near-infrared low-power sources, such as ultra-short pulsed lasers or frequency-swept lasers, would be of significant application values if they could be converted to the mid-infrared region. However, it is difficult for them to pump the OPO independently because of their low-power and limitations by fiber nonlinear effects during the amplification stages.