## 3 Experiment Results and Discussion  
The output characteristics of two fiber sources were measured firstly. The output dual-wavelength spectrum was measured using the Yokogawa AQ6370D Optical Spectrum Analyzer and is shown in Figure 2a. Two spectral peaks appeared in the figure, located at 1018 nm and 1064 nm, respectively, indicating that the wavelength interval was 46 nm. The 1064 nm spectral peak was about 20 dB lower than the 1018 nm spectral peak. Figure 2b reveals the measured interference result of 1064 nm pump laser using the Fabry-Perot interferometer. The blue and black lines represent the normalized transmissivity and scanning voltage, respectively. The figure indicates that the free spectral range (FSR) of the 1064 nm pump laser was about 4 GHz, which proved that the 1064 nm SF fiber source was in the steady operating state. In this experiment, the 1064 nm pump power was too low to reach the oscillation threshold and build an independent parametric oscillation. Thus, in order to realize an effective frequency down-conversion, the phase-matching conditions need to be met, and the 1064 nm SF pump beam also needs to strongly interact with the intracavity signal beam. For confirming the most suitable PPLN temperature, the tuning curves corresponding to 1018 nm and 1064 nm pump beams at different temperatures were calculated based on the Sellmeier equations: [15]  
\[n_{e}^{2}(\lambda,T)=a_{1}+b_{1}f(T)+\frac{a_{2}+b_{2}f(T)}{\lambda^{2}-\left( a_{3}+b_{3}f(T)\right)^{2}}+\frac{a_{4}+b_{4}f(T)}{\lambda^{2}-a_{5}^{2}}-a_{6}^ {2}\lambda^{2} \tag{1}\]  
Figure 1: Schematic diagram of the frequency down-conversion of single-frequency (SF) fiber source using intracavity difference frequency generation (DFG). \[f(T)=(T-24.5)\times(T+570.82) \tag{2}\]  
where a\({}_{1}\) = 5.756, a\({}_{2}\) = 0.0983, a\({}_{3}\) = 0.202, a\({}_{4}\) = 189.32, a\({}_{5}\) = 12.52, a\({}_{6}\) = 0.0132, b\({}_{1}\) = 2.86 \(\times\) 10\({}^{-6}\), b\({}_{2}\) = 4.7 \(\times\) 10\({}^{-8}\), b\({}_{3}\) = 6.113 \(\times\) 10\({}^{-8}\), b\({}_{4}\) = 1.516 \(\times\) 10\({}^{-4}\). The simulation results were shown in Figure 3a. The tuning curves at the same temperature were displayed in the same color. The x-coordinate of the dotted blue line was 29.82 \(\upmu\)m, which was the adopted period in the experiment. The figure reveals that there is always an intersection between the 1018 nm and 1064 nm tuning curve, and the intersection wavelength changes with the temperature. Specifically, as the temperature changed from 10 \({}^{\circ}\)C to 60 \({}^{\circ}\)C in steps of 5 \({}^{\circ}\)C, the grating period of the crystal changed from 29.65 to 29.85 \(\upmu\)m and the intersection signal wavelength changed from 1489.3 to 1490.9 nm. At the intersection, the frequency conversion of two pump beams could share the same signal beam and the phase-matching conditions were simultaneously met. The simulation results also indicate that the PPLN temperature should be adjusted to 20 \({}^{\circ}\)C to meet the phase-matching conditions. The tuning curves of 1018 and 1064 nm were further simulated and shown in Figure 3b. The intersection of the two tuning curves corresponds to the signal wavelength of 1490.6 nm at 20 \({}^{\circ}\)C. The simulated idler wavelengths at the intersection were 3210 nm and 3718 nm separately. After the PPLN temperature was set at 20 \({}^{\circ}\)C, the signal and idler spectrum were measured when the 1064 nm pump power was 340 mW and the 1018 nm pump power was 56 W. The center signal wavelength was 1492 nm with a full width at half maximum (FWHM) of 42 pm (3.5 GHz), as shown in Figure 4a. The corresponding idler spectrum was measured using a Bristol 721B Optical Spectrum Analyzer, shown in Figure 4b. The wavelengths of the two idler beams were 3206.3 nm and 3711.3 nm, which for convenience, were abbreviated as 3.2 \(\upmu\)m and 3.7 \(\upmu\)m, respectively.