# Double-Peaked Mid-Infrared Generation Based on Intracavity Difference Frequency Generation
## 3 Results and Discussion  
The 1018 nm laser is turned on and the power is increased to 43 W. The OPO process is generated in the cavity. After the signal beam is stabilized in the cavity, the ASE fiber source is turned on. The polarization period of MgO: PPLN is tuned, and two nonlinear frequency conversion processes are generated in the OPO cavity. In the experiment, it can be observed that the DFG process is generated in the OPO cavity when the pump source is first turned on. With the power of the ASE fiber source turned on to 2.1 W, the DFG idler power is stable, and the spectrum is shown in Figure 5a. The primary peak is 3695 nm, and the secondary peak is 3731 nm. Two peaks are spaced at 36 nm. Compared with the intensity of the 3695 nm peak, the intensity of the 3731 nm peak is lower. The FWHMs of the peaks at 3695 and 3731 nm are 30 and 24 nm, respectively. The polarization period is tuned at the phase-matched interval extreme values, as shown in Figure 5b. The primary peak is 3747 nm, and the secondary peak is 3770 nm. The FWHM of the two peaks is 25 and 20 nm, and the experimental results are consistent with the above simulation. In tuning the primary peak of the double-peaked mid-infrared generation from 3695 nm to 3747 nm, the intensity of the peaks is consistent, and the intensity at the long wavelength slightly decreases. Under the phase-matching condition, when the pump power is 2.1 W, the idler power generated by the DFG in the cavity is 0.18 W, and the pump-to-idler conversion efficiency is 8.5%. The tuning performance of the intracavity DFG system is further measured to generate a double-peaked idler beam in the tuning of 3688-3786 nm. As shown in Figure 6, the primary peaks are at 3725 and 3753 nm. The secondary peak intensity decreases when the primary peak is located at 3725 nm and the secondary peak of the idler beam is at 3688 nm. The spacing of the two peaks is 37 nm. The FWHMs of the primary and secondary peaks are 24 and 16 nm, respectively. In tune with the long wavelength, the secondary peak in the long wavelength also gradually decreases, as shown in Figure 6b. The spacing of the two peaks is 33 nm. The FWHM of the primary and secondary peaks are 20 and 12 nm, respectively. Figure 5: Intracavity DFG generates a double-peaked idler beam with the primary peak at (**a**) 3695 nm and (**b**) 3747 nm. By analyzing the difference between Figures 5 and 6, the idler beam in the phase-matching condition is a double-peaked spectrum with similar peak intensities. Moreover, the central wavelength of the mid-infrared spectrum is 3713-3758 nm. The primary peak is located at 3695-3747 nm. With further tuning of the polarization period, the intensity of the secondary peak in the short wavelength gradually decreases when tuning to the short wavelength. While tuning the crystal polarization period to the long wavelength, the secondary peak in the long wavelength gradually decreases. In the phase-mismatched condition, two peaks of the double-peaked mid-infrared generation have a high difference in intensity. The intracavity DFG is phase-mismatched in Figure 5a,b. In the phase-mismatching interval, it is necessary to substitute the phase-mismatched \(\Delta k\) when calculating the central wavelength and the FWHM of each peak in the phase-mismatched condition. \[\Delta k=2\pi\left(\frac{\text{n}_{\text{p}}(\lambda_{p},\theta)}{\lambda_{p} }-\frac{\text{n}_{\text{s}}(\lambda_{s},\theta)}{\lambda_{s}}-\frac{\text{n}_ {\text{i}}(\lambda_{i},\theta)}{\lambda_{i}}-\frac{1}{\Lambda}\right) \tag{6}\]  
where \(\Lambda\) is the polarization period of the crystal, and \(n_{p}\), \(n_{s}\), and \(n_{i}\) are the refractive indices of the pump beam, signal beam, and idler beam in the PPLN crystal, respectively. The signal beam generated by the 1018 nm laser is shifted as the tuning of the polarization period and the \(\Delta k\) increases. The bandwidth of the broadband mid-infrared generation decreases with an increase in \(\Delta k\).