# Double-Peaked Mid-Infrared Generation Based on Intracavity Difference Frequency Generation
## 2 Experimental Setup and Methods  
In the research of dual-wavelength mid-infrared OPOs, the current theory of multi-optical parametric oscillation is very mature and can generate wide-tuned, dual-wavelength, mid-infrared lasers. However, the development of dual-wavelength mid-infrared OPOs is limited by the shortage of pumping bandwidths and phase-matching bandwidths, which makes it challenging to generate a dual-peaked mid-infrared laser with a wide FWHM. In this paper, a scheme to generate a dual-peaked mid-infrared laser by a parametric conversion technique is considered for the demand of DIAL at 3-5 \(\upmu\)m. Assume that the phase mismatch during the covariance conversion is 0. At this time, if two parametric beams, \(\lambda_{1}\) and \(\lambda_{2}\), are injected simultaneously into the OPO cavity, the two parametric beams satisfy the conservation equation in the OPO cavity. \[\frac{1}{\lambda_{p1}}-\frac{1}{\lambda_{s1}}=\frac{1}{\lambda_{i1}} \tag{1}\]  
\[\frac{1}{\lambda_{p2}}-\frac{1}{\lambda_{s2}}=\frac{1}{\lambda_{i2}} \tag{2}\]  
\(\lambda_{p}\), \(\lambda_{s}\), and \(\lambda_{i}\) represent the pump, signal, and idler beam, respectively. Assuming that the two idler beams, \(\lambda_{i1}\) and \(\lambda_{i2}\), satisfy the parametric transition under phase-matching, two hypotheses exist for the case of generating a double-peaked idler beam. The first hypothesis states that the two injected beams are the signal beams \(\lambda_{s1}\) and \(\lambda_{s2}\), the pump beam is located at the same wavelength, and the combined beam is \(\lambda_{p}\)* by tuning the spacing \(\Delta\lambda_{s}\) of the two signal beams to regulate the peak spacing \(\Delta\lambda_{i}\) of the idler beam. Equations (1) and (2) can be associated as follows. \[\Delta\lambda_{i}=-\frac{{\lambda_{p*}}^{2}}{\left(\lambda_{s11}-\lambda_{p*} \right)\left(\lambda_{s12}-\lambda_{p*}\right)}\Delta\lambda_{s} \tag{3}\]  
There is a severe paradox in Equation (3). The transformation relationship between the signal spacing \(\Delta\lambda_{s}\) and the idler frequency spacing \(\Delta\lambda_{i}\) is inversely proportional; therefore, injecting two signal beams is not feasible. The second hypothesis states that the two injected parametric beams are the pump beam \(\lambda_{p21}\) and \(\lambda_{p22}\), and the signal beam \(\lambda_{s}\)* is located at the same wavelength. Couplings (1) and (2) are as follows. \[\Delta\lambda_{i2}=\frac{{\lambda_{s*}}^{2}}{\left(\lambda_{s*}-\lambda_{p21} \right)\left(\lambda_{s*}-\lambda_{p22}\right)}\Delta\lambda_{p} \tag{4}\]  
In Equation (4), the pump bimodal spacing \(\Delta\lambda_{p}\) is proportional to the transformation of the idler beam bimodal spacing \(\Delta\lambda_{i}\), and the scheme is theoretically feasible. When the OPO cavity is simultaneously injected into two beams with a small central wavelength spacing or a double-peaked pump source in the pumping bandwidth, a double-peaked mid-infrared laser can be generated. However, generating a double-peaked mid-infrared laser in a short wavelength range with two pump beams injected into the OPO is challenging due to increased competition. For this reason, it is necessary to explore a double-peaked pump source with more stable power after particular spectral modulation and unique technical means to make the double peaks of the pump beam simultaneously with a signal beam to generate nonlinear frequency conversion. The ASE source is the pump source in the experimental platform due to its excellent spectral flatness and stable power. The pump source and the spectrum were analyzed for the generation of the double-peaked pump source. Figure 1a shows the experimental setup of the ASE fiber source, which is a typical forward pumping structure with a one-stage amplification system. As shown in Figure 1b,c, the ASE fiber source has a narrowing bandwidth and wavelength shift during the power amplification process.