# Mid-Infrared Multispectral Gaseous Stimulated Raman Scattering Laser
## 3 Results and Discussion  
### High-Power Mid-IR Laser at 4432 nm  
In the Raman cell, with a 1064 nm laser pump, the SRS processes occurred in the mixture of H\({}_{2}\) and D\({}_{2}\) gases. When the pump laser was scattered on the vibrational transitions of H\({}_{2}\) with a Raman shift of \(\Omega_{R}^{\mathit{wilb}}(\mathrm{H_{2}})=4155\,\mathrm{cm^{-1}}\), the first Stokes (S1) at 1907 nm was generated. Similarly, when the pump laser was scattered on the vibrational transitions of D\({}_{2}\) with a Raman shift of \(\Omega_{R}^{\mathit{wilb}}(\mathrm{D_{2}})=2987\,\mathrm{cm^{-1}}\), the first Stokes (S1) at 1560 nm and the second Stokes (S2) at 2920 nm was generated. With the generation of 1560 nm and 1907 nm Raman lasers, FWM processes could occur in the Raman cell. A mid-IR Raman laser at 4432 nm would be obtained by the FWM process of the pump laser at 1064 nm, and Raman lasers at 1560 nm and 1907 nm. The scheme of the FWM process is shown in Figure 2.  
Figure 1: Schematic of the experimental setup. The inset shows the burn marks of Raman components with different wavelengths. According to [PERSON]'s research, the generated power of a 4432 nm laser by the FWM process above can be given by [19; 20]:  
\[P_{4}=Bp^{2}P_{1}P_{2}P_{3}\cdot\exp(b|\Delta k|) \tag{1}\]  
where \(B\) is a constant; \(p\) is the gas pressure in the Raman cell; \(b\) is the laser confocal parameter; \(P_{1},P_{2},P_{3}\) are the powers of the pump laser at 1064 nm, and the Raman lasers at 1560 nm and 1907 nm, separately; and \(\Delta k\) is the wave-vector mismatch caused by the dispersion of the Raman medium. In this case, the wave-vector mismatch \(\Delta k\) can be defined as:  
\[\Delta k=k_{1}+k_{2}-k_{3} \tag{2}\]  
where \(k_{1}\) and \(k_{2}\) are the wave vectors of the Raman lasers at 1560 nm and 1907 nm, separately, and \(k_{3}\) is the wave vector of the pump laser at 1064 nm. According to the Equation (1), the output power of a mid-IR laser at 4432 nm is proportional to a product of the output powers of the pump laser at 1064 nm, and Raman lasers at 1560 nm and 1907 nm, so the generation of a mid-IR laser at 4432 nm by the FWM process has no threshold. After generating a 4432 nm laser seed, it could be further amplified by the SRS process in the Raman cell. In comparison with the cascade SRS process, this work provided a much easier method to achieve a mid-IR laser output. In the Raman cell, due to the high Raman gain coefficient of the vibrational transitions, high-energy Raman lasers at 1560 nm, 1907 nm, and 2920 nm could be obtained by the SRS process in the mixture of H\({}_{2}\) and D\({}_{2}\) gases. The output powers of different Raman components were investigated and are shown in Figure 3. Had the SRS process occurred in a different gas pressure of Raman media, the conversion efficiencies of Raman components would be influenced greatly [21; 22]. To obtain high-energy mid-IR Raman laser output, the FWM process should be matched well by adjusting the partial pressure of H\({}_{2}\) and D\({}_{2}\) in the Raman cell. As shown in Figure 3a, while the 1064 nm laser pumped the mixed gas of 0.4 MPa H\({}_{2}\) and 2.0 MPa D\({}_{2}\), the output energies of the S1 Raman lasers at 1560 nm and 1907 nm were approximately equal at their highest conversion efficiencies. Similar output energies would contribute to generating a mid-IR laser at 4432 nm by the FWM process and the SRS amplification process. By slightly increasing the partial pressure of H\({}_{2}\) to 0.5 MPa, the output energy of the S1 Raman laser at 1907 nm could reach a higher level than the S1 Raman laser at 1560 nm of D\({}_{2}\), as shown in Figure 3b, which resulted in the suppression of Raman conversion at 1560 nm and 2920 nm of D\({}_{2}\). This condition was unfavorable for the generation of mid-IR Raman lasers. In the same experimental conditions, the output energy of the mid-IR laser at 4432 nm generated by the mixed gas of 0.5 MPa H\({}_{2}\) and 2.0 MPa D\({}_{2}\) was too weak to be recorded by an energy meter.