# Mid-Infrared Multispectral Gaseous Stimulated Raman Scattering Laser
## 3 Results and Discussion
### 2-6 \(\mu\)m Multispectral Mid-IR Raman Laser  
With the generation of high-energy Raman components at wavelengths of 1907 nm, 2920 nm, and 4432 nm, these Raman lasers could be used as pump sources and scattered on the rotational transition of \(\mathrm{D_{2}}\) with Raman shift of \(\mathrm{\Omega_{R}^{rot}(D_{2})=415~{}cm^{-1}}\). The circular polarization of the pump laser could contribute to reducing the threshold of the rotational Raman scattering of \(\mathrm{D_{2}}\)[26; 27]. Thus, pumped by the laser at 1064 nm with a pulse energy of 431 mJ, multispectral mid-IR Raman lasers in the range of 2-6 \(\mu\)m could be generated as: the first Stokes at 2071 nm and the second Stokes at 2266 nm, pumped by a 1907 nm laser; the first Stokes at 3322 nm and the first anti-Stokes at 2604 nm, pumped by a 2920 nm laser; the first Stokes at 5431 nm and the first anti-Stokes at 3743 nm, pumped by a 4432 nm laser. The spectra of the mid-IR Raman lasers are shown in Figure 5, and the SRS processes generating all the mid-IR Raman components in the range of 2-6 \(\mu\)m are shown in Table 1.  
Figure 4: Pulse shape (**a**) and energy conversion curve (**b**) of the mid-IR laser at 4432 nm.  
Figure 5: Mid-IR spectra of the Raman lasers generated by the 1064 nm laser pump for the mixture of \(\mathrm{H_{2}}\) and \(\mathrm{D_{2}}\) gases.  
In this work, by using a CaF\({}_{2}\) prism, the output mid-IR Raman lasers could be dispersed, and a slit could be used to select a specific wavelength that we would need in these mid-IR Raman lasers. Furthermore, some previous studies showed that it would be possible to finely tune the frequencies of Raman lasers by using wavelength-tunable laser sources, or with a tunable seed injection [28; 29].  
In our experiment, it is possible to generate the second Stokes laser at 9.2 \(\upmu\)m. However, our experimental condition makes the detection of 9.2 \(\upmu\)m very difficult. First, the optical conversion efficiency of 9.2 \(\upmu\)m is expected to lead to limitations. Even if it has a similar photon conversion efficiency to a D\({}_{2}\) second Stokes (2.92 \(\upmu\)m) Raman laser, the 9.2 \(\upmu\)m Raman laser would have much lower energy due to large quantum deficiency. Furthermore, a CaF\({}_{2}\) exit window and prism were used in this experiment. CaF\({}_{2}\) has strong absorption at 9.2 \(\upmu\)m, and this will further decrease energy of a 9.2 \(\upmu\)m laser. Output Raman lasers were dispersed by a prism, focused by a CaF\({}_{2}\) lens, with a burn mark used to know the exact position of the Raman line before the energy of the Raman laser was measured. However, in our experiment, even if a 9.2 \(\upmu\)m Raman laser was generated, it would still be very difficult to obtain the burn mark, and, consequently, the energy of the 9.2 \(\upmu\)m Raman laser could not be measured. In fact, if the second Stokes laser at 9.2 \(\upmu\)m is expected to be generated, a pure hydrogen and linear polarization pump laser should be used, and a Herriot multiple pass Raman configuration will also benefit the generation of a 9.2 \(\upmu\)m Raman laser.