## 3 Results and Discussion
### Stimulated Rotational Raman Scattering in Compressed \(H_{2}\)
Thus, the stably observed SRRS when using linear pump polarization was not associated with the inaccurate conduction of the experiment but has not yet found an exhaustive explanation. Figure 2: (**a**) The dependence of the SRRS conversion efficiency on the pump energy for circular (red solid line) and linear (black dashed line) polarizations of the laser beam at a hydrogen pressure of 3 MPa; (**b**) overall SRS spectra with circular (red solid line) and linear (black dashed line) polarizations of the laser beam at a pulse energy of 7 mJ. The axis of the quarter-wave retardation plate (\(\lambda\)/4 in Figure 1) was initially set at an angle of 0\({}^{\circ}\) to ensure the linear polarization of the laser beam. The pump energy was limited to 7 mJ to avoid damaging the MgF\({}_{2}\) windows of the Raman cell. Under the linear polarization of the pump beam, the SRRS threshold (determined at an SRRS efficiency of 1%) was reached at an energy of 2.5 mJ (Figure 2a, black dashed line). With increasing pump energy, the efficiency of SRRS conversion rose sharply to 16%. However, due to the occurrence of anti-Stokes components, upon reaching a pump energy of 4.5 mJ, the SRRS conversion efficiency gradually saturated to a maximum value of 23%. The observed spectrum was formed by several peaks at \(\sim\)1094 nm, \(\sim\)1166 nm, and \(\sim\)1251 nm (Figure 2b, black dashed line) with gradually decreasing intensity, within the measurement error corresponding to the first, second, and third Stokes orders of the cascade SRRS in hydrogen (spaced apart by a rotational Raman shift of 587 cm\({}^{-1}\)). The first of the peaks in Figure 2b is at the 1030 nm pump wavelength of the Yb:YAG laser. In addition, two more lines were observed, associated with the measurement error with SVRS at -1790 nm (based on the vibrational Raman shift of 4155 cm\({}^{-1}\)) and roughly corresponding to the rotational Stokes from this radiation at \(\sim\)1993 nm. Once the polarization of the pump beam was changed from linear to circular, a clear maximum in SRRS conversion efficiency of 33% was observed, along with a reduction in the threshold to 1.5 mJ (Figure 2a, red solid line). This was accompanied by a suppression of vibrational Stokes (Figure 2b, red solid line). Thus, it was confirmed that the circular polarization of the pump beam enhances the generation of SRRS, and this was adopted in further experiments. The dependence of the SRRS spectra on the pump pulse energy at a hydrogen pressure of 3 MPa is shown in Figure 3a. Here, the first peak corresponded to the Yb:YAG laser pump wavelength at 1030 nm. In the spectrum near the threshold at \(\sim\)3 mJ, only the first rotational Stokes line was observed. As the pump energy increased to \(\sim\)5 mJ, the saturated first Stokes, in turn, served as a pump for second-order rotational Stokes excitation. Finally, at a maximum pump energy of 7 mJ, a third-order Stokes line was also observed. Thus, when choosing optimal excitation conditions in compressed hydrogen, the efficient cascade generation of a large number of equidistant rotational Stokes lines becomes possible. Since the Raman gain is proportional to the density of the medium [23], the conversion efficiency of transient SRS strongly depends on the hydrogen pressure. Thus, the hydrogen pressure has also been optimized to ensure the most efficient generation of higher-order rotational Stokes components. Although the SRS linewidth broadens with increasing gas pressure [29], in our experiments, this effect was negligible since the pump laser linewidth exceeded the SRS linewidth [31]. The dependence of the SRS spectra on the hydrogen pressure at a pump pulse energy of 7 mJ is shown in Figure 3b. Here, as before, the first peak corresponded to the Yb:YAG laser pump wavelength at 1030 nm.