## 3 Results and Discussion
### Stimulated Rotational Raman Amplification of Supercontinuum Seed in Compressed H\({}_{2}\)  
Since the SRS process usually occurs from a very low level of spontaneous Raman scattering, the use of coherent seed radiation with higher intensity corresponding to the wavelengths of the excited Stokes modes would significantly enhance it. Therefore, in this work, we also investigated stimulated rotational Raman amplification (SRRA) seeded with WLC, which was generated in a YAG crystal using the same pump laser. We expected that by covering the wavelength range of high-order Stokes modes with a WLC seed pulse, we would be able to further increase the conversion efficiency, lower the SRRS threshold, and significantly expand its spectrum [33]. The dependance of the spectral envelope of the stimulated Raman amplification (SRA) on the hydrogen pressure at a pump pulse energy of 7 mJ is shown in Figure 5a. In contrast to SRS without a WLC seed (Figure 3b), the generation of rotational Stokes modes up to the third-order and even the fourth-order (corresponding to \(\sim\)1351 nm) was observed at hydrogen pressures of 2 MPa and 5 MPa, respectively. A vibrational Stokes line at a wavelength of \(\sim\)1795 nm and a first rotational Stokes line from it at a wavelength of \(\sim\)2007 nm were also observed in the SRA output spectra. Thus, despite the WLC radiation being of low intensity, it is still sufficient for seeding the SVRS  
Figure 5: (a) SRA spectra at different H\({}_{2}\) pressures with a pump pulse energy of 7 mJ; (b) SRRA conversion efficiency at different hydrogen pressures; (c) dependance of the best SRRA conversion efficiency (black line) and threshold energy (red solid line) on the hydrogen pressure; (d) conversion efficiency to anti-Stokes and vibrational Stokes components at a hydrogen pressure of 5 MPa. seeding also drastically reduced the SRRS threshold to 1.15 mJ at 1 MPa and 0.25 mJ at 5 MPa (Figure 5c, red solid line), which is \(-\)4 times lower compared to the case without seeding (Figure 4c, red solid line). This dependence corresponded to the reciprocal function 1.15/p (Figure 5c, red dashed line). The use of a WLC seed resulted in a significant enhancement of SRRS at all tested hydrogen pressures. As the pump energy increased, the SRRA conversion efficiency increased sharply (Figure 5b). However, due to the generation of anti-Stokes and vibrational Stokes components occurring at pump pulse energies exceeding 0.5 mJ and 1 mJ (Figure 5d), respectively, the SRRA conversion efficiency began to saturate. Accordingly, at a pump pulse energy of 7 mJ and a hydrogen pressure of 5 MPa, the conversion efficiency into anti-Stokes and vibrational Stokes components reached 10.4% and 5.8%, respectively. The best SRRA conversion efficiencies of \(\sim\)28% to 52% were achieved at pressures ranging from 1 MPa to 5 MPa, with the maximum output Stokes pulse energies exceeding 3 mJ. The widest spectrum of the SRRA comb was obtained at a hydrogen pressure of 5 MPa and a pump energy of 7 mJ (Figure 6a), and the positions of the observed peaks are given in Table 1. Accordingly, the spectral peaks detected at wavelengths of 1096 nm, 1164 nm, 1252 nm, and 1351 nm (designated as \"RS1-4\") were assigned, respectively, to the first, second, third, and fourth rotational Stokes modes, corresponding to a Raman shift of 587 cm\({}^{-1}\) in hydrogen pumped at 1030 nm. The spectrum also contained two long wavelength peaks centered at 1795 nm and 1997 nm. The first was assigned to the vibrational Stokes mode excited from the pump radiation and designated as \"VS\", and the second was assigned to the vibrational Stokes mode excited from the first rotational Stokes component (\"RS1\") and designated as \"RS1-VS\". According to the pump radiation, the SRRA output pulses were circularly polarized. The synthesized SRRA spectrum corresponded to \(\sim\)14 fs transform-limited pulses at a central wavelength of 1.2 \(\upmu\)m (Figure 6b). Thus, the cascade SRRA showed great potential