## 3 Experimental Results and Discussion  
### Stokes Lasers Generation  
The calculated wavelengths and measured spectra of the first, second, and third-order Stokes lasers from the external Raman oscillator are shown in Figure 2. The spectra of the output Stokes lasers produced at the pumping energy of 120 mJ are measured by a near-infrared spectrometer with a 7 nm wavelength resolution (Ocean optics, NIR2500). For the Ba(NO\({}_{3}\))\({}_{2}\) crystal with a Raman shift of 1047.3 cm\({}^{-1}\), the calculated wavelengths of the first, second, and third-order Stokes laser are 1197.7, 1369.5, and 1598.8 nm, respectively. For the KGW1 crystal (corresponding to E\(\parallel\)N\({}_{\text{m}}\)) with Raman shift of 901 cm\({}^{-1}\), the calculated wavelengths of the first, second, and third-order [PERSON] laser are 1177.1, 1316.7, and 1493.9 nm. For the KGW2 crystal (corresponding to E\(\parallel\)N\({}_{\text{g}}\)) with Raman shift of 768 cm\({}^{-1}\), the calculated wavelengths of the first, second, and third-order [PERSON] laser are 1158.9, 1272.1, and 1409.9 nm. By our calculation, the Kerr lens and thermal lens effects of these Raman crystals are negligible at a pumping energy of 120 mJ [33; 34]. According to a cascaded SRS model, the threshold  
Figure 1: Schematic of the discretely tunable multiwavelength visible laser based on second harmonic generation (SHG), sum frequency generation (SFG) and cascaded stimulated Raman scattering (SRS) system. M1 and M2 form the SRS cavity. HWP: half wave plate; PBS: polarization beam splitter; FR: Faraday rotator; L: lens, M: mirror; BN: Ba(NO\({}_{3}\))\({}_{2}\); DM: dichroic mirror. of SRS for Stokes laser oscillating is determined by pumping depletion, reflectivity of cavity mirrors, Raman gain coefficient, and the length of the Raman medium [35; 36]. In addition, the output energy of Stokes lasers is mostly determined by the intensity of pumping laser, length of Raman medium, and the Raman gain coefficient at the pumping wavelength [37; 38]. The optical conversion efficiency from pumping lasers to Stokes lasers is related to the parameters of the pumping pulses (i.e., the pulse duration and intensity), the features of the Raman medium (such as the length and Raman gain coefficient), and especially the reflectivity spectral profiles of the resonator mirrors [36; 39]. Therefore, the output spectra and energy of the Stokes lasers, from the aforementioned external Raman oscillator, mainly depend on the pumping intensity and transmission parameter of the output coupler, M2. At the beginning, the 1064 nm pumping laser pulses of 20 mJ are injected into the Raman oscillator, and only the 1197.7 nm laser appears, since the SRS threshold is related to the Raman gain coefficient, for which Ba(NO\({}_{3}\))\({}_{2}\) crystal is higher than KGW crystal. In addition, the reflectivity coefficient of the two cavity mirrors are almost the same at wavelengths of the three first-order Stokes lasers. As the pumping energy increases to 40 mJ, the first-order Stokes lasers of the three crystals are generated at 1197.7, 1177.1, and 1158.9 nm. Furthermore, when the energy of the pumping laser is increased to 60 mJ, three second-order Stokes lasers are generated at 1316.9, 1272.1, and 1369.5 nm in addition to the previous first-order Stokes lasers. When the energy of the pumping laser is further increased to 120 mJ, the third-order Stokes laser of KGW1 at 1493.9 nm is obtained. Moreover, there is no sign of crossed-cascaded SRS process, and higher order Stokes generation is cascaded SRS of each Raman crystal individually. The total output energy and conversion efficiency of the Stokes lasers in comparison to the energy of the 1064 nm pumping laser is presented in Figure 3. It is most apparent that the output Stokes energy (black line) progressively increases as the energy of the pumping laser increases. However, the highest optical conversion efficiency of the SRS (red line) is 40.7% at a pumping energy of 70 mJ.