## 3 Results and Discussion
### Angular Characteristics
#### 3.3.2 Lasers with an Unstable Cavity  
Radiation from Raman lasers with an unstable cavity in our case comes out through a flat output mirror in the form of a divergent spherical wave with a center of curvature located behind the pump mirror at a distance B from the output mirror. B can be calculated by the formula [12]: B = (L\({}^{2}\) + LR)\({}^{0.5}\) + L, where L is the optical length of the cavity and R is the radius of the pump mirror curvature. The output radiation can be collimated using a positive lens with a focal length F (larger than B) installed after the output mirror at a distance of d = F - B. In our setup, the lateral size of the output beam increased due to the divergence before collimation from \(\sim\)4 to \(\sim\)8 mm, so the actual beam divergence was reduced 2-fold. For a fair comparison with the results obtained for a flat-cavity laser, where the beam size was \(\sim\)4 mm, below, all angles for unstable-cavity lasers were increased 2-fold compared to those experimentally measured. The beam profiles of Raman lasers with convex mirror radii of 750 mm and 2000 mm, recorded at I\({}_{532}\) = 350 MW/cm\({}^{2}\), are shown in Figures 9 and 10. Similar to that of the laser with a flat cavity, both intensity distributions at the core are close to a super-Gaussian distribution. The recalculated angular widths of the beam profile at 1/e\({}^{2}\) level are 5.2 mrad for the laser with a 750 mm radius mirror and 10 mrad for the laser with a 2000 mm mirror. At I\({}_{532}\) = 120 MW/cm\({}^{2}\), the angular width of the 750 mm radius mirror remained unchanged, and was reduced to 7.6 mrad for the laser with a 2000 mm radius mirror. Thus, the angular core width of unstable-cavity lasers is 1.4-2.6 times less compared to flat-cavity lasers. The angular energy distribution at I\({}_{589}\) = 350 MW/cm\({}^{2}\) is represented in Figure 11. We can see that the angular energy divergence of lasers with an unstable cavity reduces with a reduction in the radius of the mirror's curvature. The divergence is 2-3 times less than that of lasers with a flat cavity (Figure 11(a)) depending on the convex mirror curvature. Figure 10: Beam profile of laser with 2000 mm convex mirror. Figure 9: Beam profile of laser with 750 mm convex mirror. Comparing the intensity and angular energy distributions at I\({}_{532}\) = 350 MW/cm\({}^{2}\), we can see that in unstable-cavity lasers, 56-58% of the total E\({}_{589}\) is concentrated in the core, whereas in flat-cavity lasers, the core contains less than 50% of the total E\({}_{589}\). Thus, use of an unstable cavity in KGW Raman lasers instead of a flat cavity results not only in a reduction in the core width, but also in a better concentration of the radiation energy within the core (in other words, a reduction in the wings). Figure 12 shows the imprints of unstable-cavity laser radiation on photosensitive paper. To compensate for the above-mentioned two-fold reduction in angular divergence, the imprints were made in the focal plane of a lens with a two-fold larger focal length (250 mm) than used for Figure 8. We can see that the size of the imprints is much lower than in Figure 8, and the structure of the wings is also different. The lower influence of the scattered light on laser generation and the lower increase in the angular divergence in an unstable cavity, compared to a flat cavity, for conventional lasers were explained in [12]. The main reason for these effects is that the scattered radiation, which propagates at large angles to the axis, is removed from an unstable resonator more quickly and is subjected to lower amplification than in a flat resonator. In Raman lasers, we assume that the reason is similar. Also, it is clear that the efficiency of parametric generation under pumping by 559 nm divergent spherical waves, which takes place in an unstable resonator, should be less than under pumping by flat-like 559 nm radiation generated in a flat cavity. Both factors work together, leading to a reduction in the intensity and angular spread of the wings.