## 3 Results and Discussion
### Angular Characteristics
#### 3.3.1 Laser with a Flat Cavity
Figure 7, left, shows an overexposed 589 nm spot on the screen, placed in the focal plane of a 125 mm lens. The spot has sharp edges and a diameter of 2.5 mm, which corresponds to an angular size of 20 mrad. The light intensity outside this spot is much lower and is not recorded. To record the structure of the wings, we made a hole with a diameter of 14 mm in the screen, through  
Figure 6: Angular energy distribution for a 4 mm pump spot and different NA values at \(\mathrm{I_{589}=350\;MW/cm^{2}}\): a—NA = 0.017; b—NA = 0.042.  
which the main part of the laser beam energy can pass (dark circle in Figure 7, right). The hole is shifted in respect to the beam axis, so we can see a part of the structured wings on the screen. For more detail, we observed the imprints made on photosensitive paper, shown in Figure 8. Here, we can observe:  
A white overexposed spot with a diameter of \(\sim\)2.5 mm, with a gray spot in the center of the overexposed area with a diameter of \(\sim\)1.6-1.8 mm. The angular size of the gray spot is 13-15 mrad, so it can be attributed to the core of the beam profile in Figure 4; the angular size of the white spot (\(\sim\)20 mrad) corresponds to that of the sharp-edged spot in Figure 7, left, where the light intensity is much higher than that in the external area. Multiple arcs with diameters of 10-16 mm and angular sizes of 80-130 mrad. During laser operation, they can be seen by the eye as bright yellow O-rings with an intensity varying in azimuth. We attribute them to the process of partially degenerative four-wave mixing (PDFWM), which accompanies Raman generation. A variety of parametric processes during stimulated Raman scattering in crystals has been studied theoretically in [18; 19] and was observed in the experiments, in general, with picosecond pumping [20; 21]. Under most experimental conditions, the PDFWM process dominated:  
\[2\;\mathrm{K_{I}}=\mathrm{K_{I+1}}+\mathrm{K_{I-1}}\]  
where \(\mathrm{K_{I}}\) is a wave vector of pump radiation and \(\mathrm{K_{I+1}}\) and \(\mathrm{K_{I-1}}\) are wave vectors of Stokes and anti-Stokes components. We believe that in our case, the 559 nm radiation also worked as a pump for PDFWM and produced Stokes radiation at 589 nm and anti-Stokes radiation at 532 nm. The latter was rejected by the highly reflective output mirror at 532 nm. The intensity and geometry of the arcs generated in KGW crystals will depend on the angle between the axes of the crystal and \(\mathrm{K_{I}}\) direction. We suspect that the appearance of  
Figure 8: Imprints of 589 nm radiation from a flat cavity laser in the focal plane of a 125 mm lens, obtained when NA = 0.042: left—at \(\mathrm{I_{532}}\) = 120 MW/cm\({}^{2}\), middle—at 220 MW/cm\({}^{2}\), right—at 350 MW/cm\({}^{2}\). Figure 7: Camera recordings of Raman laser far field radiation. NA = 0.042 mrad. Left subfigure shows an overexposed 589 nm spot, right subfigure shows the wings, which can be visible at the screen, if the central part of laser spot passed through the hole in the screen and is not recorded by the camera. multiple arcs in our experiment was caused by the relatively large angle range of partial beams produced by MLA with an NA = 0.042. As a rule, we did not see the arcs when using an MLA with an NA = 0.017 when the crystal faces were aligned in parallel with the cavity mirrors and were normal to the pump beam axis. In this case, -98% of the energy E\({}_{589}\) was propagated within 65 mrad (Figure 6(a)). However, after an appropriate crystal tilt, we observed the appearance of isolated arcs even at NA = 0.017. During high-intensity picosecond pumping, up to \(\sim\)50% of the total energy from the Raman laser can be generated due to parametric conversion [21]. In our case, we estimated the energy generated by PDFWM, from Figure 6(b), as the energy irradiated at angles above 65 mrad. This energy was about 8-10% of E\({}_{589}\) at I\({}_{532}\) = 350 MW/cm\({}^{2}\). The origin of the radiation outside the white spot (besides parametric arcs) is not clear.