## 3 Results and Discussion
### Angular Characteristics
#### 3.3.1 Laser with a Flat Cavity  
The beam profile of the flat-cavity Raman laser when the pump spot size is \(\sim\)4 mm recorded with a 400 mm lens is shown in Figure 4.  
This profile fits a super-Gaussian distribution with the core width of 14.3 mrad (at \(1/\mathrm{e}^{2}\) level). A similar distribution with the core width of 13.3 mrad was observed at \(\mathrm{I}_{532}=120\) MW/cm\({}^{2}\). The core width of the 2 mm pump spot was 8.0 mrad and remained almost unchanged within 120 MW/cm\({}^{2}<\mathrm{I}_{532}<350\) MW/cm\({}^{2}\). The width variations did not exceed \(+/-2.5\%\). The angular energy distribution is represented in Figure 5. The vertical axis shows the energy \(\mathrm{E}_{\varphi}\), irradiated within the angle \(\varphi\), expressed as a percentage of the total laser energy \(\mathrm{E}_{589}\). Figure 4: Beam profile of flat-cavity 589 nm Raman laser at \(\mathrm{I}_{532}=350\) MW/cm\({}^{2}\). Figure 5: Angular energy distribution of the radiation of Raman laser with flat cavity: a and b— for pumping spot 2.05 \(\times\) 2.4 mm at \(\mathrm{I}_{532}=120\) MW/cm\({}^{2}\) and 350 MW/cm\({}^{2}\); c and d—for spot 4.1 \(\times\) 4.4 mm at \(\mathrm{I}_{532}=120\) MW/cm\({}^{2}\) and 350 MW/cm\({}^{2}\), correspondingly. Considering Figure 5, we note that the angular energy divergence of KGW Raman lasers (EKSMA Optics, Vilnius, Lithuania) is much larger than that of conventional lasers with a similar laser pump and cavity configuration. For example, in Ti:Sapphire lasers, even with a very short flat cavity (6.5 mm) and the same beam size (\(\sim\)4 mm), 80% of the energy propagates within an angle of 8 mrad, and the angular divergence does not depend on the pump energy density (or on pump intensity) [16]. However, in Raman lasers, at \(\mathrm{I_{532}=120\;MW/cm^{2}}\), 80% of the laser energy propagates within 20 mrad; at \(\mathrm{I_{532}=350\;MW/cm^{2}}\), this angle increases to 30 mrad. A comparison of Figures 4 and 5(c,d) shows that only 60% of the total \(\mathrm{E_{589}}\) of the laser with a 4 mm pump spot is concentrated in the core (within an angle of 13.3 mrad) at \(\mathrm{I_{532}=120\;MW/cm^{2}}\) and less than 50% (within an angle of 14.3 mrad) at 350 MW/cm\({}^{2}\). For the laser with a 2 mm pump spot, 74% of the total \(\mathrm{E_{589}}\) propagates within an 8 mrad core at 120 MW/cm\({}^{2}\) and 57% at 350 MW/cm\({}^{2}\) (Figure 5(a,b)). We observe that a large part of the radiation energy in all four cases is spread over the wings of the angular distribution. When the pump intensity increases, the angular size of the core remains almost unchanged, whereas the width of the angular energy distribution increases, mainly due to wing growth. Note that when all radiation in the far field is distributed in accordance with a super-Gaussian function, then the energy within the core must exceed 88% of the total \(\mathrm{E_{589}}\)[17]. All the results described above were obtained using an MLA with a numerical aperture of NA = 0.017 (NA = p/f, where p is the pitch and f is the focal length of the MLA). Figure 6 shows the angular energy distribution of the laser with a 4 mm pump spot at \(\mathrm{I_{532}=350\;MW/cm^{2}}\), obtained using an MLA with different NA values (0.017 and 0.042). Within angles of 0-25 mrad, we observe an identical distribution for both NA values, but at angles larger than 25 mrad, the distributions begin to differ. About 10% of \(\mathrm{E_{589}}\) is irradiated at angles larger than 65 mrad when NA = 0.042, while less than 2% is irradiated when NA = 0.017. Thus, an increase in NA causes an increase in the energy irradiated at large angles. These results, in our opinion, indicate a different physical mechanism for the for wings, we took a more detailed recording of the laser's far field when NA = 0.042. The restricted dynamic range \(\mathrm{I_{MAX}/I_{MIN}}\) of the camera sensor did not allow us to simultaneously record the radiation at the beam core and the wings.