# Engineering of Ti:Sapphire Lasers for Dermatology and Aesthetic Medicine
## 2 Experimental Results and Discussion
### The Influence of Pump Fluence Distribution on Ti:Sapphire Laser Operation
At the wings, angular distribution for these pump patterns is almost identical. We again emphasize that the appearance of large wings in angular distributions for MLA and DD pumping is the most distinct feature in comparison to angular distribution for single-mode pumping, where the wings are small and almost all energy is concentrated within 15 mrad. The lateral structure of the pump patterns revealed in the far-field distribution of laser radiation is shown in Figure 13. Single-mode and DD pump patterns are axially symmetric, and the appropriate laser far-field patterns are also axially symmetric. In the case of the MLA pump pattern, which has 4 TH order rotational symmetry, the far field also consists of the same symmetry. The results obtained with 2-D MLA are shown above. In comparison, the far-field pattern, obtained with one-dimensional MLA, consisting of cylinder lenses, while having the same pitch, lens size and focal length as in two-dimensional MLA, is shown in Figure 14. This MLA produced the modulation of W only in the horizontal direction. As expected,  
Figure 9:0 Laser far-field distortion with the increase in pump fluence under pumping with one-dimensional MLA. W increases from left to right of the image by 2.9 times. Reprinted with permission from ref. [19]. Copyright 2019 © SPIE. Figure 9:1 O-rings in the far field of Ti:Sapphire laser with the single-mode pump. The right pattern was recorded when AR coatings at the crystal were damaged. Light scattering at damaged areas resulting from increased intensity of O-rings. Reprinted with permission from ref. [18]. Copyright 2019 © The Optical Society. induced scattering in the horizontal direction should be larger than in the vertical direction, so when W increased, the central spot of angular distribution increased mainly in the horizontal direction, and less in the vertical direction, parallel to the cylinder lens axis. For the MLA and DD pumping, we found that laser light intensity in the axial (normal to resonator mirror surface) direction remained almost unchanged, whereas pump fluence increased 2.9 times, and laser output energy also increased by more than three-fold. All patterns, shown in Figures 13 and 14, were recorded at equal optical attenuation and equal electronic gain of the beam profiler camera. The beam intensity profiles in Figure 13 illustrate the process of laser light energy redistribution from an axial direction to the wings when W\({}_{\text{PUMP}}\) increases. It is worth mentioning that the observed energy redistribution was analyzed and explained theoretically in [29]. Predicted and observed in [27; 28], O-rings in the far field intensity distribution (Figure 15) were also observed in our experiments under pumping with each of the three patterns, even with a single-mode pattern. The appearance of O-rings in the last case, in our opinion, might have resulted from a weak light scattering caused by dielectric coatings at Ti:Sapphire crystal and laser resonator mirrors. Damage of the Ti:Sapphire crystal AR coatings led to an increase in light scattering and, finally, an increase in the intensity of O-rings. For single-mode and DD pumping, the O-rings had a round shape, but for MLA pumping, they had a hexagonal shape (not shown here). The angular diameter of the first O-ring, shown in Figure 15, was 18.5 mrad. The calculated angular diameter was 18.95 mrad, which is in a good agreement with the experimental value. We must emphasize that scattering in the case of single-mode pumping was very week and did not lead to the synchronization of the generation. One can estimate the magnitude of phase shift [\(\delta\Psi\)]\({}_{\text{MAX}}\), induced by pump light in the Ti:Sapphire crystal using the data obtained in [25]. It was indicated that the refraction index n changed due to population inversion (electronic component) and heating (thermal component) of the crystal. The total RIC for \(\sigma\)-polarized light at 633 nm was about \(6\times 10^{-24}\) cm\({}^{3}\times\) N, where N is the concentration of exited Ti\({}^{3+}\) ions.