# 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
Nevertheless, these results show the features of laser light angular distribution caused by light scattering, so we can take into consideration the appearance of such features (first, the appearance and growing of the wings) in experimental characteristics as an indicator of intracavity light scattering. The angular distribution of Ti:Sapphire laser radiation was investigated for three different pump patterns, which are shown in Figure 9b-d. The parameters of Ti:Sapphire laser were the same for all three pump patterns: we set up a laser resonator with flat pump and output mirrors; the reflection coefficient of the output mirror was 70%, the Ti:Sapphire crystal thickness was 3 mm, the distance between mirrors was 6.5 \(\pm\) 0.3 mm, and the pump spot size was \(\sim\)5 mm. The results of the experimental investigation are shown in Figures 10-15. Figure 10 shows the angular distribution of the Ti:Sapphire laser energy under single mode pumping at two different W. Here, \(\gamma\) is a part of total laser energy, irradiated within angle \(\varphi\). Angular divergence, measured at the (1/e) level (63% of total laser energy are concentrated within this angle), was 8 mrad at W = 1 J/cm\({}^{2}\) and did not increase with the increase in pump fluence (at least, up to 1.5 J/cm\({}^{2}\)). The wings of the angular distribution were small; more than 90% of laser energy was concentrated within 12-15 mrad. Figure 10: The angular distribution of Ti:Sapphire laser energy for single-mode pumping: 1—W\({}_{\rm PUMP}\) = 1.0 J/cm\({}^{2}\); 2—W\({}_{\rm PUMP}\) = 1.5 J/cm\({}^{2}\). Reprinted with permission from ref. [18]. Copyright 2019 © The Optical Society. Figure 11: The angular distribution of Ti:Sapphire laser energy for DD at: 1—W\({}_{\text{PUMP}}\) = 0.5 J/cm\({}^{2}\); 2—W\({}_{\text{PUMP}}\) = 0.75 J/cm\({}^{2}\); 3—W\({}_{\text{PUMP}}\) = 1.05 J/cm\({}^{2}\). Reprinted with permission from ref. [18]. Copyright 2019 © The Optical Society. Figure 12: The angular distribution of Ti:Sapphire laser at W = 1.05 J/cm\({}^{2}\) for different pump patterns: 1—single-mode, 2—MLA, 3—DD. Reprinted with permission from ref. [18]. Copyright 2019 © The Optical Society. Figure 13: Far-field distribution of the Ti:Sapphire laser radiation with MLA pump. Two-dimensional patterns and the appropriate intensity profiles were recorded at the focal plane of 800 mm lens at different levels of pump fluence: left—W = 1 a. u.; middle—W = 1.7 a. u.; right—W = 2.9 a. u. Horizontal scale of the profiles is 1 mm per division. Reprinted with permission from ref. [18]. Copyright 2019 © The Optical Society. The divergence, determined as the angular diameter of the laser far-field central spot at the 1/e level, was 5.5 mrad, which is smaller than the divergence of 8 mrad determined by 50% energy. In the following, we will indicate only the value of angular divergence, determined by energy, as more relevant to our practical applications. Another situation was observed with the MLA and DD pump patterns. In both cases, the angular divergence considerably increased with the increase in W. For the DD pump pattern (Figure 11), when W was increased to 1.05 J/cm\({}^{2}\), the angular divergence at the (1/e) level increased to 13 mrad, which is much larger than the 8 mrad obtained under the single-mode pump. We should emphasize a rapid growth of the wings of angular distribution here in comparison to the wings observed under the single-mode pump. With DD pumping, when W exceeded 0.5 J/cm\({}^{2}\), more than 15% of laser energy was irradiated into angles of larger than 20 mrad. A comparison of the angular energy distribution at W = 1.05 J/cm\({}^{2}\) for all three pump patterns at the 1/e level is shown in Figure 12. We observed the minimum angular divergence of 8 mrad for the single-mode pump pattern; a larger divergence for the MLA pump pattern--11 mrad; and the largest divergence for the DD pump pattern--13 mrad. The main difference between angular distributions for MLA and DD pumping consists of the different width of the central spot, which is larger for DD.