# 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  
In spite of the good technical characteristics obtained with homogenization, the influence of pump fluence distribution, w(x, y), on Ti:Sapphire laser operation remained unclear. We describe the pumping spot patterns with different w(x, y) used in our study. Figure 9 shows 2-D patterns and the appropriate w-profiles. Pattern (a) has a structure with relatively slow variations of w. Pattern (b) has a more regular structure with diffraction O-rings at a spatial frequency \(\sim\)2.5 mm\({}^{-1}\) in the radial direction. Pattern (c) consists of different size speckles; most of them are less than 0.04 mm. Therefore, the spatial spectrum of this pattern includes frequencies \(\sim\)25 mm\({}^{-1}\) and above. MLA pattern (d) consists of horizontal and vertical interference strips with various widths and periods. At the central area of the pattern, spatial frequencies are 17-26 mm\({}^{-1}\). A bright spot at the center of the patterns (c) and (d) is caused by the part of the light transmitted through the homogenizer without phase modulation (\"0\"-order). The intensity of this spot can be reduced by the adjustment of the distance between the condenser lens and the Ti:Sapphire crystal. Figure 8: A part of HELIOS-4 laser head with assembled components for Ti:Sapphire laser: 1—mounts with Ti:Sapphire crystal and laser cavity mirrors, 2—holder with condenser lens and 3—holder with MLA. Figure 9: Two-dimensional pump beam patterns (upper row) and w-profiles (lower row) of multimode laser (a), singlemode laser (b), after homogenization of multimode laser by DD (c) and by MLA (d). Spot size at (b) is 3 mm, and at other patterns is 6 mm. Adopted with permission from ref. [17; 18]. Copyright 2019 © The Optical Society. An evident difference between patterns (a) and (b), and (c) and (d), is the high spatial frequency interference modulation of \(\mathrm{W_{PUMP}}\), caused by homogenizers in the case of pumping by the light with high coherence. In addition, the fluence modulation depth, \(\xi=2\) [\(\mathrm{w_{(MAX)}}-\mathrm{w_{(MIN)}}\)]/[\(\mathrm{w_{(MAX)}}+\mathrm{w_{(MIN)}}\)], is of the same order in all of the patterns, but slightly lower in the singlemode laser. Therefore, the differences in local gain would remain even after homogenization. This means that a simple model of multichannel generation with equalized gain cannot completely explain an improvement in temporal characteristics, which we obtained using homogenizers. In [24], it was asserted that similar interference modulation does not have a significant influence on a laser beam in high-power amplifiers and does not lead to damage of the laser crystal's input surface. Considering generation in the Ti:Sapphire laser, we must keep in mind that there are essential differences between pumping light characteristics and optical scheme configurations used for high-power amplifiers and those used in our laser:  
The use of multi-pass scheme configurations in final amplifiers leads to spatial averaging of gain within the amplified beam spot. Saturation of gain in the final amplifiers reduces the spatial modulation of fluence within the input beam, which is the result of preamplifiers pumped by Nd:YAG lasers. Indicated factors can reduce the influence of pump light interference on Ti:Sapphire laser beams in an amplifier system, but they did not in our case. The analysis of pump patterns in Figure 9 lead us to the assumption of an additional mechanism that can produce the coupling and synchronization of the generation within the Ti:Sapphire laser aperture. We suppose that the spatial modulation of pumping light leads not only to gain nonuniformity but also induces the related optical nonuniformities in the crystal due to the refraction index change (RIC) effect [25; 26]. Optical nonuniformities should cause the small-angle scattering of laser light.