# Engineering of Ti:Sapphire Lasers for Dermatology and Aesthetic Medicine
## 2 Experimental Results and Discussion
### The Characteristics of Short Cavity Ti:Sapphire Gain-Switched Laser under Nanosecond Pumping
Therefore, the pulses from different channels will appear at different times. T\({}_{\text{D}}\) is several times larger than T, so the spreading of T\({}_{\text{D}}\) may considerably increase the duration of the signal integrated from all laser apertures. To minimize spreading, we must synchronize the generation of local channels by equalizing the \(W_{\text{PUMP}}\) over laser aperture. With this purpose and for the elimination of possible damage to the Ti:Sapphire crystal by hot spots in the 532 nm beam, we arranged pump beam homogenization using two-dimensional microlens array (MLA) [21], as shown in Figure 4.  
Figure 4: Optical scheme of the Ti:Sapphire laser. Here, 1 is microlens array homogenizer; 2 is the condenser lens; 3 and 5 are pump and output mirrors, respectively; and 4 is the Ti:Sapphire active crystal. Reprinted with permission from ref. [17]. Copyright 2019 © The Optical Society. After the homogenization, we obtained the stable generation of single pulses. Ti:Sapphire laser energy and temporal characteristics were measured for different combinations of pump spot size and cavity length. The dependences of the Ti:Sapphire laser pulse duration on \(W_{\text{PUMP}}\) are shown in Figure 6.  
In general, the maximum useable \(W_{\text{PUMP}}\) can be restricted by parasitic lasing inside the Ti:Sapphire crystal [22], by crystal damage under pumping or by the appearance of the second pulse. Each of these processes can appear depending on experimental conditions. For example, parasitic generation was observed in our experiments when the cavity length was greater than 13 mm. At cavity lengths shorter than 7.5 mm, parasitic generation was always suppressed by the generation in the main laser cavity due to a larger \(T_{\text{D}}\). After pump homogenization, crystal damage appeared at \(W_{\text{PUMP}}\), exceeding the threshold of the second pulse. All experimental data shown here were recorded within the range of single pulse generation. The last point of each curve corresponds to the threshold of the second pulse. From Figure 6, one can see that when \(W_{\text{PUMP}}\) exceeded \(\sim\)1.4 J/cm\({}^{2}\), T remained almost unchanged. On the one hand, this did not allow us to achieve T \(\leq\) 460 ps. On the other hand, the low dependence of T on the \(W_{\text{PUMP}}\) resulted in the high stability of T. At maximum values of \(W_{\text{PUMP}}\), we obtained the standard deviation of T as low as 1%. The reduction in the cavity length from 7.5 to 3.8 mm did not allow us to reduce the minimum available T, and resulted only in a reduction in \(W_{\text{PUMP}}\), which is required to achieve the minimum T. This can reduce the risk of laser crystal damage by setting a lower \(W_{\text{PUMP}}\) without increasing T.  
Figure 7 shows the dependence of the Ti:Sapphire laser pulse energy, E, on pump energy, E\({}_{\text{PUMP}}\), for two pump spot sizes. In both cases, (a) and (b), the maximum E was restricted by the second pulse appearance. To achieve a larger E, we had to increase both the E\({}_{\text{PUMP}}\) and pump spot size. In our case, the maximum pulse energy of 300 mJ at  
Figure 5: Series of Ti:Sapphire laser oscillograms, obtained without pump beam homogenization. Minimum duration of the main pulse is 930 ps. Reprinted with permission from ref. [18]. Copyright 2019 © The Optical Society. Figure 6: Dependences of the pulse duration T on pump fluence \(W_{\text{PUMP}}\) for pump spot \(3.4\times 3.4\) mm (**a**) and 5.6 \(\times\) 5.6 mm (**b**). Reprinted with permission from ref. [17]. Copyright 2019 © The Optical Society. T = 520 ps was obtained with a pump spot of 7.2 \(\times\) 7.2 mm at the maximum available EPUMP of 700 mJ. Here, WPUMP did not exceed 1.35 J/cm\({}^{2}\), so the adjustment of the spot size allowed us to increase WPUMP and to achieve the same E with a shorter T.  
The best values of Ti:Sapphire laser characteristics are shown in Table 2:  
Similar characteristics were obtained using another type of homogenizer--diffraction diffusers (DD) [23].