# Ex Vivo Exposure to Soft Biological Tissues by the 2-um All-Fiber Ultrafast Holmium Laser System
## 3 Results and Discussion
After 0.3 W of USP laser radiation exposure for 60 s (18 J), a HAZ diameter of about 2000-\(\upmu\)m was measured, which is almost two times higher than that measured after the CW radiation exposure with the same power and time. It can be noted that exposure by continuous-wave laser radiation with a power of 0.5 W for 30 s (15 J) leads to comparable values of HAZ diameter to those measured after the pulsed radiation. For CW laser radiation with a power of 0.3 W, as well as for USP radiation with the lowest applied energy of 1.5 J (t = 5 s and a power of 0.3 W), formation of an ablation crater and tissue carbonization were not observed. Therefore, these values are not presented in Figure 6, which shows the dependency diagrams of the depth and diameter of the ablation (AZ) and coagulation (CZ) zones on the applied exposure energy for USP with a power of 0.3 W and CW with a power of 0.5 W radiation. As can be seen in Figure 6, the dependence of ablation depth on exposure time was weak for low applied energies, for both CW and USP radiation. The diameter of the ablation crater increased after USP radiation exposure. The maximum demonstrated ablation depth was 180 \(\upmu\)m and the diameter was 500 \(\upmu\)m. For  
Figure 5: Dependency diagram of the heat-affected zone (HAZ) diameter on the applied energy of USP and CW laser radiation for different exposure times (5, 30 and 60 s). CW radiation exposure, the ablation crater diameter was about 300 \(\upmu\)m, and dependence on exposure time was not observed. The depth of the coagulation zone, created by USP radiation with a power of 0.3 W for 60 s (18 J) and CW radiation with a power of 0.5 W for 60 s (30 J), was approximately the same. The diameter of the coagulation zone for such combinations of parameters differed slightly. Based on data reported in the literature, there are five main types of laser interaction with biological tissues. The main one is photothermal, which describes numerous effects [20]. Tissue temperature is an important parameter for this type of interaction. Understanding the change in local temperature will give insights into the effects that accompany laser exposure. [PERSON] et al. [51] confirmed in their studies that fast delivery of laser energy resulted in a sharp temperature increase, which led to the predominance of tissue vaporization over heat conduction to surrounding tissues and, as a consequence, more effective ablation and less thermal damage. On the contrary, a slow increase in temperature was accompanied by greater thermal damage. This is due to the fact that the processes of light absorption and heat generation are fast, in contrast to the heat distribution (dissipation). Therefore, temperature monitoring during laser surgery will help to reduce irreversible thermal damage and to reduce carbonization. For quantitative analysis of the single-spot experiments on muscle tissue ablation by ultrashort pulsed 2 \(\upmu\)m radiation, we have calculated the values of the ablation area (\(AA\)), specifically the area of the ablation crate and the coagulation area (\(CA\)) as an area of the coagulation zone [51; 52]. Both areas were measured on histological cross-sections using the publicly available software \"Imagej\" (National Institutes of Health) [53]. The obtained data allowed us to find the ablation efficiency (\(AE\)), measured by the ratio of the ablation area to the total irreversible thermal damage area \(\left(AE,\ \%=\frac{AA}{AA+CA}\times 100\right)\). In addition, the calculated values for USP radiation were compared with the values for CW radiation and are shown in Figure 7. The ablation area (\(AA\)) obtained after exposure by USP radiation of 0.3 W (green bars) and CW radiation of 0.5 W (red bars) increased with the growth of the applied energy. The ablation efficiency (\(AE\)) of the USP laser radiation was about 27% for 18 J of applied energy while for the CW laser radiation it was lower by 5% for 15 J of applied energy and higher by 5% for 30 J.