# Study of the Effect of Laser Radiation Parameters on the Efficiency of Lithotripsy
## 3 Results  
As mentioned above, the main idea of the study was to compare the performance of laser sources that can be used as medical lithotripers. In this regard, when comparing lasers with microsecond and femtosecond pulse durations, we evaluated the efficiency of photothermal and photoionization ablation mechanisms. As a result of our studies on a large sample (60 samples), we experimentally showed that when operating in the region of the same average output power of about 9 W, the microsecond laser's average productivity by processing a gypsum phantom is higher than that of a laser with a femtosecond pulse duration. The experimental results of the measured average productivity are shown in Figure 3a. As can be seen from the graph, in the case of a linear approximation, a microsecond laser source has a faster increase in average productivity than a femtosecond one. Qualitatively, the predominance of the efficiency of a microsecond laser source over a femtosecond one is shown in Figure 3b. It can be seen from the above bar graph that the average material removal efficiency for a microsecond laser applied is two times higher. Figure 4a shows the dependence of the average productivity on the average and the peak powers for a laser facility generating femtosecond pulses. Of note is that we achieved the best performance at the lowest peak power, which corresponds to the highest average pulse train output power. This fact can also be considered evidence of the advantage of the photothermal destruction mechanism over photoionization during urinary calculi treatment. It is also necessary to emphasize that in research practice, when treating urinary calculi with a microsecond laser, high pulse energy and a low repetition rate cause vibrations of the working end of the fiber. It leads to the random movement of the area of photothermal impact on the destructible sample. As mentioned above, in the experiment with a femtosecond laser, we used an acoustic-optical scanning system to move radiation over the sample surface, the speed and step of movement of which were chosen empirically and were not optimized. This fact, in turn, is reflected in the performance assessment. Figure 4b shows the dependence of the average productivity on the time of exposure to femtosecond pulses at the maximum average laser power. Judging by the nature of the change in the average values with increasing exposure time, it can be concluded that the key condition for obtaining high-performance indicators from a laser with a femtosecond pulse duration  
Figure 4: (**a**) Average productivity dependence on average output power (red) and peak pulse power (blue); (**b**) dependence of the average productivity on the time of exposure to femtosecond pulses. Figure 3: (**a**) Average productivity dependence on average output power for different types of lasers; (**b**) energy efficiency comparison of lasers by removing one gram of material. is the choice of the optimal time and scanning step. As you can see from the graph, at an average output power of about 9.6 W, the maximum performance level corresponded to a processing time of about 3.5 s, after which a sharp decline began. It is a consequence of the fact that the laser exposure area is processed faster compared to the scan step duration. As mentioned above, in addition to the productivity and efficiency of sample processing, the main characteristic of laser emitters used in lithotripsy is the retropulsion of the samples under study. The experimental results of the retropulsion study are presented in Figure 5. Graphical diagrams confirm the complete absence of retropulsion between the sample and the surrounding liquid when samples are exposed to femtosecond pulses. This result is explained by the superposition of several factors, among which the lower water absorption in the region of 1.03 \(\upmu\)m compared to the absorption at 1.94 \(\upmu\)m for the Tm laser source, as well as the low average radiation power. However, the main reason is the difference in the physical mechanism of material removal.