# Study of the Effect of Laser Radiation Parameters on the Efficiency of Lithotripsy
## 2 Materials and Methods
When studying the value of sample retropulsion resulting from cavitation, the object of study was immersed in the container with a NaCl solution, a linear scale applied, and placed near the zero mark on the scale. Next, we brought the fiber working end to the sample, and layered generation began. Since the study was carried out according to a static method, we placed the fiber working part close to the stone phantom and rigidly fixed it. After the laser worked for 10 s, we stopped the experiment and measured the distance the sample was displaced. It should be noted that we have chosen a static technique for studying retropulsion since it is closest to the actual operating conditions of laser lithotripers. We experimented with the effect of femtosecond pulses on urinary calculi using the commercially available ytterbium laser system TEMA-6 of AVESTA-PROJECT LLC, which makes it possible to generate linearly polarized spectrally limited pulses with a duration of 250 fs at 1032 nm with a pulse energy of up to 400 \(\upmu\)J. Thus, the pulses' peak power  
Figure 1: (**a**) Picture of test sample; (**b**) retropulsion study with microsecond pulse laser. Figure 2: (**a**) Schematic of the retropulsion experiment; (**b**) retropulsion investigation with a femtosecond laser. acting on the sample was about 1.6 GW. Due to such high peak power, radiation delivery to the calculi phantom using an optical fiber was a problem. Therefore, we decided to use bulk optics for this task. To deliver radiation to the sample, we used a system of mirrors with a focusing lens at the output (f = 25 mm). In this case, the laser radiation spot on the sample's surface was \(\sim\)300 \(\upmu\)m. Accordingly, the peak power density of radiation was 241.91 TW/cm\({}^{2}\). To create experimental conditions close to those of the microsecond laser study, the samples were preliminarily hydrated for a day and then fixed in the holder so that the test object was in the focus of radiation at each iteration of the experiment. The pulse energy during the treatment course varied from 200 to 400 \(\upmu\)J, and the exposure time from 2 to 30 s. In addition, studies were carried out at three pulse repetition rates (10, 15.6, and 25 kHz) to characterize the magnitude of the influence of the peak radiation power on the ample ablation efficiency. The pulse duration did not change throughout the experiment and was 250 fs. It is worth noting that due to the lack of a system for delivering radiation with a femtosecond pulse duration in this research work, it was necessary to use a scanning system to move the affected area over the sample. In this case, we moved the processing zone horizontally with a step of 1 mm/s. After completely passing the specified processing perimeter, the radiation was shifted deep into the sample by 1 mm. It is worth noting that the speed and number of movements were selected empirically and were not optimized, which made a negative contribution to the femtosecond laser efficiency. In addition, we studied retropulsion in water when the sample was exposed to femtosecond pulses. To measure retropulsion, the test object was placed in a rectangular pure quartz cuvette (see Figure 2a,b), which did not absorb at the laser radiation wavelength, with a linear scale applied to the bottom, and the sample was processed through the cuvette wall. It is also necessary to emphasize that one of the most significant parameters in the study of the effectiveness of laser lithotripers is the amount of heating in the environment. Since the mechanism of renal calculi destruction is photothermal ablation, during laser lithotripsy, direct absorption of infrared energy by urinary calculi leads to an accumulation of heat and a subsequent increase in temperature, which leads to renal calculus chemical decomposition. Thresholds for photothermal ablation depend on both the chemical composition and the radiological density of the stones. However, it is worth noting that urinary stones very often have a mixed chemical structure, so these thresholds are variable.