# Study of the Effect of Laser Radiation Parameters on the Efficiency of Lithotripsy
## 2 Materials and Methods  
To achieve the project's key goal, we performed several experiments comparing the performance of urinary nodule phantom destruction when exposed to laser radiation with microsecond and femtosecond pulse durations. In addition, we investigated the retropulsion effect, which plays a significant role in the course of urinary concretion removal surgery. The characteristics of the used equipment are presented in Table 1.  
To ensure a large sample of data and the result's repeatability, we carried out experimental studies of laser performance on urinary stone phantoms, which are 5 mm \(\times\) 5 mm \(\times\) 5 mm cubes, the base of which was dental plaster of the fourth-strength class (BegoStone, Bego GmbH, Bremen, Germany). The experimental sample photo is shown in Figure 1a. It is worth noting that fourth-strength dental plaster is a common material for laser lithotripsy performance studies [29]. The main objective of each experiment series was to evaluate the rate of urinary concrement destruction by laser irradiation. The task was solved by analyzing the dependence of the obtained craters' parameters on the laser radiation parameters acting on the sample. The main problem in comparing the studied lithotripsers was the different physical mechanisms of urinary stone phantom destruction. First, this was due to the difference in the working average output power of the optical radiation of the studied laser sources. In this regard, for an accurate analysis of the processes occurring when exposed to radiation of different durations on the samples under study, we have chosen reference values such as throughput and average efficiency in removing the mass of sample material in 1 s.  
In experiments with a super-pulsed Tm-fiber laser, we studied the performance using a specially developed technique, which afterward made it possible to compare the results obtained using different laser radiation sources. A key step in the experimental study was weighing the samples before and after treatment. The stone phantom was immersed in a container with physiological saline to create conditions close to the operating conditions of laser lithotripsers. The radiation was delivered to the sample by means of an optical fiber 150 \(\upmu\)m in diameter. In this case, the end of the fiber was located at a distance of 3-5 mm from the surface of the calculus. (See Figure 2b) We chose such a distance to protect the fiber end of the radiation delivery system from the impact of fractions separated from the stone. In this case, the laser radiation spot on the sample surface was \(\sim\)200 \(\upmu\)m. Accordingly, the peak power density of radiation was 4.35 GW/cm\({}^{2}\). In this case, the laser radiation profile corresponded to the Gaussian. The main series of experiments included 36 renal calculus \begin{table}
\begin{tabular}{c c c} \hline  & **Super-Pulsed Tm-Fiber** & **Femtosecond** \\  & **Laser (Microsecond)** & **Pulse Laser** \\ \hline Active medium & Tm-doped fiber laser & Bulk Yb: YAG \\ Wavelength, nm & 1940 & 1032 \\ Pulse duration & \(\geq\)0.2 ms & 250 fs \\ Pulse energy & 0.025–6 J & 400 \(\upmu\)J \\ Repetition rate & 1–2400 Hz & 10 kHz \\ Average power & \(\sim\)60 W & 10 W \\ \hline \end{tabular}
\end{table} 
Table 1: Characteristics of the used lasers. phantoms. Laser exposure was carried out with different energies from 0.2 J to 2.5 J, pulse durations from 0.2 ms to 2.5 ms, and pulse repetition rates ranging from 18 Hz to 5 Hz. The average power of laser processing and the total energy can be considered as derivatives of variable technological parameters. To measure the removed material's volume, we weighed the parts before and after laser treatment. Weighing was carried out on a non-automatic balance (Analytical Balance ME104/A). To minimize the error in measuring the sample's mass associated with moisture removal during the weighing process, the authors decided to measure the mass of dehydrated samples. In this regard, before re-weighing, the samples were dried on blotting paper for two hours.