## 1 Introduction  
During the past decades, surgical treatment for urinary stone disease changed a lot because of significant technological developments and the improvement of medical equipment [1]. The appearance of laser technologies brought innovative approaches to the surgical treatment of urinary stones. Presently, laser treatment is a generally accepted method of lithotripsy [2,3]. The efficiency of lithotripsy of different types of stones with minimum damage to the surrounding soft tissues depends on the parameters of laser radiation (wavelength, pulse duration, and power) and the properties of stones (optical, mechanical, and chemical). The prevailing mechanism of disintegration/lithotryppsy (photothermal or photoacoustic/photomechanical) depends on the intensity, energy, and duration of a pulse. It is believed that laser radiation with a pulse duration \(\tau>10\)\(\upmu\)s causes a significant increase in the temperature in the area of laser exposure in cases with minimal acoustic waves. In such cases, the material of the stone is removed by means of evaporation, melting, thermomechanical tension, and/or chemical decomposition (photothermal mechanism). On the contrary, a laser with a short pulse duration (\(<\)10 \(\upmu\)s) produces a shock wave that mechanically fragments the stone (photoacoustic mechanism) [4]. According to current EAU (the European Association of Urology) guidelines, holmium laser lithotripsy (wavelength \(\lambda\) = 2.1 \(\upmu\)m) is the gold standard [5]. In the 2000s, researchers' attention was drawn to the application of thulium (Tm)-doped fiber lasers (thulium fiber laser) with a wavelength of 1.94 \(\upmu\)m for lithotripsy [6,7]. It was established that holmium laser ablation rates were lower than for the thulium fiber laser (TFL). The TFL also produced a greater percentage by the mass of stone dust (fragments less than 0.5 mm), which can lead to lower complication rates. According to [PERSON] et al. 2019, 7% stones treated with a holmium laser are completely fragmented in \(\leq\)5 min compared 60% stones treated with TFL [8]. TFL provides greater versatility and control of pulse parameters than the holmium: yttrium aluminum garnet (Ho:YAG) laser, including advanced pulse-shaping capabilities. A comparison of ablation volumes indicated that the TFL induced significantly higher (twofold) ablation than the Ho:YAG laser [9]. Similar data were obtained by [10]. It was reported that TFL with an optimal wavelength and long pulse duration resulted in faster stone ablation and lower retropulsion in comparison to the Ho:YAG laser. It was also shown that no substantial difference in the maximum temperature rise of water around stones was observed between the two laser systems. However, fiber burnback was more pronounced for the Ho laser, especially at higher power settings [10]. These studies demonstrate that the TFL is a promising alternative laser for lithotripsy, producing higher stone ablation rates, smaller stone fragments, and respectively lower complication rates than the holmium laser. It is believed that the main mechanism of lithotripsy with 2 \(\upmu\)m range laser radiation was photothermal. It was suggested that the mechanism of laser lithotripsy included two processes. First, a quick heating of water outside the stone leads to the formation of a closed cavity with compressed vapor that expands, collapses, and creates a shock wave that destroys the stone. Second, water contained in micropores and microfissures (around 10%) expands under quick heating from laser exposure, which leads to high pressure because of its incompressibility. Additional pressure occurs when water boils, evaporates, and condenses, including the cases when blast waves are formed. All these processes lead to the disintegration of the stone [11, 12, 13]. The effectiveness of laser exposure significantly depends on the wavelength because it affects the water absorption coefficient. A higher water absorption coefficient leads to faster heating of water and the formation of a vapor bubble.