## 3 Results and Discussion
### Optical and Acoustic Tests
This mechanism of destruction prevails in cases with a so-called \"stress confinement\": \(\tau<\delta/c\), where \(\tau\)--the duration of the laser pulse, \(\delta\) = 1/\(\upmu_{\text{eff}}\)--the depth of optical penetration, \(\upmu_{\text{eff}}\)--the coefficient of effective attenuation that includes the absorption and the impact of radiation scattering, \(c\)--the sound velocity in tissue. Physically, this means that during the pulse duration, the tension provoked by fast heating cannot leave the area where the main part of the laser energy is absorbed. In such conditions, laser energy can effectively transform into mechanical or acoustic energy because of a thermoelastic response [19]. Photothermal ablation is typical for lasers with relatively long pulses (\(>\)1 \(\upmu\)m) and a relatively low intensity of laser radiation (\(<<\)10\({}^{8}\) W/cm\({}^{2}\)). The criterion of this process efficiency is described by an inequality: \(\tau<\delta^{2}/4\alpha\), where \(\alpha\)--the coefficient of thermal conductivity of the medium. Physically, this means that during the pulse duration, the heat produced in the medium as a result of laser energy absorption cannot leave the area where the main part of the laser energy is absorbed. In cases when long pulses \(\tau>>\delta/c\) are applied, the pressure relief from the heated area occurs during laser-radiation heating, and the pressure peak \(\Delta P\) is \(\tau\)-\(c\alpha\) times lower than during the realization of the conditions for photomechanical ablation [20]. In cases with even longer pulses and continuous laser radiation, the mechanism of tissue destruction associated with thermal cavitation is observed. It is characterized by the transfer of water to the near-critical or super-critical area of temperatures and pressures. Initially, due to the absorption of laser radiation, an overheated area is gradually formed in water. When the temperature comes closer to the spinodal (critical temperature T\({}_{\text{C}}\approx\)374 \({}^{\circ}\)C for the atmospheric pressure [21]), fluctuations lead to the explosive boiling of the heated liquid with the formation of a quickly expanding steam-gas cavity (bubble) [17; 22]. When the cavity reaches its maximal size, it quickly collapses and causes a pressure increase and cumulative jet and shock wave formation [23; 24], which leads to the destruction of the tissue [25]. In cases with an enclosed volume of pore space, the transition to the point of thermal cavitation is observed along with a simultaneous increase in the pressure [26]. When the water reaches the area of the spinodal (T\({}_{\text{C}}\approx\)374 \({}^{\circ}\)C), explosive water boiling occurs, and the water in pores transforms into vapor compressed to critical pressure (P\({}_{\text{C}}\approx\)22 MPa) [26; 27]. When critical parameters for the tissue are exceeded, such pressure peaks lead to the explosive destruction and formation of shock waves. Shock waves also contribute to the destruction of adjoining tissues and the outburst of particles of the fragmented material from the area of ablation [28]. The performed tests showed that for the majority of stones, the pore sizes varied from 1 to 100 \(\upmu\)m [29]. It should be noted that the above-described process of the destruction associated with the explosive boiling of water in pores of the material can be realized due to the Moses effect [30] without tight contact between the end of laser fiber and the tissue. Due to this effect, a vapor-gas bubble is formed between the fiber end and the surface of the tissue, freely transmitting laser radiation. In the present study, we used a laser with \(\lambda\) = 1.94 \(\upmu\)m, \(\tau\) = 0.4 ms, E\({}_{\text{p}}\) = 0.2 J, P\({}_{\text{p}}\) = 500 W (dusting regime).