## 3 Results and Discussion
### Optical and Acoustic Tests  
Figure 3 shows pictures taken by a high-speed camera during the disintegration of the stone phantom in the dusting mode. During laser pulse exposure, an outburst of a cloud of microscopic fragments of the material into the liquid is observed. Initially, the speed of this outburst reaches a minimum of \(\approx\)3.2 m/s, and 1 ms later, it decreases to 2.5 m/s. The frame 12 ms after the beginning of the phantom ablation clearly shows separate particles of the material outburst from the area of exposure. The size of the majority of dust-like fragments varies within the range of 1-10 \(\upmu\)m. The largest single particles reached up to 50 \(\upmu\)m in diameter. Figure 4 shows fragments of the acoustic signal registered by wideband and needle hydrophones during the laser exposure of the stone in the dusting regime. The curves contained several strong pulses with a duration of not more than 5 \(\upmu\)s. The acoustic noise in the wavetrain started with a wider and significantly lower amplitude of prepulse registered by both hydrophones. After a spike of acoustic energy, fading oscillations are observed. The curve of a wideband hydrophone (1 in Figure 4) clearly shows nearly harmonic fading oscillations with a frequency of 5 kHz. The amplitudes of maximal pulses (recalculated for the distance of 100 \(\upmu\)m) registered by a needle hydrophone were \(\approx\)40 atm and \(\approx\)27 atm. It should be noted that there was a 0.4 ms delay between the last pulse and the prepulse, which corresponded to the duration of the laser pulse. The tests showed that during the disintegration of the stone phantom in the dusting regime, the following occurred: (1) there was an active burst-like outburst of microscopic (1-50 \(\upmu\)m) fragments of the material into the liquid (pictures from a high-speed camera in Figure 3); (2) the outburst was  
Figure 4: Fragments of typical acoustic signals registered by a wide-band hydrophone (1) and needle hydrophone (2) registered during the fragmentation of the stone phantom in the dusting regime. The pressure was recalculated for the distance of 100 \(\upmu\)m.  
Figure 3: Typical pictures from a high-speed camera taken during the fragmentation of the stone phantom in a dusting regime. 1—laser fiber, 2—phantom of the stone. The time scale is in ms.  
associated with a generation of strong short pulses of acoustic signals with maximal amplitudes of dozens of atmospheres (results of the acoustic survey, Figure 4). Possible physical mechanisms explaining the registered results are described below. The main characteristic of the medium, which determines the degree of the impact of laser radiation, is the absorption coefficient. For numerous water-containing tissues, this coefficient is defined by the radiation absorption in water. For \(\lambda\) = 1.94 \(\upmu\)m, it is 129.8 \(\pm\) 0.5 cm\({}^{-1}\)[14]. When it comes to stones, it was shown that for the nearest infrared range of wavelengths, the absorption coefficient hardly depended on the stone types [19]. The dependence of the absorption coefficient in water on the radiation wavelength in the area of the local peak in the range of \(\approx\)2 \(\upmu\)m is presented in Figure 1. It can be seen that the TFL radiation is in the area of peak values, while the radiation of the Ho:YAG laser is in the edge of the peak, where the absorption coefficient is significantly lower (for 2.12 \(\upmu\)m, \(\mu_{a}\) = 26.1 \(\pm\) 0.2 cm\({}^{-1}\)). There are several mechanisms observed during the laser lithotripsy. Photomechanical ablation occurs in the regime of short pulses (\(<\)1 \(\upmu\)m) and high intensity of laser radiation (\(>\)10\({}^{8}\) W/cm\({}^{2}\)).