*3.1.2. Dependency of cavitation bubble formation during lithotripsy on fiber‐tip contact mode with the calculus surface*

Next, we analyze the effect of contact mode whereby the fiber tip is in contact with the surface of the stone phantom. After analyzing the interaction video from the high‐speed camera SA5 on the very first laser pulse hitting the stone, we observed bubble formation for both Ho:YAG and Tm:YAG laser pulses. However, in the Tm:YAG case, the bubbles can be seen more clearly because of much less debris generated by the 0.02‐J pulse (as compared to the 50× stronger pulse in Ho:YAG laser case). The bubbles generated are hemispheres because of the existence of the stone phantom and the collapse time is shown in **Figure 15**. The bubble collapse time in contact mode is ∼10–15% shorter as compared to the case without stone phantom contact for both Ho:YAG and Tm:YAG lasers.

#### *3.1.3. Transient pressure level measurement*

The transient pressure is measured by a hydrophone and its sensitivity *U*probe = *Q*probe/*C*sum, where *Q*probe is the 3.0 pC/MPa and *C*sum is the sum of probe capacity including cable (244 + 13 = 257 pF); therefore, *U*probe = 11.7 mV/MPa.

**Figure 16** shows the oscilloscope traces of laser pulse and transient pressure. The upper pic‐ ture has been created by a Ho laser pulse of 150 μs at 1 J and 10 Hz. The hydrophone end is Investigation of Laser Pulse‐induced Calculus Damage Mechanism by a High‐speed Camera http://dx.doi.org/10.5772/intechopen.69981 95

**Figure 15.** The cavitation bubble collapse time in contact mode. (a) Ho:YAG; (b) Tm:YAG.

(a) (b)

at various laser pulse lengths. For the 1‐J Ho:YAG laser pulse, at first bubble collapse of ∼500 μs (this is typically the second and the highest transient pressure of the shock wave during the laser pulse interaction with the liquid fluid, while the first transient pressure of the shock wave is at the injection of the laser pulse as demonstrated in Section 3.1.3), Note that the bubble center is ∼1 mm away from the tip of the fiber. By contrast, for the 0.02‐J Tm:YAG Q‐switched laser pulses, the first bubble collapse of ∼240 μs, the bubble center is ∼0.34 mm away from the

There is no observable difference of cavitation bubble dynamics between 273 and 365 μm

*3.1.2. Dependency of cavitation bubble formation during lithotripsy on fiber‐tip contact mode with* 

Next, we analyze the effect of contact mode whereby the fiber tip is in contact with the surface of the stone phantom. After analyzing the interaction video from the high‐speed camera SA5 on the very first laser pulse hitting the stone, we observed bubble formation for both Ho:YAG and Tm:YAG laser pulses. However, in the Tm:YAG case, the bubbles can be seen more clearly because of much less debris generated by the 0.02‐J pulse (as compared to the 50× stronger pulse in Ho:YAG laser case). The bubbles generated are hemispheres because of the existence of the stone phantom and the collapse time is shown in **Figure 15**. The bubble collapse time in contact mode is ∼10–15% shorter as compared to the case without stone phantom contact for

The transient pressure is measured by a hydrophone and its sensitivity *U*probe = *Q*probe/*C*sum, where *Q*probe is the 3.0 pC/MPa and *C*sum is the sum of probe capacity including cable (244 + 13

**Figure 16** shows the oscilloscope traces of laser pulse and transient pressure. The upper pic‐ ture has been created by a Ho laser pulse of 150 μs at 1 J and 10 Hz. The hydrophone end is

**Figure 14.** The cavitation bubble center movement at various laser pulse lengths. (a) Ho:YAG; (b) Tm:YAG.

94 Updates and Advances in Nephrolithiasis - Pathophysiology, Genetics, and Treatment Modalities

tip of the fiber.

*the calculus surface*

both Ho:YAG and Tm:YAG lasers.

*3.1.3. Transient pressure level measurement*

= 257 pF); therefore, *U*probe = 11.7 mV/MPa.

fibers.

positioned at ∼10 mm from the fiber end to prevent the probe from any possible damage due to the laser beam or pressure wave. Because the rising period of the hydrophone sensor is 45 ns, the detected transient pressure may be less than the real value. The actual pressure curve (**Figure 16**) exhibits many spikes, the first of which commences immediately after the injection of the laser pulse. This first pulse in the time sequence (from the left) represents the first shock wave. The second or the highest transient pressure peak corresponds to the first collapse of the cavitation bubble at ∼500 μs. The average transient pressure is ∼18 mV or 1.5 MPa at 10 mm away from the fiber tip. In addition, the transient pressure resulting from a 1‐J and 800‐μs pulse is less than half of that of a 150‐μs pulse.

**Figure 16.** Oscilloscope traces of laser pulse and transient pressure: injection, first bubble collapse, second bubble collapse (horizontal timescale: 200‐μs per division).

Similar sequences of transient pressure signals are shown in the lower picture, in which the first transient pressure by shock wave immediately after the injection of the laser pulse is more obvious, and the second transient pressure signal caused by bubble collapse at ∼240 μs is again the strongest one. The highest transient pressure peak generated by a 20‐mJ Tm:YAG Q‐switched laser pulse is ∼1.8 MPa at 10 mm from the fiber tip.
