**3.2. Fiber‐tip damage mechanism**

## *3.2.1. Shock wave detection*

Shock waves generated by laser pulses are disturbance waves that travel faster than sound, but quickly damp down to the speed of sound [12]. The speed of sound under water is 1484 m/s or ∼1.5 mm/μs; as such, a high‐speed camera with a frequency of ∼1 μs or 1 million fps is required for studying the dynamics of these pressure waves (with 930,000 fps, the period is ∼1 μs, while the image size is barely a few mm wide).

**Figure 17** shows a snapshot of the Schlieren image of shock wave by a 1‐J pulse with a high‐ speed camera setting of 930,000 fps. In the picture, the captured image is at the moment when the shock wave (the area indicated in the middle) leaves the fiber‐tip area and moves to the right.

**Figure 18** depicts the shock‐wave displacement curve against time. Utilizing a second‐order polynomial curve fit, we can see the speed is 1.45 mm/μs or 1450 m/s. This is consistent with the sound speed in water (1484 m/s). However, the pressure wave is quicker than the acoustic velocity at the very beginning (within the 1‐μs domain [12]). A well above 1‐million fps frame rate camera is needed (best to be 10 million fps) so that a detailed understanding of the pres‐ sure wave initiated by Holmium laser energy can be obtained.

#### *3.2.2. Fiber‐tip damage*

An additional set of tests were conducted on the thermal/mechanical damage to the fiber tip by debris by (1) varying the distance between the fiber tip and the calculus surface; (2) differing the incidence angle of the fiber to the calculus surface. According to a multicenter study of 541 pro‐ cedures, the average dose of laser energy needed for laser lithotripsy is ∼1.5 kJ [22]. Therefore,

**8.53 mm (16x128).** 

Investigation of Laser Pulse‐induced Calculus Damage Mechanism by a High‐speed Camera http://dx.doi.org/10.5772/intechopen.69981 97

**Figure 18.** The shock‐wave displacement curve against time.

**8.53 mm (16x128).** 

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

Shock waves generated by laser pulses are disturbance waves that travel faster than sound, but quickly damp down to the speed of sound [12]. The speed of sound under water is 1484 m/s or ∼1.5 mm/μs; as such, a high‐speed camera with a frequency of ∼1 μs or 1 million fps is required for studying the dynamics of these pressure waves (with 930,000 fps, the period is ∼1 μs, while

**Figure 17** shows a snapshot of the Schlieren image of shock wave by a 1‐J pulse with a high‐ speed camera setting of 930,000 fps. In the picture, the captured image is at the moment when the shock wave (the area indicated in the middle) leaves the fiber‐tip area and moves to the right. **Figure 18** depicts the shock‐wave displacement curve against time. Utilizing a second‐order polynomial curve fit, we can see the speed is 1.45 mm/μs or 1450 m/s. This is consistent with the sound speed in water (1484 m/s). However, the pressure wave is quicker than the acoustic velocity at the very beginning (within the 1‐μs domain [12]). A well above 1‐million fps frame rate camera is needed (best to be 10 million fps) so that a detailed understanding of the pres‐

An additional set of tests were conducted on the thermal/mechanical damage to the fiber tip by debris by (1) varying the distance between the fiber tip and the calculus surface; (2) differing the incidence angle of the fiber to the calculus surface. According to a multicenter study of 541 pro‐ cedures, the average dose of laser energy needed for laser lithotripsy is ∼1.5 kJ [22]. Therefore,

Q‐switched laser pulse is ∼1.8 MPa at 10 mm from the fiber tip.

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

sure wave initiated by Holmium laser energy can be obtained.

**3.2. Fiber‐tip damage mechanism**

the image size is barely a few mm wide).

*3.2.1. Shock wave detection*

*3.2.2. Fiber‐tip damage*

**Figure 17.** Shock‐wave Schlieren image with a frame interval of 1.075 μs, and the frame size is 1.07 × 8.53 mm (16 × 128).

Fibre p Shock wave

a lasing time of 2.5 min (150 s) at 1 J and 10 Hz (which is equivalent of 1.5 kJ) is used for the damage tests shown subsequently.

**Figure 19** shows images of the fiber and calculus with different incidence angles. In addi‐ tion to varying the angular setting, the fiber tip is adjusted through a range of distances from the calculus (at 2, 1, 0.5, and 0 mm). Furthermore, the fiber is translated vertically at a velocity of ∼0.4 mm/s within the 2.5‐min laser on period in the same time holding a con‐ stant spacing from the phantom.

**Figure 19.** The images of fiber and stone phantom with various incidence angles. (a) 0° incidence; (b) 45° incidence.

In **Figure 20**, we show fiber‐tip end views from fibers that underwent these various pulse conditions as well as an unused one. We utilized 1.5‐kJ laser pulse trains at the four different fibers to calculus spacing (2, 1, 0.5, and 0 mm) and with two different incidence angles (0 and 45°). It is evident from the pictures that the fiber end surface damage/deformation becomes more severe as the separation between the fiber tip and calculus surface becomes smaller after 1.5 kJ of energy delivery. Besides, the 45° incidence angle results in less end surface damage/ deformation verse 0° incidence angle for the same separation value. **Figure 21** depicts the transmission degradation of 365‐μm fiber over working distance and an incidence angle after

45 deg. incidence:

**Figure 20.** Fiber‐tip end views from unused fibers in contrast to fibers after 1.5‐kJ laser energy deliveries. Four different spacings (2, 1, 0.5, and 0 mm) between fiber tip and calculus surface, and with two different incidence angles (0 and 45°) were used for the activated fibers.

**Figure 21.** Transmission decay of a 365‐μm fiber as a function of working distance and angle of incidence after 1.5 kJ of energy delivery.

1.5 kJ of energy delivery. The transmission measurement results are consistent with the sur‐ face damage/deformation results.

#### *3.2.3. Retropulsion*

In **Figure 20**, we show fiber‐tip end views from fibers that underwent these various pulse conditions as well as an unused one. We utilized 1.5‐kJ laser pulse trains at the four different fibers to calculus spacing (2, 1, 0.5, and 0 mm) and with two different incidence angles (0 and 45°). It is evident from the pictures that the fiber end surface damage/deformation becomes more severe as the separation between the fiber tip and calculus surface becomes smaller after 1.5 kJ of energy delivery. Besides, the 45° incidence angle results in less end surface damage/ deformation verse 0° incidence angle for the same separation value. **Figure 21** depicts the transmission degradation of 365‐μm fiber over working distance and an incidence angle after

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

45 deg. incidence:

**Figure 20.** Fiber‐tip end views from unused fibers in contrast to fibers after 1.5‐kJ laser energy deliveries. Four different spacings (2, 1, 0.5, and 0 mm) between fiber tip and calculus surface, and with two different incidence angles (0 and 45°)

**Figure 21.** Transmission decay of a 365‐μm fiber as a function of working distance and angle of incidence after 1.5 kJ of

were used for the activated fibers.

energy delivery.

 New 2 mm 1 mm 0.5 mm 0 mm 0 deg. incidence:

Admittedly, our high‐speed camera is capable of 1 × 10<sup>6</sup> fps, but the field of view is limited at rates greater than 7 × 10<sup>3</sup> fps. Therefore, we find that a frame rate of 1 × 10<sup>4</sup> fps is a good com‐ promise between speed and field of view for our retropulsion study. For each measurement, the high‐speed camera recorded 10,000 images during the 1‐s interval of laser pulses interact‐ ing with the stone phantoms. Each measurement was repeated 5–10 times to improve the data quality. The video data files are analyzed using a MATLAB program. **Figure 22** shows the pen‐ dulum retropulsion test at a 10‐kfps camera frame rate and utilizing a Ho:YAG laser delivering 0.5‐J pulses at a repetition rate of 10 Hz. Hence, 10,000 data points are recorded at these condi‐ tions and are shown as a dotted curve. This dotted curve depicts the zero order of the motion and hence the displacement of the stone phantom. The apex of the movement occurs after ∼0.83 s where the phantom reaches zero velocity and begins to swing back. From this zeroth‐order curve, we can generate the first‐order curve (shown in solid) which indicates the speed of the phantom. We note that the initial speed depicted in the curve is not zero. This is due to the inad‐ equate resolution of the measurement system. At a frame rate of 1 × 10<sup>4</sup> fps, we have a 100‐μs period between camera shots. This is too coarse compared to the 240‐μs laser pulse since it will have a significant impact on the phantom within the 100‐μs time frame. Lastly, the second‐ order motion represents the acceleration of the phantom and is represented by the Dotted‐dash curve. The point that it crosses the zero acceleration line indicates the maximum speed of the phantom. Besides, the initial acceleration (multiplied by the phantom mass) is a good estima‐ tion of the average force that impacts on the pendulum by the laser pulse train within 1 s.

To further our study of retropulsion, we utilized our pendulum setup and employed various Holmium laser pulse energies impinging on a 200 mm–10 mm<sup>3</sup> stone phantom. In **Figure 23**, we show the maximum displacements of the stone phantom as a function of laser pulse energy. The increase of apex or maximum displacement with laser pulse energy is to be expected. The

**Figure 22.** Pendulum retropulsion test with 10‐kFRS camera by 0.5 J 10 Hz Ho:YAG laser.

**Figure 23.** The apex of a 200 mm–10 mm<sup>3</sup> phantom pendulum by a 10‐Hz Holmium laser.

results of displacement are 1.25 ± 0.10, 3.01 ± 0.52, and 4.37 ± 0.58 mm for 0.5, 1.0, and 1.5 J of energy per pulse, respectively.

From **Figure 22**, we can find out the initial acceleration of a 200 mm–10 mm<sup>3</sup> phantom pendu‐ lum by a 10‐Hz Holmium laser at different pulse energy level. Taking into account the mass of the stone phantom ∼2.0 g (wet and ∼1.8 g when dry), the average initial force by 10 of the 0.5‐J pulses is 3.1 × 10−5 Newton or 3.1 Dyne.

**Figure 24** reveals the average power effect of retropulsion with the 0.5‐J Holmium laser pulse train. Not surprisingly, the retropulsion increases with the average laser power applied. Apparently, the time to reach the apex increases with increasing average power. When the laser power level is increased above 25 W, the time for the phantom to come to the apex exceeds 1 s. This duration was beyond the high‐speed camera recording time in our current study. In the future, further testing should be done with increased high‐speed camera record‐ ing times (>1 s) to investigate the phantom dynamics at higher laser power levels.

**Figure 24.** The average power effect of retropulsion with Holmium laser 0.5‐J pulse train.
