2.1.

Experimental System

A schematic diagram of the experimental setup is shown in Fig. 1 . The used free-running Ho:YAG laser apparatus (Wuhan National Laboratory for Optoelectronics) can emit lasers with pulse durations of $300\mu \mathrm{s}$ [full width at half-maximum (FWHM)], wavelengths of $2.12\mu \mathrm{m}$ , a pulse energy of up to more than $2\mathrm{J}$ , and a repetition rate of up to $40\mathrm{Hz}$ . Here, the laser energy can be coupled into low $\mathrm{O}{\mathrm{H}}^{-}$ quartz fibers (length $2\mathrm{m}$ ) with different core/cladding diameters ( $200∕240\mu \mathrm{m}$ , $400∕440\mu \mathrm{m}$ , and $600∕660\mu \mathrm{m}$ , $\mathrm{NA}=0.22$ ) by a ${\mathrm{C}}_{\mathrm{a}}{\mathrm{F}}_2$ lens (diameter $20\mathrm{mm}$ , focal length $50\mathrm{mm}$ ), respectively. The distal fiber tips are all positioned in a quartz cuvette (diamensions $10\times 10\times 10{\mathrm{cm}}^3$ ) filled with pure water (drinking water) at room temperature. The fiber tips are kept at least $3\mathrm{cm}$ below the water surface to insure sufficient inertial confinement. AB, CD, and EF channels of the front pannel of a DG645 digital delay/pulse generator (Standford Research System, Sunnyvale, California) are linked with holmium laser apparatus, high-speed camera (Fastcam SA1.1, Japan, maximum frame rate of 675,000 frames per second, spatial resolution of $64\times 16\text{pixels}$ ), and oscillograph [Agilent Technologies (Santa Clara, California) InfiniVision, $350\mathrm{MHz}$ , $2\mathrm{GSa}∕\mathrm{s}$ ] via BNC connectors, respectively. The pulse widths and the voltage amplitudes of the three channels are all $100\mathrm{ns}$ and $5\mathrm{V}$ , respectively. Only after the LED light source is initiated can DG645 perform with a mode of single-shot triggering. As a result, bubble dynamics and shock waves are recorded and detected by the camera and the piezoelectric polyvinylidenefluoride (PVDF) needle hydrophone (Institute of Acoustics, Chinese Academy of Sciences, sensitivity of more than $10\mathrm{nV}∕\mathrm{Pa}$ , response time of about several tens of nanoseconds, and an active zone diameter of $0.8\mathrm{mm}$ ), respectively.

Fig. 1

Schematic diagram of the experimental setup.

The spatial beam profile of the free-running Ho:YAG laser consists of several modes. Consequently, high-order fiber modes are excited, resulting in an inhomogeneous power distribution at the distal surface of the fiber. A typical power distribution and temporal pulse shape of holmium lasers are shown in Fig. 2 .

Fig. 2

2-D thermal images of the holmium laser power distribution at the distal fiber surface at various distances from black photograph paper with a repetition rate of $5\mathrm{Hz}$ . (a) Without fiber coupler, (b) with fiber coupler and a distance of $1\mathrm{mm}$ , (c) with fiber coupler and a distance of $3\mathrm{mm}$ , and (d) temporal pulse shape of holmium laser pulse [ $300\mu \mathrm{s}$ (FWHM)].

2.2.

Hard Tissue Treatment

To study the dynamcis of ablation products induced by holmium laser pulse, some calculus as hard tissue was applied in our experiments. Calculus consisting of calcium oxalate monohydrate (COM) and uric acid (UC) constituents with a planar surface was extracted from a $22\text{-year}$ -old female patient from Tongji Hospital of Tongji Medical College of Huazhong University of Science and Technology. Analysis of the calculus constituents was important for the fragmentation process. To make the pellets, we ground ${\mathrm{KB}}_{\mathrm{r}}$ and a sample together into a powder with a mortar and pestle. The powder was then loaded into a press cell. A pellet was formed by application of adequate pressure for several minutes. We had to vary the ${\mathrm{KB}}_{\mathrm{r}}$ and sample mix to get good transparency for a better signal-to-noise ratio and to obtain absorption bands strong enough to be detected at the same time. Figure 3 shows the Fourier-transform infrared transmission spectra of potassium bromide $\left({\mathrm{KB}}_{\mathrm{r}}\right)$ -sample pellets. This work has been finished at the Analytical and Testing Center, Huazhong University of Science and Technology. A $4717\text{-}{\mathrm{cm}}^{-1}$ wavenumber corresponds to wavelength of $2.12\mu \mathrm{m}$ , as illustrated in Fig. 3.

Fig. 3

Transmission of KBr-sample pellets for calculus with COM and UC constituents. COM is calcium oxalate monohydrate, and UC is uric acid.

2.3.

Ablation Valuation with Optical Coherent Microscopy System

To obtain the quantitative ablation crater shape, an OCM system is used to measure quantitative ablation dimensions. The center wavelength is $650\mathrm{nm}$ and the bandwidth is $300\mathrm{nm}$ that provide axial and lateral resolutions of $2\mu \mathrm{m}$ with the $10\times$ objective $(\mathrm{NA}=0.3)$ . All cross sectional profiles that are at deepest position of the crater are displayed. The ablation volume can be estimated from OCM cross sections over the crater volume, and 3-D images are also given.

3.1.

Bubble Dynamics

To visualize the dynamics of the bubble formation at the distal fiber surface and in the underlying volume, high-speed photography was performed. The images for different core diameter (600, 400, and $200\mu \mathrm{m}$ , $\mathrm{NA}=0.22$ ) fibers are shown in Figs. 4, 4, 4 , respectively. In Fig. 4, the bubble formation starts $\sim 40\mu \mathrm{s}$ after laser start (ALS), and the bubble size reaches maximal value at $282\mu \mathrm{s}$ . At this time, the bubble has a characteristic pear shape and presents an excrescence (see arrow) on the wall opposite the fiber tip due to the continuing vaporization of water by the laser beam. At $454\text{-}\mu \mathrm{s}$ ALS, the proximal part of the bubble is starting to collapse rapidly. Simultaneously, a cylindrical channel, due to further vaporization by the laser, is still progressing along the axis of the fiber tip. Finally, at $488\text{-}\mu \mathrm{s}$ ALS, the complete collapse of the proximal bubble occurs with acoustic transient emission. The remnants (see arrow at $488\mu \mathrm{s}$ ) of the channel collapse at $630\mu \mathrm{s}$ and the bubble size reaches the second minimal values. After that, the bubble undergoes the last oscillation with shorter oscillation periods observed by the naked eye. At $664\mu \mathrm{s}$ , the bubble size reaches the third maximal value again, and collapses completely at $684\mu \mathrm{s}$ . The results of Fig. 4 are similar to those of Fig. 4 in the starting phase, but a very interesting feature can be observed at $368\mu \mathrm{s}$ : the bubble shape is more like a cucurbit, because it can be divided into two distinct parts. The proximal part of the bubble, still quasispherical and centered on the fiber tip, is now collapsing rapidly. Simultaneously, a spheric channel, due to further vaporization by the laser, is still progressing along the axis of the fiber tip. Finally, at $668\text{-}\mu \mathrm{s}$ ALS, the complete collapse of the proximal bubble occurs with acoustic transient emission. The remnants (see arrow at $556\mu \mathrm{s}$ ) of the channel, only partially visible at the bottom of the pictures, experiences another oscillation. After $818\mu \mathrm{s}$ , the whole bubble disappears entirely. The results of Fig. 4 are also like those of Fig. 4, but the bubble shape is more irregular and we can only observe oscillation clearly twice due to small radiation energy of the fiber tip (only $22\mathrm{mJ}$ ). The first and second maximal volumes (expand) occur at 270 and $716\mu \mathrm{s}$ (see arrows), respectively. The first and second collapse times occur at 444 and $842\mu \mathrm{s}$ , respectively. In some cases, even a third or fourth collapse generating an acoustic signal can be detected before the bubble finally dissolves into microbubbles. In these experiments, the high-speed camera is set at a speed rate of 500,000 frames per second with a spatial resolution of $256\times 16\text{pixels}$ .

Fig. 4

Sequence of images of vapor bubble formation at the fiber tips with different cord diameters. (a) $600\mu \mathrm{m}$ , (b) $400\mu \mathrm{m}$ , and (c) $200\mu \mathrm{m}$ . The laser pulse duration is $300\mu \mathrm{s}$ and the laser pulse energy is 300, 240, and $22\mathrm{mJ}$ at the distal end of 600, 400, and $200\text{-}\mu \mathrm{m}$ fibers, respectively. Times given are relative to the start of the laser pulse. A needle hydrophone is beside the $600\text{-}\mu \mathrm{m}$ fiber in (a) and (b).

3.2.

Dynamics of Ablation Products

In Fig. 5 , a calculus with COM and UC constituents as hard tissue is ablated underwater, and the surface of it is not parallel with the $600\text{-}\mu \mathrm{m}$ core diameter fiber tip. Actually, the right part of the free space (full of water) between them is bigger than that of the left. Consequently, the right ablation plume is bigger than the left ablation plume. At $\sim 18\text{-}\mu \mathrm{s}$ ALS, the ablation plume starts to occur and its size (see arrow) reaches a maximal value at $232\mu \mathrm{s}$ , and maintains a final size after $348\mu \mathrm{s}$ . To overcome the drawback of the uneven calculus surface, the fiber tip is in contact with the calculus surface, but there is still a little water between them, as illustrated in Fig. 5. At $\sim 96\text{-}\mu \mathrm{s}$ ALS, the ablation plume starts to grow and its size reaches maximal value (see arrow) at $168\mu \mathrm{s}$ . It reaches minimal value at $248\mu \mathrm{s}$ and the second smaller maximal value (see arrow) at $360\mu \mathrm{s}$ . Finally, at $392\mu \mathrm{s}$ it reaches the third minimal size of the ablation products and maintains at the same size after $392\mu \mathrm{s}$ . In Fig. 5, the ablation product volume undergoes three times oscillation because it reaches maximal values (see arrows) at 80, 200, and $334\mu \mathrm{s}$ , respectively. Accordingly, it reaches minimal values at 140, 300, and $414\mu \mathrm{s}$ , respectively, and maintains at the same value after $414\mu \mathrm{s}$ . The parameters of the high-speed camera for ablation experiments are set the same as those for cavitation effects (500,000 frames per second).

Fig. 5

Sequence of images of ablation products at the ends of different core diameter fibers: (a) 600, (b) 400, and (c) $200\mu \mathrm{m}$ . The laser pulse duration is $300\mu \mathrm{s}$ and the laser pulse energy is 300, 240, and $22\mathrm{mJ}$ at the distal end of 600, 400, and $200\text{-}\mu \mathrm{m}$ fibers, respectively. Calculus with COM and UC constituents as hard tissue is ablated underwater. Times given are relative to the start of the laser pulse.

3.3.

Pressure Measurements

Pressure transient occurrence and strength as a function of the delivering fiber diameter and laser fluence were studied. The laser-induced pressure waves generated at the submerged fiber tip were measured using a PVDF needle hydrophone. A high resistance termination on the pressure sensor yielded a signal amplitude that was proportional to the pressure. In our experiment, the absolute pressure waves were measured without a preamplifier. The diameter of the delivery fiber is an important parameter that influences pressure generation. At the same laser fluence, the diameter of the generated bubble scales with the fiber diameter. As smaller bubbles generate weaker pressure transients, the induced pressure amplitudes are significantly lower for smaller fiber diameters. On the other hand, for a given pulse energy delivered via different fibers, the induced pressure also generally decreases for decreasing fiber diameters. In this case, smaller fiber diameter means higher fluence regimes, which produce lower pressure amplitudes.¹

To avoid repeating similar results, in this work the laser-induced pressure waves generated at the submerged fiber tip, only for the $600\text{-}\mu \mathrm{m}$ core diameter fiber, are shown in Fig. 6 . The pressure amplitudes are measured at $4\text{-}\mathrm{mm}$ distance from the collapse center. The bubble formation starts at $\sim 40\text{-}\mu \mathrm{s}$ ALS in Fig. 4. A weak pressure increase $1.48\text{bar}$ associated with the bubble expansion is observed. The first pressure wave is emitted with an amplitude of $20.3\text{bar}$ $315\mu \mathrm{s}$ after bubble formation. Due to the laser pulse width of $300\mu \mathrm{s}$ , the first pressure wave induced by bubble collapse can be detected at $355\text{-}\mu \mathrm{s}$ ALS. The second, third, and fourth pressure waves with amplitudes of 12.5, 5, and $2\text{bar}$ at $4\text{-}\mathrm{mm}$ distance from the collapse center, respectively, corresponding to the second, third, and fourth bubble collapse (after rebound), can be seen at intervals of 168, 42, and $20\mu \mathrm{s}$ as shown in Fig. 6.

Fig. 6

Oscilloscope traces of the pressure transients induced by a single holmium pulse [ ${\mathrm{E}}_{\mathrm{p}}=300\mathrm{mJ}$ , $300\mu \mathrm{s}$ (FWHM), $F_1=106\mathrm{J}∕{\mathrm{cm}}^2$ ] at the submerged distal fiber tip( $600\text{-}\mu \mathrm{m}$ diameter). The increase in pressure (blue arrow) indicates the expansion of the generated bubble. The four pronounced pressure spikes after the end of a single holmium pulse are induced by the collapse of the bubble (1) and their rebounds (2, 3, and 4). Insert land II indicate the magnified pictures of the first and second pressure transients. (Color online only.)

3.4.

Comparision of Ablation Calculus in Air and Water

The distinct difference between ablation in air and ablation in a liquid environment is that the liquid confines the movement of the ablation products. Therefore, ablation in a liquid environment is accompanied by bubble formation and by mechanical effects much stronger than those observed in a gaseous environment. This bubble is essential for the transmission of optical energy to the target, and the mechansims governing its formation and subsequent dynamics have thus received attention by various researchers.^{10, 11, 12, 13, 14, 15} In cases in which the fiber tip is placed in contact with the tissue surface, the ablation products are even more strongly confined than when they are surrounded by liquid alone. Furthermore, cavitation effects always occur during laser-tissue interaction processes in a liquid environment, even if the fiber tip is in contact with the tissue surface just because of a little liquid existing at the fiber tip in real conditions. For a given radiant exposure, the confining effect of the liquid or solid results in considerably higher temperatures and pressures within the target than ablation in a gaseous environment, because the expansion and adiabatic cooling of the ablation products proceed more slowly. Therefore, in a liquid environment, there is generally a more effective transduction of the laser energy into mechanical energy.²⁰

After exposure to ten consecutive holmium laser pulses at a repetition rate of $1\mathrm{Hz}$ in air and water, ablation craters of the calculus are scanned with an OCM instrument. The cross sectional profiles are displayed in Fig. 7 . The cases can be mostly seen in actual clinical applications. To obtain the accurately quantitative ablation crater shape, an OCM system with lateral and axial resolutions of $2\mu \mathrm{m}$ was used to scan images from the $yz$ , $xz$ , and $xy$ axes directions for the calculus irradiated by holmium laser transmitted in a $200\text{-}\mu \mathrm{m}$ core diameter fiber, which are shown in the upper two rows, and $yz$ , $xz$ , and 3-D for 400 and $600\text{-}\mu \mathrm{m}$ core diameter fibers are shown in the following four rows. The height and width of the ablation craters are displayed in the corresponding images.

Fig. 7

Cross sectional topography of Ho:YAG laser-induced (ten successive lasers at a repetition rate of $1\mathrm{Hz}$ ) craters acquired with the OCM system in air and water for different core diameter fibers. Note that $xy$ direction cross-sectional topography is for only the $200\text{-}\mu \mathrm{m}$ core diameter. The output intesnity per pulse at the fiber tips (200, 400, and $600\mu \mathrm{m}$ ) can be averaged as 22, 240, and $300\mathrm{mJ}$ , respectively. h is height and w is width.

In our prior study, some conclusions can be drawn for laser ablation either in air or in water. 1. An increase in laser energy produces larger craters for all three fibers. 2. The crater shape is also affected by fiber size. At a given laser energy, irradiation with the larger fiber produces wider and shallower craters. 3. Ablation volume is a function of the fiber size and radiant exposure. 4. Even when the calculus surface is at an angle with respect to the laser beam, the ejected plume propagates in the direction normal to the calculus surface.^{15, 21, 22, 23, 24} Therefore, in this work, to study the ablation difference in air and water, three series of ablation images for 200, 400, and $600\text{-}\mu \mathrm{m}$ diameter fibers are presented in Fig. 7, respectively.

The crater diameters are always smaller than the fiber diameters used, and the crater diameters in air are smaller than those in water. But the first conclusion is not suitable for $400\text{-}\mu \mathrm{m}$ core diameter fibers, because of the 400 and $795\text{-}\mu \mathrm{m}$ crater diameters in air and water, shown in Fig. 7, are thought to be caused by calculus position shifting during the process of laser-calculus interaction. In addition, the crater height and ablation volume in air are always shorter and smaller than those in water, respectively. Finally, the important difference in the two cases is that the crater surface for underwater ablation is slippery, whereas that for in air is crude, as shown clearly in 3-D images in Fig. 7. In a word, for a certain core diameter fiber and pulse energy, ablation efficiency in water is higher than that in air.