Expert in Laser Optics, Laser Beam Shaping and Fiber/u-lens Optics; Gas, diode, diode-pumped solid state lasers and laser cavity design, for Medical, Industrial and Research applications; Semiconductor laser packaging, integration and qualification; Micro-optics design and implementation; General optical design; Production management (schedule, process optimization, quality control, trouble shooting);
Dr. Zhang joined Boston Scientific Corp since Aug 2015. Since Mar 2012, he joined AMS (an ENDO International Company) as a Principal Laser Engineer. From 1998 to 2012, he has been with Quantronix Corp., Long Island, NY, USA, as a Senior Scientist, Director and VP of R&D. From 1997 until 1998, he joined Innovative Lasers Corp., Tucson AZ, USA, as a Laser Scientist. In Jan 1996, he joined Fraunhofer-Institut fuer Lasertechnik, Aachen, Germany, as a Research Scientist. From 1992 to 1996, he was a Lecturer and Associate Professor at Modern Applied Physics Department of Tsinghua University. He was a visiting scientist at Festkoerper Laser Institute Berlin GmbH, Technical University of Berlin, from May to Sept 1994. He has published two book chapters, 31 Journal papers, 33 conference papers, more than 100 scientific reports, 1 German patent and 15 US patent (7 pending).
Specialties: Medical/Industrial laser design/application, Fiber optics for laser energy delivery, R&D/production management; LASCAD, Agile (Oracle), Edward, Winlase, Zemax, AutoCAD, Solid Edge, Mathcad and Mathematica.
Publications (21)
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The urology field's workhorse for both soft tissue and stone removal has been the flashlamp-pumped CTH:YAG or Ho:YAG (Cr3+, Tm3+, Ho3+) triple doped yttrium aluminum garnet laser for over twenty years. The transient pressure field profile of the vapor bubbles produced by laser pulses provides key insight for understanding kidney stone ablation and retropulsion. The goal of this study is to evaluate the transient pressure field profile of the vapor bubbles in water produced by the laser modes of a Ho:YAG laser. A hydrophone measures the transient pressure field from the laser lithotripsy fiber tip at various locations. Based on the symmetry of the device and environment, the transient pressure field ellipsoid can be constructed with the measurement results. The transient pressure field profiles are significantly different across the standard laser modes.
Laser lithotripsy destroys urinary stones by ablating them into fine particles and dust. However, the nature of the resulting dust is not well understood, nor how this differs for different stone types. To investigate this, canine stones of different types were ablated in vitro using a Ho:YAG laser and the resulting particles analysed to calculate a characteristic particle size distribution for each stone type. Differences in these size distributions may require the resulting stone dust to be managed differently in vivo.
As more and more small core fibers (sub 300 μm) become available for high-power laser application (>20 W), fiber distal-tip degradation during ureteroscopic laser lithotripsy (URSL) is critical. Still, it is an overlooked aspect due to its subtle effect on the ablation rate and lack of adequate characterization. The objective of this study is to measure fiber tip degradation in terms of surface modification and output beam profile. The fiber tip is a location of interest not only because fiber material performance at the tip impacts laser beam shape, power, and quality but moreover because this is the primary region where inter-procedural degradation compounds. By characterizing the degradational relationship between laser beam profile and fiber tip damage, the next generation of fiber development could be motivated to improve URSL procedures. A high-magnification microscope was used to investigate the fiber tip surface degradation due to laser dose with or without a stone phantom. A single-mode 660 nm laser beam and an M-square measurement system were used to evaluate the laser beam profile variation through lithotripsy fibers with different distal end degradations. The results show that the laser beam profile through the fiber varies with the core size of the fiber and distal end degradation.
Stone retropulsion is an inevitable side effect of laser lithotripsy. It is generally considered undesirable but can also be beneficial in techniques such as “popdusting”. It is often described by a single parameter such as a distance moved, which might not correlate with the degree to which retropulsion interferes with clinical use. We report on a novel method of measuring retropulsion in vitro using high-speed video photography to describe the stone trajectory across multiple parameters, and present results from a range of laser systems. Describing the nature of this movement may help explain how some types of retropulsion are considered clinically disruptive and others are not.
For almost 25 years, the lamp-pumped Chromium (Cr3+), Thulium (Tm3+), Holmium (Ho3+) triple doped yttrium aluminum garnet (CTH:YAG or Ho:YAG) laser has been commercialized for ureteroscopic laser lithotripsy (URSL). However, the transient pressure field profile of the vapor bubbles that arise from laser pulses has not been reported. The transient pressure field of different laser pulses provides further insight into understanding stone ablation and retropulsion. The objective of this study is to measure the transient pressure field profile of the vapor bubbles of standard and custom laser modes of a prototype CTH:YAG laser. (Disclaimers: Concept device or technology. Not available for sale. This device is not yet available for sale in the United States).
Laser lithotripsy has clinically been implemented to treat urinary stone disease by using a Ho:YAG laser system. Bubble dynamics plays an important role in determining stone ablation efficiency. The current study developed a two-phase opto-thermal model to numerically assess the bubble dynamics during laser irradiation. The simulation involved light propagation, light and water interaction, and multi-phase heat and mass transfer. The simulation was verified by the experimental setup including Ho:YAG lasers, fiber in water, and high-speed camera. Both the numerical simulations and the experimental results showed a good agreement in predicting the effects of laser pulses on the bubble dynamics.
The Ho:YAG laser has been the favored lithotripter for the treatment of urinary calculi since shortly after its introduction in the 1990s, because it can fragment all calculi compositions. The objective of this study is to simulate the transient thermal behavior of the lamp-pumped laser rod from preheating to laser operation. The simulation tool used for this study is the Ansys AIM, and the transient thermal behavior of the lamp-pumped laser rod from preheating to laser operation was reported. Optimization of the pumping pulse for the desired laser output pulse is for a future study. (Disclaimers: Concept device or technology. Not available for sale. This device is not yet available for sale in the United States).
Ureteroscopic stone dusting utilizing a high power laser system has become more popular in recent years due to the production of finer debris/remains, lower retropulsion, and shorter operation time (potential to avoid the routine use of post-operative stenting and the use of ureteral access sheaths (UAS)). Typical dusting settings are lower pulse energy (as low as 0.2 J) with higher frequency (up to 80 Hz). This study investigates the best dusting mode to produce a high ablation rate and low retropulsion. The objective of the study was to evaluate the performance of a concept optimal dusting mode. In vitro investigations of Ho:YAG laser-induced stone ablation and retropulsion were performed with a benchtop model in a highly reproducible manner using a hands-free setup and measuring the effects of multiple pulses. A systematic comparison of the performance (ablation and retropulsion) of the concept optimal dusting mode against a reference laser dusting mode was conducted. Within this benchtop test model, the optimal dusting mode had a relatively fast ablation rate while keeping retropulsion low.
The Ho:YAG laser has been the favored lithotripter for treating urinary calculus since shortly after its introduction in the 1990s because it can fragment all calculus compositions and produces less calculus migration (retropulsion) during treatment than the short-pulsed lasers. Although the lamp-pumped Ho:YAG laser has been commercialized for ureteroscopic laser lithotripsy (URS) for almost 25 years, the lamp-pumped laser rod's transient thermal behavior under an out-of-control lamp discharge has not been reported. A safety question arises: how big is the laser output under an out-of-control lamp discharge? The objective of this study is to simulate the transient thermal behavior of the lamp-pumped laser rod. The temperature profile inside the laser rod is transformed into an equivalent thermal lens, and from the time interval of the variation of the thermal lens between the lasing threshold and the maximum value of the stable region, we can estimate the level of the laser output. The simulation tool used for this study is the Ansys Fluent. The transient thermal behavior of the lamp-pumped laser rod under an out-of-control lamp discharge was reported. Optimization of the pumping pulse for the desired laser output pulse is for future study.
Stone retropulsion during laser lithotripsy results from various physical phenomena such as recoil momentum, bubble dynamics, and subsequent jet formation. Considerable stone retropulsion has been observed whereby the optical energy is converted into both mechanical and thermal energy as a distinctive bubble generation and collapse. It is hypothesized that by reducing the peak power and lengthening the pulse duration, we can reduce this conversion of optical energy into mechanical energy. This should maximize the thermal effects on the stone leading to enhanced ablation efficiency as well as less stone “chasing”. We are reporting on a new prototype Holmium laser with low pulse power and long temporal pulse durations in an attempt to minimize stone retropulsion.
Holmium:YAG laser is commonly used as an efficient technology for lithotripsy, breaking urinary stones into small particles (dust) and larger residual fragments (RF). One of the ultimate goals is to create fine dust for real-time aspiration, eliminating the need for mechanical retrieval of RFs. A recent study of stone dust definition suggests a maximum particle size of 250-µm to allow complete aspiration through the working channel of a flexible ureteroscope.
We have evaluated the particle size generation of a concept Holmium:YAG laser utilizing a pulse width modulation technique. This technology delivers numerous low-energy micro-pulses per pulse with long temporal pulse duration to potentially enable finer dust particles, better ablation rate, and reduced retropulsion. Overall, the concept device generates a high percentage of fine dust compared with prior results found in literature.
(Disclaimers: Bench Test results may not necessarily be indicative of clinical performance. The testing was performed by or on behalf of BSC. Data on file. Concept device or technology. Not available for sale. This device is not yet available for sale in the United States).
Although laser lithotripsy is now the preferred treatment option for urolithiasis due to shorter operation time and a better stone-free rate [1], the optimal laser settings for URS (Ureteroscopic lithotripsy) to enable shorter operating times remain unclear. This study aims to identify optimal laser settings for Ho:YAG laser-lithotripsy to maximize the ablation rate while minimizing the retropulsion, as well as to improve the discharge of fragments via the urinary tract. The net result will be an increase in treatment success and patient satisfaction by ameliorating the stone-free rate.
In vitro investigations of Ho:YAG laser-induced stone ablation and retropulsion were performed with a bench top model first introduced by Sroka’s group [2]. A commercial Ho:YAG laser system (Lumenis VersaPulse PowerSuite 100W, Lumenis Ltd., Yokneam, Israel) was used as the laser pulse source, with pulse energy from 0.2 J up to 1.5 J and repetition rate from 5 to 40 Hz. A DOE with two replicate points and two lack-of-fit points was performed on artificial BEGO stones of sample size 14 under reproducible experimental conditions (fiber size: 365 μm, S-LLF365 SureFlex Fiber, Boston Scientific Corporation, San Jose, CA, USA). The best fit to the experimental data was analyzed utilizing the design of experiment software, which can produce the numerical formulas for the response surfaces of ablation rate and retropulsion in terms of laser pulse parameters [3].
The coded numerical formulas for the response surfaces of ablation speed and retropulsion velocity are generated. The coded equation is useful for identifying the relative impact of the factors by comparing their coefficients. Upon examination of the laser ablation of stone phantoms (BEGO), the laser pulse energy is 1.4 times the impact of the frequency, and laser pulse peak power’s impact is the same as the frequency; while for retropulsion, the laser pulse energy is 5.8 times as the impact of the frequency, and laser pulse peak power’s impact is 13 times as the frequency; A series of laser settings for relatively efficient laser lithotripsy were identified in terms of laser pulse energy and peak power.
The laser pulse energy or peak power in reference to frequency has a higher impact coefficient to stone retropulsion as compared to stone ablation in Ho:YAG laser-lithotripsy. The most effective way to reduce stone retropulsion during laser lithotripsy is to reduce the laser pulse peak power (which has the highest impact coefficient in the coded response equation).
Laser lithotripsy is now the preferred treatment option for urolithiasis over Shock wave lithotripsy (SWL) for renal stones smaller than 1.5 cm due to shorter operation times and a better stone-free rates (from the retrospective study by E. B. Cone et al). Nonetheless, the detailed mechanism of calculus disintegration by laser pulse remains relatively unclear. One of the fundamental parameters for laser stone interaction is the ablation threshold. Richard L. Blackmon, et. al. have studied the ablation threshold for Ho: YAG and the thulium fiber lasers (TFL) in terms of the laser energy density. However, an ablation threshold in terms of peak power density would be more universally applicable. In this study, two commercially available Ho: YAG lasers were used as the laser pulse source. The fibers used in the investigation are SureFlexTM fibers, (Models S-LLF273 and S-LLF365) with 273 and 365 μm core diameters, respectively. Calculus phantoms were made of the Bego stone material with various degrees of hardness. These stone phantoms were ablated with the Ho: YAG lasers at different peak power densities. The laser pulse width was measured utilizing a 2 μm photodiode (Thorlabs DET10D), and the laser-induced crater volumes were evaluated with a 3-D digital microscope (Keyence VHX-900F). In this way, we determined the ablation threshold as a function of peak power density for the Bego stone phantoms with 3 different hardness values. Additional investigations of the ablation threshold of other stone types will be conducted in a future study.
When treating ureteral calculi, retropulsion can be reduced by using a longer pulse width
without compromising fragmentation efficiency (from the studies by David S. Finley et al. and
Hyun Wook Kang et al.). In this study, a lab build Ho:YAG laser was used as the laser pulse
source, with pulse energy from 0.2J up to 3.0 J, and electrical pump pulse width from 150 us up
to 1000 us. The fiber used in the investigation is a 365 μm core diameter fiber, SureFlexTM,
Model S-LLF365. Plaster of Paris calculus phantoms were ablated at different energy levels (0.2,
0.5, 1, 2, 3J) and with different number of pulses (1, 3, 10) using different electrical pump pulse
width (333, 667, 1000 μs). The dynamics of the recoil action of a calculus phantom was
monitored using a high-speed camera with frame rate up to 1 million frame per second
(Photron Fastcam SA5); and the laser-induced craters were evaluated with a 3-D digital
microscope (Keyence VHX-900F). A design of experiment software (DesignExpert-10,
Minneapolis, MN, USA) is used in this study for the best fit of surface response on volume of
dusting and retropulsion amplitude. The numerical formulas for the response surfaces of
dusting speed and retropulsion amplitude are generated. More detailed investigation on the
optimal conditions for dusting of other kinds of stone samples and the fiber size effect will be
conducted as a future study.
GreenLightTM procedure is an effective and economical way of treatment of benign prostate hyperplasia (BPH); there are almost a million of patients treated with GreenLightTM worldwide. During the surgical procedure, the surgeon or physician will rely on the monitoring video system to survey and confirm the surgical progress. There are a few obstructions that could greatly affect the image quality of the monitoring video, like laser glare by the tissue and body fluid, air bubbles and debris generated by tissue evaporation, and bleeding, just to name a few. In order to improve the physician’s visual experience of a laser surgical procedure, the system performance parameter related to image quality needs to be well defined. However, since image quality is the integrated set of perceptions of the overall degree of excellence of an image, or in other words, image quality is the perceptually weighted combination of significant attributes (contrast, graininess …) of an image when considered in its marketplace or application, there is no standard definition on overall image or video quality especially for the no-reference case (without a standard chart as reference). In this study, Subjective Quality Factor (SQF) and acutance are used for no-reference image quality evaluation. Basic image quality parameters, like sharpness, color accuracy, size of obstruction and transmission of obstruction, are used as subparameter to define the rating scale for image quality evaluation or comparison. Sample image groups were evaluated by human observers according to the rating scale. Surveys of physician groups were also conducted with lab generated sample videos. The study shows that human subjective perception is a trustworthy way of image quality evaluation. More systematic investigation on the relationship between video quality and image quality of each frame will be conducted as a future study.
Although laser lithotripsy is now the preferred treatment option for urolithiasis, the mechanism of laser pulse induced
calculus damage is still not fully understood. This is because the process of laser pulse induced calculus damage
involves quite a few physical and chemical processes and their time-scales are very short (down to sub micro second
level). For laser lithotripsy, the laser pulse induced impact by energy flow can be summarized as: Photon energy in the
laser pulse → photon absorption generated heat in the water liquid and vapor (super heat water or plasma effect) →
shock wave (Bow shock, acoustic wave) → cavitation bubble dynamics (oscillation, and center of bubble movement ,
super heat water at collapse, sonoluminscence) → calculus damage and motion (calculus heat up, spallation/melt of
stone, breaking of mechanical/chemical bond, debris ejection, and retropulsion of remaining calculus body). Cavitation
bubble dynamics is the center piece of the physical processes that links the whole energy flow chain from laser pulse to
calculus damage. In this study, cavitation bubble dynamics was investigated by a high-speed camera and a needle
hydrophone. A commercialized, pulsed Ho:YAG laser at 2.1 mu;m, StoneLightTM 30, with pulse energy from 0.5J up to 3.0
J, and pulse width from 150 mu;s up to 800 μs, was used as laser pulse source. The fiber used in the investigation is
SureFlexTM fiber, Model S-LLF365, a 365 um core diameter fiber. A high-speed camera with frame rate up to 1 million
fps was used in this study. The results revealed the cavitation bubble dynamics (oscillation and center of bubble
movement) by laser pulse at different energy level and pulse width. More detailed investigation on bubble dynamics by
different type of laser, the relationship between cavitation bubble dynamics and calculus damage
(fragmentation/dusting) will be conducted as a future study.
Q-switched (QS) Tm:YAG laser ablation mechanisms on urinary calculi are still unclear to researchers. Here, dependence of water content in calculus phantom on calculus ablation performance was investigated. White gypsum cement was used as a calculus phantom model. The calculus phantoms were ablated by a total 3-J laser pulse exposure (20 mJ, 100 Hz, 1.5 s) and contact mode with N=15 sample size. Ablation volume was obtained on average 0.079, 0.122, and 0.391 mm3 in dry calculus in air, wet calculus in air, and wet calculus in-water groups, respectively. There were three proposed ablation mechanisms that could explain the effect of water content in calculus phantom on calculus ablation performance, including shock wave due to laser pulse injection and bubble collapse, spallation, and microexplosion. Increased absorption coefficient of wet calculus can cause stronger spallation process compared with that caused by dry calculus; as a result, higher calculus ablation was observed in both wet calculus in air and wet calculus in water. The test result also indicates that the shock waves generated by short laser pulse under the in-water condition have great impact on the ablation volume by Tm:YAG QS laser.
Fiber-tip degradation, damage, or burn back is a common problem during the ureteroscopic laser lithotripsy procedure
to treat urolithiasis. Fiber-tip burn back results in reduced transmission of laser energy, which greatly reduces the
efficiency of stone comminution. In some cases, the fiber-tip degradation is so severe that the damaged fiber-tip will
absorb most of the laser energy, which can cause the tip portion to be overheated and melt the cladding or jacket
layers of the fiber. Though it is known that the higher the energy density (which is the ratio of the laser energy fluence
over the cross section area of the fiber core), the faster the fiber-tip degradation, the damage mechanism of the fibertip
is still unclear. In this study, fiber-tip degradation was investigated by visualization of shockwave, cavitation/bubble
dynamics, and calculus debris ejection with a high-speed camera and the Schlieren method. A commercialized, pulsed
Ho:YAG laser at 2.12 um, 273/365/550-um core fibers, and calculus phantoms (Plaster of Paris, 10x10x10 mm cube)
were utilized to mimic the laser lithotripsy procedure. Laser energy induced shockwave, cavitation/bubble dynamics,
and stone debris ejection were recorded by a high-speed camera with a frame rate of 10,000 to 930,000 fps. The
results suggested that using a high-speed camera and the Schlieren method to visualize the shockwave provided
valuable information about time-dependent acoustic energy propagation and its interaction with cavitation and
calculus. Detailed investigation on acoustic energy beam shaping by fiber-tip modification and interaction between
shockwave, cavitation/bubble dynamics, and calculus debris ejection will be conducted as a future study.
Calculus migration is a common problem during ureteroscopic laser lithotripsy procedure to treat urolithiasis. A conventional experimental method to characterize calculus migration utilized a hosting container (e.g. a “V” grove or a test tube). These methods, however, demonstrated large variation and poor detectability, possibly attributing to friction between the calculus and the container on which the calculus was situated. In this study, calculus migration was investigated using a pendulum model suspended under water to eliminate the aforementioned friction. A high speed camera was used to study the movement of the calculus which covered zero order (displacement), 1st order (speed) and 2nd order (acceleration). A commercialized, pulsed Ho:YAG laser at 2.1 um, 365-um core fiber, and calculus phantom (Plaster of Paris, 10×10×10mm cube) were utilized to mimic laser lithotripsy procedure. The phantom was hung on a stainless steel bar and irradiated by the laser at 0.5, 1.0 and 1.5J energy per pulse at 10Hz for 1 second (i.e., 5, 10, and 15W). Movement of the phantom was recorded by a high-speed camera with a frame rate of 10,000 FPS. Maximum displacement was 1.25±0.10, 3.01±0.52, and 4.37±0.58 mm for 0.5, 1, and 1.5J energy per pulse, respectively. Using the same laser power, the conventional method showed <0.5 mm total displacement. When reducing the phantom size to 5×5×5mm (1/8 in volume), the displacement was very inconsistent. The results suggested that using the pendulum model to eliminate the friction improved sensitivity and repeatability of the experiment. Detailed investigation on calculus movement and other causes of experimental variation will be conducted as a future study.
Q-switched Tm:YAG laser ablation mechanisms on urinary calculi are still unclear to researchers. In this study, dependence of water content in calculus phantom on calculus ablation performance was investigated. White gypsum cement was used as a calculus phantom model. The calculus phantoms were ablated at single pulse and contact mode in three different conditions: dry calculus in air, wet calculus in air, and wet calculus in water. Ablation volume was obtained on average 0.006, 0.008, and 0.008 mm3 in dry calculus in air, wet calculus in air, and wet calculus in water groups, respectively. There were three proposed ablation mechanisms that could explain the effect of water content in calculus phantom on calculus ablation performance, including shock wave due to bubble collapse, spallation, and microexplosion. Shock wave generation due to bubble collapse in wet calculus in water condition had negligible effect on calculus ablation as captured by a needle hydrophone and cannot be a primary mechanism for calculus ablation in this study. Increased absorption coefficient of wet calculus can cause stronger spallation process compared with that caused by dry calculus; and as a result, higher calculus ablation was observed in both wet calculus in air and wet calculus in water. Vaporization of interstitial water in porous calculus phantom can also help enhance calculus ablation efficiency. There were some limitations in this study including use of small sample size and lack of employing real urinary calculus, which should be addressed in future experiment.
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