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.
Holmium:YAG laser has been the lithotrite of choice for around 30 years in kidney stone surgery. Lasers have evolved over the years to offer higher power, increased pulse frequencies and longer pulse durations. The drivers for change have been to improve stone ablation and to minimise retropulsion. We report on a new prototype Holmium laser that fires multiple “micro-pulses” in “pulse packets” and discuss the stone phantom ablation rate results utilizing a bench model. The prototype laser demonstrated impressive stone ablation rates in our bench testing across a range of power settings. We will discuss the details of these results supporting that pulse-modulation with packets of micro-pulses are a promising technological development. (Disclaimers: Bench Test results may not necessarily be indicative of clinical performance. The testing was performed by or on behalf of BSC.
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).
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.
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.
Laser vaporization is a surgical procedure which utilizes a high power laser to quickly heat and vaporize tissue. Laser vaporization can be conducted on internal organs, such as breast or prostate, through a fiber catheter. Compared with other surgical technologies, it has excellent hemostasis capability with minimal collateral tissue damage, which may reduce hospitalization time and postoperative complications. Accurately monitoring tissue temperature during laser vaporization procedures provides important feedback to surgeons to improve surgical outcomes. Tissue cannot be vaporized if the temperature is lower than the boiling point, while high temperatures may lead to carbonization over the tissue surface, which not only reduces vaporization efficiency but also leads to postsurgical complications. However, until now, no sensing technologies have been developed to monitor tissue temperature during routine laser vaporization in clinics. Here, we report the use of blackbody radiation in the short-wave infrared range (SWIR) for tissue temperature monitoring during laser vaporization. Although blackbody radiation in SWIR is very weak for temperatures less than 100°C, the relatively low water absorption and silica fiber attenuation may allow temperature sensing in vivo. We successfully detected blackbody radiation in SWIR down to 80°C through a 2 m silica fiber. We then proved the feasibility of using blackbody radiation in SWIR to monitor tissue temperature during laser vaporization through an ex vivo tissue study. The developed technology is low-cost and can be seamlessly integrated with the fiber catheter used in laser vaporization.
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.
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.
Vaporization and coagulation are two fundamental processes that can be performed during laser-tissue ablation. We demonstrated a method allowing quasi-dynamically observing of the cross-sectional images of tissue response during ablation. The results showed that coagulation depth is relatively constant during vaporization, which supports the excellent hemostasis of green laser benign prostate hyperplasia (BPH) treatment. We also verified a new technology for real-time, in situ tissue temperature monitoring, which may be promising for in vivo tissue vaporization degree feedback during laser ablation to improve the vaporization efficiency and avoid complications.
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.
We designed a cost effective, highly efficient diode-pumped and Q-switched 532nm laser with large laser output power range and fast laser output power switching from zero Watts to any user defined operating power by adjusting the pump diode current. It is well known that pump diodes used in the solid-state Nd:YAG lasers have center wavelength around 808nm or 885nm and will shift their center wavelength while changing its drive current. The pump diode wavelength shift can be large enough to move outside of the laser gain medium absorption band. This results in lower pump absorption efficiency and hence in lower overall system optical to optical conversion efficiency for some portion of the laser output power range. A few typical ways to minimize this wavelength-shift-caused lower pump absorption effect with their trade-offs have been discussed. We report the unique pump diode wavelength of 879nm and single Nd:YAG rod design for this laser system. This system has about 38% optical to optical (879nm to 532nm) conversion efficiency. The pump current can be adjusted to tune the laser output power to anywhere within the range of 20W to 300W. The laser output power responds within 2 seconds to this input current change, and does not result in Nd:YAG rod damage. Furthermore, a high pump light absorption efficiency is maintained.
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.
The state-of-the-art beam quality from high-brightness, fiber-coupled diode laser modules has been significantly improved in the last few years, with commercially available modules now rivaling the brightness of lamp-pumped Nd:YAG lasers. We report progress in the development of these systems for a variety of applications, such as material processing and pumping of solid state and fiber lasers. Experimental data and simulation results for wavelength stabilized outputs from 200 µm diameter fibers at 975 nm for power levels greater than 200 W will be presented. The enabling technology in these products is supported by key developments in tailored diode laser bars with low slow axis divergence, micro-optics, diode laser packaging, and modular architecture.
KEYWORDS: Semiconductor lasers, Temperature metrology, Finite element methods, Data modeling, Reliability, Diodes, Heatsinks, Solid state lasers, Instrument modeling, Packaging
State-of-the-art QCW solid-state lasers are demanding ever higher brightness from the pump source-conduction cooled
diode laser stacks. The intensity of a QCW vertical stack is limited by the peak power of each diode bar and the bar
pitch. The minimum bar pitch of the existing laser diode stacks on the market is about 400um. In this paper, we present a
unique vertical diode laser stack package design to achieve a bar pitch as low as 150um, which improves the intensity of
the stack by nearly 3 times. Together with the state-of-art diode laser bar from Coherent, greater than 30kW/cm2 peak
power density is achieved from the emitting area of the vertical stack. The p-n junction temperature of the diode bars in
the device under QCW operation is modeled with FEA software, as well as measured in this research. Updated reliability
results for these diode laser stacks are also reported.
Fiber lasers have made significant progress in terms of power output, beam quality and operational robustness over the
past few years. Key to this progress has been advances in two technologies - fiber technology and 9xx nm diode laser
pump technology based on single emitters. We present the operational characteristics of our new high brightness 9xx nm
fiber laser pump sources based on diode laser bars and diode laser bar arrays and discuss the design trade offs involved
for realization of devices focused on this application. These trade offs include achieving the lowest slow axis divergence
while maintaining high wall plug efficiency and minimizing facet power density to maximize reliability.
We present results from a survey of materials used for packaging semiconductor lasers, including Cu, CuW, BeO,
diamond composite and other advanced materials. We present the results of residual bonding stress from various solders
and consider the direct effects on wavelength and spectral width. We also provide data on the second order effects of
threshold current and slow axis divergence. Additionally, we consider the heat spreading through different materials for
a laser bar and present modeled and experimental data on the thermal performance. Finally, we consider the reliability
under on-off life-testing and thermal cycling tests.
Developments in Nd-based lasers pumped on the 4I9/2→4F3/2 transition have led to
demands for increased power, brightness, and spectral stability from diode pump sources.
We describe the development of fiber coupled diode pump sources that generate >120W
of power from a 400μm, 0.22NA fiber at 88Xnm wavelengths. In order to maintain
spectral purity at these high powers, we investigated the use of Volume Bragg Gratings to
stabilize the wavelength of these multi-bar systems. A detailed study of the trade-offs
between facet reflectivity and VBG reflectivity was conducted in order to determine an
optimal combination that balances output power and locking range.
In complement to the developments in 88Xnm pumping, recent interest in eye-safe fiber
lasers have resulted in the development of Tm-doped fiber lasers pumped at 79X
wavelengths. We describe the development of fiber coupled products with >80W from a
200μm, 0.22NA fiber, including the use of optimized bar geometries to improve fiber
coupling efficiency.
We present kW QCW vertical and horizontal arrays composed of 200W bars (peak power) at 8xxnm wavelength. We
also present an unique Bar-on-Submount design using the electrically insulating submounts, which can provide a
platform for simple and flexible horizontal array construction. The p-n junction temperature of the arrays under QCW
operation is modeled with FEA software, as well as measured in this research. Updated reliability test results for these
kW arrays will be also reported. As the examples, we present the performance of the vertical arrays with > 57% Wall-Plug-Efficiency and the horizontal arrays with < 23 degree fast axis divergence (FWHM), both with 808nm wavelength.
The available wavelength for such arrays ranges from 780nm to beyond 1 um. Coherent also have the capability to
produce the array with wide and relatively uniform spectrum for athermal pumping of solid-state lasers, by integrating
diode lasers bars with different wavelength into single array.
We describe the performance of diode laser bars mounted on conductive and water cooled platforms using low smile processes. Total smile of <1μm is readily achieved on both In and AuSn based platforms. Combined with environmentally robust lensing, these mounts form the basis of multiple, high-brightness products.
Free-space-coupled devices utilizing conductively-cooled bars delivering 100W from a 200μm, 0.22NA fiber at 976nm have been developed for pumping fiber lasers, as well as for materials processing. Additionally, line generators for graphics and materials processing applications have been produced. Starting from single bars mounted on water-cooled packages that do not require de-ionized or pH-controlled water, these line generators deliver over 80W of power into a line with an aspect ratio of 600:1, and have a BPP of <2mm-mrad in the direction orthogonal to the line.
The development of mid-infrared interband diode lasers has been hindered by factors such as Auger recombination and intervalence band absorption, which become increasingly important at longer wavelengths. A number of structures have been proposed in which the effects of these processes are reduced. The maximum gain per unit volumetric current density can be used as a figure of merit for comparing different active region materials. Using this figure of merit, we compare a series of structures with band gaps near 0.3 eV (i.e., wavelengths near 4 microns). The figure of merit is obtained from gain spectra calculated using superlattice K(DOT)p theory and a combination of calculated and measured recombination rates. We show that devices based on active regions incorporating type-I InAsSb/AlInAsSb or InAsSb/InAsP quantum wells should have room temperature threshold currents 7 - 13 times smaller than those of devices based on bulk InAs. However, devices using type-II superlattice active regions should have room temperature threshold currents that are a factor of 3 - 4 times smaller than those of the type-I quantum wells. The figure of merit can also be used to determine the optimal thickness of the active region as a function of waveguide loss and optical mode width.
We demonstrate midwave infrared diode lasers than span the 3 - 4 micrometers range. Laser active regions are multiple quantum well structures with GaInSb/InAs, type-II, broken gap superlattices for the wells and GaInAsSb for the barriers. The superlattice constituents and dimensions were tailored to reduce losses from Auger recombination. AlSb/InAs superlattices are used for both n-type and p-type laser cladding regions. A device with emission at 3.2 micrometers lased up to 255 K. We have achieved 75 mW per facet at 3.0 micrometers at an operating temperature of 140 K with an 85 microsecond(s) ec input current pulse. Device output appears to be limited by resistive heating. A four-layer, strain-balanced superlattice design offers greater laser efficiency.
Mid-wave infrared lasers have been fabricated employing InAs/A1Sb superlattice cladding layers and multi-quantum well active regions consisting of Ga75In025Sb1InAs broken-gap superlattice wells and Ga75In025As023Sb,,77 barriers. Diodes demonstrated to date include lasers with emission wavelengths of 3.18j.tm at 255K, 3.40im at 195K, and 4.32p.m at 110K.
Keywords: infrared, laser, diode, superlattice, multi-quantum well
We demonstrate midwave infrared (MID-IR) diode lasers that span most of the 3 - 4 micrometers range. Laser active regions are multiple quantum well (MQW) structures with GaInSb/InAs, type-II, broken gap superlattices for the wells and GaInAsSb for the barriers. The superlattice constituents and dimensions were tailored to reduce losses from auger recombination. AlSb/InAs superlattices are used for both n-type and p-type laser cladding regions.
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