Laser Pulse Repetition Research Articles

Effects of conical nozzle configuration on impulse coupling coefficient in repetitively-pulsed laser propulsion

A dimensionless factor was introduced to deduce the analytic expression of impulse coupling coefficient for conical nozzles in the case of spherical symmetry, and a high precision impact pendulum system was used to measure impulse coupling coefficients of 15 conical nozzles with different cone angles and lengths. The expression was corrected according to experimental values. The results indicate that: 1) impulse coupling coefficient increases firstly and then decreases with augment of dimensionless length when cone angle is fixed; 2) impulse coupling coefficient decreases monotonously with augment of cone angle when dimensionless length is fixed; 3) it is of great importance for improving impulse coupling coefficient to increase the rate of laser energy deposition.

Solidification structures on carbon materials surface-melted by repetitive laser pulses

The solidification morphology of carbon materials surface-melted by laser radiation at atmospheric pressure is studied. Electron microscopy results indicate that melt solidification is accompanied by the formation of surface microstructures, presumably due to the Rayleigh—Taylor instability in the molten carbon. The instability increment and surface tension coefficient of molten carbon are estimated, and the penetration of carbon vapour into the melt during one laser pulse is examined using numerical simulation.

Effect of repetitive laser pulses on the electrical conductivityof intervertebral disc tissue

The thermomechanical effect of 1.56-μm fibre laserpulses on intervertebral disc cartilage has been studied usingac conductivity measurements with coaxial electrodes integrated with an optical fibre for laser radiation delivery to thetissue. The observed time dependences of tissue conductivitycan be interpreted in terms of hydraulic effects andthermomechanical changes in tissue structure. The laserinduced changes in the electrical parameters of the tissue areshown to correlate with the structural changes, which werevisualised using shadowgraph imaging. Local ac conductivitymeasurements in the bulk of tissue can be used to develop adiagnostic/monitoring system for laser regeneration ofintervertebral discs.

Wall-Propelled, In-Tube Propulsion with Repetitive-Pulse Laser Ablation

Proof-of-principle experiments of in-tube propulsion in which the propellant was supplied from the launch tube wall were conducted. The launch tube had a 25 mm × 25 mm square inner cross-section. A pair of opposing launch tube walls were made of polyacetal (POM), which was ablated using repetitive Transeversely-Excited Atmospheric (TEA) CO2 laser pulses. The laser beam was sent through a ZnSe window at the tube bottom to the symmetric parabolic surfaces of the projectile, and focused onto the ablator surfaces. The momentum coupling coefficient was measured from a vertical launch trajectory. When the breech of the launch tube was plugged, a momentum coupling coefficient of 4 mN-s/J at least was obtained. Even when the breech was connected to a vacuum chamber at a pressure lower than 20 Pa, the projectile could be vertically launched with a constant momentum coupling coefficient of 0.14 mN-s/J, the value of which was the same order as that obtained by directly irradiating a laser pulse onto a POM ablator.

Debris-Free High-Speed Laser-Assisted Low-Stress Dicing for Multi-Layered MEMS

We have investigated a novel debris-free high speed laser dicing technology for MEMS. The target were a Tempax or Pyrex glass and silicon and their anodically-bonded multi-layered wafers. Our technology combines three processes: dicing guide fabrication, wafer separation process, and a tape expansion. First process was the internal transformation using a nanosecond Nd:YVO4 laser. The laser pulse width and repetition rate was 11 ns and 20∼100 kHz. The laser scanning speed was over 100 mm/sec. The second process was a mechanical bending separation. The bending stress for glass and Si wafer separation was 10 and 100 MPa, respectively. At the third step, spaces between chips were opened by the tape expansion. The internal transformation fabricated in the first process worked well as the guide of the separation. The diced lines completely followed the internal transformation.

Comparison of threshold irradiances and online dosimetry for selective retina treatment (SRT) in patients treated with 200 nanoseconds and 1.7 microseconds laser pulses

Selective retina therapy (SRT) solely affecting the RPE while sparing of the photoreceptors is usually performed with a train of repetitive laser pulses of 1.7 microseconds in duration. It was our purpose to evaluate the principle feasibility of SRT with shorter 200 nanoseconds laser pulses in patients. Nineteen patients with macular disorders [diabetic maculopathy (DMP), geographic atrophy (GA), drusen maculopathy and central serous chorioretinopathy (CSR)] were treated with a prototype of a SRT laser (Nd:YLF laser; 527 nm; 1.7 microseconds and 200 nanoseconds pulse duration; 30 pulses at 100 Hz; spot size: 200 microm). Test lesions (n = 175) with increasing energy were applied at the lower arcade to determine the individual angiographic and ophthalmoscopic threshold radiant exposures (therapeutic window) before applying the central treatment lesions within these ranges additionally guided by online optoacoustic measurements. Postoperatively RPE damage was visualized and confirmed by fluorescein angiographic leakage and correlated with optoacoustic results. Additionally ED(50) damage thresholds were calculated by probit analysis. None of the short repetitive 200 nanoseconds laser pulses led to retinal hemorrhages or retinal ruptures. Nearly all of the test- and treatment lesions could be visualized by angiography indicating desired RPE damage but were ophthalmoscopically invisible suggesting intact neurosensory retinal structures. ED(50) cell damage threshold energies were significantly lower using 200 nanoseconds (99.6 microJ; n = 122) instead of 1.7 microseconds (196.3 microJ; n = 53) laser pulses. Optoacoustic and angiographic visibility correlated in 83.7% (200 nanoseconds) and 87.5% (1.7 microseconds). Selective RPE effects can safely be achieved using shorter 200 nanoseconds laser pulses in patients without adverse effects to the neurosensory retina. The required pulse energy compared to the standard 1.7 microseconds regime was reduced by about a factor of 2 suggesting a reduced heat generation and flow into adjacent tissues during the shorter laser impact and thus possibly enhancing selectivity. Optoacoustics also seem to be a viable alternative in 200 nanoseconds treatment for a non-invasive online dosimetry control system.

Moderate-Acceleration Launch Using Repetitive-Pulse Laser Ablation in a Tube

Projectile launch experiments were conducted using a new laser-driven, in-tube accelerator (LITA) in which the propellant was supplied not as pre-filled gas but from laser ablation. The launch tube had an inner diameter of 25 mm and was 1 m in length, made of acrylic. The 7.1-gram projectile had a parabolic nozzle and an ablator rod made of polyacetal that was located at the focus of the parabola. Laser pulses of 3.1 J to 3.8 were repetitively sent at a frequency of 50 Hz from a Transversely-Excited Atmospheric (TEA) CO2 laser (wavelength; 10.6 μm). Time-modulating the laser-ablative pressure, a large impulse performance was obtained with a moderate acceleration level. A momentum coupling coefficient of 1.5 mNs/J and a gross acceleration of 2.5g were obtained.

The Use of Laser Burst for Volumetric Displaying Inside Transparent Liquid

This experimental paper is about the use of plasma laser breakdown flashes as color voxels inside a vessel containing transparent liquid. A neodymium-doped yttrium aluminum garnet (Nd:YAG) Q-switch laser with a 1064 nm wavelength, 20 ns pulse duration, and 4 mJ pulse energy was used. The type of liquid, laser repetition rate, pulse energy, breakdown bubbles, and plasma spectrum were investigated. Hysteresis was found for the dependence of laser breakdown probability on the laser repetition rate for a fixed laser beam in tap water. The maximal repetition rate was found to be limited to ∼500 Hz for plasma flashing using fixed laser beams in a stationary vessel containing tap water.

Ablative Impulse Characteristics of Polyacetal with Repetitive CO2 Laser Pulses

Impulse characteristics for a polyacetal target repetitively irradiated with pulses from a transversely excited atmospheric CO 2 laser were experimentally investigated. About 10-J laser pulses were repetitively irradiated up to 110 times onto a 6.6- or 8.6-mm-diam spot on the target, which was mounted on a torsion-type impulse balance. In the first several laser pulses, the impulse and ablation rate were strongly influenced by the initial conditions of the target surface. After ten cleaning pulses, 100 pulses were irradiated in various burst modes. Successive laser pulses in a burst were irradiated at a repetition frequency of 50 Hz. The time interval between successive bursts was greater than 3 min. The momentum coupling coefficient C m was almost independent of the burst mode. With a fluence of 18.8 J/cm 2 , C m gradually increased with an increasing total number ofpulses, reaching 220 μN. s/J at an ambient pressure of 10 -2 Pa and 145 μN - ·s/J in the atmosphere. When the fluence was 31.8 J/cm 2 , C m began to decrease after about 50 pulses. C m was smaller for a smaller spot diameter. Those impulse characteristics were closely associated with target surface morphology and fluid dynamics of the ablation plume and the ambient air.

Applications of ions produced by low intensity repetitive laser pulses for implantation into semiconductor materials

This work reports experiment concerning specific applications of implantation of laser-produced ions for production of semiconductor nanocrystals. The investigation was carried out in the IPPLM within the EC STREP ‘SEMINANO’ project. A repetitive pulse laser system of parameters: energy up to 0.8 J in a 3.5 ns-pulse, wavelength of 1.06 μ m, repetition rate of up to 10 Hz, has been employed in these investigations. The characterisation of laser-produced ions was performed with the use of ‘time-of-flight’ ion diagnostics simultaneously with other diagnostic methods in dependence on laser pulse parameters, illumination geometry and target material. The properties of laser-implanted and modified SiO2 layers on sample surface were characterised with the use of different methods (XPS + ASD, Raman spectroscopy, PL spectroscopy) at the Middle East Technological University in Ankara and at the Warsaw University of Technology. The production of the Ge nanocrystallites has been demonstrated for annealed samples prepared in different experimental conditions.

Theoretical Study on the Bending Deformation of Metal Plate Induced by Repetitive Laser Shot Peening

The laser peen forming (LPF) uses high-power pulsed laser beam replacing the tiny medium balls to peen the surface of plate and generate compressive stress near the surface, the metals respond to this residual stresses by elongating at the peened surface and effectively bend the overall shape. In this paper, the deforming process of metal plate induced by repetitive pulsed laser was discussed from theory firstly, and the bending mechanism of LPF was investigated. Then a mathematical model of bending curvature concerning the depth of residual stress was presented, the influence of residual stresses on the deformation of plate was analyzed. Lastly, the line-track-peening experiment for SUS 304 plate was carried out to evaluate the reasonability of the theoretical analysis model.

“Thermal Prism” Method for Measuring Thermophysical Properties of Thin Films

Theoretical principles underlying the photothermal method for measuring the thickness and thermal properties of a thin film located between two optical elements (“sandwich”) are analyzed. The method is based on the irradiation of the assembly by repetitive pulse laser radiation. Radiation is absorbed in the film and causes heating of the optical elements by heat conduction. The element is monitored by a narrow beam of a second low-power laser propagating through the heated region. The beam is deflected due to the spatial variation of the refractive index, and the magnitude of the deflection angle as a function of time contains the relaxation and “wave” components. It is shown that the phase of the “wave” component depends on the thickness and thermophysical properties of the film. The thermophysical properties of the film can be determined, provided that the analogous properties of the optical element are measured previously or otherwise known, by comparing experimentally measured values of the phase shift with theoretical values obtained from the analytical solution of the non-stationary two-dimensional heat conduction equation.

Assessment of the precision and accuracy of thorium (232Th) and uranium (238U) measured by quadrupole based inductively coupled plasma-mass spectrometry using liquid nebulization, nanosecond and femtosecond laser ablation

The precision and accuracy of the 238U/232Th ratio were evaluated from liquid nebulization and direct solid sampling repetitive pulsed laser ablation. Nanosecond and femtosecond pulsed lasers at 266 nm wavelength were utilized for the ablation studies. The ICP-MS and sampling parameters were optimized for each procedure; flow rates, gases, laser energy and other parameters were optimized for the particular sampling approach and therefore will not be the same. The work is not a comparison per se but represents performance metrics for three optimized sampling modalities. As expected, nanosecond pulsed ablation provided the greatest inaccuracy (>30%) from the nominal 238U/232Th bulk ratio. This deviation from bulk ratio is attributed to incomplete vaporization of large particle agglomerates produced by nanosecond laser ablation. Femtosecond pulsed ablation provided inaccuracy (∼1–3%) approaching that of liquid nebulization (∼1%). In terms of temporal relative standard deviation (TRSD) and relative standard deviation (RSD), liquid nebulization provided the best precision for the 238U/232Th ratio (TRSD ∼3–5%, RSD ∼0.2–0.6%), femtosecond laser ablation (TRSD ∼5–12%, RSD ∼1%) and nanosecond laser ablation (TRSD ∼25–48%, RSD ∼9–12%). Laser ablation requires less sample to achieve these performance metrics, in some cases less than a factor of 100-times depending on the entrainment and transport efficiency.

Analysis and Optical Design of 1:1 Projection Lithography System

For the 1:1 laser projection lithography system used to achieve large-area patterning with higher resolutions as well as higher throughput, the key parameters such as the laser beam geometry, the numerical aperture of projection lens, the laser source power and the pulse repetition rate are theoretically analyzed. It is expounded the process of uniform exposure in hexagonal beam shape, the advantages and limitations of 1:1 projection owing to numerical apertures deciding the resolution, as well as the cause of choosing larger laser power and pulse repetition rate. Meanwhile, the projection lens for a unit-magnification, refractive imaging system is tentatively simulationdesigned using ZEMAX optical design software. The optimized three-dimensional layout is plotted. For the designed results, the maximum optical path difference is smaller thanλ /4 within entire visual field. The resolution for feature sizes 10μm can be achieved within depth of focus 400μm by evaluating MTF. The maximum field curvature is within 10μm and the maximum distortion is small than 0.000007%. This fulfills the demands in technical specifications.

High-precision wavelength calibration of astronomical spectrographs with laser frequency combs

We describe a possible new technique for precise wavelength calibration of high-resolution astronomical spectrographs using femtosecond-pulsed mode-locked lasers controlled by stable oscillators such as atomic clocks. Such ‘frequency combs’ provide a series of narrow modes which are uniformly spaced according to the laser's pulse repetition rate and whose absolute frequencies are known a priori with relative precision better than 10−12. Simulations of frequency comb spectra show that the photon-limited wavelength calibration precision achievable with existing echelle spectrographs should be ∼1 cm s−1 when integrated over a 4000 Å range. Moreover, comb spectra may be used to accurately characterize distortions of the wavelength scale introduced by the spectrograph and detector system. The simulations show that frequency combs with pulse repetition rates of 5–30 GHz are required, given the typical resolving power of existing and possible future echelle spectrographs. Achieving such high repetition rates, together with the desire to produce all comb modes with uniform intensity over the entire optical range, represents the only significant challenges in the design of a practical system. Frequency comb systems may remove wavelength calibration uncertainties from all practical spectroscopic experiments, even those combining data from different telescopes over many decades.

Interaction of high-power laser pulses with monodisperse gold particles

The interaction of repetitive nanosecond laser pulses with near-spherical gold nanoparticles, dispersed in an aqueous ambient and having various mean sizes, was studied for two different wavelengths 355 and 532 nm and various energies per pulse. From the in situ surface plasmon (SP), extinction measurements combined with ex situ TEM studies of the irradiated hydrosols, we conclude that the process of laser-induced fragmentation is cumulative with increasing number of subsequent pulses. Interestingly, irradiation of all parent hydrosols having different mean particle sizes (except of that one containing nanoparticles with the mean diameter of 5 nm) with laser pulses of the 532 nm wavelength and the same energy per pulse absorbed in the hydrosol resulted into hydrosols containing nanoparticles of very similar reduced mean sizes and size distributions. On the other hand, for the parent nanoparticle hydrosol with the initial mean particle diameter below 5 nm, a small increase of the mean particle size has been detected after irradiation with pulses of the same wavelength. The mean sizes of Au nanoparticles fragmented by 355 nm pulses were generally smaller than those fragmented at 532 nm.

Selective Targeting of the Retinal Pigment Epithelium in Rabbit Eyes with a Scanning Laser Beam

Selective targeting of the retinal pigment epithelium (RPE) with repetitive laser pulses that minimize thermal damage to the adjacent photoreceptors is a promising new therapeutic modality for RPE-related retinal diseases. The selectivity of an alternative, more versatile scanning approach was examined in vivo by using a broad range of scanning parameters. Acousto-optic deflectors repeatedly scanned the focus of a continuous wave (cw)-laser across the retina of Dutch belted rabbits, producing microsecond irradiation at each RPE cell. Two irradiation patterns forming separated lines (SEP) or interlaced lines (INT), different dwell times (2.5-75 micros), and repetition numbers (10 and 100 scans with 100-Hz repetition rate) were tested. Thresholds were evaluated by fundus imaging and angiography. Histology was performed for selected parameters. Selective RPE cell damage was obtained with moderate laser power. The angiographic threshold power decreased with pulse duration, number of exposures, and applying the INT pattern. Ophthalmoscopic thresholds, indicating onset of thermal coagulation, were higher than twice the angiographic threshold for most tested parameters. Histology confirmed selective RPE cell damage for SEP irradiation with 7.5 and 15 micros; slower scan speeds or closed lines caused photoreceptor damage. A cw-laser scanner can be set up as a highly compact and versatile device. Selective RPE damage is feasible with dwell times up to 15 micros. Greatest selectivity is achieved with short exposure times and separated scan lines. Interlaced lines and long exposure times facilitate heat conduction into photoreceptors. A scanner is an attractive alternative for pulsed selective targeting, because both selective targeting and thermal photocoagulation can be realized.

Control of tissue mechanics upon the repetitive-pulse laser heating of cartilage

The viscoelastic properties of laser-irradiated cartilages are studied with optoacoustic methods upon the thermal excitation of mechanical oscillations by repetitive-pulse laser radiation. The effect of laser power, pulse duration, repetition rate, and irradiation time on the shape of the optoacoustic signal is analyzed. It is demonstrated that the optoacoustic response of the cartilage to the repetitive-pulse radiation of a fiber laser depends on the softening of the tissue upon the variation in its shape. Under repetitive-pulse laser irradiation, the optoacoustic response of the cartilage depends on the mechanical characteristics of the biotissue (elastic modulus, hydraulic permeability, and thickness). A simple model that makes it possible to estimate the contribution of the viscoelastic properties to the formation of the optoacoustic response at various laser repetition rates is proposed.

Laser Fabrication of Low Resistivity Electrode on Glass

A method for low resistivity resistor on transparent substrate with laser induced plasma assisted by ablation and laser annealing was described. The diffusion distribution for metallization was measured with XPS. The surface profile was analysed with SEM. The minimum resistivity we got was 0.1 /Sq. It demonstrates that the electronic thin film obtained has a very good conductivity. The most important factors for forming electrically conducting films on the transparent substrate are laser fluence, beam scanning and pulse repetition rate. This technology will be useful in the fabrication of LCD electrode, glass defused resistor and circuit on glass.

Laser repetitive pulse heating influence of pulse duty on temperature rise

Laser repetitive pulse heating of the solid surface is considered and influence of duty cycle in the repetitive pulses on the temperature rise and temperature difference at the surface is examined. Laser power distribution with Gaussian profile is assumed and 3D heating model is accommodated in the heating analysis. A numerical method is employed to solve governing equations of heat transfer. It is found that the rate of surface temperature rise is high and temperature difference is large for high duty cycle during the consecutive pulse irradiation. This in turn enhances the thermal cyclic loading of the heated surface.