Laser Repetition Research Articles

Research on a different method to reach the saturate limit of titanium aluminide alloy surface mechanical and fatigue properties by laser shock process

Laser shock peening is one of the best method to enhance the mechanical and fatigue properties of titanium aluminide alloys. Most research depends on the pressure model produced by Professor Fabrro, based on Von-Mise stress yield standard, when the pressure is 2 Hugoniot elastic limit (HEL), the plastic deformation reaches the maximum limit and when the pressure is 2.5HEL The stress release from the edge of laser shock area, This research discussed the titanium aluminide alloys surface mechanical and fatigue properties after reducing the pressure of laser shock and inducing the repetitions of laser shock properly. When the pressure reached HEL the material began plastic deformation, in the experiment environment there are several factors affect the results, so the pressure should be more than 1HEL and less than 2HEL-2.5HEL. As the same repetitive times the effective of 2HEL-2.5HEL pressure is better than the less pressure but with the repetitive times rise the 2HEL-2.5HEL pressure specimens would be cracked and the less pressure specimens mechanical and fatigue properties still increase. The results show that with inducing the repetitions and reducing the pressure the material surface hardness improved nearly 25% and the fatigue life increase nearly 40% in high fatigue cycle, the microstructure is similar but the less pressure more repetitive times specimens contains dislocation cells. It indicates that with 2HEL-2.5HEL pressure (the maximum plastic deformation) the mechanical and fatigue properties reached to its peak after 3 times LSP and with 1.5 HEL pressure 8 times repetitive laser shock can reach to the saturate limit of the material itself, that means with less pressure and more repetitive laser shock is more accurately to get close to the saturate limit of the material.

Cortical Pain Processing in the Rat Anterior Cingulate Cortex and Primary Somatosensory Cortex

Pain is a complex multidimensional experience encompassing sensory-discriminative, affective-motivational and cognitive-emotional components mediated by different neural mechanisms. Investigations of neurophysiological signals from simultaneous recordings of two or more cortical circuits may reveal important circuit mechanisms on cortical pain processing. The anterior cingulate cortex (ACC) and primary somatosensory cortex (S1) represent two most important cortical circuits related to sensory and affective processing of pain. Here, we recorded in vivo extracellular activity of the ACC and S1 simultaneously from male adult Sprague-Dale rats (n = 5), while repetitive noxious laser stimulations were delivered to animalÕs hindpaw during pain experiments. We identified spontaneous pain-like events based on stereotyped pain behaviors in rats. We further conducted systematic analyses of spike and local field potential (LFP) recordings from both ACC and S1 during evoked and spontaneous pain episodes. From LFP recordings, we found stronger phase-amplitude coupling (theta phase vs. gamma amplitude) in the S1 than the ACC (n = 10 sessions), in both evoked (p = 0.058) and spontaneous pain-like behaviors (p = 0.017, paired signed rank test). In addition, pain-modulated ACC and S1 neuronal firing correlated with the amplitude of stimulus-induced event-related potentials (ERPs) during evoked pain episodes. We further designed statistical and machine learning methods to detect pain signals by integrating ACC and S1 ensemble spikes and LFPs. Together, these results reveal differential coding roles between the ACC and S1 in cortical pain processing, as well as point to distinct neural mechanisms between evoked and putative spontaneous pain at both LFP and cellular levels.

Synthesis of convection velocity and turbulence measurements in three-stream jets

Key flow regions linked to jet noise sources are investigated through comparison of convection velocity and turbulence measurements in high speed three-stream nozzles. A time-resolved Doppler global velocimetry (DGV) instrument was applied in the Nozzle Acoustic Test Rig (NATR) at NASA's Aero-Acoustic Propulsion Lab to measure 250 kHz repetition laser scattering signals arising from seeding particles at 32 points in high speed flow. Particle image velocimetry (PIV) measurements previously reported by NASA provided mean velocity and turbulence intensity. Results for convection velocity of the particle concentration field were obtained from the DGV data for three-stream nozzle configurations using a cross-correlation approach. The three-stream cases included an axisymmetric and an asymmetric, offset nozzle configuration. For the axisymmetric case, areas of high convection velocity and turbulence intensity were found to occur 4 to 6 area-equivalent nozzle diameters downstream from the nozzle exit. Comparison of convection velocity between the axisymmetric and offset cases show this same region as having the greatest reduction in convection velocity due to the offset, up to 20% reduction for the offset case compared to the axisymmetric case. These findings suggest this region along the centerline near the end of the potential core is an important area for noise generation with jets and contributes to the noise reductions seen from three-stream offset nozzles.

Nm-sized cryogenic hydrogen clusters for a laser-driven proton source.

A cryogenic hydrogen cluster-jet target is described which has been used for laser-plasma interaction studies. Major advantages of the cluster-jet are, on the one hand, the compatibility to pulsed high repetition lasers as the target is operated continuously and, on the other hand, the absence of target debris. The cluster-jet target was characterized using the Mie-scattering technique allowing to determine the cluster size and to compare the measurements with an empirical formula. In addition, an estimation of the cluster beam density was performed. The system was implemented at the high power laser system ARCTURUS, and the measurements show the acceleration of protons after irradiation of the cluster target by high intensity laser pulses with a repetition rate of 5 Hz.

Detuning dependent Rabi oscillations of a single molecule

A single organic dye molecule at cryogenic conditions is resonantly excited in a confocal microscope. Under strong laser illumination it undergoes Rabi oscillations. Mathematically, this was well described and had been experimentally implemented. These oscillations can be measured as side-bands on their resonance fluorescence, e.g. in the Mollow-Triplet. An alternative method is to research this effect by an analysis of the single molecule anti-bunched photon statistics. This has been performed in this work. Here we research on the detuning dependence of this signal—it is experimentally demanding since the utilized laser might drift or single emitters are not necessarily spectrally stable enough, such that the spectrum can be measured indefinitely. We therefore apply a measurement technique in which the photon correlation signal is acquired in detuning dependent steps. This is performed by continuous laser sweeps over the single molecule excitation spectrum. A single recording of the anti-bunched photons takes 20–50 ms. After approx. 1 h of repetitive laser detunings a full anti-bunching curve is reconstructed for each spectral position. An alternative technique with 100 ns laser pulses allows us to acquire a set of comparable data. Our study is derived from a single dibenzanthanthrene molecule with a natural linewidth of 2π×16 MHz. It emits under resonant excitation more than 380.000 photons per second. Under spectral detuning, Rabi-oscillations are observed up to ΩRabi = 2π×160 MHz.

Quality improvement and mode evolution of high-Q lithium niobate micro-disk induced by “light annealing”

High-quality factor microresonators are a key component in photonic integrated circuits. However, it is more difficult to precisely engineer a single component after fabrication as integration density gets higher. In this work, we improve the quality factor of the fabricated resonator by femtosecond laser shots. The high repetition laser pulses scatter into the cavity, and then the localized high-density light field introduces a light refining and “annealing” process that may restore the lattice disorders. The intrinsic quality factor of a measured mode can be promoted from 2.17×10 5 to 1.84×10 6. Moreover, increasing the shot power may kill the high order modes, and only the fundamental mode survives. This method may inspire new potential application in femtosecond laser modification in photonic integrated circuits.

Predictive model and parametric analysis of laser marking process on gallium nitride material using diode pumped Nd:YAG laser

Laser marking is one of the effective and efficient ways of permanent marking process in the different engineering materials like composites, ceramics and any other hard metals and alloys. The present investigation deals with the experimental analysis on laser marking process using diode pumped Nd:YAG laser considering several process parameters for example lamp current, pulse repetition rate and laser scanning speed. The Response Surface Methodology (RSM) based experimental design was adopted to study in-depth the influence of these parameters on marking performance criteria such as mark width, mark depth and mark intensity. Based on experimental results, the second order mathematical models were established. Multi performance optimized parametric combination was achieved to obtain high quality marking using developed models and desirability functions. Moreover, in order to determine the most influencing parameter on the responses, Sensitivity Analysis (SA) was performed on the test results obtained from experimentation as well as predictive models. SEM and optical microscopic images were also analyzed to examine the quality aspects of laser marking on gallium nitride.

Quantitative photochemical scaling model for femtosecond laser micromachining of ophthalmic hydrogel polymers: effect of repetition rate and laser power in the four photon absorption limit

We report on the effect of laser repetition rate on the refractive index (RI) change or phase change induced by the femtosecond micromachining hydrogel-based ophthalmic materials. A repetition rate tunable femtosecond ytterbium (Yb) doped fiber laser was used to study the relation between the laser-induced phase change and the pulse repetition rate in the range of 1 MHz to 60 MHz. We present both the qualitative and quantitative results of the laser-induced phase change obtained in hydrogel-based contact lenses at different repetition rates and discuss the effect of repetition rate on the magnitude of the phase shift, the optical damage threshold and the maximum achievable phase change just below the optically-induced damage threshold. A photochemical model derived in the four photon absorption limit with pulse overlapping is employed to fit the experimental data obtained at four different repetition rates, 5 MHz, 10 MHz, 15 MHz and 60 MHz. This work contributes to the current knowledge of the response of hydrogel polymers to various laser irradiation parameters and the optimization of laser repetition rates enables femtosecond micromachining of ophthalmic materials at a lower average power and a faster writing speed.

Effect of process parameters on the formation of single track in pulsed laser powder bed fusion

There has been increasing interest in Laser Powder Bed Fusion (L-PBF) of metallic materials as a promising manufacturing technology. Although most L-PBF systems utilize laser beams with continuous wave emission (L-PBF(CW)), the possibility of using pulsed lasers (L-PBF(P)) has become available in some industrial L-PBF machines over the past few years. Previous studies suggest that the use of pulsed lasers could enable larger control of heat input and melt pool formation during the process, and could thus enable improvement of spatial resolution and feature sizes in L-PBF. In this study, the experiments were implemented using a pulsed laser in combination with continuous scanning movement instead of the ‘point-and-shoot’ method typically used by industrial L-PBF(P) machines of today. The experiments were executed using a trial L-PBF system (IPG ytterbium fiber laser, wavelength 1075 nm) for gas-atomized stainless steel 316L powder on compositionally similar substrates. Single tracks were melted with three different pulse lengths (50, 100, and 200 µs) by using a constant layer thickness of 50 µm, while varying pulse repetition rate, scanning speed and laser power based on six preset values of volume energy density (VED) of 36-120 J/mm3. In order to allow a comparison to be made, additional samples were manufactured by using the CW emission of the same laser. It was observed that the L-PBF(P) samples yielded narrower tracks in comparison to the samples manufactured using CW emission. In addition, the results of the experiments show that, while maintaining constant VED values, decreasing the pulse length or scanning speed decreased the widths of the tracks and their penetration into the substrate. Consequently, it was noticed that shorter pulse lengths require more overlap between consecutive pulses in order to produce continuous tracks. Pulsed emission shows potential for improving the spatial resolution of the L-PBF process.

Numerical Modeling of 193-nm Excimer Laser Ablation on CR-39 Polymer

Laser polymer processing has evolved into an important field of applied and fundamental research. Allyl diglycol carbonate or CR-39 is considered an important polymer for use in optical and medical applications since it is a hard and infusible thermoset plastic that is insoluble in all solvents. The temperature distribution of CR-39 when subjected to irradiation during 193-nm excimer laser ablation was investigated. An effective simulation was developed to reflect the effects of polymer thermal diffusion, laser fluence, beam geometry, and number of pulses. A two-dimensional finite element model was used to predict the temperature distribution and ablation depth in the plastic CR-39 polymer under the 193-nm laser ablation process. The photochemical threshold ablation fluence was 750 mJ/cm2 for a single laser pulse, but dropped to 240 mJ/cm2 for repetitive laser pulses. At higher fluence, thermal effects exhibited a slight contribution to the ablation process. For fluence ≥ 750 mJ/cm2, the ablation depth increased abruptly. It can thus be inferred that the rate of temperature increase with pulse number increased concomitantly with the fluence. Thus, thermal effects are a significant factor in the ablation process.

Biomimetic surface structures in steel fabricated with femtosecond laser pulses: influence of laser rescanning on morphology and wettability.

The replication of complex structures found in nature represents an enormous challenge even for advanced fabrication techniques, such as laser processing. For certain applications, not only the surface topography needs to be mimicked, but often also a specific function of the structure. An alternative approach to laser direct writing of complex structures is the generation of laser-induced periodic surface structures (LIPSS), which is based on directed self-organization of the material and allows fabrication of specific micro- and nanostructures over extended areas. In this work, we exploit this approach to fabricate complex biomimetic structures on the surface of steel 1.7131 formed upon irradiation with high repetition rate femtosecond laser pulses. In particular, the fabricated structures show similarities to the skin of certain reptiles and integument of insects. Different irradiation parameters are investigated to produce the desired structures, including laser repetition rate and laser fluence, paying special attention to the influence of the number of times the same area is rescanned with the laser. The latter parameter is identified to be crucial for controlling the morphology and size of specific structures. As an example for the functionality of the structures, we have chosen the surface wettability and studied its dependence on the laser processing parameters. Contact angle measurements of water drops placed on the surface reveal that a wide range of angles can be accessed by selecting the appropriate irradiation parameters, highlighting also here the prominent role of the number of scans.

Characteristics of UV-induced bulk damage in large-aperture fused silica from full-sized beam tests

Laser-induced bulk damage in fused silica is studied on a large-aperture laser system with 6 J/cm2, 5-ns shot sequences at 351 nm. The lateral dimension of bulk damage is found to propagate exponentially with shot number in a wide range of growth rates, similar to the stochastic trend as displayed by rear-surface damage. Resemblance in the morphology between bulk and surface damage is revealed offline by microscopic observations. Arising from the accumulated effect of fluence variation, the repetitive laser exposures tend to create an increased number of bulk damage, the tendency of which is applied to evaluate the number of damage initiations via simulations.

A 250 m Direct Time-of-Flight Ranging System Based on a Synthesis of Sub-Ranging Images and a Vertical Avalanche Photo-Diodes (VAPD) CMOS Image Sensor

We have developed a direct time-of-flight (TOF) 250 m ranging Complementary Metal Oxide Semiconductor (CMOS) image sensor (CIS) based on a 688 × 384 pixels array of vertical avalanche photodiodes (VAPD). Each pixel of the CIS comprises VAPD with a standard four transistor pixel circuit equipped with an analogue capacitor to accumulate or count avalanche pulses. High power near infrared (NIR) short (<50 ns) and repetitive (6 kHz) laser pulses are illuminated through a diffusing optics. By globally gating the VAPD, each pulse is counted in the in-pixel counter enabling extraction of sub-photon level signal. Depth map imaging with a 10 cm lateral resolution is realized from 1 m to 250 m range by synthesizing subranges images of photon counts. Advantages and limitation of an in-pixel circuit are described. The developed CIS is expected to supersede insufficient resolution of the conventional light detection and ranging (LiDAR) systems and the short range of indirect CIS TOF.

A numerical approach for a comparative study of laser drilling process under single and repetitive pulse

AbstractAn axisymmetric model is developed to study laser drilling process under a single pulse as well as repetitive laser pulse. The laser pulse irradiated on the surface of the workpiece is volumetric and Gaussian in nature. The laser irradiated surface is subjected to convective‐radiative boundary condition while rest of the surfaces are insulated. Finite volume method is used to discretize the domain under consideration. The resulting algebraic equations are solved with the help of the tridiagonal matrix algorithm to find temperature distribution throughout the domain. The enthalpy‐porosity method is used to track the solid‐liquid interface generated during the laser melting process. Convective heat transfer occurs inside the generated melt pool. The current model is first used to validate the results of the existing literature and as the results agreed well, further studies are made to find out the advantages of using repetitive laser pulse over single pulse laser source for laser drilling process for the same laser energy and total heating duration. Vaporization has been avoided during the process and metal removal occurs through melting only. The present numerical model can provide some insight for practical laser drilling process.

Selective drug-free cancer apoptosis by three-dimensional self-targeting magnetic nickel oxide nanomatrix.

To develop a drug-free strategy addressing limitations of current cancer therapy. A 3D self-assembled magnetic nickel oxide (NiO) nanomatrix is synthesized using femtosecond pulsed laser to mimic extracellular matrix. The tunable laser pulse-interaction time and repetition rate aided in generating programmable NiO nanomatrix chemistry. The nanomatrix mimicked extracellular matrix in physical configuration and properties presenting favorable cues to cancerous HeLa cell and fibroblast cell adhesion and proliferation without cytotoxicity. The 3D nanomatrix structure altered HeLa cell behavior and induced apoptosis cancer apoptosis with an evidence of increased endocytosis when compared with fibroblast cells. The results demonstrate the availability of new potential avenues for magnetic drug-free cancer therapeutics.

Momentum predictability and heat accumulation in laser-based space debris removal

Small space debris objects of even a few centimeters can cause severe damage to satellites. Powerful lasers are often proposed for pushing small debris by laser-ablative recoil toward an orbit where atmospheric burn-up yields their remediation. We analyze whether laser-ablative momentum generation is safe and reliable concerning predictability of momentum and accumulation of heat at the target. With hydrodynamic simulations on laser ablation of aluminum as the prevalent debris material, we study laser parameter dependencies of thermomechanical coupling. The results serve as configuration for raytracing-based Monte Carlo simulations on imparted momentum and heat of randomly shaped fragments within a Gaussian laser spot. Orbit modification and heating are analyzed exemplarily under repetitive laser irradiation. Short wavelengths are advantageous, yielding momentum coupling up to ∼40 mNs / kJ, and thermal coupling can be minimized to 7% of the pulse energy using short-laser pulses. Random target orientation yields a momentum uncertainty of 86% and the thrust angle exhibits 40% scatter around 45 deg. Moreover, laser pointing errors at least redouble the uncertainty in momentum prediction. Due to heat accumulation of a few Kelvin per pulse, their number is restricted to allow for intermediate cooldown. Momentum scatter requires a sound collision analysis for conceivable trajectory modifications.

Launching swarms of microsatellites using a 100 kW average power pulsed laser

Transferring small payloads from Earth to low Earth orbit (LEO) and to interplanetary space using laser ablation propulsion is a new application of pulsed laser ablation propulsion, which offers parameters not available with chemical propulsion. In earlier work, we showed how to use a 1 MW average power pulsed laser to launch a 25 kg satellite from LEO to a Mars Hohmann transfer orbit in 18 minutes. Here, we discuss a lower-power application that can achieve the same result, and also cheaply and rapidly launch swarms of microsatellites into geosynchronous orbit (GEO) for communication, Earth observation, and observation of assets in GEO. The result will be a network of satellites with different functions that is robust to failures. Laser propulsion offers orbit/launch mass ratio m/M impossible with chemical rockets as well as low launch cost/kg. The main purpose in this paper is to illustrate satellite launch with repetitive pulse lasers at realistic laser average power levels. The benchmark laser has 80 ps pulsewidth, 5 kJ pulse energy, 355 nm wavelength (third Nd harmonic), and 20 Hz pulse repetition rate, using the “L’ADROIT” laser station design. To minimize laser power, we employ eight loops of steadily increasing apogee, propelled at perigee by a L’ADROIT laser in LEO, followed by a circularization “burn” at apogee using a second L’ADROIT in GEO with similar laser power. We estimate the final mf/M ratio for these maneuvers to be 43%–53%, delivering 10.8 kg into a Mars Hofmann transfer orbit and 13.2 kg to GEO. The ablated mass is contained in a spherical shell surrounding the satellite payload composed of a mixture of metal powder and polyoxymethylene. The mixture is tailored to the mission to give a matched specific impulse Isp and momentum coupling coefficient Cm. For the GEO insertion, the ablation shell splits and is re-entered after one orbit by a final “burn” from the GEO laser.

Novel scintillator-based x-ray spectrometer for use on high repetition laser plasma interaction experiments.

The characterisation of x-rays from laser-plasma interactions is of utmost importance as they can be useful for both monitoring electron dynamics and also applications in an industrial capacity. A novel versatile scintillator x-ray spectrometer diagnostic that is capable of single shot measurements of x-rays produced from laser-plasma interactions is presented here. Examples of the design and extraction of the temperature of the spectrum of x-rays produced in an intense laser-solid interaction (479 ± 39 keV) and the critical energy from a betatron source (30 ± 10 keV) are discussed. Finally, a simple optimisation process involving adjusting the scintillator thickness for a particular range of input spectra is demonstrated.

High-contrast high-intensity repetitive petawatt laser.

We report the generation of 63J of broadband pulse energies at 0.1Hz from the J-KAREN-P laser, which is based on an OPCPA/Ti:sapphire hybrid architecture. Pulse compression down to 30fs indicates a peak power of over 1 PW. High temporal contrast of 1012 prior to the main pulse has been demonstrated with 10J output energy. High intensities of 1022 W/cm2 on target by focusing a 0.3 PW laser with an f/1.3 off-axis parabolic mirror have been achieved. Fundamental processes of laser matter interaction at over 1022 W/cm2 intensities belong to a new branch of science that will be the principal research task of our infrastructure.

Parametric Analysis of Pulsed Laser Melting Process

An axisymmetric numerical model has been developed to study the laser drilling of a cylindrical work piece under repetitive Gaussian laser pulse. Finite volume method has been used to discretize the energy equation. Convective and radiative heat losses have been considered from the laser irradiated surface. The resulting algebraic equation has been solved with the help of Tri-Diagonal Matrix Algorithm (TDMA) to know the temperature distribution in the work piece. The enthalpy porosity method has been employed to capture the moving solid-liquid interface evolving due to melting of the work piece subjected to laser energy. The laser source irradiated on the work piece has been considered as volumetric laser source instead of surface laser source which is rarely found in the existing literature. The present model is validated first with the existing literature. It has been found that, the results agree well. A parametric analysis has been done to know the effects of different laser parameters such as duty cycle, energy, pulse width and frequency etc. on the temperature field and the size of the melt pool when the work piece is subjected to repetitive laser pulse. The numerical results can provide some guidance for practical laser drilling process.