Top-hat Beam Profile Research Articles

Absolute Quantification of Glutathione Using Top-Hat Optics for IR-MALDESI Mass Spectrometry Imaging.

Infrared matrix-assisted laser desorption electrospray ionization (IR-MALDESI) uses an infrared laser to desorb neutral biomolecules with postionization via ESI at atmospheric pressure. The Gaussian profile of the laser with conventional optics results in the heating of adjacent nonablated tissue due to the energy profile being circular. A diffractive optical element (DOE) was incorporated into the optical train to correct for this disadvantage. The DOE produces a top-hat beam profile and square ablation spots, which have uniform energy distributions. Although beneficial to mass spectrometry imaging (MSI), it is unknown how the DOE affects the ability to perform quantitative MSI (qMSI). In this work, we evaluate the performance of the DOE optical train against our conventional optics to define the potential advantages of the top-hat beam profile. Absolute quantification of glutathione (GSH) was achieved by normalizing the analyte of interest to homoglutathione (hGSH), spotting a dilution series of stable isotope labeled glutathione (SIL-GSH), and analyzing by IR-MALDESI MSI with either the conventional optical train or with the DOE incorporated. Statistical comparison indicates that there was no significant difference between the quantification of GSH by the two optical trains as evidenced by similar calibration curves. Results support that both optical trains can be used for qMSI without a change in the ability to carry out absolute quantification but providing the benefits of the top-hat optical train (i.e., flat energy profile and square ablation spots)-for future qMSI studies.

Development and Application of Prototype System Based on Light-Emitting Diode Arrays (660 nm) with a Top Hat Beam Profile in Order to Optimize Photobiomodulation Protocols for Treatment of Radiation-Induced Oral Mucositis in Rats.

Background: Oral mucositis (OM) is a common adverse effect of radiation to the head and neck. Recent research has shown that extra oral photobiomodulation (EO-PBM) reduces the severity of OM. However, appropriate EO-PBM therapy parameters for OM severity reduction have not been documented. Objective: This work aims to optimize EO-PBM radiation parameters for lowering the severity of radiation-induced OM in rats by establishing a photobiomodulation (PBM) treatment system based on light-emitting diode arrays with top-hat beam profile. Methods: The 36 rats are separated into 2 control groups and 4 groups receiving PBM treatment. The PBM groups are exposed to irradiance between 4 and 24 J/cm2 at 660 nm. The cheek pouch mucosa is removed after scarification for biochemical and histological examination. Student's t-test, and one-way analysis of variance (ANOVA) followed by Tukey's Multiple were applied to compare the statistical significance of differences between control groups and PBM treatment groups. Results: Statistical analysis reveals that PBM irradiation at 12 J/cm2 (200 sec) with a flatness of 0.8 and a diameter of 3 cm substantially decreased the level of inflammatory cytokines compared with the positive control group. Conclusions: Our results indicate that the designed treatment PBM system is capable of delivering the optical parameters necessary for therapeutic treatment.

Next steps in high-repetition-rate laser development for Thomson scattering

Design of a next generation high-rep-rate laser system is underway, aiming for a maximum rep rate of 100 kHz for 1 ms. This will be a “pulse-burst” laser, which is a type of heat-capacity laser. Heat-capacity laser operation is characterized by a burst of pulses of limited duration, with burst length ≤100 ms and pulse rep rate ≥1 kHz. Waste heat accumulates in the laser rod during the burst. This heat is deposited evenly throughout the rod volume, with very little heat removed during the burst, such that temperature rises evenly across the rod radius. Thus beam distortion due to thermal gradients is small. Heat is removed from the rodafter the burst, with typically tens of seconds between bursts. Pulse-burst operation of flashlamp pumped Nd:YAG lasers is a cost-effective route to high-rep-rate capability. Pulse-burst laser systems with “fast burst” rep rates in the range of 10–20 kHz have been built for the Thomson scattering diagnostics on MST, NSTX-U, and LHD. For Thomson scattering diagnostic application, the typical requirements are 1064 nm, 1–2 J/pulse, ≤30 ns FWHM pulse, with a top-hat beam profile. A major requirement for this next generation laser system is flexibility in burst sequence programs, ranging from 1 kHz for 100 ms to 100 kHz for 1 ms, and a variety of scenarios in between so that operation can be tailored to plasma experiment requirements. Flashlamp pumping will be used for this next generation laser because it is inexpensive and flexible. A new switch-regulated flashlamp driver will provide improved flashlamp control at lower cost. Additional design issues such as the optimum flashlamp pumping spectrum and optimum Nd doping will be addressed. The use of commercial off-the-shelf components will be maximized in this laser system so that pulse-burst systems can be developed and built by others for new applications.

Enhanced surface effects and optical property modulation of Ge2Sb2Te5 by pulsed laser irradiation

In this work, Ge2Sb2Te5 (GST) thin films are irradiated by a 1064 nm pulsed laser heat treatment system with different beam profiles. The surface effects induced by different laser conditions are studied systematically by atomic force microscope, spectroscopic ellipsometry, and Raman spectroscopy. It is found that a top-hat beam profile with uniform intensity distribution demonstrates the advantages of a non-destructive and homogeneous surface, which is critical for large-scale processing uniformity. The threshold laser fluence for the amorphization process is predicted by simulation and further proved by the laser irradiation experiment to be 27.9 mJ/cm2 at 1 ns pulse width. We further show that modulation of complex refractive indices of GST thin films can be achieved with different duty ratios (spatial ratio of amorphization part) from 0% to 100%. Our approach paves the way for the precise control of the optical properties of PCMs in emerging optical applications such as photonic switches, optical memories, and all-optical neural networks.

More than Ninety Percent of the Light Energy Emitted by Near-Infrared Laser Therapy Devices Used to Treat Musculoskeletal Disorders Is Absorbed within the First Ten Millimeters of Biological Tissue.

There is increasing interest in the application of near-infrared (NIR) laser light for the treatment of various musculoskeletal disorders. The present study thoroughly examined the physical characteristics of laser beams from two different laser therapy devices that are commercially available for the treatment of musculoskeletal disorders. Then, these laser beams were used to measure the penetration depth in various biological tissues from different animal species. The key result of the present study was the finding that for all investigated tissues, most of the initial light energy was lost in the first one to two millimeters, more than 90% of the light energy was absorbed within the first ten millimeters, and there was hardly any light energy left after 15-20 mm of tissue. Furthermore, the investigated laser therapy devices fundamentally differed in several laser beam parameters that can have an influence on how light is transmitted through tissue. Overall, the present study showed that a laser therapy device that is supposed to reach deep layers of tissue for treatments of musculoskeletal disorders should operate with a wavelength between 800 nm and 905 nm, a top-hat beam profile, and it should emit very short pulses with a large peak power.

Study of Laser Backside Ohmic Contact Formation of SiC-Ni Interface to Evaluate the Process Influence on the Electrical Characteristics

Within this paper, we will present the results of a study on the ohmic contact formation process with nanosecond (ns) pulsed UV lasers. For the study we compared two laser processes: The base line process with a 100-300 ns pulsed laser with Gaussian beam profile and the 3D-Micromac AG process with a 50-100 ns pulsed laser with top hat beam profile. The forward voltage characteristics at wafer level was analyzed and proves a clear benefit of the top hat laser process. Besides, the forward voltage characteristics of a second run was performed to analyze the influence of increasing energy density to the electrical characteristic of heat sensitive front side structures. Also with high energy density no negative influence could be detected.

Experimental Study on Laser-Induced Surface Damage of a Single-Crystal Nickel-Based Superalloy Under Continuous Wave Fiber Laser Scanning Process

Abstract Single-crystal (SC) nickel-based superalloy castings offer high-temperature mechanical properties that result in superior gas turbine engine performance and durability. These castings undergo various precision machining operations to remove a significant amount of material while manufacturing. Here, nickel-based superalloys are one of the most difficult materials to be cut. Therefore, novel concepts are being employed to improve their machinability including lowering their surface strength. This paper presents the introduction of laser-induced surface damage (LISD) on a second-generation SC nickel-based superalloy using a continuous wave (CW) fiber laser. Laser scanning experiments were performed on SC specimens in the as-cast condition with a laser power of 1000 W, a beam diameter of 1.2 mm, and scanning speeds from 5.5 mm/s to 16.5 mm/s. The cross-sections of the laser-irradiated surfaces were investigated by measuring the irradiated geometries (IRG), microstructural changes, microsegregations, solidification cracking, and heat affected zone (HAZ). The IRG shows the conduction mode of penetration with a high width-to-depth ratio under a bigger beam diameter and top-hat type beam profile. The IRG boundaries have irregular profiles due to the dissolution of interdendrite regions and eutectic phases. The IRG showed fine dendrites and solidification cracks with reduced microsegregation levels. The solidification cracking is mainly attributed to thermal stresses and the microcracking in HAZ is attributed to the dissolution of low melting Mo and Ti eutectics. The evolved HAZ ranges from 15% to 20% of the IRG depth. The LISD volume is evaluated as IRG plus HAZ for removal by machining process.

A comparative study on the adhesive bonding strength of unidirectional, 4-Harness Satin weaved and plain weaved carbon fibre reinforced polymer after KrF excimer laser pre-treatment

Several types of carbon fibre reinforced polymers (CFRP) are available based on the appearance of the carbon fibre laminates. In this study, three distinct types of CFRPs, namely, unidirectional (UD CFRP), 4-Harness Satin weaved (4HSW CFRP) and plain weaved (PW CFRP) were selected to compare the bonding strength performance of specific CFRP/CFRP adhesive joints after KrF laser pre-treatment. KrF excimer laser with top hat beam profile was utilized. This treatment increased the Sq and Sdr values significantly for all three CFRP types. KrF laser affected the matrix polymer C-N bonds and converted the tertiary and secondary aliphatic amines to secondary and primary amines, respectively, after de-polymerization. Single lap shear stress (SLSS) test revealed that the average adhesive bonding strength achieved respectively for UD CFRP, 4HSW CFRP and PW CFRP after KrF laser pre-treatment were 32.80, 23.97 and 35.21 MPa which were 104.34%, 47.24% and 39.72% higher than their pristine counterparts. Repeatability of the SLSS results were more for weaved pattern CFRPs. Adhesive failure reduced drastically and presence of cohesive failure, thin layer cohesive failure and fibre tear failure were prevalent in SLSS for all considered CFRPs. Comparison of all results showed that the best performance was obtained for PW CFRP.

Selective Large-Area Retinal Pigment Epithelial Removal by Microsecond Laser in Preparation for Cell Therapy.

PurposeCell therapy is a promising treatment for retinal pigment epithelium (RPE)-associated eye diseases such as age-related macular degeneration. Herein, selective microsecond laser irradiation targeting RPE cells was used for minimally invasive, large-area RPE removal in preparation for delivery of retinal cell therapeutics.MethodsTen rabbit eyes were exposed to laser pulses 8, 12, 16, and 20 µs in duration (wavelength, 532 nm; top-hat beam profile, 223 × 223 µm²). Post-irradiation retinal changes were assessed with fluorescein angiography (FA), indocyanine green angiography (ICGA), and optical coherence tomography (OCT). RPE viability was evaluated with an angiographic probit model. Following vitrectomy, a subretinal injection of balanced salt solution was performed over a lasered (maximum 13.6 mm2) and untreated control area. Bleb retinal detachment (bRD) morphology was then evaluated by intraoperative OCT.ResultsWithin 1 hour after irradiation, laser lesions showed FA and ICGA leakage. OCT revealed that large-area laser damage was limited to the RPE. The angiographic median effective dose irradiation thresholds (ED50) were 45 µJ (90 mJ/cm2) at 8 µs, 52 µJ (104 mJ/cm2) at 12 µs, 59 µJ (118 mJ/cm2) at 16 µs, and 71 µJ (142 mJ/cm2) at 20 µs. Subretinal injection over the lasered area resulted in a controlled, shallow bRD rise, whereas control blebs were convex in shape, with less predictable spread.ConclusionsLarge-area, laser-based removal of host RPE without visible photoreceptor damage is possible and facilitates surgical retinal detachment.Translational RelevanceSelective microsecond laser-based, large-area RPE removal prior to retinal cell therapy may reduce iatrogenic trauma.

Compact Beam Homogenizer Module with Laser-Fabricated Lens-Arrays

We report on manufacturing of a compact beam homogenizer module including two lens arrays and an aperture. Lens arrays are fabricated by an all laser-based technology employing a precise femtosecond pulsed laser ablation and a CO2 laser polishing step. Each lens array is processed revealing a high contour accuracy and a roughness of 25 nm. The 8x8 lens arrays are designed to have a square footprint to generate a quadratic Top-Hat beam profile and focal length of 10 mm to realize compact packaging. Firstly, the lens arrays are tested in an experimental setup using commercial lens holders with their functionality being demonstrated by shaping a uniform 4.5 mm squared Top-Hat beam profile, as being calculated. Afterwards, a 3D printer is used to additively manufacture the housing for the beam homogenizer module having a length of only 16 mm. After assembling the laser-fabricated lens arrays and a laser-cutted aperture into the housing, the functionality of the miniaturized module is proven.

Crack mitigation in Laser Powder Bed Fusion processed Hastelloy X using a combined numerical-experimental approach

Laser Powder Bed Fusion (LPBF) is an additive manufacturing (AM) technique associated with large thermal gradients and thermal strains resulting from the inherent nature of the process. Due to the non-equilibrium solidification conditions, certain alloys suffer from thermally induced micro-cracking, which hinders their exploitation. In this paper, we propose a novel approach towards the processing of a defect-free nickel-based superalloy, Hastelloy X, by means of Laser Powder Bed Fusion (LPBF). A combined experimental and numerical approach was used in order to evaluate the influence of the laser beam intensity distribution and melt track overlap on the temperature evolution, solidification behavior and crack formation. The use of a laser beam with narrow diameter and Gaussian beam intensity distribution leads to keyhole mode melting and a steep cooling rate, whereas a large diameter beam with top-hat intensity distribution leads to the formation of wide and shallow conduction mode melt pools, with a lower cooling rate and consequently a coarser sub-grain microstructure. The decrease of hatch spacing did not show a significant change in the local thermal history within a melt pool when a Gaussian beam was used. On the other hand, with the laser with the top-hat beam profile was used, an overall increase in the temperature of the layer and significantly lower cooling rates were observed. As a result, by decreasing the hatch spacing to 125 µm, micro-crack free Hastelloy X material was produced by LPBF for the first time, using a commercially available powder, simply by optimizing the scanning strategy and without modification of the chemical composition of the alloy. The material in the as-build condition exhibited a higher yield tensile strength (500 ± 17 MPa) than conventionally produced Hastelloy X (340 MPa), with elongation of 31.5 ± 3.0%.

Diffractive optical elements in single crystal diamond.

We demonstrate the design, fabrication, and experimental characterization of near-field binary phase transmission diffractive optical elements (DOEs) in single crystal diamond. Top-hat and arbitrary pattern DOE beam shapers were numerically optimized using an iterative Fourier transform algorithm (IFTA). Commercially available single crystal diamond plates (3mm×3mm×0.3mm) were patterned using hardmask deposition (α-Si), e-beam lithography, and O2 plasma-based diamond reactive ion etching. The resulting binary phase relief patterns were characterized using scanning electron microscopy (SEM) and atomic force microscopy (AFM). Experimental characterization of the single crystal diamond DOEs in transmission at λ=532nm confirms excellent uniformity of the resulting top-hat beam profile as required in copper welding applications.

Laser transfer technique using wavefront correction and beam homogenizers in Thomson scattering diagnostics

This paper suggests the use of two optical elements for laser transfer in Thomson scattering diagnostics, which are widely used for other laser applications. YAG lasers amplified by flash lamps (repetition frequency of 50–100 Hz having a few Joules output) can sometimes have adverse effects such as peaked beam profiles and wavefront distortion. We used a beam homogenizer, which is a refractive optical element, to change the beam profile from peaked to non-peaked at a designated position. Using a super-Gaussian output profile when amplifying lasers facilitated longer—by a factor of 1.5 (=300 mm/200 mm) compared with top-hat beam profile—sustained distances, i.e. the distance along the laser path having a non-peaked profile. In terms of beam quality, wavefront distortion broke the beam profile in the far field. We used a deformable mirror to correct and flatten the wavefront of the beam. A YAG laser having a wavefront distortion of 0.46λ peak-to-valley was used to test the application of the deformable mirror, the results of which improved wavefront distortion to a wavelength of 0.1λ peak-to-valley.

Spectral pulse shaping of a 5 Hz, multi-joule, broadband optical parametric chirped pulse amplification frontend for a 10 PW laser system.

We present a broadband optical parametric chirped pulse amplification (OPCPA) system delivering 4J pulses at a repetition rate of 5Hz. It will serve as a frontend for the 1.5kJ, <150 fs, 10PW laser beamline currently under development by a consortium of National Energetics and Ekspla. The spectrum of the OPCPA system is precisely controlled by arbitrarily generated waveforms of the pump lasers. To fully exploit the high flexibility of the frontend, we have developed a 1D model of the system and an optimization algorithm that predicts suitable pump waveform settings for a desired output spectrum. The OPCPA system is shown to have high efficiency, a high-quality top-hat beam profile, and an output spectrum demonstrated to be shaped consistently with the theoretical model.

Mechanical properties of laser beam welded similar and dissimilar aluminum alloys

Two approaches were used for laser beam welding of similar and dissimilar joints of AA7075 and AA5182 that aim to overcome the weldability problems of high-strength Al-Zn-Mg-Cu alloys. The first approach implies the use of vanadium foil as additional filler material, while the second implies the use of a fiber laser with a large beam diameter and a top-hat beam profile. Although both approaches result in an improved weld quality, in terms of weld appearance, porosity and cracking, the resulting mechanical properties differ considerably. The addition of vanadium leads to a local increase of microhardness in the fusion zone. However, the tensile strength of these joints is lower as for fiber laser welded joints. In direct comparison fiber laser welded joints exhibit also higher formability as the joints welded with vanadium foil. The highest formability is obtained for dissimilar joints with the medium-strength Al-Mg alloy. Due to the unavoidable softening in the weld zone of heat treatable aluminum alloys, the formability of the joints is inferior in comparison to the base materials. In addition, the positive effect of post-weld heat treatment, surface milling and warm forming on the resulting mechanical properties of similar and dissimilar joints is discussed.

High-energy Nd:YAG laser system with arbitrary sub-nanosecond pulse shaping capability.

We report on a laser system capable of generating high-energy (>270 mJ) temporally shaped pulses at 1064 nm with 0.43-ns shaping resolution. The pulses are generated by modulation of a continuous-wave seed laser and subsequent amplification by a dual-stage grazing-incidence Nd:YVO4 "bounce" amplifier and a Nd:YAG power amplifier (all quasi-continuous diode-pumped). The system produces pulses with a high-quality top-hat spatial beam profile with up to 0.6 GW of peak power and 44 W of average power, a power stability of 0.22% rms, and fully programmable complex temporal shapes.

Laser cladding of Inconel 625-based composite coatings reinforced by porous chromium carbide particles

Inconel 625/Cr3C2 composite coatings were produced via a laser cladding process using Cr3C2 reinforcing particles presenting an open porosity of about 60%. A laser cladding system used consisted of a direct diode laser with a rectangular beam spot and the top-hat beam profile, and an off-axis powder injection nozzle. The microstructural characteristics of the coatings was investigated with the use of scanning electron microscopy and X-ray diffraction. A complete infiltration of the porous structure of Cr3C2 reinforcing particles and low degree of their dissolution have been achieved in a very narrow range of processing parameters. Crack-free composite coatings having a uniform distribution of the Cr3C2 particles and their fraction up to 36vol% were produced. Comparative erosion tests between the Inconel 625/Cr3C2 composite coatings and the metallic Inconel 625 coatings were performed following the ASTM G 76 standard test method. It was found that the composite coatings have a significantly higher erosion resistance to that of metallic coatings for both 30° and 90° impingement angles. Additionally, the erosion performances of composite coatings were similar for both the normal and oblique impact conditions. The erosive wear behaviour of composite coatings is discussed and related to the unique microstructure of these coatings.

Power-dependent upconversion quantum yield of NaYF4:Yb3+,Er3+ nano- and micrometer-sized particles - measurements and simulations.

Photophysical studies of nonlinear lanthanide-doped photon upconverting nanoparticles (UCNPs) increasingly used in biophotonics and photovoltaics require absolute measurements of the excitation power density (P)-dependent upconversion luminescence (UCL) and luminescence quantum yields (ΦUC) for quantifying the material performance, UCL deactivation pathways, and possible enhancement factors. We present here the P-dependence of the UCL spectra, ΦUC, and slope factors of the different emission bands of representative 25 nm-sized oleate-capped β-NaYF4:17% Yb3+, 3% Er3+ UCNPs dispersed in toluene and as powder as well as ΦUC of 3 μm-sized upconversion particles (UCμP), all measured with a newly designed integrating sphere setup, enabling controlled variation of P over four orders of magnitude. This includes quantifying the influence of the beam shape on the measured ΦUC and comparison of experimental ΦUC with simulations utilizing the balancing power density model of the Andersson-Engels group and the simulated ΦUC of UCμP from the Berry group, underpinned by closely matching decay kinetics of our UC material. We obtained a maximum ΦUC of 10.5% for UCμP and a ΦUC of 0.6% and 2.1% for solid and dispersed UCNPs, respectively. Our results suggest an overestimation of the contribution of the purple and an underestimation of that of the red emission of β-NaYF4:Yb3+,Er3+: microparticles by the simulations of the Berry group. Moreover, our measurements can be used as a guideline to the absolute determination of UCL and ΦUC.

Nanosecond laser nanostructuring of fused silica surfaces assisted by a chromium triangle template

The well-reproducible, fast and cost-effective nanostructuring is a big challenge for laser methods. The laser nanostructuring of fused silica assisted by chromium nanotriangles was studied using a KrF excimer laser (λ=248nm, Δtp=25ns, top hat beam profile). Therefore, a fused silica substrate was covered with periodically ordered polystyrene (PS) spheres with a diameter of 1.59μm. Subsequently, this system was covered with 30nm chromium by electron beam evaporation. Afterwards the PS spheres were removed and the bare and resultant periodic Cr triangles were irradiated. The laser irradiation with high laser fluences resulted in a removal of the chromium and in localized modifications of the fused silica like a localized ablation of the fused silica. The resultant structures were studied by scanning electron (SEM) and atomic force microscopy (AFM) as well as the surface composition was analysed by energy-dispersive X-ray spectroscopy (EDX). The laser process allows the production of well-defined periodic hole structures into the fused silica surface where the resultant surface structure depends on the laser parameters. The multi-pulse irradiation of the Cr/SiO2 sample with moderate laser fluences (Φ∼650mJ/cm2) allows the fabrication of periodic pyramidal-like structures (depth Δz=130nm).

Direct Diode Laser Cladding of Inconel 625/WC Composite Coatings

Metal matrix composite coatings, composed of Inconel 625 alloy and tungsten carbide (WC), have been produced by laser cladding using a high power direct diode laser with a rectangular laser beam spot and a top-hat beam profile. The primary goal of the investigation was to understand the role of the shape of WC particles and the heat input level on the quality of the composite coating system used, especially on its erosion behaviour. The results indicated that angular WC particles are more susceptible to dissolution in the molten pool than spherical. However, the composite coatings containing angular WC particles exhibited significantly higher erosion resistance than those with spherical WC for both normal and oblique impacts. This is directly attributed to the excellent mechanical interlocking of the angular WC in the matrix. The WC/matrix interfacial decohesion has been observed in the coatings containing spherical WC at the oblique impact.