Reflector Laser Research Articles

Laser-based remote detection of leaf wetness

Pesticide-free agricultural strategies need new tools for disease prevention. Better than early detection of disease, detection of conditions favorable to their appearance can be a progress. In the case of fungal diseases, the presence of water on the plant surface is necessary. In order to detect remotely this presence early and at the scale of a crop field, we propose a low-cost solution based on laser reflection. Here, experimental results in a controlled environment are presented on both hydrophobic and hydrophilic leaves (rapeseed Brassica Napus and grapevine Vitis Vinifera, respectively). We first assess the water detection on a leaf surface by recreating the dew formation process. We next evaluate the influence of the scanning measurement and leaf inclination on the detection to get closer to in-field conditions. Results show that this method is very sensitive on both types of leaves. Water detection is possible from a low surface coverage with a high temporal precision at 1 m. In the hydrophobic case, water on a leaf surface leads to an increase of the detected signal up to three times compared to a dry leaf. The corresponding minimum surface coverage detectable at 1 m is evaluated at 1.6% thanks to 2D ray-tracing numerical simulations. In the hydrophilic case, on the contrary, water on a leaf surface leads to a decrease of the detected signal by almost half. For both types, the dew detection delay is contained under 5 min and can be improved. Finally, the presented results pave the way to a field application.

Influence of recoil pressure, mushy zone flow resistance and reflectivity on melt pool shape in laser powder bed fusion simulation

Melt pool dimensions (depth: D and width: W) are strongly correlated with defects generated in laser powder bed fusion. The melt pool dimensions have been evaluated by CFD simulation including solid–liquid–gas phase change and laser multiple reflection. Such a multi-physics simulation requires a large number of parameters. Preliminary simulations for parameter identification require a lot of time and effort. The parameter sensitivity to the melt pool dimensions is important; however, the systematic evaluations have hardly been found in the literature. In this study, we performed systematic parametric evaluation for three parameters related to recoil pressure, flow resistance in solid–liquid mushy zone and Fresnel laser reflection. As a result, the D values increased with increasing recoil pressure, but the W values did not. The flow resistance force influenced the velocities in the mushy zone but not the melt pool dimensions. The D increased with increasing reflectivity, but the W did not. The melt pool dimensions varying recoil pressure, flow resistance force and reflectivity were all inconsistent with the experimental melt pool dimensions. In order to agree with the experimental melt pool dimensions, the problems to be solved were discussed by comparing the previous studies.

Modelling the influences of powder layer depth and particle morphology on powder bed fusion using a coupled DEM-CFD approach

Optimising the quality of metal parts produced by additive manufacturing requires an understanding of how the powder characteristics impact both layer spreading and the subsequent powder melting. Here we present a simulation study which couples models of powder spreading (by the Discrete Element Method) and powder bed fusion by laser beam (by CFD). Previous simulations of these processes have mostly assumed idealised smooth surfaces, spherical particles and a single laser track. Here we seek a more realistic description by spreading over a previously-melted (rough) surface and by melting a number of tracks laid side-by-side to mimic the crosshatched scans typically used during part production. The effects of powder morphology (via a range of particle shapes including spheres, disks, ellipsoids and cuboids) and layer depth on spreading and subsequent melting have been investigated. We find that the fraction of laser energy transferred to the part and the melt pool volume both increase rapidly with the volume of powder deposited, due to the combined effects of multiple laser reflections and the insulating nature of the powder layer. In contrast, particle shape has very little effect on the overall melting behaviour of the powder, with the volume deposited and uniformity of coverage being the key determining factors. These observations suggest that the use of cheaper non-spherical powders is feasible provided that a sufficiently uniform coverage can be achieved at small layer depths.

Role of B4C in the directed energy deposition of aluminum alloy 6061/B4C composite using core-shell powder for crack annihilation, pore mitigation, and grain refinement

Despite the high demand for additive manufacturing of high-strength aluminum alloys (60XX and 70XX), high-strength aluminum alloys are one of the most difficult materials to manufacture through metal additive manufacturing (MAM) because of their high laser reflectivity, thermal conductivity, low viscosity, and large solidification range. Thus, MAM products of high-strength aluminum alloys are characterized by pores, cracks, and irregular surface morphology. In the present study, the stable MAM behavior of aluminum matrix composites was achieved by adding B4C reinforcement to the feedstock powder in the form of a novel core-shell powder. The aluminum alloy 6061, which is notorious for its difficulty in MAM, was selected as the matrix. The core-shell B4Cp/6061 MMC powder was prepared for MAM via directed energy deposition (DED). The B4C addition effectively mitigated the formation of cracks, pores, and balling during DED. In addition, the grains were refined, thus preventing anisotropy and improving the mechanical properties. Furthermore, the favorable effect of B4C addition during DED was found to be caused by the formation of AlB2 during solidification. These findings provide a basic understanding of the promising potential of MMCs fabricated by MAM.

Dynamic Analysis of Aerosol Release from a Pressurized Metered Dose Inhaler Combined with a Valved Holding Chamber Using Simplified Laser Photometry.

Background: A pressurized metered dose inhaler combined with a valved holding chamber (pMDI+VHC) is used to prevent upper airway complications and improve the efficiency of inhaled drug delivery; however, the aerodynamic behavior of the released particles has not been well investigated. This study aimed at clarifying the particle release profiles of a VHC using simplified laser photometry. Methods: An inhalation simulator comprised a computer-controlled pump and a valve system that withdrew aerosol from a pMDI+VHC using a jump-up flow profile. A red laser illuminated the particles leaving VHC and evaluated the intensity of the light reflected by the released particles. Results: The data suggested that the output (OPT) from the laser reflection system represented particle concentration rather than particle mass, and the latter was calculated as OPT × instantaneous withdrawn flow (WF). Summation of OPT hyperbolically decreased with flow increment, whereas summation of OPT × instantaneous flow was not influenced by WF strength. Particle release trajectories consisted of three phases, namely increment with a parabolic curve, flat, and decrement with exponential decay phases. The flat phase appeared exclusively at low-flow withdrawal. These particle release profiles suggest the importance of early phase inhalation. The hyperbolic relationship between WF and particle release time revealed the minimal required withdrawal time at an individual withdrawal strength. Conclusions: The particle release mass was calculated as laser photometric output × instantaneous flow. Simulation of the released particles suggested the importance of early phase inhalation and predicted the minimally required withdrawal time from a pMDI+VHC.

Efficient proton-carbon ions acceleration by the plasma-beat-wave mechanism in the presence of the axial magnetic field

It is possible to generate the low-divergence ion bunches through the interaction of equally or slightly different counter-propagating laser pulses. In this paper, in the framework of plasma beat wave, we have numerically simulated the simultaneous acceleration of carbon/hydrogen ion beams. Using a two-dimensional particle-in-cell simulation, we have shown that in an HC plasma mixture with an optimal hydrogen ratio of 1:5, the laser reflectivity coefficient reduces up to about 4.6%. This condition may provide the acceleration of low-divergence H+ and C4+ ions beam. The cut-off energy for protons and C4+ ions are about 27 MeV and 410 MeV, respectively at n H = 0.2 n C. The values increase by about 15.6% for protons and 21% for C4+ at n H = 0.7 n C. In the presence of an axial magnetic field, the energy absorption arrives at its maximum at the values of Ωs/ω p = 0.1, where Ωs and ω p are the gyro-frequency and plasma frequency, respectively. Here, the average kinetic energy of the accelerated ions raises by 17.9% and 7.3% for carbon and proton ions, respectively. Compared to the magnetic field-free case, divergence angles were suppressed at approximately 24.2% and 20.3% for the carbon ions and hydrogen ions, respectively.

Multi-phase flow simulation of powder streaming in laser-based directed energy deposition

As a promising additive manufacturing technique, laser-based directed energy deposition has been widely applied in part fabrication, surface cladding, and part restoration. During the powder feeding process, the gas, particles, and laser interact and further affect the molten pool flow. To study the powder feeding process, a multi-physics gas-particle multi-phase flow model coupled with a laser ray-tracing heat source model is developed to predict the powder particle temperature, velocity, and concentration distribution as well as laser energy loss, which is then validated against experimental results. The single-nozzle simulation shows that powder particles out of the nozzle follow a radial Weibull distribution instead of a 2D Gaussian distribution from the inlet of the nozzle. In the four-nozzle powder feeding process, the particles converge further after passing the focal plane, and setting the focal plane above the molten pool surface could potentially increase the powder catchment. Moreover, the laser ray-tracing model incorporates laser reflection and laser incident angle into laser energy loss calculation, which would be more accurate than those models based on the Beer-Lambert law. Additionally, by increasing the powder feed rate or decreasing the carrier gas flow rate, the laser energy loss linearly increases. The predicted powder velocity, temperature distribution, and attenuated laser can be further implemented into the molten pool flow model to more comprehensively study the interaction between particles and the molten pool in the future.

Application of existing algorithms for classification and clustering of laser reflection points (k-Means, DBSCAN, SVM) to solve territorial planning problems

Objective. The aim of the study is to generalize the experience of using and reveal the features of methods for classifying and clustering images obtained by laser scanning.Method. The study is based on the use of algorithms for recognition, clustering and classification of data obtained using laser scanning.Result. A brief review of existing algorithms used for clustering images obtained by laser scanning has been carried out. The possibility of using sequentially the K-means and DBSCAN algorithms for clustering in relation to objects of various shapes is shown. The possibilities of using algorithms for the classification and clustering of laser reflection points (k-Means, DBSCAN, SVM) in the framework of solving territorial planning problems are analyzed.Conclusion. The use of the presented algorithms makes it possible to form data arrays that, when superimposed on each other subsequently and further processed, make it possible to obtain even more accurate representations of objects and territories in territorial planning documents, and in the future, based on the use of machine learning methods and processing of data matrices available in memory, get more accurate information about objects on the ground.

Single-pass cutting of frosted glass via change of laser incident medium.

We report a water medium-assisted composite laser cutting (WMACLC) technology for what is believed to be the first time to achieve single-pass separation of frosted glass (FG). The water medium was used to flatten the surface of FG to reduce the diffuse reflection and random refraction of the incident laser. The simulation results of picosecond pulsed laser Bessel beam (PPLBB) intensity distribution in FG showed that the peak intensity in the presence of water can reach about 24 times and 2.3 times that in the absence of water when the PPLBB is 0.08 mm and 0.3 mm below the upper surface of FG, respectively. A PPLBB with higher intensity can be formed along the thickness direction to realize the material modification. A coaxial CW laser provides the thermal tensile stress required for separation. Finally, high-quality separation of FG was achieved using the WMACLC technology with a speed of 50 mm/s. No deviation in the separation track and no edge collapse occurred. The roughness Sa of the separated sidewall is less than 0.3 µm.

Study of the performance signature based ternary and quaternary III–V compounds for laser diodes in optical communication system

Abstract This study outlines the performance based ternary and quaternary III–V compounds in optical communication system. We have employed aluminum dopant ratio to check the effects in the spectral and thermal performance characteristics of various structures of optical diode sources based ternary and quaternary III–V compounds. The laser diode threshold current, reflection loss, corner wavelength, and optical gain are tested under various dopant aluminum ratios. The laser diode rise time and data flow rate are examined under thermal effects and various dopant levels. The laser diode corner wavelength and reflection loss can be enhanced by the increase of aluminum dopant ratio. The basic Al x In1−x Sb laser diode structure is the best candidate according to its high flow data rates in compared to other laser structures under the same thermal effects.

An innovative method for measuring the three-dimensional water surface morphology of unsteady flow using light detection and ranging technology

High-resolution free-surface elevation measurement is challenging due to the rapid evolution of unsteady flow. In this study, using dam-break flow as an example, the applicability of a low-cost sensor, the Intel® RealSense™ L515 solid-state light detection and ranging (LiDAR) probe, for the free-surface elevation measurement was evaluated. A white inorganic pigment was added to the water to improve the water's laser reflection. A series of dam-break flow experiments were systematically conducted, including asymmetric flow with obstacles in downstream rivers. The results demonstrate that the single-point time-domain process and water surface profile as measured by the LiDAR probe showed good consistency with acoustic displacement meter measurements with uniform correction of the free-surface elevation deviation. The proposed method's advantage is that the unsteady flow's free surface can be obtained with high temporal and spatial resolution; in addition, the volume flow rate and cross-sectional average velocity can be indirectly obtained, thus providing a useful reference for measuring the free surface of unsteady flow under laboratory conditions.

A Pre-Pruning Quadtree Isolation Method with Changing Threshold for ICESat-2 Bathymetric Photon Extraction

The new generation of spaceborne laser altimeter, the Ice, Cloud, and Land Elevation Satellite-2 (ICESat-2), which can simultaneously generate laser reflections on the water surface and underwater, is a potential data source for exploring water depth in nearshore environments. To achieve this scientific goal, accurate bathymetric photon extraction is needed. This study proposed a pre-pruning quadtree isolation (PQI) method with changing threshold. Firstly, the pre-pruning step is introduced. Photons are transformed into different levels in the isolated quadtree structure according to spatial density. Then, the frequency histogram of photon elevation and isolated level (IL) is generated, the IL thresholds in different depth ranges are calculated by the Otsu method, and the bathymetric photons are extracted. The results in the Culebra archive show that this method achieved a 92.71% F1 score. Noise rate and water depth are the main factors affecting the extraction of sounding photons. When the photon density gradually increases from 2–4 pts/m to 6–8 pts/m, the F1 score of PQI decreases by no more than two percent. In different depth ranges, the extraction results of PQI are also better than those of comparison methods. Therefore, PQI can provide reliable theoretical support for nearshore areas lacking water depth data.

Lidar Reflective Tomography of the Target Under Incomplete View State

The lidar reflective tomography (LRT) system transmits a laser signal and obtains laser reflection projections of the target, which shows great potential for further long-distance noncooperative target detection. However, the received projections are normally in an incomplete view state. Hence, in this article, an improved algebraic reconstruction technique (ART) utilizing the sparse regularization model and nonlocal means (NLMs) algorithm is introduced and proposed for LRT reconstruction to restore incomplete signals or projections. By using the designed LRT outfield system, the comparative experiments are carried out to validate the effectiveness of the proposed method. By considering different investigation states, the improved NLM-ART sparse method shows great capability for LRT of noncooperative targets in long distance.

Light-trapping carbon nanotube forests in glass fibre-reinforced thermoplastic prepregs for efficient laser-assisted automated fibre placement

Laser-assisted automated fibre placement (LATP) is an emerging technology for manufacturing thermoplastic composite components. However, major challenges are still encountered for the LAFP process for glass fibre (GF) reinforced composites due to the high near-infrared (NIR) laser reflectance and transmission as well as resulting poor absorption of prepreg constituents. Inspired by the efficient light-trapping and photothermal characteristics of carbon nanotube (CNT) forest, we fabricate CNT forest onto the GF surface by rapid and low‐cost flame synthesis method, to enhance the photothermal conversion efficiency of poly-ether-ether-ketone (PEEK)/GF prepregs during LAFP process. CNT forests with different lengths, alignments and densities were produced by changing flame synthesis parameters. The spectroscopy results show that CNT forest suppresses transmission and reflection from GF and crystals in GF/PEEK prepreg within the NIR wavelength range at near normal incidence, decreasing transmission and reflectance to 0.3% and 7.9%, respectively, and increasing absorption to 91.8% at 1080 nm wavelength. Furthermore, CNTs forest improves the heating rate of GF/PEEK prepreg by more than 700% for a wide range of incident angles. Consequently, CNT forest enables a 400% increase in placement speed and a 79% reduction in energy consumption during LAFP process, while making negligible voids in the interlayer.

Determining the laser absorptivity of Ti-6Al-4V during laser powder bed fusion by calibrated melt pool simulation

Additive manufacturing processes such as laser powder bed fusion (LPBF) are now routinely used for advanced medical and aerospace components. However, moving from these bespoke applications to high-volume commercial manufacturing requires higher levels of predictability and control of part properties. Detailed simulations of the LPBF process can assist this transition by providing understanding of defect mechanisms and guiding paths to improvement. An accurate value of laser absorptivity for the material is critical for LPBF simulation but the published literature contains surprisingly little absorptivity data applicable to actual LPBF operating conditions. Here we determine the in-situ laser absorptivity of the alloy Ti-6Al-4V during LPBF to be 0.27 +/- 0.03, for a laser wavelength of 1.07 μm. Our technique involves calibrating melt pool CFD simulations against single-track experiments conducted over a range of energy densities and can be extended to other materials. The simulations incorporate multiple laser reflections and cover the transition from conduction to keyhole mode. We also discuss physical mechanisms that may be responsible for changes in the absorption behaviour at high laser energy density which are observed in this and other work.

Ongoing Challenges of Laser-Based Powder Bed Fusion Processing of Al Alloys and Potential Solutions from the Literature-A Review.

Their high strength-to-weight ratio, good corrosion resistance and excellent thermal and electrical conductivity have exponentially increased the interest in aluminium alloys in the context of laser-based powder bed fusion (PBF-LB/M) production. Although Al-based alloys are the third most investigated category of alloys in the literature and the second most used in industry, their processing by PBF-LB/M is often hampered by their considerable solidification shrinkage, tendency to oxidation, high laser reflectivity and poor powder flowability. For these reasons, high-strength Al-based alloys traditionally processed by conventional procedures have often proved to be unprintable with additive technology, so the design and development of new tailored Al-based alloys for PBF-LB/M production is necessary. The aim of the present work is to explore all the challenges encountered before, during and after the PBF-LB/M processing of Al-based alloys, in order to critically analyse the solutions proposed in the literature and suggest new approaches for addressing unsolved problems. The analysis covers the critical aspects in the literature as well as industrial needs, industrial patents published to date and possible future developments in the additive market.

High-flux positron generation via the ultra-intense laser irradiating density-modulated plasmas

To investigate plasma density during the Breit–Wheeler positron generation, a comparative study of four plasma targets is performed via the PIC (particle-in-cell) code EPOCH. When an ultra-intense laser (2.8×1023 Wcm−2) is incident, more positrons with high energy are generated in the increasing density plasmas. The positron yield is already 1.5×108 with a cutoff energy of 2 GeV at t=37T0. It is demonstrated that increasing density plasmas will enhance gamma photon radiation and positron generation. In increasing density plasmas, under-dense plasmas favor electron acceleration, and over-dense plasmas will induce laser reflection. Cross sections of the Compton back-scattering and the BW positron generation are both increased via high-energy electrons colliding with the reflected laser. In addition, increasing the laser intensity will directly enhance positron generation. This investigation will further facilitate high-flux positron generation and application.

Microstructure and dynamic microhardness of additively manufactured (TiB2+TiC)/AlSi10Mg composites with AlSi10Mg and B4C coated Ti powder

In-situ synthesis of ceramics hybrid reinforced Al matrix composites (AMCs) by laser powder bed fusion (L-PBF) induced combustion reaction is a promising strategy to strengthen Al alloys. However, the in-situ formed microstructures are highly sensitive to the stability of mixed multi-powder feedstocks, which is a challenge to fabricate AMCs. This work develops a novel approach to obtain B4C coated Ti particles (hereafter as B4C@Ti) by collosol spraying to prepare (B4C@Ti)/AlSi10Mg composite powders. L-PBF technique was used to design and manufacture (TiB2+TiC)/AlSi10Mg composites by collosol coated powders. The effects of the coating process on the powder characteristics and phase structural evolution were systematically investigated, and the microstructures and dynamic microhardness of associated composites were monitored to reveal its underlying in-situ formation mechanism. Results indicate that the introduced B4C@Ti particles reduced the laser reflectivity of AlSi10Mg powders by 2.0 % and improved the K-M absorption factor by 4.8 %. The formation of Ti3SiC2 transition phase in the as-built composites was inhibited, and the supersaturated Si dissolution resulted in the lattice constant (a = 4.0466 Å) and interplanar spacing (d(111) = 2.3363 Å) of α-Al matrix phase increasing to 4.0504 Å and 2.3385 Å, respectively. The nanoscale TiB2 and TiC ceramic phases were in-situ formed within the elemental transition zone (ETZ) around the residual B4C particles, and the limited titanium source in the AlSi10Mg molten pool generated only TiB2 rather than TiB. The average values of dynamic microhardness of the L-PBF as-built composites concomitantly increased from 1.37 ± 0.08 GPa to 1.80 ± 0.13 GPa.

Early stages of polycrystalline diamond deposition: laser reflectance at substrates with growing nanodiamonds.

The chemical vapor deposition of polycrystalline diamond (PCD) films is typically done on substrates seeded with diamond nanoparticles. Specular laser reflectance has been used in tandem with a continuous film model to monitor the thickness of these films during their deposition. However, approaches to gain information on properties that strongly affect film morphology, such as the areal density of seeds, remain largely unexplored. This work outlines a strategy for using laser reflectance measurements to refine the monitoring of film thickness during deposition, estimate the mean equivalent radii and the areal density of seeds, and estimate growth incubation periods. We present a general model based on the Rayleigh theory of scattering for laser reflectance at substrates with growing nanoparticles that captures the early stages of PCD deposition. We test our model experimentally by depositing diamond under identical conditions on silicon substrates with various seed densities and by comparing seed densities obtained by scanning electron microscopy to those determined by our strategy. We also explore the different deposition stages for which our model and a continuous film model can be used safely. In addition to providing guidelines for characterizing PCD deposition, this work may also advance the general understanding of nanoparticle growth and formation.

Research on Infrared Emissivity and Laser Reflectivity of Sn1−xErxO2 Micro/Nanofibers Based on First-Principles

Sn1−xErxO2 (x = 0%, 8%, 16%, 24%) micro/nanofibers were prepared by electrospinning combined with heat treatment using erbium nitrate, stannous chloride and polyvinylpyrrolidone (PVP) as raw materials. The target products were characterized by thermogravimetric analyzer, X-ray diffrotometer, fourier transform infrared spectrometer, scanning electron microscope, spectrophotometer and infrared emissivity tester, and the effects of Er3+ doping on its infrared and laser emissivity were studied. At the same time, the Sn1−xErxO2 (x = 0%, 16%) doping models were constructed based on the first principles of density functional theory, and the related optoelectronic properties such as their energy band structure, density of states, reflectivity and dielectric constant were analyzed, and further explained the mechanism of Er3+ doping on SnO2 infrared emissivity and laser absorption from the point of electronic structure. The results showed that after calcination at 600°C, single rutile type SnO2 was formed, and the crystal structure was not changed by doping Er3+. The calcined products showed good fiber morphology, and the average fiber diameter was 402 nm. The infrared emissivity and resistivity of the samples both decreased first and then increased with the increase of Er3+ doping amount. When x = 16%, the infrared emissivity of the sample was at least 0.71; and Er3+ doping can effectively reduce the reflectivity of SnO2 at 1.06 μm and 1.55 μm, when x = 16%, its reflectivity at 1.06 μm and 1.55 μm are 50.5% and 40%, respectively, when x = 24%, the reflectivity at 1.06 μm and 1.55 μm wavelengths are 47.3% and 42.1%, respectively. At the same time, the change of carrier concentration and electron transition before and after Er3+ doping were described by firstprinciple calculation, and the regulation mechanism of infrared emissivity and laser reflectivity was explained. This study provides a certain experimental and theoretical basis for the development of a single-type, light-weight and easily prepared infrared and laser compatible-stealth material.