Laser Profile Research Articles

Generalization of the PIV loss-of-correlation formula introduced by Keane and Adrian

In 2D particle image velocimetry (PIV), the loss-of-correlation due to out-of-plane motion or light-sheet mismatch has two effects. First, it reduces the probability of detecting a valid vector. Second, it increases the uncertainty measured in velocity fields. The loss-of-correlation is commonly determined by the \(F_\mathrm {O}\) factor, which was initially proposed by Keane and Adrian (Appl Sci Res 49:191–215, 1992). However, the present study demonstrates that the validity of the original \(F_\mathrm {O}\) definition is confined to cases with identical laser intensity profiles. As light sheets usually differ in width and shape, the proposed definition is of limited use in reality. To overcome this restriction, a new definition for \(F_\mathrm {O}\) is proposed which covers the effect of light-sheet pairs with different shapes and widths. The proposed improvement was validated by means of synthetic PIV images based on various light-sheet profiles. The loss-of-correlation for the images was compared to the theoretical solution based on the light-sheet profiles. The results show that the new definition of \(F_\mathrm {O}\) accurately predicts the loss-of-correlation for all tested laser mismatches and agrees with the old definition for the ideal case involving identical light sheets. Based on the revised \(F_\mathrm {O}\) definition, prediction of loss-of-correlation due to light-sheet mismatches and misalignment is now possible using a laser profiling camera. This allows the optimization of a laser prior to any PIV measurements. For the case of out-of-plane motion, the loss-of-correlation can also be estimated from the correlation function of the PIV images. Thus, it is possible to optimize the laser alignment to match the flow conditions while setting up an experiment. These findings help experimentalists to understand and control the sources of errors associated with out-of-plane effects and help to minimize the measurement uncertainty.

Analysis of Cerium Oxide Doped Anti-Corrosion Coatings By Scanning Electrochemical Microscopy and Electrochemical Impedance Spectroscopy

Previously, a protocol was developed to use SECM (scanning electrochemical microscopy) to screen for the effectiveness of new anti-corrosion coatings produced by the Navy.[i]Briefly, increasingly positive potentials at the Al substrate are applied while the tip is in the substrate generation tip collection (SGTC) mode. Poor coatings show significant tip currents while good ones do not. The current work used this protocol on coatings provided by engineers from Naval Air Systems Command that had been doped with varying amounts of CeO. Initial voltage applications showed coating breakdown with increasing current at ever more positive potentials. However, a maximum was seen and at potentials above 2V, the coatings appear to have improved. In an effort to elucidate what caused this effect, Electrochemical Impedance Spectroscopy (EIS), laser profiling, and x-ray fluorescence studies were performed to look for changes in the coating. Preliminary EIS results (Figure 1) indicate the resistance due to pores increased after higher potential application hinting the pores may be becoming smaller.[ii]The value of the real component of the complex impedance, where the left side of the semicircle begins, is the sum of the uncompensated resistance and pore resistance. Spectra were taken after the Al substrate was exposed to the indicated DC potential. While not large, it can be seen the resistance increased as the applied potential increased. X-ray results as well as laser profiling (not shown) indicate the coating at and near the surface is changing. The X-ray data show other components of the coating (Ti, Fe, etc) are migrating to/from the surface. The laser generated topographical data also shows morphological changes on the surface all of which may be the cause of the pore size reduction. All data were taken at the U.S. Naval Academy, an undergraduate only institution, by one student. Projects such as this where the midshipmen conduct two semesters of research under the supervision of a professor, became part of USNA’s chemistry major over a decade ago. While not compulsory, some 70% of the majors elect this option and most result in at least a published conference proceeding. This is even more remarkable in that the students do not have the flexibility to conduct experiments outside the normal school day. [i] Lee, C., Calhoun, M., Dorriety, W., Hanrahan, R., Lancaster, F., Calhoun, R., ECST, 66 (30) 65-71 (2015). [ii] Loveday, D., Peterson, P., Rodgers, B., JCT Coatings Tech, Oct, 88-93 (2004). Figure 1

Ultraviolet laser transverse profile shaping for improving x-ray free electron laser performance

The photocathode rf gun is one of the most critical components in x-ray free electron lasers. The drive laser strikes the photocathode surface, which emits electrons with properties that depend on the shape of the drive laser. Most free electron lasers use photocathodes with work function in the ultraviolet, a wavelength where direct laser manipulation becomes challenging. In this paper, we present a novel application of a digital micromirror device (DMD) for the 253 nm drive laser at the Linear Coherent Light Source. Laser profile shaping is accomplished through an iterative algorithm that takes into account shaping error and efficiency. Next, we use laser shaping to control the X-ray laser output via an online optimizer, which shows improvement in FEL pulse energy. Lastly, as a preparation for electron beam shaping, we use the DMD to measure the photocathode quantum efficiency across cathode surface with an averaged laser rms spot size of $59\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$. Our experiments demonstrate promising outlook of using DMD to shape ultraviolet lasers for photocathode rf guns with various applications.

Morphological hysteresis in the evolution of beach profiles under sequences of wave climates - Part 1; observations

Novel series of experiments are presented that demonstrate morphological hysteresis in the evolution to equilibrium of beach profiles under sequences of different wave climates. The experiments were conducted in a wave flume at medium scale using both monochromatic and random waves, representing 2D conditions. Beach profiles were obtained with high spatial resolution at frequent intervals with a laser profiler, from which shoreline location, bar position and sediment transport rates were derived. Experiments were conducted for sequences of wave climates, where a sequence comprised of 6–13 sequential tests, each commencing with the beach profile from the preceding test. Each test was run until equilibrium conditions were obtained and had a constant wave height, increased or decreased relative to the preceding test. Cyclical conditions were also included, with erosive and accretive wave conditions of short durations alternating through multiple cycles, so that equilibrium conditions were not reached during a test. With a sequence of increasing wave heights, the relationship between the shoreline position and the bulk cross-shore sediment transport, at equilibrium, was non-monotonic, indicating a maximum in the landward sediment transport rate. For test series comprised of a sequence of increasing wave heights followed by a sequence of decreasing wave heights, morphological hysteresis was observed in the equilibrium shoreline position and bulk cross-shore sediment transport, such that shoreline recession, or offshore transport, continued in some instances after reductions in wave height. This is inconsistent with classical equilibrium type shoreline evolution models. However, when equilibrium conditions were not reached, in the cyclic sequences, no such morphological hysteresis was observed and a dynamic equilibrium is reached. The morphological hysteresis occurs because of the decay, stranding, or increased relative depth, of the breaker bar following a reduction in wave height, often in conjunction with a new breaker bar generated by further offshore transport in the inner surf zone. Similar sequences of morphological response are evident in field data and larger scale tests in the literature. Finally, it is shown that the morphological hysteresis can be explained using the classical equilibrium beach state model of Wright et al. (1985) by introducing the concept of a subsequent alternate active beach state, which may occur following a change in wave conditions.

Laser radiation pressure proton acceleration in gaseous target

An analytical model for hole boring proton acceleration by a circularly-polarized CO2 laser pulse in a gas jet is developed. The plasma density profile near the density peak is taken to be rectangular, with inner region thickness l around a laser wavelength and density 10% above the critical, while the outside density is 10% below the critical. On the rear side, plasma density falls off rapidly to a small value. The laser suffers strong reflection from the central region and, at normalized amplitude a0≥1, creates a double layer. The space charge field of the double layer, moving with velocity vfzˆ, reflects up-stream protons to 2vf velocity, incurring momentum loss at a rate comparable to radiation pressure. Reflection occurs for vf≤ωpzflm/mp, where m and mp are the electron and proton masses, zf is the distance traveled by the compressed electron layer and ωp is the plasma frequency. For Gaussian temporal profile of the laser and parabolic density profile of the upstream plasma, the proton energy distribution is narrowly peaked.

A 3D Laser Profiling System for Rail Surface Defect Detection.

Rail surface defects such as the abrasion, scratch and peeling often cause damages to the train wheels and rail bearings. An efficient and accurate detection of rail defects is of vital importance for the safety of railway transportation. In the past few decades, automatic rail defect detection has been studied; however, most developed methods use optic-imaging techniques to collect the rail surface data and are still suffering from a high false recognition rate. In this paper, a novel 3D laser profiling system (3D-LPS) is proposed, which integrates a laser scanner, odometer, inertial measurement unit (IMU) and global position system (GPS) to capture the rail surface profile data. For automatic defect detection, first, the deviation between the measured profile and a standard rail model profile is computed for each laser-imaging profile, and the points with large deviations are marked as candidate defect points. Specifically, an adaptive iterative closest point (AICP) algorithm is proposed to register the point sets of the measured profile with the standard rail model profile, and the registration precision is improved to the sub-millimeter level. Second, all of the measured profiles are combined together to form the rail surface through a high-precision positioning process with the IMU, odometer and GPS data. Third, the candidate defect points are merged into candidate defect regions using the K-means clustering. At last, the candidate defect regions are classified by a decision tree classifier. Experimental results demonstrate the effectiveness of the proposed laser-profiling system in rail surface defect detection and classification.

3D dynamics of debris flows quantified at sub-second intervals from laser profiles

We use pairs of parallel mounted laser profile scanners to measure main debris-flow variables in two debris-flow channels in central and southern Switzerland. The scanners measure the instantaneous cross-sectional geometry of debris flows at rates of 25–100 Hz, and we apply large-scale particle image velocimetery to estimate velocity. The scanners also provide direct measurements of flow depth. From these data, we were able to estimate debris-flow depth, velocity and discharge for 16 out of 17 events. These results are consistent with discharge estimated from a system of geophones and a radar gauge for two available datasets. We also investigated debris-flow geometry to quantify rheology-controlled cross-flow convexity and found that four events manifest strong surface convexity at their surge fronts where we expect the largest boulders and low pore-fluid pressures. The scanners provide a completely new view of debris-flow dynamics and channel morphology and present novel opportunities to measure discharge and investigate debris-flow geometries.

Optimization of the temperature profile of a high-powered strontium bromide laser

The subject of investigation is a new high-powered strontium bromide (He– $$\hbox {SrBr}_{2})$$ vapor laser emitting in the infrared spectrum. The aim of the study is to investigate the possibilities for optimizing the thermal mode of the laser by enhancing its operating characteristics. Based on a previous analytical self-consistent model of the authors, a model of the device temperature profile is presented under the conditions of forced convection. The model consists of quasi-stationary heat transfer equation with appropriate set of boundary conditions to investigate the influence of the cooling processes in the laser tube with transverse external air cooling. An assessment is performed of the nature of the main physical processes accompanying the distribution of heat energy from the center of the tube and its interaction with the ambient environment under forced convection. Through simulations, it has been found that when air flow rate changes, the operating temperature can vary widely within 100 K. It is also illustrated how to increase the supplied electric power while maintaining the maximum working temperature at the center of the laser tube. The model allows for detailed examination of the thermal processes, it does not require temperature measurements, and it can be used for different geometric dimensions of the laser tube, values of the supplied electric power, coefficients of thermal conductivity of the structural materials, and other operational and design parameters. The developed model is applicable to a wide range of metal vapor lasers, and other closely related ones.

Charge production studies from [formula omitted] photocathodes in a normal conducting RF gun

This work discusses the behavior of electron bunch charge produced in an L-band normal conducting radio frequency gun from Cs2Te photocathodes illuminated with ps-long UV laser pulses and presumed homogeneous flattop laser transverse distribution. The measured charge shows the expected linear dependence in the quantum efficiency limited emission regime at low laser pulse energies. At higher laser pulse energy, the measured charge in the space charge limited emission regime should saturate, assuming an ideal homogeneous flattop laser transverse distribution. However, this behavior is not observed experimentally. Instead of saturating, the measured charge continues to increase with laser pulse energy, albeit with much weaker dependence than in the quantum efficiency limited emission regime. Simulations with the space charge particle tracking code ASTRA show that the charge saturates as expected using a homogeneous flattop laser transverse distribution. The discrepancy between simulations and measured excess charge may be attributed to the presence of unintentional Gaussian-like decaying radial halo beyond the core of the otherwise presumed homogeneous flattop core. The rate of increase of the measured charge at high laser pulse energies seems to be proportional to the amount of halo despite charge saturation in the core of the transverse laser radial profile. By utilizing core + halo particle distributions based on measured radial laser profiles, ASTRA simulations and semi-analytical emission models reproduce the behavior of the measured charge for a wide range of RF gun and laser operational parameters within the measurement uncertainties.

Development of laser target sight-on system based on multiple transmission through a tapered glass capillary for ion microbeam irradiation

SynopsisAn estimation method of micro-meter sized laser profile created by the laser beam transmission through a tapered glass capillary is developed for the target sight-on system of ion microbeam irradiation to living cells. In our system, both the laser and the ions are designed to be transmitted through the same capillary so that the laser spot is surely on the ion track. By selecting the excitation light of a fluorescent protein in cell nucleus, the fluorescent volume can be confirmed as the targeted area.

Enhanced Second Harmonic Generation Efficiency via Wavefront Shaping

Optical second harmonic generation is a fundamental nonlinear effect with a large impact on laser technology and optical imaging/sensing. For most practical applications of second harmonic generation, high conversion efficiency is required. However, many techniques used to achieve high efficiency are limited to fabrication methods and optical energy requirements. Here, we investigate and demonstrate substantial enhancement in the conversion efficiency of second harmonic generation in a nonlinear crystal via the application of wavefront shaping. In a one-dimensional understanding of second harmonic generation, a phase offset applied to the fundamental wave has no effect on the intensity of the generated light. We show that when the pump field is not a plane wave, enhanced conversion efficiency can be controlled by the application of a phase mask to the fundamental beam. This investigation of the dependence of conversion efficiency upon the transverse phase profile of the incident pump laser yields the prom...

Using a constructive pavement texture index for skid resistance screening

Skid resistance measurement is one of the major measurements used to assess road safety. Typical devices used for measuring skid resistance include single point checking type and continuous measurements type. However, conducting periodic full roadway network skid resistance survey can be a relatively time consuming and difficult task because of the limited water carrying capacity used by all kinds of devices, including the continuous types. On the contrary, measurement of pavement surface texture can be conducted easily and continuously by most profilers. This research investigates the relation between pavement skid resistance and surface texture. The focus is on the development of a texture index that can serve as a screening indicator to filter out the high-risk pavement sections for further skid resistance measurement. Two measuring devices, GripTester (GT) and National Taiwan University laser profiler (NTU Profiler), were used to collect data on pavement skid resistance and texture in a network of 555 sections, respectively. The pavement texture index of the mean difference of elevation (MDE) is calculated at a 1mm profile interval, which is a very promising indicator to represent pavement skid resistance. Through a sensitivity study, the MDE threshold of 0.15mm is observed to provide a relatively reasonable screening function that filters out 29.01% of the entire surveyed network for further skid resistance inspection. It averted more than 70% skid resistance measurement capacity of the entire network. The accuracy of correct judgment by the selected threshold is about 80%, only 3.78% of roadway sections considered for further inspection of the recommended list are not included.

Tailoring residual stress profile of Selective Laser Melted parts by Laser Shock Peening

The paper describes a new approach in controlling and tailoring residual stress profile of parts made by Selective Laser Melting (SLM). SLM parts are well known for the high tensile stresses in the as – built state in the surface or subsurface region. These stresses have a detrimental effect on the mechanical properties and especially on the fatigue life. Laser Shock Peening (LSP) as a surface treatment method was applied on SLM parts and residual stress measurements with the hole – drilling method were performed. Two different grades of stainless steel were used: a martensitic 15-5 precipitation hardenable PH1 and an austenitic 316L. Different LSP parameters were used, varying laser energy, shot overlap, laser spot size and treatments with and without an ablative medium. For both materials the as-built (AB) residual stress state was changed to a more beneficial compressive state. The value and the depth of the compressive stress was analyzed and showed a clear dependence on the LSP processing parameters. Application of LSP on SLM parts showed promising results, and a novel method that would combine these two processes is proposed. The use of LSP during the building phase of SLM as a “3D LSP” method would possibly give the advantage of further increasing the depth and volume of compressive residual stresses, and selectively treating key areas of the part, thereby further increasing fatigue life.

Effect of groundwater table rising on the performance of a Swedish-designed gravel road

Abstract Sweden has a wide network of gravel roads. When the water table levels rise during the freeze-thaw and wet seasons, the bearing capacity of the roads will decrease and rut depths increase. The road will suffer a small deterioration every time a vehicle travel over it. Therefore, it has been decided to investigate the long-term performance of a Swedish designed gravel road using an accelerated pavement testing facility. A trial gravel road was constructed and exposed to various levels of ground water table which simulate the weather changes in Sweden. The structure of the trial gravel road consisted of 15 cm (base and surface layer), 20 cm sub-base layer and 260 cm subgrade soil. The trial road was tested using the Heavy Vehicle Simulator (HVS) and the vertical surface deformations and the vertical dynamic deformations were measured by means of laser profile measurement and eMU coils respectively. At the end of 85 000 HVS passes, the tested road suffered from serious deformations as discussed in details in this paper. Furthermore, a transverse trench section was excavated at the end of HVS trafficking in order to investigate the possible mechanism contributing to the tested road failure.

DESENVOLVIMENTO, CONSTRUÇÃO E VALIDAÇÃO DO PERFILÔMETRO A LASER

O objetivo deste trabalho foi desenvolver, construir e validar um perfilômetro a laser para mensurar as áreas mobilizada e elevada do solo, espessura média da camada mobilizada e o empolamento do solo obtido do resultado do trabalho de diferentes implementos e/ou máquinas. Construiu-se uma armação retangular de 1500 mm x 3000 mm, com tubos retangulares para suporte do carrinho de leitura. Este movimenta-se milimetricamente nas coordenadas x e y, para referenciar a posição do sensor a laser, que obtém a coordenada z. Para validação do perfilômetro a laser, compararam-se as variáveis de mensuração com o perfilômetro utilizado tradicionalmente em pesquisas, constituído por réguas paralelas posicionadas em suporte com perfuração equidistante ao longo de seu comprimento, através das quais deslizam hastes perpendiculares ao comprimento das réguas paralelas. As variáveis de mensuração não apresentaram significância estatística quando comparados os parâmetros recolhidos pelos diferentes perfilômetros. Em conclusão, o uso do perfilômetro proposto neste trabalho, foi o que apresentou maior agilidade na coleta, aquisição e armazenamento de dados, além de não modificar a estrutura do perfil analisado por tratar-se de um método de leitura a laser, sem contato com a amostra.

Effects of alpha stopping power modelling on the ignition threshold in a directly-driven inertial confinement fusion capsule

The alpha-particle energy deposition mechanism modifies the ignition conditions of the thermonuclear Deuterium-Tritium fusion reactions, and constitutes a key issue in achieving high gain in Inertial Confinement Fusion implosions. One-dimensional hydrodynamic calculations have been performed with the code Multi-IFE [R. Ramis and J. Meyer-ter-Vehn, Comp. Phys. Comm. 203, 226 (2016)] to simulate the implosion of a capsule directly irradiated by a laser beam. The diffusion approximation for the alpha energy deposition has been used to optimize three laser profiles corresponding to different implosion velocities. A Monte-Carlo package has been included in Multi-IFE to calculate the alpha energy transport, and in this case the energy deposition uses both the LP [C.K. Li and R.D. Petrasso, Phys. Rev. Lett. 70, 3059 (1993)] and the BPS [L.S. Brown, D.L. Preston, and R.L. Singleton Jr., Phys. Rep. 410, 237 (2005)] stopping power models. Homothetic transformations that maintain a constant implosion velocity have been used to map out the transition region between marginally-igniting and high-gain configurations. The results provided by the two models have been compared and it is found that - close to the ignition threshold - in order to produce the same fusion energy, the calculations performed with the BPS model require about 10% more invested energy with respect to the LP model.

Improving the performance of time-domain pulsed echo laser profile using tunable lens.

We theoretically and experimentally examined the effects of the different positions between receiving lens (RL) and detector, namely, defocus, off-axis, and tilt, on the time-domain pulsed echo laser profile (TDPELP). Results show that distortions including saturation and broadening of TDPELP are obtained, regardless of the position between RL and detector. Thus, we adjust the focal length of RL to successfully obtain an optimal TDPELP using tunable lens under extreme situations, such as too strong or too weak intensity of the TDPELP.

A Study of the Static/Kinetic Friction Behavior of PTFE-Based Fabric Composites

ABSTRACTThree different polytetrafluoroethylene (PTFE)-based fabric composites were prepared. The static/kinetic friction behaviors of these composites under different loads and speeds were studied. A 3D laser microscope and profile measurement apparatus were used for analysis of the morphology and weave structure of the composites, and the contact temperature of these composites under different loads and speeds was monitored continuously using a high-precision thermal resistor. In addition, a dynamic mechanical analysis (DMA) apparatus was used to explore the thermal and mechanical properties of PTFE-based fabric composites. The results demonstrated that speed/load, weave structure, and fiber form have an important influence on static and kinetic friction behavior of the fabric composites. Generally, the static friction coefficient is greater than the kinetic friction coefficient, except when considering light load conditions. Under light load conditions, the static friction coefficient is equal to the kinetic friction coefficient. In addition, the kinetic friction coefficient first increased and then decreased with increased speed, but the static coefficient increased first and then remained at an almost constant value. At all sliding speeds, the static and kinetic friction coefficients of tape yarn composites are better than those of the multifilament yarn composites. Weave structure has no effect on the static friction coefficient, but it has a significant influence on the kinetic friction coefficient.

Simplified model for fast optimization of a free-electron laser oscillator

A simplified one-dimensional theoretical model for free-electron laser oscillator (FELO) calculation which reserves the main physics is proposed. Instead of using traditional macroparticles sampling method, the theoretical model takes advantage of low gain theory to calculate the optical power single-pass gain in the undulator analytically, and some reasonable approximations are made to simplify the calculation of power growth in the cavity. The theoretical analysis of single-pass gain, power growth, time-dependent laser profile evolution and cavity desynchronism are accomplished more efficiently. We present the results of infrared wavelength FELO and X-ray FELO with the new model. The results are validated by simulation with GENESIS and OPC.

Nano and micro structured targets to modulate the spatial profile of laser driven proton beams

Nano and micro structured thin (μ m-scale) foils were designed, fabricated and irradiated with the high intensity laser system operating at LLC (Lund Laser Centre, Sweden) in order to systematically study and improve the main proton beam parameters. Nano-spheres deposited on the front (laser irradiated) surface of a flat Mylar foil enabled a small enhancement of the maximum energy and number of the accelerated protons. Nano-spheres on the rear side allowed to modify the proton beam spatial profile. In particular, with nanospheres deposited on the rear of the target, the proton beam spatial homogeneity was clearly enhanced. Silicon nitride thin foils having micro grating structures (with different step dimensions) on the rear surface were also used as targets to influence the divergence of the proton beam and drastically change its shape through a sort of stretching effect. The target fabrication process used for the different target types is described, and representative experimental results are shown and discussed along with supporting 3D particle-in-cell simulations.