Reflector Laser Research Articles

Imaging techniques for defect detection of fiber reinforced polymer‐bonded civil infrastructures

The present article provides a state-of-the-art review of imaging techniques used for defect detection of fiber reinforced polymer (FRP)-bonded civil engineering structures. Compared to the conventional techniques by equipping a structure with stress wave sensors, the contactless imaging techniques feature efficient instrumentation, convenient data acquisition, and rapid evaluation in the procedures of nondestructive testing (NDT). Recently, a substantial progress in utilizing electromagnetic waves for development of imaging NDT techniques (e.g., synthetic aperture radar, infrared thermography, laser shearography, and laser reflection technique) with the purpose to identify debonding or delamination of FRP-bonded structural systems has been made. As another electromagnetic wave-based imaging technique, X-ray computed tomography is promising for exploration of the damage evolution in this structural system, despite little application in structural health monitoring of real infrastructures. Apart from these imaging techniques, more recently, there have been computer-aided motion magnification techniques for defect identification. The motion magnification technique only requires a digital camera and a computer with vision algorithm, which can amplify the motion of defect region and render it visible in a video. This advanced imaging technique achieves high-resolution measurement, simultaneous full-field inspection, and straightforward defect identification in the scene. In this review, both Eulerian motion magnification and phase-based motion magnification techniques for structural visual inspection are presented and discussed. Furthermore, the present article recommends the combination of imaging NDT techniques with artificial intelligence approaches (e.g., deep learning algorithms) to realize the automated and efficient defect detection towards FRP-bonded civil infrastructures.

In situ laser reflectivity to monitor and control the nucleation and growth of atomically thin 2D materials*

The growth of atomically thin two-dimensional (2D) layered and other quantum materials is typically performed without in situ monitoring or control. Here, a simple laser reflectivity approach is demonstrated to provide in situ control over sub-monolayer thickness and growth kinetics during pulsed laser deposition (PLD) of MoSe2 layers. First, the general technique is presented with emphasis on designing the maximum sensitivity of the optical contrast through consideration of Fresnel’s equations with proper choice of layer thickness, substrate, and laser monitoring wavelength, incidence angle, and laser polarization. Then the 632.8 nm optical reflectivity of MoSe2 layers on SiO2/Si substrates was predicted and compared with in situ monitoring of MoSe2 growth by PLD under actual growth conditions using a probe HeNe laser beam. The measurements showed high sensitivity and excellent agreement with MoSe2 surface coverages calculated from atomic resolution STEM analysis of 2D layers deposited in arrested growth experiments. Growth kinetics revealed by these measurements showed sigmoidal nucleation and growth stages in the formation of the 2D MoSe2 layers that are described by a simple model, indicating the promise of the laser reflectivity technique for in situ monitoring and control of 2D materials deposition.

Resonant sputtered neutral mass spectrometry using multiple reflections of laser to counterbalance Doppler broadening

Resonant laser sputtered neutral mass spectrometry can greatly enhance the ionization efficiency of specific neutral sputtered atoms through resonant excitation using tunable laser(s). However, the laser beam cannot irradiate the entire volume where sputtered particles are moving, because the laser beam is focused. In contrast, for an extremely low concentration of target elements, it is necessary to increase the laser irradiation efficiency. Irradiation efficiency depends on both spatial coverage and resonance wavelength matching. The former can be increased by increasing the laser path length within the sputtered neutral plume. The latter depends on the relationship between the laser spectral linewidth and the Doppler broadening width caused by the velocity of sputtered neutral particles. The authors have developed a new method to improve the ionization probability of neutral particles by reflecting the laser using mirrors to increase the ionization efficiency based on the concept mentioned above.

Thermal conduction in isotope diamond thin films studied by pump-probe laser reflectivity measurement

We study the thermal conductivity of isotope-controlled diamond thin films between 0.001% and 99% 13C contents using an ultrafast pump-probe laser experiment. We measure the surface temperature change through the reflectivity change of the probe light pulse after heating with the pump light pulse, compare it with a theoretical calculation, and estimate the thermal conductivity. The Brillouin-oscillation appears in the reflectivity change, which is used for determining the film thickness and then removed in determining the thermal conductivity. The theoretical calculation reveals that the diamond film should be thicker than 1 μm for obtaining appropriate thermal conductivity. Our results indicate that the thermal conductivity of the 50%-13C diamond thin film is much lower than the reported value, suggesting unavailability of reported values of bulk materials for thin films and necessity for measurement for the thermal conductivity of thin films.

Nanosecond-millisecond combined pulse laser drilling of alumina ceramic.

A nanosecond-millisecond combined pulse laser (CPL) drilling method was proposed for drilling alumina ceramic. The total energy consumption of the CPL drilling was 1/7 of that of a conventional millisecond laser, and the drilling quality was better. The simulation results demonstrated that, due to the nonuniform reflection of the millisecond laser in the keyhole, the ellipse keyhole ablated by the off-axis incident nanosecond pulses had no effect on the circularity of the through hole. In addition, the multireflection of the laser in the keyhole enhanced the absorption, so the keyhole ablated by the nanosecond pulses could be used as a target for limiting the absorption of the subsequent millisecond pulses. In this context, the keyhole could be used to reduce the hole diameter if the subsequent millisecond laser had a bigger spot size, and this CPL drilling method could be used as an effective group hole drilling method.

Laser reflection as a catalyst for direct laser acceleration in multipicosecond laser-plasma interaction

We demonstrate that laser reflection acts as a catalyst for superponderomotive electron production in the preplasma formed by relativistic multipicosecond lasers incident on solid density targets. In 1D particle-in-cell simulations, high energy electron production proceeds via two stages of direct laser acceleration: an initial stochastic backward stage and a final nonstochastic forward stage. The initial stochastic stage, driven by the reflected laser pulse, provides the preacceleration needed to enable the final stage to be nonstochastic. Energy gain in the electrostatic potential, which has been frequently considered to enhance stochastic heating, is only of secondary importance. The mechanism underlying the production of high energy electrons by laser pulses incident on solid density targets is of direct relevance to applications involving multipicosecond laser-plasma interactions.

Diffusion bonded Be neutron target using 8MeV proton beam

The development of the high intensity compact neutron source is mainly conducted by using accelerators for medical purposes. Recently, a lot of compact neutron sources have been developed, and most of them are for boron neutron capture therapy (BNCT). Compared with the common accelerators used for industries, accelerators for BNCT have to accelerate a 100 times larger current of charged particles because of the low conversion efficiency of neutron moderators. To attain a reliable target for the BNCT neutron source, two obstacles have to be overcome; radiation damage (blistering) and heat issue. We introduce diffusion bonding method aiming to make defect free Be neutron target and 1 MPa latent heat water-cooling system. We also install beam expansion optics using quadrupole and octupole magnets to reduce the charge density. Our facility has been operating for more than three years from the commissioning period and we report the recent situation as following. 1) Diffusion bonding method was applied to the solid neutron target of Be with latent heat cooling system. 2) Amount of integrated coulomb value of protons implanted to the Be target is nearly 3000 C now 3) Amount of neutron dose rate was measured. 4) A laser reflection microscope (LRM) method has been developed to observe the neutron target condition (especially radiation damage) through viewport with a few meters working distance. 5) Direct observation of the surface of the Be target was done during the period of improving nearly target vacuum and residual radiation conditions. From these evidences, we conclude our neutron target already produces neutrons capable to treat nearly 500 patients of malignant melanoma.

Enabling high-performance 3D printing of Al powder by decorating with high laser absorbing Co phase

Pure Al with high laser reflectivity is essentially incompatible with laser powder bed fusion. The retention of a large number of unmelted particles leads to inferior geometrical quality and mechanical properties of printed pure Al parts. In the present study, we propose decorating Al with a small amount of high laser absorbing Co nanoparticles on the surface of Al powders to reduce laser reflectivity and improve printability. The experimental results confirmed that the approach was very effective. The near homogenous dispersion of Co slightly modified the surface chemical composition and roughened the powder surface. This approach completely melted the particles and eliminated the internal pores, thereby favorably tuning the geometrical dimensions. Additionally, the introduction of Co provided solid solution strengthening and precipitation hardening via dispersion of second-phase Al9Co2 with a coherent interfacial relationship with the Al matrix. The tensile properties of printed Al parts were comparable to commercial medium-strength Al alloys at an optimal Co-content of 0.5 wt.%.

BeamDelta: simple alignment tool for optical systems

BeamDelta is a tool to help align optical systems. It greatly assists in assembling bespoke optical systems by providing a live view of the current laser beam position and a reference position. Even a simple optical setup has multiple degrees of freedom that affect the alignment of beam paths. These degrees of freedom rise exponentially with the complexity of the system. The process of aligning all the optical components for a specific system is often esoteric and poorly documented, if it is documented at all. Alignment methods used often rely on visual inspection of beams impinging on pinholes in the beam path, typically requiring an experienced operator staring at diffuse reflections for extended periods of time. This can lead to a decline in accuracy due to eye strain, flash blindness as well as symptoms such as headaches and, possibly, more serious retinal damage. Here we present BeamDelta a simple alignment tool and accompanying software interface which allows users to obtain accurate alignment as well as removing the necessity of staring at diffuse laser reflections. BeamDelta is a robust alignment tool as it doesn't require any precise alignment itself.

Skin Imaging Using Ultrasound Imaging, Optical Coherence Tomography, Confocal Microscopy, and Two-Photon Microscopy in Cutaneous Oncology.

With the recognition of dermoscopy as a new medical technology and its available fee assessment in Korea comes an increased interest in imaging-based dermatological diagnosis. For the dermatologist, who treats benign tumors and malignant skin cancers, imaging-based evaluations can assist with determining the surgical method and future follow-up plans. The identification of the tumor's location and the existence of blood vessels can guide safe treatment and enable the use of minimal incisions. The recent development of high-resolution microscopy based on laser reflection has enabled observation of the skin at the cellular level. Despite the limitation of a shallow imaging depth, non-invasive light-based histopathologic examinations are being investigated as a rapid and pain-free process that would be appreciated by patients and feature reduced time from consultation to treatment. In the United States, the current procedural terminology billing code was established for reflectance confocal microscopy in 2016 and has been used for the skin cancer diagnosis ever since. In this review, we introduce the basic concepts and images of ultrasound imaging, optical coherence tomography, confocal microscopy, and two-photon microscopy and discuss how they can be utilized in the field of dermatological oncology.

Different lasers reveal different skin microcirculatory flowmotion - data from the wavelet transform analysis of human hindlimb perfusion

Laser Doppler flowmetry (LDF) and reflection photoplethysmography (PPG) are standard technologies to access microcirculatory function in vivo. However, different light frequencies mean different interaction with tissues, such that LDF and PPG flowmotion curves might have distinct meanings, particularly during adaptative (homeostatic) processes. Therefore, we analyzed LDF and PPG perfusion signals obtained in response to opposite challenges. Young healthy volunteers, both sexes, were assigned to Group 1 (n = 29), submitted to a normalized Swedish massage procedure in one lower limb, increasing perfusion, or Group 2 (n = 14), submitted to a hyperoxia challenge test, decreasing perfusion. LDF (Periflux 5000) and PPG (PLUX-Biosignals) green light sensors applied distally on both lower limbs recorded perfusion changes for each experimental protocol. Both techniques detected the perfusion increase with massage, and the perfusion decrease with hyperoxia, in both limbs. Further analysis with the wavelet transform (WT) revealed better depth-related discriminative ability for PPG (more superficial, less blood sampling) compared with LDF in both challenges. Spectral amplitude profiles consistently demonstrated better sensitivity for LDF, especially regarding the lowest frequency components. Strong correlations between components were not found. Therefore, LDF and PPG flowmotion curves are not equivalent, a relevant finding to better study microcirculatory physiology.

Controlling ceramic-reinforcement distribution in laser cladding of MMCs

Laser cladding (LC) using blown powder is an important technique among laser deposition processes, able to perform coating, repair, manufacturing and prototyping. However, deposition of fine ceramic-reinforced MMC clads is challenging due to the divergence of fine particles. This leads to a non-uniform distribution of reinforcements and reduced clad performance. The high laser reflectivity of some metals such as Al also limits material deposition efficiency. In this work, the effect of a powder satelliting process on improving laser-deposited Al–TiC clads on aluminium 6082-T6 substrates was investigated. A simple-blended feedstock was also deposited for comparison. The effect of process parameters of laser energy and powder deposition density on clad characteristics of clad area, dilution and porosity were evaluated, and optimised parameters were identified. Two deposition mechanisms are proposed to explain the differences in the behaviour of both feedstocks within LC. The satellited clads yielded an increase of 29% and 113% in the deposition efficiency and TiC fraction, respectively, compared to the blended clads. The satellited clads yielded a mean increase of 60% in matrix microhardness and a mean reduction of 64% in specific wear rate compared with the blended clads indicating a clear improvement in the mechanical properties.

Opto Mechanical Design Aspects of High Power Laser Mirrors

Reflecting Optics is a critical component of any High Power Laser (HPL) based System. With the advancements in Laser Sources, power is continuously increasing with decrease in operating wavelength. This has lead toeven more sophisticated upto mechanical design of HPL optics. A large portion of the incident power is reflected and the rest is absorbed and transmitted in various stages. This absorbed power is very detrimental which affects the system performance and may even lead to catastrophic system failure. The mechanism of laser reflection, absorption and transmission is explained in a simple way. General requirements and desired properties of substrate materials are listed. Various metallic and non metallic substrate materials are tabulated along with their physical and mechanical properties. The materials are compared on the basis of their Figure of Merit (ratio of thermal conductivity to coefficient of thermal expansion) andstrength to weight ratio (E/ρ). Finally thermo structural analysis is carried out fora specific design input and results are formulated.

Experimental investigations of thermal modal parameters for a C/SiC structure under 1600 °C high temperature environment

This work aims to investigate the thermal–mechanical behavior of a carbon fiber reinforced silicon carbide (C/SiC) structure under 1600 °C high temperature environment. In this test, both the contact and non-contact measurement are used. Four laser vibrometers were used for non-contact measurement. The laser reflection characteristics of C/SiC surface under high temperature were tested. The fiber Bragg grating sensor (FBG) sensor network measures both strain and temperature of the test piece, by means of wavelength shifts due to the tensile stress on a Bragg grating. The energy spectrum of the signal is obtained by the time–frequency analysis method, and the peak value is extracted by an algorithm to obtain the time–frequency characteristic of different modes. The result shows that the modal frequencies of C/SiC structure under time-varying thermal loads change with the temperature of the structure, which has a great influence on the modal characteristics of the structure. Besides, this study indicates that the FBG sensor is suitable for structural health monitoring of C/SiC structure under 1600 °C high temperature environment. The results provide an important basis for the dynamic performance analysis and safety design of structure under high-temperature thermal-vibration conditions.

Research on projection alignment method in laser reflection tomography

Laser reflection tomography has broad application prospects in the field of long-distance detection of space targets. The key step in the imaging process is to align the multi-angle laser range profiles of the target to the target’s center of rotation. For non-cooperative target, it is difficult to extract the target’s center of rotation directly from the echoes, but laser range profiles can be aligned based on the similarity of adjacent profiles. In this paper, the multi-angle laser range profiles are aligned to the target’s center of rotation based on the average range profile envelope method, and the equivalent size of the target is estimated from laser range profiles. Furthermore, the approximate range of the rotation center is estimated, then the target image is reconstructed by iteration. The applicability of this method in laser reflection tomography is analyzed by simulation model, and the feasibility of the method is also verified by experiments.

Facile bioactive yeast cell templated synthesis of laser stealth antimony doped tin oxide hollow microspheres

Hollow antimony-doped tin oxide (ATO) microspheres with a diameter of 2.0–3.0 μm and ellipsoidal morphology were successfully prepared through a co-precipitation route. The prepared ATO microspheres possessed more Sb5+ and 73.5 m2 g−1 specific surface area. These ATO hollow microspheres showed a widened infrared absorption phenomenon in the infrared band of 850–2000 nm. The near-infrared reflection was obviously lower than ATO nanoparticles. Additionally, the reflectance of ATO hollow microspheres was approximately 7% at 1064 nm, indicating better laser reflection properties.

Intelligent crack extraction and analysis for tunnel structures with terrestrial laser scanning measurement

An automatic and intelligent method for crack detection is significantly important, considering the popularity of large constructions. How to identify the cracks intelligently from massive point cloud data has become increasingly crucial. Terrestrial laser scanning is a measurement technique for three-dimensional information acquisition which can obtain coordinates and intensity values of the laser reflectivity of a dense point cloud quickly and accurately. In this article, we focus on the optimal parameter of Gaussian filtering to balance the efficiency of crack identification and the accuracy of crack analysis. The innovation of this article is that we propose a novel view of the signal-to-noise ratio gradient for Gaussian filtering to identify and extract the cracks automatically from the point cloud data of the terrestrial laser scanning measurement.

A General Strategy for Enhancing 3D Printability of High Laser Reflectivity Pure Aluminum Powder

The current printable materials are mainly limited to Fe, Ti, Co, and Ni-based alloys because they have good laser absorptivity. However, a number of Al alloys that are highly reflective to a laser are incompatible with selective laser melting. We demonstrate here that the coating with a small amount of high laser-absorbing Ni on Al powder reduces laser reflectivity, improves its printability, and increases tensile strength to ~ 1.5 times that of printed pure Al.

Growth dynamics and light scattering of gold nanoparticles in situ synthesized at high concentration in thin polymer films.

Background: Numerous optical applications of nano-objects require a dispersion of the nanoparticles in a dielectric matrix. In order to achieve high particle concentrations, one can, as an alternative, directly grow the particles in a polymer or an inorganic film by, e.g., thermal annealing.Results: Simple laser reflection experiments showed that this growth process induced light scattering at the film/air interface. We report on this phenomenon, considering the growth dynamics of gold nanoparticles in a polymer film. The scattering of light was studied by measuring the bi-directional reflection distribution function. In parallel with the observation of enhanced scattering, imaging ellipsometry in dynamics mode showed that local values of the ellipsometric angles Ψ and Δ were strongly modified by the annealing process.Conclusion: A diffraction pattern corresponding to local modifications of the optical properties of the film gradually appeared, which turned out to be the signature of the growth of the Au nanoparticles. Moreover, the monitoring of the statistical distribution of the ellipsometric angles during annealing helped evidencing two regimes in the dynamics of the nanoparticle growth and in the optical response of the nanocomposite.

Reutilization of workpiece-reflected energy assisted laser machining of metallic materials with high laser reflectivity

The reutilization of workpiece-reflected energy assisted laser machining (REALM), combining nanosecond laser and the optimized optical system, was developed skillfully to modify the surface of metallic materials with high laser reflectivity. The characteristics of REALM were clarified via the theoretical analysis and the 6061 Aluminum machining test. It was observed that with respect to normal laser machining (NLM), the total energy utilization ratio and the processing efficiency for REALM could be significantly enhanced. Moreover, the nominal ablation threshold of aluminum for REALM could be decreased. These superiorities of REALM stem from its specific regime of reuse of the laser energy scattered and reflected, and thus the enhancement of the effective laser energy density and the effective utilization of absorbed energy.