Reflected Laser Beam Research Articles

Experimental Verification of Laser Radar Cross Section Model for Complex Targets with Gaussian Beam Irradiation

The reflection of a Gaussian laser beam from a flat Lambert disk is considered theoretically. It was found that the results of experimental measurements of the reflected beam power as a function of the disk radius at various distances from the photodetector to the target are in good agreement with the theoretical model. It was shown that when the radius of the laser beam is greater than the dimensions of the probed complex target this target can be replaced by an equivalent Lambert disk with the same laser radar cross section.

Adaptive slicing in powder bed fusion of metals using a laser beam—Investigation on productivity, laser absorption, geometrical accuracy, and thermal conditions

Powder bed fusion of metals using a laser beam (PBF-LB/M acc. to DIN EN ISO/ASTM 52900) has reached market maturity. In addition to developing new materials and enabling new applications, the industry focuses on increasing productivity and reducing costs. In this context, increasing the layer thickness can increase productivity but often leads to a deterioration of surface quality and part density. Using variable layer thicknesses depending on the manufactured geometry is well-known from filament-based material extrusion processes and is called adaptive slicing. This study investigates the manufacturing aspects of adaptive slicing using AlSi10Mg. Laser beam reflectance of the different powder layer thicknesses is quantified using diffuse reflectance infrared Fourier transform spectroscopy. The process window is identified and analyzed, focusing on the achievable productivity and required energy input. Furthermore, the suitability of layer thicknesses and processing parameters is analyzed by measuring the dimensional accuracy and process stability of overhanging structures. Heat input, heat dissipation, and potential heat-up are investigated and compared to conventional processes using part-scale thermal simulations. In this study, parameters are developed for layer thicknesses of 120 μm with an almost threefold increase in productivity in nonoverhanging structures with a part density above 99.7%. Further, adaptive slicing can increase productivity in PBF-LB/M while decreasing the impact on part quality. Future work will focus on automated algorithms to optimize and automize adaptive slicing.

Measurement of early time outer laser beam reflection inside a cylindrical hohlraum at the National Ignition Facility

In a National Ignition Facility (NIF) cylindrical hohlraum, any specular reflection (“glint”) off the wall from the outer cone (incident angle of 50° and 44° relative to the surface of the wall) laser beam will irradiate the capsule poles. If the glint power is sufficiently large during the picket (early time) of the laser pulse, it may seed high-mode perturbations on the capsule surface that can grow during the implosion. To quantify the glint power on the capsule during the picket by the outer beams, we performed dedicated experiments on NIF using a flat witness foil as a surrogate for the capsule in a half-hohlraum target. We found that the measured glint power is approximately comparable to simulations using a nominal electron conduction flux limiter of f = 0.15, and over an order of magnitude lower than that predicted using f = 0.03 in the wall. Based on our current understanding, we conclude that the glint from the outer beams plays an insignificant role in capsule drive asymmetry.

Simulation of the reflection of a high energy laser beam at the sea surface for hazard and risk analyses

The application of a high energy laser beam in a maritime scenario necessitates a laser safety concept to prevent injury to personnel or uninvolved third parties from uncontrolled reflections of laser light from the sea surface. Therefore, it is crucial to have knowledge of the amount and direction of reflected laser energy, which varies statistically and depends largely on the dynamics of the wavy sea surface. These dynamics are primarily influenced by wind speed, wind direction, and fetch. An analytical model is presented for calculating the time-averaged spatial intensity distribution of the laser beam reflected at the dynamic sea surface. The model also identifies the hazard areas inside which laser intensities exceed a fixed exposure limit. Furthermore, as far as we know, our model is unique in its ability to calculate the probabilities of potentially eye-damaging glints for arbitrary observer positions, taking into account the slope statistics of gravity waves. This is a critical first step toward an extensive risk analysis. The simulation results are presented on a hemisphere of observer positions with fixed radii from the laser spot center. The advantage of the analytical model over our numeric (dynamic) model is its fast computation time. A comparison of the results of our new analytical model with those of the previous numerical model is presented.

Detection of rising damp and material changes on hyperboloid cooling tower shells based on the intensity of the reflected laser beam

Detection of rising damp and material changes on hyperboloid cooling tower shells based on the intensity of the reflected laser beam

Precision tailoring of laser beam propagation in a single crystalline diamond tool for in situ laser-assisted diamond turning.

The characteristics of laser beam propagation within a diamond tool critically influence the applied thermal softening capability of in situ laser-assisted diamond turning (In-LAT). In the present work, we perform optical geometric analysis, optical simulation and experimental validation to propose a novel diamond tool configuration for precisely tailoring laser beam propagation in In-LAT. First, the characteristics of laser beam propagation in the current In-LAT diamond tool are theoretically and experimentally explored. Second, according to the issues discovered in the current In-LAT diamond tool, an improved tool configuration based on the total internal reflection of a laser beam within the diamond tool is proposed, aiming for promoting refraction of the laser beam from the rake face of the diamond tool as well as eliminating the reflection of laser beam to tool holder. Finally, the optimization of laser beam incident position is carried out for achieving the superior profile and intensity of the emitted laser spot. Current work provides rational laser beam propagation for improving the thermal-softening capability of an In-LAT diamond tool.

Polarization-dependent, beam reflection-regulated hole shaping dynamics visualized by in-situ imaging of ultrafast laser drilling process

Flexible hole-shape control during ultrafast laser micro-drilling requires in-depth understanding of the processing dynamics, which remains a challenge. We use an in-situ imaging method to visualize the dynamic evolution of hole shape during the ultrafast laser drilling of metals. The direct visualization of the hole shaping dynamics shows that the laser beams are transmitted towards the hole bottom with polarization-dependent reflections on the hole wall. There exists a depth threshold from which the reflected laser beams start to dominate the hole shape under all conditions, and the depth threshold is jointly determined by the pulse energy and polarization of the laser, the trepanning radius, and the material. Due to the polarization-dependent beam reflections, the hole shape evolves uniformly under circularly polarized laser, whereas in the case of linearly polarized laser, two distinct shape evolution patterns are observed on the sections parallel and perpendicular to the polarization direction. These results provide fresh understanding on hole shaping dynamics, and the proposed in-situ imaging method is a powerful platform for the in-depth research of laser drilling process.

Unravelling keyhole instabilities and laser absorption dynamics during laser irradiation of Ti6Al4V: A high-fidelity thermo-fluidic study

A 3D multi-phase thermo-fluidic solver is implemented to investigate the laser-melt pool interaction dynamics and keyhole instabilities during stationary and moving laser irradiation. The physical phenomena such as phase changes, thermocapillary driven flow, multiple reflections of the Gaussian laser beam and beam trapping, vaporization recoil pressure, and heat losses (convective, radiative, and evaporative) have been incorporated in the model. A detailed parametric study has been carried out to investigate the influence of the laser beam spot diameter and the traversal speed on the laser absorption dynamics and melt pool characteristics. For both the stationary and moving laser irradiation, it is found that during the initial transient stage, the melt pool morphology belongs to the conduction melting mode with laser absorption according to the normal incidence condition of the Fresnel equation. Later, depending on the irradiating laser intensity, a melt transition occurs from conduction to the keyhole mode of melting, during which laser absorption rises sharply. The laser absorption and the vapor depression morphology in the keyhole regime show substantial oscillations (periodic and non-periodic) due to the complex hydrodynamics of the recoil pressure, the surface-tension dynamics, and convection-driven flow resulting in the opening and closing of the vapor depression. These oscillations are quantified, and a direct correlation of the laser absorptance with the keyhole depression depth has been observed for stationary laser irradiation. The keyhole closure mechanism and bubble trapping-ejection phenomena have been explored during the solidification stage. The numerically predicted melt pool dimensions match well with the in-house performed experimental measurements and published benchmarking experiments combining integrating sphere radiometry and in-situ X-ray radiography.

Rectangular Amplitude Mask-Based Auto-Focus Method with a Large Range and High Precision for a Micro-LED Wafer Defects Detection System.

Auto-focus technology plays an important role in the Micro-LED wafer defects detection system. How to accurately measure the defocus amount and the defocus direction of the Micro-LED wafer sample in a large linear range is one of the keys to realizing wafer defects detection. In this paper, a large range and high-precision auto-focus method based on a rectangular amplitude mask is proposed. A rectangular amplitude mask without a long edge is used to modulate the shape of the incident laser beams so that the spot shape distribution of the reflected laser beam on the sensor changes with the defocus amount of the wafer sample. By calculating the shape of the light spots, the defocus amount and the defocus direction can be obtained at the same time. The experimental results show that under the 20× microscopy objective, the linear range of the auto-focus system is 480 μm and the accuracy can reach 1 μm. It can be seen that the automatic focusing method proposed in this paper has the advantages of large linear range, high accuracy, and compact structure, which can meet the requirements of the Micro-LED wafer defects detection equipment.

Effect of using reflective target materials on enhancing UV nanosecond laser machining of sapphire

The machining of sapphire, a hard and transparent material, is of significant interest in various industries due to its exceptional properties. Traditional machining methods for sapphire suffer from drawbacks such as low efficiency and poor processing quality. Laser machining, particularly using ultraviolet (UV) nanosecond lasers, has emerged as a promising alternative for machining sapphire. In this article, we explored the use of reflective target materials to enhance laser energy absorption during the machining of sapphire. The effect of different reflective target materials, including aluminum and silicon, on the laser machining process was examined. Parametric calculations and finite-difference time-domain (FDTD) simulation were performed to evaluate the laser intensity at the top surface of sapphire when a reflective target material was employed. The results demonstrated that the use of reflective target materials leads to increased laser intensity due to the interference of incident and reflected laser beams, thus improving material removal efficiency. Moreover, experimental observations confirmed the effectiveness of using reflective target materials in enhancing the machined depth of sapphire. For example, the machined depth of 21.6 µm with a power density of 1.7 × 108 W/cm2 increased to 23.3 µm and 26.5 µm by using silicon and aluminum target materials. It is found that using aluminum target led to higher machined depth than silicon owing to its higher reflectivity.

Cost-effective measurement of magnetostriction in nanoscale thin films through an optical cantilever displacement method

A cost-effective method for the quantitative characterization of the magnetostrictive effect in thin films is presented. In this method, a sample's magnetostriction is extrapolated from the tip displacement of a thin-film magnetostrictive cantilever. The tip displacement is measured by monitoring the position of a reflected laser beam using two differentially coupled photodiode positioning sensors. In contrast with alternative optical deflection-angle devices designed for educational purposes, the detection limit of our setup resolves submicron-level displacements from nanoscale thin films. The efficacy of the system is demonstrated through measurements using amorphous 200-nm thick Terfenol-D/Si (100) bimorph cantilevers. In these measurements, magnetostriction values of 106 ± 3.5 ppm at ±4300 Oe applied field were attained, where the voltage noise floor was ±0.05 V (a cantilever displacement uncertainty of ±70 nm). In-plane (IP) and out-of-plane (OOP) magnetization curves and crystallographic x-ray diffraction (XRD) were performed to determine the magnetic behavior and confirm the amorphous nature of the films, respectively. The experimental methods and material characterization systems demonstrated here enhance the understanding of complex magnetic phenomena and introduce common measurement techniques to better equip students with the skills for insightful analysis of fundamental magnetic physics.

A coupled ray-tracing based CFD and cellular automaton model for predicting molten pool formation and microstructure evolution in narrow gap laser welding

Laser beam reflection plays a significant role in the temperature development history during various laser material processes. Different from other processes where laser beam reflection main happens inside the keyhole, in narrow gap laser welding (NGLW), the laser beam would interact with the sidewalls multiple times before existing the narrow groove, which would have a great impact on the defect formation as well as the grain structure development. However, the underlying mechanism of this process remains unknown in multi-pass NGLW. In this study, an integrated ray-tracing based CFD and cellular automaton (CA) modelling framework is established to investigate the impact of laser beam reflection on molten pool formation and microstructure evolution in multi-pass NGLW. From the CFD simulation, it is found that the overall laser beam absorption of NGLW is greater than laser metal deposition (LMD) while smaller than laser powder bed fusion (LPBF) process. Besides, the overall absorptivity in NGLW is increased 28% when considering the ray reflection during the first welding pass. The absorptivity would increase and then stabilise through the multi-pass welding process, which is attributed to the change in incident angle and number of reflections. Additionally, it is found that the undercut defect is more likely to occur with ray reflection due to the excessive temperature rise on the sidewalls, while lack of fusion is observed in the case without ray reflection. By incorporating the thermal history of each welding pass with CA, the impact of laser beam reflection on grain structure development is evaluated. Columnar grains would govern the microstructure evolution in the case with ray reflection, which grow epitaxially from the fusion line to the molten pool centre. Grain coarsening would occur around the lack of fusion region in the case without ray reflection. Moreover, the analysis on grain size distribution is conducted and a greater maximum grain size is obtained in the case without ray reflection as a result of lower cooling rate caused by lack of fusion. However, a higher grain size increase is obtained in the case with ray reflection during multiple welding passes, and larger grains are expected to form if more welding passes are to be conducted. The simulated molten pool morphology and grain structure are compared with the experimental observation, and a good agreement is achieved.

Effect of fibre orientation on the light scattering during laser transmission welding

During laser welding of thermoplastic (LTW) composites reinforced with short fibres, a divergence of the laser beam is observed due to internal refraction of the beam at each matrix–fibre interface. This phenomenon leads to scattering of the laser beam in this heterogeneous medium, resulting in reduction of energy reaching the weld interface. This work presents a numerical study of the effect of fibre orientation in fibre-reinforced composites during the laser transmission welding simulation. A three dimensional (3D) numerical structure is generated to take into account the real microstructure of a composite material reinforced with short fibres. The information of fibre volume fraction, fibre length distribution and fibre orientation distribution is extracted from injection moulding simulations. Fibre orientation distribution function is extracted from fibre orientation tensors. An algorithm is implemented to trace rays propagating through composite material at two scales: microscopic and macroscopic. This algorithm uses optical properties of the composite material to simulate laser beam reflection and refraction in a complex structure. Laser beam scatter at the weld interface is simulated in 3D geometries with various fibre orientation distributions. The effect of fibre orientation on the light scattering phenomenon of laser is compared for different cases of fibre orientation. The simulation results are validated experimentally.

Electromagnetic biaxial scanning mirror based on 3D printing and laser patterning

Scanning mirror, as an optical vector scanning actuator, which can control the deflection and reflection of laser beam, and plays an important role in laser scanning, laser imaging and laser radar. However, the current stage of micro-electro-mechanical systems (MEMS) micromirror suffers from high cost and long preparation cycle. In order to solve these problems, a novel and lost-cost electromagnetic biaxial scanning mirror is proposed based on 3D printing and laser patterning. The mechanical structure of the biaxial scanning mirror is made of acrylonitrile butadiene styrene (ABS) material. The copper foil accurately patterned by ultraviolet (UV) laser is used as the driving coil, and the annular neodymium magnet set provides the radial magnetic field. The experimental results show that the scanning mirror can achieve vertical scanning with an optical angle of 32.4° and horizontal scanning with an optical angle of 16.5° under the resonance excitation of 119 Hz and 250 Hz, respectively. The proposed biaxial scanning mirror has the advantages of low cost and rapid preparation, which has potential applications in intelligent household, regional monitoring, industrial cameras, etc. A novel and lost-cost electromagnetic biaxial scanning mirror is proposed based on 3D printing and laser patterning. The mechanical structure of the biaxial scanning mirror is made of acrylonitrile butadiene styrene (ABS) material. The copper foil accurately patterned by ultraviolet (UV) laser is used as the driving coil, and the annular neodymium magnet set provides the radial magnetic field. The scanning mirror can achieve vertical scanning with an optical angle of 32.4° and horizontal scanning with an optical angle of 16.5°, which has low cost, short preparation cycle, and has potential applications in intelligent household, regional monitoring, industrial cameras, and intelligent household appliances. • A novel biaxial electromagnetic scanning mirror based on 3D printing and laser patterning is proposed, which has the advantages of low cost and rapid preparation. • The mechanical structure of the biaxial scanning mirror is made of acrylonitrile butadiene styrene (ABS) material. The copper foil accurately patterned by ultraviolet (UV) laser is used as the driving coil. These make the scanning mirror have lower cost and faster preparation cycle. • The scanning mirror can achieve vertical scanning with an optical angle of 32.4° and horizontal scanning with an optical angle of 16.5°. Compared with similar polymer scanning mirrors, it has a larger angle.

A novel and cost-effective optical detection of high magnitude current and magnetic pulses through a metallic cantilever

We report a novel optical technique to measure short duration high magnitude current and magnetic pulses based on deflection of a macroscopic ferromagnetic cantilever. Deflection of the cantilever as a transducer takes place due to the attraction by an electromagnet. The reflected laser beam from a small thin mirror at tip of the cantilever is scanned over two spatially distinct photodetectors. Different high magnitude magnetic pulses are produced by discharging a capacitor bank through an inductive coil with a ferrite core. The response of the sensor is observed at different charging limits of the capacitor bank and spatial intervals between cantilever and inductive coil. A repeatable and linear response is detected by the devised sensors in the range 158.53–380.47 A current and 0.19–0.48 T magnetic field with sensitivity of 39.15 A kV−1 and 50.98 mT kV−1 for current and magnetic field amplitudes respectively in response to 2.5–6.0 kV charging of the capacitor bank. The proposed technique is remote, nondestructive, cost-effective and has a large dynamic range.

Simultaneous formation of two-fold rotation symmetry with charge order in the kagome superconductor CsV3Sb5 by optical polarization rotation measurement

We report the observation of two-fold rotation symmetry in charge density wave (CDW) state in the newly discovered Kagome superconductor CsV$_3$Sb$_5$. Below its CDW transition temperature ($T_{CDW}$), the polarization rotation of the reflected laser beam promptly emerges and increases close to about 1 mrad, and the rotation angle shows two-fold rotation symmetry. With femtosecond laser pulse pumping, the rotation angle can be easily suppressed and then recovers in several picoseconds accompanied with coherent oscillations. Significantly, the oscillations in the signal also experience a 180 degree periodic change. Our investigation provides clear optical evidence for the formation of nematic order with two-fold rotation symmetry just below $T_{CDW}$. The results imply a immediate development of nematicity and possible time-reversal symmetry breaking in CDW state of CsV$_3$Sb$_5$.

Numerical and experimental study of thermal fluid flow and keyhole dynamic in laser welding of aluminum alloy assisted by electromagnetic field

A steady electromagnetic field with high magnetic flux density was designed to improve weld appearance and suppress porosity defect of aluminum alloy laser welded joint. A comprehensive numerical model considering the multiple reflection of laser beam, the magnetohydrodynamics (MHD), and the vapor shear stress on free surface of keyhole was established to reveal the effect of electromagnetic field on thermal fluid flow and keyhole dynamic. A sufficient Lorentz force (∼1.255 × 106 N/m3) is induced when magnetic flux density reaches to 500 mT. The fluids flow downward, forward, and upward in the upper, middle, and lower region are inhibited by the Lorentz force, respectively. Therefore, less heat exists in the bottom and more heat accumulates in the middle edge of the weld pool, leading to the smaller weld depth, larger length at half depth, and disappeared necking when an electromagnetic field is used. Moreover, the impact of vortex on the rear keyhole wall is weakened by the Lorentz force, which decreases the oscillation and breakup of keyhole. This work can help improve the understanding concerning influence of the electromagnetic field on thermal fluid flow behavior in laser welding and provide guidelines for the suppression of porosity and splash defects.

Polarization Image Laser Line Extraction Methods for Reflective Metal Surfaces

In this work, we propose a novel pipeline method for laser line extraction from images with a polarization image sensor. The proposed method is specifically developed for strong laser beam reflections from metal surfaces. For the preprocessing stage, we propose a demosaicing algorithm for color polarizer filter array (CPFA) sensors. This can be implemented by using either one-quarter or full resolution of the sensor. In addition, we propose two methods for optimizing the information available in a 12-channel color polarization image. The first method is based on the minimum linearly polarized irradiance, and the second method is based on the linear polarization intensity. These preprocessing and optimization methods are combined with laser line extraction methods. The laser line extraction is done with either the polarized finite impulse response (FIR) center of gravity (COG), where the laser line coordinates are computed from the filtered laser intensity distribution, or with the polarized FIR-Peak, where the laser line coordinates are calculated from the first derivative of the filtered laser signal. The performance of the proposed algorithms is studied experimentally using a laser line scanner assembly, made of a polarization camera, and a laser line projector operating in the blue wavelength range.

Specular reflections (“glint”) of the inner beams in a gas-filled cylindrical hohlraum

We report on the experimental measurement of specular reflection (“glint”) of laser beams off the hohlraum wall in inertial confinement fusion experiments at the National Ignition Facility. In a hohlraum, glinted light can escape the opposite laser entrance hole of the hohlraum and is a potential laser energy loss mechanism. The total measured glint on the inner cones of beams is measured to be less than 8 TW (when using the full National Ignition Facility laser), which is <2% of incident peak power. The simulated x-ray flux exceeds the measurement by 10%–20%, and glinted laser light is unable to account for this discrepancy. Similar inner beam glint was measured for ρ = 0.3 and 0.6 mg/cc gas fill hohlraums, but no glint was detected for 1.2 mg/cc densities. Inner beam glint is dominated by the lowest angle 21.5 beams within a 23.5 quad, and it is at most 30% sensitive to different quad polarization arrangements.

Laser additive manufacturing of a 316L/CuSn10 multimaterial coaxial nozzle to alleviate spattering adhesion and burning effect in directed energy deposition

Multimaterial additive manufacturing is aimed at the integration of functionality and mechanical performance of dissimilar materials for various applications. In this work, a 316L/CuSn10 multimaterial coaxial nozzle was designed and additively manufactured by laser powder bed fusion (LPBF) for alleviating spattering adhesion and burning effect in directed energy deposition. The applicability of the 316L/CuSn10 nozzle was then validated in the laser-based directed energy deposition (L-DED) process. The interfacial diffusion zone of the 316L/CuSn10 structure was measured with a length of ~400 μm, indicating a well bonding between 316L and CuSn10. A high laser power (P = 230 W) and a low scanning speed (v = 300 mm/s) adopted for printing CuSn10 on 316L could lead to “keyhole mode”, resulting in a large penetration depth of the pre-solidified layer and wide diffusion zone at the interface. The spattering behavior of the 316L/CuSn10 nozzle applied in the L-DED process showed that the heterogeneity of dissimilar materials could reduce the adverse effect of the adhesion of spattering droplets. High laser reflection and high thermal conductivity of CuSn10 could relieve the burning effect of the reflected laser beam on the tip of the 316L/CuSn10 nozzle. Therefore, the utilization of CuSn10 on the nozzle tip could improve its service life. These findings may serve as a new way for low-cost and rapid mass production of the coaxial nozzles that can be applied in the L-DED technique.