Intensity Distribution Of Laser Beam Research Articles

Influence of the laser-beam intensity distribution on the performance of directed energy deposition of an axially fed metal powder

This paper investigates the influence of laser-beam intensity distribution (LBID) on the performance of the annular laser-beam directed energy deposition (DED) process with axial powder delivery. Three different LBIDs: Gaussian-like (G-LBID), top-hat-like (TH-LBID) and ring (R-LBID) at two LBID diameters were used. The process performance was characterised qualitatively in terms of the melt-pool shape and the process stability and quantitatively by powder-catchment efficiency, selected geometrical and metallurgical properties of the clad. The observed influence of LBID on process performance, as determined by the relationship between LBID and powder stream density distribution (PSDD), decreased with increasing mean surface-energy density and was more significant at larger LBID diameter. The highest powder-catchment efficiencies (90% and 87%) were achieved with the G-LBID and TH-LBID, whose high-intensity centre is aligned with the peak of the Gaussian-like PSDD. A lower powder-catchment efficiency of 77% was achieved with the R-LBID, whose high-intensity region is located at the edge of the melt pool with minimum powder density. However, this also results in the highest and most uniform dilution, the highest metallurgical bond ratio and the lowest lack of fusion porosity at the clad-substrate interface. In addition, the process was stable at the lower values of mean surface-energy density with R-LBID, while balling instability was observed with G-LBID and TH-LBID. It can be concluded that the use of R-LBID at lower values of mean surface-energy density improves the performance of the DED process with axial powder feed in terms of process stability and metallurgical properties of the clad.

Cutting thick aluminum plates using laser fusion cutting enhanced by dynamic beam shaping

Cutting thick plates is affected not only by the laser power but also by the cut kerf width and the melt flow dynamics that determine the ejection of the molten material. Employing the same laser beam intensity distribution for various thicknesses is the limiting factor when cutting thicker plates. This paper investigates fiber laser fusion cutting of 25 mm thick aluminum with dynamic beam shaping (DBS). While both static and longitudinal dynamic intensity distributions fail to cut this thickness with a 4 kW laser power, a cut through is achieved using annular and elliptical intensity distributions. However, an improvement of 45% in cutting speed can be achieved using an elliptical intensity distribution compared to an annular one. In order to understand the effect of the beam shape, an infrared thermal camera is used to study lateral heat propagation when using different process parameters. Moreover, to analyze the melt flow when changing the DBS frequency, high-speed imaging is utilized to observe the molten material inside the cut kerf. Finally, the cut edge quality is investigated for different cutting conditions.

Fiber laser cutting of steel plate by twin spot beam setting in scanning direction

This study investigated the effects of laser beam intensity distribution on the reduction of dross height in fiber laser cutting of a steel plate with 3.2 mm thickness. A twin-spot beam was produced by splitting a single Gaussian beam into two beams using a special axicon lens, and these beams were set in the scanning direction for cutting experiments. The power ratio of two beams (R:F = Rear power:Front power) was varied to discuss the intensity balance for the effective reduction of dross. After cutting experiments, ray tracing analysis was conducted using an optical analysis to calculate the absorbed power density distributions in the kerf. A smaller dross height of 18 μm can be achieved at a power ratio of R:F = 8:2, and its value is lower than that by a single Gaussian beam. At a power ratio of R:F = 8:2, the front beam of lower power is irradiated at the upper part of the workpiece, and the rear beam of higher power is absorbed at the lower part of the workpiece. Thus, effective heat input to the lower part of the workpiece can contribute to a reduction of the dross height. Variation of power ratio in the rear and the front beams is effective in controlling the cutting front shape, and the uniformity of absorbed power in the thickness direction can be improved by setting the rear beam of about four times higher power to the front beam of lower power to obtain a smaller dross height in the case of a 3.2 mm steel plate.

A Simulation of Air Lasing Seeded by an External Wave in a Femtosecond Laser Filament.

Air lasers induced by femtosecond laser filaments play an important role in remote sensing applications. Few studies have been dedicated to the spatial distribution of external-seeded air laser radiation in the laser filament based on the numerical simulation method, which can pave the way to understanding the mechanism of the external-seeded air lasing process during filamentation. In this study, numerical simulations of the propagation of an air laser seeded by an external plane wave with a wavelength of 391 nm during femtosecond laser filamentation were performed. The results indicated that the air laser's beam intensity distribution varies from a ring pattern to a donut pattern when the filament length and nitrogen ion density are raised as a result of the defocusing and lasing effects of the filament plasma. Here, the ring pattern is formed by several thin rings, while the donut pattern refers to a notably thicker, ring-like structure. In addition, it has been demonstrated that the air laser's beam power would increase exponentially versus the filament length and the nitrogen ion density. The knowledge about the angular distribution of air lasing could be important for optimizing the detection geometry of the LIDAR setup, including the view angle and the size of the collecting optical component.

Long-term repeatability improvement using beam intensity distribution for laser-induced breakdown spectroscopy

The relatively low measurement repeatability has long been considered as a major obstacle to the widespread use and commercialization of laser-induced breakdown spectroscopy (LIBS). Although many efforts have been made to improve the signal repeatability in the short term, how to improve the long-term signal repeatability is critical in practical applications and has rarely been studied. Moreover, the mechanisms behind the degradation of long-term repeatability are not fully revealed. This study proposes a new method to improve the long-term repeatability of LIBS measurement, which modifies the spectral intensity based on laser beam intensity distribution. It first pre-processes the beam intensity distribution profiles and spectral intensity. Then the relationship between the relative deviations of beam and spectral intensities is modelled using Partial Least Squares Regression (PLSR). The proposed method was tested on copper and silicon samples, and the spectra and laser beam intensity distribution were recorded for more than thirty days. Day-to-day variations in beam intensity distribution were observed. Such variations can lead to changes in spectral intensity, resulting in degraded signal repeatability. By modifying the spectral intensity, the long-term signal repeatability was improved. Specifically, in terms of day-mean spectral intensity, the valid correction rates were above 70% for both of copper silicon sample in most cases. Long-term RSD decreased from ∼13.5% to ∼4% for copper and decreased from ∼10.7% to 6.5% for silicon sample. These results indicate that the proposed method provides a viable method for improving the long-term repeatability of LIBS measurement.

Melt flow and cutting front evolution during laser cutting with dynamic beam shaping

A dramatic increase in the industrial demands for tailoring the laser beam profile in laser cutting has led to significant performance improvements thanks to the recently emerging dynamic beam shaping technology. Nonetheless, the underlying reasons for such process improvements are not fully understood yet, and mainly theoretical assumptions are available. This paper uses several high-speed videography strategies to evaluate and compare the melt flow dynamics and cutting front geometry during 4 kW fiber laser cutting of 15 mm plates with static and oscillating beams. The absorption behavior on the cutting front is analyzed through trim-cut observation, and a substantial difference of around 40% is observed between these two laser beam intensity distributions. Moreover, a distinctive molten material ejection mechanism is found during linear beam oscillation, being beneficial for fusion cutting and detrimental for flame cutting. Finally, based on the experimental results, a former explanatory model from the literature is enhanced and further ways to optimize the beam profile during laser cutting are proposed.

Utilizing a Diffractive Focus Beam Shaper to Enhance Pattern Uniformity and Process Throughput during Direct Laser Interference Patterning.

Uniform periodic microstructure formation over large areas is generally challenging in Direct Laser Interference Patterning (DLIP) due to the Gaussian laser beam intensity distribution inherent to most commercial laser sources. In this work, a diffractive fundamental beam-mode shaper (FBS) element is implemented in a four-beam DLIP optical setup to generate a square-shaped top-hat intensity distribution in the interference volume. The interference patterns produced by a standard configuration and the developed setup are measured and compared. In particular, the impact of both laser intensity distributions on process throughput as well as fill-factor is investigated by measuring the resulting microstructure height with height error over the structured surface. It is demonstrated that by utilizing top-hat-shaped interference patterns, it is possible to produce on average 44.8% deeper structures with up to 60% higher homogeneity at the same throughput. Moreover, the presented approach allows the production of microstructures with comparable height and homogeneity compared to the Gaussian intensity distribution with increased throughput of 53%.

Experimental and numerical investigation of thick plate laser cutting using dynamic beam shaping

Further improvement of quality and productivity are some of the most relevant laser cutting process challenges. One way of affecting the process performance is manipulating the laser beam intensity distribution. Within this work, the feasibility of enhancing the cutting speed and edge quality using dynamic beam shaping in 4 kW fiber laser cutting of 15 mm stainless steel is investigated. Moreover, in order to evaluate the effect of dynamic beam shaping parameters on the process zone, a simplified thermal simulation model is introduced. As a result, the effect of longitudinal beam oscillation on the cutting efficiency is studied experimentally and explained using numerical calculation. The obtained results have shown a significant improvement for laser fusion cutting compared with a static top-hat intensity distribution. In particular, the cutting speed can be increased up to 45 % with considerably better quality. Finally, further steps to optimize the dynamic beam shaping technology for laser cutting applications are proposed.

Defining speckle-based spectral metric from a linear array detector for characterizing the laser focusing spot

Speckle modulation in the laser beam projection scenarios such as adaptive optics, remote sensing, and active imaging has been a long-standing challenge that causes intensity fluctuation and further degrades the performance of the system. Speckle statistical characteristics coupled with the feature of the laser beam intensity distribution can be obtained by analyzing the speckle variation through a single point detector or an image sensor. A single point detector has the advantage in high sampling rate but may lose speckle spatial information, while the image sensor has a high spatial resolution but slow sampling rate. In this paper, we defined a dynamic speckle metric based on a linear array detector to estimate the encircled energy of the far-field spot on the diffuse target, which well balanced the trade-off between sampling rate and spatial resolution. The potential of the metric is analyzed using a physics-based speckle simulation and experimental verification. Both results agree with each other, showing that the metrics based on the speckle-field spatiotemporal spectrum analysis are monotonically dependent on the target focused-spot size.

The effect of laser beam intensity distribution on weld characteristics in laser welded aluminum alloy (AA5052)

The distribution of laser beam intensity can play a key role in laser welding, since it is deeply related to the temperature gradient and cooling rate of the melt pool. In this study, aluminum alloy butt welding was conducted using a normal Gaussian beam and modulated beams with various intensity profiles, produced with a diffractive optical element (DOE). Their effect on the weld characteristics was investigated. Welding with the modulated beams generally required larger heat input for full penetration compared with the Gaussian beam welding (GBW) since laser energy was spread by the core and ring beam combination. This laser energy spreading reduced weld defects and produced a funnel-shaped weld cross-section in the modulated beam welding (MBW). Microstructure analyses found that the GBW and MBW had different grain growth behaviors based on the shape of the melt zone boundary. A straight melt zone boundary formed along the vertical direction induced columnar grain growth along the horizontal direction in the GBW. For the MBW, it grew along the diagonal direction, because the melt zone boundary had a slanted shape. The highest tensile strength (208.2 MPa, which corresponded to ~ 91% of the tensile strength of the base material) was obtained with the MBW, owing to reduced average grain size. It was found that the re-crystallization process induced by laser welding significantly increased the grain size of the original material.

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

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

МАТЕМАТИЧЕСКОЕ МОДЕЛИРОВАНИЕ РАСПРОСТРАНЕНИЯ ПУЧКОВ ЛАЗЕРНОГО ИЗЛУЧЕНИЯ В НЕРАВНОВЕСНОЙ ДВИЖУЩЕЙСЯ ОБЛУЧАЕМОЙ НЕЙТРОНАМИ ПЫЛЕВОЙ ПЛАЗМЕ, СОДЕРЖАЩЕЙ НАНОЧАСТИЦЫ УРАНА

The use of uranium nanoparticles dispersed in a laser-active medium instead of traditionally used methods of heterogeneous nuclear pumping of active gas medium can increase the fraction of energy carried by fission fragments from the condensed phase into the gas medium by more than an order of magnitude. This creates the conditions for increasing the efficiency of nuclear into optical radiation energy conversion. However, the scattering and absorption of laser radiation by an active medium containing uranium nanoparticles are important factors preventing the laser radiation generation in such a medium. Mathematical modeling of the generation and amplification of laser radiation in a moving spatially inhomogeneous nuclear-excited neutron-irradiated argon-xenon medium containing uranium nanoparticles injected into a cylindrical laser-active dusty element, taking into account optical inhomogeneities caused by the dust component. The process of amplification and intensity distribution of a master oscillator laser beam in a moving spatially inhomogeneous nuclear-excited argon-xenon neutron-irradiated medium containing uranium nanoparticles has been studied for the first time. Data were also obtained on the propagation of a light beam near a relatively sharp dust component concentration decrease in moving gas medium. As a brief result: moving spatially inhomogeneous nuclear-excited neutron-irradiated argon-xenon medium containing uranium nanoparticles does not have a pronounced destructive effect on the propagation of laser radiation in it, on condition that the light flux is not wider than the dust component.

Morphological characteristics of nanoholes induced by single-shot femtosecond laser ablation of borates and aluminate silicates

Single-shot femtosecond laser ablation experiments with linearly and circularly polarized light were conducted in order to investigate the morphological characteristics of surface nanostructures in lithium borate crystals and glasses, a strontium borate crystal, lanthanide containing borate crystals, and aluminate silicate crystals: Li2B4O7 (LTB) and LiB3O5 (LBO) crystals and Li2O⋅2B2O3 (LTB) and Li2O⋅3B2O3 (LBO) glasses; SrB4O7 (SBO) crystal; Li6Gd(BO3)3 (LGB) and LaSc3(BO3)4 (LSB) crystals; and Ca2Al2SiO7 (CAS) and CaSrAl2SiO7 (CSAS) crystals. In the present study, the material and laser polarization dependance of the morphology of nanoholes was examined in these crystals and glasses. A single nanohole or two holes (a primary hole and a secondary hole) were observed in the borate and aluminate silicate crystals. The size of the nanohole is not restricted by the diffraction limit but instead is dependent on the laser fluence and the materials. It is suggested that the formation of these secondary nanoholes in the studied crystals is attributed to a spontaneous reshaping of the incoming Gaussian pulse into a Gaussian–Bessel pulse. In the LTB and LBO crystals, nanoholes (both primary and secondary holes) with subwavelength sides exhibit a quadrilateral (approximately square or rectangular) morphology, regardless of linear or circular polarization. The sides of the quadrilateral nanoholes lie approximately in the {h h 0} planes on the LTB crystal and in the ({h 0 0} and {0 0 l}) planes on the LBO crystal. We found that the nanohole morphology did not reflect the spatial distribution of the laser intensity. These phenomena were the first observations on the anisotropic morphology of nanoholes. These morphologies do not correspond to the circular symmetric pattern of the Gaussian intensity distribution of the incoming laser beam. This is contrary to the expectations based on the generally accepted laser ablation mechanism. The quadrilateral nanoholes could be an inherent morphology in the LTB and LBO crystals. The morphology of the quadrilateral holes in the LTB and LBO crystals is considered to reflect the continuous BO33− and/or BO45− network structure in their respective tetragonal or orthorhombic unit cells, in which self-tapped excitons are formed in an initial process under multiphoton excitation. In contrast, the SBO, LGB, LSB, CAS, and CSAS crystals and the LTB and LBO glasses exhibit circular nanoholes with subwavelength diameters independent of the laser polarization, the structure, or the composition. The isotropic morphology of nanoholes in these samples reflects the circular pattern of the Gaussian intensity profile of the focused laser beam.

Investigation of 3 dB Optical Intensity Spot Radius of Laser Beam under Scattering Underwater Channel.

An important step in the design of receiver aperture and optimal spacing of the diversity scheme for an underwater laser communication system is to accurately characterize the two-dimensional (2D) spatial distribution of laser beam intensity. In this paper, the 2D optical intensity distribution and 3 dB optical intensity spot radius (OISR) are investigated due to the dominating optical intensity of laser beam being within the 3 dB OISR. By utilizing the Henyey–Greenstein function to compute the scattering angles of photons, the effects of the scattering underwater optical channel and optical system parameters on 3 dB OISR are examined based on the Monte Carlo simulation method. We have shown for the first time that in the channel with a high density of scattering particles, the divergence angle of the laser source plays a negligible role in 3 dB OISR. This is an interesting phenomenon and important for optical communication as this clearly shows that the geometric loss is no longer important for the design of receiver aperture and optimal spacing of the diversity scheme for the underwater laser communication system in the highly scattering channel.

Comprehensive modeling of the lithographic errors in laser direct write

Patterns written by laser direct write have a critical dimension (CD) bias dependence on the dose similar to other direct write methods, such as electron beam lithography, which can be explained by the exposure intensity distribution (EID) of the laser beam. In this study, a comprehensive model based on the EID is proposed to understand the pattern CD bias dependence on the dose, which is known as the exposure latitude. This model was supported by the results of the exposure tests on MicroChem S1800 resist on an Si wafer using the Heidelberg DWL66+ laser writer. The exposure latitudes of the patterns at both micrometer and submicrometer scale were measured. At the micrometer scale, the exposure latitudes were found to have no measurable dependence on the pattern size and the local pattern density. This conclusion does not hold at the submicrometer scale where the length scale is comparable to the width of the laser beam. This study proposes a way to convert the experimental exposure latitude data into the EID functions and fit these functions by assuming a Gaussian laser beam intensity distribution. All empirical observations are found to agree with the predictions made by the EID model. The authors also show that this model can help achieve dimensional accuracy, especially when there is a change in the exposure process. Moreover, it can be used to analyze and minimize the CD inconsistency between different runs found in the experiments. To reduce such inconsistency, overexposing combined with a negative bias applied to the pattern is suggested.

Thermal-stress distribution and damage characteristics of three-junction GaAs solar cell irradiated by continuous laser beam

According to the theory of Fourier heat transfer and thermal stress field, a model on thermal damage of three-junction GaAs solar cell is established. The temperature and thermal-stress distribution inside the solar cell, which irradiated by continuous laser, are calculated. Results show that the temperature and thermal-stress distribution are corresponded to the intensity distribution of the incident laser beam. The maximum values of the compressive and tensile stress increase with the increasing of the power density and waist radius of the incident laser. When the waist radius is 0.5 cm, the melting damage is the main damage pattern as the power density is below about 40 W/cm2. However, when the laser power density is higher the stress damage will change to the main damage form. As the waist radius increases, the change in damage processes will also occur at lower laser power density.

In situ real-time beam monitoring with dielectric meta-holograms.

A novel approach for performing in situ and real-time beam monitoring, based on dielectric meta-hologram, is proposed and demonstrated. The ultrathin dielectric meta-hologram projects a portion of the beam power onto a screen to provide a visual indicator of the spatial intensity distribution of a Gaussian laser beam, as well as its waist position along the optical axis. Specifically, we demonstrate simple monitoring of the spot size, astigmatism, lateral position, and position along the optical axis of the beam. Good agreement is found with both theory and conventional knife-edge beam profiler measurements. This in situ beam monitoring approach could provide a highly useful tool for numerous optical applications.

Customized laser beam intensity distribution for the laser surface treatment of geometrically convoluted components

The Laser Surface Hardening Process of real components must achieve a uniform hardened profile to get constant mechanical properties and must deal with the energy distribution in the proximities of geometrical singularities to prevent them from overheating. Classical approaches like the overlapped laser tracks or uniform beams by fixed optical arrangements introduce the problems of annealing and lack dynamic adaptability capability, respectively. The present work uses a galvanometric scanner to get a user-defined laser intensity distribution, by iteratively scan a flat pattern, capable of creating uniform hardened profiles and dynamically adapting in real time to handle geometrical singularities. The experimental setup to obtain it and its characteristics as a function of the scanning frequency of the flat pattern are presented. Experimental results were obtained in crankshafts of automotive engines achieving uniform hardened profiles and good surface finish around the geometrical singularities.

Calibration and testing of a Raman hyperspectral imaging system to reveal powdered food adulteration.

The potential adulteration of foodstuffs has led to increasing concern regarding food safety and security, in particular for powdered food products where cheap ground materials or hazardous chemicals can be added to increase the quantity of powder or to obtain the desired aesthetic quality. Due to the resulting potential health threat to consumers, the development of a fast, label-free, and non-invasive technique for the detection of adulteration over a wide range of food products is necessary. We therefore report the development of a rapid Raman hyperspectral imaging technique for the detection of food adulteration and for authenticity analysis. The Raman hyperspectral imaging system comprises of a custom designed laser illumination system, sensing module, and a software interface. Laser illumination system generates a 785 nm laser line of high power, and the Gaussian like intensity distribution of laser beam is shaped by incorporating an engineered diffuser. The sensing module utilize Rayleigh filters, imaging spectrometer, and detector for collection of the Raman scattering signals along the laser line. A custom-built software to acquire Raman hyperspectral images which also facilitate the real time visualization of Raman chemical images of scanned samples. The developed system was employed for the simultaneous detection of Sudan dye and Congo red dye adulteration in paprika powder, and benzoyl peroxide and alloxan monohydrate adulteration in wheat flour at six different concentrations (w/w) from 0.05 to 1%. The collected Raman imaging data of the adulterated samples were analyzed to visualize and detect the adulterant concentrations by generating a binary image for each individual adulterant material. The results obtained based on the Raman chemical images of adulterants showed a strong correlation (R>0.98) between added and pixel based calculated concentration of adulterant materials. This developed Raman imaging system thus, can be considered as a powerful analytical technique for the quality and authenticity analysis of food products.

Study of an annular laser beam based axially-fed powder cladding process

An annular laser beam based powder cladding head, which enables an axial powder feeding and variation of the laser beam intensity distribution (LBID) on the workpiece surface is presented. The influence of typical LBIDs, including Ring, Tophat(−), Tophat(+), and Gaussian-like, on a cladding process has been characterized based on the process and melt pool visualization, powder catchment efficiency, clad layer geometry, and porosity. The results showed that the most stable process without plasma formation but with low dilution and porosity of the clad layer can be achieved within the range from a Ring to a Tophat(−) LBID. Additionally, axial powder feeding results in a high powder catchment efficiency above 80%.