Energy Distribution Of Laser Beam Research Articles

Analysis of atmospheric turbulence and vibration impact on the performance of satellite–ground laser communication beam

AbstractWith satellite laser communication, advantages can be achieved in high communication rate, precise beam pointing, high confidentiality, and light‐small terminal. Satellite–ground (SG) laser links also play a crucial role as trunk links. However, the SG link is influenced by atmospheric turbulence and platform vibration, these two factors will cause unstable link tracking. This paper focuses on the viewpoint of beam energy transmission, using the low‐frequency harmonic phase compensating method to establish the random phase screen. On the basis of this model, the laser beam energy distribution and the spot mass positioning error are analyzed considering turbulence taking vibration which is not considered in previous studies. The simulation results show that the spot mass center (SMC) is impressionable in strong turbulence under platform vibration, but in a weak turbulent environment, SMC is insensitive to vibration when turbulence and vibration are at the same noise level.

Windowed Fourier transform and cross-correlation algorithms for molecular tagging velocimetry

Simulated and experimental molecular tagging velocimetry (MTV) images have been analyzed with a technique commonly used to process grid images on surfaces, the windowed Fourier transform with local spectrum analysis (WFT-LSA). A systematic synthetic image study of the modulation transfer function (MTF) and error tendencies of the WFT-LSA was performed and compared with a PIV-style cross-correlation algorithm to see if advanced strategies such as iterative image deformation can improve analysis of gridded images with high noise levels. Testing of single-pass algorithms showed that in typical MTV images, the WFT-LSA yields significantly lower bias errors than cross-correlation (CC) at displacements greater than 1 pixel but slightly higher random error at all displacements and image conditions. Analysis of the MTF shows that CC provided better resolution of spatial fluctuations than the WFT-LSA in many combinations of grid size and interrogation window. Tests of image deformation algorithms showed that the gap in performance between CC and WFT-LSA is maintained even as both methods improve. Additionally, WFT-LSA and CC methods are applied to real MTV experiments in high-speed gas jet flows. A preliminary analysis of phosphorescence lifetime provided by acetone vapor excited at 266 nm is used for assessing the required gas speeds for making MTV application feasible. The application of WFT-LSA to real MTV images demonstrates the ability of the algorithm to handle further real-world effects that could not be considered in the synthetic image analysis, like reduced signal-to-noise ratio and non-uniform intensity of the tagging grid across the image introduced by the actual laser beam energy distribution. With experimental images, CC is more accurate with shear flows but less robust to high noise levels than WFT-LSA, as predicted by the synthetic image analysis.

How the laser beam energy distribution effect on laser surface transformation hardening process; Diode and Nd:YAG lasers

The laser surface transformation hardening (LSTH) of AISI 4130 steel by 1600 W high power diode laser (HPDL) and 700 W Nd:YAG lasers were investigated in this present study. The distribution influence, and the lasers beam shape; Top-hat in the HPDL and Gaussian distribution in the Nd:YAG laser, have been studied on the geometrical dimensions, and micro-hardness in hardened area (i.e.; depth, width, and angle of entry of hardened profile), micro-hardness deviation (MHD) from the row steel in geometric dimensions, and the ferrite’s percentage in hardened layer center. Microstructure evaluation of the laser hardened areas were performed by FE-SEM and optical microscopy. Based on the results, maximum hardness was created with the HPDL, and the geometrical dimension was more than the Nd:YAG laser. Also, MHD, and minor phase of ferrite in the HPDL laser surface hardening than the hardened layer in Nd:YAG laser, which is related to the higher laser absorption. Results show that, the hardened zone of HPDL is about 698 HV0.1 with 1.02 mm depth, while for Nd:YAG laser is about 698 HV0.1 with 0.98 mm depth. Comparing the results with the furnace hardening heat treatment (FHT) demonstrated that the hardness in diode laser and Nd:YAG laser hardening are 1.38 and 1.22 times of the hardness in FHT, respectively.

Adjusting the catalytic properties of cobalt ferrite nanoparticles by pulsed laser fragmentation in water with defined energy dose

Highly active, structurally disordered CoFe2O4/CoO electrocatalysts are synthesized by pulsed laser fragmentation in liquid (PLFL) of a commercial CoFe2O4 powder dispersed in water. A partial transformation of the CoFe2O4 educt to CoO is observed and proposed to be a thermal decomposition process induced by the picosecond pulsed laser irradiation. The overpotential in the OER in aqueous alkaline media at 10 mA cm−2 is reduced by 23% compared to the educt down to 0.32 V with a Tafel slope of 71 mV dec−1. Importantly, the catalytic activity is systematically adjustable by the number of PLFL treatment cycles. The occurrence of thermal melting and decomposition during one PLFL cycle is verified by modelling the laser beam energy distribution within the irradiated colloid volume and comparing the by single particles absorbed part to threshold energies. Thermal decomposition leads to a massive reduction in particle size and crystal transformations towards crystalline CoO and amorphous CoFe2O4. Subsequently, thermal melting forms multi-phase spherical and network-like particles. Additionally, Fe-based layered double hydroxides at higher process cycle repetitions emerge as a byproduct. The results show that PLFL is a promising method that allows modification of the structural order in oxides and thus access to catalytically interesting materials.

万瓦级高功率激光焊接时金属蒸气与熔池耦合行为研究现状

高功率激光焊接技术是实现厚板连接的先进技术，在现代装备制造中具有广阔的应用前景。本文分别从试验研究和理论仿真研究综述了高功率激光深熔焊接厚板过程中激光与材料的相互作用特征、蒸气与熔池相互作用规律和机理。在高功率激光深熔焊接时，由于各种不稳定因素包括焊接参数的变化，引起焊接小孔内激光能量分布的变化，从而改变局部材料的气化率，使得小孔内气流状态发生改变，蒸气流体与熔池流体之间的力平衡关系发生改变，促使孔壁和熔池的波动。实现良好焊接效果和焊接质量的前提是必须保证焊接蒸气和熔池流动状态的稳定性，控制焊接小孔的动态平衡，从而实现稳定的焊接过程。 High power laser welding technology is an advanced technology to realize thick plate connection, and has broad application prospects in modern equipment manufacturing. In this paper, the in-teraction characteristics of laser and materials, the interaction law between vapor and molten pool and its mechanism during high power laser deep penetration welding of thick plate were summarized from experimental research and theoretical simulation. In high power laser welding, due to various unstable factors including the change of welding parameters, the energy distribution of laser beam was changed inside the keyhole, thus the local material gasification rate was changed. Then the flow state inside the keyhole was changed from equilibrium between the vapor fluid and the molten pool to the unequilibrium. Thus, the keyhole wall and the molten pool became to fluctuate. The premise of achieving good welding quality is to ensure the stability of the vapor flow and molten pool state, and control the dynamic balance of the welding hole.

Laboratory setup for fiber Bragg gratings inscription based on Talbot interferometer

An experimental setup for the fiber Bragg gratings (FBGs) inscription by use of Talbot interferometer has been described in this paper. A KrF excimer laser system Master Oscillator–Power Amplifier CL-7550 (Optosystems Ltd, Russia) was used as the UV radiation source in the experimental setup. In order to control laser beam quality, thus to provide FBGs effective writing, the laboratory setup includes: spectral width control system, based on Fabry–Perot interferometer; laser beam energy distribution control system; monitoring system of laser pulse energy density on the optical fiber; and control system of optical fiber to the laser beam relative position. FBGs of type I inscription results in a single-pulse and multi-pulse modes are presented.

Experimental Approbation of Selective Laser Melting of Powders by the Use of Non-Gaussian Power Density Distributions

Experimental results on laser beam modulation at selective laser melting (SLM) are presented. The modulation is a possible way to improve the efficiency of the SLM process. The optical diagnostics shows the energy loss up to 30%. This could be a consequence of high thermal gradients in the melt pool resulted by the Gaussian power density distribution. The Gaussian distribution can be changed to the flat-top one or to the inverse-Gaussian (donut) one. An experimental stand with a 200W laser source was developed. Twenty single tracks for each of the three modes were obtained for a Co-Cr alloy. The samples were studied by scanning electronic microscopy (SEM) on irregularity. Optical diagnostics by high velocity camera (HVC) shows that the use of the non-Gaussian laser beam distributions can significantly reduce the width of the free-of-powder consolidation zone, which is considered as the main reason for irregularity of single tracks. A better metallurgical bonding of the single tracks with the substrate was obtained by the use of the flat-top laser beam. Both of these facts show a significant influence of the laser beam energy distribution on the energy loss at SLM, especially for high power laser sources. The observed escape of granules shows a possible influence of the dynamic factor. The use of the non-Gaussian distributions can enhance 3D parts, for example, improve the geometrical tolerance and decrease the residual porosity.

Improving interfacial reaction nonhomogeneity during laser welding–brazing aluminum to titanium

Heterogeneous interfacial reactions were easily found along the Ti/Al interface due to high temperature gradient during laser welding–brazing of Ti/Al dissimilar alloys. To improve the nonhomogeneity, relative uniform energy distribution of laser beam and appropriate groove were attempted. The effects of these attempts on the nonhomogeneity of interfacial reactions were investigated by finite element method (FEM) numerical simulation and experimental validation. The results indicate that the V-shaped groove can make the interface roughly parallel to the isotherm of the temperature field. Moreover, the rectangular spot laser can further improve homogenization of the interfacial reaction along the interface in comparison with circular spot laser. Tensile test results show that the combination of rectangular spot laser welding–brazing and V-shaped groove can effectively control the fracture of Ti/Al joints in the seam in a wide processing parameters window, and the average tensile strength reaches 278 MPa.

Damage mechanism and morphology characteristics of chromium film in femtosecond laser rear-side ablation

In this paper, damage mechanism and morphology characteristics of chromium film in femtosecond laser rear-side ablation are investigated. The film removing process includes two key sub-processes: the laser ablation dynamic process and subsequent breaking and ejecting dynamic process. Film morphology in rear-side ablation is determined by the interrelation between the laser energy and the film strength. When lower laser energy is used, residual out-layer film is relative thick and tends to break into some large fragments, which results in an irregular ablation shape. While when higher pulse energy is used, thinner residual film with weaker break strength breaks into small fragments, the ablation quality improves correspondingly. Besides laser energy and film property, energy distribution of laser beam also affects the ablation quality. In experiments, this kind of effect is researched by changing the focal position. It is found that ripples, which are familiar nano-structures in front-side ablation, also exist in rear-side ablation. These ripples are formed initially at the interface between quartz substrate and film, and their coverage varies with the energy distribution. Additionally, increasing number of scans is an effective method to shorten the period of ripples.

Study of an Optical Device Used to Homogenize a Laser Beam. Application to Emissivity Measurements on Semitransparent Materials at High Temperature

On an experimental apparatus designed to measure the directional spectral emissivity of semitransparent materials at high temperature (2000°C), a specific optical device (kaleidoscope) is mounted in order to homogenize the energy distribution of a CO2 laser beam, which is used as a heating source. An objective of this work, for operating in the best conditions, is to obtain a uniform temperature of the tested sample. This study, based on Fourier optics, focuses on a square aperture kaleidoscope. A model was developed to simulate the energy distribution of the laser beam, at any transverse plane on the optical path. The final objective is to simulate the energy distribution on the sample surface to optimize the homogenization device taking into account the surface temperature gradient induced by the local energy distribution. To validate this model, quantitative comparisons of theoretical simulations and experimental thermal spots are performed.

Photometric and clinical assessment of localized UVB phototherapy systems for the high‐dosage treatment of stable plaque psoriasis

BACKGROUND: Ultraviolet‐B (UVB) light sources are widely used for the safe and effective treatment of inflammatory skin conditions. The recent commercial introduction of fiber‐coupled UVB phototherapy systems facilitates the selective exposure and treatment of localized psoriasis plaques while permitting the safe use of high‐dosage treatments.OBJECTIVE: In this study, the performance characteristics and clinical outcome of psoriasis treatments were assessed when using two technologically distinct sources of high‐intensity, fiber‐optically delivered therapeutic UVB.METHODS: A pulsed, monochromatic 308 nm excimer laser and a continuous‐wave, incoherent UVB light source were compared using photosensitive recording papers, images captured with a CCD camera, and on the healthy and lesional skin of ten psoriasis patients.RESULTS: Beam profile analyses and minimal erythema dose (MED) test spots revealed distinct energy distribution patterns from the two devices. The Guassian‐type laser beam energy distribution complicated MED determinations, whereas skin exposed to light from the incoherent UVB system developed a more uniform erythema. Both systems cleared the treated psoriasis plaques equivalently, requiring no more than two to five weeks of high‐dose treatments.CONCLUSION: When used at equally erythemogenic high doses, both systems produced rapid plaque clearance with minimal side effects. Unlike conventional phototherapy, localized UVB minimizes exposure to the healthy skin, making it suitable for patients with mild to moderate psoriasis, individuals with recalcitrant plaques and for the successful treatment of lesions occurring on most body sites.