Hollow Laser Beam Research Articles

Temperature behavior of metal surface with moving annular hollow laser heat source

The temperature distribution characteristics of a moving annular hollow laser beam irradiated on the substrate are studied in this paper. First, through the linear superposition property of potential function, we derive the analytical solution of temperature distribution and the cooling rate of the moving ideal annular hollow heat source. The entransy dissipation rate per unit volume of the annular hollow laser beam is more uniform than that of the Gaussian laser beam. The “ω”-shaped profile of the cross-sectional melt temperature is predicted. Second, we set up an actual annular hollow heat source expression, and two parameters (hollow ratio and heat source hollow area) are defined to describe the heat transfer characteristics of the actual annular hollow heat source. Experiments are carried out to validate transient numerical results. The maximum relative error between the experiments and the simulations is 6.39%, and the average relative error is 3.16%. In the simulation, we compare the profile temperature field of the annular hollow heat source with that of the Gaussian heat source. The profile temperature field in simulation for the actual annular hollow heat source is in agreement with that predicted in the analytical solution for the ideal annular hollow heat source. The effects of scanning speed, power intensity, hollow ratio, and heat source hollow area on the temperature field are demonstrated. The obtained characteristics provide theoretical support for laser materials engineering applications.

Investigation of effect of electron temperature on intensity and efficiency of terahertz generated by laser beating in inhomogeneous plasma

We report terahertz (THz) wave generation based on mixing of two radially polarized hollow Gaussian laser beams in periodic density plasma. We incorporate the effect of plasma electron temperature on THz intensity and efficiency. Our investigations reveal that emitted THz is radially polarized. Brightness and efficiency of THz significantly increase with plasma electron temperature. Our scheme predicts upto 2.4-fold enhancement in THz conversion efficiency with increase in plasma electron temperature from 0–10 KeV. We observe that effect of plasma electron temperature is more prominent when difference of laser frequency matches with plasma frequency i.e. The profile of THz depends on the laser parameters and it is independent of plasma electron temperature. A high THz field (∼10 MV cm−1) and high conversion efficiency (∼6%) is predicted for optimized pump laser and plasma parameters.

Power and efficiency enhancement of THz radiation in a plasma by dark hollow laser beams: Effects of magnetic field

The power and efficiency of terahertz radiation (THz) in a rippled density plasma by dark hollow laser beams are very small due to the collisional effect. We show that when a magnetic field is applied perpendicularly to the plasma, the THz radiation power and efficiency significantly enhance. Also, the magnetic field changes the THz radiation profile to a profile similar to a dark hollow laser beam. The mutual effect between the laser beams and THz radiation in plasma, which is important in high energy transfer between laser beams and THz radiation, is taken into account. We find that the maximum efficiency obtained in our scheme is more than four orders of magnitude greater than the efficiency in the absence of magnetic field.

Tailored Transverse Mode Vertical-Cavity Surface-Emitting Laser: Hollow Laser Beam Emission through Circle Aperture

Vertical-cavity surface-emitting lasers that can be controlled to emit hollow laser beam in certain LP21 mode are proposed and fabricated. A circular inverted surface relief with precisely designed diameter and thickness is introduced through theoretical simulation. The fabricated device shows a stable single LP21 mode operation with excellent robustness of mode and power to temperature. Preliminary analysis attribute the excellent performance of the device to the inverted surface relief, leading to hollow near-field spot distribution.

Propagation characteristics of a hollow Gaussian laser beam in a tapered plasma channel

The propagation characteristics of a hollow Gaussian laser beam (HGB) in a tapered plasma channel are studied by simultaneously considering the effects of relativistic self-focusing, ponderomotive self-channeling, and preformed channel focusing. A differential equation of the laser width parameter is derived by using the paraxial like approximation, and the critical surface that classifies the evolution behaviors of the HGB into focusing and defocusing is obtained theoretically. For verification, some numerical results are also given. It is found that the focusing ability of the HGB is proportional to the density of the tapered plasma channel, but it weakens with the increase in the order of the HGB. Furthermore, we also find that for the Gaussian laser beam, when the transverse plasma density is homogeneous, its focusing ability is stronger than that of the HGB with n = 3, while the trend just reverses when the transverse plasma density is parabolic.

Optical-trapping of particles in air using parabolic reflectors and a hollow laser beam.

We present an advanced optical-trapping method that is capable of trapping arbitrary shapes of transparent and absorbing particles in air. Two parabolic reflectors were used to reflect the inner and outer parts of a single hollow laser beam, respectively, to form two counter-propagating conical beams and bring them into a focal point for trapping. This novel design demonstrated high trapping efficiency and strong trapping robustness with a simple optical configuration. Instead of using expensive microscope objectives, the parabolic reflectors can not only achieved large numerical aperture (N.A.) focusing, but were also able to focus the beam far away from optical surfaces to minimize optics contamination. This design also offered a large free space for flexible integration with other measuring techniques, such as optical-trapping Raman spectroscopy, for on-line single particle characterization.

Laser-induced subwavelength structures by microdroplet superlens.

Nanoscale patterns on rigid or flexible substrates are of considerable interest in modern nanophotonics and optoelectronics devices. Subwavelength structures are produced in this study by using a laser beam and microdroplets that carry nanoparticles to the deposition substrate. These droplets are generated from an aqueous suspension of nanoparticles by electrospray and dispensed through a conical hollow laser beam so that laser-droplet interactions occur immediately above the substrate surface. Each microdroplet serves the dual role as a nanoparticle carrier to the substrate and as a superlens for focusing the laser beam to a subwavelength diameter. This focused beam vaporizes the droplet and sinters the nanoparticles on the substrate. The deposition of subwavelength nanostructures and thin films on a silicon wafer are demonstrated in this paper. This process may be applied to produce novel nanophotonics and electronics devices involving a variety of subwavelength patterns including an ordered array of semiconductor nanoparticles.

The study of high-power hollow-beam laser diode

High-Power VCSEL is one of the most promising semiconductor laser sources with more advantages such as lower threshold current, compact size, efficiently fiber coupling, long lifetime compared to other types of lasers. Thanks to its slice feature, VCSEL has been the most potential light source emitting hollow laser beam. In this experiment, part of center area of P-DBRs was etched to increase the threshold of the center area of VCSEL and force carriers to gather on the edge of the VCSEL. As a result, a hollow laser beam with an output power of 350 mW and a circular optical filed distribution has been obtained.

Terahertz radiation for medical application

Terahertz (THz) radiations focused at multiple positions are quite useful for their medical application. For this, we propose to employ beating of dark hollow Gaussian laser beams (DHGBs) of different beam orders in plasma. In view of realistic situations, we also include the collisions between the electrons and the neutrals. Because of a special intensity gradient distribution in DHGBs, the emitted THz radiation is found to have a unique field profile with multiple focii. The dependence of plasma density, order of the beams and collision frequency on the THz field amplitude is discussed. The important feature to consider hollow Gaussian laser beams is having the same power at different beam orders. It is found that the amplitude of the emitted radiation is highly sensitive to the beam order of the incident lasers. An optimization of the plasma parameters and the lasers characteristics can lead to the enhancement in the THz intensity and the efficiency of the mechanism.

Control of peaks of terahertz radiation and tuning of its frequency and intensity

We propose to employ dark hollow laser beams (DHBs) for obtaining multifocal terahertz (THz) radiations which are quite useful in medical applications. The number of peaks in the THz field can be suppressed if we apply external magnetic field and use intense lasers so that the relativistic effects are prominent. It means we can achieve strong bifocal THz radiation with this scheme. However, multifocal THz radiation can be reconstituted by increasing the beating frequency of the lasers. The separation of the peaks can be controlled by wisely choosing the orders of the lasers, and also by varying the strength of the magnetic field. The magnetic field is found to enhance the THz field, and also it tunes the frequency of the THz field. The DHBs of higher and equal orders are found to be most significant for efficient THz radiation generation with respect to the role of the magnetic field and density ripples.

Closed-loop control of variable width deposition in laser metal deposition

In laser metal deposition (LMD) technology, multi-track overlapping is usually used for the deposition of the unequal width thin-walled part. In the present study, a single-track deposition with the variable width method was proposed to improve deposition accuracy and efficiency, and reduce repeated heating. The hollow laser beam internal powder feeding nozzle was applied to realize variable laser spot deposition. Based on the developed deposition height sensor, a closed-loop control system and PI controller were developed and tested. A segmented deposition height control method was developed. The unequal width thin-walled part was divided into several segments along the scanning direction, and each segment was matched with different laser spot sizes. The segments were controlled to have unequal widths, but with an equal height. A thin-walled part with a width range of 5.08–2.14 mm and a “blade-shaped” part with a width range of 6.18–2.64 mm were deposited. The control curves of most segments were stable and convergent. The microstructure and properties of the formed part varied with wall width. All width areas achieved the compact and small size of the grains, as well as high microhardness.

Generation of a localized hollow laser beam using crossed nonlinear optical crystals

A nonlinear optical approach to generate a localized hollow laser beam and its array using crossed nonlinear ZnSe crystals is proposed. We calculate the intensity distributions of the hollow beam and its propagating properties in free space, and study the dependences of the dark spot size of the hollow beam on the incident beam waist radius and the crystal length. Our studies show that the longitudinal size of the hollow beam can be reduced by ∼660 times than that generated by a single nonlinear crystal. Such laser beam with a dark hollow region and a large intensity gradient can be used to trap cold neutral atoms, molecules and microscopic particles.

Electron Acceleration by Beating of Two Intense Cross-Focused Hollow Gaussian Laser Beams in Plasma**Supported by United Arab Emirates University for Financial under Grant No. UPAR (2014)-31S164

This paper presents propagation of two cross-focused intense hollow Gaussian laser beams (HGBs) in collisionless plasma and its effect on the generation of electron plasma wave (EPW) and electron acceleration process, when relativistic and ponderomotive nonlinearities are simultaneously operative. Nonlinear differential equations have been set up for beamwidth of laser beams, power of generated EPW, and energy gain by electrons using WKB and paraxial approximations. Numerical simulations have been carried out to investigate the effect of typical laser-plasma parameters on the focusing of laser beams in plasmas and further its effect on power of excited EPW and acceleration of electrons. It is observed that focusing of two laser beams in plasma increases for higher order of hollow Gaussian beams, which significantly enhanced the power of generated EPW and energy gain. The amplitude of EPW and energy gain by electrons is found to enhance with an increase in the intensity of laser beams and plasma density. This study will be useful to plasma beat wave accelerator and in other applications requiring multiple laser beams.

Opto-aerodynamic focusing of aerosol particles

ABSTRACTWe describe a new method for focusing and concentrating a stream of moving micron-sized aerosol particles in air. The focusing and concentrating process is carried out by the combined drag force and optical force that is generated by a double-layer co-axial nozzle and a focused doughnut-shaped hollow laser beam, respectively. This method should supply a new tool for aerosol science and related research.Copyright © 2018 American Association for Aerosol Research

Generation of terahertz radiation by intense hollow Gaussian laser beam in magnetised plasma under relativistic-ponderomotive regime

This paper explores the self-focusing of hollow Gaussian laser beam (HGLB) in collisionless magnetized plasma and its effect on the generation of THz radiation in the presence of relativistic-ponderomotive nonlinearity. The relativistic change of electron mass and electron density perturbation due to the ponderomotive force leads to self-focusing of the laser beam in plasma. Nonlinear coupling between the intense HGLB and electron plasma wave leads to generation of THz radiation in plasma. Resonant excitation of THz radiation at different frequencies of laser and electron plasma wave satisfies proper phase matching conditions. Appropriate expressions for the beam width parameter of the laser beam and the electric vector of the THz wave have been evaluated under the paraxial-ray and Wentzel-Kramers Brillouin approximations. It is found that the yield of THz amplitude depends on the focusing behaviour of laser beam, magnetic field, and background electron density. Numerical simulations have been carried out to investigate the effect of laser and plasma parameters on self-focusing of the laser beam and further its effect on the efficiency of the generated THz radiation.

基于中空激光光内送粉技术的扇形结构件激光成形研究

基于中空激光光内送粉技术的扇形结构件激光成形研究

Stimulated Brillouin backscattering of hollow Gaussian laser beam in collisionless plasma under relativistic–ponderomotive regime

AbstractStimulated Brillouin backscattering of an intense hollow Gaussian laser beam (HGLB) from collisionless plasma has been investigated under relativistic–ponderomotive regime. The main feature of considered hollow Gaussian laser beam is having the same power at different beam orders with null intensity at the center. Backscattered radiation is generated due to nonlinear interaction between main beam (pump beam) with pre-excited ion acoustic wave (IAW). Modified coupled equations has been set up for the beam width parameters of the main beam, ion-acoustic wave, back-scattered wave, and back reflectivity of stimulated Brillouin scattering (SBS) with the help of the Wentzel–Kramers–Brillouin approximation, fluid equations and paraxial theory approach. These coupled equations are solved analytically and numerically to study the laser intensity in the plasma, the variation of amplitude of the excited IAW and back reflectivity of SBS. The back reflectivity of SBS is found to be highly sensitive to the order of the HGLB, intensity of main laser beam, and plasma density for typical laser and plasma parameters. The focusing of main laser beam (hollow Gaussian) and IAW significantly affected the back reflectivity of SBS. The results show that the self-focusing and back reflectivity is enhanced for higher order modes of HGLB.

Laser metal deposition with spatial variable orientation based on hollow-laser beam with internal powder feeding technology

In this study, a hollow-laser beam with internal powder feeding (HLB-IPF) head is applied to achieve non-horizontal cladding and deposition of overhanging structure. With the features of this head such as uniform scan energy distribution, thin and straight spraying of the powder beam, the deposition in spatial variable orientation is conducted using a 6-axis robot. During the deposition process the head keeps tangential to the growth direction of the part. In the experiment, a “vase” shaped metal part with overhanging structure is successfully deposited, and the largest overhanging angle achieves 80° to the vertical direction. The “step effect” between cladding layers is completely eliminated with the best surface roughness of Ra=3.864µm. Cross section of cladding layers with unequal height are deposited for angle change. Test results indicate that the formed part has uniform wall thickness, fine microstructure and high microhardness.

The influence of the substrate-inclined angle on the section size of laser cladding layers based on robot with the inside-beam powder feeding

Almost all of the application of laser cladding technology are based on horizontal reference plane at present, and this way has greatly limited its extensive application in non-horizontal reference plane. The substrate-inclined angles from 0° to 150° and continuously variable postures of coaxial powder nozzle have been achieved by changing the programming to control the laser processing robot adopting the technology of “hollow laser beam and internal powder feeding.” The influence rules of the substrate-inclined angle on the section size of cladding layers were investigated, and the forces applied onto the molten pool were analyzed. The experimental results show that the section height of single track decreases, the section width of single track does not have significant variations, and the displacement offset between the axis by the peak point of cladding layer and the axis by center position of the molten pool is firstly increased and then decreased with the growth of the substrate-inclined angle. And the large-inclined angle of coaxial powder nozzle is applied and accomplished to clad on non-horizontal reference plane. A new process method is provided for laser cladding, repairing, and forming on non-horizontal reference plane by the inside-beam powder feeding technology.

Intense source of cold cesium atoms based on a two-dimensional magneto–optical trap with independent axial cooling and pushing**Project supported by the National Natural Science Foundation of China (Grant No. 11304177).

We report our studies on an intense source of cold cesium atoms based on a two-dimensional (2D) magneto–optical trap (MOT) with independent axial cooling and pushing. The new-designed source, proposed as 2D-HP MOT, uses hollow laser beams for axial cooling and a thin pushing laser beam to extract a cold atomic beam. With the independent pushing beam, the atomic flux can be substantially optimized. The total atomic flux maximum obtained in the 2D-HP MOT is 4.02 × 1010 atoms/s, increased by 60 percent compared to the traditional 2D+ MOT in our experiment. Moreover, with the pushing power 10 μW and detuning 0Γ, the 2D-HP MOT can generate a rather intense atomic beam with the concomitant light shift suppressed by a factor of 20. The axial velocity distribution of the cold cesium beams centers at 6.8 m/s with an FMHW of about 2.8 m/s. The dependences of the atomic flux on the pushing power and detuning are studied in detail. The experimental results are in good agreement with the theoretical model.