Laser Beam Spot Size Research Articles

Second harmonic generation of self-focused Hermite-Gaussian laser beam in collisional plasma

This work presents a theoretical approach for self-focusing as well as second harmonic generation of a Hermite-Gaussian (HG) laser beam in collisional plasma. On incidence in plasma, high electric field of HG laser beam causes the nonuniform distribution of plasma electrons as a result of collisional nonlinearity and thus creates the refractive index gradient which causes the wavefront to collapse about the centre and form a tight focus. Moment theory approach has been used to derive the coupled differential equations governing the spot size of the HG laser beam in both the transverse directions x and y. Second harmonics of the HG laser beam is generated by the excitation of the electron plasma wave at pump frequency which further interacts with the pump beam to generate the radiation with double the frequency of the later. Different mode indices of HG laser beam have significant effect on the self-focusing as well as on second harmonic generation of the laser beam in plasma. It has been observed that for higher mode indices (TEM02) second harmonic yield (SHY) is greater.

Perturbative approach to the self-focusing of intense X-ray laser beam propagating in thermal quantum plasma

In this theoretical study, the problem of self-focusing of an X-ray intense laser beam in the thermal quantum plasma is studied. Using a relativistic fluid model and taking into account the hydrodynamic pressure of degenerate electrons in the zero temperature limit, the nonlinear momentum equation of electrons is solved by means of a perturbative method and the nonlinear current density of the relativistic degenerate electrons is obtained. Saving only the third-order nonlinearity of the laser beam amplitude, a nonlinear equation describing the interaction of a laser beam with the quantum plasma is derived. It is shown that considering the nonlinearity of the system through solving the nonlinear equation of the degenerate electron leads to the originally different wave equation in comparison to outcomes of the approach in which the permittivity of longitudinal waves of quantum plasma is problematically extended to the relativistic case. The evolution of the laser beam spot size with the Gaussian profile is considered, and the effect of quantum terms on the self-focusing quality is studied. It is shown that considering quantum effects leads to the decrease in the self-focusing property and the effect of Bohm tunneling potential is more dominant than the degenerate electron pressure term.

Comparative study of AISI 304L to AISI 316L stainless steels joints by TIG and Nd:YAG laser welding

In the present study, investigation and comparison laser and tungsten inert gas (TIG) welding techniques for dissimilar butt joint of 304L and 316L stainless steel sheets was carried out. The effects of voltage and laser beam spot size during laser welding and effects of voltage, traveling speed and presence of backing gas during TIG welding on yield strength, elongation, hardness and weld width were investigated. Moreover, in order to examine the mechanical and metallurgical behavior of the weld, microstructure analysis was used. Results showed that welding depth of TIG welding was higher than laser method. It was observed that strength and toughness of samples decreased by increasing voltage in TIG and laser welding. Additionally, by increasing beam spot size in laser welding, strength, toughness and hardness increased. Also, the presence of backing gas during TIG welding resulted in the increase of strength, toughness and hardness of the samples.

A physical modeling and predictive simulation of the laser cladding process

A 3D finite element simulation model of the laser cladding process has been developed taking into account heat transfer, fluid flow, surface tension and free surface movement. All input parameters and data, which are independent of the process parameters but depend only on the material and machine properties, have been obtained from measurements. Thereby the melt pool and the resulting surface contour can be simulated without compromising assumptions or calibration, because the machine parameters are the only variable input parameters of the model. Thus, the model can easily be transferred to other material combinations or other machines. For the surface contour calculation a modified height function method is applied. The model surface follows this contour as an arbitrary Lagrangian Eulerian (ALE) method is used allowing for mesh deformations. The model was implemented using the commercial finite element software COMSOL Multiphysics and validated by comparing the simulation results with caloric measurements of the effective heat input and metallographic cross sections from experiments, where the nickel-base alloy MetcoClad® 625 in powder form was deposited on structural steel S235JRC + C and the process parameters of laser power, feed speed, laser beam spot size and powder mass flow were varied within a range of at least 50% of their mean value each. The maximum deviation of the simulation results compared to the experimental data regarding track geometry is 14% for the parameter sets without weld defects so that these parameter sets could be industrially applied, whereas the average deviation of track width and height is below 5.1%. Moreover, the maximum difference of the simulated absorptivity compared to the measurement results is 6.9%, while the average difference is only 1.7%, meaning the model shows a high predictive capability especially regarding the heat input.

Exact solution of thermal response in a three-dimensional living bio-tissue subjected to a scanning laser beam

At present, laser plays an important role in biomedical treatments and surgical techniques. To guarantee patients’ safety, a deeply understanding of thermal responses of biological tissues during laser-tissue interaction is required. The existing analytical researches usually explore the bio-thermal process based on a one-dimensional heat conduction model because of the complication of the governing equations in three-dimensional case. The present study developed an analytical solution for the laser-tissue thermal interaction with a moving heat source based on a three-dimensional DPL thermal transfer model. The effects of the capillary vessel system and metabolism were under consideration. The expression of temperature in the cuboid bio-tissue was derived analytically by utilizing the Green’s function approach and its accuracy was verified through agreement with a numerical simulation. The influences of the laser moving speed, the spot size of laser beam and the two parameters of relaxation times were discussed.

Cost Effective Profiling of Neodymium-Doped Vanadate Laser Pulses

As environmental conditions and component degradation and failure are known to affect the performance of ultrafast lasers, it is important to monitor their state in any nonlinear optical study. This may be achieved by measuring the temporal width of the laser pulses using an autocorrelator. In this work, an autocorrelator for measuring the pulsewidth of mode-locked lasers was custom-built using pieces of equipment usually found in a typical ultrafast optics laboratory. The assembled equipment was tested using SESAM (Saturable Semiconductor Absorber Mirror) mode-locked neodymium-doped vanadate (Nd:YVO4) laser having manufacturer specified average output power of 1.6 W and 10 ps pulsewidth. Using the background-free autocorrelation technique, the pulse width of the laser was measured to be 10.4 ps. This type of autocorrelator is cost effective and may be handy in situations where research funds are limited; a scenario commonly experienced in research laboratories of developing countries. Additionally, the processes involved in assembling the autocorrelator provide a useful learning experience for new researchers. The study also outlined the processes involved in modifying an existing autocorrelation setup in order to measure laser beam spot size; a useful parameter in nonlinear optical studies.

레이저 폴리싱에서 레이저 빔의 다양한 경로조건에 따른 표면거칠기 분석 연구

In laser polishing, a laser beam is used to melt the surface of a work piece to improve surface quality. An optimum combination of process controlling parameters and state of laser beam output is the key strength of laser polishing. Laser beam output power, feed rate, path interval, and spot size are critical parameters in the laser polishing process effect on the thermal state of surface quality. In this study, experimental tests demonstrate that it is an optimum value of the proper processes condition in the pulse laser and CW laser system. The proposed dominant controlling parameters, method for examining morphology; several experimental conditions; and details from performance improvement of surface roughness are presented.

Evolution of a circularly polarized laser beam in an obliquely magnetized plasma channel

AbstractThe evolution of the spot size and amplitude of a circularly polarized laser beam propagating in a plasma channel embedded in an obliquely applied magnetic field has been investigated. The wave equation describing the evolution of the radiation field is set up and a variational technique is used to obtain the equations governing the evolution of the spot size and amplitude. Numerical methods are used to analyze the evolution of the laser beam spot size and amplitude. It is seen that the amplitudes of the two transverse components of the electric field of the laser beam evolve differently, since they are driven by unequal current densities. This leads to the conversion of a circularly polarized laser beam into an elliptically polarized beam, under appropriate conditions.

Self-Focusing and de-Focusing of Intense Left- and Right-Hand Polarized Laser Pulse in Hot Magnetized Plasma in the Presence of an External Non-Uniform Magnetized Field

In this paper, self-focusing of an intense circularly polarized laser beam in the presence of a non-uniform positive guide magnetic field with slope constant parameter δ in hot magnetized plasma, using Maxwell’s equations and relativistic fluid momentum equation is investigated. An envelope equation governing the spot-size of laser beam for both of left- and right-hand polarizations has been derived, and the effects of the plasma temperature and magnetic field on the electron density distribution of hot plasma with respect to variation of normalized laser spot-size has been studied. Numerical results show that self-focusing is better increased in the presence of an external non-uniform magnetic field. Moreover, in plasma density profile, self-focusing of the laser pulse improves in comparison with no non-uniform magnetic field. Also, with increasing slope of constant parameter of the non-uniform magnetic field, the self-focusing increases, and subsequently, the spot-size of laser pulse propagated through the hot magnetized plasma decreases.

Self-focusing and de-focusing of intense left and right-hand polarized laser pulse in hot magnetized plasma: Laser out-put power and laser spot-size

In this work, self-focusing of an intense circularly polarized laser pulse in hot magnetized plasma using Maxwell’s equations and relativistic fluid momentum equation is investigated. An envelope equation governing the spot-size of laser beam for both of left- and right-hand polarizations has been derived and the effects of the plasma temperature and magnetic field on the spatial electron density distribution of hot plasma with respect to variation of normalized laser spot-size has been studied. Numerical results indicate that with increasing the magnetic field strength self-focusing of the right-hand polarization laser pulse increases, while for the left-hand polarization, laser de-focusing enhances. Furthermore, it is found that for right-hand polarization of the incident laser, when the external magnetic field strength enhances the laser spot-size reduces and laser pulse becomes focused. In fact, effect magnetic field on the density profile in right-hand polarization is more prominent. Also it is shown that plasma temperature has a significant role in the, laser spot-size, laser out-put power and the variation of plasma density.

The Effect of Laser Beam Welding Parameters onto the Evolving Joints Geometry

In our research the effect of a new type of laser beam parameters during the laser welding have been investigated with 80 different welding parameters. The laser welding parameters such as the laser power, laser beam spot size on the surface and feed rate greatly affect the resulting weld geometry. S235 grade steel has been used. The operating equipment was a Trumpf 4001 4 kW disk laser with a diameter of 100 microns optical fiber. The effect of different welding parameters were evaluated from the metallographic cross-section of the welded joints. This article describes the effect of the different laser beam focusing and the welding feed rate.

Self-focusing of a laser beam in the rippled density magnetoplasma

In the present paper, we have investigated the self-focusing of intense laser beam in rippled density magnetoplasma. The differential equation for the beam width parameter is obtained by following WKB and paraxial approximations and solved it numerically. The behaviour of beam width parameter with dimensionless distance of propagation is extensively investigated for different values of laser and plasma parameters and is strongly influenced by the magnetic field. The results are discussed and presented graphically. It is seen that the density ripple of suitable wave number increases the self-focusing property of laser beam in magnetoplasma. Meanwhile, an increase in the values of magnetic field increases the strength of the self-focusing, especially in shorter distance of propagation. Furthermore, increase in plasma frequency (plasma density) causes more reduction in the spot size of laser beam and consequently increases the rate of self-focusing of laser beam in magnetoplasma.

Excimer laser ablation of aluminum: influence of spot size on ablation rate

The dependence of ablation rate of an Al alloy on laser beam spot size (10–150 µm) was investigated using an ArF excimer laser operating at a wavelength of 193 nm and pulse width less than 4 ns. Ablation was conducted in air at a fluence of 11 J cm−2 and at a repetition rate of 20 Hz. Surface morphology and depth of craters produced by a variable number of laser pulses were characterized using optical and scanning electron microscopy. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) was used as an additional diagnostic technique to estimate the amount of material ablated from craters produced by a laser beam of different diameters. Laser beam spot size and number of laser pulses applied to the same spot were found to influence crater morphology, ablation rate, shape and amount of particles deposited at or around the crater rim. Ablation rate was found to be less dependent on spot size for craters greater than 85 µm. A four-fold increase in ablation rate was observed with decreasing crater size from 150 µm to 10 µm.

Measurement and reliability issues in resonant mode cantilever for bio-sensing application in fluid medium

Cantilevers immersed in liquid experience viscous damping and hydrodynamic loading. We report on the use of such cantilevers, operating in the dynamic mode with, (i) frequency sweeping and (ii) phase locked loop methods. The solution to reliability issues such as random drift in the resonant peak values, and interference of spurious modes in the resonance frequency spectrum, are explained based on the actuation signal provided and laser spot size. The laser beam spot size and its position on the cantilever were found to have an important role, on the output signal and resonance frequency. We describe a method to distinguish the normal modes from the spurious modes for a cantilever. Uncertainties in the measurements define the lower limit of mass detection (mmin). The minimum detection limits of the two measurement methods are investigated by measuring salt adsorption from phosphate buffer solution, as an example, a mass of 14 pg was measured using the 14th transverse mode of a m × 100 μm × 1 μm silicon cantilever. The optimized measurement was used to study the interaction between antibody and antigen.

The investigation of laser shock peening effects on corrosion and hardness properties of ANSI 316L stainless steel

Laser shock peening (LSP) is known as a post processing surface treatment which can improve the mechanical properties of some materials. Shock waves are generated by confining the laser-induced plasma to cause a large pressure shock wave over a significant surface area. In the present study, effects of LSP on the electrochemical corrosion and micro hardness properties of 316L stainless steel alloy were investigated by changing the laser parameters such as the laser spot size, the average number of impacts, and the laser intensity. Since laser parameters do not cover the desired region of LSP, we have to use the proper design of experiment method, in which the D-optimal design of MATLAB was selected. Results revealed that by increase in irradiance, number of impacts and spot size of laser beam, improvement in the surface micro hardness, and corrosion resistance is achieved. Also, due to unexpected drop into the outcome of our experiments, it was found that the contamination of the transparent overlay and reduction of the absorption coefficient of the absorbent layer play a key role to reduce the efficiency of the mechanical impacts. So, by changing the experimental conditions, even better results are expected.

Nonlinear propagation of Gaussian laser beam in an inhomogeneous plasma under plasma density ramp

Nonlinear propagation of Gaussian laser beam in an inhomogeneous plasma under plasma density ramp is studied. The differential equation for beam width parameter is derived by parabolic wave equation approach under paraxial approximation. For different values of plasma density, laser intensity and initial beam width, the behavior of beam width parameter with the normalized propagation distance is examined. It is found that the beam width parameter decreases with a higher rate. The frequency of the oscillations increases while amplitude decreases and the laser beam focuses up to long distance. Further, increase in relative plasma density causes reduction in the amplitude of spot size of laser beam near the propagation axis.

Second harmonic generation of self-focused Cosh-Gaussian laser beam in collisional plasma

This paper presents theoretical investigation of effect of self-focusing of Cosh-Gaussian (ChG) laser beam on second harmonic generation (SHG) in collisional plasma. Due to non uniform intensity distribution along the wave front of ChG laser beam non uniform Ohmic heating of plasma electrons takes place. Non uniform heating of plasma the laser beam leads to self-focusing of the laser beam which in turn produces strong density gradients in the transverse direction. The generated density gradients excite electron plasma wave at pump frequency that interacts with the incident laser beam to produce its second harmonics. Following moment theory approach in Wentzel–Kramers–Brillouin (W.K.B) approximation second order differential equation governing the evolution of spot size of laser beam with distance of propagation has been derived. Numerical simulations have been carried out to investigate the effect of laser intensity as well as plasma parameters on self-focusing of the laser beam and also on yield of second harmonics. Results predict that ChG laser beams show smaller divergence as they propagate and, thus, lead to enhanced conversion efficiency of second harmonics.

Femtosecond Laser Ablation of Polymer Thin Films for Nanometer Precision Surface Patterning

Femtosecond laser ablation of ultrathin polymer films on quartz glass using laser pulses of 100 fs and centered at λ=400 nm wavelength has been investigated for nanometer precision thin film patterning. Singleshot ablation craters on films of various thicknesses have been examined by atomic force microscopy, and beam spot diameters and ablation threshold fluences have been determined by square diameter-regression technique. The ablation thresholds of polymer film are about 1.5 times smaller than that of quartz substrate, which results in patterning crater arrays without damaging the substrate. In particular, at a 1/e 2 laser spot diameter of 0.86 μm, the smallest craters of 150-nm diameter are fabricated on 15-nm thick film. The ablation thresholds are not influenced by the film thickness, but diameters of the ablated crater are bigger on thicker films than on thinner films. The ablation efficiency is also influenced by the laser beam spot size, following a w 0q -0.45 dependence.

Electron plasma wave excitation by beating of two q-Gaussian laser beams in collisionless plasma

AbstractThis paper presents a scheme for excitation of an electron-plasma wave (EPW) by beating two q-Gaussian laser beams in an underdense plasma where ponderomotive nonlinearity is operative. Starting from nonlinear Schrödinger-type wave equation in Wentzel–Kramers–Brillouin (WKB) approximation, the coupled differential equations governing the evolution of spot size of laser beams with distance of propagation have been derived. The ponderomotive nonlinearity depends not only on the intensity of first laser beam, but also on that of second laser beam. Therefore, the dynamics of one laser beam affects that of other and hence, cross-focusing of the two laser beams takes place. Due to nonuniform intensity distribution along the wavefronts of the laser beams, the background electron concentration is modified. The amplitude of EPW, which depends on the background electron concentration, is thus nonlinearly coupled with the laser beams. The effects of ponderomotive nonlinearity and cross-focusing of the laser beams on excitation of EPW have been incorporated. Numerical simulations have been carried out to investigate the effect of laser and plasma parameters on cross-focusing of the two laser beams and further its effect on EPW excitation.

Investigation of Laser Parameters in Silicon Pulsed Laser Conduction Welding

In this paper, laser welding of silicon in conduction mode is investigated numerically. In this study, the effects of laser beam characteristics on the welding have been studied. In order to model the welding process, heat conduction equation is solved numerically and laser beam energy is considered as a boundary condition. Time depended heat conduction equation is used in our calculations to model pulsed laser welding. Thermo-physical and optical properties of the material are considered to be temperature dependent in our calculations. Effects of spatial and temporal laser beam parameters such as laser beam spot size, laser beam quality, laser beam polarization, laser incident angle, laser pulse energy, laser pulse width, pulse repetition frequency and welding speed on the welding characteristics are assessed. The results show that how the temperature dependent thermo-physical and optical parameters of the material are important in laser welding modeling. Also the results show how the parameters of the laser beam influence the welding characteristics.