Intensity Distribution Of Laser Pulse Research Articles

Phonon transport in a curved aluminum thin film due to laser short pulse irradiation

Laser short-pulse heating of a curved aluminum thin film is investigated. The Boltzmann transport equation is incorporated to formulate the heating situation. A Gaussian laser intensity distribution is considered along the film arc and time exponentially decaying of pulse intensity is incorporated in the analysis. The governing equations of energy transport in the electron and lattice sub-systems are coupled through the electron-phonon coupling parameter. To quantify the phonon intensity distribution in the thin film, equivalent equilibrium temperature is introduced, which is associated with the average energy of all phonons around a local point when the phonon energies are redistributed adiabatically to an equilibrium state. It is found the numerical simulations that electron temperature follows similar trend to the spatial distribution of the laser pulse intensity at the film edge. Temporal variation of electron temperature does not follow the laser pulse intensity distribution. The rise of temperature in the electron sub-system is fast while it remains slow in the lattice sub-system.

Evolution of laser pulse shape in a parabolic plasma channel

During high-intensity laser propagation in a plasma, the group velocity of a laser pulse is subjected to change with the laser intensity due to alteration in refractive index associated with the variation of the nonlinear plasma density. The pulse front sharpened while the back of the pulse broadened due to difference in the group velocity at different parts of the laser pulse. Thus the distortion in the shape of the laser pulse is expected. We present 2D particle-in-cell simulations demonstrating the controlling the shape distortion of a Gaussian laser pulse using a parabolic plasma channel. We show the results of the intensity distribution of laser pulse in a plasma with and without a plasma channel. It has been observed that the plasma channel helps in controlling the laser pulse shape distortion. The understanding of evolution of laser pulse shape may be crucial while applying the parabolic plasma channel for guiding the laser pulse in plasma based accelerators.

Laser pulse heating of steel mixing with WC particles in a irradiated region

Laser pulse heating of steel mixing with tungsten carbide (WC) particles is carried out. Temperature field in the irradiated region is simulated in line with the experimental conditions. In the analysis, a laser pulse parameter is introduced, which defines the laser pulse intensity distribution at the irradiated surface. The influence of the laser parameter on the melt pool size and the maximum temperature increase in the irradiated region is examined. Surface temperature predictions are compared with the experimental data. In addition, the distribution of WC particles and their re-locations in the treated layer, due to combination of the natural convection and Marangoni currents, are predicted. The findings are compared to the experimental data. It is found that surface temperature predictions agree well with the experimental data. The dislocated WC particles form a streamlining in the near region of the melt pool wall, which agree with the experimental findings. The Gaussian distribution of the laser pulse intensity results in the maximum peak temperature and the maximum flow velocity inside the melt pool. In this case, the melt pool depth becomes the largest as compared to those corresponding to other laser pulse intensity distributions at the irradiated surface.

Non-equilibrium energy transport and entropy production due to laser short-pulse irradiation

Laser short-pulse heating of a nano-size wire is considered and entropy generation rate is predicted during the heating pulse. The analytical solution of the heat equation is obtained using the Lie point symmetry for the laser short-pulse heating. The nano-size wire is assumed to be symmetric along its y-axis. Laser pulse intensity is considered to be Gaussian at the irradiated surface while the exponential decay of the laser pulse is incorporated in the time domain. It is found that surface temperature variation in the lattice subsystem almost follows the laser pulse intensity distribution at the surface. Entropy generation rate attains low values along the symmetry axis and it increases considerably in the region of the nano-size wire edges. This behavior is associated with the temperature gradient, which attains high values in the region close to the nano-size wire edge.

Spatially Fourier-encoded photoacoustic microscopy using a digital micromirror device

We have developed spatially Fourier-encoded photoacoustic (PA) microscopy using a digital micromirror device. The spatial intensity distribution of laser pulses is Fourier-encoded, and a series of such encoded PA measurements allows one to decode the spatial distribution of optical absorption. The throughput and Fellgett advantages were demonstrated by imaging a chromium target. By using 63 spatial elements, the signal-to-noise ratio in the recovered PA signal was enhanced by ∼4×. The system was used to image two biological targets, a monolayer of red blood cells and melanoma cells.

Intensity distribution of a capillary-discharge 46.9 nm soft X-ray laser

We realize a Ne-like Ar 46.9 nm soft X-ray laser pumped by a capillary discharge. The study of the laserpulse-intensity distribution is important for applications of soft X-ray lasers. The intensity distribution demonstrates the gain distribution, plasma radius, and axial plasma density that contribute to the study of the laser-pulse formation. To measure the intensity in different positions of the X-ray laser spot, we moved transversally an X-ray diode (XRD) assembled with a slit. We obtain the onedimensional intensity distribution. We find a laser divergence (FWHM) of 4.0 mrad. According to the gain-guided model, we calculate the intensity distribution. The measured divergence of 4.0 mrad roughly corresponds to a plasma radius a approximately equal to 230–250 μm and on-axis electron density ne≈ 8.0∙1018−9.0∙1018 cm−3. The results of calculations indicate that the divergence of the intensity distribution increases when the plasma radius decreases and the on-axis electron density increases.

The effect of atomic potential on the above threshold ionization

We investigate theoretically the influence of the long-range and short-range potentials on the plateau structure of the above threshold ionization. In a considerable range of laser parameter, the above threshold ionization spectra of the atoms in the long-range potential always exhibit a clear double-plateau structure; as for the atoms with a short-range potential, the boundary of the double-plateau in photoelectron spectra is no longer clear, and with the decrease of laser intensity, it transits from the double-plateau to the single-plateau gradually. The numerical simulation based on classical analysis and quantum mechanics illustrates that in different model potentials, the distinction of ionization rates as well as the difference of the electronic elastic rescattering cross-sections results in the difference of plateau structures. In addition, the influence of intensity distribution of laser pulse on the phenomenon is discussed.

Laser induced melt pool formation in titanium surface: influence of laser scanning speed

PurposeA model study of laser heating process including phase change and molten flow in the melt pool gives physical insight into the process and provides useful information on the influence of melting parameters. In addition, the predictions reduce the experimental cost and minimize the experimental time. Consequently, investigation into laser control melting of the titanium alloy becomes essential. The purpose of this paper is to do this.Design/methodology/approachLaser repetitive pulse heating of titanium surface is investigated and temperature field as well as Marangoni flow in the melt pool is predicted using finite volume approach. The influence of laser scanning speed and laser pulse parameter (defining the laser pulse intensity distribution at the workpiece surface) on temperature distribution and melt size is examined. The experiment is carried out to validate temperature predictions for two consecutive laser pulses.FindingsThe influence of laser scanning speed is significant on the melt pool geometry, which is more pronounced for the laser pulse parameter β=0. Temperature predictions agree with the thermocouple data obtained from the experiment.Research limitations/implicationsAlthough temperature dependent properties are used in the simulations, isotropy in properties may limit the simulations. The laser canning speed is limited to 0.3 m/s, which is good for surface treatment process, but it may slow for annealing treatments.Practical implicationsThe results are very useful to capture insight into the melting process. In addition, the influence of laser scanning speed and laser pulse intensity distribution on the melt formation in the surface vicinity is well presented, which will be useful for those working on laser surface treatment process.Originality/valueThe work is original and findings are new, which demonstrate the influence of laser parameters on the melt pool formation and resulting Marangoni flow.

Laser heating of titanium and steel: Phase change at the surface

Laser heating of titanium and steel surface is carried out and temperature field as well as the size of the melt pool developed at the irradiated surface is predicted. The repetitive laser pulses are introduced and Marangoni effect is incorporated in the analysis to obtain the flow field in the melt pool. The laser pulse parameter is introduced to account for the influence of laser pulse intensity distribution on temperature field and the melt size. The influence of laser scanning speed on heating is also incorporated in the simulations. An experiment is carried out to verify temperature predictions at the surface. It is found that temperature predictions agree well with the experimental data. The influence of laser pulse intensity parameter and laser scanning speed on temperature and melt pool size is significant, which is more pronounced for steel.

Laser repetitive pulse heating and melt pool formation at the surface

In this paper, laser repetitive pulse heating of steel surface is conducted to predict temperature and melt pool geometry in the irradiated region. The enthalpy porosity method is incorporated to account for the phase change in the irradiated surface. The flow field in the melt pool is simulated after considering the Marangoni effect. To examine the influence of the laser pulse intensity distribution on the formation and flow field in the melt pool, the laser pulse intensity parameter (β) is introduced in the analysis. An experiment is conducted to compare the melt pool geometry predicted from the simulation and that obtained from the experiment. The melt pool size is significantly affected by the intensity parameter (i.e., the melt pool is too shallow for high-intensity parameters). The predicted melt pool geometry is in agreement with the experimental results.

Laser melting of carbide tool surface: Model and experimental studies

Laser controlled melting is one of the methods to achieve structural integrity in the surface region of the carbide tools. In the present study, laser heating of carbide cutting tool and temperature distribution in the irradiated region are examined. The phase change process during the heating is modeled using the enthalpy–porosity method. The influence of laser pulse intensity distribution across the irradiated surface ( β) on temperature distribution and melt formation is investigated. An experiment is carried out and the microstructural changes due to laser consecutive pulse heating is examined using the scanning electron microscope (SEM). It is found that melt depth predicted agrees with the experimental results. The maximum depth of the melt layer moves away from the symmetry axis with increasing β.

レーザーフラッシュ法熱拡散率測定における試料の異方性の影響

An analytical study on thermal diffusivity for anisotropic materials using a laser flash method was carried out to clarify the applicability of the method which has been established for an isotropic materials. A measurement error for anisotropic materials caused by the laser pulse intensity distribution was estimated analytically using a finite element calculation code. In this paper the analytical result on the measurement error of the anisotropic materials was presented and the applicability of the laser flash method to the anisotropic material was discussed.