Simulation Of Laser Beam Propagation Research Articles

Effect of Water and Aerosols Absorption on Laser Beam Propagation in Moist Atmosphere at Eye-Safe Wavelength of 1.57 μm

To realize optical wireless power transmission, atmospheric propagation of eye-safe wavelength (1.57μm) laser beams was theoretically investigated. Laser beams are affected by the presence of water vapor and aerosols which absorb and scatter the laser energy. The scattering coefficients of water molecules and aerosols were estimated to be about 6.3 × 10^-7 and 5.6 × 10^-5 m^-1, respectively, at wavelength (λ₀) of 1.57μm. Furthermore, the absorption coefficients of moist air at 30% relative humidity and aerosols were estimated to be about 6.16 × 10^-3 and 2.52 × 10^-5 m^-1, respectively, at λ₀ = 1.57μm. Then simulation of laser beam propagation in the moist atmosphere at λ₀ = 1.57μm was performed using these coefficients. Under the condition of no wind, the beam intensity decreases rapidly with increasing the length <i>z</i> and the rate of decrease slows down as the beam radius (ω) increases. When <i>z</i>_h is defined as the <i>z</i> where the normalized intensity is halved, the <i>z</i>_h (= 25 m) at ω = 20 mm when input power <I>P</I> = 10 W is about three times longer than that (= 8 m) when <I>P</I> = 100 W. This result indicates that the thermal distortion of laser beams due to accumulated heat around the <i>z</i> axis becomes more conspicuous as the optical power increases. The effect of this thermal beam distortion can be weakened when the laser beam is subject to crosswinds. Under the condition of gentle uniform wind with wind velocity <i>v</i> = 5 m/s, propagation of laser beams with ω = 20 mm was studied when <I>P</I> = 100 W. The <i>z</i>_h (= 105 m) when <i>v</i> = 5 m/s is about 13 times longer than that (= 8 m) when <i>v</i> = 0 m/s. Thus, under conditions of <i>v</i> = 5 m/s and 30% relative humidity, laser beams with <I>P</I> = 100 W and ω = 20 mm can propagate over 100 m without damaging the initial beam shape at λ₀ = 1.57μm.

Electrodynamic simulation of laser beam propagation in waterjet-guided laser processing

Waterjet-guided material processing is a technique that combines the capabilities of laser material processing with water jetting. In this study, we have investigated laser beam propagation in a waterjet column by numerically solving the Maxwell equations and the heat equation. A 1064 nm laser and its frequency-doubled 532 nm laser were chosen for the simulations, and the finite-difference time-domain (FDTD) method was employed for solving the Maxwell equations. The coupling effect of the laser and waterjet was simulated with different numerical apertures and different waterjet column diameters. Extensive investigations on laser absorption phenomena regarding the outer surface geometries of the waterjet (cylindrical shape with and without sinusoidal perturbation), and temperature distributions were also conducted. To the best of the authors' knowledge, this is the first electrodynamic simulation of laser beam propagation and interaction with a water column in waterjet-guided laser processing. Some interesting findings concerning laser beam absorption characteristics inside a waterjet column were revealed.

Transient simulation of laser beam propagation through turbulent cutting gas flow

Abstract For many laser machining applications, an assist gas is required. However, for applications with high pressure or temperature gradients, the density of the assist gas is not homogeneous. Thus, the laser propagation is influenced by the density properties within the turbulent gas. In this article, an overview and example results of the influence of a nozzle geometry on the light propagation in two dimensions are presented.

Giant irradiance spikes in laser beam propagation in volume turbulence: analysis and impact

In the wave-optics numerical simulations of laser beam propagation in volume atmospheric turbulence, we observe irregular appearance of giant intensity spikes with amplitudes exceeding the diffraction-limited intensity value by a factor of ten or even more. The presence of giant spikes may explain the existing significant difference between the scintillation index theoretical prediction and the results obtained in numerical simulation and experimental measurements. The giant spikes’ physics-based origin, probability of appearance, and impact on irradiance scintillation index are analyzed.

Accounting for the effect of large-scale atmospheric inhomogeneities in problems of laser radiation propagation along long high-altitude paths

We show that large-scale atmospheric inhomogeneities observed experimentally can significantly affect optical radiation propagation along long paths, and it is impossible to take into account this effect within the classical turbulence model. We suggest a new algorithm for numerical simulation of laser beam propagation along long low-inclined atmospheric paths with large-scale atmospheric inhomogeneities, which allows one to consider beam refraction and focusing at large-scale inhomogeneities along with beam distortions due to small-scale inhomogeneities and regular refraction in a vertical plane.

Effectiveness of the subharmonic method in problems of computer simulation of laser beam propagation in a turbulent atmosphere

The results of the analysis of the effectiveness of the subharmonic method for simulating large-scale turbulent inhomogeneities of the refractive index in problems of laser beam propagation in a turbulent atmosphere are presented in comparison with experimental data.

Simulation of laser beam propagation with a paraxial model in a tilted frame

We study the Schrödinger equation which comes from the paraxial approximation of the Helmholtz equation in the case where the direction of propagation is tilted with respect to the boundary of the domain. In a first part, a mathematical analysis is made which leads to an analytical formula of the solution in the simple case where the refraction index and the absorption coefficients are constant. Afterwards, we propose a numerical method for solving the initial problem which uses the previous analytical expression. Numerical results are presented. We also sketch an extension to a time dependent model which is relevant for laser–plasma interaction.

Analyses of laser-plasma interactions in National Ignition Facility ignition targets

A capability to analyze laser-plasma interactions (LPI) for ignition targets to be fielded at the National Ignition Facility has been developed and exercised. LPI in these targets may cause direct energy loss (backscatter) or energy redirection (beam spray, deflection, and energy transfer). These analyses range from analyzing the gain exponents for backscatter and beam spray to performing massively parallel, three-dimensional simulations of laser beam propagation in the most promising candidate ignition target designs. In the former assessment, ignition designs are iterated to reduce the gain exponent values. In the latter, beam propagation simulations are performed to analyze the reflectivity and beam transmission of speckled laser beams in the computed plasma profiles of the ignition targets. In current ignition designs, laser reflectivity is calculated to be well below 10%.

Monte Carlo simulation of laser beam propagation in a plane layer of the erythrocyte suspension: comparison of contributions from different scattering orders to the angular distribution of light intensity

The scattering phase functions of light are obtained for a layer of the erythrocyte suspension by the Monte Carlo method. At the erythrocyte concentration corresponding to a whole blood, these functions substantially differ from the phase function of a single erythrocyte. Contributions from the low-order and multiple scattering to the light intensity measured at different angles are compared. It is shown that scattering of light from a suspension layer of thickness of about 100 μm to the forward half-plane is mainly determined by the low-order scattering (by snake photons), whereas scattering to the back half-plane is mainly determined by multiple scattering. The possibility of using the diffuse approximation for the theoretical description of scattering is analysed.

Calculations of wave propagation through statistical random media, with and without a waveguide

Simulations of laser-beam propagation through atmospheric turbulence and acoustic pulse propagation though ocean internal waves in the presence of a transverse waveguide are described. Both these problems are amenable to the parabolic wave equation for propagation in the forward direction. In the optical case, the question treated is the irradiance variance and spatial spectrum. In the ocean case, pulse propagation over long ranges is investigated. Determining the travel time of a pulse requires expanding the numerical simulation in a broadband, multifrequency calculation that takes even more time. Much effort has been expended in approximating the propagation by rays, so that the trajectory of the energy propagating from a given source to a given receiver can be tracked, and coherence calculations can be ray-based, using semi-classical formulas. This paper reviews the comparisons of several analytic ray-based approximations with numerical parabolic-equation simulations to determine their accuracies.

Propagation of focused and multibeam laser energy in biological tissue.

The results of a Monte Carlo simulation of laser beam propagation in turbid media are presented. The study was performed to determine whether using a focused beam or multiple beams instead of a single collimated beam could improve subsurface laser energy delivery in biological tissue. A parametric study was carried out to determine both the laser fluence at a target depth and the ratio of fluence at the target over surface fluence as a function of tissue properties and the mode of energy delivery. It was found that the reduced scattering coefficient was the primary determinant as to whether multibeam or focused beam delivery could be effective. A focused beam was found to be extremely effective in increasing fluence at the target if the dimensionless reduced scattering coefficient was less than 2. The delivered fluence, however, was found to be extremely sensitive to tissue properties. A five-beam laser system was found to be less effective at increasing fluence at the target than a focused beam; but the fluence delivered by a five-beam system was far less sensitive to tissue properties, thereby making accurate dosimetry more feasible.

Simulation of laser beam propagation through an axisymmetric dense plasma

A numerical simulation is reported of the parameters of laser radiation after its propagation through an axisymmetric dense plasma. It is shown that, for typical laser plasma and Z pinch parameters, the depolarisation of probe radiation is considerably less than in the case of the magneto-optical Faraday effect. The conclusions drawn from this simulation are supported by experimental probing of a laser plasma. New direct methods for measuring the magnetic induction and the frequency of electron—ion collisions are proposed.

Numerical simulation of laser beam propagation in three-dimensional random media: beam splitting and patch formation

Abstract The results of numerical simulations of far-field, focused beam propagation in a homogeneous, Kolmogorov field are compared with theoretical predictions. It is shown that the spot structure at the focal plane depends on two parameters: the diffraction parameter ηD, which is defined as the ratio of the Fresnel length 2Z/kD 0 to the initial beam diameter D 0, and an analogous turbulence parameter ηtu=2Z/k(ρ0 LT)2, where Z is the focal distance, k is the wavenumber in vacuum, and ρ0 LTis the long-term far-field coherence length. For ηtu≫1 the beam is broken into multiple patches of typical size that is smaller than the transmitter diffraction scale. It is found that patches are also formed when ηtu≤1 provided the initial diameter D 0 is much bigger than ρ0 LT. The structure of the spot when ηtu≪1 depends on the value of ηD. For ηD≫1, the beam intensity pattern can be very distorted, however the spot is not dispersed.

Diffraction modeling of a space relay experiment

Optical system modeling is particularly important for simulation of laser beam propagation over large distances. This paper describes the diffraction modeling of the propagation of a laser beam between two points on the surface of the earth, using relay mirrors on one or more satellites. Because of the importance of the atmospheric and propagation effects, computer modeling is necessary to develop understanding of the first-order design parameters. Some of the significant design parameters are characterization of the adaptive mirror for the atmospheric aberration correction, size and finish of the beam expander, apertures of the relay mirror, and acceptable levels of jitter. An example of a propagating laser beam is presented, demonstrating the three-dimensional nature of this model.