Laser Beam Propagation Research Articles

Relativistic calculation of the electron momentum shift in tunneling ionization

SynopsisWe describe a procedure for the solution of the time-dependent Dirac equation. The procedure is based on the relativistic generalization of the matrix iteration method. We use this procedure to study electron momentum distribution along the laser beam propagation direction for the process of the tunneling ionization of hydrogen atom. We found, in agreement with the experimental observations [1], that relativistic effects lead to appreciable deviation of the distribution from the strict left-right symmetry present in the non-relativistic case. The expectation value of the momentum along the laser beam propagation direction grows linearly with intensity and follows closely the behavior of the expectation value of the kinetic energy divided by the speed of light. These features agree with the experimental results [1].

Stimulated Brillouin backscattering of a ring-rippled laser beam in collisionless plasma

AbstractThe effect of the propagation of a ring-rippled laser beam in the presence of relativistic and ponderomotive non-linearities on the excitation of ion-acoustic wave (IAW) and resulting stimulated Brillouin backscattering in collisionless plasma at relativistic powers is studied. To understand the nature of propagation of the ring ripple-like instability, a paraxial-ray approach has been invoked in which all the relevant parameters correspond to a narrow range around the irradiance maximum of the ring ripple. Modified coupled equations for growth of ring ripple in the plasma, generations of IAW and back-stimulated Brillouin scattering (SBS) are derived from fluid equations. These coupled equations are solved analytically and numerically to study the intensity of ring-rippled laser beam and excited IAW as well as back reflectivity of SBS in the plasma for various established laser and plasma parameters. It is found that the back reflectivity of SBS is enhanced due to the strong coupling between ring-rippled laser beam and the excited IAW. The results also show that the back reflectivity of SBS reduce for higher intensity of the laser beam.

Terahertz-to-infrared emission through laser excitation of surface plasmons in metal films with porous nanostructures.

We report on the investigation of terahertz-to-infrared (THz-to-IR) thermal emission that relies on the excitation of surface plasmons in metal films deposited on a substrate with randomly ordered nanoscale pore arrays. The THz-to-IR radiation was observed both in the direction of laser beam propagation and the reverse direction. The intensity ratio between backward and forward radiation is exponentially dependent on the nominal thickness of the porous metal films. The findings are discussed in view of the proposed generation mechanism based on propagating surface plasmon polaritons on both air/metal and metal/substrate interfaces.

Wave structure function and long-exposure MTF for laser beam propagation through non-Kolmogorov turbulence

Abstract The degrading effects of atmospheric turbulence on an imaging system can be characterized by the atmospheric modulation transfer function (MTF). In this paper, we derived analytically a new expression for the wave structure function (WSF) of a Gaussian-beam based on the weak fluctuation theory. We assumed the beam-wave is propagating through a horizontal path experiencing isotropic and homogeneous non-Kolmogorov atmospheric turbulence where the power spectrum has a generalized spectral power-law exponent which varies between 3 and 4 instead of the fixed classical Kolmogorov power-law exponent of 11/3. Using the WSF, we derived mathematical expressions for the spatial coherence radius and the long-exposure turbulence MTF of the Gaussian-beam wave. These new expressions were used to analyze the influence of power-law variations and beam sizes on the WSF and quality of imaging systems. The simulation results show that different exponent values produce varying effects on both WSF and imaging systems.

Measurement system with high accuracy for laser beam quality.

Presently, most of the laser beam quality measurement system collimates the optical path manually with low efficiency and low repeatability. To solve these problems, this paper proposed a new collimated method to improve the reliability and accuracy of the measurement results. The system accuracy controlled the position of the mirror to change laser beam propagation direction, which can realize the beam perpendicularly incident to the photosurface of camera. The experiment results show that the proposed system has good repeatability and the measuring deviation of M² factor is less than 0.6%.

Excitation of electron plasma wave by filamented laser beam and third-harmonic generation in magneto plasma

AbstractThe combined effects of filamentation and magnetic field on the third-harmonic generation of electromagnetic beams have been investigated considering extended paraxial rays in magneto plasma. The analysis is done using eikonal method in which eikonal and other relevant quantities are extended up to fourth power of r. The time scale of laser beam is chosen such that the relativistic mass variation of electron becomes dominated source of nonlinearity in refractive index. The expression for coupling between ultra-intense laser beam and electron plasma wave due to relativistic nonlinearity has been deduced. Interaction of the seed plasma wave with the incident filamented laser beam excites the plasma wave and generates third harmonics. The expressions for plasma wave power and third-harmonic power have been derived. The effect of the magnetic field on the power of plasma wave and the third-harmonic power has been carried out. The role of magnetic field has been found to decrease the power of plasma wave and so as the power of third harmonic. Our results can be helpful for various laser plasma diagnostics experiments in which magnetic field present externally or generated spontaneously in the high-power laser–plasma interaction.

Near-vacuum hohlraums for driving fusion implosions with high density carbon ablatorsa)

Recent experiments at the National Ignition Facility [M. J. Edwards et al., Phys. Plasmas 20, 070501 (2013)] have explored driving high-density carbon ablators with near-vacuum hohlraums, which use a minimal amount of helium gas fill. These hohlraums show improved efficiency relative to conventional gas-filled hohlraums in terms of minimal backscatter, minimal generation of suprathermal electrons, and increased hohlraum-capsule coupling. Given these advantages, near-vacuum hohlraums are a promising choice for pursuing high neutron yield implosions. Long pulse symmetry control, though, remains a challenge, as the hohlraum volume fills with material. Two mitigation methodologies have been explored, dynamic beam phasing and increased case-to-capsule ratio (larger hohlraum size relative to capsule). Unexpectedly, experiments have demonstrated that the inner laser beam propagation is better than predicted by nominal simulations, and an enhanced beam propagation model is required to match measured hot spot symmetry. Ongoing work is focused on developing a physical model which captures this enhanced propagation and on utilizing the enhanced propagation to drive longer laser pulses than originally predicted in order to reach alpha-heating dominated neutron yields.

Determining the phase and amplitude distortion of a wavefront using a plenoptic sensor.

We have designed a plenoptic sensor to retrieve phase and amplitude changes resulting from a laser beam's propagation through atmospheric turbulence. Compared with the commonly restricted domain of (-π,π) in phase reconstruction by interferometers, the reconstructed phase obtained by the plenoptic sensors can be continuous up to a multiple of 2π. When compared with conventional Shack-Hartmann sensors, ambiguities caused by interference or low intensity, such as branch points and branch cuts, are less likely to happen and can be adaptively avoided by our reconstruction algorithm. In the design of our plenoptic sensor, we modified the fundamental structure of a light field camera into a mini Keplerian telescope array by accurately cascading the back focal plane of its object lens with a microlens array's front focal plane and matching the numerical aperture of both components. Unlike light field cameras designed for incoherent imaging purposes, our plenoptic sensor operates on the complex amplitude of the incident beam and distributes it into a matrix of images that are simpler and less subject to interference than a global image of the beam. Then, with the proposed reconstruction algorithms, the plenoptic sensor is able to reconstruct the wavefront and a phase screen at an appropriate depth in the field that causes the equivalent distortion on the beam. The reconstructed results can be used to guide adaptive optics systems in directing beam propagation through atmospheric turbulence. In this paper, we will show the theoretical analysis and experimental results obtained with the plenoptic sensor and its reconstruction algorithms.

Efficient propagation of ultra-intense laser beam in dense plasma

Ultra intense laser propagation in extended, dense plasma is investigated through optical and proton probing. When a >1 kJ, 10 ps laser propagates into a long-density scale length plasma, channel formation was observed up to 0.6 nc from the analysis of optical probe images. The proton track analysis shows the formation of strong electric and magnetic fields along the plasma channel, which may lead to the observed collimated electron beam on the laser axis. These results are promising for the feasibility of the direct irradiation scheme of fast ignition.

Self-shaping of a relativistic elliptically Gaussian laser beam in underdense plasmas

AbstractSelf-shaping and propagation of intense laser beams of different radial profiles in plasmas is investigated. It is shown that when a relativistic elliptically Gaussian beam propagates through an underdense plasma, its radial profile will self-organize into a circularly symmetric self-similar smooth configuration. Such self-similar propagation can be attributed to a soliton-like structure of the laser pulse. The anisotropic electron distribution results in a circular electric field that redistributes the electrons and modulates the laser pulse to a circular radial shape.

Relativistic self-focusing of intense laser beam in thermal collisionless quantum plasma with ramped density profile

Propagation of a Gaussian x-ray laser beam has been analyzed in collisionless thermal quantum plasma with considering a ramped density profile. In this density profile due to the increase in the plasma density, an earlier and stronger self-focusing effect is noticed where the beam width oscillates with higher frequency and less amplitude. Moreover, the effect of the density profile slope and the initial plasma density on the laser propagation has been studied. It is found that, by increasing the initial density and the ramp slope, the laser beam focuses faster with less oscillation amplitude, smaller laser spot size and more oscillations. Furthermore, a comparison is made among the laser self-focusing in thermal quantum plasma, cold quantum plasma and classical plasma. It is realized that the laser self-focusing in the quantum plasma becomes stronger in comparison with the classical regime.

Relativistic calculation of the electron-momentum shift in tunneling ionization

We describe a procedure for the solution of the time-dependent Dirac equation. The procedure is based on the relativistic generalization of the matrix iteration method. We use this procedure to study electron-momentum distribution along the laser-beam propagation direction for the process of the tunneling ionization of a hydrogen atom. We found, in agreement with the experimental observations [C. T. L. Smeenk, L. Arissian, B. Zhou, A. Mysyrowicz, D. M. Villeneuve, A. Staudte, and P. B. Corkum, Phys. Rev. Lett. 106, 193002 (2011)], that relativistic effects lead to appreciable deviation of the distribution from the strict left-right symmetry present in the nonrelativistic case. The expectation value of the momentum along the laser-beam propagation direction grows linearly with intensity and follows closely the behavior of the expectation value of the kinetic energy divided by the speed of light. These features agree with the experimental results [C. T. L. Smeenk, L. Arissian, B. Zhou, A. Mysyrowicz, D. M. Villeneuve, A. Staudte, and P. B. Corkum, Phys. Rev. Lett. 106, 193002 (2011)].

Optical characterisation and modelling for oblique near-infrared laser heating of carbon fibre reinforced thermoplastic composites

The optical behaviour of a carbon fibre reinforced thermoplastic composite material is investigated for a near infrared laser heating process applied to automated composite tape placement. A nip point heating strategy in laser tape placement results in a shadow before the nip point on both the incoming tape and substrate. The moderate laser angle relative to the surface of the composite leads to reflections in the cavity formed by the tape and the substrate, reducing the shadow. An optical ray tracing model can provide valuable insight to the interaction of the laser with the composite, as well as detailed estimation of the irradiance distributions. This paper provides the foundations for such a model, describing an optical characterisation process and formulation of appropriate models to capture the composite surface and laser source behaviour. A micro-half-cylinder surface treatment was shown to give a good approximation of the anisotropic scattering behaviour of the composite. Angular dependent reflectance was described well by Fresnel equations. An approximation of the laser beam profile and propagation is also presented.

Propagation of Gaussian beams family through a Kerr-type left-handed metamaterial

In this paper the propagation of elegant Hermite-cosh-Gaussian, elegant Laguerre Gaussian, and Bessel Gaussian beams through a Kerr left-handed metamaterial (LHM) slab have been studied. A split-step Fourier method is used to investigate the propagation of laser beams through this media. Numerical simulation shows that Gaussian beams have different focusing behaviors in a Kerr LHM slab with positive or negative nonlinearity. Beam focusing happens in slabs with positive nonlinearity and not in slabs with negative nonlinearity; however, negative nonlinearity is required for a Kerr LHM slab to act like a lens. Additionally, the focusing properties of beams can be controlled by controlling the thickness of the slab or the input power of the incident beam. A multilayer structure is also proposed to have beam focusing by thinner slabs and passing longer distances.

Discrete Energies of a Weakly Outcoupled Atom Laser Beam Outside the Bose–Einstein Condensate Region

We consider the possibility of a discrete set of energies of a weakly outcoupled atom laser beam to the homogeneous Schrodinger equation with anisotropic harmonic trap in Cartesian coordinates outside the Bose–Einstein condensate region. This treatment is used because working in the cylindrical coordinates is not really possible, even though we implement the cigar-shaped trap case. The Schrodinger equation appears to replace a set of two-coupled Gross– Pitaevskii equations by enabling the weak-coupling assumption. This atom laser can be produced in a simple way that only involves extracting the atoms in a condensate from by using the radio frequency field. We initially present the relation between condensates as sources and atom laser as an output by exploring the previous work of Riou et al. in the case of theoretical work for the propagation of atom laser beams. We also show that even though the discrete energies are obtained by means of an approaching harmonic oscillator, degeneracy is only available in two states because of the anisotropic external potential

RESEARCH ON THE RECTILINEAR MOTION SYSTEM OF THE GANTRY MODULE / TILTINIO MODULIO TIESIAEIGIŠKUMO PALAIKYMO SISTEMOS TYRIMAS

The paper submits experimental researches into guidance on the shuttle gantry module. The article has determined the influence of the roughness intensity of soil surface on the accuracy of guidance on the shuttle gantry module. The gantry module worked in automatic mode. A laser beam generator emitting a vertical beam has been placed at the end of the experimental field. A laser beam catcher equipped with photo diodes has been mounted onto the replaceable support of the positioned gantry module. Chassis deviations have been determined in two ways: when a laser beam trap with equipped a photodiode and the positioning have been mounted on the left chassis and when the laser beam equipped with a photodiode trap has been mounted in the middle of the beam, and the positioning wheel has been left on the left chassis. The conducted researches have shown that the roughness of the soil surface influences the accuracy of guidance. In the first case, deviations of the left chassis evolved from 45 mm to 98 mm, whereas those of the right one - from 38 mm to 150 mm. For determining the accuracy of guidance on the gantry module in the second case, deviations of the left chassis varied from 50 mm to 115 mm while those of the left one from 36 mm up to 120 mm. Šiame darbe pateikiami šaudyklinio tiltinio modulio (ŠTM) tiesiaeigiškumo palaikymo sistemos eksperimentinių tyrimų rezultatai. Ištirta dirvos paviršiaus nelygumų įtaka ŠTM tiesiaeigiškumo tikslumui. ŠTM dirbo automatiniu režimu tiesiaeigiškumą palaikant lazerine įranga. Lazerio spindulių generatorius, skleidžiantis vertikalų spindulių pluoštą modulio kurso link, statomas lauko gale. Lazerio spindulių pluošto gaudytuvas su fotodiodais montuojamas ant modulio stovo. Tiesiaeigiškumas buvo nustatomas dviem būdais: kai lazerio spindulių pluošto gaudytuvas su fotodiodais ir pozicionavimo ratukas buvo sumontuoti ant kairės važiuoklės; kai lazerio spindulių pluošto gaudytuvas su fotodiodais buvo sumontuoti sijos viduryje, o pozicionavimo ratukas paliktas ant kairės važiuoklės. Nustatyta, kad dirvos paviršiaus nelygumai turi įtakos ŠTM tiesiaeigiškumui. Nustatant pirmuoju būdu kairiosios važiuoklės nuokrypiai kito nuo 45 mm iki 98 mm, o dešiniosios nuo 38 mm iki 150 mm. Nustatant tiltinio modulio tiesiaeigiškumą antruoju būdu kairiosios važiuoklės nuokrypiai kito nuo 50 mm iki 115 mm, o dešiniosios – nuo 36 mm iki 120 mm.

Coaxial propagation of Laguerre–Gaussian (LG) and Gaussian beams in a plasma

AbstractThis paper investigates the non-linear coaxial (or coupled mode) propagation of Laguerre–Gaussian (LG) (in particular L01 mode) and Gaussian electromagnetic (em) beams in a homogeneous plasma characterized by ponderomotive and relativistic non-linearities. The formulation is based on numerical solution of non-linear Schrödinger wave equation under Jeffreys–Wentzel–Kramers–Brillouin approximation, followed by paraxial approach applicable in the vicinity of intensity maximum of the beams. A set of coupled differential equations for spot size (beam width) and phase evolution with space corresponding to coupled mode has been derived and numerically solved to determine the propagation dynamics. Using focusing equation a critical condition describing the self-trapped (i.e., spatial soliton) mode of laser beam propagation in the plasma has been discussed; as a consequence oscillatory focusing/defocusing of the beams in coupled mode propagation have been analyzed and presented graphically. As an important outcome, significant enhancement in the intensity of LG beam is noticed when it is coupled with the Gaussian mode.

Underwater optical communication performance for laser beam propagation through weak oceanic turbulence.

In clean ocean water, the performance of a underwater optical communication system is limited mainly by oceanic turbulence, which is defined as the fluctuations in the index of refraction resulting from temperature and salinity fluctuations. In this paper, using the refractive index spectrum of oceanic turbulence under weak turbulence conditions, we carry out, for a horizontally propagating plane wave and spherical wave, analysis of the aperture-averaged scintillation index, the associated probability of fade, mean signal-to-noise ratio, and mean bit error rate. Our theoretical results show that for various values of the rate of dissipation of mean squared temperature and the temperature-salinity balance parameter, the large-aperture receiver leads to a remarkable decrease of scintillation and consequently a significant improvement on the system performance. Such an effect is more noticeable in the plane wave case than in the spherical wave case.

Wavefront analysis of the laser beam propagating through a turbid medium

Laser beam propagation through a scattering suspension of polystyrene microspheres in distilled water is studied theoretically and experimentally. The dependence of wavefront aberrations on the particle concentration is investigated. The existence of symmetric wavefront aberrations of the laser beam passed through a turbid medium is shown.

EXPERIMENTAL VERIFICATION OF THE TURBULENT EFFECTS ON LASER BEAM PROPAGATION IN SPACE

In this work, we have modified an existing experimental setup to fully classify the thermal effects on a laser beam propagating in air. Improvements made to the setup include a new, more powerful laser, a precision designed turbulence delivery system, an imbedded pressure sensor, and a platform for height adjustability between the laser beam and the turbulence model. The setup was not only able to reproduce previous results exactly but also allowed new data for the turbulence strength C2n, the Rytov variance (scintillation) and the coherence diameter (Fried’s parameter) to be successfully measured. Analysis of the produced interferograms has been discussed using fast Fourier transforms. The results confirm, within the Kolmogorov regime, that phase and intensity fluctuations increase relative to temperature. The turbulent region exhibited very strong disturbances, in the range of 1.1 × 10–12 m–2/3 to 2.7 × 10–12 m–2/3. In spite of the strong turbulence strength, scintillation proved otherwise, since the condition for a weak turbulence environment was determined in the laboratory and a low scintillation index was to be expected. This is as a result of the relatively short propagation distances achieved in the laboratory. In the open atmosphere, path lengths extend over vast distances and in order for turbulent effects to be realized, the turbulence model must generate stronger turbulence. The model was, therefore, able to demonstrate its ability to fully quantify and determine the thermal turbulence effects on a propagating laser beam.