Laser Beam Propagation Research Articles

Застосування наноструктурованих люмінофорів у системах об’ємного оптичного запису

The principles of the organization of volume optical recording systems and synthesis of recording medum based on nanostructured pyrazoline luminophores were analyzed. Multi-layer photoluminescent media architecture with sandwich-structure of information and intermediate layers, information coding with lands’ length and peripheral areas was proposed. It was shown that thickness of the information layers depends on the on the information elements depth, while thickness of the intermediate layers depends on optical system objective lens numerical aperture and must be chosen big enough to divide readout signal from different layers. While all the structure of the disc is transparent and homogeneous the parasitic signal will be caused mostly by photoluminescece and absorption of pits areas where laser light is unfocused. For big quantity of layers the level noise will got significant value, so it was suggested to distinguish readout signal as a variable one. A mathematical model for reading data from a multi-layer photoluminescent media is proposed. To make a proper analysis of multi-layer photoluminescent media optimal characteristics it was developed computer simulation of the readout laser beam propagation process. There was built digital interference pattern using input parameters of the multi-layer photoluminescent drive and then calculated the path of the every single light point up to the objective lens. A methodology for estimating the parameters of a storage media based on volumetric optical recording systems which uses comparing of the system architecture parameters and the characteristics of the read-out signal has been developed. The algorithm for determining the optimal architecture of the storage device through the solution of the problem of finding an extremum is proposed.

On the general equivalence of the Fried parameter and coherence radius for non-Kolmogorov and oceanic turbulence

This article aims to discuss the general equivalence between two main metrics used in laser beam propagation through random media and imaging: the Fried parameter and the coherence radius. In particular, we show that their relationship deduced for Kolmogorov and non-Kolmogorov turbulence stays valid for other media as long as their spectral power laws are equal.

Scintillation analysis of LIDAR systems operating in weak-to-strong non-Kolmogorov turbulence: unresolved target case

We use a non-Kolmogorov power spectrum for theoretical investigation of laser beam propagation in the double-passage problem: transmitter–target–receiver. The major application of our work is the light detection and ranging (LIDAR) system operating in the zone of the atmosphere where non-Kolmogorov turbulence may be present. We show that the scintillation index is substantially affected by non-Kolmogorov turbulent channels at any turbulence strength. We consider the case of a small unresolved target and our analysis is valid for both the bistatic and monostatic configuration of the LIDAR system, in which the latter leads to the enhanced backscattering effects. Additionally, the paper contains a discussion of the simulation approach to light propagation through non-Kolmogorov turbulence. The simulation results and a discussion of their agreement with the theory are included. We found that the theory and the results of the simulations agree only to a certain extent. The “giant spikes” that occur when propagating through “deep turbulence” are responsible for the lack of better agreement. Their influence on the scintillation index and also on the probability density function of intensity probably requires a theoretical approach.

Megafilament in air formed by self-guided terawatt long-wavelength infrared laser

The diffraction-compensated propagation of high-power laser beams in air could open up new opportunities for atmospheric applications such as remote stand-off detection, long-range projection of high-energy laser pulses and free-space communications. Here, we experimentally demonstrate that a self-guided terawatt picosecond CO2 laser beam forms in air a single centimetre-scale-diameter megafilament that, in comparison with a short-wavelength laser filament, has four orders of magnitude larger cross-section and guides many joules of pulse energy over multiple Rayleigh distances at a clamped intensity of ~1012 W cm–2. We discover that this megafilament arises from the balance between self-focusing, diffraction and defocusing caused by free carriers generated via many-body Coulomb-induced ionization that effectively decrease the molecular polarizability during the long-wavelength laser pulse. Modelling reveals that this guiding scheme may enable transport of high-power picosecond infrared pulses over many kilometres in the 8–14 μm atmospheric transmission window. A terawatt picosecond CO2 laser beam is shown to form a centimetre-scale-diameter filament in air that is capable of carrying several joules of energy.

Algorithm of the Parallel Sweep Method for Numerical Solution of the Gross–Pitaevskii Equation with Highest Nonlinearities

In this paper, we for the first time introduce a numerical scheme the solution of a nonlinear equation of the Gross–Pitaevskii type (GP) or the nonlinear Schrodinger equation (NLSE) with highest nonlinearities, which provides implementation of a complete set of motion integrals. This scheme was parallelly implemented on a non-uniform grid. Propagation of a ring laser beam with non-zero angular momentum in the filamentation mode is studied using the implemented numerical scheme. It is shown, that filaments under exposure to centrifugal forces escape to the periphery. Based on a number of numerical experiments, we have found the universal property of motion integrals in the non-conservative case for a given class of equations. Research of dynamics of angular momentum for a dissipative case are also presented. We found, that angular moment, particularly normed by initial energy during filamentation process, is quasi-constant.

Energy transfer between lasers in low-gas-fill-density hohlraums

We investigate cross-beam energy transfer (CBET), where power is transferred from one laser beam to another via a shared ion acoustic wave in hohlraums with low-gas-fill density as a tool for late-time symmetry control for long-pulse (greater than $10\phantom{\rule{0.28em}{0ex}}\mathrm{ns}$) inertial confinement fusion (ICF) and laboratory astrophysics experiments. We show that the radiation drive symmetry can be controlled and accurately predicted during the foot of the pulse (until the rise to peak power), which is important for mitigating areal density variations in the compressed fuel in ICF implosions. We also show that the effective inner-beam drive after CBET is much greater than observed in previous high-gas-filled-hohlraum experiments, which is thought to be a result of less inverse bremsstrahlung absorption of the incident laser light and reduced (by more than 10 times) stimulated Raman scattering (and Langmuir wave heating). With the inferred level of inner-beam drive after transfer, we estimate that more than $1.25$ times larger plastic capsules could be fielded in this platform with sufficient laser-beam propagation to the waist of the hohlraum. We also estimate that a full-scale plastic capsule, $1100\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{m}$ in capsule radius, would require $\ensuremath{\sim}1--2\phantom{\rule{0.16em}{0ex}}\AA{}$ of $1\ensuremath{\omega}$ wavelength separation between the outer and inner beams to achieve a symmetric implosion in this platform.

Coherent propagation of laser beams in a small-sized system of weakly coupled optical light guides

The features of the self-action of a wave field in a small-sized optical system consisting of $2N$ identical light guides, which are disposed equidistantly along a ring, and an isolated fiber in the center are studied analytically and numerically. Stable exact solutions are found for intense wave beams in such a system, allowing long-distance coherent transport of radiation in a set of optical fibers. The total beam power in this case can exceed several times the critical power of self-focusing in the continuous medium. This is clearly manifested for the out-of-phase spatial distribution of ${u}_{n}={(\ensuremath{-}1)}^{n}f$, which is stable at arbitrary powers. Direct numerical simulation of the nonlinear unidirectional wave propagation equation confirms the stability of found wave field distributions.

Toward Defeating Diffraction and Randomness for Laser Beam Propagation in Turbulent Atmosphere

A large distance propagation in turbulent atmosphere results in disintegration of laser beam into speckles. We find that the most intense speckle approximately preserves both the Gaussian shape and the diameter of the initial collimated beam while loosing energy during propagation. One per 1000 of atmospheric realizations produces at 7km distance an intense speckle above 20\% of the initial power. Such optimal realizations create effective extended lenses focusing the intense speckle beyond the diffraction limit of vacuum propagation. Atmospheric realizations change every several milliseconds. We propose to use intense speckles to greatly increase the time-averaged power delivery to the target plane by triggering the pulsed laser operations only at times of optimal realizations. Resulting power delivery and laser irradiance at the intense speckles well exceeds both intensity of diffraction-limited beam and intensity averaged over typical realizations.

Response to "Comment on 'Water-water correlations in electrolyte solutions probed by hyper-Rayleigh scattering'" [J. Chem. Phys. 149, 167101 (2018)

The Comment proposes that the discrepancy between two second harmonic scattering (SHS) experiments for D2O electrolyte solutions with 100 ns and 190 fs laser pulses is due to unexplained processes for 100 ns pulses that give non-quadratic power dependence for the second harmonic signal. However, the different power dependence of the second harmonic scattering signals measured with 100 ns and 190 fs laser pulses is due to changes in laser beam propagation and focal intensity caused by the combined effect of thermal defocusing and Kerr lens self-focusing. Non-quadratic power dependence does not explain the discrepancy in the second harmonic scattering results.

Resonant third harmonic generation of super-Gaussian laser beam in a rippled density plasma

Resonant third harmonic generation of super-Gaussian laser beam in rippled density plasma is studied. Both the relativistic and ponderomotive nonlinearities are included in the analysis, and the study is done for self-guided laser beam. The quasi-static component of ponderomotive force creates electron density depression in the beam region while the second harmonic component leads to second harmonic density oscillations, leading to third harmonic generation. The relativistic mass variation supplements these processes with same order of contributions. The ripple provides the phase matching and requisite ripple wave number decreases with the frequency of the laser.

Generation of high-average-power ultra-broadband infrared radiation

High-average-power ultra-broadband mid-IR radiation can be generated by illuminating a nonlinear medium with a multi-line laser radiation. Propagation of a multi-line pulsed CO2 laser beam in a nonlinear medium, e.g., gallium arsenide or chalcogenide, can generate directed broadband IR radiation in the atmospheric window (2–13 μm). A 3D laser code for propagation in a nonlinear medium has been developed to incorporate extreme spectral broadening resulting from the beating of several wavelengths. The code has the capability to treat coupled forward and backward-propagating waves, as well as transverse and full linear dispersion effects. Methods for enhancing the spectral broadening are proposed and analyzed. Grading the refractive index radially or using a cladding will tend to guide the CO2 radiation and extend the interaction distance, allowing for enhanced spectral broadening. Nonlinear coupling of the CO2 laser beam to a backward-propagating reflected beam can increase the rate of spectral broadening in the anomalous dispersion regime of a medium. Laser phase noise associated with the finite CO2 linewidths can significantly enhance the spectral broadening, as well. In a dispersive medium, laser phase noise results in laser intensity fluctuations. These intensity fluctuations result in spectral broadening due to the self-phase modulation mechanism. Finally, we present propagation through a chalcogenide fiber as an alternative for extreme spectral broadening of a frequency-doubled CO2 multi-line laser beam.

Enhancement of X-ray emission from nanocolloidal gold suspensions under double-pulse excitation.

Enhancement of X-ray emission was observed from a micro-jet of a nano-colloidal gold suspension in air under double-pulse excitation of ultrashort (40 fs) near-IR laser pulses. Temporal and spatial overlaps between the pre-pulse and the main pulse were optimized for the highest X-ray emission. The maximum X-ray intensity was obtained at a 1–7 ns delay of the main pulse irradiation after the pre-pulse irradiation with the micro-jet position shifted along the laser beam propagation. It was revealed that the volume around gold nanoparticles where the permittivity is near zero, ε ≈ 0, accounts for the strongest absorption, which leads to the effective enhancements of X-ray emission.

The effects of atmospheric turbulence on laser beam propagation in a closed space—An analytic and experimental approach

The main purpose of this article was to derive a formula for the prediction of laser beam deviations under conditions of air turbulence in closed space. The theoretical analysis yielded its own formula for the prediction of the spatial variability of the laser beam. In this work, only the analysis of the impact of air turbulence on changes in laser wave propagation was discussed, while other meteorological parameters (pressure, humidity) and particles of pollution (dust, smoke) were ignored. The suitability of the formula was checked in relation to industrial metrology. Finally, the experimental setup and the method for measuring the coefficient of turbulence structure Cn were described.

Laser space debris cleaning: elimination of detrimental self-focusing effects

A ground-based laser system for space debris cleaning requires pulse power well above the critical power for self-focusing in the atmosphere. Self-focusing results in beam quality degradation and is detrimental for the system operation. We demonstrate that, for the relevant laser parameters, when the thickness of the atmosphere is much less than the focusing length (that is, of the orbit scale), the beam transit through the atmosphere produces the phase distortion only. The model thus developed is in very good agreement with numerical modeling. This implies that, by using phase mask or adaptive optics, it may be possible to eliminate almost completely the impact of self-focusing effects in the atmosphere on the laser beam propagation.

The effect of atmospheric turbulence on the quality of complete and partially coherent of flat-topped beam with circular, rectangular and elliptical symmetry

The laser beam propagation in irregular environments (e.g. atmosphere and ocean) has many applications, especially in free-space optical (FSO) communication systems. The growing use of such communications in applications such as laser, optical telecommunication radars and some information processing techniques has caused the growth and advancement of researchers in this field. The study of the properties of laser-emission in a turbulent atmosphere is a fundamental issue in laser physics and its applications. It is necessary to investigate the influence of atmospheric turbulence on the quality factor (M2-factor or propagation factor) of laser beams with various symmetries to control the types of laser beams in a turbulent atmosphere. In this article, the M2-factor of complete and partially coherent of flat-topped (PCFT) beams with circular, rectangular and elliptical symmetries are investigated in turbulent atmosphere. The results show that the intensity of three beams, with the propagation in the turbulent atmosphere becomes less and with the lower coherence length the intensity gets less. With an increasing wavelength of beams, M2-factor decreases and thus the quality of the beams is improved. The effect of the turbulent atmosphere on the partially coherent beam is much less than a complete coherent beam, thus the coherent reduction of the laser space is proposed as an approach to decrease the effects of the atmosphere. By decreasing the refractive index structure, the M2-factor of the beams becomes less so the quality of the beam becomes better. The investigations show that the PCFT beams have the best quality with elliptical symmetry and the worst quality with the circular symmetry in the turbulent atmosphere. In a complete unlike partially coherent at the beginning of the path, circular beam quality is better than rectangular and elliptical beams but circular beam quality becomes worse after a distance.

Self-focusing and defocusing of cosh Gaussian laser beam in the presence of nonlinearity of ponderomotive force and temperature gradient

In this paper, the propagation of Cosh Gaussian laser beam and its interaction with isothermal plasma without temperature gradient as well as the effect of the exponential electron temperature gradient are investigated. Here the ponderomotive nonlinearity force is effective mechanism. This force can modify the electron density distribution. All the investigations are carried out for different initial plasma temperatures. Using Maxwell’s equations we obtained the nonlinear second-order differential equation of the dimensionless beam-width parameter (f) on the distance of propagation for several initial electron temperatures and exponential temperature variations. These equations are solved numerically by taking WKB and paraxial approximation. Under the effect of initial electron temperature, self-focusing and defocusing of hyperbolic cosine (cosh) Gaussian laser beam is distinguished. Furthermore, the exponential temperature gradient cause to stationary propagation mode breaks, and self-focusing or defocusing properties is observable.

Effect of Critical Beam Radius on Self-focusing of cosh-Gaussian Laser Beams in Collisionless Magnetized Plasma

Effect of critical beam radius on self-focusing of cosh-Gaussian laser beams in collisionless magnetized plasma under ponderomotive nonlinearity forms the main core of present work. To investigate propagation dynamics of cosh-Gaussian laser beams in collisionless magnetized plasma, well established parabolic equation approach under WKB and paraxial approximations is employed. Our study is crucially pivoted on the concept of critical curve and subsequent determination of numerical interval for decentered parameter to sustain the competition between diffraction and self-focusing during the propagation of laser beam. Additionally, in the present study an interesting feature in the self-focusing region of the critical curve has been attempted for different values of decentered parameter.

Optical sectioning in multifoci Raman hyperspectral imaging

AbstractIn this study, we compared the depth discrimination and speed performance of multifoci Raman hyperspectral imaging with the reference standard of a single laser point confocal Raman mapping. A liquid crystal spatial light modulator was employed for the generation of multifoci laser beams, and a digital micromirror device was used as a software‐configurable reflective pinhole array. The patterns of the laser foci and pinhole array can be rapidly changed without requiring any hardware alterations. Confocal patterns with different distance‐to‐size ratios were tested and compared. After optimization of the laser‐foci pattern, we demonstrated the feasibility of multifoci Raman hyperspectral microscopy for recording depth‐resolved molecular maps of biological cells (Acanthamoeba castellanii trophozoites). Micrometric depth discrimination and short acquisition times (20 min for single plane confocal image) were achieved.

Magnetic field generation by amplitude modulated laser pulse in a rippled plasma

The mechanism of a self-generated magnetic field by an amplitude modulated laser beam propagation through a rippled plasma is analyzed. The amplitude modulated laser beam exerts a ponderomotive force on plasma electrons, giving them oscillatory velocities at the modulation frequency. The oscillatory velocity couples with the electron density ripple and drives a nonlinear current. This nonlinear current generates a magnetic field. The magnetic field amplitude shows the resonant enhancement at the laser modulation frequency close to the electron plasma frequency, proportional to the ripple density and the laser intensity. In the relativistic case, it also scales directly with the electron plasma frequency and inversely proportional to the square of laser spot size and can approach the order of several hundred-kilogauss.

An open source virtual laboratory for the Schrödinger equation

A simple Python-based open source software library for the numerical simulation of the linear or nonlinear time-dependent Schrödinger equation in one and two dimensions is presented. The integration is performed using a first-order split-step pseudospectral method, relying on the fast Fourier transform. The software library could be useful for undergraduate courses in elementary quantum mechanics, wave optics and computational physics. It could also be of interest for graduate students working with nonlinear waves, in frameworks such as laser beam propagation in nonlinear optical materials, matter waves within ultracold gases, dark matter or superfluid dynamics, among others. The discussion is complemented by solved examples and suggestions for educational applications of the code.