Laser Beam Propagation Research Articles

Vortex revivals and Fermi-Pasta-Ulam-Tsingou recurrence.

We study the self-trapped vortex-ring eigenstates of the two-dimensional Schrödinger equation with focusing Poisson and cubic nonlinearities. For each value of the topological charge l, there is a family of solutions depending on a parameter that can be understood as the relative importance of the cubic term. We analyze the perturbative stability of the solutions and simulate the fate of the unstable ones. For l=1 and l=2, there is an interval of the family of eigenstates for which the initial profile breaks apart but subsequently reconstructs itself, a process that can be interpreted as a nontrivial nonlinear oscillation between the vortex and an azimuthon. This revival provides a compelling realization of a recurrence of the Fermi-Pasta-Ulam-Tsingou type. Outside this interval, the vortices can be stable (for small cubic terms) or unstable and nonrecurrent (for large cubic terms). We argue that there is a crossover between these regimes that resembles a strong stochasticity threshold. For l≥3, all solutions are unstable and nonrecurrent. Finally, we comment on the possible experimental implementation of this phenomenon in the context of nonlinear laser beam propagation in thermo-optical media.

Time Series Irradiance Synthesizer for Optical GEO Satellite Downlinks in 5G Networks

Next generation 5G networks generate a need for broadband, low latency and power efficient backhauling and data-relay services. In this paper, optical satellite communications links, as an integrated component of 5G networks, are studied. More specifically, the Geostationary (GEO) satellite-to-ground optical communication link is investigated. Long-term irradiance statistics based on experimental measurements from the ARTEMIS program are presented and a new time series generator related to the received irradiance/power fluctuations due to atmospheric turbulence is reported. The proposed synthesizer takes into consideration the turbulence-induced scintillation effects that deteriorate the laser beam propagation, on the assumption of the Kolmogorov spectrum. The modeling is based on Rytov theory regarding weak turbulence conditions with the incorporation of first order stochastic differential equations. Summing up, the time series synthesizer is validated in terms of first and second order statistics with experimental results from the European Space Agency‘s ARTEMIS experimental optical downlink and simulated received power statistics for various weather conditions are presented using the proposed validated methodology. Some important conclusions are drawn.

Implementing Artificial Intelligence in Predicting Metrics for Characterizing Laser Propagation in Atmospheric Turbulence

The effects of a laser beam propagating through atmospheric turbulence are investigated using the phase screen approach. Turbulence effects are modeled by the Kolmogorov description of the energy cascade theory, and outer scale effect is implemented by the von Kármán refractive power spectral density. In this study, we analyze a plane wave propagating through varying atmospheric horizontal paths. An important consideration for the laser beam propagation of long distances is the random variations in the refractive index due to atmospheric turbulence. To characterize the random behavior, statistical analysis of the phase data and related metrics are examined at the output signal. We train three different machine learning algorithms in tensorflow library with the data at varying propagation lengths, outer scale lengths, and levels of turbulence intensity to predict statistical parameters that describe the atmospheric turbulence effects on laser propagation. tensorflow is an interface for demonstrating machine learning algorithms and an implementation for executing such algorithms on a wide variety of heterogeneous systems, ranging from mobile devices such as phones and tablets to large-scale distributed systems and thousands of computational devices such as GPU cards. The library contains a wide variety of algorithms including training and inference algorithms for deep neural network models. Therefore, it has been used for deploying machine learning systems in many fields including speech recognition, computer vision, natural language processing, and text mining.

Imaging Michelson interferometer for a low-density gas jet characterization.

A new optical probing method with increased interferometric sensitivity for a low-density gas jet characterization is presented. The proposed technique employs a Michelson interferometer with a self-imaging object arm, which enables the relay imaging of the object on itself, and in this way, the phase sensitivity of the device is increased by a double propagation of the object laser beam through the gas jet. The wavelength of 405 nm was chosen to further increase the sensitivity by increasing the probe wavenumber. A low-density argon gas jet with various backing pressures was characterized by our method and by a commonly used Mach-Zehnder interferometer setup showing the expected twofold increase in the signal to noise ratio in the double pass configuration.

Twisted laser beam propagation in a novel one-dimensional photonic crystal defected by an absorbent plasma layer

A novel tunable filter including the absorbent plasma layer affected by twisted electromagnetic beam propagation is indicated. The presented photonic crystal is composed by SiO2 and free space with one absorbent plasma defect layer, and that the transmittance and reflectance spectra are studied by using Transfer Matrix Method. It is found out that the transmittance spectrum can be adjusted by the plasma absorption rate, vortex charge number or intensity of incident laser beam. The dependence of the transmittance, reflectance and absorptivity spectra to the transverse coordinate is surveyed, particularly when a change in vortex charge number is applied.

Propagation of Cylindrical Vector Laser Beams in Turbid Tissue-Like Scattering Media

We explore the propagation of the cylindrical vector beams (CVB) in turbid tissue-like scattering medium in comparison with the conventional Gaussian laser beam. The study of propagation of CVB and Gaussian laser beams in the medium is performed utilizing the unified electric field Monte Carlo model. The implemented Monte Carlo model is a part of a generalized on-line computational tool and utilizes parallel computing, executed on the NVIDIA Graphics Processing Units (GPUs) supporting Compute Unified Device Architecture (CUDA). Using extensive computational studies, we demonstrate that after propagation through the turbid tissue-like scattering medium, the degree of fringe contrast for CVB becomes at least twice higher in comparison to the conventional linearly polarized Gaussian beam. The results of simulations agree with the results of experimental studies. Both experimental and theoretical results suggest that there is a high potential of the application of CVB in the diagnosis of biological tissues.

Engineering equation for filamentation self-focusing collapse distance in atmospheric turbulence.

The propagation of high peak-power laser beams in real atmospheres will be affected by both linear and nonlinear effects contained therein. Atmospheric turbulence usually will induce decreases in the filamentation self-focusing collapse distance for refractive index structure parameter increases. This paper provided the first validated analytical equation for predicting the nonlinear self-focusing collapse distance based on a modification of Petrishchev's and Marburger's theories. It shows that the estimate of the peak power to critical power at range in turbulence is modified to be the product of the transmitted peak power to critical power ratio times a multiplicative factor derived from Petrishchev's turbulence equations. This estimate is used in the Marburger distance equation to yield a predicted self-focusing collapse distance. This approach was compared to previous NRL's HELCAP computer simulation results and showed good agreement. The HELCAP simulations capability has shown good agreement between its results and a previously published laboratory-scale experiment. The analytical approach in this paper may provide a guide for further numerical simulations, more formal theoretical developments and field experiments.

Resonator to Laser Cavity Decoupling Interface for Chemical Oxygen Iodine Laser

In the present work, the conventional chemical oxygen iodine laser (COIL) source has been technologically upgraded and successfully tested after implementation of decoupling interface between laser cavity and resonator. In the technique, anti-reflection coated fused silica windows mounted in suitable mechanical assemblies are placed between resonator mirrors and laser cavity in such a way that the generated laser beam is out coupled through the resonator during the laser operation. The implemented decoupling assemblies isolate the caustic environment produced in low pressure laser cavity from that of resonator mirrors. Thus requirement of using isolation valves and cavity limbs between laser cavity and resonator as in conventional COIL source is eliminated. Such decoupling mechanism therefore effectively reduces the number of associated components as well as overall length of the laser source which in turn further reduces the overall weight of the laser making it suitable for use onto a mobile platform. Moreover the technique provides accrued benefits in terms of reduction in readiness time and checking of optical alignment of the laser source at will in practical operation scenarios.

Second harmonic generation of q-Gaussian laser beam in plasma channel created by ignitor heater technique

AbstractThis paper presents a scheme for second harmonic generation (SHG) of q-Gaussian laser beam in plasma channel created by ignitor heater technique. The ignitor beam creates plasma by tunnel ionization of air. The heater beam heats the plasma electrons and establishes a parabolic density profile. The third beam (q-Gaussian beam) is guided in this plasma channel under the combined effects of density nonuniformity of the plasma channel and relativistic mass nonlinearity of the plasma electrons. The propagation of q-Gaussian laser beam through the plasma channel excites an electron plasma wave at pump frequency that interacts with the incident laser beam to produce its second harmonics. The formulation is based on finding the numerical solution of the nonlinear Schrodinger wave equation for the fields of the incident laser beams with the help of moment theory approach. Particular emphasis is put on dynamical variations of the spot size of the laser beams and conversion efficiency of the second harmonics with distance of propagation.

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.

Formation of required distributions on the basis of decomposition by vortex eigen functions of a bounded non-paraxial propagation operator

The solution of the problem of overcoming the diffraction limit based on the representation of an optical signal in the form of a superposition of communication modes matched with the vortex eigenfunctions of a bounded (in the object and spectral regions) nonparaxial propagation operator in free space is considered. Nonparaxial propagation of laser beams is described using an expansion in terms of conic waves based on the m-th order Fourier-Hankel transform. The eigenfunctions of such an operator, which have near-unity eigenvalues, determine the number of degrees of freedom and characteristics of an optical signal transmitted without distortion over a given distance. Based on the considered approach, a parametric method was developed for solving the inverse diffraction problem, including overcoming the diffraction limit.

Peculiarities in the Distribution of Temperature under the Influence of a Laser Beam in a Multilayered Medium

One of the most widely used ways for modeling the propagation of laser beams in biological tissues is through the Monte Carlo method. This article focuses on the distribution of temperature in human skin based on the results of the Monte Carlo method for a laser beam with wavelength of 800. A mathematical model of the heat transfer process, based on the solution of the differential equations through the finite element method, was introduced. Simulations of the laser beam propagation and the distribution of temperature in the skin were performed for two types of beams. The results of this work can be used to make an assessment of the energy levels of pulse laser radiation with different durations, during which a damage of biological tissue is observed, as well as to create standards for the safe use of laser systems.

Stronger self-focusing of cosh-Gaussian laser beam under exponential density ramp in plasma with linear absorption

A theoretical model is purposed which explains propagation of the cosh-Gaussian laser beam in the plasma with exponential density transition. Also the influence of linear absorption on self-focusing of the beam is taken into account and it may be helpful in achieving enhanced self-focusing of laser beam. The nonlinearity arises because of the relativistic motion of electrons, is considered for the penetration of laser in plasma. By considering the expression of dielectric function, we have set up differential equation for the beam width parameter. Further, the effect of decentred parameter is studied which results in stronger self-focusing of cosh-Gaussian laser beam. Stronger self-focusing of laser beam is also noticed on account of the increased plasma frequency.

Selective Delamination upon Femtosecond Laser Ablation of Ceramic Surfaces

We report on the experimental observation of selective delamination of semi-transparent materials on the example of yttria-stabilized zirconia ceramics upon femtosecond laser processing of its surface with low numerical aperture lens. The delamination of a ceramic layer of dozens of micrometers takes place as a by-side effect of surface processing and is observed above the surface ablation threshold. The onset of delamination (delamination threshold) depends on the degree of overlap of the irradiation spots from consecutive laser pulses upon beam scanning over material surface. Analysis of the delaminated layer indicates that the material undergoes melting on its both surfaces. The mechanism of delamination is identified as a complex interplay between the optical response of laser-generated free-electron plasma and nonlinear effects upon laser beam propagation in semi-transparent ceramics. The discovered effect enables controllable laser microslicing of brittle ceramic materials.

Polarimetric laser beam diffraction in a tokamak plasma

In this paper the method of complex geometrical optics (CGO) is applied to the analysis of polarimetric laser beam propagation and diffraction in tokamak plasma. The paraxial CGO reduces the problem of beam diffraction in inhomogeneous medium to the set of ordinary differential equations, which significantly simplifies calculation as compared with full-wave approach. Numerical simulations for Gaussian beam, propagating in the poloidal plane of the inhomogeneous thermonuclear plasma, reveal differences in beam parameters evolution (beam path, width and phase front curvature) in comparison with homogeneous media. Such additional distortion of the beam must be taken into account in polarimetric system modelling. The presented approach may also be applied in other measurement techniques that use electromagnetic beams, like interferometry or refractometry – with limitations discussed in the text.

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.

Anharmonic Bloch oscillations in a coupler composed of two parallel optical fibre arrays

The effects of laser beam propagation in a coupler composed of two parallel optical-fibre arrays are studied by the coupled-mode method taking into account the interaction of a given fibre with the nearest neighbours and the linear dependence of the propagation constant on the fibre number. It is shown that, due to the complex structure of each subsystem, the structure of light intensity spatial distribution becomes significantly complicated in the system under study. The occurrence of space limited transverse light diffraction is predicted.

Conical refraction output from a Nd:YVO4 laser with an intracavity conerefringent element.

A conical refraction (CR) laser based on an a-cut Nd:YVO4 laser was demonstrated. By using a KGW crystal as a CR element, a typical laser with a Gaussian intensity output profile was transformed into a laser with conically refracted output. The CR laser delivered 220mW of output power for 500mW of pump power. The separation of the laser gain medium and the CR element reduced the complexity of the pumping scheme, and resulted in the generation of well-behaved CR laser beams with outstanding quality. The presented approach is power scalable and offers a unique possibility of studying the transformation of a Gaussian laser mode into a conically refracted one in a laser cavity.

Nonlinear dynamic instability of Landau-quantized graphene inside an optical ring cavity

Controlling dynamic behaviors, such as instability and chaos, in various systems has been a central matter of nonlinear physics in recent years. Here we employ an effective density-matrix method for the study of laser beam propagation through an optical ring cavity containing Landau-quantized graphene nanostructures. By taking into account the linear and nonlinear phase shifts, we detailedly investigate and characterize the nonlinear dynamic behaviors of the transmission field and the results show that the system exhibits dynamic instability. The start and termination, as well as the periodicity of this dynamic oscillations inside the optical cavity can be manipulated by the experimentally achievable parameters, such as the laser intensity and frequency detuning. Such dynamic responses are attributed to the competition between optical saturation of the incident field and population oscillations induced by the control field in graphene system.

Plasmon-mediated discrete diffraction behaviour of an array of responsive waveguides.

We investigate the discrete diffraction phenomenon in a Polymer-Liquid Crystal-Polymer Slices (POLICRYPS) overlaying a random distribution of gold nanoparticles (AuNPs, plasmonic elements). We study the propagation of a CW green laser beam through the waveguide structure as a function of beam polarization, laser intensity and sample temperature. It turns out that the plasmonic field created at the interface between AuNPs and POLICRYPS waveguides enables and stabilizes the optical field propagation within the responsive nematic liquid crystal channels. The active role of the liquid crystal is pointed out by a polarization, temperature and beam divergence experimental analysis and evidenced by a peculiar trumpet-shaped discrete diffraction pattern. Theoretical simulations confirm that the observed optical behavior is governed by the interaction of the nematic liquid crystal with optical and plasmonic fields.