Vortex Source Research Articles

Twisted Light in a Single‐Crystal Fiber: Toward Undistorted Femtosecond Vortex Amplification

AbstractIntense femtosecond optical vortices with spatially structured amplitude and spiral phase front give rise to novel phenomena in light–matter interactions and strong‐field physics. However, current femtosecond vortex sources exhibit a poor power handling capability and amplification remains an open challenge due to a number of inherent technical difficulties. Here, it is demonstrated that a single‐crystal fiber laser amplifier is particularly well‐suited to directly amplify a femtosecond optical vortex without pulse stretching and compression in the time domain, while still maintaining the spatial properties associated with a clear central singularity and a spiral phase front, i.e., a well‐defined amount of orbital angular momentum (OAM).The optical nonlinearity experienced by such twisted light is verified to be substantially weaker compared to a fundamental mode beam where supercontinuum generation and spatial distortion are observed. The simple design and straightforward power scaling capability pave the way toward ultrahigh‐intensity femtosecond singular laser sources with an arbitrary topological charge. Such ultrafast OAM light sources are expected to help reveal complex physical phenomena in light–matter interactions and expand the applications to attoscience with X‐ray vortices, laser plasma acceleration, and micromachining.

Vortices nucleation by inherent fluctuations in nematic liquid crystal cells

Multistable systems are characterized by exhibiting domain coexistence, where each domain accounts for the different equilibrium states. In case these systems are described by vectorial fields, domains can be connected through topological defects. Vortices are one of the most frequent and studied topological defect points. Optical vortices are equally relevant for their fundamental features as beams with topological features and their applications in image processing, telecommunications, optical tweezers, and quantum information. A natural source of optical vortices is the interaction of light beams with matter vortices in liquid crystal cells. The rhythms that govern the emergence of matter vortices due to fluctuations are not established. Here, we investigate the nucleation mechanisms of the matter vortices in liquid crystal cells and establish statistical laws that govern them. Based on a stochastic amplitude equation, the law for the number of nucleated vortices as a function of anisotropy, voltage, and noise level intensity is set. Experimental observations in a nematic liquid crystal cell with homeotropic anchoring and a negative anisotropic dielectric constant under the influence of a transversal electric field show a qualitative agreement with the theoretical findings.

Activity characteristics of the northeast moving Tibetan Plateau vortices in summer

The Tibetan Plateau Vortex (TPV) is mainly divided into east moving (E-TPV), northeast moving (NE-TPV) and so on according to its moving path after moving off the Tibetan Plateau (TP). Based on the database of TPV from 1979 to 2018, this paper mainly makes statistical analysis on the NE-TPV in summer. According to its moving-off position, NE-TPV is subdivided into westward vortex (denoted as NEI-TPV) and eastward vortex (denoted as NEII-TPV). In terms of source, structure, circulation, moving speed, life history and influence on precipitation, the characteristics of two types of NE-TPVs were analyzed and compared with E-TPV to reveal the activity characteristics of the NE-TPVs and the similarities and differences between them. The results show that: (1) The NEI-TPV is mainly generated in the northwest of the TP, while the NEII-TPV and E-TPV are generated in the whole TP. There are mainly three centers of vortex sources: west, middle and east. The eastern sources of TPV with different paths differ greatly in north-south location and concentration. The eastern source of NEI-TPV is northward and concentrated, while the E-TPV is southward and dispersed, and NEII-TPV is between the former two. (2) The intensity of NEII-TPV is stronger than that of NEI-TPV before moving off, and the ascending motion of these two is located near the vortex center. After moving off, the relative vorticity and vertical velocity of both of them weakened significantly, and the maximum upward movement of NEI-TPV moved to the northeast of the vortex, while the NEII-TPV is similar to E-TPV, and its rising region is located in the southeast of the vortex. (3) The moving direction before moving off and moving-off position of TPV is mainly affected by the 200 hPa westerly fluctuations, while after moving off, the moving direction is mainly affected by 500 hPa troughs and ridges. Among them, the Hetao High-Pressure Ridge is particularly important in blocking NE-TPV. (4) Besides the influence of altitude difference, the moving speed of TPV is closely related to its moving direction, so the TPV moving eastward move faster. (5) NEII-TPV and E-TPV have longer mean life history than NEI-TPV, meanwhile, they often result in a wider range of precipitation and a higher proportion of rainstorms. The precipitation of NEI-TPV is mainly located in the Hexi Corridor. While the precipitation of NEII-TPV is mainly located in the eastern part of Northwest China.

Turbulence Modeling for Leading-Edge Vortices: An Enhancement Based on Experimental Data

Low-aspect ratio wing planforms, such as delta wings, experience the predominance on their aerodynamic characteristics of large-scale leading-edge vortices separating around the leading edges. Due to their important application for highly maneuverable aircraft, the physical and aerodynamic understanding related to the vortex flow is of primary relevance. The investigation process is routinely performed with numerical simulations employing Reynolds-averaged Navier–Stokes equations. As the vortex grows in intensity, the limitation of ordinary models reduces the accuracy grade. Scale-resolving or more complex turbulence models can increase the accuracy, but the computational cost prohibits the application to a large envelope of cases. In the context of this work, the enhancement of a one-equation eddy-viscosity model is employed. The model is improved by formulating additional vortex source terms exclusively active inside the vortex-flow region and by employing a calibration procedure that is integrated into an iterative automated process where experimental data are used as targets for optimizing the model. The accuracy is enhanced for a cluster of cases around the calibration target, and it shows the potential of the application to large datasets that include several geometric and flow condition variations.

Tunable Optical Vortex from a Nanogroove-Structured Optofluidic Microlaser

Optical vortices with tunable properties in multiple dimensions are highly desirable in modern photonics, particularly for broadly tunable wavelengths and topological charges at the micrometer scale. Compared to solid-state approaches, here we demonstrate tunable optical vortices through the fusion of optofluidics and vortex beams in which the handedness, topological charges, and lasing wavelengths could be fully adjusted and dynamically controlled. Nanogroove structures inscribed in Fabry-Pérot optofluidic microcavities were proposed to generate optical vortices by converting Hermite-Gaussian laser modes. Topological charges could be controlled by tuning the lengths of the nanogroove structures. Vortex laser beams spanning a wide spectral band (430-630 nm) were achieved by alternating different liquid gain materials. Finally, dynamic switching of vortex laser wavelengths in real-time was realized through an optofluidic vortex microlaser device. The findings provide a robust yet flexible approach for generating on-chip vortex sources with multiple dimensions, high tunability, and reconfigurability.

The Interaction of Two Unsteady Point Vortex Sources in a Deformation Field in 2D Incompressible Flows

The Interaction of Two Unsteady Point Vortex Sources in a Deformation Field in 2D Incompressible Flows

Tunable vortex beam generation using an optical parametric oscillator with an antiresonant-ring interferometer.

We report a high-average-power picosecond optical vortex source tunable in the near-infrared, using an antiresonant-ring (ARR) interferometer internal to an optical parametric oscillator (OPO) in combination with an external cylindrical lens for astigmatic mode conversion. The ARR OPO is tunable in the signal across 1457-1647 nm with a vortex intensity profile and up to 1 W of average power at 1602 nm. The corresponding idler is tunable over 3006-3945 nm in a Gaussian intensity profile with as much as 1.6 W at 3168 nm. The vortex signal and the Gaussian idler exhibit passive power stability better than 1.7% rms and 1.3% rms, respectively, over >1h. The signal pulses have a Gaussian duration of <19ps with a time-bandwidth product of ΔτΔν<3.6 across the tuning range.

High-power, electronically controlled source of user-defined vortex and vector light beams based on a few-mode fiber amplifier

Optical angular momentum (OAM)-based structured light beams provide an additional degree of freedom for practical applications ranging from optical communication to laser-based material processing. Many techniques exist for generating such beams within laser sources and these primarily rely upon the use of specially designed optical components that limit laser power scaling and ready tunability of the topological charge and polarization of the output OAM beams. Here we show that some of these limitations can be overcome by employing a computer controlled reflective phase-only spatial light modulator to adaptively tailor the input (and subsequent output) beam wavefront and polarization in a few-mode fiber amplifier. In this way modal-coupling-induced beam distortion within the fiber amplifier can be mitigated and we are able to generate at will any desired supported spatial mode guided in the fiber, including conventional linearly polarized (LP) modes, scalar OAM modes, and cylindrical vector modes, at average powers &gt; 10 W and with a peak power of &gt; 11 kW . Our results pave the way to the realization of practical high-power structured laser sources with tunable chirality and polarization.

Effect of vortex and entropy sources in sound generation for compressible cavity flow

The present study reports the effect of different source terms on the near and far-field acoustic characteristics of compressible flow over a rectangular cavity using hybrid computational aeroacoustics methodology. We use a low dispersive and dissipative compressible fluid flow solver in conjunction with an acoustic perturbation equation solver based on the spectral/hp element method. The hybrid approach involves calculating the base fields and the acoustic sources from a fluid simulation in the first step. In the next step, the acoustic solver utilizes the variables to predict the acoustic propagation due to the given sources. The validation of the methodology against benchmark cases provides quite accurate results while compared against the existing literature. The study is then extended to assess the importance of the entropy source term for the flow over a rectangular cavity. The predictions of hybrid simulations with vortex and entropy source terms reproduce the perturbation pressure values very close to the existing direct numerical simulation results. Moreover, the results suggest that the use of just the vortex source terms over-predicts the perturbation pressure near the source region. Finally, we have carried out detailed simulations with all the source terms to investigate the noise sources for compressible flow over the cavity for different Mach number ranges (M=0.4,0.5,0.6,0.7,1.5). The obtained acoustic spectra and the sound directivity are in close agreement with the reference experiment.

Control of THz Surface Plasmons by Geometric Phases

Surface plasmons (SPs) are expected to have a wide range of applications in many fields, so they have recently attracted much attention. However, most of the previous studies achieved the manipulation of SPs through designing the structure of the individual meta-atom. When developing the next generation of integrated photonic devices and components, it is essential to seek out new methods of software control, which enable more diverse modulation and higher efficiency. Here, the tunable emission of SPs with metasurfaces is systematically studied. SPs are a source of on-chip plasmonic vortices (PVs). To verify the controllability of the directional excitation of SPs, we designed beam deflectors with different angles of surface waves (SWs). Furthermore, PVs with different topological charges were generated by arranging spatially varied microslits. The proposed control strategy provides a common platform for various promising applications, such as on-chip generation of the propagation control of SPs and PVs.

Probing arbitrary Laguerre–Gaussian beams and pairs through a tilted biconvex lens

Laguerre–Gaussian (LG) laser beams carrying orbital angular momentum are attractive vortex sources for a variety of photonic applications. In this work, we investigate the probing method based on a tilted biconvex lens for LG beams and pairs. We theoretically derive the formulation of the general astigmatic transformation of an LG vortex beam with non-zero radial index p and topological charge l after passing through a tilted biconvex lens. It is found that at a certain position after the focus of the lens, the field intensity of the LG beam is converted into a ( p+1) by ( p+|l|+1) slanted matrix of bright spots. The indices of p and l are obtained by examin-ing the converted intensity patterns. Theoretical calculations are in good agreement with experiments using LG beams with p up to 2 and l up to 34 emitted from a nonplanar ring vortex oscillator. Furthermore, we also found the method can also be used to estimate the mode comp-onents, the ratio of beam intensity and phase relationships of any two superposed LG beams.

Two-dimensional plan source, vortex and vortex source

Modern hydraulic structures require highly reliable water-supply channels, free-flow pipes and open spillways. Therefore, they must be built with considering dynamic properties of the affecting flow. The theory of one-dimensional open flows cannot answer many questions raised by the hydraulic engineering practice. Hence, this paper considers two-dimensional graphical open flows, with separate particular models following from the general theory, important for designing couplings, curves in more complex constructions (two different channels) required in hydraulic engineering (Hydraulics technical structures) (HTS). And the more such models are obtained, the more opportunities will appear for scientists and designers to synthesize more complex structures required in HTS.This study identifies three elements, and three simplest models of two-dimensional graphic open water flows similar to flat models: a source, a vortex and a vortex source. The number of such individual models will continue to increase and expand the range of possibilities for designing more complex water technical objects (WTO).The models are obtained analytically from solving the system of two-dimensional graphical open potential water flows in the plane of the velocity hodograph. The elementary construction of each model includes an analytical solution for the potential function and the stream function.The paper considers the problem of determining the parameters of a two-dimensional graphical open water flow at any point in the flow: a source, a vortex, a vortex source. The practical significance of the models lies in the possibility to use the results by the designers of hydraulic structures both at the first stage of solving problems and at the subsequent ones, with flow resistance forces taken into account.The study also represents the transition method from the plane of the flow velocity hodograph to the flow diagram by integrating the models in the plane of the velocity hodograph from the condition of the connection between the considered planes.

Повышение энергоэффективности шахтных центробежных насосов

The increase in the efficiency and competitiveness of mining enterprises is limited by the insufficient efficiency and adaptability of the currently used centrifugal pumps. Using the vortex theory of turbomachines, Theorems Stokes’ and Helmholtz, the principles of hydrodynamic analogy and superpositions, a mathematical model of the hydrodynamic calculation of centrifugal pumps with adaptive vortex sources integrated into the impeller blades is obtained. A significant influence on the hydrodynamic parameters and adaptability of pumps of the energy characteristics of adaptive vortex sources has been proved. Criteria for the similarity of the hydrodynamic process of fluid flow in the interscapular channels of impellers and adaptive vortex sources and their influence on the hydrodynamic characteristics of pumps are obtained. Mathematical and experimental modeling uses a regression equation to calculate the parameters of vortex chambers and their impact on the efficiency and adaptability of pumps. The optimal geometric parameters of the vortex chambers, the diameter of which does not exceed 5…7 % of the impeller diameter, increase the hydrodynamic loading by at least 13 %, the nominal efficiency. not less than 6 %, adaptability not less than 8 %. On the basis of the proposed developed mathematical model, after the positive test results obtained on the laboratory pump K 20/30, tests were carried out on the CNS 300-300 pump

Modeling underwater propagation of acoustic vortex beams by ray methods

Acoustic vortex beams have application potentials in the fields of object manipulation and high-speed communication. Due to the additional azimuthal features, modeling the propagation of vortex beams usually requires three-dimensional (3-D) models, which are of high computational cost, especially for large-scale simulations. Here, the two-dimensional (2-D) BELLHOP ray-tracing model is used to simulate the 3-D propagation of acoustic vortex beams in the deep, long-range ocean environment for a low computational cost. The vortex source is constructed by multiple point sources, each with a predefined initial phase to form the vortex wavefront. The 3-D vortex acoustic field is modeled by a sum of the fields simulated for the individual sources in 2-D environments. The results reveal that the traditional 2-D ray-tracing model is an effective tool to study vortex beam propagation in inhomogeneous media, with less complexity compared to finite element simulations using COMSOL Multiphysics.

ДВУХМЕРНЫЙ В ПЛАНЕ ВИХРЕИСТОЧНИК

In this paper, we show in principle a method for solving the problem of a two-dimensional vortex source in terms of flow. The material of the article relates to mathematical modeling of classical two-dimensional problems in terms of potential flows. The resulting solution is similar to the results for a flat vortex source and is an important result for the development of the analytical theory of two-dimensional in terms of potential water flows. Other solutions for real potential two-dimensional flows are considered, which are clarified by the authors, which confirms the relevance of the work.

Circular Optical Arrays Using Waveguide Fed 45° Angled Mirrors

An array of optical waveguide fed 45° metal-coated mirrors are proposed and validated by simulation, for generating optical scalar and vector vortices. Subwavelength element-to-element spacings are achieved, which is not possible in traditional grating-based phased arrays. This vortex source can be superior over other methods like phase masks which are not planar process integrable with lasers and phase modulators. A simple plane wave reflection model is used to analyze the mirror and the results are matched with reasonable tolerance against simulation results. The spin and orbital angular momentum charges are directly calculated from the field values for quantifying the performance. Four- and eight-element arrays are studied. The matching of prediction with simulation has been good for linear, azimuthal and radial polarization. Good angular momentum conversion efficiencies are obtained for up to orbital angular momentum charges ${l}$ = ±2. Important aspects of generating vortices with circular polarization are brought out. Directional beams with >10 dB gains are achieved. This is the first report in which a planar dielectric optical waveguide array with subwavelength spacing has achieved good angular momentum control for linearly, azimuthally, radially and circularly polarized beams, in the same geometry.

The emergence of inertial waves from coherent vortex source in strongly coupled dusty plasma

The evolution of isotropic, nondispersive, inertial waves emerging from an unsteady initial coherent vortex source is studied for strongly correlated dusty plasma using two-dimensional molecular dynamics simulation. In this study, the effects of azimuthal speed of a vortex source, strong correlation, large screening, and the compressibility of the medium on the propagation of generated inertial waves have been presented. It has been observed that these inertial waves only exist when the angular speed or azimuthal speed of the vortex source (U0) is larger than the transverse sound speed (Ct) of the system. The estimated speed of the nonlinear wave (CNLW) is found to be always larger and close to longitudinal sound speed (Cl) of the system for the range of coupling and screening parameters studied. We find that spontaneously generated inertial wave speed in dusty plasma is suppressed by the compressibility and dust-neutral drag of the system and is less sensitive to coupling strength. We also report a transition from “incompressible to compressible” flow. This transition is found to depend on the screening parameter and azimuthal speed of the vortex source. The existence of a critical Mach number Mc≈0.35 is found (where Mc=U0/Cl), above which inertial waves are found to exist, indicating the compressible nature of the wave.

Detailed force modelling of the secondary load cycle

Steep wave passage around vertical circular cylinders is associated with an additional force peak occurring after the main peak: the secondary load cycle. The secondary load cycle for a focused wave group typical of offshore wind turbine foundations at 33 m full-scale water depth is investigated in scale 1 : 50. Ensemble-averaged force, front face pressures and free surface elevation measurements are used as the basis for the investigation. A two-phase free-surface Reynolds-averaged Navier–Stokes solver is validated against generic cases of turbulent flow over a wall, wave-boundary layer flow for a Reynolds number, , of , two-dimensional (2-D) drag on a cylinder for and 2-D oscillatory flow for three combinations of and Keulegan–Carpenter number, , respectively. The solver is next applied to reproduce ensemble-averaged experimental results of the focused wave group and a good match for the inline force and free surface elevation is found along with a good match for the measured front face pressures. The numerical solution for the focused wave is next analysed in detail to explain the cause of the secondary load cycle. We find that the secondary load cycle is confined to an upper region ranging from just above the still water level to 1.5 cylinder diameters below. By a further break down of the pressure field into contributions from the individual terms of the vertical Navier–Stokes equation, we find that the local force peak in the secondary load cycle is mainly caused by suction effects around the still water level on the back side, contributed through the material time derivative of the vertical velocity, . The suction occurs due to the rapid decrease of water level below the generated water column at the back of the cylinder, which at this time has only just begun its downward motion. The first force local minimum in the secondary load cycle is aided by the hydrostatic pressure from the water column while the second local minimum of the secondary load cycle is aided by wash-down effects on the front side. Finally, the role of the observed vortices behind the cylinder is discussed and compared to reference computations with slip conditions. The results confirm findings from earlier slip boundary studies that the global force history through the secondary load cycle is not strongly affected by the boundary layer. The source of vortices behind the cylinder, observed in both sets of computations is discussed.

Численное моделирование сложных течений газа в концентрированных огненных вихрях

This article presents the results of numerical simulation of free fire vortices arising in laboratory conditions. The authors demonstrate the possibility of obtaining such concentrated fire vortices in a series of experimental studies conducted under the supervision of A. &amp;nbsp;Yu. &amp;nbsp;Varaksin, a corresponding member of the Russian Academy of Sciences, at the Joint Institute for High Temperatures of the Russian Academy of Sciences.&lt;br&gt; The authors propose to consider the analytical and numerical studies of arising complex swirling gas flows during local heating of a metal underlying surface by several sources from the point of view of gas dynamics. When considering complex flows of a heating gas as a motion of a viscous, heat-conducting, and compressible continuous medium, the complete system of Navier&amp;nbsp;— Stokes equations is used. The proposed initial-boundary conditions made it possible to numerically determine the main gas-dynamic characteristics of the resulting three-dimensional and unsteady gas flows in free fire vortices.&lt;br&gt; The calculation results showed that during the formation of fiery vortices, several stages are distinguished in their development. The first stage is characterized by the occurrence of local gas flows diverging in the radial direction from the heating regions. The second stage is accompanied by the formation in the regions of the location of the heating sources of local vortices with opposite spin directions. The third stage is characterized by the fact that from smaller vortices due to the intense influx of external air a common large thermal vortex is formed, which receives a positive twist under the influence of the Coriolis force. At the fourth stage, with an increase in the rotation speed, a decrease in the vertical dimensions of the thermal vortex and its decay into several small ones is observed. Thus, the completion of the life cycle of one concentrated vortex is replaced by the formation of a new one. For the initial parameters, the lifetime of the concentrated thermal vortex is about one minute.

Tunable ultraviolet vortex source based on a continuous-wave optical parametric oscillator

We report a continuous-wave (cw) optical parametric oscillator (OPO) generating optical vortices tunable in the ultraviolet (UV). Based on MgO:sPPLT as the nonlinear crystal, the singly resonant OPO is pumped by a cw vortex beam in the green, and deploying intracavity sum-frequency generation (SFG) between the undepleted pump and the Gaussian resonant signal in the crystal of BiB3O6, it can generate optical vortices of order, luv=1 and 2, tunable across 332-344nm in the UV with a maximum power of 12mW. Due to conservation of orbital angular momentum in the parametric process, the OPO also produces a non-resonant idler output beam in a vortex spatial profile of order li=1 and 2, identical to the pump vortex, with the signal beam in Gaussian distribution. The idler vortex is tunable across 1172-1338nm with maximum output power of 1.3W.