Vortex Source Research Articles

Topological-charge-tunable and wavelength- switchable vortex laser enabled by a helically twisted high-absorption few-mode erbium-doped fiber.

Vortex beams carrying orbital angular momentum (OAM) offer a solution for enhancing spatial degrees of freedom, particularly in conjunction with wavelength division multiplexing, which can significantly boost data capacity for optical communication. Addressing the increasing demand for high information-carrying capacity, we present a dynamically tunable OAM laser source in this study. We demonstrate a ring-cavity vortex fiber laser employing intra-cavity mode conversion through a helically twisted high-absorption few-mode erbium-doped fiber (HA-FM-EDF). The constructed vortex fiber laser exhibits wavelength switchability via an integrated Sagnac loop, facilitated by a homemade ring-core fiber. Furthermore, topological-charge tunability is achieved through the utilization of twisted HA-FM-EDF with varying helical pitches. To our knowledge, this marks the first successful implementation of two-dimensional multiplexing of wavelength and OAM in a vortex fiber laser. The OAM laser serves as a versatile vortex source with high tunability and flexibility, holding significant potential for deployment in ultrahigh-speed/ultrahigh-capacity communications, ultrahigh-resolution imaging, and ultrahigh-sensitivity sensing applications.

Efficient approach for generating vortex sources with arbitrary orbital angular momentum in acoustic experiments

Abstract In theoretical research framework of acoustics or optics, how to provide stable and efficient experimental vortex sources with arbitrary orbital angular momentum (OAM) (especially with larger OAM) is a highly challenging research topic. Here, we propose and demonstrate the general principle of two different methods to generate vortex sources with arbitrary OAM, based on the point-sources array and acoustic metamaterials, respectively. Specifically, the general synthetic law is summarized from the analytical perspective behind generating two-dimensional vortex waves using different point sources with different phases, and the design flexibility of acoustic metamaterials is also utilized to provide an ideal solution for generating vortex sources with larger OAM. Besides, we qualitatively and quantitatively determine the OAM of generated vortex waves through simple formulas, and briefly discuss the applicability and stability of two different methods with complementary advantages. The principles of vortex sources generation revealed in this work provide direct theoretical support for the experimental exploration of interactions between multiphysics fields and complex media, with potential applications in vortex fields manipulation and OAM detection.

Perfect space-time vortices.

We introduce the concept of perfect space-time vortices (PSTVs) that can exist in media with anomalous dispersion. If the topological charge of a PSTV is not too large, the spatiotemporal intensity distribution of the vortex field does not depend on the magnitude of the topological charge. We show theoretically how a PSTV can be realized in the optical context through spatiotemporal focusing of a Bessel-Gaussian space-time optical vortex source that is placed in the focal plane of a space-time lens composed of an ordinary lens and a time lens with matched spatial and temporal focal lengths.

Coherent beam combining of optical vortices.

We experimentally demonstrate the power scaling of optical vortices using the coherent beam combining technique, encompassing topological charges ranging from ℓ = 1 to ℓ = 5 realized on the basis of a Yb-doped fiber short-pulsed laser system. The combining efficiency varies from 83.2 to 96.9% depending on the topological charge and beam pattern quality generated by the spatial light modulators. This work is a proof of concept for using a coherent beam combining technique to surpass the physical power/energy limitation of any single source of optical vortices, regardless of the generation methods employed. These results open a pathway to power scaling of optical vortices with diverse applications in science and industry by utilizing advances in light-matter interactions.

Adding a Vorticity Source to the Hasegawa-Wakatani Equations

Instabilities in fusion reactor plasma are an area of ongoing research as they represent the primary hurdle in producing an ongoing fusion reaction for fusion energy production. To study one possible source of these instabilities, computational simulations to solve the modified Hasegawa-Wakatani Equations and the evolution of edge plasmas through time are required. This work shows that the addition of a source of vortices leads to edge instabilities using the TOKAM2D turbulence code. This is an important step into developing our understanding of how instabilities form in plasmas and maintaining a fusion reaction.

Circularly coherent vortex beams optimized for propagation through turbulence

Self-focusing partially coherent beams with circular coherence have shown high potential for robust propagation through atmospheric turbulence. In this paper, we introduce a criterion to approximate the degrading effects of turbulence and we show how the coherence of the source can be optimized to generate a beam with the highest stability in turbulence. To test our prediction, we analytically compare the turbulence propagation of the OAM spectrum of circularly coherent Gaussian vortex sources with three different coherence parameters. It is shown that by satisfying the introduced optimizing conditions, we can minimize the adverse effects of turbulence on the OAM spectrum.

Effects of increasing orbital number on the field transformation in focused vortex beams

Acoustic vortex beams possess helical wavefronts characterized by an orbital number n. In the presence of focusing, the toroidal vortex ring for n = 1 moves out of the focal plane z = d and toward the source with increasing n, accompanied by a departure from its toroidal shape. In this talk, ray theory is developed to explain this field transformation. An expression is derived for the radius and cross-sectional area of the annular channel formed by the family of rays emanating from a thin circular ring centered at the origin in the source plane z = 0. This expression leads to explicit results for the field amplitude and caustic coordinates due to a focused vortex source with an arbitrary axisymmetric amplitude distribution. Comparison with field calculations in which diffraction is included demonstrate that the caustics describe the redistribution of the global maximum and its shift toward the source plane with increasing n. For moderate focusing gains (of order 10 or 20), a toroidal vortex ring for n = 1 transforms with increasing n into a spheroidal surface in the prefocal region 0 < z < d having volume nλd 2/6, where λ is the wavelength, inside of which exists a shadow zone. [C.A.G. was supported by the ARL:UT McKinney Fellowship in Acoustics.]

The motion of a circular foil in the field of a fixed point singularity: Integrability and asymptotic behavior

A finite-dimensional model is developed, which describes the motion of a balanced circular foil with proper circulation in the field of a fixed vortex source. The motion of the foil has been studied in two special cases: that of a fixed vortex and that of a fixed source. It is shown that in the absence of proper circulation, the fixed vortex and the fixed source have the same impact on the motion of the foil. However, adding nonzero proper circulation leads to qualitative differences in the foil's dynamics. For a fixed vortex, there exist three types of motions: the fall on a vortex in finite time, periodic and quasiperiodic motion around the vortex. The investigation of this case reduces to analysis of a Hamiltonian system with one degree of freedom. Typical phase portraits and graphs of the effective potential of the system are plotted vs the distance between the geometric center of the foil and the vortex. For a fixed source, two types of motions are possible: the fall on the source in finite time and unbounded escape from the source. For small intensities of the source, the asymptotics of escape to infinity is constructed.

Asymmetric conversion of arbitrary vortex fields via acoustic metasurface

Asymmetric manipulation of acoustic waves has gained significant attention due to its rich physical properties and potential application prospects. In this study, we design and demonstrate a planar acoustic metasurface (AM) that enables asymmetric conversion for vortex fields with arbitrary orbital angular momentum (OAM) to different plane waves by placing the same vortex source at different focusing points of above and below. This asymmetric effect is caused by the spatial asymmetry of vortex wave, and AM achieves the conversion of two types of waves through directional compensation of phases. Numerical demonstrations and acoustic experiments further validate this asymmetric phenomenon, and the deflection angle of converted plane waves is qualitatively and quantitatively confirmed by a more general formula. Our work enriches the research meta-system of acoustic wave physics and holds potential applications in underwater acoustic communication and OAM-based devices.

Plasmonic vortex beam emitter

Terahertz vortices prompt numerous advanced applications spanning classical and quantum communications, sensing, and chirality-based detection, owing to the inherent physical properties of terahertz waves and orbital angular momentum (OAM). Nonetheless, existing methodologies for generating terahertz vortices face challenges such as unalterable topological charges and intricate feed networks. To address these limitations, we propose a novel approach to generate multi-mode and tunable vortex beams based on chiral plasmons. Through eigenmode analysis, the uniform helical gratings are demonstrated to support chiral plasmons carrying OAM. By leveraging their vortex characteristics and introducing modulation into the periodic system, these chiral plasmons are alternatively diffracted into high-purity vortex radiations according to the Bragg law. To validate the theory, the vortex beam emitter is fabricated and measured in the microwave regime based on the modulated scheme. Experimental results confirm the emission of vortex beams with desirable phase distributions and radiation patterns. Our findings highlight the potential of chiral plasmons as seeds for tunable and compact vortex radiation, offering promising applications in tunable vortex sources.

High-order femtosecond vortices up to the 30th order generated from a powerful mode-locked Hermite-Gaussian laser

Femtosecond vortex beams are of great scientific and practical interest because of their unique phase properties in both the longitudinal and transverse modes, enabling multi-dimensional quantum control of light fields. Until now, generating femtosecond vortex beams for applications that simultaneously require ultrashort pulse duration, high power, high vortex order, and a low cost and compact laser source has been very challenging due to the limitations of available generation methods. Here, we present a compact apparatus that generates powerful high-order femtosecond vortex pulses via astigmatic mode conversion from a mode-locked Hermite-Gaussian Yb:KGW laser oscillator in a hybrid scheme using both the translation-based off-axis pumping and the angle-based non-collinear pumping techniques. This hybrid scheme enables the generation of femtosecond vortices with a continuously tunable vortex order from the 1st up to the 30th order, which is the highest order obtained from any femtosecond vortex laser source based on a mode-locked oscillator. The average powers and pulse durations of all resulting vortex pulses are several hundred milliwatts and <650 fs, respectively. In particular, 424-fs 11th-order vortex pulses have been achieved with an average power of 1.6 W, several times more powerful than state-of-the-art oscillator-based femtosecond vortex sources.

The Rotational Doppler Effect of Twisted Photons in Scattered Fields

AbstractAlong with broad applications of the linear Doppler effect, the rotational Doppler effect (RDE) of a structured light source carrying orbital angular momentum (OAM) has attracted significant attention for applications ranging from optical sensors to Doppler cooling. However, the high‐purity structured source's low energy efficiency and unknown optimal OAM parameters have significantly degraded the RDE's performance in previous work. Here, instead of utilizing an optical vortex source, the Doppler features are analyzed in scattered twisted photons carrying OAM with a common light source, and it is demonstrated that the RDE induced by a typical rotator can be extracted using a spiral phase spatial filter (SPSF) at the receiver. This model reveals that a rotating rough surface scatters abundant twisted photons carrying varied OAM values, and the OAM spectrum distribution is modulated by its angular coherence of spatial signature. Furthermore, common surfaces with different autocorrelation structures on received signals using the SPSF method are analyzed theoretically and experimentally. Such a scheme facilitates rotator detection with a generalized nonvortex simple source and dozen–fold improved efficiency with robustness against noncoaxial problems. These demonstrations open a path for studying and applying scattered twisted photons during light detection.

High-Order Sinc-Correlated Model Vortex Beams

We propose a new partially coherent vortex source model in which the spatial correlation function is a sinc function on the difference from the q-th power of the coordinates of two points of the source field. The beam radiated by such source is termed the high-order sinc-correlated model vortex (SCMV) beam. We derived the propagating formula of the cross-spectral density (CSD) function for SCMV beams in atmospheric disturbances. On the basis of the derived analytical expression, the behavior of the spectral density of the SCMV beams propagating in free space and atmosphere turbulence was investigated under comparative analysis. The results show that the spectral densities of such beams exhibited interesting novel features, which were significantly different from those of the trivial vortex beams.

Red, orange, and dual wavelength vortex emission from Pr:WPFGF fiber laser using a microscope slide output coupler.

Visible vortex beams have a large array of applications; however, the sources are often large or complex. Here, we present a compact vortex source with red, orange, and dual wavelength emission. This Pr:Waterproof Fluoro-Aluminate Glass fiber laser uses a standard microscope slide as an interferometric output coupler, yielding high quality first order vortex modes in a compact setup. We further demonstrate the broad (∼5 nm) emission bands in the orange (610 nm), red (637 nm) and near-infrared regions (698 nm), with the potential for green (530 nm) and cyan (485 nm) emission. This is a low-cost, compact and accessible device giving high quality modes for visible vortex applications.

Vortex-induced quasi-shear polaritons

Hyperbolic shear polaritons (HShPs) emerge with widespread attention as a class of polariton modes with broken symmetry due to shear lattices. We find a mechanism of generating quasi-HShPs(q-HShPs). When utilizing vortex waves as excitation sources of hyperbolic materials without off-diagonal elements, q-HShPs will appear. In addition, these asymmetric q-HShPs can be recovered as symmetric modes away from the source, with a critical transition mode between the left-skewed and right-skewed q-HShPs, via tuning the magnitude of the off-diagonal imaginary component and controlling the topological charge of the vortex source. It is worth mentioning that we explore the influence of parity of topological charges on the field distribution and demonstrate these exotic phenomena from numerical and analytical perspectives. Our results will promote opportunities for both q-HShPs and vortex waves, widening the horizon for various hyperbolic materials based on vortex sources and offering a degree of freedom to control various kinds of polaritons.

Theoretical Analysis of Continuous-Wave Mid-Infrared Optical Vortex Source Generated by Singly Resonant Optical Parametric Oscillator

Due to the important application in the study of vibrational circular dichroism and helical dichroism of chiral molecules, the tunable vortex beam at mid-infrared region has attracted increasing attention. Based on orbital angular momentum (OAM) conservation in nonlinear interactions, the vortex pumped singly resonant optical parametric oscillator (SRO) is recognized as a versatile source of coherent vortex radiation providing high power and broad wavelength coverage from a single device. However, the low parametric gain and high oscillation threshold under continuous wave (cw) pumping has so far been the most challenging factor in generating cw tunable vortex beams. To predict the output characteristic of vortex pumped SRO, a theoretical model describing the vortex pumped SRO is needed. In this study, the theoretical model describing the vortex pumped SRO is set up under collimated Gaussian beam approximation. Output characteristics of different SROs are simulated numerically. By proper selection of pump scheme (such as double-pass pumping scheme), the vortex pumped mid-infrared SRO can oscillate at a relatively low pump power. By controlling the gain (mode overlap ratio between the pump and resonant wave in the nonlinear crystal) and loss (employing a spot-defect mirror with different defect size as the output coupler) of the resonant signal mode in the SRO, the OAM of the pump beam can be directionally transferred to a specific down converted beam. The transfer mechanism of the OAM among the pump light and the down-converted beams and factors affecting the transfer are studied. Our study provides the guidelines for the design and optimization of vortex pumped SRO under cw operation.

On the flow dynamics of wall-mounted low-aspect-ratio rotating cylinders

The flow physics and vortical features attributed to a low-aspect-ratio circular cylinder cantilevered on a laminar boundary layer plate and rotated about its respective axial coordinate were investigated experimentally using Stereo Particle Image Velocimetry. The investigation documents the three-dimensional spatial evolution of on-body dynamics to deep-wake phenomena. In disseminating the more salient global characteristics of the resultant flowfield, the analysis focuses on the particular case of an aspect ratio of 1 driven in continuous rotary motion at a speed ratio of 3. The two key findings are (i) vortex source: a wall-normal swirl develops and reorients into a streamwise vortex at the cylinder's mid-diameter location due to the heavy influence of the incoming boundary layer, local fluid entrainment, and proximity to the wall. (ii) Enhanced downwash: in return, the streamwise vortex primarily drives the production of additional downwash in the wake, which appends to the downwash generated by the cylinder's free end. A two-dimensional planar particle image velocimetry campaign in service of a parametric survey reveals the sensitivity of streamwise vortex production and downwash to rotational rate and cylinder height relative to boundary layer thickness. The streamwise vortex production tends to deteriorate when the cylinder is immersed in the boundary layer due to reduced downwash.

Observation of Vortex Hall Effect in Double Negative Index Materials

AbstractOptical orbital angular momentum (OAM) Hall effect requires the interaction between different angular momenta, which hosts great potential in the manipulation of light. Dozens of studies have revealed that this interaction depends on the materials that the waves pass through. Negative refraction has given rise to many exceptional phenomena in the past decades, which greatly inspires us to investigate the interaction between OAM and double negative index materials (DNMs). It is demonstrated that in‐plane optical vortex waves can exhibit transverse shift through DNM, and such a phenomenon is also called vortex Hall effect. From numerical calculations, a linear relationship is observed between the transverse shift and the topological charge l for a fixed wavelength. The transverse shift originates from the transition of the caustics of the vortex source through DNM. The general theory reported in this work can be applied in more than electromagnetic (EM) systems. Acoustic experiment is performed to support the proposed theory due to the equivalence between acoustic and EM systems. The vortex Hall effect is then observed experimentally utilizing acoustic space‐coiling metamaterials, which can be effectively regarded as DNM. This phenomenon will help to understand the superoscillations of the vortex source, as well as bring significant applications in on‐chip signal de‐multiplexing.

Lensless ghost imaging of a partially coherent vortex source in atmospheric turbulence

The image quality of ghost imaging was mainly degraded because of the beam wander and scintillation caused by atmospheric turbulence. References show that the vortex phase can effectively deduce the negative effects and improve the critical resolution and image quality of ghost imaging in free space which can be used to improve its quality through turbulence. In this study, we investigate the quality of the ghost imaging of a Gaussian Schell-model vortex source with different topological charges and coherent widths during propagation through atmospheric turbulence with different strengths using numerical simulations. The results show that the image quality is improved with increasing topological charge in turbulence, which was further proved via experimental measurements. Our study can be applied to free space optical communication and information encryption based on ghost imaging systems to reduce the influence of turbulence.

Generation and verification of optical vortices with controlled phase based on coherent beam combining

Optical vortices (OVs) with controllable orbital angular momentum (OAM) distributions have potential applications in optical communication and optical manipulation. However, the source of optical vortices with segmented phase gradients generated by existing methods can be used only at a short distance because of their low power. In this study, based on coherent combining technology, we proposed a method to generate a controlled-phase optical vortex (COV). Compared with traditional OVs, the magnitude and direction of the local OAM of the COV are controllable. The transmission characteristics of the COV in free space were numerically examined using a split-step Fourier transform algorithm. We theoretically and experimentally proved the feasibility of the coherent combining technology to generate an COV and proved that it has the properties of non-diffraction and self-healing.