Orbital Angular Momentum Beams Research Articles

Spin-textured neutron beams with orbital angular momentum

We present a rigorous theoretical framework underpinning the technique of spin-echo modulated small-angle neutron scattering (SEMSANS), and show how the technique can be extended in order to generate spin-textured neutron beams with orbital angular momentum (OAM) via birefringent neutron spin-polarization devices known as magnetic Wollaston prisms. Neutron OAM beams are mathematically characterized by a ``cork-screw'' phase singularity ${e}^{i\ensuremath{\ell}\ensuremath{\phi}}$ about the propagation axis where $\ensuremath{\ell}$ is the OAM quantum number. To understand the precise relationship between the emergent OAM state and the variety of spin textures realized by various setups, we have developed a path-integral approach that in the interferometric limit makes a judicious use of magnetic Snell's law. We show that our proposed technique produces a complex two-dimensional pattern of spin-OAM entangled states which may be useful as a probe of quantum magnetic materials. We compare our path-integral approach to the well-known single-path Larmor precession model and present a pedagogical derivation of magnetic Snell's law of refraction for both massive and massless particles based on Maupertuis's action principle.

108.96 nm Broadband Generation of Second-Order OAM Beam Using an Angular Modulated Cascading LPFG

Broadband generation of orbital angular momentum (OAM) beam in fiber has a lot of applications in optical communications and fiber sensing. However, the efficient broadband generation of second-order OAM beam using long-period fiber grating (LPFG) is still a great challenge due to the low intermodal overlap. In this paper, we achieved a 108.96 nm efficient broadband generation of second-order OAM beams with a conversion efficiency over 95% in experiment using an angular modulated cascading LPFG with low insertion loss. By changing the refractive index modulation area in certain angle, not only the coupling between fundamental mode and second-order mode in fiber can be greatly enhanced, but also second-order even and odd modes are generated with approximately equal magnitudes, which is very beneficial to high-purity OAM beams generation. Finally, by choosing proper cascading parameters and an adaptive refractive index modulation fabrication strategy, we achieved a LPFG mode converter with a conversion efficiency over 95% in a 108.96 nm from 1520.96 nm to 1629.92 nm. The insertion loss of the LPFG is measured only ∼0.48 dB.

Distortion Compensation for Orbital Angular Momentum Beams: From Probing to Deep Learning

Beams carrying orbital angular momentum (OAM), has attracted wide interests recently, due to their favorable potential in communications, optical tweezers, remote detection, etc. However, distortion will come once OAM beams propagate through inhomogeneous media like atmospheric turbulence. Such distortion will broaden the orbital angular momentum spectrum, thus introduce interchannel crosstalk. These phenomena will adversely affect practical applications, for instance, the rising bit-error-rate in OAM-based data transmission. Therefore, distortion correction for OAM beams, an important means to improve the robustness of OAM beams when propagating in atmospheric turbulence, must be developed. In this paper, we overview recent advances on distortion compensation of OAM beams, including the influence of atmospheric turbulence on OAM beams, and multiple wavefront correction schemes. In addition, toward the engineering realization of OAM based free space data-transmission, we propose to apply TSVMD-AR model to predict atmospheric turbulence, and expect to further improve the distortion correction efficiency.

Orbital Angular Momentum Beams for High-Capacity Communications

There is a growing interest in structured light, especially orbital angular momentum (OAM) beams, due to its unique amplitude and phase structure. Multiple orthogonal OAM beams can be utilized to enable high-capacity communication systems in various scenarios. This review will discuss the properties, challenges, advances, and perspectives of OAM-based optical communications, including the following (a) basic multiplexing and encoding links; (b) OAM-based communication in free space and fiber; (c) technical hurdles; (d) mitigation approaches; (e) encoded quantum communications; (f) advanced topics including different frequency ranges and novel beams; and (g) ecosystem and commercialization.

Dual Concentric-Ring-Core Fiber With Four Zero-Dispersion Wavelengths for Beyond Three-Octave OAM Supercontinuum Generation

Orbital angular momentum (OAM) beams, featuring with unique spatial field distributions, have been extensively investigated and applied into a wide range of fields. In most applications, OAM beams covering a certain frequency band are usually required, and supercontinuum (SC) generation is a feasible method to provide broadband OAM source. As a short pulse is incident into a specially designed optical fiber with flat dispersion over broad bandwidth, a broadband SC could be generated along propagation due to the nonlinear spectral broadening. In this work, we propose a dual concentric-ring-core fiber design to achieve near-zero and flat dispersion profile over 3007-nm wavelength range with four zero-dispersion wavelengths for the OAM mode. The precise position of the zero-dispersion wavelengths can be tailored by varying the structural variables and the germanium-doped concentration of the silica-based fiber. The performance of the spectral broadening in terms of flatness and bandwidth is investigated under different input pulse and propagation conditions. Simulation results depict that the OAM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3,1</sub> mode supercontinuum spectrum can achieve beyond three-octave spanning from 445 to 3942 nm at −40 dB level by pumping a 50-fs 600-kW Gaussian pulse at the central wavelength of 1900 nm into a 2-mm designed fiber. By further optimizing the proposed fiber structure for the OAM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1,1</sub> mode with <60 ps/(nm·km) dispersion variation over a 3210-nm bandwidth, the generated supercontinuum can cover nearly three octaves.

OAM-Basis Wavefront Single-Pixel Imaging via Compressed Sensing

Utilizing a complex-number basis for single-pixel imaging (SPI) a spatial wavefront in high quality is still an open issue. To solve this issue, here, we proposed a method called OAM-basis spatial wavefront single-pixel imaging (WSPI) via compressed sensing (CS) for reconstructing a spatial wavefront in high quality. In there, we generate the orbital angular momentum (OAM) beams as the complex-number basis for the measurement, and propose to use the TVAL3 CS algorithm for the wavefront reconstruction, for the first time to the best of our knowledge. The reconstruction method is appropriate for WSPI using a complex-number basis. Both simulations and experiments show that our method is superior to the existing second-order correlation method. It provides a promising complex-number-basis-based WSPI technique for high-quality wavefront reconstruction. Thus, it may find applications involving optical communication, optical detection and imaging, optical information processing and encryption, and so on.

Multiple Orbital Angular Momentum Beams with High‐Purity of Transmission‐Coding Metasurface (Adv. Theory Simul. 4/2023)

Broadband Transmission-Coding Metasurface with Multi-Vortex-Beam In article number 2200842, Si Jia Li and co-workers develop a new type of transmission-coding metasurface by changing the size of metal patches based on the principle of propagation phase and phase superposition, which digitally realize the vortex waves with wide-band, high-purity, and multiple orbital angular momentum (OAM) beams simultaneously. This transmission-coding metasurface with multi-vortex-beam has potential applications in the OAM multiplexing communication and multi-target positioning in wireless systems.

Highly Dispersive Optical Fiber for Orbital Angular Momentum Modes

Orbital angular momentum (OAM) modes, featured by the unique properties of the helical phase front and annular intensity profile, have shown enormous potential for a wide range of applications. Optical fibers with ring-shape profile have been experimentally proven for supporting stable OAM beam propagation. In most instances, managing the chromatic dispersion is of vital importance in a variety of optical fiber systems. In this paper, we summarize optical fibers with concentric ring cores for extremely dispersive OAM modes. The properties of the OAM mode in double ring-core fiber are presented, including the effective refractive index, effective mode area, power integral ratio, chromatic dispersion, and loss. Photonic crystal fiber and triple ring-core fiber, which share the same operation principle for dispersive OAM modes as the one in the dual-ring fiber, are also briefly introduced.

Integrated Phased Array for Scalable Vortex Beam Multiplexing

Orbital angular momentum (OAM) modes have low model interactions during fiber propagation at data center distances, and thus are suitable for ultra-high capacity systems at low digital signal processing. Generating OAM modes using free-space setups is useful for proof-of-concept experiments, but is not a scalable solution. We use an optical phased array (OPA) with two-dimensional antennas for on-chip circularly polarized OAM beam generation. Our previous work demonstrated an OAM multiplexer for lower-order modes. In this work, we demonstrate an OAM multiplexer that supports a record of 46 (23 per polarization) simultaneous spatial modes up to OAM order 11. We also improve the crosstalk performance of our multiplexer. We incorporate an intensity tuning capability that substantially improves the OAM quality by enabling a uniform power distribution across the antennas. The worst-case crosstalk for the supported OAM5 to OAM11 are found experimentally to be better than <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$-12$</tex-math></inline-formula> dB, with OAM10 achieving <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$-17.2$</tex-math></inline-formula> dB.

3D Printed Multi-Beam OAM Antenna Based On Quasi-Optical Beamforming Network

In this paper, a multi-beam orbital angular momentum (OAM) antenna is proposed. By employing 3D printed millimeter-wave quasi-optical beamforming network (BFN), the emitted OAM wave can realize passive beam scanning capability. The antenna is composed of a pillbox BFN, a slotted waveguide antenna (SWA) array, and several full-dielectric phase elements (PEs) on the top of the SWA array. The required phase distribution of the OAM beams is turned by the phase shifts of the PEs. The pillbox BFN can generate the input waves with different phase gradients for the SWA array, and thus steer the beam direction. Moreover, the folded structure of the pillbox BFN can greatly decrease the size of the antenna. The proposed topology can be easily expanded to realize more beam number, higher directivity, and other OAM modes for its periodical structure. To verify the performance of the proposed antenna, a prototype is fabricated and measured. Two types of 3D printing technology are used, i.e., selective laser melting (SLM) for the pillbox BFN and the SWA array, and fused deposition modeling (FDM) for the PEs. The application of the 3D printed method greatly helps to decrease the fabrication difficulty. The measured results show that the antenna can achieve five +1 mode beams with ±20° 3 dB beam coverage range. Moreover, due to the full-metal structure of the BFN and the SWA array, the proposed design can achieve the minimum efficiency of 83.2%, which is much better than the other printed circuit broad (PCB) based OAM antennas. The advantages of low-cost beam scanning capability and the extensibility make the proposed antenna topology a good choice for future OAM communication and sensor systems.

Orbital Angular Momentum Encoded Beam Tracking and Wavefront Sensing

The working principle of conventional Shack-Hartmann-type wavefront sensors is based on a wavefront division by two-dimensional microlens arrays and a Poynting vector mapping by comparing the focal displacements to reference measurements. At complex phase distortions or steep phase gradients, however, wavefront reconstruction can suffer from ambiguities by crossing or overlapping neighboring sub-beams, or even by a complete loss of beam trajectories. Moreover, the recognition of sub-beams can additionally be affected by parasitic reflections at optical surfaces, or scattering in a medium. A possible solution of the problem is to work with arrays of well distinguishable sub-beams. Recently, high-resolution pixelated liquid crystal spatial light modulators became available which enable to flexibly generate spatially structured light and thus to realize advanced wavefront sensors with individually encoded beams. In our experimental study we investigated the specific advantages of orbital angular momentum beams for a robust beam tracking and wavefront reconstruction. As a key point for a wavefront reconstruction from vortex arrays, the reliable detection of spiral-like image features is addressed and related strategies are proposed.

Scattering analysis of a Laguerre Gaussian beam for target detection.

The propagation and scattering characteristics of a Laguerre Gaussian (LG) beam and Gaussian beam are theoretically and experimentally comparatively studied. The phase of the LG beam is almost free of scattering when the scattering is weak, resulting in a much weaker loss of transmission compared with the Gaussian beam. However, when scattering is strong, the phase of the LG beam is completely disturbed by it, and the transmission loss of the LG beam is stronger than that of the Gaussian beam. Moreover, the phase of the LG beam becomes stabler as the topological charge increases, and the beam radius increases as well. Therefore, the LG beam is suitable for short-range target detection in a weak scattering medium, not for long-range in a strong scattering medium. This work will contribute to the development of target detection, optical communication, and other applications of orbital angular momentum beams.

Generalized spiral transformation for high-resolution sorting of vortex modes.

We achieve high-resolution sorting of the orbital angular momentum (OAM) of light with two bespoke diffractive optical elements using the generalized spiral transformation. The experimental sorting finesse is 5.3, approximately two times better performance than what has been reported. These optical elements will be useful for optical communication based on OAM beams and can be easily extended to other fields that use conformal mapping.

Integrated circuits based on broadband pixel-array metasurfaces for generating data-carrying optical and THz orbital angular momentum beams

Abstract There is growing interest in using multiple multiplexed orthogonal orbital angular momentum (OAM) beams to increase the data capacity of communication systems in different frequency ranges. To help enable future deployment of OAM-based communications, an ecosystem of compact and cost-effective OAM generators and detectors is likely to play an important role. Desired features of such integrated circuits include generating and detecting multiple coaxial OAM beams, tunability of OAM orders, and operation over a wide bandwidth. In this article, we discuss the use of pixel-array–based metasurfaces as OAM transmitters and receivers for mode division multiplexing (MDM) communications in near-infrared (NIR) and terahertz (THz) regimes.

Image reconstruction of scattered vortex light field based on deep learning

Vortex beams propagating through a scattering medium generate a random field that obstructs the retrieval of original information carried by the vortex beam. In this paper, a new method based on an improved generative adversarial network (GAN) to reconstruct scattered object information carried by a vortex beam is proposed. First, particular holograms were designed to achieve imaging while preserving the vortex beam's orbital angular momentum property. Then, an improved GAN was used to realize the reconstruction from speckle images after the vortex beams pass through the scattering medium. In order to evaluate the image reconstruction ability of the proposed GAN, three types of vortex beams with different topological charges and three types of scattering media with different roughness are employed in experiments, and 15,000 speckle-object pairs collected are used to train five GAN models. The results show that each test dataset's average Pearson correlation coefficient reaches 0.886, 0.934, 0.922, 0.903, and 0.886, which means that the formed speckle field of vortex beam still retains certain information and the improved GAN have excellent ability in restoring original information from scattered vortex light field.

Achromatic Broadband Multi‐Layer Diffraction Mode Multiplexing

AbstractEfforts have been made in recent years to achieve broadband mode‐division multiplexing (MDM) for wavelength and mode hybrid multiplexing optical interconnect. However, most multiplexers are suffering from either strong chromatic aberration or material dispersion, like planar lightwave circuits, mode selective couplers, and single‐layer diffraction elements. The multi‐layer diffraction structure provides a method for achieving both mode conversion and broadband properties. However, the reported multi‐layer diffraction MDM can usually only achieve high efficiency working at a specific narrow bandwidth due to the monochromatic coherent light design. Here, a general achromatic broadband multi‐layer diffraction model is proposed to compensate the dispersion by wavefront matching of discrete multi‐wavelength optical fields. Using this model, an achromatic orbital angular momentum beam generator and an achromatic linearly polarized mode multiplexer for coarse wavelength‐division multiplexing (CWDM) optical interconnections are designed. Compared with the monochromatic designs, the generator exhibits lower wavelength dependence and the multiplexer doubles the bandwidth with mode purity exceeding 91%. Additionally, signal transmission over a 5‐km fiber is demonstrated experimentally in the CWDM‐MDM hybrid communication system. This work represents a solid step toward practical implementation of achromatic MDM and may find applications in the design of various achromatic devices.

High-accuracy recognition of perfect optical vortex orbital angular momentum modes based on F + GS-MSVM model

The recognition of vortex beam orbital angular momentum (OAM) is the basis for achieving high-speed communication, but the atmospheric turbulence (AT) causes distortion of phase fronts of vortex beams, which hinders the recognition of OAM modes. In this work, we propose and investigate a joint scheme (F + GS-MSVM) of combining Gerchberg-Saxton (GS) algorithm and multi-class support vector machine (MSVM) model that can achieve high recognition accuracy (RA) of OAM modes of perfect optical vortex (POV) beam which is focused(F) by a lens at the receiver under turbulence environment. We verify the feasibility of the scheme by comparing F + GS-MSVM with other three algorithms recognition performances under various turbulence-strength levels and different beam parameters in unknown turbulent environment. The simulation results show that the RA of our F + GS-MSVM model in arbitrary turbulent environments achieves more than 98%. In unknown turbulence environment, the F + GS-MSVM model can obtain a RA of 97.1% under different hollow radius, 99% under different beam waist radius, 86% under different wavelengths and 98.2% under different transmission distances. The results demonstrate the scheme is characterized by high RA, fast speed, wide range, and high portability.

Resolution Analysis of Coincidence Imaging Based on OAM Beams With Equal Divergence Angle

The multi-mode orbital angular momentum (OAM) beams can be effectively applied in target detection when the divergence angles are equal. The resolution threshold of the coincidence imaging system with multi-mode OAM is analyzed assisted by the subspace projection method. First, the relationship between the imaging resolution and the correlation function (CF) of the measurement mode is obtained based on the first-order correlation algorithm for the multi-mode OAM-based coincidence imaging (MM-OAMCI). Then, the feature vector of each imaging unit is extracted to distinguish the imaging point in the discretized imaging plane. Finally, the resolution threshold is given based on three factors, i.e., the feature vector, the noise, and the reconstruction algorithm. Specifically, the resolution of the MM-OAMCI system is determined by the deployment of the transmitting array, the reconstruction algorithm and the noise caused by the system design, the imaging modeling, and the environment noise. The super-resolution capability of the multi-mode OAM coincidence imaging system is validated by the simulations and experiments. The imaging results show that the resolutions in elevation and azimuth of the reconstructed image can be improved largely compared with the traditional microwave coincidence imaging (TMCI) methods, which is consistent with the theoretical analysis results.

Generation of various vortex beams based on transformation materials

A flexible vortex beam generator (VBG) is proposed based on transformation optics (TO) to convert input plane waves to various output vortex beams. The methodology is a geometrical one based on the topological equivalence between the vortex beam cross section disc and a belt, turning the in-plane curvilinear distributed phase or polarization structures in the disc to be redistributed around the belt. Consequently, abundant vortex beams can be exported by simply configuring the incidence angles and polarization directions of the plane waves. The topology and wave propagation characteristics behind the system are investigated, and TO is used to obtain the desired materials by adopting a specific space mapping. Some sample numerical simulations are described to verify the feasibility of the proposed design, including several complex orbital angular momentum and cylindrical vector beams. Due to its flexibility and simplicity, the proposed VBG is likely to have extensive application prospects.

60 nm Span Wavelength-Tunable Vortex Fiber Laser with Intracavity Plasmon Metasurfaces

Wavelength-tunable vortex fiber lasers that could generate beams carrying orbital angular momentum (OAM) hold great attention in large-capacity optical communications. The wavelength tunability of conventional vortex fiber lasers is, however, limited by a range of ∼35 nm due to narrow bandwidth and/or insertion loss of mode conversion components. Optical metasurfaces apart from being compact planar components can flexibly manipulate light with high efficiency in a broad wavelength range. Here, we propose and demonstrate for the first time, to the best of our knowledge, a metasurface-assisted vortex fiber laser that can directly generate OAM beams with customizable topological charges. Due to the designed broadband gap-surface plasmon metasurface, combined with an intracavity tunable filter, the laser enables an OAM beam with a center wavelength continuously tunable from 1015 to 1075 nm, nearly twice of other vortex fiber lasers ever reported. The metasurface can be designed at will to satisfy requirements for either low pump threshold or high slope efficiency of the laser. Furthermore, the cavity-metasurface configuration can be extended to generate higher-order OAM beams or more complex structured beams in different wavelength regions, which greatly broadens the possibilities for developing low-cost and high-quality structured-beam laser sources.