Conventional Orbital Angular Momentum Research Articles

High-Efficiency Multi-Channel Orbital Angular Momentum Multiplexing Enabled by the Angle-Dispersive Metasurface.

Orbital angular momentum (OAM) multiplexing of electromagnetic (EM) waves is of great significance for high-speed wireless communication and remote sensing. To achieve high-efficiency OAM multiplexing for multi-channel incident EM waves, this paper presents a novel angle-dispersive meta-atom structure, which can introduce the required anti-symmetric phase dispersion as well as high transmission efficiency for OAM multiplexing. These meta-atoms are then arranged delicately to form an angle-dispersive metasurface working at the X band, which enables three-channel OAM multiplexing by converting highly directional transverse-magnetic (TM) waves incident from 0 and ±45° to coaxial OAM beams with l = 0 and ±2 modes, respectively. The simulation and experimental results reveal that the proposed metasurface can convert a higher proportion of energy to the required OAM modes compared to the conventional OAM multiplexing metasurfaces, which can significantly improve the coaxial transmission efficiency of multi-channel OAM multiplexing.

Generation and free-space transmission characterization of bottle vortex beam

Vortex beams carrying orbital angular momentum (OAM) with the doughnut-shaped intensity distribution can be employed in free-space optical (FSO) communication links to circumvent obstructions. However, the size of the receiver aperture is proportional to the size of obstructions, which seriously constrains the application of OAM beams in this scenario. In this paper, we propose a method to generate bottle vortex beams (BVBs) with a parabolic trajectory by manipulating the radial phase distribution of conventional OAM beams. Meanwhile, the trajectory of BVBs generated are highly compatible with the predesigned trajectory by using this method. Moreover, we evaluate the free-space transmission performance of BVBs under atmospheric turbulence and limited receiving aperture. The results show that BVBs have better OAM FSO communication link performance compared with conventional OAM beams and Bessel beams. In addition, the performance of the BVBs circumventing obstructions is further investigated. The simulation results show that when setting the atmospheric turbulence strength D/r0 = 2 and the obstruction size of 40 mm, the average received optical power of the BVBs captured by a limited receiving aperture diameter (d = 40 mm) is improved about 7 dB and 3 dB compared to conventional OAM beams and Bessel beams, respectively.

Turbulence-resistant high-capacity free-space optical communications using OAM mode group multiplexing.

Twisted light carrying orbital angular momentum (OAM), which features a helical phase front, has shown its potential applications in diverse areas, especially in free-space optical (FSO) communications. Multiple orthogonal OAM beams can be utilized to enable high-capacity FSO communication systems. However, for practical OAM-based FSO communication links, atmospheric turbulence will cause serious power fluctuations and inter-model crosstalk between the multiplexed OAM channels, impairing link performance. In this paper, we propose and experimentally demonstrate a novel OAM mode-group multiplexing (OAM-MGM) scheme with transmitter mode diversity to increase system reliability under turbulence. Without adding extra system complexity, an FSO system transmitting two OAM groups with a total of 144 Gbit/s discrete multi-tone (DMT) signal is demonstrated under turbulence strength D/r0 of 1, 2, and 4. In our experiments, the proposed OAM-MGM scheme helps to achieve bit-error-rate (BER) mostly less than 3.8 × 10-3 under turbulence strength D/r0 of 1 and 2 with a total transmitted power of 10 dBm. Compared with the conventional OAM mode multiplexed system, the system interruption probability decreases from 28% to 4% under moderate turbulence strength D/r0 of 2.

Sub‐THz 3D printed lens based on diffractive neural network for low‐cost detection of orbital angular momentum states

AbstractAccurate detection of orbital angular momentum (OAM) is one of the critical techniques for OAM‐related communications. However, conventional OAM identification approaches are often complicated and costly. In this letter, a convenient method based on diffraction neural network (DNN) is presented for OAM states detection in the sub‐terahertz (sub‐THz) regime. The optimal phase distribution of the lens for OAM state detection is obtained from DNN training. Then, a prototype based on three‐dimensional printing is fabricated according to the training results. Numerical simulation and experiment are performed at 140 GHz to verify the proposed approach. The results show that the proposed method can accurately detect the OAM state with lower complexity. This convenient detection approach can provide a novel pathway for realizing low‐cost THz OAM receiving systems, promoting the development of future communication technology.

Structured Beamforming Based on Orbital Angular Momentum Mode-Group

The structured beams, which can manipulate arbitrary intensity and phase of electromagnetic rays, are significant in communication and sensing. The vortex electromagnetic wave carrying orbital angular momentum (OAM) exhibits the phase distribution of the helical wavefront, which can be viewed as a complete set of eigenmodes for electromagnetic waves. It provides a flexible beamforming method to achieve the azimuthal pattern diversity by the spatial superposition of multiple specific OAM modes. We have proposed a novel form of OAM wave with a two-dimensional structure called plane spiral OAM (PS-OAM), which is conducive to overlapping different modes. Based on this, the PS-OAM mode-group can be employed to modify the spatial beam structure and form structured electromagnetic beams. This kind of beam not only retains the inherent vorticity and orthogonality of the OAM waves but also tackles the difficulties in applications stemming from the singularity-caused dark zone and beam divergence of conventional OAM waves. In this paper, the construction and manipulation of the structured beams are investigated, and the properties are analyzed. The potential applications in multiple-input multiple-output (MIMO) communication, orthogonal multiplexing communication, and spatial field digital modulation (SFDM) communication are envisioned.

Broadband Diffractive Graphene Orbital Angular Momentum Metalens by Laser Nanoprinting

Optical beams carrying orbital angular momentum (OAM) play an important role in micro-/nanoparticle manipulation and information multiplexing in optical communications. Conventional OAM generation setups require bulky optical elements and are unsuitable for on-chip integration. OAM generators based on metasurfaces can achieve ultracompact designs. However, they generally have limited working bandwidth and require complex designs and multistep time-consuming fabrication processes. In comparison, graphene metalenses based on the diffraction principle have simple designs and can be fabricated by laser nanoprinting in a single step. Here, we demonstrate that a single ultrathin (200 nm) graphene OAM metalens can integrate OAM generation and high-resolution focusing functions in a broad bandwidth, covering the entire visible wavelength region. Broadband graphene OAM metalenses with flexibly controlled topological charges are analytically designed using the detour phase method considering the dispersionless feature of the graphene material and fabricated using ultrafast laser nanoprinting. The experimental results agree well with the theoretical predictions, which demonstrate the accuracy of the design method. The broadband graphene OAM metalenses can find broad applications in miniaturized and integrated photonic devices enabled by OAM beams.

Orbital angular momentum deep multiplexing holography via an optical diffractive neural network

Orbital angular momentum (OAM) mode multiplexing provides a new strategy for reconstructing multiple holograms, which is compatible with other physical dimensions involving wavelength and polarization to enlarge information capacity. Conventional OAM multiplexing holography usually relies on the independence of physical dimensions, and the deep holography involving spatial depth is always limited for the lack of spatiotemporal evolution modulation technologies. Herein, we introduce a depth-controllable imaging technology in OAM deep multiplexing holography via designing a prototype of five-layer optical diffractive neural network (ODNN). Since the optical propagation with dimensional-independent spatiotemporal evolution offers a unique linear modulation to light, it is possible to combine OAM modes with spatial depths to realize OAM deep multiplexing holography. Exploiting the multi-plane light conversion and in-situ optical propagation principles, we simultaneously modulate both the OAM mode and spatial depth of incident light via unitary transformation and linear modulations, where OAM modes are encoded independently for conversions among holograms. Results show that the ODNN realized light field conversion and evolution of five multiplexed OAM modes in deep multiplexing holography, where the mean square error and structural similarity index measure are 0.03 and 86%, respectively. Our demonstration explores a depth-controllable spatiotemporal evolution technology in OAM deep multiplexing holography, which is expected to promote the development of OAM mode-based optical holography and storage.

Experimental Study of Plane Spiral OAM Mode-Group Based MIMO Communications

Spatial division multiplexing using conventional orbital angular momentum (OAM) has become a well-known physical layer transmission method over the past decade. The mode-group (MG) superposed by specific single-mode plane spiral OAM (PSOAM) waves has been proved to be a flexible beamforming method to achieve the azimuthal pattern diversity, which inherits the spiral phase distribution of conventional OAM wave. Thus, it possesses both beam directionality and vorticity. In this article, it is the first time to show and verify a novel PSOAM MG based multiple-in–multiple-out (MG-MIMO) communication link experimentally in a line-of-sight (LoS) scenario. A compact multimode PSOAM antenna is demonstrated experimentally to generate multiple-independent controllable PSOAM waves, which can be used for constructing MGs. After several proof-of-principle tests, it has been verified that the beam directionality gain of MG can improve the receiving signal-to-noise ratio (SNR) level in an actual system. Meanwhile, the vorticity can provide another degree of freedom (DoF) to reduce the spatial correlation of MIMO system. Furthermore, a tentative long-distance transmission experiment operated at 10.2 GHz has been performed successfully at a distance of 50 m with a single-way spectrum efficiency of 3.7 bits/s/Hz/stream. The proposed MG-MIMO may have potential in the long-distance LoS backhaul scenario.

Arbitrary Vortex Beam Synthesis With Donut-Shaped Metasurface

This article presents an efficient synthesis method for generating vortex beams based on donut-shaped orbital angular momentum (OAM) metasurfaces. It not only allows one to synthesize the vortex beam with an arbitrary combination of OAM modes and arbitrary mode energy distribution, but also avoids the undesired phase singularities in the conventional OAM metasurfaces. Three specific examples are implemented to verify the effectiveness of the synthesis method including single-mode, multimode, and equal-amplitude vortex beams. Based on the proposed constraint conditions, a donut-shaped metasurface is designed to generate a high-purity single-mode vortex beam. To generate high-performance multimode beams, a shape-related tailoring is further introduced to avoid the undesired phase singularities of the multimode OAM metasurfaces. Finally, the synthesized equal-amplitude vortex beams are generated and verified experimentally over a wide frequency range, which demonstrates robust and precise control of the vortex beams.

Free-space optical communication with quasi-ring Airy vortex beam under limited-size receiving aperture and atmospheric turbulence.

Vortex beams carrying orbital angular momentum (OAM), which feature helical wavefronts, have been regarded as an alternative degree of freedom for free-space optical (FSO) communication systems. However, in practical applications, atmospheric turbulence and limited-size receiving aperture effects will cause OAM modal degradation and seriously reduce the received power. In this paper, by controlling the radial phase distribution of conventional OAM beams, quasi-ring Airy vortex beams (QRAVBs) are successfully generated in the experiments to increase the received power under the limited-size receiving aperture conditions. By employing 72-Gbit/s 16-ary quadrature amplitude modulation (16-QAM) discrete multi-tone (DMT) signals, we successfully demonstrate free-space data transmission with QRAVBs in the experiments. Moreover, the transmission performance of QRAVBs under atmospheric turbulence is also evaluated. Comparing with conventional OAM beam and Bessel beam, the obtained results show that QRAVBs can achieve higher received power and better BER performance under limited-size receiving aperture and atmospheric turbulence conditions.

Dual-frequency multiple compact vortex beams generation based on single-layer Bi-spectral metasurface

Vortex beams carrying orbital angular momentum (OAM) have attracted considerable attention owing to the potential to expand channel capacity of microwave and optical communication. However, the OAM generations usually suffer from divergence along propagation. In this work, we proposed a strategy to generate multiple vortex beams with compact energy distributions based on a single-layer reflective metasurface. First, the mechanism is developed for the generation of multiple compact vortex beams. Then, an advanced single-cell bi-spectral meta-atom, which is composed of a double C-shaped slot resonator and a modified double C-shaped resonator, is proposed to actualize independent geometric phase controls at two frequencies. As an illustrative example, a dual-frequency metasurface that can achieve four compact vortex beams (two beams at each frequency) with different OAM modes at 9 and 13 GHz is designed, and each OAM beam features a much more compact energy distribution compared to the conventional OAM beam. The measured results agree very well with the simulated results, which validate the proposed design methodology.

Experimental BER Performance of Quasi-Circular Array Antenna for OAM Communications

Orbital angular momentum (OAM), providing an additional degree of freedom, has attracted much attention for spectral efficiency and its potential in communications. The need for more compact OAM-based communication systems has inspired research into quasi-circular arrays (QCAs) producing quasi-OAM beams. In this letter, a QCA-based communication system prototype is designed to transmit and receive quasi-OAM beams of dominant mode order l=-1, over a free-space link at 8.5 GHz. The performance of quasi-OAM beams generated by QCAs is analyzed through bit-error-rate (BER) measurements in a lab setting. The BER results are then analyzed against BER of a conventional OAM circular array and augmented quasi-OAM by a QCA to investigate their performance. Furthermore, the intermode interference is analyzed by adding mode l=0. The results indicate the feasibility of a quasi-OAM communication system, and proved to be similar, or better than conventional OAM-based systems, where there is no mode cross-talk. Augmenting OAM modes is also proven to make a quasi-OAM communication link similar to conventional OAM, in the presence of cross-talk.

Direct Generation of OAM Mode-Group and Its Application in LoS-MIMO System

A partial slotted curved waveguide leaky-wave antenna which can directly generate orbital angular momentum (OAM) mode-groups (MG) with high equivalent OAM order l <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">e</sub> = ±40 at 60 GHz is proposed in this letter. This partial arc slotted antenna is designed based on the circular traveling-wave antenna which can generate a single mode conventional OAM wave, so it can be termed as a partial arc transmitting (PAT) scheme compared with the full 2π aperture slotting of the circular traveling-wave antenna. The full-wave simulation results show that the generated OAM MGs present a high gain beam with a helical phase distribution. Thus it can be used in a MG based line-of-sight (LoS) multiple-in-multiple-out system (MG-MIMO), whose performance is numerically analyzed. These MGs can improve the signal-to-noise ratio (SNR) and decrease the spatial correlation of sub-channels, thereby the MG-MIMO has a better performance in terms of capacity or BER than conventional MIMO.

Generation of High-Order Bessel Orbital Angular Momentum Vortex Beam Using a Single-Layer Reflective Metasurface

In this paper, we propose a method to generate a high-order Bessel orbital angular momentum (OAM) vortex beam by using a single-layer reflective metasurface, which integrates the advantages of small size, high efficiency, insensitive polarization and stable incident angles. An offset-fed horn configuration is adopted to overcome the feed-blockage effect, and a subwavelength unit cell with rotational symmetry is designed to cover the reflection phase variation range of 360° in different incident angles. The metasurface is simulated, fabricated and measured at the center frequency of 10 GHz, and the results validate that a second-order Bessel OAM vortex beam can be generated effectively. By comparing the field distributions between the Bessel OAM vortex beam and the conventional OAM vortex beam, we find that the generated Bessel OAM vortex beam is obviously more convergent and has more stable field distributions. The single-layer reflective metasurface is simple and flexible to shape the wavefront to be a Bessel OAM vortex wave, which has the potential to be used in a wide range of applications.

Capacity Modelling and Performance Analysis of OAM-OFDM Wireless Communication Systems

The Orbital Angular Momentum (OAM) wireless communication technology has received much attention in recent years for its natural orthogonality between different OAM states. Combining the OAM technology with the Orthogonal Frequency Division Multiplexing (OFDM) technology, we proposed a new OAM-OFDM wireless communication system to explore a new approach for enhancing the transmission capacity of the wireless communication system. Atmospheric turbulence is an important factor for influencing the capacity of the OAM-OFDM wireless communication system. In consideration of the effect of the atmospheric turbulence, we derived a crosstalk model of the proposed OAM-OFDM wireless communication system. Furthermore, a capacity model of the OAM-OFDM wireless communication system is proposed in consideration of the effect of atmospheric turbulence on both the OAM and OFDM signals. Compared with the conventional OAM wireless communication system, the capacity performance of the proposed OAM-OFDM wireless communication system is significantly improved and the average improvement is 751%.

Generation of unconventional OAM waves by a circular array

In recent years, orbital angular momentum (OAM) waves have been applied to improve the performance of radar target detection and imaging. However, the phase singularity of conventional OAM beams and the doughnut-shaped intensity distribution will degrade the performance of target detection. To solve the divergence problem in OAM multiplexing systems, the generation method of OAM beams propagating along a transverse plane is proposed using a uniform circular array, which is validated by full-wave simulation. Compared with conventional OAM beams, simulation results indicate that the generated beams with different OAM modes all propagate along the transverse plane while maintaining the regular wavefront shapes. This work can provide the development of new radar paradigm with technical support.

Quasi-Orbital Angular Momentum (Q-OAM) Generated by Quasi-Circular Array Antenna (QCA)

Orbital Angular Momentum (OAM), as a property of Electromagnetic (EM) fields has recently been proposed for Radio and Microwave communications. This paper investigates a new class of OAM radiation patterns for Radio and Microwave applications, namely, Quasi-OAM radiation patterns, induced by a proposed Quasi-Circular Array Antenna (QCA). Simulations and Experiments show that Quasi-OAM waves can be induced and preserved in the far-field using the proposed QCA apertures and configurations, demonstrating non-integer dominant OAM modes corresponding to l = +1 and l = −1 with a directional quasi intensity and rotational 2π phase profiles. The proposed method in this work significantly reduces aperture size and cost by using Quasi-Circular Arrays of NQ = 5 and NQ = 6 elements in lieu of conventional OAM circular aperture arrays with N = 8 elements.

Topological photonic orbital-angular-momentum switch

The large number of available orbital angular momentum (OAM) states of photons provides a unique resource for many important applications in quantum information and optical communications. However, conventional OAM switching devices usually rely on precise parameter control and are limited by slow switching rate and low efficiency. Here we propose a robust, fast and efficient photonic OAM switch device based on a topological process, where photons are adiabatically pumped to a target OAM state on demand. Such topological OAM pumping can be realized through manipulating photons in a few degenerate main cavities and involves only a limited number of optical elements. A large change of OAM at $\sim 10^{q}$ can be realized with only $q$ degenerate main cavities and at most $5q$ pumping cycles. The topological photonic OAM switch may become a powerful device for broad applications in many different fields and motivate novel topological design of conventional optical devices.

GENERATION OF OPTICAL VORTEX BEAMS BY COMPACT STRUCTURES

Orbital angular momentum (OAM) has been an enthralling topic of research from fundamental physics to technological applications since its discovery in 1992. Many techniques have been developed to generate OAM such as using spiral phase plates (SPPs), computer-generated holograms (CGH) and cylindrical mode converters, etc. However, these methods are based on bulky optics that cause a major hindrance for their exploitation in the emerging area of micron-nanophotonics. Recently, generation of such helical beams by micron-sized structures has gained momentous interest to make their appliance possible at small scale. This paper recapitulates these endeavors of minuscule optical vortex generation. Five different types of compact optical vortex generators, i.e., downscaling of conventional OAM generators, plasmonic vortex lens (PVLs), metasurfaces, integrated OAM emitters and subwavelength apertures are brought under discussion.