Electromagnetic orbital angular momentum (OAM) has attracted a lot of interest recently in the field of communication systems. OAM-carrying radio beams can be generated and detected by several devices. In this paper, we use a circular phased antenna array to generate OAM-carrying beams that possess ring-shaped intensities and helical phase fronts. We also propose and validate a communication system that uses OAM-based multiplexing. A circular antenna array is used to generate the superimposed OAMmode by using appropriate excitation settings. The transmission characteristics of OAM modes with respect to different transmission distances are investigated. We find that the OAM-based multiplexing technology is only feasible within a certain distance range. The obtained experimental results are consistent with simulation and numerical calculation results.

Generation and Transmission of OAM-Carrying Vortex Beams Using Circular Antenna Array

Orbital angular momentum (OAM) has aroused a widespread interest in many fields, especially in telecommunications due to its potential for unleashing new capacity in the severely congested spectrum of commercial communication systems. Beams carrying OAM have a helical phase front and a field strength with a singularity along the axial center, which can be used for information transmission, imaging and particle manipulation. The number of orthogonal OAM modes in a single beam is theoretically infinite and each mode is an element of a complete orthogonal basis that can be employed for multiplexing different signals, thus greatly improving the spectrum efficiency. In this paper, we comprehensively summarize and compare the methods for generation and detection of optical OAM, radio OAM and acoustic OAM. Then, we represent the applications and technical challenges of OAM in communications, including free-space optical communications, optical fiber communications, radio communications and acoustic communications. To complete our survey, we also discuss the state of art of particle manipulation and target imaging with OAM beams.

Orbital Angular Momentum Waves: Generation, Detection and Emerging Applications

Recently, vortex beam carrying orbital angular momentum (OAM) for radio communications has attracted much attention for its potential of transmitting multiple signals simultaneously at the same frequency, which can be used to increase the channel capacity. However, most of the methods for getting multi-mode OAM radio beams are of complicated structure and very high cost. This paper provides an effective solution of generating dual circularly-polarized (CP) dual-mode OAM beams. The antenna consists of four dual-CP elements which are sequentially rotated 90 degrees in the clockwise direction. Different from all previous published research relating to OAM generation by phased arrays, the four elements are fed with the same phase for both left-hand circular polarization (LHCP) and right-hand circular polarization (RHCP). The dual-mode operation for OAM is achieved through the opposite phase differences generated for LHCP and RHCP, when the dual-CP elements are sequentially rotated in the clockwise direction. The measured results coincide well with the simulated ones, which verified the effectiveness of the proposed design.

Experimental Array for Generating Dual Circularly-Polarized Dual-Mode OAM Radio Beams.

Recently, vortex beam carrying orbital angular momentum (OAM) for radio communications has attracted much attention for its potential of transmitting multiple signals simultaneously at the same frequency, which can be used to increase the channel capacity. However, most of the methods for getting multi-mode OAM radio beams remain up to now mainly at simulation level, since their implementations are of complicated structure and very high cost. This paper provides an effective design of generating five-mode OAM radio beams by using the Rotman lens-fed antenna array. The Rotman lens is a viable beamforming approach instead of electronically scanned arrays for its low cost and the ease of implementation. The lens-fed array employs a two-layer structure for size reduction and the lens body and the antenna array are segregated by a common ground plane to eliminate spurious radiation and thus improve the performance of the OAM beams. The measured results coincide with the simulated ones, which verified the effectiveness of the proposed design for generating multi-mode OAM beams.

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Rotman Lens-Based Circular Array for Generating Five-mode OAM Radio Beams

The generation of vortex electromagnetic (EM) waves carrying orbital angular momentum (OAM) has attracted more and more attention due to its special characteristics and potential applications. In this paper, the circularly polarized vortex EM waves with different OAMs have been generated at 2.4 GHz by a novel conical conformal patch antenna (CCPA) consisting of a conical substrate, a correspondingly conformed circular-ring metallic patch, and a single-feed point. First, we analyze the radiated field of a standard circular-ring patch, in which a circularly polarized TM $_{nm}$ mode excited by two coaxial feeds can generate the circularly polarized OAM wave with the topological charge of $l = (n - 1$ ). Then, a pair of the slot is added to the CCPA for obtaining a simpler structure with a single feed point. By adjusting the size of the slot and the angle between the slot and the single-feed point, two orthogonal modes with the same amplitude and a relative 90° phase shift can be obtained for generating the circularly polarized vortex wave. In addition, we have also tried to use an additional conical horn outside of the CCPA for enhancing the performance of the antenna. Finally, in order to validate the theoretical results, we have also fabricated and measured the CCPA, from which we can confirm that our design is efficient for generating the vortex waves.

Generating Circularly Polarized Vortex Electromagnetic Waves by the Conical Conformal Patch Antenna

This paper provides an effective solution of generating OAM-carrying radio beams with all three polarizations: the linear, the left-hand circular, and the right-hand circular polarizations. Through reasonable configuration of phased antenna array using elements with three polarizations, the OAM radio waves with three polarizations for different states can be generated. The vectors of electric fields with different OAM states for all three polarizations are presented and analyzed in detail. The superposition of two coaxial OAM states is also carried out, and the general conclusion is provided.

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Generation of OAM Radio Waves with Three Polarizations Using Circular Horn Antenna Array

This paper presents a simple structure for the realization of an $N$ -way reconfigurable power divider. With this $N$ -way reconfigurable power divider, $N$ different transmission modes with $2^{N}-1$ states can be realized by the use of only $3N-1$ p-i-n diodes. In each transmission mode, the input port and transmission ports are all matched and high isolations between transmission ports are obtained. To realize matching performances under different transmission modes, a reconfigurable multiple-impedance transformer network (RMITN) is introduced at the input port of the proposed divider. With the parameters of $N-1$ p-i-n diodes under different bias conditions, $N$ different load impedances can be all transformed into $Z_{0}$ (port impedance) by RMITN, where load impedances can be real or complex impedances. In addition, no lumped elements, such as dc blocking capacitors, are required in the proposed reconfigurable divider. For verification, a 5-GHz four-way reconfigurable power divider is designed and fabricated. Four different kinds of transmission modes (four-path mode, three-path mode, two-path mode, and single-path mode) with 15 states are realized by the prototype. The measured transmission losses at 5 GHz in the four-path, three-path, two-path, and single-path modes are less than 0.78, 1.32, 1.9, and 1.74 dB, respectively; while in those modes, the isolations between different ports are higher than 30.2, 31.5, 24, and 27.4 dB, respectively.

An $N$ -Way Reconfigurable Power Divider

A truncated circular cone slot array antenna that radiates a circularly polarized (CP) conical beam is presented. The direction into which the conical beam radiates depends mainly on the tilt angle of the outer conductor as well as the proper arrangement of the positions of its two slot arrays. The CP property is realized by introducing quasi-perpendicular slots that are cut into the outer conductor of a truncated circular cone. To illustrate the performance characteristics of this CP conical beam antenna, the tilt angle is set to 45° and the beam angle is pointed at 45° from boresight within the operating band. A prototype was fabricated and tested. The measured results demonstrate that its −10 dB impedance bandwidth ( $\vert S_{{11}}{\rm{\vert \, dB) is 720 MHz, from 5.05 to 5.77 GHz; and its 3 dB axial-ratio (AR) bandwidth is 500 MHz, from 5.5 to 6.0 GHz. Its realized gain over this AR bandwidth ranges from 4.84 to 5.8 dB. The designed beam direction can be easily changed to meet specific application requirements. Moreover, it can be readily reconfigured to be a conformal antenna mounted on the top of a high-speed mobile platform.

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Truncated Circular Cone Slot Antenna Array That Radiates a Circularly Polarized Conical Beam

A new type of perforated dielectric flat reflectarray for the generation of OAM-carrying radio waves is presented. The reflectarray is introduced to transform the spherical wave emanating from the feed into a vortex beam after reflection through the azimuthal variation of the effective dielectric permittivity. Through perforating the substrate with holes of various diameters, an azimuthal variation of the permittivity is obtained and the corresponding azimuthal phase-compensation creates a vortex, thus an OAM wave is generated. The reflectarray antenna works in the millimeter frequency band, and can be used to generate the OAM states of l =+1.

Perforated dielectric antenna reflectarray for OAM generation

A horn lens antenna designed for the generation of OAM-carrying radio waves is presented. The conventional horn antenna is integrated with a single-layer perforated planar dielectric lens by covering its aperture. Through varying the diameter of holes, an azimuthal variation of the permittivity is obtained and the corresponding azimuthal phase delay creates a vortex, thus an OAM wave is generated. The lens antenna works in the millimeter frequency band, and can be used to generate the OAM states of l =+1 or −1.

Liang Liu

Papers

Generation of OAM Radio Waves with Three Polarizations Using Circular Horn Antenna Array

An $N$ -Way Reconfigurable Power Divider

Truncated Circular Cone Slot Antenna Array That Radiates a Circularly Polarized Conical Beam

Perforated dielectric antenna reflectarray for OAM generation

Design of a horn lens antenna for OAM generation