In this paper, we review the generation of vortex beams carrying orbital angular momentum in the microwave domain. We firstly present the theory of Laguerre–Gaussian beams where it is demonstrated that they carry such type of momentum. We further provide an overview of the classical methods used to generate orbital angular momentum vortex beams, which rely on two main methods; plane wave to vortex wave conversion and direct generation using radiating antennas. Then, we present recent progress in the physics of metasurfaces devoted to the generation of vortex beams with a discussion about reflective and transmissive metasurfaces for plane wave to vortex wave conversion as well as methods to reduce the intrinsic divergence characteristics of vortex beams. Finally, we conclude on this rapidly developing research field.

A Review of Orbital Angular Momentum Vortex Beams Generation: From Traditional Methods to Metasurfaces

Orbital angular momentum (OAM) for radio communications has the potential of simultaneously transmitting multiple signals at the same frequency and time resources. This multiplies the achievable channel capacity at a given bandwidth by increasing the available number of simultaneous data streams. One downside of OAM radio communication is the requirement of multimode radio frequency (RF) hardware at both ends of a link, i.e., the transmitter and the receiver. This is not always practically viable, especially as we move toward low-profile receivers in future communication devices. In this work, we present a novel method of OAM-based radio communication with enhanced physical layer security that requires only a single-antenna receiver. We first present the system architecture, then we design and realize a Rotman lens-based circular antenna array transmitter operating at 5.8 GHz. We then experimentally verify the capability of the hardware to create multiple modes. As a proof-of-concept, we propose a communication system that simultaneously uses modes 0 and +1 of the OAM beamformer and in doing so show how a single-antenna receiver can be used for data recovery. We first identify a general analog modulation expression and use the proposed system to transmit digitally modulated data stream to a single-antenna-equipped receiver. A precommunication training sequence is required to realize the proposed approach. The experimental results verify the simulated predictions.

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Physical Layer Secure Communication Using Orbital Angular Momentum Transmitter and a Single-Antenna Receiver

Vortex waves have potential applications in wireless communication, because of their advantages in improving channel-transmission efficiency. However, the peculiar divergence of vortex waves seriously limits their propagation distance for communication. Thus the authors propose an innovative method for collimating vortex waves at microwave frequencies, based on transformation optics. Their all-dielectric device can reduce the divergence angle of vortex waves over a wide bandwidth, and collimate vortex beams with multiple modes.

Design and Validation of an All-Dielectric Metamaterial Medium for Collimating Orbital-Angular-Momentum Vortex Waves at Microwave Frequencies

Vortex beams of light carrying orbital angular momentum (OAM) show promise for a wide range of microwave applications. However, classical OAM wave generators in the rf band are generally hindered by narrow operating bandwidth, complicated feed structures, or high loss. This study presents a method for OAM wave generation using the concept of spatial transformation at microwave frequencies. The all-dielectric device implementation enables broadband operation, and the technique efficiently generates vortex waves carrying OAM modes, thus illustrating the practicality of employing spatial transformations to realize innovative microwave devices.

All-Dielectric Transformed Material for Microwave Broadband Orbital Angular Momentum Vortex Beam

In this paper, an ultra-low profile anisotropic holographic metasurface is proposed to generate a linearly polarized high-order Bessel vortex beam carrying orbital angular momentum with predesigned topological charge. Based on the leaky-wave theory and optical holographic principle, the anisotropic impedance pattern can be properly mapped by shaping the quasi-periodic metasurface with different meta-atom sizes. Compared with the prevalent spatial wave-modulated metasurface, this surface wave-modulated holographic metasurface effectively transforms a reference wave excited by a feeding source in a single point at the center of the antenna to a leaky high-order Bessel vortex beam without any extra air feeding, which enables a unique characteristic of error-free alignment between the air feeding and designed metasurface. The good agreement between the numerical simulation and the measured result demonstrates that the proposed approach can be employed to launch a linearly polarized high-order Bessel vortex beam with an arbitrary topological mode of interest.

Launcher of high-order Bessel vortex beam carrying orbital angular momentum by designing anisotropic holographic metasurface

In this letter, a phase-gradient metasurface (MS) lens whose elements satisfy high transmittance and desired transmission phase is proposed to achieve collimation of vortex beams generated by an orbital angular momentum (OAM) radiating source. Two different phase profiles, hyperbolic and conical, are calculated and simulated. In addition, a prototype based on the conical phase profile is fabricated and experimentally tested, which verifies that the MS lens can realize effective collimation functionality. The proposed integrated lens-antenna system is able to provide new possibilities for OAM applications in radio wireless communication, imaging, and sensing.

https://iopscience.iop.org/article/10.7567/1882-0786/ab2c1d/pdf

Design and validation of a metasurface lens for converging vortex beams

The invention provides a low-frequency wave transmission and high-frequency wave absorption frequency selection device. The device comprises MxN frequency selection surface units distributed periodically, wherein each frequency selection unit comprises a first square dielectric plate and a second square dielectric plate which are stacked up and down; 2x2 ITO thin films in the shape of a regular polygon or a regular polygon ring are pasted to the upper surface of each first dielectric plate, and the characteristic of high-band electromagnetic wave broadband absorption is realized by determiningthe size of each ITO thin film; and a metal patch is printed on the lower surface of each second dielectric plate, wherein a quasi-Jerusalem cross-shaped gap is etched on the metal patch, and the characteristic of low-band electromagnetic wave bidirectional transmission is realized by determining the size of the quasi-Jerusalem cross-shaped gap. The device has both the characteristic of low-bandbidirectional wave transmission and the characteristic of high-band wave absorption, the bandwidth of a wave absorption band is expanded, and the device can be applied to the field of wireless communication technologies.

Low-frequency wave transmission and high-frequency broadband wave absorption frequency selection device

The invention provides a method for generating omni-directional circularly polarized vortex electromagnetic waves, and solves the technical problems that in the prior art, omni-directional vortex electromagnetic waves are low in gain, and circular polarization is difficult to achieve. The implementation process comprises the following steps of: designing the lens shape of a cylindrical metasurfacelens component; setting parameters of the lens component; calculating expected vortex electromagnetic wave phase distribution and metal layer geometric rotation angle distribution of the initial model; establishing a lens component initial model and performing full-wave simulation; calculating convergence phase distribution and metal layer geometric rotation angle distribution of a final model; and obtaining a final model of the lens component, and carrying out full-wave simulation verification on the final model. A planar metasurface lens formed by periodically arranging transmission unit structures is conformal to a cylindrical surface to form a cylindrical metasurface lens component. The method for generating omni-directional circularly polarized vortex electromagnetic waves has the advantages of the relatively high gain, the relatively high circular polarization purity and the simple feeding, and can be applied to the field of communication.

Method for generating omni-directional circularly polarized vortex electromagnetic waves

Vortex electromagnetic waves carrying orbital angular momentum (OAM) have been widely discussed for potential applications in wireless communications. Belonging to the Laguerre–Gaussian beams family, such type of waves present a hollow conical shape and divergence characteristics along with a directional radiation. In this paper, an innovative method to produce omnidirectional OAM beams based on spatial transformation is proposed at microwave frequencies. As a proof-of-concept demonstration, a lens with omnidirectional radiation in the horizontal plane is designed and simulated with an incident vortex beam carrying the OAM mode l = +2. The designed lens can be potentially implemented with an all-dielectric medium showing a gradient permittivity distribution. Furthermore, the proposed lens presents good performances over a wide operational bandwidth spanning from 8 to 17 GHz. By converting the directional beam to an omnidirectional one, the proposed method opens the door to the potential development of microwave vortex antenna systems.

Die Li

Papers

Design and validation of a metasurface lens for converging vortex beams

Low-frequency wave transmission and high-frequency broadband wave absorption frequency selection device

Method for generating omni-directional circularly polarized vortex electromagnetic waves

A metamaterial lens based on transformation optics for horizontal radiation of OAM vortex waves