Generation of OAM beams with multiple modes with concentric spiral sub-array

By controlling the radial phase distribution of OAM beams, we successfully generate OAM beams with desired transmission trajectories, which can suppress the intrinsic divergence of conventional OAM beam at different transmission distance.

We design and fabricate a pixel-array-based THz integrated circuit for the generation and multiplexing of multiple OAM beams. We experimentally demonstrate a 20-Gbit/s QPSK free-space THz link at 0.317 THz by multiplexing OAM +1 and –1 using the integrated circuit.

In this paper, we proposed an active metasurface in reflection manner that can generate reconfigurable OAM vortex beams with high purity in the X-band. The metasurface has a high reflectance of 0.94 and achieves a phase coverage of 320° between 9.8 ​GHz and 11 ​GHz. Then, by encoding the phase distribution of the meta-atoms, various OAM vortex beams including ±1, ±2, ±3, and ±4 orders are generated, where the purity of all modes can be above 70%. Moreover, the metasurface can also deflect the OAM beam with a certain angle while maintaining high purity, which can be applied to reduce the influence of the alignment deviation between transmitting and receiving antennas during the communication processes. As a validation, the metasurface composed of 30 ​× ​30 meta-atoms is fabricated and measured. Both simulation and measurement results demonstrate the capability of the proposed metasurface to generate reconfigurable OAM beams with high purity, indicating the application potentials of proposed meta-devices in future OAM communications.

The design and testing of computer-generated holograms for complex light illumination are introduced and described. The high-order beams carrying angular momentum (OAM) are generated by means of novel spiral phase plates (SPP) for the control of the radial index. Besides the common helical profile, these phase plates exhibit radial π-discontinuities located at the zeros of the associated Laguerre polynomial. SPP samples were fabricated by electron beam lithography over glass substrates coated with a polymethylmethacrylate (PMMA) resist layer. A numerical code based on iterative Fourier transform algorithm has been developed for the computation of phase-only diffractive optical element for illumination under OAM beams. Numerical analysis and preliminary experimental results confirm the applicability of these computer-generated holograms as high-security optical elements for anti-counterfeiting applications.

In the realm of wave propagation through turbulent media, the spectrum of the orbital angular momentum of optical vortex beams is known to undergo symmetric broadening. However, the evolution of beams that are initially azimuthally asymmetric represents a distinct phenomenon. In this work, we have developed an analytical model describing the propagation of asymmetric OAM beams through the so-called Kolmogorov turbulence. Our results describe how the perturbation strength and the initial beam properties lead to a nonsymmetric spectrum of OAM modes. These findings lay the groundwork for further use of asymmetric fields that propagate in inhomogeneous media and their applications such as communications and sensing.

We report on the generation of fractional vortex beams with an all-fiber device incorporating a multi-mode interferometer and a few-mode fiber. Upon combining different modes at the input, complex patterns resembling OAM beams are generated.

The circular phased array is one of the main methods to generate radio frequency orbital angular momentum (RF-OAM) beams with high purity and capability of OAM-beams steering, but OAM beams generation and steering are only related to angle, not range dimensions. Therefore, the mode of OAM beams can be easily received in a wide range and intercepted by unintended eavesdroppers in the beam axis. As a promising solution, a range-angle dependent OAM beams transmission scheme is highlighted by the frequency diverse array (FDA) for the sake of the OAM secure transmission in this paper. In order to realize the range-angle dependent OAM beams, a special frequency interval between adjacent antenna elements in the FDA is designed and illustrated. The modes of OAM beams generated by the FDA can only be observed in the legal target location. Numerical results show that the scheme can effectively realize the safe transmission of OAM beams, i.e. , reducing the possibility of the OAM beams being tapped, improving the OAM-beams transmission performance, and enhancing the energy efficiency with low complexity.

In this paper, a novel transmission metasurface structure for the generation and beam direction control of vortex beams is presented. The metamaterial unit is composed of three layers of dielectric material and four layers of metal patches, wherein the metal layer is composed of a square ring and a central pattern. By changing the length of the central pattern, full coverage of phase can be achieved at 10 GHz, and the cell structure has a good transmission coefficient. The article briefly describes and analyzes the vortex beam generation method, realizing +1 mode OAM beam generation with high gain of 14.48dBi and beam direction control. The simulation results show that the transmitarray can be used in beam scanning. The directionality makes the information easy to be received and demodulated, which could be beneficial to the practical application of OAM in communication.

In this paper, the design of single layer metasurface for generating wideband OAM beam from 12–23 GHz with small divergence angle is presented. The proposed OAM metasurface is composed by a <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$30\times 30$</tex> unit cells distributed in a square lattice in the <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$xy$</tex> -plane. The phase compensation required at each unit cell across the metasurface aperture was achieved by rotating the metallic resonators to the desired angles. The proposed metasurface was fed by a circular polarization (CP) horn antenna. The proposed metasurface converts the EM waves coming from the horn feed with spherical phase-front to reflected OAM beam with helical phase-front. The full-wave simulation results show that OAM beams of modes <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$M=1$</tex> and <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$M=2$</tex> can be generated from 12 GHz to 23 GHz with a mode purity more than 92% and divergence angles around 16°. At the moment, prototypes of the proposed metasurfaces are being fabricated, and the measured results will be presented.

Propagation of OAM beams through atmospheric turbulence: comparison of simulation and experiment

Orbital angular momentum shift-keying communication through atmospheric turbulence and high scattering channel using a deep learning decoder

There exist a variety of methods and platforms for generation of OAM beams. However, all these techniques imply that beams pass through an optical component and thus, when pulsed beam is used, the pulses can acquire dispersive broadening and distortion. The design of compact and efficient devices for OAM generation requires that these effects be characterized and quantified, and a set of parameters and techniques for their treatment should be developed. Here, the spatio-temporal properties of the ultrashort vortical pulses are analyzed numerically. Case studies will be presented. The results obtained are characterization of the effects of dispersion, geometry and discretization in the numerical modeling of ultrashort vortical pulses under various propagation conditions, and can serve as a basis for design of new optical devices.

By using finite-element method (FEM) in the helical coordinates, the OAM modes are studied in a chirally-coupled-ring fiber with a ring, an inner and a pair of twisted side cores. The effects of the side cores on the effective indices of OAM modes are simulated in detail. It is shown that chirally-coupled-ring fiber has flexible design and great potential on the generation and transmission of OAM beams.

Generation of Highly-Pure OAM Beams with Simple Slotted SIW Antenna Array

Generation of oblique Airy-OAM beam using reflective metasurface