Multiple Orbital Angular Momentum Beams Research Articles

Dual-band metasurface generating multiple OAM beams independently in full polarizations.

In this paper, a dual-band metasurface (MS) generating multiple orbital angular momentum (OAM) beams independently in full polarizations is proposed. First, the design principle of controlling full polarizations independently is analyzed. Second, the frequency selective surface is introduced to the meta-atom design that ensures the meta-atom operates at Ku- and Ka-band independently, while, at each band, sixteen optimized meta-atoms realize the high reflection amplitude and enough phase coverage. Next, the optimized dual-band meta-atom controlling full polarizations independently is utilized to design the MS, which could generate eight independent OAM beams including the x-polarized, y-polarized, left hand circularly polarized, and right hand circularly polarized OAM beams at dual-band. Finally, the MS is designed, fabricated, and measured. Both simulated and measured results verify that the proposed MS could generate multiple OAM beams in full polarizations at dual-band, showing the perspective in the OAM-based area, such as the wireless communication, target detection, and security encryption.

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.

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.

Multiple Orbital Angular Momentum Beams with High‐Purity of Transmission‐Coding Metasurface

AbstractVortex electromagnetic wave carrying orbital angular momentum (OAM) is expected to break through the shortage of spectrum resource because of its multiple independent modes. Nowadays, there are various methods to generate OAM vortex beam. Based on the concepts of digital metasurface and propagation phase, a broadband 3‐bit coding transmission metasurface is designed and illustrated by simulation and measurement. The elements exhibit the different phase by changing the size of metal patch, and the focusing phase is introduced to reduce the divergence of OAM beams. Based on the principle of phase superposition, the wide‐band, high‐purity, and multi‐beam OAM vortex waves can be generated by the transmission‐coding metasurface (TCMS). The experimental results verify that TCMS realizes the multiple OAM beams along different directions simultaneously from 9 to 10.5 GHz, which are consistent with the theoretical results. The high purity more than 90% of OAM mode is demonstrated by both the simulated and measured results. The TCMS provides a potential and effective scheme for OAM multiplexing and multi‐target positioning in wireless systems.

A THz Integrated Circuit Based on a Pixel Array to Mode Multiplex Two 10-Gbit/s QPSK Channels Each on a Different OAM Beam

We experimentally demonstrate a THz integrated circuit with a pixel-array-based orbital-angular-momentum (OAM) mode converter for the generation and multiplexing of multiple data-carrying OAM beams. The integrated device consists of (i) two coupling spikes, which serve as inputs and can couple THz signals from hollow metallic waveguides to the device; (ii) two mode expansion structures, which expand the mode profiles from the waveguide inputs to match the OAM mode converter; and (iii) a 3.6 × 3.6 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> pixel-array-based OAM mode converter, which vertically emits and multiplexes two free-space THz data-carrying OAM beams with different mode orders. The generated OAM order depends on the corresponding input port. Two different designs of the integrated OAM emitters show the generation and multiplexing of (a) OAM +1 and –1, and (b) OAM +1 and –2 at the center frequency of ∼ 0.317 THz. Crosstalk lower than –15 dB between the OAM modes can be achieved. Specifically, the OAM +1/–1 emitter has a 3-dB power bandwidth of ∼15 GHz. Using the OAM +1/–1 emitter for a free-space link at a 0.317-THz carrier frequency, we experimentally demonstrate 20-Gbit/s quadrature-phase-shift-keying (QPSK) transmission by multiplexing OAM +1 and –1. Compared to a single-beam transmission, the multiplexed link induces a ∼1-dB signal-to-noise ratio (SNR) penalty at the bit error rate (BER) of 3.8×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−3</sup> .

Dual-Band Spin-Decoupled Metasurface for Generating Multiple Coaxial OAM Beams

In this article, we propose a shared-aperture metasurface (MS) combining spin-decoupled strategy to generate four coaxially circularly polarized (CP) beams carrying orbital angular momentum (OAM) at Ku-band and Ka-band. First, the frequency selective surface (FSS) is adopted to suppress the mutual coupling between the Ku-band and the Ka-band. Next, at each band, the spin-decoupled meta-atom is designed to realize the independent control for dual circular polarizations. Subsequently, the designed MS illuminated by CP feed source would generate left-hand CP (LHCP) beam with OAM mode +1 and right-hand CP (RHCP) beam with OAM mode −1 at Ku-band; meanwhile, the LHCP beam with OAM mode +2 and the RHCP beam with OAM mode −2 are generated at Ka-band. Finally, the MS is designed, manufactured, and measured. Both simulated and measured near-field and far-field performance verify that the proposed MS can generate four independent CP OAM beams along the same spatial axis at dual bands. On the other hand, the experimental results and the corresponding theoretical analysis verify that the proposed MS illuminated by a linear-polarized (LP) feed source could also generate eight correlative LP OAM beams at dual bands.

Bifunctional Full-Space Metasurface Combining Pancharatnam–Berry and Propagation Phases

A bifunctional full-space metasurface (FS-MS) capable of generating reflective multiple vortex beams and focusing transmissive electromagnetic waves within different frequency bands is presented in this letter. The generation of vortex beams is based on the Pancharatnam–Berry (PB) phase, which exhibits the property of full phase control by rotating the orientation angle of an I-shaped patch. Meanwhile, this PB unit is backed by a metallic square loop frequency selective surface (FSS), which behaves as stopband and passband responses around 18 GHz and 12 GHz, respectively. By encoding PB phase to generate a certain phase response, four reflective vortex beams carrying orbital angular momentum (OAM) are achieved at a higher frequency band under normal incidence. On the other hand, the incident waves operating at a lower frequency band can pass through such an FSS-backed PB unit and be focused by a three-layer composite unit consisting of a polarization-conversion unit sandwiched by two orthogonal polarizers. Therefore, upon illumination of normal incident plane waves, the proposed FS-MS works as a multiple OAM beams generator and a focusing lens simultaneously. The proposed design can motivate the realizations of FS-MS and hold potential applications in intelligent housing systems.

Compensating the distorted OAM beams with near zero time delay

Vortex beams, carrying orbital angular momentum (OAM), have great potential to increase the information capacity of optical communication systems for the orthogonality and infinite mode number. For OAM beams propagating in free space, however, the atmospheric turbulence may cause mode distortions and hinder their utilization in practice. In this work, we propose a kind of diffractive deep neural network (D2NN) to compensate the distorted OAM beams. Different from those D2NNs reported before, the network reported here is dissipative, rather than unitary. In our system, the common features of various wavefront distortions are extracted and compensated, while the random distortions are filtered out by a diaphragm, which is achieved by constructing an improved loss function. The results show that multiple OAM beams with different degrees of distortions can be compensated simultaneously, and good agreement between simulations and experiments is obtained. The D2NN based OAM beam compensating reported here will greatly improve the robustness and efficiency of free space optical communication.

Orbital angular momentum of light for communications

Structured light, especially beams carrying orbital angular momentum (OAM), has gained much interest due to its unique amplitude and phase structures. In terms of communication systems, multiple orthogonal OAM beams can be potentially utilized for increasing link capacity in different scenarios. This review describes challenges, advances, and perspectives on different aspects of the OAM-based optical communications, including (a) OAM generation/detection and (de)multiplexing, (b) classical free-space optical communication links, (c) fiber-based communication links, (d) quantum communication links, (e) OAM-based communications in different frequency ranges, (f) OAM-based communications using integrated devices, and (g) novel structured beams for communications.

Single-layer substrate wideband reflectarray with dual-polarization for multiple OAM beams

In this paper, a single-layer wideband dual-linearly polarized reflective element is developed operating from 13GHz to 15.5GHz. The developed reflective metasurface can produce orbital angular momentum (OAM) vortex waves with required modes, beam numbers, direction, x-, y- or both x- and -y polarizations. To demonstrate this versatility, three reflectarrays with multiple modes, multiple beams, and both x- and -y polarizations are designed, and one is fabricated and measured to validate the design. The measured peak gain, aperture efficiency, divergence angle is 19dBi, 11.54%, 6°, respectively. The designed reflectarray has potential applications for high communication rate.

Perspectives on advances in high-capacity, free-space communications using multiplexing of orbital-angular-momentum beams

Beams carrying orbital-angular-momentum (OAM) have gained much interest due to their unique amplitude and phase structures. In terms of communication systems, each of the multiple independent data-carrying beams can have a different OAM value and be orthogonal to all other beams. This paper will describe the use of multiplexing and the simultaneous transmission of multiple OAM beams for enhancing the capacity of communication systems. We will highlight the key advances and technical challenges in the areas of (a) free-space and fiber communication links, (b) mitigation of modal coupling and channel crosstalk effects, (c) classical and quantum systems, and (d) optical and radio frequency beam multiplexing.

Modal coupling and crosstalk due to turbulence and divergence on free space THz links using multiple orbital angular momentum beams

Orbital-angular-momentum (OAM) multiplexing has been utilized to increase the channel capacity in both millimeter-wave and optical domains. Terahertz (THz) wireless communication is attracting increasing attention due to its broadband spectral resources. Thus, it might be valuable to explore the system performance of THz OAM links to further increase the channel capacity. In this paper, we study through simulations the fundamental system-degrading effects when using multiple OAM beams in THz communications links under atmospheric turbulence. We simulate and analyze the effects of divergence, turbulence, limited-size aperture, and misalignment on the signal power and crosstalk of THz OAM links. We find through simulations that the system-degrading effects are different in two scenarios with atmosphere turbulence: (a) when we consider the same strength of phasefront distortion, faster divergence (i.e., lower frequency; smaller beam waist) leads to higher power leakage from the transmitted mode to neighbouring modes; and (b) however, when we consider the same atmospheric turbulence, the divergence effect tends to affect the power leakage much less, and the power leakage increases as the frequency, beam waist, or OAM order increases. Simulation results show that: (i) the crosstalk to the neighbouring mode remains < − 15 dB for a 1-km link under calm weather, when we transmit OAM + 4 at 0.5 THz with a beam waist of 1 m; (ii) for the 3-OAM-multiplexed THz links, the signal-to-interference ratio (SIR) increases by ~ 5–7 dB if the mode spacing increases by 1, and SIR decreases with the multiplexed mode number; and (iii) limited aperture size and misalignment lead to power leakage to other modes under calm weather, while it tends to be unobtrusive under bad weather.

OAM-Generating Transmitarray Antenna With Circular Phased Array Antenna Feed

A transmitarray antenna (TAA) with a small-scale circular phased array antenna (PAA) feed is proposed in this article to generate orbital angular momentum (OAM)-carrying radio beams. After analyzing the operating mechanisms of the OAM-generating PAA-fed TAA, its numerical model is further developed to determine the TAA element arrangement and the PAA feed excitation coefficients. Furthermore, a three-layer element with polarization-conversion property and the traditional patch antenna element are identified as the elements of the TAA and PAA feed, respectively. Benefiting from the intrinsic multibeam capabilities of the TAA, the steering of the zero-OAM-mode beam is firstly discussed, where a conical scanning across an angular range of ±25° is realized. Meanwhile, the creation of multiple pure or mixed OAM beams with helical phase fronts is also presented, where the superposition of multiple OAM states provides more possibilities to increase the channel capacity and spectrum efficiency. To facilitate the experiments, an $8\times 8$ Butler matrix with a bandwidth over 30% is designed, which integrated with the array-fed TAA also provides an attractive solution to simultaneously generate multiple OAM modes without requiring any active modules. The experimental results agree reasonably well with the numerical ones, validating the feasibility of our idea to design a high-gain OAM-generating PAA-fed TAA.

Orbital angular momentum beamforming for index modulation with partial arc reception

A novel beamforming (BF) method for orbital angular momentum (OAM) index modulation (IM) scheme is proposed in this Letter. Since the OAM wave has an inverted cone-shaped beam structure, only partial phase plane can be received during long distance transmission. Based on this partial phase plane information, an OAM–IM system can be established. Since the receiver is only located on a partial arc of the cone-shaped beam, the energy of the whole beam cannot be fully received. However, by adjusting the combinations of multiple OAM beams, the energy of the cone-shaped beam will be converged to a specific direction where the receiver is located. Compared with the traditional BF method, as a new degree of freedom, the OAM–BF method can not only increase the flexibility of beam steering, but also realise the IM information transmission on the partial receiving phase plane.

Holographic-Inspired Multiple Circularly Polarized Vortex-Beam Generation with Flexible Topological Charges and Beam Directions

Exploiting the orbital angular momentum (OAM) of light in applications has emerged as a key subject in optics and photonics. Here the authors discuss a holographic technique in the context of generating radio waves that carry OAM. Using suitable reflectarray patterns, a linearly polarized plane wave can be converted to a vortex beam with a spiral phase profile over a wide frequency range---or to multiple OAM beams with flexible beam directions, topological charges, and polarization states. These results are rather interesting for $e.g.$ expanding the capacity of wireless communication systems.

Two-dimension and high-resolution demultiplexing of coaxial multiple orbital angular momentum beams.

A high-resolution demultiplexing method is proposed for the separation of coaxial multiple orbital angular momentum (OAM) beams which are created by a self-designed computer-generated hologram (CGH) encoded with logarithmic spiral phases. The logarithmic spiral phase, consisting of both azimuthal and radial phase, enables the two-dimensional demultiplexing of the OAM beams. The simulation and experiment results show that, with our method, the coaxial multiple OAM beams can be effectively separated in the specified plane. The overlap between the adjacent channel is greatly reduced by the high-resolution demultiplexing method, which is based on an optical geometrical transform and a coherent copy technique. The proposed method has great potential in increasing the number of spatial channels available for OAM multiplexing transmission system.

Concentric UCAs Based Low-Order OAM for High Capacity in Radio Vortex Wireless Communications

Due to the potential capacity-boosting for wireless communications, radio vortex wireless communication (RowComm) over orthogonal states/modes of orbital angular momentum (OAM) has been paid much attention to in recent years. A uniform circular array (UCA), as an efficient and convenient antenna structure, can transmit/receive multiple OAM beams with different OAM-modes simultaneously when the transmitter and receiver are aligned. However, for high-order OAMmodes, the OAM beams are divergent accompanied by severe attenuation. Thus, it is difficult to directly use high-order OAM-modes to achieve high capacity for RowComms. To obtain high capacity potentially offered by OAM-modes, in this paper we transform the singular UCA into the concentric UCAs, where high capacity can be achieved using multiple parallel low-order OAM-modes instead of all high-order OAM-modes, to increase the capacity of transmitter-receiver aligned Row-Comms. In particular, we study two cases: 1). concentric UCAs based RowComms without co-mode interference; 2). concentric UCAs based RowComms with co-mode interference. We propose a co-mode-interference-free and a co-mode-interference-contained mode-decomposition schemes to recover transmitted signals for these two cases, respectively. Additionally, we develop optimal power allocation schemes to maximize the capacity for these two cases. Numerical simulations are presented to validate and evaluate that our developed concentric UCAs based low-order RowComms can significantly increase the capacity as compared with that of singular UCA based RowComms.

An Orbital Angular Momentum (OAM) Mode Reconfigurable Antenna for Channel Capacity Improvement and Digital Data Encoding

For purpose of utilizing orbital angular momentum (OAM) mode diversity, multiple OAM beams should be generated preferably by a single antenna. In this paper, an OAM mode reconfigurable antenna is proposed. Different from the existed OAM antennas with multiple ports for multiple OAM modes transmitting, the proposed antenna with only a single port, but it can be used to transmit mode 1 or mode −1 OAM beams arbitrary by controlling the PIN diodes on the feeding network through a programmable microcontroller which control by a remote controller. Simulation and measurement results such as return loss, near-field and far-field radiation patterns of two operating states for mode 1 and mode −1, and OAM mode orthogonality are given. The proposed antenna can serve as a candidate for utilizing OAM diversity, namely phase diversity to increase channel capacity at 2.4 GHz. Moreover, an OAM-mode based encoding method is experimentally carried out by the proposed OAM mode reconfigurable antenna, the digital data are encoded and decoded by different OAM modes. At the transmitter, the proposed OAM mode reconfigurable antenna is used to encode the digital data, data symbol 0 and 1 are mapped to OAM mode 1 and mode −1, respectively. At the receiver, the data symbols are decoded by phase gradient method.

Orbital-angular-momentum-based reconfigurable optical switching and routing

Orbital angular momentum (OAM) has gained interest due to its potential to increase capacity in optical communication systems as well as an additional domain for reconfigurable networks. This is due to the following: (i) coaxially propagated OAM beams with different charges are mutually orthogonal, (ii) OAM beams can be efficiently multiplexed and demultiplexed, and (iii) OAM charges can be efficiently manipulated. Therefore, multiple data-carrying OAM beams could have the potential capability for reconfigurable optical switching and routing. In this paper, we discuss work involving reconfigurable OAM-based optical add/drop multiplexing, space switching, polarization switching, channel hopping, and multicasting.

Design of on-chip N-fold orbital angular momentum multicasting using V-shaped antenna array.

We design a V-shaped antenna array to realize on-chip multicasting from a single Gaussian beam to four orbital angular momentum (OAM) beams. A pattern search assisted iterative (PSI) algorithm is used to design an optimized continuous phase pattern which is further discretized to generate collinearly superimposed multiple OAM beams. Replacing the designed discrete phase pattern with corresponding V-shaped antennas, on-chip N-fold OAM multicasting is achieved. The designed on-chip 4-fold OAM multicasting exploiting V-shaped antenna array shows favorable operation performance with low crosstalk less than -15 dB.