Ultimate Capacity Analysis of Orbital Angular Momentum Channels

Traditional wireless links employing OAM (orbital angular momentum) beams make use of their spatial orthogonality in order to increase their spectral efficiency, by carrying a separate data stream on each beam. This requires that the OAM antenna axes are aligned with the line of sight of the link, and are capable of radiating and receiving high-purity orthogonal OAM beams. Due to the deep spatial nulls of these beams (except the zero-order OAM beam) on their axes, the received signal strengths are much lower than from conventional beams of comparable-sized antennas, limiting the signal-to-noise ratio (SNR) and the reach of such links, or requiring very large receive antennas in terms of wavelengths.Here we describe an alternative technique for recovering received OAM beams based on pseudo-Doppler techniques [1]-[3] which avoid the above limitations of traditional OAM links. In this work, the receiving antennas are not aligned with the OAM beam axes, so the received signals are stronger, the receivers and their antennas are fewer and simpler, and the reach is longer than in traditional wireless OAM links. We demonstrate for the first time, by simulations and experiments performed on an outdoor 100m link at 28 GHz, that it is possible to recover 4 independent data streams carried in a common 20MHz channel on 4 co-aperture OAM beams using the technique developed in [1], with only 2 receiver chains. The transmitting OAM antenna radiates all OAM beams with the same cone angles and the 2 receiving antennas are set on their peaks off-axis in the far field, 20cm apart and tangent to the OAM cones, unlike in traditional OAM wireless links [4]. Salient aspects of the pseudo-Doppler OAM recovery algorithm, its simulation in an end-to-end link, and key design features of the demonstration hardware and signal-processing are presented.

As a pathbreaking approach to generate orbital angular momentum (OAM) beam, uniform circular array (UCA) has been extensively researched and applied. However, the impact of the imperfect UCA (IUCA) converted from a UCA with the position of antenna elements on the purity of the generated OAM beam is rarely considered. In order to verify the effect of position deviation on the OAM beam, this letter deduces the analysis model of the IUCA and the OAM mode purity model. The OAM mode purity refers to the weight of the intensity of the "expected OAM mode." Numerical simulation results show that the OAM beam generated by IUCA is a superposition of OAM beams with a series of different modes, and this results in a certain OAM beam received being contaminated. Compared with the radius deviation, the azimuth deviation of antenna elements has more obvious influence on the OAM mode purity. Moreover, low-mode OAM beams are more suitable for long-distance transmission. Properly increasing the antenna array radius can improve the purity of high-order OAM modes.

In the realm of free-space optical (FSO) communication systems, the utilization of orbital angular momentum (OAM) multiplexing has garnered significant attention. However, OAM beams are subject to atmospheric turbulence (AT) during transmission in free space, resulting in distortions of the phase front, intermodal crosstalk, and mode-dependent loss (MDL). While techniques such as adaptive optics and digital signal processing can mitigate these effects, inconsistencies in OAM mode performance persist due to MDL. Some studies have evaluated an MDL-impaired mode division multiplexing system for non-unitary multiple input multiple output (MIMO) channels based on maximum likelihood detection. Nevertheless, the computational complexity remains considerably higher compared to simpler yet suboptimal advanced MIMO decoders, such as successive interference cancellation (SIC) with zero-forcing/minimum mean square error (ZF/MMSE). On the other hand, an OAM mode and radial index are two distinct indices of Laguerre-Gaussian (LG) modes, with the potential to carry a large amount of information. To the best of our knowledge, no prior studies have explored the impact and mitigation strategies for MDL induced by atmospheric turbulence in an FSO system utilizing both OAM mode and radial index multiplexing with MMSE-SIC receiver. This paper introduces OAM multiplexing holography to generate OAM modes. We perform both analytical and numerical analyses of the orbital angular momentum spectra associated with distorted single-mode and multiplexed LG beams. This is achieved by generating holograms, incorporating random phase screens along the propagation path, and varying the refractive index structure constants. We mathematically develop and simulate an MMSE-SIC receiver for OAM multiplexed FSO for different radial values for the first time and compared its BER to Zero Forcing-SIC and K-best detectors. Additionally, orthogonal space-time block coding is employed at the transmitter to enhance the overall performance of the system.

Vector orbital angular momentum (OAM) beams, described by higher-order Poincaré (HOP) sphere, are generalized forms of waves carrying OAM with an inhomogeneous polarization of wavefronts. We construct all-dielectric metasurfaces with adjustable amplitude, polarization, and phase to generate arbitrary vector OAM beams. The metasurface is composed of two pairs of silicon nanopillars arranged alternately. Using the interference effect of the four meta-atoms related to the circular polarization, combined with the propagation and geometric phases, two OAM beams with controlled amplitude, phase, and equal topological charge but opposite signs can be obtained under the incidence of orthogonally circularly polarized lights. For the x linearly polarized light, arbitrary vector OAM beams on the HOP sphere are generated via the superposition of the above OAM beams. Additionally, the evolution process of the beam on the longitude and latitude of the Poincaré sphere is revealed by changing the amplitude and phase of the two OAM beams. This work provides a simple, effective, and flexible method for realizing vector OAM beams while having potential implications for the generation and manipulation of vectorial light fields at the micro-nano scale.

Orbital angular momentum (OAM) beams have attracted great attention owing to their excellent performances in imaging and communication. In this paper, a dual-arm Archimedean spiral antenna (DASA) is proposed to generate multiple OAM states with positive and negative values by feeding at the inner and outer ends, respectively. The topological charge of radiated vortex waves is reconfigurable by tuning the operating frequency. Dual-mode OAM states are generated at different working frequencies (l = ±1 at 3 GHz, l = ±2 at 4 GHz, and l = ±3 at 4.8 GHz). Both the simulation and measurement results demonstrate that OAM beams can be generated effectively by the DASA. In addition, a conical cavity is used to increase the gain of the proposed DASA for more than 5 dBi in comparison to the traditional cylindrical cavity. Furthermore, the qualities of the generated OAM modes by the proposed DASA have been evaluated at different operating frequencies of 3 GHz, 4 GHz, and 4.8 GHz, respectively. The OAM modes purities of l = −1, −2, −3, 1, 2, and 3 are predominate with the proportion of about 81%, 70%, 74%, 78%, 77%, and 75%, respectively. Our results demonstrate that the proposed DASA has great potentials in OAM multiplexing communication systems.

Orbital angular momentum (OAM) beams allow for increased channel capacity in free-space optical communication. Conventionally, these OAM beams are multiplexed together at a transmitter and then propagated through the atmosphere to a receiver where, due to their orthogonality properties, they are demultiplexed. We propose a technique to demultiplex these OAM-carrying beams by capturing an image of the unique multiplexing intensity pattern and training a convolutional neural network (CNN) as a classifier. This CNN-based demultiplexing method allows for simplicity of operation as alignment is unnecessary, orthogonality constraints are loosened, and costly optical hardware is not required. We test our CNN-based technique against a traditional demultiplexing method, conjugate mode sorting, with various OAM mode sets and levels of simulated atmospheric turbulence in a laboratory setting. Furthermore, we examine our CNN-based technique with respect to added sensor noise, number of photon detections, number of pixels, unknown levels of turbulence, and training set size. Results show that the CNN-based demultiplexing method is able to demultiplex combinatorially multiplexed OAM modes from a fixed set with >99% accuracy for high levels of turbulence-well exceeding the conjugate mode demultiplexing method. We also show that this new method is robust to added sensor noise, number of photon detections, number of pixels, unknown levels of turbulence, and training set size.

The capacity of free-space optical (FSO) communication links could potentially be increased by the simultaneous transmission of multiple orbital angular momentum (OAM) beams. For such an OAM multiplexing approach, one requires the collection of adequate power as well as proportion of the phase front for a system with minimal crosstalk. Here we study the design considerations for an OAM-multiplexed free-space data link, analyzing the power loss, channel crosstalk, and power penalty of the link in the case of limited-size receiver apertures and misalignment between the transmitter and the receiver. We describe the trade-offs for different transmitted beam sizes, receiver aperture sizes, and mode spacing of the transmitted OAM beams under given lateral displacements or receiver angular errors. Through simulations and some experiments, we show that (1) a system with a larger transmitted beam size and a larger receiver aperture is more tolerant to lateral displacement but less tolerant to the receiver angular error, and (2) a system with a larger mode spacing, which uses larger OAM charges, suffers more system power loss but less channel crosstalk; thus, a system with a small mode spacing shows a lower system power penalty when system power loss dominates (e.g., a small lateral displacement or receiver angular error), whereas that with a larger mode spacing shows a lower power penalty when channel crosstalk dominates (e.g., a larger lateral displacement or receiver angular error). This work could be beneficial to the practical implementation of OAM-multiplexed FSO links.

With the rapid growth in the number of mobile devices and user connectivity, the demand for higher system capacity and improved quality-of-service is required. As the demand for high-speed wireless communication grows, numerous modulation techniques in the frequency, temporal, and spatial domains, such as orthogonal frequency division multiplexing (OFDM), time division multiple access (TDMA), space division multiple access (SDMA), and multiple-input multiple-output (MIMO), are being developed. Along with those approaches, electromagnetic waves’ orbital angular momentum (OAM) is attracting attention because it has the potential to boost the wireless communication capacity. Antenna electromagnetic radiation can be described by a sum of Eigen functions with unique eigenvalues, as is well known. In order to address such issues, the millimeter-wave (mmWave) communication is proposed which is considered as one of the potential technology for 5G wireless networks. The intrinsic feature of all electromagnetic waves is OAM. The OAM beams’ unique qualities have led to a slew of new uses. Broadband OAM generators, on the other hand, have gotten very little attention, especially in the mmWave frequency band. The use of OAM in conjunction with mmWave can reduce the beam power loss, enhance the received signal quality, and hence increase the system capacity. The transmitter and receiver antennas must be coaxial and parallel to achieve precise mode detection. The proposed mmWave integrated with OAM system model is discussed in this study. The channel model is created using the channel transition characteristics. The simulation results demonstrate that the proposed system model is a good way to boost the system capacity.

Orbital Angular Momentum (OAM) multiplexing is a new multiplexing dimension after the existing multiplexing technologies. OAM and MIMO technology are combined to the OAM-MIMO, which can greatly improve the channel capacity of wireless optical communication. The coding and modulation of information, the generation, transmission and separation of OAM superposition states are studied, the influence of atmospheric turbulence on OAM superposition states is discussed. It lays a theoretical foundation and provides an experimental basis for the application of OAM multiplexing technology.

As the dominant information carrier in water, the acoustic wave is widely used for underwater detection, communication and imaging. Even though underwater acoustic communication has been greatly improved in the past decades, it still suffers from slow transmission speeds and low information capacity. The recently developed acoustic orbital angular momentum (OAM) multiplexing communication promises a high efficiency, large capacity and fast transmission speed for acoustic communication. However, the current works on OAM multiplexing communication mainly appear in airborne acoustics. The application of acoustic OAM for underwater communication remains to be further explored and studied. In this paper, an impedance matching pentamode demultiplexing metasurface is designed to realize multiplexing and demultiplexing in underwater acoustic communication. The impedance matching of the metasurface ensures high transmission of the transmitted information. The information encoded into two different OAM beams as two independent channels is numerically demonstrated by realizing real-time picture transfer. The simulation shows the effectiveness of the system for underwater acoustic multiplexing communication. This work paves the way for experimental demonstration and practical application of OAM multiplexing for underwater acoustic communication.

Artificial intelligence methods for the phase control of laser arrays have been extensively developed in the recent years. To tailor high-power and fast switchable orbital angular momentum (OAM) beams by the coherent combination of laser arrays, complex phase control difficulties arise, and the introduction of deep-learning (DL) methods to learn the mapping from the intensity information to the relative phases of array elements offers a promising solution. In this work, we theoretically propose a DL-based phase control scheme that can customize OAM beams with desirable topological charges from laser array systems, which relies on the acquisition of the optical field information in the view of angular domain. When the system in closed loops, the optical field and phase errors information of laser array are extracted and decoupled in the angular domain perspective by using an OAM demultiplexer. The processed optical field of the coherently combined beam serves the DL-based phase control servo to realize the efficient phase stabilization, thus ensuring the controllable customization of OAM beams. This work could provide a useful reference on the intelligent control of laser array systems for structured light beams customization.

In this paper, a design of circular single-sideband (SSB) time-modulated array (TMA) for generating orbital angular momentum (OAM) beams is proposed. It increases signal bandwidth significantly compared with conventional TMAs for generating vortex waves. Theoretical analysis and simulated results of the proposed SSB TMA modulated by the optimized four-step pulse are reported. Higher harmonic efficiency and total efficiency (98.73% and 64.15%) are achieved simultaneously. And mode 1, 2, and 3 OAM beams with mode purity up to 97.3%, 97.2%, and 79.6% are generated at the first positive harmonic frequency.

We experimentally demonstrate turbulence-resilient free-space optical (FSO) coherent communications with multiple multiplexed data channels in different orthogonal domains, including mode, polarization, and wavelength. The turbulence resiliency is enabled by utilizing pilot-assisted optoelectronic mixing of the received beams, wherein one pilot is being mixed with its corresponding beam. By transmitting additional continuous-wave (CW) pilot tones, whose frequencies are offset from the data-carrying beams, and mixing all pilot tones and beams in a single free-space-coupled photodetector (PD) at the receiver, the turbulence-induced modal coupling could be efficiently suppressed. The paper first discusses an experimental demonstration of a turbulence-resilient mode-division-multiplexed (MDM) FSO communication link using two orbital angular momentum (OAM) beams. Each OAM beam carries an independent 2-Gbit/s quadrature-phase-shift-keying (QPSK) data channel. Experimental results indicate that (i) the channel crosstalk under both the weaker and stronger turbulence distortions are measured to be lower than ∼−25.7 dB, (ii) the turbulence-induced OAM modal coupling could be effectively suppressed for the emulated 15 random turbulence realizations, and (iii) the error-vector magnitudes (EVM) of the multiplexed QPSK data channels are measured to be lower than by 17.4% and 19.8% under the relatively weaker and stronger turbulence effects, respectively. The paper subsequently describes an experimental demonstration of a turbulence-resilient FSO link with eight 0.5-Gbit/s QPSK data channels multiplexing two OAM beams ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">l</i> = +1 & <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">l</i> = −2), two polarizations, and two wavelengths. The measured bit-error rates (BERs) of all data channels under turbulence effects are below the 7% forward-error-correction (FEC) limit, with power penalties less than ∼3.7 dB.

The Orbital Angular Momentum (OAM) carried by light beams with helical phasefront (vortex beams) has been widely employed in many applications such as optical tweezers, optical drives of micro-machines, atom trapping, and optical communication. OAM provides an additional dimension (diversity) to multiplexing techniques, which can be utilized in addition to conventional multiplexing methods to achieve higher data rates in wireless communication. OAM beams have been thoroughly studied and used in the optical regime but in the mm-wave and THz-wave region, they are still under investigation. In these frequency bands, there are difficulties associated with beam-splitting and beam-combining processes as well as with the use of spiral phase plates and other methods for OAM generation, since the wavelength is much larger compared to those at optical frequencies, leading to higher diffraction losses. The present paper describes the natural generation of high-power OAM modes by gyro-type vacuum electron devices with cylindrical interaction circuit and axial output of the generated rotating higher-order transverse electric mode TEm,n, where m &gt; 1 and n are the azimuthal and radial mode index, respectively. The ratio between the total angular momentum (TAM) JN and total energy WN of N photons is given by m/ω, where ω is the angular frequency of the operating mode, which in a gyrotron oscillator is close to the TEm,n-mode cutoff frequency in the cavity. Therefore, m/ω = Rc/c, where Rc is the caustic radius and c the velocity of light in vacuum. This means that the OAM is proportional to the caustic radius and at a given frequency the same for all modes with the same azimuthal index m. Right-hand rotation (co-rotation with the electrons) corresponds to a positive value of m and left-hand rotation to negative m. The corresponding OAM mode number (topological charge) is l = m – 1. Circularly polarized TE1n modes only possess a Spin Angular Momentum (SAM: s = ±1). TE0n modes have neither SAM nor OAM. This is the result of the photonic (quasi-optical) approach to derive the TAM of modes generated in gyrotrons. The same result follows from the electromagnetic (EM) wave approach for the TAM within a given waveguide volume per total energy of the EM wave in the same volume. Such high-power output beams with very pure higher-order OAM, generated by gyrotron oscillators or amplifiers (broadband) could be used for multiplexing in long-range wireless communications. The corresponding mode and helical wavefront sensitive detectors for selective OAM-mode sorting are available and described in the present paper.

Structured light carrying orbital angular momentum (OAM) opens up a new physical dimension for studying light–matter interactions. Despite this, the complex fields created by OAM beams still remain largely unexplored in terms of their effects on surface plasmons. This paper presents a revelation of anomalous plasmon excitations in single particles and plasmon couplings of neighboring nanorods under OAM beams, which are forbidden using non-OAM sources. The plasmon excitation of single nanoparticles is determined both by photon spin angular momentum (SAM) and OAM and influenced by the locations of the nanoparticles. Specifically, when SAM and OAM are equal in magnitude and opposite in direction, a pure plasmon excitation along light propagation direction is achieved. Two plasmon dipoles show end-to-end antibonding coupling and side-by-side bounding coupling, which are the opposite of the typical couplings. Furthermore, we observe Fano resonance with a nanorod dimer: one aligned along light propagation direction acting as the bright mode and the other aligned along the global polarization direction of light acting as the dark mode, which is the opposite of the usual plasmonic Fano resonance. By taking advantage of the unique property of the OAM source, this investigation presents a novel way to control and study surface plasmons, and the research of plasmon behavior with OAM would open new avenues for controlling electromagnetic waves and enriching the spectroscopies with more degrees of freedom.