Vortex Smith–Purcell radiation generation with holographic grating

A 3D-printed sub-terahertz lens for manipulating quasi-non-diffractive orbital angular momentum (OAM) waves is proposed in this paper. The proposed lens comprises all-dielectric elements that impose desired phase profiles on the incident waves by varying their height. Based on the concept of the Bessel beam launchers and the beam deflectors, quasi-non-diffractive OAM waves with a pre-defined deflection angle can be produced by the proposed lens. For verification, two lenses were 3D printed for producing OAM waves with a non-diffractive depth of 55.58λ <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</inf> (λ <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</inf> is the free-space wavelength at 140 GHz). Measured by a terahertz imaging camera, the desired quasi-non-diffractive OAM waves were observed behind the proposed lenses with expected non-diffractive performance and a large deflection angle of up to 47°.

The orbital angular momentum (OAM) wave has shown great potential for improving radar imaging and underwater communication performance due to its helical wavefront phase and infinite orthogonal modes. However, there are currently no known applications of this technology in underwater imaging. In this paper, we employed acoustic OAM wave for underwater imaging and established transceiver signal models using the uniform circular array. We concurrently achieved two-dimensional imaging of azimuth and elevation angles, which differs from radar imaging. We proposed a matching process for the echo signal in the modal domain, the OAM wave beam image's sidelobe decreased by 7.9 dB in the elevation direction and 6.1 dB in the azimuth direction compared to the plane wave, with the mainlobe decreased by 0.2° in the elevation direction and 0.4° in the azimuth direction. Furthermore, this paper introduced OAM wave high-resolution image reconstruction based on the orthogonal matching pursuit (OMP) algorithm. Finally, we implemented broadband acoustic OAM wave for underwater imaging and introduced an image reconstruction method based on the modal domain OMP algorithm. Simulation results demonstrate that the use of OAM wave in underwater imaging is feasible, and the proposed scheme can achieve high-resolution imaging.

SummaryUtilization of planar printed uniform circular antenna arrays (UCA) to generate the orbital angular momentum (OAM) carrying waves in the millimeter‐wave (mmWave) frequency band is advantageous from the viewpoints of easy signal modulation and mode reconfiguration, low cost, low profile, and straightforward integration with the existing broadband wireless infrastructure. The OAM mmWave UCA are highly promising as the enablers of very high transmission data rates required by hybrid 5G/6G optical and wireless communication systems by complementing and enhancing other technologies currently in use. Therefore, we here contribute a detailed electromagnetic analysis of important constraints of such antenna arrangements aimed at short‐range multimode OAM wave transmission. We investigate (i) the required antenna array dimensions and optimized UCA arrangements for a particular link range and (ii) the corresponding mode structure of OAM waves in the plane of receiving arrays. Four relatively simple antenna configurations operating in the 60‐GHz band are compared. Theoretical assumptions based on ideal OAM modes are critically assessed and, using state‐of‐the‐art numerical electromagnetic analysis, compared to realistically generated OAM waves. The proposed “cyclic transmission setup” resulted in much lower unwanted field components in the region of receiving arrays. RMS magnitudes of unwanted modes are on average about 64% of the received mode, in comparison with 80% (up to 94%) for sequential transmission. The observed mode impurities and mode mixing effects at the receiver indicate the need to dedicate more attention to the system‐level design, the development of efficient receiving arrays, the MIMO processing, and the stream separation.

Orbital Angular Momentum (OAM) has been considered as a new dimension in wireless transmission. In Multiple Input Multiple Output (MIMO) system, OAM can also be utilized as the new Degrees of Freedom (DoF) to manipulate the beamforming instead of the traditional Channel State Information (CSI) feedback, so that the computation complexity of the beamforming dramatically decreases, as well as the capacity of the transmission is maintained approaching the upper bound of the MIMO system. For example, this beamforming manipulation with OAM is extremely efficient in the Line of Sight (LoS) MIMO system. In this paper, the mathematical model of the DoF in beamforming manipulation in MIMO with OAM is established. The computation complexity is evaluated, and compared in LoS environment within MIMO, MIMO-OAM, and OAM. The quantitatively simulation confirms the fact that MIMO-OAM has the best trade-off between the capacity and the computation complexity, which gives a consolidated evidence that the MIMOOAM scheme can be adopted as an efficient beamforming scheme in LoS backhaul links between the base station and the self backhaul small cell station, or Near Field Communications (NFC) directly between mobile terminals. Due to these promising applications, MIMO-OAM may become one of the candidate technologies in Beyond 5G (B5G) mobile communications.

EM waves with finite orbital angular momentum (OAM) have shown potential for a dramatic increment in the RF channel capacities. Generation of OAM waves using microstrip antennas makes the most economical configuration and thus can further boost the research toward realization of RF OAM channels. So far, all such microstrip configurations that generate OAM waves require more than one feed, especially for modes higher than the fundamental. This article gives a new and simple OAM antenna design method for radiating OAM waves of desired topological charge using a direct single point feed connected to a circular microstrip antenna (CMA) for corresponding mode. The presented method manipulates cylindrical harmonics using tiny modifications in the CMA. Subsequently, a generalized equivalent circuit model has been given for the same. Additionally, the presented method has also been qualitatively extended to annular ring microstrip antennas (ARMAs). Simulated and measured results have been presented to corroborate the proposed concept.

In order to extend the applications of vortex waves, we propose a water-immersed rectangular horn antenna array for generating underwater vortex waves carrying the orbital angular momentum (OAM). Firstly, a single dielectric-loaded rectangular horn antenna with the central frequency of 2.6 GHz was designed for generating underwater electromagnetic (EM) waves. Due to the supplementing dielectric-loaded waveguide in this single antenna, the problems with difficult sealing and fixation of the feed probe could be solved effectively. The simulation results show that it has a good impedance characteristics (S11 &lt; −10 dB) and reasonable losses (less than 3.5 dB total for two antennas and a coaxial line) from 2.5 GHz to 2.7 GHz. Experiments on the single antenna were also carried out, which agree well with the simulations. Based on the designed single antenna, the water-immersed rectangular horn antenna array was proposed, and the phase gradient from 0~2π was fed to the horn antennas for generating underwater OAM waves. The simulation results demonstrate high fidelity of the generated OAM waves from the intensity and phase distributions. The purity of the generated OAM modes was also investigated and further verifies the high fidelity of the generated OAM waves. The generated high-quality OAM waves meet the requirements for underwater applications of OAM, such as underwater communication and underwater imaging.

A pivotal issue of the conventional optical fiber communications networks is to meet the explosively increasing requirement in data traffic. In order to meet this ever-increasing demand, there have been a lot of research and industrial development efforts to utilize the photon in various dimensions such as wavelength division multiplexing (WDM), space division multiplexing (SDM), mode division multiplexing (MDM) and so on. Fueled by emerging bandwidth-hungry applications, orbital angular momentum (OAM) modes and their multiplexing have recently gained much attention due to its special doughnut-shaped intensity distribution, as well as its unique helical phase wavefront with the theoretically infinite topological value. The OAM modes with different topological charge values are orthogonal to each other, which has provided a new degree of freedom in MDM. In this paper, we propose and design a Ge-doped air-core ring fiber, which can support numerous OAM modes. By varying the mole fraction of GeO2 and adjusting the structure parameter, including the air-core radius and the GeO2-doped ring width, we study the influence of the different fiber parameters on the total supported OAM mode number. The hollow silica fiber with a 50-μm air core and a 1.5-μm thickness of Gedoped ring is designed in simulation to support fiber eigenmodes up to HE112,1 and EH107,1. This provides 436 OAM modes at 1550 nm while maintaining radially single mode condition. Moreover, it can support more than 400 OAM modes from 1260 nm to 1625 nm, covering O, E, S, C, and L bands.

The applications of orbital angular momentum (OAM) carried beam are limited by its on-axis phase singularity and wide-angle directivity. In this paper, a new form of OAM wave termed as plane spiral OAM wave is proposed. Different from general OAM wave propagating along the axial direction, plane spiral OAM wave has the distinguishing characteristic of transverse propagation. Hence it has no singularity along the propagation path and divergence problem no longer exists. A traveling-wave circular slot antenna excited by a 90° hybrid coupler is used to generate plane spiral OAM waves. Due to the ring horn outside the antenna, energy is focused on the radial plane and the generated waves are propagating in the transverse direction. The measured results of a fabricated prototype with OAM states l=±3 for 10 GHz operation validate its good performance and characteristic of plane spiral. The potential applications of plane spiral OAM waves, e.g., plane spiral OAM based multiple-in-multiple-out (MIMO) system and plane spiral OAM based beam-forming method, are also detailedly discussed.

The generation of rapidly tunable optical vortex (OV) beams is one of the most demanding research areas of the present era as they possess orbital angular momentum (OAM) with additional degrees of freedom that can be exploited to enhance signal‐carrying capacity by using mode division multiplexing and information encoding in optical communication. Particularly, rapidly tunable OAM devices at a fixed wavelength in the telecom band stir extensive interest among researchers for both classical and quantum applications. This article demonstrates the realistic design of a Si‐integrated photonic device for rapidly tunable OAM wave generation at a 1550‐nm wavelength by using an ultra‐low‐loss phase change material (PCM) embedded with a Si‐ring resonator with angular gratings. Different OAM modes are achieved by tuning the effective refractive index using rapid electrical switching of Sb2Se3 film from amorphous to crystalline states and vice versa. The generation of OAM waves relies on a traveling wave modulation of the refractive index of the micro‐ring, which breaks the degeneracy of oppositely oriented whispering gallery modes. The proposed device is capable of producing rapidly tunable OV beams, carrying different OAM modes by using electrically controllable switching of ultra‐low‐loss PCM Sb2Se3.

Optical vortices are a new kind of laser beam and receiving more and more attention currently.The complex amplitudes of optical vortices comprise a helical term exp (il),with l being the topological charge and the azimuthal angle.Each photon in optical vortices carries the orbital angular momentum (OAM) with a value of lħ,where ħ is the Planck's constant divided by 2.The topological charge l is the eigenvalue of optical vortices,and determines the helical wavefront distribution,thus also known as OAM state.Moreover,such an OAM state can be an infinite integer state. And vortices with various OAM states are orthogonal to each other,making it possible to be employed in high capacity data-transmission system.In addition,the above unique features contribute to their widely applications in lots of areas such as optical tweezers and spanners,rotation detection,quantum entanglement,etc.In these applications,detecting OAM states is basic,and greatly significant.Recently researchers have developed lots of approaches to detecting the OAM states,including the methods of interference,diffraction gratings,metasurface,etc.Of such approaches,the scheme of diffraction gratings is the simplest and most widely used,where one or more diffraction gratings are employed. When optical vortices propagate through such gratings,the OAM states are acquired immediately through capturing and analyzing the distinct OAM-related diffraction patterns.In this review,we focus on the techniques of detecting OAM states through diffraction gratings,which have been demonstrated by our group and other researchers.Some of the main detection gratings,including double-slit,triangle aperture and slit,angular slit,cylindrical lens,graduallychanging-period grating,annular grating,are introduced.In addition,schemes like composite fork grating,Dammann vortex grating and integrated Dammann grating,are presented to detect the OAM state for coaxial multiplexed vortices. Besides diagnosing OAM state,measuring the intensity proportion of each OAM channel,known as OAM spectrum, in multiplexed vortices is also necessary in some cases.Therefore we also introduce the ways to measure the OAM spectrum,e.g.the OAM mode sorter,the gray-scale algorithm.

Orbital angular momentum (OAM) communication is considered as a potential key technology for the sixth generation (6G) mobile communication system. To fully understand and characterize the propagation properties of OAM waves, such as reflection and transmission, in the indoor multipath propagation and the interaction with building materials, OAM channel modeling and target detection can be achieved. However, the special spiral phase wavefront structure of OAM creates divergence, which makes the theoretical study of OAM wave propagation characteristics and channel modeling face great challenges. In this paper, a measurement campaign is carried out in the 38 GHz band to measure the reflection of OAM waves on various building materials, such as glass, wood panels, and PVC, and to analyze the results of the received power angular spectra and propagation coefficients. The reflection curves of OAM and plane waves are compared, and the mean square error (MAE) between the theoretical Fresnel propagation coefficients and the measured values are calculated. The results show that with the introduction of OAM wave modes, it is difficult for the classical propagation model to accurately describe the propagation characteristics of OAM waves. Based on this, a Bessel modified propagation model is proposed in this paper, which introduces the fluctuation component in the OAM primary reflection path, accurately expresses the fluctuation of OAM waves in real situations, and calculates the error between the modified OAM propagation model and the measured values, which is better than the classical model, and lays a foundation for the further modification of the OAM propagation model by utilizing the actual data of OAM propagation. The results show that it is better than the classical model.

The structured beams, which can manipulate arbitrary intensity and phase of electromagnetic rays, are significant in communication and sensing. The vortex electromagnetic wave carrying orbital angular momentum (OAM) exhibits the phase distribution of the helical wavefront, which can be viewed as a complete set of eigenmodes for electromagnetic waves. It provides a flexible beamforming method to achieve the azimuthal pattern diversity by the spatial superposition of multiple specific OAM modes. We have proposed a novel form of OAM wave with a two-dimensional structure called plane spiral OAM (PS-OAM), which is conducive to overlapping different modes. Based on this, the PS-OAM mode-group can be employed to modify the spatial beam structure and form structured electromagnetic beams. This kind of beam not only retains the inherent vorticity and orthogonality of the OAM waves but also tackles the difficulties in applications stemming from the singularity-caused dark zone and beam divergence of conventional OAM waves. In this paper, the construction and manipulation of the structured beams are investigated, and the properties are analyzed. The potential applications in multiple-input multiple-output (MIMO) communication, orthogonal multiplexing communication, and spatial field digital modulation (SFDM) communication are envisioned.

The vortex electromagnetic (EM) wave carrying orbital angular momentum (OAM) offers a new degree of freedom for radar techniques, although to date its application to synthetic aperture radar (SAR) imaging has not yet been widely reported. In this letter, a SAR imaging model based on vortex EM waves is derived. Ku-band vehicle-borne vortex EM waves SAR experiment using phased uniform circular array antennas is reported for the first time. A detailed comparison with conventional plane EM waves SAR imaging is given in terms of signal-to-noise ratio and azimuth resolution. The experimental results demonstrate the effectiveness of the proposed imaging model, and indicate that vortex EM waves can be exploited in SAR imaging with higher azimuth resolution than plane EM waves. OAM, which is completely independent of frequency and polarization, is a new degree of freedom in SAR imaging. This work shows the new applications of vortex EM waves, which paves the way for the development of novel SAR systems.

This paper presents orbital angular momentum (OAM) multiplexing transmission with Butler matrices implemented on a sub-terahertz (sub- THz) band for the first ever to our best knowledge. Sub- THz bands enable us to utilize a wide frequency bandwidth for high-capacity wireless transmission. However, the baud rate becomes extremely high as the bandwidth widens, so digital spatial equalization becomes a significant hurdle for implementation. We therefore consider it practical to move part of the function over to analog devices. A Butler matrix is an analog device that can perform discrete Fourier transform (DFT) calculations, and a combination of the Butler matrix and uniform circular array (UCA) can be used to perform the generation and separation of OAM waves. In this paper, we present 4×4 Butler matrices for the generation and separation of four OAM waves (OAM modes 0, ±1, 2) and demonstrate a physical layer data rate of more than 100 Gbit/s for OAM multiplexing transmission over a 10-GHz bandwidth on a sub-THz band.

In recent years, orbital angular momentum (OAM) waves have been applied to improve the performance of radar target detection and imaging. However, the phase singularity of conventional OAM beams and the doughnut-shaped intensity distribution will degrade the performance of target detection. To solve the divergence problem in OAM multiplexing systems, the generation method of OAM beams propagating along a transverse plane is proposed using a uniform circular array, which is validated by full-wave simulation. Compared with conventional OAM beams, simulation results indicate that the generated beams with different OAM modes all propagate along the transverse plane while maintaining the regular wavefront shapes. This work can provide the development of new radar paradigm with technical support.