Broadband Orbital Angular Momentum Research Articles

Analysis and Design of Broadband OAM Array Antenna

This paper presents a Uniform Circular Array (UCA) antenna of crossed-dipole that can excite vortex waves in a wide frequency range from 3.5 GHz to 8.4 GHz. In the design process, the theoretical derivation of the influence on the Orbital Angular Momentum (OAM) when the antenna elements are arrayed in Co-directional Antenna Array (CAA) and Rotational Antenna Array (RAA) is given respectively. The fixed mode of the dipoles CAA is achieved by feeding each element with equal amplitude and 90∘ phase difference produced by the broadband feeding network. Furthermore, the proposed broadband OAM array antenna has been fabricated and measured to verify the predicted properties. The vortex electromagnetic wave with +1 mode could be excited in the bandwidth of 82.35%. Simulated and measured results are in good agreement. The proposed OAM array antenna is simple in design principle, compact in structure and low in profile, making this array antenna an excellent candidate for broadband OAM communication systems.

All-fiber function devices for twisted lights.

Lights carrying orbital angular momentum (OAM), also called twisted lights, have been applied in fields of optical manipulation, imaging, quantum communication, and mode-division-multiplexing (MDM) optical communication systems. Traditional approaches for manipulating twisted lights carrying OAM in free space paths such as Q-plates, spiral phase plates (SPPs), and spatial light modulators (SLMs) that are usually affected by diffraction effect and imperfect alignment between different optical components, limiting the practical applications of twisted lights. Here we design, fabricated, and package all-fiber function devices for twisted light carrying OAM such as all-fiber broadband OAM generator, all-fiber OAM (de)multiplexer, all-fiber OAM & WDM coupler, and all-fiber OAM 1 × 2 coupler. Base on coupled mode theory and phase-matching condition, twisted light can be generated and detected by pre-tapered single mode fiber (SMF) fusing with multi-mode fiber (MMF). The results show that the proposed all-fiber function devices for twist light have large working broadband (at least C band), high purity (above 95%), and low insert loss (less than 3 dB). The proposed devices will open a reliable way for twisted light applied in optical fiber communications and optical interconnections.

Broadband orbital angular momentum beam generation based on polarization-insensitive reflect array

Electromagnetic waves carrying orbital angular momentum (OAM) can be applied in various regions. Much research endeavor has been devoted to developing broadband and polarization-insensitive OAM beams that are preferred in many intriguing applications. This work reports a broadband polarization-insensitive millimeter-wave OAM generator based on high-refractive-index brick-shaped dielectric resonators and studies the relationship between the permittivity of the bricks and OAM beam bandwidth. Based on proper design of the resonators, the broadband, polarization-insensitive, and full phase control can be realized in a broad millimeter-wave band. It is also found that the resonator with higher permittivity leads to a wider OAM beam bandwidth. This structure and conclusion can find a variety of applications in imaging, lithography, and information processing.

Dual Concentric-Ring-Core Fiber With Four Zero-Dispersion Wavelengths for Beyond Three-Octave OAM Supercontinuum Generation

Orbital angular momentum (OAM) beams, featuring with unique spatial field distributions, have been extensively investigated and applied into a wide range of fields. In most applications, OAM beams covering a certain frequency band are usually required, and supercontinuum (SC) generation is a feasible method to provide broadband OAM source. As a short pulse is incident into a specially designed optical fiber with flat dispersion over broad bandwidth, a broadband SC could be generated along propagation due to the nonlinear spectral broadening. In this work, we propose a dual concentric-ring-core fiber design to achieve near-zero and flat dispersion profile over 3007-nm wavelength range with four zero-dispersion wavelengths for the OAM mode. The precise position of the zero-dispersion wavelengths can be tailored by varying the structural variables and the germanium-doped concentration of the silica-based fiber. The performance of the spectral broadening in terms of flatness and bandwidth is investigated under different input pulse and propagation conditions. Simulation results depict that the OAM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3,1</sub> mode supercontinuum spectrum can achieve beyond three-octave spanning from 445 to 3942 nm at −40 dB level by pumping a 50-fs 600-kW Gaussian pulse at the central wavelength of 1900 nm into a 2-mm designed fiber. By further optimizing the proposed fiber structure for the OAM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1,1</sub> mode with <60 ps/(nm·km) dispersion variation over a 3210-nm bandwidth, the generated supercontinuum can cover nearly three octaves.

Ultra-Wideband OAM Mode Generator Based on a Helical Grating Written in a Graded-Index Few-Mode Fiber

An ultra-wideband orbital-angular-momentum (OAM) mode generator has been proposed and demonstrated both theoretically and experimentally, which is based on a helical long period fiber grating (HLPG) but written in a graded-index few-mode fiber (GIFMF). Unlike all the HLPG-based broadband OAM generators reported to date, where the utilized HLPG (fabricated only in step-index fibers) all were operated at wavelengths very close to the dispersion-turning point (DTP), the GIFMF-based HLPG proposed in this study is, however, operated at wavelengths far from its DTP, which provides a new and robust method enabling to produce a ultra-wideband OAM mode generator with an ultra-high conversion-efficiency and yet with a great flexibility in controlling the central wavelength of itself. As a typical example, the first-order OAM mode generator with a central wavelength of ∼1510 nm, a bandwidth of ∼235 nm, and a conversion-efficiency larger than 99% (∼20.7 dB) has been successfully obtained. To the best of our knowledge, this is the first report for practical demonstration of the GIFMF-based HLPG.

Wave-band-tunable optical fiber broadband orbital angular momentum mode converter based on dispersion turning point tuning technique.

A wave-band-tunable optical fiber broadband orbital angular momentum (OAM) mode converter based on a helical long-period fiber grating (HLPFG) and dispersion turning point (DTP) tuning technique is demonstrated both theoretically and experimentally. The DTP tuning is achieved by thinning the optical fiber during the HLPFG inscription. As a proof of concept, the DTP wavelength of the LP1,5 mode is successfully tuned from the original ∼2.4 µm to ∼2.0 µm and ∼1.7 µm. With the help of the HLPFG, broadband OAM mode conversion (LP0,1→LP1,5) is demonstrated near the 2.0 µm and 1.7 µm wave bands. This work addresses a longstanding problem that the broadband mode conversion is limited by the intrinsic DTP wavelength of the modes and provides a new, to the best of our knowledge, alternative for broadband OAM mode conversion at the desired wave bands.

A reflective metasurface for broadband OAM vortex wave generation

In this paper, we proposed a reflective metasurface which controls the phase delay of the electromagnetic (EM) wave through geometric phase manipulation. By coding the metasurface unit cells in different orientation angles, an orbital angular momentum (OAM) wave can be obtained through the metasurface reflection. A broadband reflective OAM wave with mode l = −1 was demonstrated which was experimentally observed from 8.3 GHz to 11.6 GHz. The mode spectrum of OAM wave was analyzed and the spectrum weight was about 0.82 through numerical calculation and about 0.47 through experimental measurement for the mode l = −1. The realization of this broadband OAM wave has the potential application to improve the OAM wave based communication capacity.

Generation of orbital angular momentum light based on mechanically induced long-period fiber gratings using 3D printing technique

• An OAM beam generator was proposed with broadband and tunable operation wavelength. • OAM generation was achieved by employing 3D-printed mechanically induced few-mode fiber gratings. • By adjusting applied pressures and inclination angle of few-mode fibers, the operation wavelength can be tuned. • The operational bandwidth can be broadened by employing cascaded few-mode fiber gratings. We propose and experimentally demonstrate an orbital angular momentum (OAM) beam generation method based on mechanically induced few-mode fiber gratings fabricated using 3D printing technique. Experimental results indicate that the transmission depth at resonance wavelength of few-mode fiber gratings with axially uniform refractive index could be adjusted when different lateral pressures are applied onto the few-mode fibers, and hence mode coupling efficiency between the fundamental core mode and LP 11 mode could be controlled in this way. In addition, by changing the inclination angle of the few-mode fibers with respect to direction perpendicular to the mask grooves, the resonance wavelength of the few-mode grating could be changed as well. Besides uniform fiber gratings, we have also investigated the spectral broadening property of the few-mode cascaded fiber grating, and a 10-dB spectral bandwidth up to 114 nm is obtained experimentally. Our proposed OAM light generation method based on mechanically induced fiber gratings using 3D printing technique possesses several desirable merits such as low cost, ease of operation, flexibility in grating structure and no damage to fiber medium, which is anticipated to provide low lost and flexible solution to generation of broadband OAM light for future OAM-based communication systems.

Suspended Metasurface for Broadband High-Efficiency Vortex Beam Generation

Electromagnetic (EM) waves carrying orbital angular momentum (OAM) exhibit phase vortex and amplitude singularity. Broadband OAM generation with high efficiency is highly desired with suggested applications such as broadband imaging and communications. In this paper, suspended metasurface structure achieving low-Q factor is proposed to realize broadband phase control and excellent reflection efficiency. Broadband vortex beam generation with OAM order of 1 and 2 are realized using the proposed suspended structure. Furthermore, by analyzing different metasurface aperture phase distribution schemes, the efficiency of the OAM generator is maximally achieved. The designs are validated by simulation and measurement. The proposed OAM generators work across 4–10 GHz with efficiency higher than 82%. This design provides a route to broadband metasurface realization and high efficiency OAM generation.

Beamforming Performance Analysis of Millimeter-Wave 5G Wireless Networks

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.

A High-Efficiency and Broadband Folded Reflectarray Based on an Anisotropic Metasurface for Generating Orbital Angular Momentum Vortex Beams

In this paper, a low-profile folded reflectarray (FRA) assembled by an anisotropic metasurface is proposed to generate high-efficiency and broadband orbital angular momentum (OAM) vortex waves in the Ka band. In order to suppress the mutual coupling and increase the polarization conversion efficiency, a dual-polarization phase matching method (DPPMM) for helical wavefront construction is developed to achieve minimum phase compensation error in the orthogonal direction. A compact subwavelength anisotropic meta-atom with orthogonal || |-shaped structure is designed to achieve flexible wavefront manipulation and high-resolution phase compensation for x and y-polarization incident waves. After the optimization design, nearly 100% cross-polarization conversion efficiency for the anisotropic metasurface can be achieved at the working frequency of 12 GHz. To validate the proposed design method, an OAM FRA with 24×24 meta-atoms is fabricated and measured. The measurement results show that the aperture efficiency of the proposed FRA carried with the OAM mode l = 1 is significantly improved and reaches 20.2%. Moreover, the OAM bandwidth and 3-dB gain bandwidth reach 22.5% and 25%, respectively.

Wideband and high efficiency orbital angular momentum generator based on bi-layer metasurface

A broadband and high-efficieny bi-layer metasurface is proposed in this paper. The unit cell of the metasurface is formed by symmetrically etching two cross-type metal patches on both sides of a dielectric plate. Furthermore, the two metal patches have a displacement of half a period along the &lt;i&gt;y&lt;/i&gt;-axis. By employing the displacement, the transmission bandwidth of the bi-layer metasurface is significantly expanded. In order to obtain a physical insight into bandwidth broadening, a π-type equivalent circuit that presents the electromagnetic coupling between within the bi-layer metasurfaces is successfully extracted to investigate the influence of electromagnetic coupling on transmission performance. The results show that by shifting the metal patches along the &lt;i&gt;y&lt;/i&gt;-axis by half a period, the coupling impedance (&lt;i&gt;Z&lt;/i&gt;&lt;sub&gt;12&lt;/sub&gt; or &lt;i&gt;Z&lt;/i&gt;&lt;sub&gt;21&lt;/sub&gt;) of bi-layer metasurface can be significantly modified, which further changes the electromagnetic coupling of the bi-layer metasurface. Correspondingly, the impedances &lt;i&gt;Z&lt;/i&gt;&lt;sub&gt;p&lt;/sub&gt; and &lt;i&gt;Z&lt;/i&gt;&lt;sub&gt;s&lt;/sub&gt; in the π-type circuit are changed to approximately meet the resonant condition of circuit in broadband, resulting in the bandwidth expansion of the proposed device. By using Pancharatnam-Berry phase theory, we redesign the proposed metasurface unit cell into a broadband orbital angular momentum generator. The simulation and measurement results verify that the bi-layer metasurface can convert a left-hand circularly polarized wave into a right-hand circularly polarized wave carrying orbital angular momentum in a frequency range between 11 GHz and 12.8 GHz, demonstrating the performance of device.

Broadband High-Efficiency Electromagnetic Orbital Angular Momentum Beam Generation Based on a Dielectric Metasurface

Electromagnetic waves carrying orbital angular momentum (OAM) have motivated a wealth of applications. Achromatic or broadband OAM beams are preferred in many applications and have been extensively explored in previous research endeavors. In view of the low efficiency of the previous reported broadband OAM generators, this work proposes a metasurface composed of high-refractive-index dielectric resonators to achieve broadband OAM beam generation with high efficiency. An efficient method for designing the resonators is presented, which indicates that proper adjustment of the dimensions and rotation of the dielectric resonators can render broadband and high-efficiency full phase control. A broadband high-efficiency OAM generator is engineered based on the resonators from 18 to 28 GHz as a proof-of-concept example and validated by both simulation and experiments. Above 70% polarization conversion efficiency and above 65% operation efficiency are measured throughout the entire design band. Such structure provides an alternative mechanism for realizing broadband high-efficiency microwave, millimeter-wave, terahertz or optical OAM generation and can find potential applications in high-capacity communications and imaging.

Ultra-broadband conversion of OAM mode near the dispersion turning point in helical fiber gratings

Based on the dual-resonance principle around the dispersion turning point (DTP), for the first time, an ultra-broadband orbital angular momentum (OAM) mode converter formed by the helical long-period fiber grating (HLPG) is proposed. The converter used for delivering the OAM operation with 3-dB bandwidth of 287 nm, which is about 7 times of general OAM converters and has only one mode. Furthermore, by chirping the HLPG working around DTP, a flat-top broadband OAM mode converter with bandwidth of ∼182 nm@3 dB is conveniently achieved. The flatness of spectrum can be increased by apodizing and optimizing the length of the chirped HLPG. Subsequently, we significantly developed a flat-top broadband rejection filter with &gt;30 dB bandwidth of a high level of ∼100 nm@1 dB by double-cascading the HLPG. It is shown that the performances of the OAM mode converter and the flat-top broadband rejection filter can be remarkably improved by accomplishing the DTP in the mode phase-matching for the HLPG.

Broadband vortex beam generating for multi‐polarisations based on a single‐layer quasi‐spiral metasurface

A reflective metasurface based on the quasi-spiral element is proposed to achieve broadband orbital angular momentum vortex beams for multi-polarisations. The quasi-spiral element can provide linear and smooth phase responses covering about 360° from 10 to 15 GHz with a relative bandwidth about 40% by adjusting the length of four phase delay lines. Additionally, the metasurface can generate vortex beams with optional charges in vertical, horizontal and circular polarisations which are determined by the polarisation of feed antenna. Finally,we have designed and fabricated a 24 × 24 cm2 metasurface prototype. The simulated and measured results show that the reflective metasurface can generate broadband vortex beams with orbital angular momentum effectively. As the structure is simple, lightweight and easy to fabricate, the proposed method is applicable to generate broadband vortex beams for multi-polarisations.

Vortex chirality-dependent filtering in helically twisted single-ring photonic crystal fibers.

Helically twisted microstructured optical fibers have a wide application prospect in the field of optical vortex communications. In this paper, a helically twisted single-ring photonic crystal fiber (HS-PCF) is proposed for orbital angular momentum (OAM) vortex modes selective filtering. And the theoretical framework of OAMs filtering is also constructed. Positive and negative OAM vortexes have different transmission losses in HS-PCF, and the loss difference between them increases significantly after the twist rate reaches a certain value. Such fibers can filter out OAMs with a certain sign of the topological charge (depending on the handedness of the thread), while dissipating oppositely charged OAMs. In addition to the general OAMs, i.e., zero-order radial vortex modes, the helically twisted fiber also performs a good selective filtering for the first-order radial vortex modes. Remarkably, the filtering bandwidth of HS-PCF is very broad, covering four communication bands from O- to C-band. This kind of fiber can be used as a broadband OAMs filter.

High-efficiency, broadband all-dielectric transmission metasurface for optical vortex generation

Orbital angular momentum (OAM) generated by metamaterials has interesting properties in many applications. But in near infrared communication wave bands, broadband OAM generators have rarely been investigated. Here, we report an approach to design a high efficiency, broadband all dielectric transmission metasurface working in a telecom wave band from 1300 nm to 1700 nm. Simulated OAM generators with a topological charge of m = 1 and m = 20 present good mode purity up to 99.99% and high energy efficiency up to 97.38%. The proposed broadband all-dielectric OAM generator is meaningful to reduce dispersion-induced strong distortions and exploit new applications such as broadband object detection and multiplexed communication.

Metasurface-based broadband orbital angular momentum generator in millimeter wave region.

Orbital angular momentum (OAM) is an intrinsic property that all electromagnetic waves can carry. Interesting properties of OAM beams have enabled many novel applications. But broadband OAM generator has rarely been investigated, especially in the millimeter wave frequency band. In this work, a broadband OAM generator applying a metallic reflective metasurface operating from 59 to 70 GHz is designed, simulated, fabricated and measured. Both simulation and experiment results demonstrate that broadband millimeter wave OAM beams with good quality can be reliably launched by the designed metasurface. The proposed broadband OAM generator alleviates malfunctions caused by dispersion and provides new possibilities of multiplexing for millimeter wave communication applications.

A 2-D FFT-Based Transceiver Architecture for OAM-OFDM Systems With UCA Antennas

Radio orbital angular momentum (OAM) provides another perspective of spatial multiplexing to improve the spectrum efficiency. However, multipath induces severe intraand interchannel crosstalk. To solve the problem in a uniform circular array (UCA)-based OAM system, we first incorporate the effect of sign changing of OAM reflection in modeling the multipath OAM channel. Then, we propose a transceiver architecture for broadband OAM orthogonal frequency-division multiplexing (OFDM) wireless communication systems, which uses baseband digital 2-D fast Fourier transform (FFT) rather than existing radio frequency analog phase shifters to generate and receive the OAM-OFDM signal, thus reducing energy consumption and hardware cost. At last, a flexible 2-D FFT algorithm is developed. Analysis and simulation results show that compared with the traditional row-column FFT algorithm, the proposed 2-D FFT algorithm could reduce the multiplication complexity by 41MN log2 N, where N and Mare the number of UCA antenna elements and the number of subcarriers, respectively.

4.8‐Gbit/s broadband wireless communication link at radio frequency using orbital angular momentum and polarisation multiplexing

Recently, the ability of orbital angular momentum (OAM) of electromagnetic fields at radio frequency to multiplex multiple beams has been investigated. Such OAM multiplexing can be used to increase channel capacity and spectral efficiency. However, the studies of OAM multiplexing are restricted to narrow band. In this Letter, the authors experimentally demonstrate a 4.8-Gbit/s broadband OAM and polarisation multiplexing wireless link in which the operating frequency is from 2.1 to 2.7 GHz, with a relative bandwidth about 25%. The proposed wireless communication link over 2.5 m achieves a spectral efficiency of ∼8 bit/s/Hz by using two independent OAM beams for each of two polarisations. All four independent OAM channels propagate along the same beam axis with crosstalk less than −13.3 dB from 2.1 to 2.7 GHz. All four channel signals are recovered with a bit error rate of 3.5 × 10−3 when the signal-to-noise ratio is 24 dB.