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This paper introduces a 400 Gbps free space optics (FSO) system to meet the expected surge in the demand for data traffic. The proposed system is based on using 4-level pulse amplitude modulation (PAM-4) signals in addition to polarization division multiplexing and orbital angular momentum (OAM) multiplexing schemes. To reduce spectrum usage, only a single wavelength of 1550 nm is used. A 400 Gbps is transmitted over two polarization states and eight OAM beams. The signal on each polarization carries the data of four different OAM beams ( LG 0 0 , LG 15 0 , LG 40 0 , and LG 80 0 ). The performance of the proposed model is examined under the influence of light haze, medium haze, heavy haze, low fog, medium fog, heavy fog, wet Snowfall, dry snowfall, light dust storms, medium dust storms, and heavy dust storms. The maximum transmission range and the Bit Error Rate (BER) are used for evaluating the system performance. The simulation results reported the longest FSO link with ranges from 800 to 1475 m under haze conditions. A slight decrease in these ranges is achieved under fog conditions (ranges from 450 to 859 m). The FSO highest attenuation is experienced for HDS case, which results in the shortest range of 73 m. These ranges are calculated at BER Forward Error Correction limit of 3.8 × 10–3. Subsequently, the proposed system can be used for the next 6G applications due to its simplicity in design and high transmission capacity. [ABSTRACT FROM AUTHOR]

Holographic imaging as one of promising image techniques reconstructs object image using the amplitude and phase information scattered from objects. Programmable metasurface composed of digitally coding unit cells can arbitrarily manipulate amplitude and phase of the scattering wave by tailoring operation states of the unit cells, and thus achieves high‐resolution imaging reconstruction. However, the conventional metasurface can only achieves a desired holographic image on an imaging plane. This paper presented a time‐space coding 2‐bit metasurface to overcome this challenge. By rapidly switching PIN diodes loaded in metasurface unit cell according to the customized time sequences, the magnitude and phase of the ±1st order harmonic fields can be fully controlled, achieving the hologram imaging at the higher order harmonics. The proof‐of‐concept measurements are given to demonstrate the versatile holographic imaging functions at the ±1st order harmonics including two different holographic images on an imaging plane, holographic imaging and beam scanning, and holographic imaging and orbital angular momentum (OAM) wave generation, which undoubtedly enriches the applications of the programmable metasurfaces in the field of the holographic imaging. [ABSTRACT FROM AUTHOR]

In this paper, a high-speed transmission system that combines a multimode fiber (MMF) with Free Space Optics (FSO) is proposed to accommodate the rapid growth of traffic. As Orbital Angular Beams Multiplexing (OAM) plays a crucial role in 6G networks by enhancing transmission capacity, four OAM beams ( LG 0 , 0 , LG 0 , 20 , LG 0 , 40 , and LG 0 , 40 ) are utilized in this study. Additionally, the Optical Code Division Multiple Access (OCDMA) technique is employed, known for its high level of confidentiality and the ability to allow multiple channels to transmit data simultaneously. To ensure ubiquitous data transmission, two channels: MMF and FSO are used. Accordingly, in our proposed MMF/FSO system, we employ four OAM beams, with each carrying four OCDMA channels assigned distinct Permutation Vector (PV) codes. With a data transmission rate of 10 Gbps on each OCDMA channel, the overall capacity of the system amounts to 160 Gbps (10 Gbps × 4 OCDMA channels × 4 OAM beams). Moreover, the performance is investigated and evaluated using two scenarios. The first assumes an FSO range of 5 km and Clear Air (CA) weather conditions, with a variable MMF length. In contrast, the second scenario maintains a fixed MMF length of 1 km while altering the FSO span. Weather conditions like CA and rain conditions; Light Rain (LR), Medium Rain (MR), and Heavy Rain (HR) are considered while evaluating the system performance in the second case. The bit error rate (BER), travelling distances in the channel either MMF or FSO, and eye diagrams are among the metrics used for evaluation. They all provide information about the received signal performance. Finally, the obtained results for the proposed MMF/FSO system, utilizing OAM beams and PV codes, indicate that in the first scenario, the system can support a total capacity of 160 Gbps across all channels. The performance is noteworthy, with a BER below 10 - 5 and a wide eye opening, enabling successful transmission over 6.2 km (1.2 km MMF length + fixed 5 km FSO span). For the second scenario, the achievable distance is decreased due to the attenuation caused by rain. It becomes 2.62 km (1 km MMF + 1.62 km FSO span), 5.2 km (1 km MMF + 1.2 km FSO span), and 1.77 km (1 km MMF + 0.77 km FSO span), under LR, MR, and HR, respectively. [ABSTRACT FROM AUTHOR]

Future advances in information transmission technology require better capacities for fiber optics communication. As a new degree of freedom, the orbital angular momentum (OAM) has attracted extensive attention in the field of mode division multiplexing. Herein, an anti-resonance fiber (ARF) is designed with three different materials, SF11, LaSF09, and SiO2, for the negative curvature tube. Systematic analysis and optimization are carried out to determine the number of layers of the negative curvature tubes in the cladding, the number of second-layer cladding tubes, and thickness of the annular area. The finite element analysis software COMSOL is utilized to calculate the effective refractive index difference, dispersion, limiting loss, and other characteristics. The ARF can stably transmit 86 OAM modes in the wavelength range between 1.2 and 2.0 μ m, and the minimum refractive index difference is greater than 1 × 1 0 − 4 , which effectively reduces the mode abridgment. The nonlinear coefficients of the HE 2 3 , 1 mode and the OAM purity of HE 1 , 1 mode are 0.34 km − 1 ⋅ W − 1 and 99.43%, respectively, thus facilitating stable transmission of OAM modes for a long transmission distance. The anti-resonance fiber has significant application potential in long-distance network information transmission, relay communication, and other communication areas. [ABSTRACT FROM AUTHOR]

In this paper, a 3D conformal meta-lens designed for manipulating electromagnetic beams via height-to-phase control is proposed. The structure consists of a 40 × 20 array of tunable unit cells fabricated using 3D printing, enabling full 360° phase compensation. A novel automatic synthesizing method (ASM) with an integrated optimization process based on genetic algorithm (GA) is adopted here to create the meta-lens. Simulation using CST Microwave Studio and MATLAB reveals the antenna's beam deflection capability by adjusting phase compensations for each unit cell. Various beam scanning techniques are demonstrated, including single-beam, dual-beam generation, and orbital angular momentum (OAM) beam deflection at different angles of 0°, 10°, 15°, 25°, 30°, and 45°. A 3D-printed prototype of the dual-beam feature has been fabricated and measured for validation purposes, with good agreement between both simulation and measurement results, with small discrepancies due to 3D printing's low resolution and fabrication errors. This meta-lens shows promise for low-cost, high-gain beam deflection in mm-wave wireless communication systems, especially for sensing applications, with potential for wider 2D beam scanning and independent beam deflection enhancements. [ABSTRACT FROM AUTHOR]

Polarization detection has been widely used in various fields, especially in free-space optical communication associating with orbital angular momentum (OAM). However, the bulky optical devices and the complexity of the detection process have hindered the integration and miniaturization of communication systems. Herein, a novel polarization detection method is proposed to directly identify the polarization state of incident light based on the normalized spectra of the sorted OAM modes which are converted from a composite vortex beam (CVB) to a set of spatial distinct strip type beams (STBs). The polarization states of light beam can be apparently distinguished via the combination of different STBs without redundant detection elements and the system demonstrates great robustness under specific atmospheric turbulence as well. The introduction of such a polarization detection method in future communication links will obtain lower complexity and consumption, which is very favorable regarding OAM communications. [ABSTRACT FROM AUTHOR]

In this paper, an orbital angular momentum (OAM) mode reconfigurable antenna based on a uniform circular array (UCA) is presented. The phase superposition method is presented to overcome the limitation of the number of array units on the mode order in the conventional four‐element UCA antenna, which is limited to generating first‐order OAM beams. The unit phase is introduced by a bifilar helical antenna in this method, and the unit phase is superimposed with the feed phase of the UCA to enable the generation of 3rd‐order OAM beams from the four‐element UCA. At the same time, the DC signal‐controlled feed network assigns three distinct feed phases to the UCA, which realizes the reconfigurability of the three OAM beams. To achieve mode reconfigurability, 20 p‐i‐n diodes were integrated as switches in the feed network for controlling the ON/OFF status of p‐i‐n diodes in the feed network using DC signals. Finally, simulated and measured results show that three OAM beams (lz = −1, −2, and −3) have been generated in the frequency band from 10.6 to 11.8 GHz. [ABSTRACT FROM AUTHOR]

In this study, the generalized Lorenz–Mie theory (GLMT) is used to calculate the scattered electromagnetic field components of vortex Bessel beams (VBB) by a foggy atmosphere. The interaction of VBB with scattering particulates is very distinct due to its complex optical fields and orbital angular momentum (OAM). By employing the integral localized approximation (ILA), the beams shape coefficients (BSCs) for VBB are evaluated. To describe the VBB, the BSCs are considered as a crucial part in the GLMT. Numerical results on extinction efficiency, scattering efficiency, absorption efficiency, radiation pressure efficiency, albedo factor, and scattering asymmetry parameter are presented in relation to the OAM mode index. Also, results on generalized Gamma distribution as a function of particle diameter for diverse visibility factors are presented and discussed. Moreover, attenuation associated with advection fog and radiation fog for different OAM mode index versus visibility is explored using GLMT. The computations show that fog attenuation is highly influenced by the OAM mode number. The numerical results dictate distinct signatures regarding VBB that there exists higher transmission of optical VBB propagation in the specific foggy environments. Moreover, this research work has potential applications to study the scattering dynamics of the OAM in marine atmosphere, rainy atmosphere, and environmental optics. [ABSTRACT FROM AUTHOR]

In this paper, a novel phase offset (PO)-aided index modulation (IM) for orbital angular momentum (OAM) communication, called OAM-PO-IM, is proposed to enhance capacity. The proposed system conveys two types of index information including an active mode combination (AMC) and misalignment of PO states. This method leverages the PO state induced by the selective activation of antenna elements within a uniform circular array (UCA). Specifically, a UCA is divided into multiple uniform circular subarrays (UCSAs) in the transmitter, and additional index bits are allocated according to the activation state of the UCSAs. In addition, the modified Bessel function (MBF) of the first kind is used to derive the channel model to allow a straightforward determination of the optimal AMC for the OAM-IM system. To detect index and symbol bits, the receiver provides a maximum likelihood (ML) detector and a new low-complexity ML detector using the log-likelihood ratio (LLR). The effectiveness of the proposed OAM-PO-IM is verified through comprehensive simulations that benchmark its performance against the existing OAM and OAM-IM in terms of sum capacity, bit error rate (BER), and energy efficiency. Compared with the OAM-IM, the proposed OAM-PO-IM provides a capacity gain of additional $\log _{2}{G}$ bps/Hz, where G denotes the number of UCSA. Furthermore, this scheme improves the gain by approximately 1 dB at a BER of $10^{-4}$ for active mode $L_{A}=2$ at 6 bps/Hz.

This manuscript proposes the design of a 28 GHz antenna system that is capable of generating at the same time five orbital angular momentum (OAM) modes with aligned far-field antenna patterns. The antenna system includes a radial line slot antenna (RLSA), based on radial waveguide technology, which is fed by two Butler networks: one is $8 \times 8$ and the other is $16 \times 16$ . The Butler matrix networks are able to generate various progressive phase states for the RLSA to enable simultaneous generation of five concentric OAM beams with right hand circular polarization (RHCP). The proposed design could be utilized for increasing spectral efficiency by multiplexing a separate data stream onto each transmitted OAM beam and performing partial-aperture OAM reception in line-of-sight (LOS) communications in 6G. The experimental results performed on the antenna prototype reveal the effective performance of the proposed antenna system.

The aim of this paper is to design a novel compact and wideband feed for monopulse antenna using the Orbital Angular Momentum (OAM) modes of 0 and 1. The OAM mode 0 produce the sum pattern and the OAM mode 1 generate the difference pattern. Based on the correspondence between the OAM mode 0 and the circular polarized (CP) TE11 mode in circular waveguide, as well as between the OAM mode 1 and the circular polarized TE21 mode, the feed structure consists of two circular waveguides with different radius. In the waveguides with smaller and larger radius, the CP TE11 mode and TE21 mode are excited respectively, using the novel proposed mode coupler. Each mode coupler has two pairs of arms, one of which excites the TEi1 mode (i = 1,2) and the other the orthogonal ones (TE $^{\bot }$ i1, i = 1,2) with 90-phase difference inside the circular waveguide. The frequency bandwidth of the proposed feed is from 9.65-10.85 GHz (11.7%) and the isolation between TE11 and TE21 mode couplers is -30 dB. The frequency bandwidth of the proposed structure is very higher than the state of the art references. Moreover, it has low side lobe levels as well as compact structure with high power handling. These specifications prove the ability and capability of the proposed structure in radar applications.

The orbital angular momentum (OAM) states are used for OAM modulation and OAM multiplexing in free-space optical (FSO) communication systems to enhance the transmission quality and capacity. Although a few impactful methods had been developed to measure static OAM states, the fast and accurate recognition of dynamic OAM states in FSO communications remains a challenge due to the OAM crosstalk easily induced by the atmospheric turbulence. In this paper, we present a new simple statistical recognition method realized by utilizing the turbulence-induced crosstalk distribution, other than the routine way to use the spatial intensity patterns. The proposed method is based on acquiring the priori OAM crosstalk distribution of the training optical data, and then using similarity analysis to recognize the OAM states of the information optical data. Its availability and accuracy are validated through the computer simulations. The recognition accuracy under relatively strong turbulence conditions can be improved by using wavefront compensation and specific distance calculation.

This paper demonstrates the significance of employing the infinitesimal dipole modeling (IDM) in analyzing uniform circular array-orbital angular momentum (UCA-OAM) near-fields in terms of electromagnetic (EM) energy densities. The electric and magnetic field, and the EM energy densities of UCA-OAM waves are theoretically analyzed and numerically estimated where the spatial distribution of EM Lagrangian density and complex Helicity is visualized. It is validated that the IDM is more accurate than the conventional far-field assumption in evaluating the near-fields of UCA-OAM waves as the EM Lagrangian density and the complex Helicity are clearly depicted based on the IDM, while they are unobservable with the far-field assumption. The visualization of EM Lagrangian density and complex Helicity can be beneficial in designing UCA-OAM communication system as it suggests the mutual coupling between antennas by describing the regions of reactive EM energy densities and non-orthogonal EM fields in the near-field region.

Two patch antennas that can generate two or four orbital angular momentum (OAM) modes are designed and verified to work at frequency band from 2.2GHz to 2.7GHz. One antenna is composed of two single OAM mode monopole patch antenna systems with one common ground plane and it can generate two OAM modes: mode $l =1$ and mode $l = -1$ . This antenna is designed, simulated, fabricated and tested. Its reflection coefficients, radiation patterns, phase distributions and isolation between two different OAM mode channels are obtained. Phase distributions of the two OAM modes are measured to show the effectiveness of the antenna to generate two OAM modes. Channel isolation between these two OAM modes is 25.72dB at 2.598GHz and more than 15dB at the frequency band from 2.585GHz to 2.62GHz. The measurement results have shown that this two-OAM-mode antenna can produce two isolated channels at the same frequency band. Another monopole patch antenna has been designed and verified by simulation to be able to generate four OAM modes. This four-OAM-mode antenna provides an example that this kind of multiplexed monopole patch antennas can multiplex more OAM modes together.

In this paper we document for the first time some of the effects of self-healing, a property of orbital-angular-momentum (OAM) or vortex beams, as observed on a millimeter-wave experimental communications link in an outdoors line-of-sight (LOS) scenario. The OAM beams have a helical phase and polarization structure, and have conical amplitude shape in the far field. The Poynting vectors of the OAM beams also possess helical structures, orthogonal to the corresponding helical phase-fronts. Due to such non-planar structure in the direction orthogonal to the beam axis, OAM beams are a subset of “structured light” beams. Such structured beams are known to possess self-healing properties when partially obstructed along their propagation axis, especially in their near fields, resulting in partial reconstruction of their structures at larger distances along their beam axis. This has practical implications in siting antennas where obstructions due to people or vegetation may occur. Various theoretical rationales have been proposed to explain, model and experimentally verify the self-healing physical effects in structured optical beams, using various types of obstructions and experimental techniques. Based on these models, we hypothesize that any self-healing observed will be greater as the OAM order increases. Here we observe the self-healing effects for the first time in structured OAM radio beams, in terms of communication signals and channel parameters rather than beam structures. We capture the effects of partial near-field obstructions of OAM beams of different orders on the communications signals, and provide a physical rationale to substantiate that the self-healing effect was observed to increase with the order of OAM, agreeing with our hypothesis.

In this paper, we propose divergence intensity (DI) and mode combinations for multiple orbital angular momentum (OAM) users to enhance the sum capacity. Generally, OAM multiplexing has beam divergence (BD) limiting the achievable link distance and degrading performance. The effective OAM channel model implies DI which is considered as intensity of OAM mode. In addition, the near–far mode combination is recommended to be sub-optimal compared with different mode combinations which types are sequential and uniform. The sub-optimal mode combination outperforms the other two mode combinations because channel gain shares the DI effect. The performance of the proposed scheme is derived and simulated in terms of normalized DI, capacity per user, and sum capacity.

Vortex beams carrying orbital angular momentum (OAM) provide a new degree of freedom for light waves in addition to the traditional degrees of freedom, such as intensity, phase, frequency, time, and polarization. Due to the theoretically unlimited orthogonal states, the physical dimension of OAM is capable of addressing the problem of low information capacity. With the advancement of the OAM optical communication technology, OAM router devices (OAM-RDs) have played a key role in significantly improving the flexibility and practicability of communication systems. In this review, major breakthroughs in the OAM-RDs are summarized, and the latest technological standing is examined. Additionally, a detailed account of the recent works published on techniques related to the OAM-RDs has been categorized into five areas: channel multicasting, channel switching, channel filtering, channel hopping, and channel adding/extracting. Meanwhile, the principles, research methods, advantages, and disadvantages are discussed and summarized in depth while analyzing the future development trends and prospects of the OAM-RDs. [ABSTRACT FROM AUTHOR]

A geometric metalens is presented that can achieve a three‐dimensional focusing curve embedded with on‐trajectory tailored phase distribution. The proposed design strategy is highlighted by flexible modulations on both the focusing trajectory and the on‐trajectory gradient phase and by the independent designability between them. The coexisting intensity gradient force and phase gradient force are numerically investigated, which can jointly provide the versatile abilities to push particles along the curve in the direction of phase increasing. This design strategy offers more controllability degrees of freedom on a complex optical field and is promising to miniaturize the contactless system of multifunctional optical manipulation on tiny particles. [ABSTRACT FROM AUTHOR]

In this article, a novel compact dual‐band uniform circular array (UCA) is proposed to generate the conical‐shaped orbital angular momentum (OAM) beam with high azimuthal symmetry for vehicular communication. Most antenna designs proposed in the past for obtaining conical‐shaped radiation patterns have complex structures and are very expensive. However, the generation of conical‐shaped patterns by an OAM antenna provides simplicity in the structural realization and is compact for integrating with transceiver circuits. By selecting circularly polarized (CP) patches as array elements of a UCA, conical‐shaped OAM beams can be generated. The proposed UCA consists of a simple feed network with eight dual‐band CP elements. The required phase delay for generating OAM beam (l $l$ = +1 mode) is obtained by rotating the CP elements 45∘ $4{5}^{\circ }$ in clock wise direction. The UCA is 150 mm × $\times $ 140 mm in size, fabricated on a FR4 substrate, and experimentally tested. The measured results confirm that the generated OAM beam has conical‐shaped radiation patterns at 5.3 and 5.77 GHz. The generated OAM beam has a peak gain of 10 dBi with high mode purity and a divergence angle of ±20∘ $\pm 2{0}^{\circ }$ in both simulated and measured findings. [ABSTRACT FROM AUTHOR]

Orbital angular momentum (OAM) modes can offer high density and high-capacity communication. The traditional phased array antenna can only produce a limited number of OAM beam modes l, usually less than half of the number of array elements (N): −N/2 < lmax < N/2. An OAM antenna array for generating high-order OAM modes is proposed in this letter. The proposed antenna array consists of a ring patch antenna that can generate vortex waves with OAM mode l = 1 or −1 and a phase-shifting feeding network. By adding different feed excitation signals to each element, the generated beam carries a higher-order mode: l = N or −N, breaking the previous limitations. Near-field measurements were conducted on antenna arrays composed of 3, 4, and 5 elements, revealing a high degree of correspondence between their phase distribution and radiation patterns with numerical simulation results. This alignment further substantiates the practical efficacy of this approach in significantly enhancing the generation of high-order OAM modes within antenna arrays. This advancement improves component utilization efficiency, reduces system complexity, and meets the high demands for spectral resources and channel capacity in future communication applications. [ABSTRACT FROM AUTHOR]

Recently, different research articles have given more emphasis to the design of high-speed free space optical (FSO) links using various multiplexing techniques to achieve optimum long reach under adverse weather conditions. This research paper presents a spectral-efficient, robust design of a novel, 2 × 120 Gb/s, hybrid multiplexed FSO system based on orthogonal frequency division multiplexing (OFDM). The 16-QAM modulated multiplexed data is sent over the helical structure of the orthogonal laser modes to increase the optical link's capacity, whereas OFDM is used to mitigate the fading induced by the atmospheric turbulence. The in-phase (I) and quadrature phase (Q) based optical modulators, along with the digital signal processing (DSP) unit, are used at the receiver to take care the high-speed data below the acceptable FEC limit. The system performance is evaluated in terms of scatter plot, link distance, optical signal-to-noise ratio (OSNR), and received optical power under strong turbulence, heavy rain and snow conditions to achieve the maximum transmission range of 2.5 km for an acceptable threshold bit error rate of ≤ 2 × 10−3. The simulation results illustrate the receiver sensitivity of − 21.71 dBm and OSNR requirements of 32.98 dB for the B2B case under clear weather conditions. [ABSTRACT FROM AUTHOR]

Summary: Utilization 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. [ABSTRACT FROM AUTHOR]

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Signals transporting orbital angular momentum (OAM) have gained significant interest due to their unique structure and capability for increasing the channel capacity. OAM beams are orthogonal and can be multiplexed to increase link capacity in different scenarios. The collective combination of radio OAM modes and OFDM modulations in outdoor conditions has not been extensively explored. This paper depicts an analysis of multiple OAM modes transmitted as narrowband subcarriers using orthogonal frequency division multiplexing (OFDM) modulation with outdoor free space optical channel measurements based on OAM modes. OAM-OFDM simulations were conducted to investigate the performance of QPSK, 16QAM, and 64QAM baseband modulation and demodulation mechanisms, with code rates ½ and ¾ respectively. Four outdoor OAM free space optical channel characteristics parameters were applied. The throughput, bit error rate (BER), and packet error rate (PER) were evaluated. The results show that OAM +4 Mode outperforms other OAM modes in terms of BER and PER. In terms of the throughput, 64 QAM at code rate ¾ outperforms other coding schemes and code rates, for all OAM modes.

Conventional metasurfaces based on geometric phase are constrained to spin‐locked phase profile, resulting in mirrored functionalities for different spins. A single flat device that enables independent manipulation of wavefronts in two orthogonal circularly polarized channels is of paramount importance in wireless and optical communications. In this work, by tuning the dimension and rotation angle of H‐shaped meta‐atoms to synthesize propagating phase and geometric phase, spin‐dependent plasmonic metasurfaces are presented to manipulate circularly polarized waves in the visible band. To verify the capability of spin‐dependent wavefront manipulation, three metasurfaces are implemented. The first metasurface generates vortex beams with orbital angular momentum (OAM) l = 1 under left‐handed circularly polarized (LCP) incidence and l = 2 under right‐handed circularly polarized (RCP) incidence. By introducing convolution operation, the second metasurface is capable of producing vortex beams with different OAMs and different directions for two spins. The third metasurface produces dual‐beam and quad‐beam with different OAMs for different circular polarizations. This scheme can provide a new pathway in ultracompact nanophotonic devices and systems. [ABSTRACT FROM AUTHOR]

Signals transporting orbital angular momentum (OAM) have gained significant interest due to their unique structure and capability for increasing the channel capacity. OAM beams are orthogonal and can be multiplexed to increase link capacity in different scenarios. The collective combination of radio OAM modes and OFDM modulations in outdoor conditions has not been extensively explored. This paper depicts an analysis of multiple OAM modes transmitted as narrowband subcarriers using orthogonal frequency division multiplexing (OFDM) modulation with outdoor free space optical channel measurements based on OAM modes. OAM-OFDM simulations were conducted to investigate the performance of QPSK, 16QAM, and 64QAM baseband modulation and demodulation mechanisms, with code rates ½ and ¾ respectively. Four outdoor OAM free space optical channel characteristics parameters were applied. The throughput, bit error rate (BER), and packet error rate (PER) were evaluated. The results show that OAM +4 Mode outperforms other OAM modes in terms of BER and PER. In terms of the throughput, 64 QAM at code rate ¾ outperforms other coding schemes and code rates, for all OAM modes. [ABSTRACT FROM AUTHOR]

This paper proposes a novel 4 × 10 Gb/s underwater optical wireless communication system based on orbital angular momentum (OAM) multiplexed beams. The proposed transmission uses four OAM beams based on Laguerre-Gaussian profiles, each transporting data at 10 Gb/s at 532 nm operating wavelength. The system performance is evaluated for five different types of water environments, i.e., pure sea (PS), clear ocean (CO), coastal ocean (CS), harbor I (HI), and harbor II (HII) by measuring the received optical power (ROP), underwater transmission distance, bit error rate, beam divergence angle, eye-diagrams, and receiver aperture diameter. The obtained results show that the PS has the longest underwater propagation distance of 10 m, while HII has the shortest range of 2.62 m due to high attenuation at ROP of approximately −18 dBm. The underwater span achieved at the same ROP value for CO, CS, and HI are 8 m, 6.1 m, and 3.85 m, respectively. [ABSTRACT FROM AUTHOR]

In this paper, a 3D conformal meta-lens designed for manipulating electromagnetic beams via height-to-phase control is proposed. The structure consists of a 40 × 20 array of tunable unit cells fabricated using 3D printing, enabling full 360° phase compensation. A novel automatic synthesizing method (ASM) with an integrated optimization process based on genetic algorithm (GA) is adopted here to create the meta-lens. Simulation using CST Microwave Studio and MATLAB reveals the antenna’s beam deflection capability by adjusting phase compensations for each unit cell. Various beam scanning techniques are demonstrated, including single-beam, dual-beam generation, and orbital angular momentum (OAM) beam deflection at different angles of 0°, 10°, 15°, 25°, 30°, and 45°. A 3D-printed prototype of the dual-beam feature has been fabricated and measured for validation purposes, with good agreement between both simulation and measurement results, with small discrepancies due to 3D printing’s low resolution and fabrication errors. This meta-lens shows promise for low-cost, high-gain beam deflection in mm-wave wireless communication systems, especially for sensing applications, with potential for wider 2D beam scanning and independent beam deflection enhancements.

A novel radar cross-section (RCS) reduction metasurface is designed, fabricated, and experimentally characterized to flexibility control wavefront and efficiency generated orbital angular momentum (OAM). The metasurface is composed of several square loops with different reflection phases. Utilizing the property of the OAM phase singularity, the metasurface presents a significant RCS reduction along the propagation axis. Simulation results show that RCS reduction over 40 dB for normal incident plane waves at 7.6 GHz. For further clarity the physical mechanism of the RCS reduction, the near-field and 3D far-field features of the metasurface are discussed in detail. Two prototypes are fabricated and experimented with different topological charges. Good agreement is achieved between full-wave simulations and measurements. The design methodology provides an alternative to low-RCS and wavefront modulation design.

We proposed and numerically investigated a new circular hollow photonic crystal fiber made of LiNbO3 as the high-index ring material with silica as the background layer supporting orbital angular momentum modes. The proposed fiber supports > 120 orbital angular momentum modes for signal transmission with flat dispersion ~ 1.6 ps-nm− 1 km− 1 in the O + E + S + C + L and U Bands. The high effective mode index difference of ~ 10− 3 and high fiber bending tolerance (~ 0.1 cm) across the optical bands are notable features of the architecture. The proposed photonic crystal fiber has the potential to be utilised in high-bandwidth fiber communication systems. [ABSTRACT FROM AUTHOR]

To meet the needs of future wireless optical networks, this paper introduces a high-speed, hybrid multiplexed, coherent free-space optical (FSO) communication system that integrates an orbital angular momentum (OAM) multiplexed signal with an orthogonal frequency division multiplexing (OFDM) technique. Two independent QAM polarized beams, each carrying in-phase and quadrature (I/Q) phase 16-QAM-OFDM modulated data, are combined using mode division multiplexing (MDM) to increase the capacity of the proposed system. The reason of choosing OFDM is its capability to support higher data rate, and mitigating intersymbol interference (ISI). The signal is detected using a coherent detection-based digital signal processing (DSP) algorithm at the receiver end. The proposed hybrid FSO system is evaluated in low and heavy dust environments using bit error rate (BER), link distance, optical signal-to-noise ratio (OSNR), and received optical power performance matrices. The simulation results demonstrate the successful transmission of a 120 Gb/s single carrier over the longest link ranges of 1.5 and 0.40 km, respectively, under low and heavy dust weather environments below the signal degradation threshold value (forward error correction (FEC) limit) of BER 2.2 × 10-3 in strong turbulent conditions. [ABSTRACT FROM AUTHOR]

Vortex electromagnetic (EM) waves, which carry orbital angular momentum (OAM), can provide more information of the target in radar imaging and target detection. Pulsed radar is often used in current OAM radar design. However, compared with the pulsed radar, the frequency-modulated continuous-wave (FMCW) radar is small in size, light in weight, and low in power consumption, which can be applied into many practical scenarios. Therefore, this letter investigates into the OAM radar based on the FMCW system. It can be found that, apart from the inherent characteristics of small blind area and high measurement accuracy in FMCW system, the OAM-FMCW radar also owns an additional degree of freedom, which is independent of the frequency, finally leading to the result that the FMCW radar will not be affected by the range and velocity of the target when performing azimuth detection. Simulation results are given to validate the corresponding analysis of the proposed system. [ABSTRACT FROM AUTHOR]

Orbital angular momentum (OAM) multiplexing is a recently considered solution for enhancing wireless and free-space optical communications channel capacity, whether implemented separately or in combination with existing multiplexing techniques. The theoretically infinite number of paraxially propagating and mutually orthogonal OAM modes is expected to increase the channel capacity. However, the orthogonality for different OAM modes has been shown to decrease for far link range distances, and the paraxiality of the OAM beams is not very good for small radiating sources. Based on the current knowledge, OAM beams are most likely to be used for short-range communications. Many models of the electromagnetic (EM) fields carrying the OAM neglect the fact that the OAM beam sources could be electrically large or introduce other approximations that are appropriate for far-field analysis only. An in-depth analysis of the short-range properties of OAM EM fields is still lacking. To address this problem, we propose the use of the infinitesimal (Hertz) dipole method customized for the analysis of the OAM EM fields. This technique can model the positioning and basic radiation properties of separate antennas or antenna sub-arrays that are the building blocks of OAM arrays exactly and efficiently. Similar modeling can represent the OAM sources for free-space optical communications. We focus here on the uniform circular antenna arrays and provide an in-depth analysis of what can and cannot be expected, in the best case, in their utilization. We assume low losses, which is a common assumption for many methods, except for computationally much more demanding full-wave simulations. The obtained results indicate the need to simultaneously optimize the transmission of all planned OAM modes and allow estimates of the link distances that could provide adequate OAM wave reception in various cases. [ABSTRACT FROM AUTHOR]

Vortex electromagnetic (EM) waves, with different orbital angular momentum (OAM) modes, have the ability to distinguish the azimuth of radar targets, and then the two-dimensional reconstruction of the targets can be achieved. However, the vortex EM wave imaging methods in published research have no ability to obtain the elevation of the targets, and thus, the three-dimensional spatial structure and richer feature information of the radar target cannot be obtained. Therefore, a three-dimensional imaging method of vortex EM waves with integer- and fractional-order OAM modes is proposed in this paper, which can realize a three-dimensional reconstruction of a radar target based on a uniform circular array (UCA) with two-step imaging. First, the vortex EM wave with integer- and fractional-order OAM modes is generated, and the echo model with different OAM mode types is established. Thereafter, the echo with integer order is processed to obtain the range-azimuth image by fast Fourier transform (FFT). Then, in order to realize the three-dimensional reconstruction, the echo with fractional order is processed by utilizing the butterfly operation and analyzing the characteristics of the fractional Bessel function. Moreover, the resolution and reconstruction precision of the azimuth and elevation are analyzed. Finally, the effectiveness of the proposed method is verified by simulation experiments. [ABSTRACT FROM AUTHOR]

Orbital angular momentum (OAM) has recently attracted increasing interest in optical communications for capacity scaling by OAM mode‐division multiplexing (MDM). OAM (de)multiplexer is crucial to the success of OAM‐MDM communications. Scalable and efficient dense OAM demultiplexing is highly desired, but full of challenges. Here, a quasi‐wavelet conformal mapping method is proposed and demonstrated to implement scalable and efficient dense OAM demultiplexing. The OAM mode is divided at the input plane into multiple concentric rings, which are mapped to multiple tilted plane waves and arranged side by side in a line. The engineered transformed light beams with periodic extension and increased length enable narrow focused spot width, reduced beam overlap, and suppressed demultiplexing crosstalk. The quasi‐wavelet conformal mapping method is compared with conventional log‐polar transformation scheme and hybrid log‐polar and fan‐out technique, and the crosstalk matrix for OAM demultiplexing is measured. Efficient OAM demultiplexing is demonstrated for 15 OAM modes (OAM−7 to OAM+7) with a maximum crosstalk of −12.1 dB in the experiment. Moreover, the method is also applied to system‐level data‐carrying OAM‐MDM communications with favorable performance. The demonstrated quasi‐wavelet conformal mapping method may pave the way for future ultrahigh capacity dense OAM‐MDM communications with a large number of OAM modes. [ABSTRACT FROM AUTHOR]

Vortex beams' orbital angular momentum (OAM) has important application value in optical communication and other fields. Accurate measurement of their OAM topological charges is required for the application of vortex beams in the field of optical communication. As a type of convolutional neural network, DenseNet which combines the “Data Augmentation with Expansion” method, was introduced to measure the fractional topological charge of OAM images in the case of few samples. The OAM images used in this paper generally do not have high brightness, which simulates ultra-long-distance transmission situations in reality. The experiment shows that the classification accuracy of DenseNet network reaches 98.77%, with an average training time of about 1100 seconds, which is short. Our results showcase the potential of convolutional neural network approach to further study the OAM light in the application of free-space optical communication.

This study delves into the potential of harnessing the orbital angular momentum (OAM) property of electromagnetic waves in near-field and line-of-sight scenarios by utilizing large intelligent surfaces, in the context of holographic multiple-input multiple-output (MIMO) communications. The paper starts by characterizing OAM-based communications and investigating the connection between OAM-carrying waves and optimum communication modes recently analyzed for communicating with smart surfaces. Subsequently, it proposes implementable strategies for generating and detecting OAM-based communication signals using intelligent surfaces and optimization methods that leverage focusing techniques. Then, the performance of these strategies is quantitatively evaluated through simulations. The numerical results show that OAM waves while constituting a viable and more practical alternative to optimum communication modes are sub-optimal in terms of achievable capacity.

The abundant spectrum resources and low beam divergence of the terahertz (THz) band, can be combined with the orthogonal propagation property of orbital angular momentum (OAM) beams to multi-fold the capacity of wireless communication systems. Here, a reflective metasurface (RMTS) is utilized to enhance the coverage of the high gain THz OAM beams by enabling the non-line-of-sight (NLoS) component by reshaping the planar wavefront of the incident wave into the helical wavefront, so that it is re-directed towards the direction of interest. This can contribute to alleviating the concern of the small aperture size, since improving the channel capacity can be achieved at the low spectrum blocks of the THz band (larger aperture size). For validation, three $90 \times 90$ mm RMTSs are simulated, fabricated, and tested in the frequency range 90–110 GHz, to re-direct single and dual OAM beams towards the desired location.

Antennas that generate orbital angular momentum (OAM) have the potential to significantly enhance the channel capacity of upcoming wireless systems. This is because different OAM modes that are excited from a shared aperture are orthogonal, which means that each mode can carry a distinct stream of data. As a result, it is possible to transmit multiple data streams at the same time and frequency using a single OAM antenna system. To achieve this, there is a need to develop antennas that can create several OAM modes. This study employs an ultrathin dual-polarized Huygens' metasurface to design a transmit array (TA) that can generate mixed-OAM modes. Two concentrically-embedded TAs are used to excite the desired modes by achieving the required phase difference according to the coordinate position of each unit cell. The prototype of the TA, which operates at 28 GHz and has a size of 11 × 11 cm 2 , generates mixed OAM modes of −1 and −2 using dual-band Huygens' metasurfaces. To the best of the authors' knowledge, this is the first time that such a low-profile and dual-polarized OAM carrying mixed vortex beams has been designed using TAs. The maximum gain of the structure is 16 dBi. [ABSTRACT FROM AUTHOR]

Laguerre–Gaussian (LG) beam scattering by a perfect electromagnetic conductor (PEMC) cylinder coated with a metamaterial is analyzed. In order to obtain the LG beam scattering from PEMC cylinder, the beam expressions are modified by considering the LG potential and plane wave scattering. The influence of beam parameters such as; the radial mode (p) , azimuthal mode (l) , and beam waist radius (w 0) is studied. The normalized bistatic radar cross section can be amplified or repressed using these parameters. A comparison of our results for p = 0 and l = 0 (fundamental Gaussian beam) by a PEMC coated cylinder is carried out and indicates good agreement for plane wave scattering. [ABSTRACT FROM AUTHOR]

A digital coding metasurface is proposed to generate multi-beam and multi-mode orbital angular momentum (OAM) wave in the full space. The 3-bit coding elements are designed for multifunctional integrated digital coding metasurface (MIDCMS) based on the propagation phase theory. The variable-sized phase-shift scheme is applied on metal structures to realize different functionalities in linearly polarized (LP) incidence. Furthermore, the phase distribution of MIDCMS is determined by the pattern convolution theorem. The transmitted and reflected wavefronts are manipulated simultaneously through MIDCMS. As a proof of concept, a metasurface device was fabricated and measured for the four-beam OAM with +1 mode in transmission space and the four-beam OAM with −1 mode in reflection space. Both experimental results and simulated values verify that the proposed MIDCMS can effectively realize the full-space multi-beam multi-mode OAM generation and improve the capacity of information channels, which has potential applications for comprehensive implementations in the future wireless communication systems.

Vortex beams carrying orbital angular momentum (OAM) provide a new degree of freedom for light waves in addition to the traditional degrees of freedom, such as intensity, phase, frequency, time, and polarization. Due to the theoretically unlimited orthogonal states, the physical dimension of OAM is capable of addressing the problem of low information capacity. With the advancement of the OAM optical communication technology, OAM router devices (OAM-RDs) have played a key role in significantly improving the flexibility and practicability of communication systems. In this review, major breakthroughs in the OAM-RDs are summarized, and the latest technological standing is examined. Additionally, a detailed account of the recent works published on techniques related to the OAM-RDs has been categorized into five areas: channel multicasting, channel switching, channel filtering, channel hopping, and channel adding/extracting. Meanwhile, the principles, research methods, advantages, and disadvantages are discussed and summarized in depth while analyzing the future development trends and prospects of the OAM-RDs.

In this work, we have theoretically studied the resonant four-wave mixing (FWM) in a four-level double Lambda (Λ) atomic system in connection with orbital angular momentum (OAM) transfer from probe to generated signal beam. The effect of the relaxation process is studied in the forward as well as backward FWM. Based on the semiclassical model, our analysis shows a strong dependence of conversion efficiency on spontaneous decay and decoherence rates. From the intensity and phase profile, we have confirmed the OAM nature of the generated signal beam. The physical explanation is given for the dependence of efficiency on the decay rate of the excited atomic state. We have shown that decoherence present in the system always leads to a deleterious effect on conversion efficiency. Our presentation treats forward and backward FWM in a unified way in the context of OAM transfer and sheds light on the parameter dependence of conversion efficiency.