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This paper proposes a new method of realizing multimode OAM beams with almost the same divergence angles. The theoretical relationship between the divergence angle of the OAM beam and its radiation source is presented, and the radiation source distributions for various mode OAM beams with the same divergence angle range are discussed. In order to v...
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... Well-known methods of generating OAM beams including the helical phase plate [14,15], the rotating paraboloid [16,17], the uniform circular array (UCA) [18,19], and the metasurface modulator [20,21] have already been proposed. The metasurface aperture is an effective electromagnetic control method since the metasurface can be designed with multiple degrees of freedom, such as geometry of the metasurface units, dispersion, and polarization response, etc. ...
... The performance comparison between the FDMM OAM modulator and existing works is shown in table 2. Compared with the MM-OAM-generation methods in [17,18], and [31], there is no complex feed network in the proposed method. Compared with designs in [30] and [32], this work can simultaneously generate different modes of OAM beams with approximately equal divergence angles. ...
Traditional multi-mode orbital angular momentum (OAM) beams suffer from low energy efficiency in detection since the divergence angle tends to increase with increasing order of the OAM mode. Thus, OAM beams with equal divergence angle are highly desirable. In this paper, a frequency-diverse multi-mode (FDMM) OAM modulator with an equal divergence angle is proposed. The modulator is composed of an OAM modulation metasurface and a spatial filter (SF) with narrow passband. Firstly, the OAM modulation metasurface is designed based on multiple annular regions with different radii in the metasurface aperture. Each mode of OAM beam is generated at a corresponding frequency point in one region. Thus, the metasurface aperture can generate different modes of OAM beams at different frequencies (i.e., FDMM OAM beams) simultaneously. Then, the SF with narrow passband is proposed through synthesizing multi-layer frequency selective metasurfaces, aiming to reduce interferences between different modes of OAM beams. Finally, the FDMM OAM modulator is realized by superposing the SF on the OAM modulation metasurface. The FDMM OAM modulator can generate 6 modes of OAM beams (i.e., , and modes) at 12.5 GHz, 13.0 GHz and 13.6 GHz, respectively. A multi-mode OAM coincidence imaging model based on the FDMM OAM modulator is established. The simulation and experiment results show that the proposed FDMM OAM modulator can improve the coincidence imaging performance assisted by an optimization algorithm.
... However, those studies mainly concentrated on the microwave and optical domains, and in contrast, rather seldom researches are studied in the THz field. Additionally, vortex beams gradually spread along the radial direction on the transverse plane during propagation, limiting further practical application in long-range wireless transmission [43,44]. Henceforth, generating spatially stable THz vortex waves carrying OAM in free space without changing their initial field distribution at any plane orthogonal to the direction of propagation becomes an urgent issue that merits further study. ...
Bandwidth, orbital-angular momentum (OAM) divergence, and mode purity are the three critical issues for the practical terahertz orbital angular momentum manipulation, especially in the next sixth-generation (6G) communication system. Here we propose the broadband high-order Bessel vortex beam carrying multiple OAM modes reflective metasurface in the terahertz domain. The simulation results agree with the theoretical expectation, and the diffracting divergence of OAM vortex beam characteristics has been alleviated. The research on the relationship between the varieties of lattice type and mode purity is also relatively scarce. Henceforth, a comparison study has been conducted between three lattice types, i.e., square lattice, triangular lattice, and concentric ring lattice. And corresponding results of the relationship of mode purity with those lattice types show that the concentric ring lattice has the best performance.
... The OAM antennas based on traveling wave structures are proposed and optimized such as metallic waveguide slot antenna . However, such antenna using metallic waveguide is relatively heavy and difficult to fabricate compared with substrate integrated waveguide (SIW) antenna such as a bifocal parabolic reflector antenna with half mode SIW feeds (Zhang et al. 2021). Moreover, the antenna based on traveling wave structures could radiate four or eight or even more OAM modes through multiple ring structures (Zhang et al. 2017a) while the radiation characteristics of each OAM mode such as main beam direction could be obtained or tuned through phase mode analysis (Yao et al. 2017) and spectrum analysis (Gao et al. 2020). ...
This paper demonstrates the design and simulation analysis in association with fabrication measured results of an orbital angular momentum (OAM) substrate integrated waveguide (SIW) slot antenna working at 10 GHz in X band. Simple feeding networks including 90° hybrid couplers are introduced to excite traveling waves inside four separated circular resonators radiating eight OAM modes beams. The antenna is composed of two PCB layers where the circular resonators and feeding networks are located, respectively. The measured results including S-parameters and radiation patterns are in good agreement with the simulation results in HFSS.
Angular momentum (AM) multiplexing technique provides multiple information channels for electromagnetic waves (EMWs) modulation at the same frequency, improving spectrum efficiency effectively. Among AM multiplexing techniques, the patch antenna has the advantage of minimization, simple feeding circuits, and easy integration. Nevertheless, limited reports provide theoretical guidance for designing AM multiplexing patch antennas. Herein, we propose a scheme to achieve AM multiplexing via a shared-aperture patch antenna. The radiating structure consists of multiple radiating patches sharing a common physical aperture, each of which is responsible for producing different AM-carrying waves. By analyzing the theoretical model built on a dipole representation of the patch antenna, the shapes and the desired current patterns of these patches can be determined according to the AM mode of the expected EMWs. Besides, the sizes and feeding structures of these patches can be studied by investigating their radiating characteristics using characteristic mode (CM) theory. For proof-of-concept, a prototype was designed, fabricated, and measured for the generation of four orthogonal AM-carrying waves coaxially at 5.8GHz. The experimental results agree reasonably well with the simulated ones, demonstrating that the proposed scheme provides an efficient guidance for the design of AM multiplexing patch antennas.
In this paper, the detections of targets’ longitudinal and angular accelerations, including uniform and variable accelerations, have been investigated, based on vortex electromagnetic (EM) waves carrying orbital angular momentum (OAM), in which the time–frequency analysis approaches are used to process the echo signal. Compared with other methods, the Choi-Williams distribution (CWD) is more suitable for detecting, which has better energy concentration and less cross term. Meanwhile, the multi-OAM modes method has been proposed to decouple the longitudinal and angular accelerations and detect the concrete acceleration coefficients. And the detection errors have been analyzed by transmitting the OAM waves with different mode combinations of l = 1, 3, 5 at 3 GHz. The results indicate that increasing mode intervals and decreasing topological charges can relatively improve detection accuracy in an appropriate sampling frequency range. In addition, the proposed method is also applicable to the case that the transmission modes are impure in practical application. And the directions of the longitudinal and angular accelerations can be acquired by sending OAM with different amounts of power. This work paves the way of detecting 3-dimensional velocity of complex target.
