Electromagnetic (EM) vortex can realize high resolution imaging in forward-looking observation geometry without relative motion between the radar system and targets. However, the current research about EM vortex imaging only exploits integral orbital angular momentum (OAM) beams, which limit the application in the real scene with the existence of noise. An EM vortex enhanced imaging method based on fractional OAM beams is proposed against noise effect in this letter. Firstl the generation method of fractional OAM beams with phase distribution of nonintegral OAM mode is proposed using uniform circular array, and the corresponding features are analyzed. Subsequently, the imaging model and scheme with fractional OAM beams are provided and the principle for enhancing imaging performance is derived. Finally, results by Monte Carlo simulations demonstrate that the proposed vortex imaging method based on fractional OAM beams is robust against noise effect and can achieve better imaging performance in low signal-to-noise ratio (SNR) environment, which is of prospect for overcoming the shortage of poor imaging performance in low OAM order and complex noise scenario. This letter benefits the development of EM vortex imaging.

The electromagnetic (EM) vortex imaging has demonstrated superior performance in target detection and imaging with azimuthal super-resolution. However, the restricted elevation resolution degrades the acquisition of target spatial information, which limits the development of the radar imaging technology based on orbital angular momentum (OAM). This article offers a solution to achieve the 3-D EM vortex imaging, by effectively utilizing relative motion between radar and target in the line-of-sight (LOS) direction. First, the forward-looking radar imaging scenario is presented, the 3-D echo model is derived, and the characteristics are analyzed as well. Second, the imaging method, based on the back-projection (BP) and spectrum estimation method, is proposed to obtain the target’s 3-D focused image. Furthermore, the influence factors about the elevation resolution are analyzed by the point spread function (PSF). Finally, simulations are carried out to verify the effectiveness of the theoretical analyses.

This paper investigates electromagnetic (EM) vortex imaging based on dual coupled orbital angular momentum (OAM) beams. An OAM-generating scheme is proposed to generate radio waves carrying various dual coupled OAM at the X-frequency band and the experimental results of radar imaging in an anechoic chamber are illustrated for the first time. The uniform circular array (UCA) is exploited to generate dual coupled OAM beams and the radiated fields are presented by both theoretical analysis and full-wave EM simulation. Subsequently, the mathematical imaging model is established and the target reconstruction method is proposed. The relative imaging error (RIE) is introduce to evaluate the imaging performance. The imaging results indicate that the proposed method can have a better performance than EM vortex imaging without OAM-multiplexing and save half time of OAM beams transmitting. Moreover, the imaging experimental results in the anechoic chamber validate the effectiveness of the proposed method. This paper applies OAM-multiplexing beams to radar imaging and improves the vortex imaging efficiency, which can benefit the applications of OAM beams and the design of OAM-based radar systems.

To evaluate the interaction between the orbital angular momentum (OAM) beams and the electrically-large standard objects, the backward scattering characteristics of perfect electrical conductor (PEC) sphere and PEC cone are studied. Firstly, the incident vortex electromagnetic field carrying OAM is generated by Hertizian dipole arrays. Subsequently, the backward scattered field is calculated by physical optics (PO) algorithm. The phase profiles and OAM spectra are analyzed in detail, which indicates that the scattered field for axial symmetric object still keeps the main characters of vortex field due to angular momentum conservation. Finally, the radar cross section (RCS) for PEC sphere and cone are calculated, respectively. The backward scattering features show significant differences when OAM beams change their topological charges. Consequently, given a specific propagation direction, compared to plane waves, more information will be offered by the OAM beams for object detection and recognition.

Detection of the rotational objects in arbitrary position, using vortex electromagnetic (EM) waves carrying orbital angular momentum (OAM), is studied in this paper. In addition to the traditional linear Doppler shift, the rotational Doppler shift in OAM-based radar system can provide more detailed features of the object. Firstly, the Doppler detection model in OAM-based radar system is established, the linear Doppler and rotational Doppler frequency shifts are derived and verified. Subsequently, the detection method of rotational object using vortex EM waves, including an effective linear and rotational Doppler separation and a motion parameter estimation method, is proposed. Theoretical and simulation results validate the effectiveness of the proposed method, and more detailed rotational parameters can be obtained by using vortex EM waves in OAM-based radar system, compared to the method using plane waves in conventional radar system.

Vortex electromagnetic (EM) wave carrying orbital angular momentum (OAM) has been exploited in radar imaging realms in recent years. However, the radiation energy pattern caused by the Bessel amplitude modulation limits the OAM-based applications in the long distance. To solve the radiation energy divergence problem in radar realms, the imaging method based on waveform diversity (WD) and rotational array is developed in this article. The energy hollow along the beam axis is avoided by the orthogonal waveform design during the radar illumination process. Then, the echo demodulation approach is proposed, and the corresponding imaging model is established. Furthermore, a highly efficient imaging scheme based on rotational array is designed, where the OAM multiplexing approach is proposed and large OAM index can be used for imaging by virtual synthesis. Both simulation results and experimental results in an anechoic chamber are provided to demonstrate the effectiveness. This work can promote the application of OAM technology in realistic scenarios.