Study On The Theory And Method Of Electromagnetic Vortex Imaging

The exploitation of diversity in time,frequency and polarization domains can provide increased flexibility,capabilities and robustness to conventional sensing systems.Apart from these,the wavefront modulation ability of electromagnetic(EM)waves has attracted increasing attention.Orbital angular momentum(OAM)is a significant physical observable,which has not been fully exploited.While the OAM is modulated on the conventional EM wave,it leads to helical wavefront profile.Thus,the vortex EM wave forms,which is capable of improving the EM wave's information transfer and acquisition capacities.In this paper,based on the fundamental of EM vortex,the vortex EM wave is applied in radar imaging applications.We are dedicated to solving the fundamental and scientific issues in EM vortex imaging,which includes the principle of EM vortex imaging,the generation and optimization method of vortex EM waves for radar imaging,measurement approaches of vortex EM waves and target scattering characteristics,and target reconstruction methods.All the problems listed above are resolved,and a series of new thought and approaches are proposed,in this paper.Firstly,the research background and merits are presented.The intrinsic character of vortex EM waves and OAM is introduced.By comparing with conventional EM fields,the distributions of vortex fields in linearly polarization and circularly polarization are analysed,and the propagation characteristics are studied as well.This work can provide theoretical foundation for the study on the vortex EM waves' applications in radar imaging.Subsequently,the imaging models of “multiple-in-multiple-out” and “multiple-in-single-out” modes are established by analysing the principle of EM vortex imaging,and the imaging methods based on FFT and back projection are proposed.The imaging model in squint mode is derived as well and the corresponding imaging method is developed.Compared with conventional array imaging,the features of EM vortex imaging are summarized and the advantages are shown,which provides the prospect for improving imaging resolution in the azimuthal dimension.The generation method of vortex EM waves is proposed aiming at its applications for radar imaging.The mathematical model is derived and several approaches using different array configurations are developed to generate vortex EM waves.According to directional element and concentric uniform circular arrays(CUCAs),the main-lobe direction is collimated and the side lobes are suppressed for different OAM mode numbers.Furthermore,a compact and high-gain plane horn antenna is designed to generate vortex EM waves at X frequency band with CUCAs.Thirdly,the measurement of vortex EM waves and the target scattering characteristics are another important issue to be solved.A fast estimation method of OAM mode number is proposed based on the conversation of angular momentum(AM).The OAM-generating system at X frequency band is designed and the influences of array errors on the generation of vortex EM waves are analysed.A measurement experiment is performed in the anechoic chamber and the measured field is analysed using the proposed methods in this paper.In analogy to conventional radar cross section(RCS),the definition of OAM-based radar cross(ORCS)is proposed.The interaction between vortex EM waves and the target is analysed,the analytical expressions of ORCS is derived,and the ORCS characteristic curves for the metal plate and cylinder are obtained,for the first time.This research work can benefit the study on the principle of target identification and imaging algorithms.Finally,two types of imaging algorithms are developed,including the FFT-based or power estimation density(PSD)based method,and the sparse Bayesian learning(SBL)based method.The EM vortex imaging model is established using the linear frequency modulation(LFM)signal and the uniform circular array(UCA).Based on the CUCAs,the optimization method of the radiation pattern,the model establishment,and imaging processing method are proposed.The merits are also demonstrated by comparison with UCA.The effectiveness of the proposed imaging method in this paper and its robustness against noise is validated by a proof-of-concept imaging experiment,which indicate that the vortex EM waves can be readily used to achieve high resolution.Moreover,the sparse representation model is derived,and target reconstruction approaches based on SBL framework are proposed.The imaging model after beam steering is also derived and the improvement of imaging performance is shown by simulation results.A practical application processing the real-world data from a proof-of-concept experiment in an anechoic chamber is provided.Our research and achievements build the system of the theory and method of EM vortex imaging,which not only enriches the fundamental theory and application of EM vortex,but also provides beneficial guidance for the development of new paradigm of radar detection imaging technology.