Research On The Imaging Theory And Method By Electromagnetic Vortex Wave

In recent years,the electromagnetic vortex wave carrying orbital angular momentum(OAM)has attracted much attention of researchers.Different from plane wave,the phase front of electromagnetic vortex wave has a spiral structure,which is believed to have the potential to improve the information acquisition ability from the target.Theoretically,electromagnetic(EM)vortex wave has infinite OAM modes,and different modes are orthogonal to each other.This feature has been applied in wireless communication by transmitting information in different modes to improve the channel capacity,which is expected to solve the shortage of the wireless spectrum resources.In addition,EM vortex wave has been studied in radar detection,the scattering fields of target present different characteristics compared with plane wave due to the special phase wavefront,which may be potential in radar anti stealth.In radar imaging,how to use different modes of electromagnetic vortex waves to extract more abundant information from the target has become a hotspot.This dissertation focuses on the generation of electromagnetic vortex wave and its applications in radar imaging area including the following aspects:Firstly,the basis theory of orbital angular momentum of electromagnetic vortex wave and the orthogonalily between different modes that can be used to extrat certain mode are introduced.It is deduced and proved that the EM vortex wave can be decomposed into a superposition of plane waves that propagating in different directions.Then the generation of EM vortex wave by phased arrray is introduced and the characteristics of the radiated field with different parameters such as the radius and element number of the array are studied.Considering that the radiation pattern of a single UCA has high side-lobes at high OAM mode,the optimized concentric uniform circular array(CUCA)is introduced which can be applied in the fast target acquisition.Secondly,the application in active radar imaging of the EM vortex wave is studied including staring imaging and synthetic aperture radar(SAR)imaging.The resolutions in elevation and azimuth dimensions for staring imaging are derived using the correlation function.Then high resolution two dimensional imaging is realized using sparse reconstruction theory.In addition,an equivalent fractional order OAM mode is proposed for EM vortex imaging.The proposed method is applied to the "MIMO" scheme to overcome the aliasing problem in azimuth angle to achieve high-resolution.Then,the EM vortex wave is introduced to SAR to further improve the resolution.The traditional back projection(BP)method is modified to compensate the additional vortex phase and SAR imaging is realized in the side view and down view configuration.Thirdly,the EM vortex wave is introduced in passive millimeter wave imaging to improve the resolution.The imaging scene is discretized into small cells,and in order to solve the unknown brightness temperature,different OAM modes are used during the scanning scene to form the equations.Since the maximum mode is limited by the aperture size of the antenna,compressed sensing is introduced to realize high-resolution in a few modes by utilizing the sparsity of the imaging scene in the transform domain.Lastly,considering that the EM vortex wave diverges along the propagation path,the generation and imaging method for focused OAM beams are studied.Starting from the plane vortex beam,the beam focusing on multi-mode superposition is introduced.Then the focused OAM beam is used in staring imaging and SAR imaging.In staring imaging,the focused beam has advantages in suppressing the interference of false targets;while in SAR imaging,three-dimensional reconstruction of the observation area can be realized with only one flight using the improved three-dimensional BP algorithm,and high-resolution is realized using the sparse reconstruction algorithm.In addition,the focused OAM beam based on CUCA is introduced.By adjusting the main lobes of different modes to a same elevation angle,the radiation pattern is focused at azimuth angle.The scheme is more flexible to achieve the superposition of any group of OAM modes and the scanning in elevation angle which is potential in radar detection.The research and achievements of this paper further improve the theory and methods of the EM vortex based radar imaging,and provide a reference for the research and applications of new radar imaging.