| The wave carraying Orbital Angular Momentum(OAM)is a kind of special electromagnetic wave,whose wavefront phase is helical structure.Compared with plane waves,OAM waves can not only carry information in time domain,frequency domain and polarization domain,but also provide a new degree of freedom for information modulation.Due to the distinguish characteristics,OAM waves have great application in wireless communication,radar imaging,detection of moving targets and other fields.So OAM waves have attracted extensive attention of scholars.In this thesis,the generation of OAM waves,the principle of Doppler detection and inverse scattering imaging are explored.The main work is summarized as follows:To date,various methods to generate OAM beams have been continuously proposed,yet the generation of a small-divergence-angle and high-purity OAM beams is still considered as a big challenge.Based on the principle of metasurface beam manupulation,a transmissive metasurface antenna that can generate OAM beam is designed.The simulation and experimental results show that the proposed antenna can generate OAM beams wih high mode purity and small divergence angle.Then,the rotational Doppler effect under non-ideal conditions is emphatically studied,and the echo signal of OAM radar is derived in detail.The angular Doppler frequency shift induced by the OAM radar solve the problem of "blind zone" existing in plane wave radar detection of rotating target.To measure the angular velocity of the slow rotating object,this thesis analyzes the situation that the center between the phase center of the OAM wave is not coincident with the center of rotation of the object,and the angular velocity is measured with the designed OAM antenna.The measurement error can be controlled within1%.These results show that the detection technology based on OAM waves can be complementary to the existing detection technology based on plane wave in the detection of rotating targets,improving the ability of radar to acquire target information.Finally,based on the phase characteristics of OAM waves,the application of OAM waves in inverse scattering imaging is preliminarily explored.Based on the OAM inverse scattering imaging model,Tikhonov regularization method was used to solve the linearized imaging equation,and the effect of the purity of OAM waves on imaging quality was discussed.The simulation results show that the higher the purity of OAM waves is,the better the imaging quality is.Therefore,the design of an antenna generating high purity OAM waves has positive significance for the development of inverse scattering imaging. |
