Micro-motion Target Detection Based On Non-diffraction Orbital Angular Momentum

With the increasing requirement of transmission capacity,orbital angular momentum(OAM)vortex wave has aroused a widespread interest as a momentum property of electromagnetic waves,which describes the rotation of the wave front.The electromagnetic wave carrying OAM uses the angular momentum mode to carry information,which has a higher dimensional of information modulation compared with traditional plane wave and has a better resolution in target detection.However,the application of OAM waves has slowed down due to the divergence characteristics of vortex waves in long-distance detecting.In addition,the application of vortex waves is focused on the detection of stationary targets,and the research in target micro-motion is relatively few.This thesis established target detection systems in microwave field and terahertz field,respectively,and made a preliminary study of micro-motion detection based on non-diffraction orbital angular momentum.The main research contents and innovation results achieved are as follows:(1)A microwave target detection system is established using a Metasurface.For the existing vortex waves are mostly diffracted and the dispersion characteristics have not been improved,a single-layer reflective Metasurface is designed that can generate a Bessel vortex beam in X-band with plane wave excitation.After adding scatterers in the radiation field,and the influence of the axial and radial micro-motion on the scattering phase is analyzed.The results show that the amplitude of the scattered field intensity decreases by19.7V/m when the target object moves from 50 mm to 300 mm in the axial direction,and the amplitude of the scattered field intensity decreases by 16.8V/m when the micro-movement of the target in the radial direction increases from 50 mm to 200 mm,and the scattered field distribution is incomplete at 200 mm from the propagation axis.Then a method to identify micro-motion target is proposed.Firstly,the form of target micro-motion is identified according to the distribution integrity of scattering intensity and phase.Then,the characteristics of target micro-motion are further obtained according to the divergence of scattering intensity and the phase.Therefore,the rule of target detection can be obtained according to the scattering intensity amplitude and phase distribution.The detection model analyzes the scattering characteristics of the target micro-motion under the irradiation of non-diffraction vortex beam and provides a technical support for improving the detection capability of micro-motion targets.(2)A platform is built to establish a detection system in terahertz band to detect the micro-motion thrombus.In view of the promising application of non-diffraction vortex beams in the terahertz band,two types of non-diffraction vortex waves are introduced to study the interaction between terahertz non-diffraction vortex waves and thrombus.An improved electromagnetic simulation algorithm is used to build a simulation platform for electromagnetic field simulation and numerical calculation of the thrombus model,and the scattering phase distribution of thrombus micro-motion for terahertz beams is analyzed from two perspectives: off-axis micro-motion and rotation around the axis.The results show that the scattering phase diverges and transitions to higher-order modes when the thrombus is 10 mm away from the axis,and the vortex is incomplete when the thrombus is rotated around the axis at an angle of 90°.It is demonstrated that the terahertz non-diffraction vortex beam has great significance for the detection of thrombus micro-motion and provides a method for terahertz human medical diagnosis.Based on the study above,the results based on scattering field show that the non-diffraction orbital angular momentum has a better identification in target detection compared with plane waves.And the scattering phase results after micro-motion of the target also indicate the feasibility of non-diffraction vortex waves in micro-motion target detection,which provides an important method to investigate the rule between non-diffraction vortex waves with micro-motion targets.