Research On Far-field High-resolution Imaging Technique Based On Electromagnetic Time Reversal

The dissertation mainly investigates the high-resolution imaging of source targets and scatterers based on electromagnetic time reversal technique,with help of evanescent wave conversion structures.With the development of electromagnetic imaging techniques,the requirements to the performance of imaging systems are promoting,such as long-range imaging,super-resolution imaging,multi-dimensional imaging,and so on.The self-adaptive focusing property of time reversal electromagnetic wave,synchronously in spatial and time domain,makes it an excellent choice for far-field imaging applications.To realize the sub-diffraction imaging of source targets,two kinds of evanescent wave conversion structures,small perfect electric conducting(PEC)ball arrays and grating-like plate,are designed and researched in-depth.Meanwhile,on considering of the development requirements for imaging of scatterers,a new method named time reversal demodulation imaging is proposed.Firstly,to break the polarization limitations to source targets of traditional far-field super-resolution imaging systems,a fast solution model for the calculation of time reversal electromagnetic fields of source targets loaded with a small PEC ball array is established.In addition,a far-field two-dimensional imaging system,which is insensitive to the polarization of source targets,is constructed and analyzed.By investigating the time reversal theory of electromagnetic field in-depth,the time reversal electromagnetic field distributions with excitation sources of electric dipole and magnetic dipole are researched,respectively.When excited by dipoles,the equivalent dipole model of a small PEC ball array is proposed.Then,the corresponding fast solution method is analyzed according to time reversal cavity theory.Meanwhile,the source target imaging results are discussed in detail.Compared with the calculation of using commercial full-wave simulation software,the establishing of this equivalent model and fast solution method improves the analysis efficiency obviously.What is more,we can have a further understanding of the internal mechanism through this analytical analysis.Finally,the problem that traditional far-field super-resolution imaging systems are sensitive to the polarization of source targets is solved thanks to the rotational symmetry of the PEC balls.By extending the loaded PEC ball array two-dimensionally,two-dimensional super-resolution imaging of source targets can be finally realized.Subsequently,by considering of the limitation of traditional far-field imaging system that the targets should be placed on some specific locations,a kind of grating-like evanescent wave conversion structure is designed to assist the super-resolution imaging of source targets,which locates on anywhere of the imaging plane.From the essence of realizing super-resolution,the mechanism of realizing evanescent wave conversion by surface plasmon is studied at first.Then,according to the principle of spatial domain modulation,a grating-like microwave plasmonic structure is designed with its conversion effect being verified by simulation.Afterwards,according to the experimental conditions,the specific steps of real time reversal experiments in laboratory environment are given.At the same time,for handling parameter selection error,which is easy to make when carrying out time reversal experiment,some simulations are carried out to verify the correct selection.Compared with traditional structures,which realize evanescent wave conversion based on the principle of resonance or equivalent double-curvature lens,this kind of structure has no strict restriction on the position of original source target.The problem that the initial source must be located directly above the resonant structure,which is encountered by aforementioned traditional imaging systems,will not occur in the proposed system.Finally,to meet the increasing needs of scatterer imaging and by noticing that there are some intrinsic differences from source target imaging,a two-dimensional demodulation time reversal scatterer imaging scheme,which is based on the principle of spatial modulation of grating-like structure,is proposed.By further investigating the mechanism of evanescent wave conversion by grating-like structure quantitatively,the conversion efficiency is calculated.While introducing the imaging method,the difference between actual time reversal experiment and ideal time reversal cavity is considered,with the corresponding normalization method being proposed.By analyzing the one-to-one relationship between near and far field spatial spectra during evanescent wave conversion,a demodulation method is proposed to recover the evanescent field distribution.Then,the corresponding time reversal imaging method related to scatterer imaging application is proposed.Meanwhile,thanks to the flexibility of the auxiliary structure,the evanescent information of targets in different directions can be achieved by rotating the grating-like structure.Afterwards,the two-dimensional imaging of targets can be achieved by combining the information in all directions.Compared with time reversal imaging without demodulation,the imaging resolution of the method being proposed can be enhanced obviously,since more evanescent information have been used to form the image.