Research On Theoretical And Experimental Study Of Terahertz Tomography

Terahertz radiation,also known as terahertz wave,sub-millimeter wave or far-infrared electromagnetic radiation,the frequency of electromagnetic waves in this band is between 0.1THz ~ 10THz(wavelength is 3mm ~ 30?m),and its wavelength should be less than millimeter wave,between infrared light and millimeter wave,so it has the characteristics of microwave and infrared.Similar to visible light,infrared radiation,X-rays and other radiations,it can be used as a light source for imaging objects.Because terahertz waves have a strong ability to penetrate non-polar materials,they can be used for imaging and non-destructive analysis of the internal structure of objects.Thanks to this feature,terahertz imaging has great application potential in the fields of security inspection,aerospace,and non-destructive testing of materials.Moreover,as a new imaging method,it can complement other imaging methods.Tomography technology is an imaging technology that can three-dimensionally image the internal structure and material distribution of an object.In other wavebands,this technology has been effectively applied,such as X-ray CT technology which is widely used in the field of medical security,OCT technology,and so on.Terahertz also has good penetrating power to objects,so these technologies can be also applied to the terahertz band.Unlike X-rays,since terahertz has only 4meV of extremely low photon energy,the terahertz wave will not produce ionization effect on the detected objects and has high security,so it can be effectively used as a supplementary detection method for X-CT.This paper mainly conducts theoretical and experimental research on terahertz tomography,that is,transmission terahertz tomography and reflective terahertz tomography,and analyzes the factors that affect resolution.Among them,the transmission type terahertz CT imaging technology uses terahertz impatt solid-state source to generate continuous waves,transmit the measured object,and use a Golay-Cell detector for signal detection.It has the characteristics of high power and high sensitivity.It uses a filtered back projection algorithm and simultaneous algebraic reconstruction technique process the experimental data to obtain the sample tomogram.The terahertz reflection tomography uses the terahertz TDS system to produce terahertz pulse to radiate the object and detect the distribution of different structures below the surface depth of the object,a novel PPEX algorithm was used to process the signal reflected back to the detector and reconstruct the fault structure below the surface depth of the sample measured.The main research contents and main work of this paper are as follows:1.Investigate the theory of terahertz tomography.Starting from Maxwell's equations,the imaging fundamental wave equation is obtained,and the nature of the terahertz tomography problem is explained.Then two approximations are used to linearize the problem simplify.2.Based on the terahertz tomography model,the central slice theorem and other related theories,use Matlab to write a filtered back projection algorithm(FBP)and simultaneous algebraic reconstruction technique(SART)to restore the projection data collected in the experiment.The classic brain Phantom is used to simulate and verify the two algorithms.3.Set up a transmission tomography system using experimental equipment such as Impatt solid-state source,Golay-Cell detector,etc,and use Labview language to write related control and data acquisition program,and perform scanning experiments on various types of samples.Using the above two algorithms,the tomographic image reconstruction of multiple samples was successfully implemented.4.According to related theories,a reflection tomography system was built using the terahertz time-domain spectroscopy system,and a relatively new algorithm was introduced to conduct experiments on multi-layer paper samples.The position and shape of the paper are restored,and the pattern on the paper is also restored.