Research On Terahertz Frequency Modulation Continuous Wave Imaging Technology

Terahertz(THz)frequency modulated continuous wave(FMCW)imaging system has many advantages such as high range resolution,good receiving sensitivity and simple structure.These advantages make it have a broad application prospect in the fields of industrial nondestructive testing and human security inspection.However,there are many factors restricting the wide application of THz FMCW imaging technology in the practical industrial application,such as the frequency nonlinearity of the transmitting signal of the THZ FMCW system,the fringe noise of the THz frequency modulated continuous wave grating scanning imaging system and the imaging speed of THz FMCW imaging system restrict.This paper studies the above problems and puts forward correspkonding solutions.The research results of this paper provide theoretical and technical support for the wide application of THz nondestructive testing(NDT).The work of this paper mainly includes the following aspects:Firstly,the nonlinear causes of the THz FMCW system’s output signal are analyzed,and a new method is proposed to estimate the frequency spectrum corresponding to the target point by windowing Fourier transform and perform non-linear correction of the THz frequency modulated continuous wave system’s output signal according to the estimated frequency.In the 3rd chapter,the nonlinear correction method is verified by numerical simulation and real experiment.In this paper,the verification experiment proves that the output beat frequency spectrum after nonlinear correction can accurately indicate the position of the target in the 60 mm neighborhood of the reference position in the theoretical resolution range,and the signal to noise ratio of the target signal peak is high and clearly discernible.Besides,the system can effectively carry out three-dimensional(3D)imaging of the two-layer structure of the sample and the results of 3D imaging can truly reflect the information of each layer of the sample and accurately reflect the optical path between each layer,after nonlinear correction.Secondly,the causes of fringe noise in the imaging results of THz FMCW grating scanning imaging system are analyzed and an algorithm based on two-dimensional continuous wavelet transform is proposed,according to the characteristics of two-dimensional continuous wavelet transform,to extract the defect region from the THz nondestructive testing results under the fringe noise.The validity of this method is verified by conducting nondestructive testing experiments with Prefabricated defect samples.The experimental results show that the proposed algorithm can effectively and accurately extract the defect contour from the imaging results of the THz FMCW grating scanning imagi1 ng system under the fringe noise.Thirdly,the principle of the THz FMCW rotation mirror imaging system is analyzed,and its imaging optical path is given.A method of three-dimensional image reconstruction based on the THz FMCW rotation mirror imaging system’s scanning data is presented.