Research And Development Of Three-dimensional Terahertz Continuous Wave Imaging Radar For Nondestructive Inspection

Nondestructive testing(NDT)technology plays an important role in ensuring product quality and providing early warning against operation failure.Previous researches have developed several detection methods,including X-ray method,ultrasonic detection,magnetic powder,eddy current,infrared thermal image,laser holography and microwave detection.Each of them has its own application occasions and limitations.In recent years,terahertz(THz)band of the electromagnetic spectrum has attracted increasing attention,because of its unique advantages,and terahertz imaging technology has developed into a new non-destructive testing approach,supplementing traditional NDT technologies mentioned above.The terahertz band covers the frequency region of 100 GHz ~ 10 THz,between infrared and microwave,encompassing the transition zone from electronics to optics.Due to the particularity of this frequency band,THz imaging technology has many unique and excellent characteristics in NDT.Its photon energy is low,especially compared to the widely used X-ray method in industry.So it will not produce harmful photoionization in biological tissues and is safe for test materials and operators.It has strong water absorption.Even with high-power radiation,the damage to human body is only limited to the skin.It is highly penetrating and is not susceptible to Mie scattering.For most dry and nonpolar dielectrics such as foam,plastics,wood,and ceramics,it is more translucent than visible and infrared light.But it is not as indistinguishable from the dielectric as X-rays due to excessive penetration.In addition,its spatial resolution is high and the wavelength is short enough to provide millimeter or sub-millimeter resolution,which can meet the needs of most NDT applications.With careful consideration of the transmission loss,output power,detection depth,working time,stability and engineering practicability,a three-dimensional(3D)THz continuous wave(CW)imaging radar is proposed and designed in this dissertation.The main research contents are as follows:The type and principle of CW system are analyzed,and the whole system structure is determined.For pursuing 3D imaging,with fast frequency sweep and the best anti-interference ability to detect low-speed or static targets,wideband linear frequency modulated continuous wave(LFMCW)is selected as the radiation waveform.Due to the extraordinary difficulty of designing stable local oscillator and fundamental mixer in THz band,and considering the signal to noise ratio and dynamic range,the super heterodyne detection structure is determined to have the most advantages through comparative analysis.In order to improve ranging accuracy and 3D imaging performance,a fully coherent form for THz band is proposed.With consideration of the application background of NDT,a single transceiver antenna design is adopted,which not only improves the accuracy of ranging and locating,but also has high practicability.Finally,for solid-state electronic technology,the THz frequency is extremely high,and two generation methods are designed.One is to achieve THz band directly by frequency multiplication from the microwave,which is regarded as the form of full multiplication.The other is that millimeter wave is generated by multiplication of each of the two low frequencies,and then this signal is multiplied to THz band,which is denoted as the form of frequency mixing-multiplying.In terms of cost performance and engineering practicality,the latter becomes the first choice.After the overall structure determined,the parameters of the 3D THz imaging radar are formally proposed as the main research objective.Due to the expensive solid-state electronic components and few references for system development in the THz band,two sets of verification platforms have been established using existing instruments and devices in the laboratory before finalizing the hardware design,which help to explore the feasibility of ultra-wideband,analyze the selection of center frequency and prove the correctness and effectiveness of LFMCW,frequency mixing-multiplying technology and fully coherent super heterodyne structure.And both are tested with actual samples to fully verify the overall structure.Then,the hardware schematic diagram of the 3D THz imaging radar is designed,the output results of whole system are pre-studied for the first time by ADS software to determine the performance and parameter of each component,which also proves the validity of the schematic diagram.Among them,the most special is the microwave linear-frequency modulation source,which is also the most important and the most difficult to realize.According to the parameter requirements,it is designed independently based on the ultra-wideband phase-locked frequency synthesis technology and processed by professionals,thus realizing ultra-large bandwidth terahertz solid-state electronic system under the existing domestic technology.All devices are tested one by one to confirm that they satisfy the requirements of each link.Afterwards,they are connected to make up the entire hardware.In the meantime,the branch links are tested to ensure the smooth and correct signal transmission.At last,the whole hardware system is debugged and verified,and the key parameters such as transmitting power,dynamic range,wideband spectrum linearity,system stability and spatial resolution are tested.Meanwhile,in order to improve the spectral linearity and ranging accuracy,the factors affecting the spectrum linearity are analyzed based on time-frequency analysis and the system calibration process is designed.Finally,the 3D THz imaging system was implemented with a center frequency of 200 GHz,a bandwidth of up to 47 GHz,an average power of 0.5 mW,a dynamic range of more than 60 dB,a frequency sweep time as short as 2.8 ms,and an operating distance of more than 1 m.In real aperture,its cross-range and range resolution are 2.1 mm and 3.2 mm respectively,and the cross-range resolution can be further improved to 1.5 mm by a synthetic aperture radar imaging algorithm.At present,it has the largest bandwidth,the highest bandwidth center frequency ratio and the best range resolution near 200 GHz in the world.At the same time,it has a leading frequency sweep rate.Besides the hardware,it is indispensable to explore data processing and image reconstruction algorithms,so as to realize 3D imaging.In this dissertation,the common applications of NDT are discussed,which are divided into four categories and corresponding algorithms are developed.The first is the accurate measurement of the object's thickness and distance with wide ranges.Spectrum correction and refinement based on Fast Fourier transform and modern power spectral estimation are presented.The second is surface detection or internal detection of objects with low refractive index.Two frequency-domain synthetic aperture radar algorithms based on free-space Green's functions are employed.The third is the interior detection of materials with large refractive index.A new modified algorithm based on half space-Green's functions and explosion source model is proposed.The fourth is the measurement of the surface morphology of inclined or curved objects.A new algorithm combining frequency interference with phase unwrapping is proposed and demonstrated,which breaks through the limitation of bandwidth and can accurately detect objects with certain inclination and curvature.Electromagnetic simulation of the above-mentioned scenarios are carried out based on the finite difference time domain approach,and the corresponding algorithms are applied to verify their effectiveness.For a variety of applications of NDT,data processing and reconstruction algorithms are discussed in detail,and algorithms from other fields are innovatively applied to terahertz imaging detection.In particular,the third and fourth algorithms are newly proposed or substantially improved on the basis of the original algorithms,and their imaging performance is shown to be significantly better.In the above algorithms,refractive index is a requisite input parameter.Therefore,Fresnel formulation analytical method based on terahertz time-domain spectral system is used to solve for the refractive index of a given sample.Finally,the hardware and reconstruction algorithms are combined: all kinds of actual samples are irradiated and scanned by the hardware to obtain the intermediate frequency signals,and then the appropriate reconstruction algorithm is selected to realize their 3D imaging and the imaging results are excellent.The self-developed 3D THz imaging radar has the advantages of the ultra wideband,high bandwidth center frequency ratio,great range resolution and fast frequency sweep.And the data processing and reconstruction algorithms have the directivity and preciseness.These lay the foundation for the realization of high-quality 3D imaging and provide an effective way for nondestructive testing of non-metallic materials.