Research On Nondestructive Testing Technology Based On Terahertz Imaging

Nondestructive testing technology plays a critical role in guaranteeing product quality and early warning of equipment failure,and is an essential and effective tool for industrial development.In recent years,terahertz imaging technology has attracted more and more attention due to its unique advantages and has been widely used in biomedical imaging,security inspection,radar detection,nondestructive testing and other fields.Therefore,it is of great significance to study the application of terahertz imaging technology in nondestructive testing.In this thesis,the research status of terahertz nondestructive testing technology at home and abroad is firstly investigated,mainly including the nondestructive testing based on terahertz pulse wave imaging and terahertz continuous wave imaging.According to the current research progress,terahertz time-domain spectroscopy imaging,terahertz computer tomography imaging and terahertz frequency modulated continuous wave imaging are used to investigates the nondestructive testing technology.The main research topics are as follows:(1)A reflective all-fiber terahertz time-domain spectroscopy imaging system was built using a terahertz fiber-optic time-domain spectroscopy system and a two-dimensional translation stage and variable angle device.Experiments were conducted on thinner samples such as chips and access cards,and the internal structures of them were successfully imaged with high imaging resolution,but the method has a long scanning time and is not suitable for imaging thick samples.(2)The imaging principle of terahertz computer tomography imaging and the filtering back projection algorithm for image reconstruction were analyzed.Then a terahertz computer tomography imaging system was built by using a vector network analyzer and a rotating translation platform.The two-dimensional cross-section imaging of the PMI foam with inclusions and cavity defects was performed,and it was found that the interior of thicker objects could be imaged.However,this method is not suitable for the detection of samples with large refractive index and the scanning time is long.(3)The working principle of linear frequency modulated continuous wave is analyzed,and a terahertz frequency modulated continuous wave imaging system is designed using a vector network analyzer as the transmitting source,with a frequency modulated bandwidth of 140 GHz and a range resolution of 1.095 mm in free space.In order to improve the distance measurement accuracy,a spectrum correction algorithm and spectrum zooming algorithm based on fast Fourier transform is proposed.The imaging methods include real aperture imaging,which directly images the extracted echo data,and synthetic aperture imaging,which uses imaging algorithms to achieve digital focusing for imaging.Two frequency domain synthetic aperture imaging algorithms,range shift migration algorithm and phase shift migration algorithm,are proposed for image reconstruction.Finally,according to the established imaging system,three different samples are inspected by the above methods.The results show that the focal plane imaging of the focused case has the highest azimuthal resolution and is suitable for two-dimensional high-resolution imaging.The azimuthal resolution of synthetic aperture imaging is also very good,and it can achieve better focusing effect on the sample in a thicker range,which is suitable for the three-dimensional image reconstruction of the target.In summary,terahertz time-domain spectroscopy imaging system,terahertz computer tomography imaging system and terahertz frequency modulated continuous wave imaging system were built to successfully achieve high quality imaging of different types of materials in this thesis.The feasibility of these schemes for nondestructive testing is verified,which provides an effective way for nondestructive testing in different situations.