Three-Dimensional Terahertz Continuous Wave Imaging Radar for Nondestructive Testing

In recent years, our research group has been devoting substantial efforts to the research and development of active all-solid-state electronic terahertz (THz) continuous wave imaging systems for nondestructive testing, which is currently benefitting from the increasing amount of transmitting power, high performance/cost ratio and adaptability to engineering. In this paper, an in-house developed broadband linear frequency modulated continuous wave (LFMCW) three-dimensional (3D) THz imaging system is described, and two sets of experimental platforms are set up to assist the planning and completion of the research, including a narrow-band LFMCW 3D imaging radar and a wide-band stepped-frequency modulated continuous wave 3D imaging radar. For 3D imaging systems, to cope with demanding scenarios and to achieve excellent imaging performance, various reconstruction algorithms are explored. The first is a spectral refinement and correction approach based on fast Fourier transform and modern spectral estimation for accurate thickness measurement. The second is the synthetic aperture radar imaging algorithm for surface detection or internal detection of objects with lower refractive index. The third is a 3D reconstruction algorithm based on half space Green's function and the exploding source model for the interior detection of materials with higher refractive index. The fourth is the frequency interference algorithm combining phase unwrapping to measure uneven and nonplanar surfaces. Exploiting these systems, along with the associated experimental platforms and reconstruction algorithms, we successfully implemented non-destructive testing for objects with various defects and of different materials, such as polymer boards with voids, and foam with inclusions.