Broadband Millimeter-Wave Imaging Radar-Based 3-D Holographic Reconstruction for Nondestructive Testing

A self-designed all-solid-state electronic 3-D millimeter-wave imaging radar is demonstrated and applied in holographic imaging for nondestructive testing of polymer materials. Full heterodyne detection is implemented in the system, which is based on a sawtooth linear frequency-modulated continuous wave, with a center frequency of 195 GHz, a bandwidth of 42 GHz, and an average power of 0.5 mW. A suitable calibration method is employed to solve the problem caused by the nonlinearity of frequency modulation. Performance parameters are established experimentally, including range resolution (3.2 mm, better than theoretical value) and lateral resolution, ranging from 1.45 to 4 mm at imaging distances from 10 to 70 mm. Simulation and experimental studies ascertain the effectiveness of the system in examining internal defects in combination with imaging algorithms, a classical range migration algorithm for surface reflection based on free-space Green’s function, and a modified subsurface imaging algorithm based on a rigorous half-space Green’s function. For surface defects and defects inside the material whose refractive index is close to air, the performances of both are similar. However, in imaging interior defects buried in a medium with substantially different refractive indices, the modified algorithm is superior, as illustrated by testing of Teflon blocks containing internal voids. Furthermore, convincing experimental evidence shows that the system can image slightly bent and tilted samples.