3D THz-TDS SAR Imaging by an Inverse Synthetic Cylindrical Aperture

Terahertz time-domain spectroscopy (THz-TDS) is a promising tool for high-resolution 3D imaging of objects due to the high center frequency and bandwidth compared to microwave systems. In addition, terahertz waves have a higher penetration depth than visible or near-infrared radiation. Typically, optics are used to focus the terahertz radiation onto an object under test. This limits the imaging capability in the axial dimension to the depth of field and limits simple imaging of complex surfaces. In this work, we adapt a backpropagation algorithm from synthetic aperture radar (SAR) imaging to reconstruct high-resolution 3D images from time-domain traces acquired with a lensless THz-TDS system. For this purpose, an inverse cylindrical aperture is used and an equation that describes the maximum achievable resolution as a function of the beam pattern, the bandwidth, and the length of the synthetic aperture is derived. The calculated resolution is $960 \mu \text{m}$ for the linear dimension of the aperture and 75 $\mu \text{m}$ for the rotationally symmetric dimension of the cylindrical aperture for a bandwidth of $2\,\text {THz}$ . The resolving power is verified by measurements on metallic and dielectric objects. In the future, this method can be used for non-destructive testing of objects with complex shaped surfaces and internal structures.