| Terahertz time domain spectroscopy (THz-TDS) is a coherent measurement technology. Using THz-TDS, the phase and amplitude of the THz pulse at each frequency can be determined. Like Radar, THz-TDS also provides time information that allows us to develop various three-dimensional (3D) terahertz (THz) tomographic imaging modalities.; 3D THz imaging has potential in such application as non-destructive inspection. The broadband nature of the THz wave also allows the 3D THz imaging to extract spectroscopic information for a target. The interaction between a coherent THz pulse and an object provides rich information about the object under study, therefore, 3D THz imaging is a very useful tool to inspect or characterize dielectric and semiconductor objects. For example, 3D THz imaging can be used to detect and identify the defects inside a space shuttle insulation tile.; The 3D THz tomographic imagings we investigated are: terahertz diffraction tomography (THz DT), terahertz computed tomography (THz CT), terahertz binary lens tomography and terahertz digital holography. THz DT uses the THz wave as a probe beam to interact with a target, and then reconstructs the 3D image of the target using the THz waves scattered by the target. THz CT is based on geometrical optics and inspired from X-ray CT. THz binary lens tomography uses the frequency dependent focal length property of binary lenses to obtain tomographic images of an object. For a single frequency wave, an ideal binary lens is identical to a conventional lens. It images a 2D object with an object distance that satisfies the lens law. When the wavelength changes, the focal length of the binary lens changes and the object distance required by the lens law also changes, therefore, this binary lens will image another 2D object onto the same image plane. THz 3D holography combines Radar and conventional holography technology. By separating the multiple scattered THz waves of different scattering orders, we used a digital holography method to reconstruct the sparely distributed scattering centers.; In this dissertation, we studied the 3D THz imaging optics based on the electromagnetic wave theory. We proposed and demonstrated the method to realize high resolution THz CT. THz CT with a resolution better than 0.5 mm has been achieved. We investigated the relationship between THz imaging system and the reconstructed images in THz DT, THz binary lens tomography and 3D THz holography. This research can be used to develop a fast THz tomography system, which has particular potential applications in security screening and in situ noninvasive inspection. |
