| Scope and method of study. The purpose of this study was to demonstrate, characterize, and theoretically model a novel imaging system operating in the frequency range of approximately 0.1 to 2.5 terahertz (THz). Aside from optoelectronic THz generation and reception, this quasi-optic system uses optical methods exclusively for imaging, unlike other existing THz imaging systems. This allows high-resolution imaging without the need for image reconstruction. A large aperture spherical telescope mirror is used form a complete, real, diffraction limited, THz image of an object, which is then sampled. Since the THz illumination is in the form of transform-limited pulses, the system also provides range information which permits three-dimensional imaging. The data produced by the system are coherent, time-domain, electric-field images with two transverse dimensions and one temporal dimension.; Since the system is coherent, it is also shown that synthetic aperture techniques can be used to further increase transverse resolution. This extension is called synthetic phased-array THz imaging and is a method whereby multiple, diffraction-limited, coherent THz images are recorded and later superposed to form a sharper image.; Findings and conclusions. The images of several steel objects are presented and illustrate the coherent resolution features. Based on the normal configuration, the system is able to easily resolve objects separated by less than 1.4 mm in the transverse directions and about 80 mum in the range direction. Several phased-array images are also presented and show a dramatic increase in the transverse resolving power of the system. In the phased-array configuration, the system is shown to resolve two objects transversely separated by only 391 mum.; Finally, three theoretical models, which accurately describe the THz imaging system, are presented. All three models employ both geometric and diffractive optics methods to generate theoretical images that are found to agree very well with the experimental data. |
