Terahertz (>0.3THz) active imaging systems

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2016
İdikut, Fırat
Imaging systems based on terahertz waves are becoming an integral part of commercial and military screening applications. In this thesis, the prototype of active scan THz imaging system was constructed for detection of concealed objects at standoff distance longer than 5m. The system was mounted on a platform that can adjust in height, tilt and azimuthal angle. The methods of generation and detection of THz signal are based on Schottky diode rectifiers and Schottky diode mixers. The wavelength of the continuous wave (CW) source is 880μm and measured output power was 7mW. The detector was Schottky diode based mixer and heterodyne detection method was used. The source and detector are constructed in a transceiver architecture using a directional coupler specially designed for this frequency. The leakage due to the directional coupler limited the dynamic range of the system to at most 20dB. The optical design of the active scan THz imaging system is based on Confocal Gregorian Geometry (CGG). In this geometry, the main elliptical mirror and secondary parabolic mirror were manufactured by machining bulk aluminum. The final optical system utilized 6 mirrors in total: a flat mirror to turn the beam, a short focal length parabolic reflector to collimate the beam, an XY scanning galvanometer mirrors assembly utilizing beryllium mirrors, and the main as well as secondary manufactured mirrors. Before testing the system, the output frequency of the source was determined by using dichroic filter, a metal mesh of patterned circular holes. The XY galvanometer mirror assembly allowed for imaging at 2 frames/s speed. The final constructed system allowed for imaging a 0.5m x 0.5m field of view with 1.5cm resolution at a standoff distance of about 6m. By using the prototype, it was shown that obscured objects causing specular reflections was successfully detected from standoff distance. A metal stick, behind clothes and having 3cm thickness, was imaged.
Citation Formats
F. İdikut, “Terahertz (>0.3THz) active imaging systems,” M.S. - Master of Science, Middle East Technical University, 2016.