Active THz imaging system with improved frame rate

We present a scanning THz-camera with active illumination. Three different fully electronic transceiving techniques are evaluated: The first employs a commercial 230-320GHz frequency-modulated continuous-wave system with a harmonic mixing detector. Its bandwidth allows a ranging resolution in the mm-range. The second one is based on heterodyne detection operating at 645GHz with sub-harmonic mixer and provides a dynamic range beyond 100 dB. Like in the first system, we employ local illumination (THz beam focused on observed pixel). The third one equals the second one, but utilizes global illumination of the scene. In all cases, the scanning optics consists of a Cassegrainian telescope with a primary mirror diameter of 23 cm and a scanning mirror, which is spinning about a slightly tilted axis which itself is slowly rotated in a perpendicular direction to provide the second scan-dimension. With a typical distance of 0.5m between the scanning mirror and the object plane, the field of view covers several 100cm2. While the fast mirror axis spins with about 660RPM, the slow axis turns with at least 1 deg. per second and the data acquisition samples about 40000 points for each THz-image. Single-pixel detectors are used; the frame acquisition time is below 10 s. The development of a video-rate multi-pixel imager with up to 32 sub-harmonic mixers as detectors is in progress.