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.
We report on the realization of two active fully electronic THz cameras operating at different frequencies (645 GHz and
300 GHz) and room temperature. Active illumination together with the frequency modulation continuous wave approach
allows us to implement unique features, such as phase-sensitive detection, working-distance selection and the
suppression of spurious reflections. With both systems we are able to acquire images with more than 50000 pixels (phase
and amplitude) in 9 seconds. The dynamic range exceeds 35 dB and we achieve a subwavelength depth resolution due to
the measurement of the phase. The typical object distance is about 50-100 cm and the image size is on the order of
hundreds of cm2. With frequency modulation of the source we are furthermore able to detect the position form objects up
to an accuracy of a few mm.
The development of active THz cameras with the potential for video-rate operation is an emerging and exciting research field. With our currently realized 645 GHz system we achieve scan rates of a few seconds with a one-pixel heterodyne detector and two-dimensional fast rotational scanning. The active illumination allows to resolve the object topography with subwavelength resolution. Within the next evolution step we will realize an active 812 GHz system incorporating a 1x32-pixel heterodyne detector array with one-dimensional scanning. This will allow video-frame-rates for images (amplitude and phase) with approximately 2000 pixels. But for large fields of view the quasioptical system must be optimized to minimize the aberrations inherent in all optical systems. We show, with the use of the optical software package Zemax, how to design, simulate and optimize such quasioptical systems for the one-dimensional 1x32-pixel heterodyne detector array. The resulting quasioptical system is diffraction-limited over the field of view (20 cm x 30 cm) at the design working distance of 4 m and has an adjustable focus optics for distances from 2 m up to 6 m.