We report on the development of an active stand-off imaging system operating in the 80 GHz - 110 GHz frequency
range. 3D real-time imaging is enabled by a combination of a mechanically scanned one-dimensional conventional
imaging projection with a rotating metallic reflector and a two-dimensional synthetic imaging reconstruction with a
linear array of transmitter (Tx) and receiver (Rx) elements. The system is conceived, in order to allow a resolution better
than 1cm both in lateral, as well as in range directions by using a multi-view imaging geometry with an aperture larger
than 2 m x 2 m. The operation distance is 8.5 - 9 m. The 2D synthetically reconstructed imaging planes are derived from
the correlation of 20 sources and 24 coherent detectors. Range information is obtained by operating in a frequency
modulated continuous wave (FMCW) mode. Real-time imaging is enabled by implementing the synthetic image
reconstruction algorithms on a general purpose graphics processing unit (GPGPU) system. A multi-view imaging
geometry is implemented, in order to enhance the imaging resolution and to reduce the influence of specular reflections.
An active system for stand-off imaging operating in a frequency range from 234 GHz to 306 GHz is presented. Imaging
is achieved by combining a line array consisting of 8 emitters and 16 detectors with a scanning cylindrical mirror system.
A stand-off distance of 7-8 m is achieved using a system of mirrors with effective aperture of 0.5 x 0.5 meter.
Information about range and reflectivity of the object are obtained using an active FMCW (frequency modulated
continuous wave) radar operation principle. Data acquisition time for one line is as short as 1 ms. Synthetic image
reconstruction is achieved in real-time by an embedded GPU (Graphical Processing Unit).
The ability of terahertz and millimeter-wave imaging to detect suspicious hidden objects underneath or in luggage
has led to increased interest in these techniques. Several approaches have been demonstrated in the past few
years, amongst which active, all-electronic terahertz imaging has proven to be particularly adapted for safety
and security applications. It combines a large dynamic range and the ability to perform range measurements
with increased spatial resolution. At the French-German Research Institute of Saint Louis (ISL), we use an
all-electronic 3D imaging system for a comprehensive study on various suspicious objects and cloth types. We
demonstrate an enhanced detection capability for hidden suspicious objects if the range information is extracted
and visualized in appropriate ways.
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.
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.
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.
We report the realization of a hybrid system for stand-off THz reflectrometry measurements. The design combines the best of two worlds: the high radiation power of sub-THz micro-electronic emitters and the high sensitivity of coherent opto-electronic detection. Our system is based on a commercially available multiplied Gunn source with a cw output power of 0.6 mW at 0.65 THz. We combine it with electro-optic mixing with femtosecond light pulses in a ZnTe crystal. This scheme can be described as heterodyne detection with a Ti:sapphire fs-laser acting as local oscillator and therefore allows for phase-sensitive measurements. Example images of test objects are obtained with mechanical scanning optics and with measurement times per pixel as short as 10 ms. The test objects are placed at a distance of 1 m from the detector and also from the source. The results indicate diffraction-limited resolution. Different contrast mechanisms, based on absorption, scattering, and difference in optical thickness are employed. Our evaluation shows that it should be possible to realize a real-time multi-pixel detector with several hundreds of pixels and a dynamic range of at least two orders of magnitude in power.
A recent study initiated by the European Space Agency aimed at identifying the most promising technologies to significantly improve on the generation of coherent electromagnetic radiation in the THz regime. The desired improvements include, amongst others, higher output powers and efficiencies at increasingly higher frequencies, wider tunability and miniaturization. The baseline technologies considered revolve around Photomixing and novel laser based technologies compared to all electronic techniques. Some of the most significant findings will be presented together with technological developments and experimental results selected for medium to short term development. These technologies include advanced p-i-n photomixer with superlattice structures and, THz quantum cascade lasers. Recent results achieved in these fields will be put into the potential perspective for the respective technology in the future.
Terahertz imaging is an emerging modality, with potential for medical applications, using broadband sub-picosecond electromagnetic pulses in the range of frequencies between 100 GHz and 100 terahertz (THz). Images can be formed using parameters derived from the time domain, or at the range of frequencies in the Fourier domain. The choice of frequency at which to image may be an important factor for clinical applications. Image quality as a function of frequency was assessed for a terahertz pulsed imaging system by means of; (i) image noise measurements on a specially designed step wedge, and (ii) modulation transfer functions (MTF) derived from a range of spatial frequency square wave patterns. It was found that frequencies with larger signal magnitude gave lower image noise, measured using relative standard deviation (standard deviation divided by mean) for uniform regions of interest of the step wedge image. MTF results were as expected, with higher THz frequency signals demonstrating a consistently higher MTF and higher spatial frequency limiting resolution than the lower THz frequencies. There is a trade-off between image noise and spatial resolution with image frequency. Higher frequencies exhibit better spatial resolution than lower frequencies, however the decrease in signal power results in a degradation of the image.