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Terahertz imaging solutions for industrial non-destructive testing applications receive an increasing attention. Besides the need for real-time acquisition and image processing, 3D feature localization and large field of views of several tenths to a few meters are in the focus of interest. For this purposes sparse line arrays of transmitters (Tx) and receivers (Rx) offer an optimal approach to reduce costs, hardware complexity and potentially the required measurement time of the system, while providing a comparable imaging quality to full array based approaches. But in the presence of dominant specular reflections, sparse arrays can lead to an inhomogeneous illumination which impairs images and hampers their correct interpretation. We propose an experimentally verified method to simulative asses the design of sparse line arrays, taking in consideration the scattering characteristics of the object under test.

Imaging systems operating in the sub-terahertz frequency range have proven to be highly attractive devices for non-destructive testing applications. Especially frequency modulated system concepts have shown a great potential for volume inspection in industrial process and quality control, but often lack fast image acquisition times. We report on the development of a line array system for fast sub-terahertz imaging applications. The realized system employs a sparse array consisting of 12 emitters and 12 heterodyne receivers and operates in the W frequency band between 75 GHz and 110 GHz. The applied stepped-frequency continuous-wave radar concept provides 35 GHz modulation bandwidth for fast volume inspections of large test objects in combination with a band-conveyor.

Frequency modulated terahertz imaging systems have shown a great potential for volume inspections in the industrial process and quality control. However, many system concepts lack the capability to encompass a large field of view with fast data acquisition speed, while providing significant depth information. Based on our recently outlined system design of a terahertz imaging solution for the industrial process control, we report on the realization of a stepped-frequency modulated sparse array with 12 emitters and 12 heterodyne receivers operating in the frequency range from 75 GHz to 110 GHz. The system is ready to perform imaging tasks and will be in a final step combined with a band-conveyor for fast terahertz image acquisition.

By suitable design it is possible to achieve quasi-ballistic transport in semiconductor nanostructures over times up to the ps-range. Monte-Carlo simulations reveal that under these conditions phase-coherent real-space oscillations of an electron ensemble, generated by fs-pulses become possible in wide potential wells. Using a two-color pump-and-probe technique we have been able to observe this new phenomenon in excellent agreement with the theoretical predictions. Apart from its fundamental significance, ballistic transport in nanostructures can also be used for high-efficiency coherent THz-sources. The concept of these THz-emitters and its experimental confirmation will also be presented.

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 receive r (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 geomet ry 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. Keywords: THz, millimeter-wave, imaging, stand-off, FMCW, deramping, synthetic reconstruction. *matthias.kahl@uni-siegen.de

We investigated an all-electronic fmcw millimeter / Terahertz wave computed tomography system with short acquisition time and operating frequencies of 230–320 GHz or alternatively 70–110 GHz. The 3D computed tomography reconstruction of several measured examples is presented.

We present an all-electronic 3D THz imaging system employing synthetic reconstruction techniques with operating frequencies of 230–320 GHz or alternatively 70–110 GHz. We investigate the reconstructed 3D images of a few examples.

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).

We report on the realization of two active, fully electronic THz cameras, both operating at room temperature, but at different frequencies (645 GHz and 300 GHz, respectively). Active illumination together with the frequency-modulated-continuous-wave (FMCW) approach allows us to implement unique features such as phase-sensitive detection, suppression of spurious reflections and high resolution ranging. The measuring of the phase of the THz wave allows for sub-wavelength depth resolution while the ranging capabilities enable us to acquire an absolute depth map of objects under test with accuracy in the mm range. With both systems, we are able to acquire images with more than 55,000 pixels (phase and amplitude) in 9 seconds and the dynamic range exceeds 35 dB. The typical object distance is about 75–150 cm and the image size is in the order of hundreds of cm2 suitable for stand off detection of concealed weapons.

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 100cm 2 . 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.

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.

An active stand-off imaging system operating at 230–320 GHz is presented. Imaging is achieved by combining a line array consisting of 8 sources and 16 detectors with a scanning cylindrical mirror system. The stand-off distance is 8 meters and the effective aperture of the system is 0.5 meters by 0.5 meters. Range and intensity information of the object are obtained using an active FMCW (frequency modulated continuous wave) radar 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 data indicates a spatial resolution of about 2 cm and the range resolution of about 2 mm. To analyze the performance of the system, high-precision measurements employing a vector network analyzer are presented. An outlook on further developments is given.