Holger Quast

Continuous-wave all-optoelectronic terahertz imaging

We present an all-optoelectronic THz imaging system based on photomixing of two continuous-wave laser beams using photoconductive antennas. For a specific biological sample, we compare continuous-wave THz imaging and pulsed THz imaging at 1 THz with respect to data-acquisition time and signal-to-noise ratio, and discuss image formation from both amplitude and phase data. In addition, we introduce the application of hyperboloidal lenses which allow tighter focusing and a corresponding improvement in spatial resolution compared to off-axis paraboloidal mirrors.

All-optoelectronic continuous wave THz imaging for biomedical applications

We present an all-optoelectronic THz imaging system for ex vivo biomedical applications based on photomixing of two continuous-wave laser beams using photoconductive antennas. The application of hyperboloidal lenses is discussed. They allow for f-numbers less than 1/2 permitting better focusing and higher spatial resolution compared to off-axis paraboloidal mirrors whose f-numbers for practical reasons must be larger than 1/2. For a specific histological sample, an analysis of image noise is discussed.

3D-terahertz-tomography for material inspection and security

We demonstrate all-electronic 3D Terahertz tomographic imaging with an extraordinary dynamic range of higher than 40 dB combined with an extremely short acquisition time of 240 μs per pixel. Next to several application examples we discuss the potential for real-time operation for production line and security applications.

All-optoelectronic continuous-wave terahertz systems.

We discuss the optoelectronic generation and detection of continuous–wave terahertz (THz) radiation by the mixing of visible/near–infrared laser radiation in photoconductive antennas. We review attempts to reach higher THz output–power levels by reverting from mobility–lifetime–limited photomixers to transit–time–limited p–i–n photodiodes. We then describe our implementation of a THz spectroscopy and imaging–measurement system and demonstrate its imaging performance with several examples. Possible application areas of THz imaging in the biomedical field and in surface characterization for industrial purposes are explored.

Investigation of foam and glass fiber structures used in aerospace applications by all-electronic 3D Terahertz imaging

We investigate how all-electronic Terahertz (THz) / Millimeter wave imaging allows the evaluation of special material structures used in aerospace applications. One of the structures is a foam material intended for the isolation of cryogenic tanks, the other a glass fiber composite sample intended for several applications.

All-optoelectronic CW THz-imaging

Summary from only given. We demonstrate cw THz imaging with complex sample and compare the results with images taken with a state-of-the-art pulsed system based on an Ti:sapphire amplifier laser operating at 1-kHz repetition rate.

THz all-electronic 3D imaging for safety and security applications

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.

300 GHz imaging with 8 meter stand-off distance and one-dimensional synthetic image reconstruction

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).

Perspectives of Continuous-Wave Optoelectronic THZ Imaging

During the last fifteen years, tremendous advances in the ability to generate and detect THz (far-infrared) radiation optoelectronically have led to an increased interest in the application of this part of the electromagnetic spectrum in areas as diverse as solid-state physics, chemistry, microelectronics, and medical imaging. The progress of THz optoelectronics is based mainly on the efficiency of the photomixing techniques developed for optical femtosecond (fs) pulses which allow the generation and detection of broadband, coherent THz pulses with a frequency content from a few GHz to many THz. A great advantage of these techniques is the fact that both the generation and detection process are synchronized to the same laser source via optical gating thus enabling the sensitive measurement of both the amplitude and phase of the electric THz field in a quasi-stroboscopic way. The photoconductive or electrooptic photomixers are operated at room temperature thus ridding far-infrared spectroscopy of the burden of detection at cryogenic conditions. Tracing of the THz wave-form in the time domain permits the extraction of both the real and imaginary part of the dielectric function of the object under investigation.

3D-Terahertz-Bildgebung für die zerstörungsfreie Materialprüfung

Zusammenfassung Terahertz- und Millimeterwellen sind gesundheitlich unbedenkliche elektro-magnetische Wellen im Bereich von 100–10.000 Gigahertz. Bei diesen Frequenzen sind die meisten nicht-leitfähigen Materialien transparent, sodass eine kontaktlose, zerstörungsfreie Prüfung der inneren Struktur moderner (Kunststoff-Komposite-) Werkstücke mit einer Orts- und Tiefenauflösung im Submillimeterbereich möglich ist. Die hier vorgestellte vollelektronische Technologie ermöglicht hochqualitative Bilder in einer extrem kurzen Messzeit. Abstract Terahertz- and millimeter waves are electromagnetic waves with a frequency between 100 and 10000 Gigahertz. Most non-conductive materials are transparent at these frequencies, permitting a non-tactile 3D non-destructive testing of the inner structure of modern (plastic-composite-) materials with a sub-millimeter depth and spatial resolution. The all-electronic technology presented here delivers high-quality images in very short measurement times.