One of the most prominent applications of terahertz time-domain spectroscopy is the spectral investigation of materials covered by visibly opaque objects. Therefore, terahertz waves are well suited to inspect the content of mail. We report on our work on mail inspection in this spectral range including machine design, optical layouts, data analysis, and implementations.

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.

Letter bombs are an increasing problem for public authorities, companies and public persons. Nowadays every big company uses in his headquarters inspection system to check the incoming correspondence. Generally x-ray systems are used to inspect complete baskets or bags of letters. This concept which works very fine in big company with a large postal center is not usable for small companies or private persons. For an office environment with a small number of letters x-ray systems are too expensive and oversized. X-ray systems visualize the wires and electric circuits inside the envelope. If a letter contains no metallic components but hazard materials or drugs, the dangerous content is invisible for the most low-cost x-ray systems. Millimeter wave imagining systems offer the potential to close this gap.

Combining different chemometric tools we demonstrate the reliable identification of substances out of real-world terahertz-spectra. Systematic evaluation of the spectra allows for the distinction between spectral properties arising from samples geometry versus true material absorption.

Terahertz (THz) spectroscopy is a powerful technique for identification of dangerous materials like drugs and explosives, especially when hidden under optically opaque concealments. Multiple-reflections within such sample systems lead to additional spectral features which could be misinterpreted as artificial material absorption bands. We show that combinations of different chemometric tools help reducing such misinterpretations. However, an angle-depending detection scheme leads to a much more powerful artifact suppression. We received experimental data which are in good agreement with our numerical predictions. Hence, the overall false alarm rate of our pattern-recognition algorithm is significantly reduced.

Recently the essential amino acid tryptophan has attracted attention in cancer research, as its metabolism regulates antitumor immune responses and tumor-intrinisic properties. Measurement techniques to determine tryptophan concentrations of aqueous solutions are therefore vastly important for ongoing research in this field. Recently, Terahertz spectroscopy has illustrated its high potential to be utilized for the characterization of bio-crystals and bio-molecules. We have developed a method to detect and quantify tryptophan based on the parallel-plate waveguide (PPWG) technology together with a commercially available terahertz time domain spectroscopy (TDS) system called “T-SPECTRALYZER F” providing a spectral bandwidth from 0.1 THz to 5 THz. As Terahertz waves are strongly absorbed by water, a measurement of aqueous solutions is a challenging task. In our setup, parallel-plate waveguides are used to detect low tryptophan concentrations, in principle, in solution. Drop-casting the solution into the waveguide forms a dry homogeneous film after evaporation of the solvent. A spectroscopic analysis of the transmission spectrum of the waveguide allows for a determination of the tryptophan concentration as the detection limit is drastically improved by the use of waveguides. In order to increase the detection sensitivity of this measurement technique the terahertz setup was encapsulated in a dry air box to reduce water vapor effects. Here we introduce the working mechanism of “TSPECTRALYZER F” and present the spectral evaluation procedures applied. Finally, we show the improvement of the detection sensitivity using a terahertz time-domain spectroscopy system together with PPWG technology. © (2017) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.