Active terahertz imaging with high acquisition rates and its application for moisture sensing

In this paper we give a short introduction on THz imaging and exemplarily present possible applications in the paper fabrication and in the security sector. Then we describe a method for depth imaging with a frequency-swept continuous-wave (FMCW) electronic THz source. In the electromagnetic spectrum, the frequency range between microwaves and the infrared is often called the THz gap, because it has been rarely used in technology, because of a lack suitable sources and detectors. One can approach this gap from both sides, as from high-frequency electronics or longwavelength optics. Some properties make THz waves interesting for applications. First, due to the low photon energy, the risk of harm through ionization seems negligible. Many materials like paper, plastics and fabrics are transparent to THz radiation, while water absorbs and metals reflect the radiation. Due to its long wavelengths, the radiation is insensitve to surface roghness. Finally, many organic substances have characteristic spectroscopic signatures in the THz range. Several possible applications of THz radiation in imaging have been demonstrated, taking advantage of one or more of the properties mentioned in the paragraph above. Sections 2 and 3 give two examples, while Section 4 demonstrates a way for depth imaging.