We demonstrate concepts for compact and cost effective THz technology based on semiconductor diode lasers. In detail,
we analyze diode laser based THz sources and detectors. Continuous wave THz radiation is generated by two color diode
lasers either with external photomixers or direct difference frequency generation in the diode laser. For time domain THz
sampling applications we present a suitable mode-locked diode laser system. Further we present a method to detect THz
radiation with diode lasers at room temperature: A THz signal coupled into the active region of a diode laser results in a
variation of the voltage across the p-n-junction.
We demonstrate the detection of metallic and nonmetallic foreign bodies in chocolate using pulsed terahertz imaging. Investigating the shape of the temporal waveform allows for the discrimination between wanted ingredients like nuts on one hand and foreign bodies like stone, glass, or plastic particles on the other hand. Yet, the intensity image alone does not provide enough information to evaluate the quality of the chocolate bar. To achieve a low false-alarm rate it is important to measure the height profile of the sample and to include the measured results in the image-processing step. Our results show that terahertz imaging can be used for the detection of contaminations in chocolate bars. Furthermore, other kinds of dry food can be investigated with our technique.
The worldwide production volume of polymers is still rising exponentially and the number of applications for plastic
components steadily increases. Yet, many branches within the polymer industry are hardly supported by non-destructive
testing techniques. We demonstrate that terahertz (THz) spectroscopy could be the method of choice to ensure high-quality
polymer products. Applications range from the in-line monitoring of extrusion processes and the quality control
of commodities in a mass production up to a total inspection of high-tech safety relevant products. Furthermore, we
present an extension to THz time-domain spectroscopy in the form of a new data extraction algorithm, which derives the
absorption coefficient, the refractive index and the thickness of a sample with very high precision in a single pass.
Apart from that, we discuss the ability of THz systems for quality control of polymeric compounds. Here, it is essential
to monitor the additive content as well as additive inhomogeneities within the mixture. Recently, we built a fiber-coupled
THz spectrometer for in-line monitoring of compounding processes. Additionally, we demonstrate the potential of THz
systems for the non-destructive and contactless testing of structural components. THz imaging is capable of analyzing
material thicknesses, superstructures, the quality of plastic weld joints, and of detecting flaws in components.
Plastics and THz form a very fruitful symbiosis. In return, plastics industry can provide THz systems with custom-tailored
components, which have very attractive properties and extremely low costs. Examples of this development are
photonic crystals or polymeric Bragg filters, which have recently been demonstrated.
Polymers cover the whole range from commodities to high-tech applications. Plastic products have also gained in
importance for construction purposes. This draws the attention to joining techniques like welding. Common evaluation of
the weld quality is mostly mechanical and destructive. Existing non-destructive techniques are mostly not entirely
reliable or economically inefficient. Here, we demonstrate the potential of terahertz time-domain spectroscopy imaging
as a non-destructive testing tool for the inspection of plastic weld joints. High-density polyethylene sheets welded in a
lap joint with varying quality serve as samples for terahertz transmission measurements. Imperfections within the weld
contact area can clearly be detected by displaying the transmitted intensity in a limited frequency range. Contaminations
such as metal or sand are identified since they differ significantly from the polymer in the terahertz image. Furthermore,
this new and promising technique is capable of detecting the boundaries of a weld contact area. Aside from revealing a
contrast between a proper weld joint and no material connection, the size of an air gap between two plastic sheets can be
determined by considering the characteristic frequency-dependent transmission through the structure: The spectral
positions of the maxima and minima allow for the calculation of the air layer thickness.
In this paper we show that the real emission frequency of a photoconductive dipole antenna operating in
conjunction with photomixers is not its natural resonant frequency, but a frequency where the antenna's input resistance
is the highest and the corresponding mismatch with the source is the smallest. We also introduce a new kind of antenna
that offers a much higher input resistance to the photomixer and hence enhances the efficiency of the continuous-wave