We present a compact, robust, and transportable fiber-coupled THz system for inline monitoring of polymeric
compounding processes in an industrial environment. The system is built on a 90cm x 90cm large shock absorbing
optical bench. A sealed metal box protects the system against dust and mechanical disturbances. A closed loop controller
unit is used to ensure optimum coupling of the laser beam into the fiber. In order to build efficient and stable fiber-coupled
antennas we glue the fibers directly onto photoconductive switches. Thus, the antenna performance is very
stable and it is secured from dust or misalignment by vibrations. We discuss fabrication details and antenna performance.
First spectroscopic data obtained with this system is presented.
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.
We present two-dimensional photonic-crystal waveguides for fluid-sensing applications in the sub-terahertz range. The
structures are produced using a standard machining processes and are characterized in the frequency range from 67 to
110 GHz using a vector network analyzer. The photonic crystal consists of an air-hole array drilled into a high-density
polyethylene block. A waveguide is introduced by reducing the diameter of the holes in one row. The holes can be
loaded with liquid samples. For all structures we observe photonic band gaps between 97 and 109 GHz. While the pure
photonic crystal shows the deepest stop band (28 dB), its depth is reduced by 5 dB when inserting a waveguiding
structure. The depth of the photonic band gap is further reduced by several decibels depending on the refractive index of
the liquid that is inserted. With this type of fluid sensor we can clearly distinguish between cyclohexane and
tetrachloromethane with refractive indices of 1.42 and 1.51, respectively. The results are in good agreement with
theoretical calculations based on the 2D finite-difference time-domain (FDTD) method.
To evaluate the potential of THz imaging systems for mail and luggage inspection we study a set of letters containing different hazardous items. The samples are investigated with three different THz systems available in our group: A microwave based system working around 100 GHz, a THz time-domain system and a THz gas laser. We provide a comparative discussion on our results and the advantages and disadvantages of each system.