N. Krumbholz, C. Jansen, M. Scheller, T. Müller-Wirts, S. Lübbecke, R. Holzwarth, R. Scheunemann, R. Wilk, B. Sartorius, H. Roehle, D. Stanze, J. Beckmann, L. von Chrzanowski, U. Ewert, M. Koch
We present a handheld fiber-coupled terahertz spectrometer operating at a center wavelength of 1550 nm. The key
elements are a fs-fiber laser, a fiber stretcher delay line and fiber-coupled antennas, which contain novel InAlAs-InGaAs
multi layer chips. First experimental data obtained with this system demonstrates its great potential and robustness. In
addition, we investigate different hazardous and harmless liquids in reflection geometry. These experiments show that
liquids are in principle distinguishable by terahertz spectroscopy. Finally, first steps towards an algorithm that allows for
an extraction of the liquids dielectric properties are discussed. The algorithm works for the analysis of reflection data
even if the liquid is located inside a container.
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.
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.
Dielectric mirrors are widely used in optical setups for spectral regions such as UV, visible, as well as IR. Yet, for the rapidly growing field of terahertz spectroscopy dielectric multilayer optics are sparsely utilized. But with low-loss materials high quality THz optics can be obtained. We present two approaches for the realization of highly effective dielectric THz mirrors. First, four thin slices of high-resistivity silicon and five common polypropylene (PP) foils were alternately stacked together to obtain a broad reflection band. This stop-band blueshifts with increasing angles of incidence. But due to the high index step between Si and PP a band from 0.32 to 0.375 THz always remains the stopband for all incidence angles and both the s- and p-polarization. The measurement data obtained in reflection and transmission geometry are reproduced well by numerical simulations. With a minor change of the layer sequence a microresonator is obtained which reveals a sharp transmission peak at around 0.3 THz within the reflection band. The second material system consists of ceramic laminates of alumina (A) and alumina-zirconia (AZ). Measurements on 12.5 pairs of A/AZ layers yield a strong stop-band from 0.3 to 0.37 THz at normal incidence, which again match numerical simulations. The big advantage of the ceramic mirror is the rugged, quasimonolithic design of the sintered multilayer structure.
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