- Start: 01.07.2008
- Duration: 30 +12 months
- Funding: FWF-Translational Programme
- Project Nr. L 507-N520
- Goal: Developement of a polarization-sensitive, scanning THz-System
THz-Imaging System for Industrial Material Investigations
One main goal of the FWF-Translational project “THz-Imaging System for Industrial Material Investigations” was the development of a specific THz-imaging system for material investigations. The system, which was realized as a laboratory setup, uses pulsed THz radiation and operates in reflection mode. It therefore enables depth-resolved imaging of samples. A schematic of the setup is shown in Fig. 1 left. One specific feature is the use of a large-aperture, two-axis galvano-scanner for raster-scanning of the THz-beam, in contrast to common systems where mainly the sample is moved. Thus, also bulky samples can be investigated without any problems. Furthermore, the system allows recording of transversal images parallel to the sample surface, in addition to conventional cross-sectional images (see Fig. 1 right).
The developed system exhibits further advantages: for optimization, a specific THz-scanning lens as well as a novel measurement routine was developed, which enables adjusting of the phase of the THz-pulse during the measurement. By this, the curvature of the image plane (in one direction; resulting from the use of a two-axis galvano-scanner) can easily be eliminated directly during the measurement. The effect of phase correction is shown in Fig. 2 for a test sample (left: without, right: with phase correction). Consequently, imaging of planar structures can be performed more efficient and up to 20 times faster than without the correction. For more details see Ref. [5,7]
Further goal of the project was the investigation and implementation of polarization-sensitive (PS) THz-imaging by exploiting the polarization properties of THz radiation.
The investigation of changes in the polarization of electro-magnetic radiation passed through a material allows deducing specific properties, such as birefringence due to anisotropies in the sample. This effect is exploited by photoelastic measurements using visible light. Drawback of this technique is that a transparent model of the sample has to be fabricated. For polarisation-sensitive optical coherence tomography (PS-OCT), an interferometric reflection technique, the polarization properties of light in the near infrared (NIR) are used to determine birefringence within samples in a depth resolved way. This method is well suited for semitransparent and turbid media, but is limited to near-surface regions of the sample only due to the low penetration depth of the NIR radiation. Thus, the relatively large penetration depth of THz-radiation for many dielectric materials is a major advantage compared to OCT.
In the frame of the project, the concepts originally developed for PS-OCT for the investigation of birefringence (and consequently of anisotropies) were adopted and evaluated for PS-THz-imaging for the first time. Therefore, the scanning THz-system was upgraded by polarization-optical components as well as a specific THz-detector. This detector is a multi-contact antenna, which allows the detection of the horizontal and vertical polarization component on one chip. The antenna structures were developed by the project partner IHF (see “Partner”), where several different arrangements of the electrodes were realized, tested and characterized (see Ref. ).
The obtained results showed that with the proposed method information about anisotropies of a material can be obtained, where an algorithm was developed for extracting birefringence and the orientation of the optical axis (i.e., the direction of anisotropy) from the measured data. An important advantage of our method is that there is no more need for multiple measurements with different polarization orientations of the THz-radiation, as it is the case for the methods introduced so far. Thus, the scanning PS-THz imaging system allows imaging of birefringence and of the optical axis in a relatively efficient way, as we could show, e.g., for fibre-reinforced polymer samples. For more details see Ref. [1,3]
The developed THz-system was in particular tested for industrial applications, such as samples from automotive , polymer or solar-thermal industries . As an example, Fig. 3 shows the detection of defects in an adhesive layer used for fixing a metal cylinder on a plastic plate . There, the defects (indicated by the arrows) could clearly be identified in a single transversal image. It is noted that the image was recorded through the plastic plate (opaque for visible light).
As potential applications of PS-THz imaging the analysis of anisotropies and strains in polymers, plastics and fibre-reinforced composite materials were identified. In the frame of the project, the high potential of the technique could clearly be demonstrated in particular for different fibre-reinforced polymers.
The project was performed within the frame of the D-A-CH agreement in collaboration with the Terahertz Systems Group (TSG) of the Institute for High-Frequency Engineering (IHF) of the Technical University of Braunschweig (Germany). The IHF was concerned with the development of optimized THz-emitters as well as special PS-THz detectors.
Stefan Katletz et al., “PS-THz imaging: analysis of orientation and birefringence”, IRMMW-THz 2012 (Sept. 2012, Wollagong, Australia).
Gerald Steinmaurer, Christian Hofer, Harald Dehner, Stefan Katletz, and Karin Wiesauer, „InSolTec – Qualitätssicherung von solarthermischen Kollektoren durch innovative Sensorik“, 22. Symposium Thermische Solarenergie (May 2012, Bad Staffelstein, Germany).
Stefan Katletz, Michael Pfleger, Harald Pühringer, Nico Vieweg, Benedikt Scherger, Bernd Heinen, Martin Koch and Karin Wiesauer, „Polarization sensitive imaging with pulsed THz radiation“, in Proc. International THz Conference 2011 in Villach, Austria, Österreichische Computergesellschaft Wien, pp. 63 – 67 (2011).
Michael Pfleger, Stefan Katletz, Harald Pühringer, Nico Vieweg, Benedikt Scherger, Bernd Heinen, Maik Scheller, Martin Mikulics, Martin Koch, and Karin Wiesauer, “Comparison of polarization-sensitive detection methods of THz radiation”, in Proc. International THz Conference 2011 in Villach, Austria, Österreichische Computergesellschaft Wien, pp. 149 – 154 (2011).
Stefan Katletz, Michael Pfleger, Harald Pühringer, Nico Vieweg, Benedikt Scherger, Bernd Heinen, Martin Koch, and Karin Wiesauer, „Efficient Terahertz En-face Imaging“, Opt. Express 19, 23042–23053 (2011).
H. Stephani, B. Heise, S. Katletz, K. Wiesauer, D. Molter, J. Jonuscheit, and R. Beigang, "A Feature Set for Enhanced Automatic Segmentation of Hyperspectral Terahertz Images", IMVIP 2011 - Irish Machine Vision and Image Processing Conference, pp. 117-122 (2011). DOI: http://doi.ieeecomputersociety.org/10.1109/IMVIP.2011.34, ISBN: 978-0-7695-4629-2.
Stefan Katletz, Nico Vieweg, Benedikt Scherger, Bernd Heinen, Martin Koch, and Karin Wiesauer, „Phase correction for rapid en-face scanning with pulsed terahertz radiation“, Proc. SPIE 8195, 81951L (2011); DOI: 10.1117/12.900994.
K. Ezdi, B. Heinen, C. Jördens, N. Vieweg, N. Krumbholz, R. Wilk, M. Mikulics, K. Wiesauer, S. Katletz, and M. Koch, “Pulsed THz antennas – a time-domain approach”, in Proc. IRMMW-THz 2009 in Busan, South Korea, (2009).
Henrike Stephani, Michael Herrmann, Karin Wiesauer, Stefan Katletz, and Bettina Heise, “Enhancing the interpretability of Terahertz Data Through Unsupervised Classification”, in Proc. XIX IMEKO World Congress Fundamental and Applied Metrology 2009 in Lisbon, Portugal, (2009).