Frank Ospald

Aeronautics composite material inspection with a terahertz time-domain spectroscopy system

Abstract. The usability of pulsed broadband terahertz radiation for the inspection of composite materials from the aeronautics industry is investigated, with the goal of developing a mobile time-domain spectroscopy system that operates in reflection geometry. A wide range of samples based on glass and carbon fiber reinforced plastics with various types of defects is examined using an imaging system; the results are evaluated both in time and frequency domain. The conductivity of carbon fibers prevents penetration of the respective samples but also allows analysis of coatings from the reflected THz pulses. Glass fiber composites are, in principle, transparent for THz radiation, but commonly with significant absorption for wavelengths >1  THz. Depending on depth, matrix material, and size, defects like foreign material inserts, delaminations, or moisture contamination can be visualized. If a defect is not too deep in the sample, its location can be correctly identified from the delay between partial reflections at the surface and the defect itself.

Generation and detection of terahertz radiation up to 4.5 THz by low-temperature grown GaAs photoconductive antennas excited at 1560 nm

Generation and detection of terahertz radiation at 1560 nm based on low-temperature grown GaAs photoconductive antennas are demonstrated. The dependence of the DC photo current and terahertz peak amplitude on the illumination power are measured and found to follow a superlinear power law proposed for sequential charge carrier excitation via midgap states. A terahertz time domain spectroscopy system employing low-temperature grown GaAs photoconductive antennas pumped at 1560 nm reaches a bandwidth of 4.5 THz and a peak signal to noise ratio of 29 dB, while the reference measurement at 780 nm reaches 40 dB and a bandwidth of approximately 3 THz.

Comparison of terahertz technologies for detection and identification of explosives

We present results on the comparison of different THz technologies for the detection and identification of a variety of explosives from our laboratory tests that were carried out in the framework of NATO SET-193 “THz technology for stand-off detection of explosives: from laboratory spectroscopy to detection in the field” under the same controlled conditions. Several laser-pumped pulsed broadband THz time-domain spectroscopy (TDS) systems as well as one electronic frequency-modulated continuous wave (FMCW) device recorded THz spectra in transmission and/or reflection.