Joachim Jonuscheit

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.

Compact fiber-coupled terahertz spectroscopy system pumped at 800 nm wavelength.

Photonic terahertz (THz) technology using femtosecond (fs) lasers has a great potential in a wide range of applications, such as non-destructive testing of objects or spectroscopic identification of chemical substances. For industrial purposes, a THz system has to be compact and easily implementable into the particular application. Therefore, fiber-coupled THz systems are the key to a widespread use of THz technology. In order to have flexible THz emitters and detectors near infrared fs light pulses have to be sent through optical fibers of considerable length. As a consequence, the fibers dispersion has to be compensated for and nonlinear effects in the fiber have to be minimized. A fiber-based THz time-domain spectroscopy system of high stability, flexibility, and portability is presented here.

Nondestructive testing potential evaluation of a terahertz frequency-modulated continuous-wave imager for composite materials inspection

Abstract. The sub-terahertz (THz) frequency band has proved to be a noteworthy option for nondestructive testing (NDT) of nonmetal aeronautics materials. Composite structures or laminates can be inspected for foreign objects (water or debris), delaminations, debonds, etc., using sub-THz sensors during the manufacturing process or maintenance. Given the harmless radiation to the human body of this frequency band, no special security measures are needed for operation. Moreover, the frequency-modulated continuous-wave sensor used in this study offers a very light, compact, inexpensive, and high-performing solution. An automated two-dimensional scanner carrying three sensors partially covering the 70- to 320-GHz band is operated, using two complementary measurement approaches: conventional focused imaging, where focusing lenses are used; and synthetic aperture (SA) or unfocused wide-beam imaging, for which lenses are no longer needed. Conventional focused imagery offers finer spatial resolutions but imagery is depth-limited due to the beam waist effect, whereas SA measurements allow imaging of thicker samples with depth-independent but coarser spatial resolutions. The present work is a compendium of a much larger study and describes the key technical aspects of the proposed imaging techniques and reports on results obtained from human-made samples (A-sandwich, C-sandwich, solid laminates) which include diverse defects and damages typically encountered in aeronautics multilayered structures. We conclude with a grading of the achieved results in comparison with measurements performed by other NDT techniques on the same samples.

Highly accurate thickness measurement of multi-layered automotive paints using terahertz technology

In this contribution, we present a highly accurate approach for thickness measurements of multi-layered automotive paints using terahertz time domain spectroscopy in reflection geometry. The proposed method combines the benefits of a model-based material parameters extraction method to calibrate the paint coatings, a generalized Rouards method to simulate the terahertz radiation behavior within arbitrary thin films, and the robustness of a powerful evolutionary optimization algorithm to increase the sensitivity of the minimum thickness measurement limit. Within the framework of this work, a self-calibration model is introduced, which takes into consideration the real industrial challenges such as the effect of wet-on-wet spray in the painting process.

Combless broadband terahertz generation with conventional laser diodes

We present a novel technique to generate a continuous, combless broadband Terahertz spectrum with conventional low-cost laser diodes. A standard time-domain spectroscopy system using photoconductive antennas is pumped by the output of two tunable diode lasers. Using fine tuning for one laser and fine and coarse tuning for the second laser, difference frequency generation results in a continuous broadband THz spectrum. Fast coarse-tuning is achieved by a simple spatial light modulator introduced in an external cavity. The results are compared to multi-mode operation for THz generation.

The influence of surface roughness on THz reflection measurements

In order to improve the performance of THz timedomain spectroscopy for the detection of hazardous substances in the field of civil security, the influence of surface roughness was studied. We performed transmission and reflection mesurements of smooth and rough metallic and dielectric surfaces varying the degree of surface roughness and the tilt angle of the sample. The results were compared to calculations based on radar theory with good agreement.

Simulation and experiment of terahertz stand-off detection

Terahertz (THz) spectroscopy is a promising technique for the stand-off detection and characterization of hidden objects. The THz band is particularly well suited firstly because THz radiation penetrates many dielectrics like clothes and secondly because many potentially hazardous substances have characteristic signatures in the THz spectral region. In order to utilize the full potential of THz radiation for detecting possible hazards and recognizing characteristic signatures, disturbing influences must be accounted for. We have performed experiments and simulations in order to investigate the limits of terahertz stand-off detection. A special emphasis is paid on humidity in ambient air and properties of the sample like surface roughness, alignment and interfaces. Water vapor absorption strongly affects the THz spectra. Since the absorption lines are strong and narrow, the calculation must be precise. We have checked various models well-known in meteorology covering the infrared and the microwave region of the electromagnetic spectrum. By extending the models into the THz region, an accurate description of the measured spectral absorption is achieved. In our studies transmission spectra for different substances were tested. In a reflection scheme metallized sandpaper of various grit sizes was used to determine the influences of different surface properties. Further measurements were performed for different tilt angles to analyze the influence of the surface roughness. We are currently creating a look-up table to show which parts of the THz spectrum can be used for THz stand-off detection.

Coherent electro-optical detection of terahertz radiation from an optical parametric oscillator

We report the realization of coherent electro-optical detection of nanosecond terahertz (THz) pulses from an optical parametric oscillator, which is pumped by a Q-switched nanosecond Nd:YVO4 laser at 1064 nm and emits at approximately 1.5 THz. The beam profile and wavefront of the THz beam at focus are electro-optically characterized toward the realization of a real-time THz camera. A peak dynamic range of approximately 37 dB/radical Hz is achieved with single-pixel detection.

A pulsed THz Imaging System with a line focus and a balanced 1-D detection scheme with two industrial CCD line-scan cameras

We present a pulsed terahertz imaging system with a line focus intended to speed up measurements. A balanced 1-D detection scheme working with two industrial line cameras is used. The instrumental characteristics are determined.

Fasergekoppeltes Terahertz-Spektroskopiesystem (Fiber-coupled Terahertz Spectroscopy System)

Üblicherweise werden Terahertz-Zeitbereichsspektroskopie-Systeme in Freistrahlanordnung gepumpt. Durch die Verwendung geeigneter Lichtwellenleiter, über die die infraroten Laserpulse dann mehrere Meter weit übertragen werden, erreicht man eine Entkopplung des optischen Bereiches von dem Terahertz-Messbereich. Dadurch wird das System kompakter und gegenüber störenden äußeren Einflüssen robuster. Im Folgenden wird die glasfaserbasierte Übertragungstechnik der optischen Femtosekundenpulse präsentiert. Als mögliches Anwendungsgebiet der glasfasergekoppelten Terahertz-Systeme wird im Bereich der zerstörungsfreien Materialprüfung die Messung dünner Schichten sowohl in Transmission als auch in Reflexion diskutiert. The use of broadband laser pumped Terahertz (THz) systems for industrial applications strongly relies on the reliability and stability of the system. It has to be compact and easy to be implemented into the particular application. Fiber-coupled THz systems are the key to widespread use of these systems. In order to couple THz emitters and detectors to fibers with lengths of several meters the dispersion and the nonlinear properties of fibers have to be taken into account. This is discussed in this article. As a possible application of the fiber-coupled Terahertz system in the field of nondestructive testing the measurements of the thicknesses of thin films are shown. These measurements are performed in both transmission and reflection.