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We report on the realization of a 3D terahertz imaging system for industrial radome inspection. The system has been designed to be operated within a machining center for radome manufacturing. In order to provide the required flexibility in respect to image resolution and penetration depth, we developed a system, which combines two frequency-modulated continuous-wave terahertz sensing units, operating...
We present a highly accurate self-calibrating approach to investigate ceramic thermal barrier coatings on metallic substrates using terahertz (THz) technique in reflection geometry. The proposed approach enables the simultaneous determination of the complex refractive index and the coating thickness. Hence, a pre-calibration of the specimen under test is not required.
Terahertz imaging solutions for industrial non-destructive testing applications receive an increasing attention. Besides the need for real-time acquisition and image processing, 3D feature localization and large field of views of several tenths to a few meters are in the focus of interest. For this purposes sparse line arrays of transmitters (Tx) and receivers (Rx) offer an optimal approach to reduce...
Imaging systems operating in the sub-terahertz frequency range have proven to be highly attractive devices for non-destructive testing applications. Especially frequency modulated system concepts have shown a great potential for volume inspection in industrial process and quality control, but often lack fast image acquisition times. We report on the development of a line array system for fast sub-terahertz...
We present a novel numerical approach to decrease the limits of the minimum paint thickness measurements of individual layers in multilayered structures using terahertz pulsed technique in reflection geometry. This method combines the benefits of model-based material parameters extraction, a generalized transfer matrix method, and an evolutionary optimization algorithm. The proposed approach has been...
Frequency modulated terahertz imaging systems have shown a great potential for volume inspections in the industrial process and quality control. However, many system concepts lack the capability to encompass a large field of view with fast data acquisition speed, while providing significant depth information. Based on our recently outlined system design of a terahertz imaging solution for the industrial...
Ultra wide-band frequency-modulated continuous-wave imaging systems operating in the lower terahertz regime have proven to be highly desirable for non-destructive testing applications. However, either quasi-optical configurations or the characteristics of typically used antennas as well as temperature-dependent nonlinear frequency sweeps restrict the achievable spatial resolution and reliability of...
We report on the development of a stepped frequency continuous wave THz imaging system for process control in industrial environments. The system is based on a sparse line array consisting of 12 emitters and 12 heterodyne receivers operating in a frequency range between 75 GHz and 110 GHz. The applied multiple-input and multiple-output technique allows the reconstruction of THz images with 144 virtual...
We present a novel approach to determine the individual layer thickness in a dielectric multilayer sample using pulsed terahertz spectroscopy in reflection geometry. In a first step, the optical parameters of each layer have to be determined. Based on these parameters, we simulate the reflected THz-pulse from the multilayer system and compare it to the measurement. A genetic algorithm is used to determine...
Terahertz waves can provide in-depth information on defects for structural health monitoring of composite materials. This paper describes the technology of a continuous-wave and a time-domain terahertz system operating on a 2-D and 3-D motion platform to provide 3-D high spatial resolution. The system as well as the overall detection performance will be described.
Concerning several attacks using letter bombs within Europe in the last couple of years and the highly rising number of new drugs, there is a need for new kinds of detection devices for explosives and drugs. We present a postscanner on basis of Terahertz (THz) spectroscopy using novel chemometric methods for the evaluation of detected THz fingerprints.
Recently, there has been a significant interest in employing high frequency radar imagery for many industrial oriented and security applications. The quality control of aeronautics composite multi-layered materials and structures through Non-Destructive Testing (NDT) is the main focus of this study. The main motivations for using this technology are: it allows penetration of most non-metal and non-polarized...
We present measured terahertz radiation patterns from antennas made of low temperature grown gallium arsenide on semi-insulating gallium arsenide and low temperature grown gallium arsenide on high resistivity silicon. A strong dependence on the substrate used for the terahertz source can be shown.
The global increase of plastic production has led to an increase in coating materials available for plastics, as many plastics need to be protected from environmental factors such as light, solvents and other chemicals. For this reason the coating of plastics is a useful example for the coating thickness measurement on non-metal surfaces. Analogies can also be found in many other areas such as the...
We report realization of a THz camera based on electro-optical detection of THz radiation from an optical parametric oscillator, using a silicon-based CCD camera as the optical detector. The dynamic range of 16.5 dB for a 6000-frame average (2-minute data acquisition time) is limited by laser noise.
In a new approach we report on a THz imaging system with arrays of horn-type photoconductive switches emitting in-plain. Aim is to combine the flexibility and sensitivity of THz antennas with the measurement speed of electro-optic sampling.
We have built a THz system for one-dimensional imaging with a THz line focus and electro-optic detection based on two line-scan cameras in a balanced arrangement. The system uses pulsed THz radiation generated with a fast-repetition-rate (80 MHz) femtosecond laser and an electrically chopped dipole antenna.
In order to improve the performance of THz time-domain 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 measurements 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...
Many chemicals, including hazardous substances, can be distinguished based on their spectral fingerprints in the terahertz range under laboratory conditions. In a real-world standoff environment, additional factors such as water vapor absorption in humid air, clothes and packaging materials, and the surface properties of the particular sample make this task more difficult. We have performed measurements...
Terahertz spectroscopy has become a well-established tool under laboratory conditions but terahertz stand-off detection in the real world is hampered by a number of influences related to the environment and the sample properties such as air humidity, surface roughness and angle of detection. A terahertz stand-off system must be calibrated in order to account for these influences. We have performed...
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