Design and Implementation of a Frequency Modulated Continuous Wave Terahertz Tomography System for the Development of Hybrid Reconstruction Algorithms

Abstract: The electromagnetic properties of synthetic materials at terahertz frequencies offer great opportunities for non-destructive testing in quality assurance or production process control. Terahertz tomography systems providing three-dimensional images of specimens have several advantages over conventional X-ray or ultrasonic systems including the opportunity of a spectral analysis of the material properties. However, terahertz tomography systems are still a current field of research and the demand for applicable improvements in both, system design and reconstruction algorithms is still highly topical.

The here developed tomographic imaging system is one of the first hybrid systems that combines transmission and reflection system architecture. Transmission terahertz tomography using reconstruction algorithms similar to X-ray tomography allows to analyse material properties but resolution in the propagation direction of the beam and use algorithms based on the synthetic aperture radar principle adopted from ultrasound tomography. The increased amount of collected information in a hybrid system combining both architectures enables the development of more efficient algorithms providing significantly higher resolution.


The hybrid system consists of an electronic terahertz source and four heterodyne detectors which simultaneously measure the transmitted and reflected radiation. The system works similar to a frequency-modulated continuous wave radar. It sweeps the emitted signal frequency between 70 and 110 GHz, detecting both, amplitude and phase of every single frequency component. As advancement the development of a system using optically generated terahertz pulses allowing spectral analysis of specimens is already planned. The time of flight is determined by detecting the frequency difference between the emitted and received signal. Telescopes consisting of two lenses provide facile alignment of optics as well as the opportunity to modify beam parameters by exchanging lenses easily. One rotational and two translational stages are used for scanning the specimen. First results show reconstructed three-dimensional images of samples with dimensions up to 140 millimetres where the development of hybrid reconstruction algorithms promises significant improvement in resolution.