Search

Applications of multistatic sensor arrays, which operate in the terahertz-region, are increasingly gaining importance for non-destructive testing purposes. Continuous enhancements in processing capabilities of CPUs and GPUs allow to generate volumetric images with superior speeds compared to alternative raster-scan systems. However, this requires efficient reconstruction- and correction algorithms. In addition to the system design, the choice of signal processing methods has a decisive influence on the resolution, image quality and measurement duration. In this contribution, we discuss some aspects of this signal processing along with specific examples.

Many established terahertz imaging modalities are on one side restricted by the tradeoff between resolution and field of view such as in the case of focal plane arrays and on the other side suffer from a limited depth of field such as in the case of quasi-optical terahertz imaging configurations. Furthermore, typical scanning solutions require time-consuming measurement procedures and restrict significant potential industrial deployments of terahertz imaging technology. Imaging with sparse multistatic line arrays in combination with digital beam forming (DBF) techniques enables us to overcome these limitations and offer three-dimensional (3D) terahertz image reconstructions of the object. This contribution addresses the design of such terahertz imaging systems from a general point of view with the focus on the design of sparse line arrays, while considering objects scattering properties. Based on this design concept, the realization of a novel highly sparse 3D terahertz imaging system is discussed. The sparse line array of the system is operating within a frequency range from 75 to 110 GHz and is used in combination with a conveyor in order to generate a synthetic sampling aperture. The system is capable to generate 3D terahertz images with tens of megavoxels at feed motions of up to a few 10 cm/s. Also, a sparse array design in regard to an imaging system operating at 240 GHz with integrated SiGe sensor elements is discussed. In addition, three different DBF algorithms are compared in regard to their computational efficiency.

Utilizing composite materials instead of metals allows to reduce weight while maintaining mechanical strength. Fiber-Reinforced Thermoplastics (FRT) have very competitive mechanical properties and additionally offer the capability for mass production, which makes the material particularly attractive in the automotive and aircraft sectors. Typical defects in FRT are dominated by fiber shifts and cracks. We show that 3D THz imaging using a FMCW approach can resolve such defects and is therefore a suitable tool for nondestructive testing applications, specifically for the quality control in FRT mass production.

THz imaging and sensing has demonstrated a wide-ranging application potential. However, the transfer of such basic applicability observations to real-world application scenarios is severely obstructed by fundamental limitations imposed by the comparatively long wavelength of this analytic technique. In this presentation, an overview of recent signal processing developments for the enhancement of the analytic performance of THz imaging and sensing systems is presented. The first part of the presentation introduces advanced signal processing techniques to enhance the spectroscopic investigation capability of THz systems. Experiments are performed at particularly difficult application situations, including inter alia very thin material systems or measurements with strongly absorptive features beyond the signal to noise limitations of spectroscopic instrumentation. Model- based numeric procedures for spectroscopic investigation with pulsed THz systems are derived, which enhance the analytic material data quality by two orders of magnitude in comparison to established numeric procedures. Furthermore, computer vision based blind-deconvolution superresolution approaches are introduced, which allow the unassisted increase of imaging resolution beyond the diffraction limit. Experiments performed with a FMCW- based THz imaging system operating from 514 - 640 GHz demonstrate a resolution increase by a factor of 2.3 beyond the diffraction limit, without requiring any prior knowledge on the point-spread function size or shape of the imaging system, but based on a direct analysis of the imaging data of an unknown target sample.

Multiple-input multiple-output (MIMO) imaging systems in the terahertz frequency range have a high potential in the field of non-destructive testing (NDT). With such systems it is possible to detect defects in composite materials, for example cracks or delaminations in fiber composites. To investigate mass-produced products it is necessary to study the objects in close to real-time on a conveyor without affecting the production cycle time. In this work we present the conception and realization of a 3D MIMO imaging system for in-line investigation of composite materials and structures. To achieve a lateral resolution of 1 mm, in order to detect such small defects in composite materials with a moderate number of elements, precise sensor design is crucial. In our approach we use the effective aperture concept. The designed sparse array consists of 32 transmitters and 30 receivers based on planar semiconductor components. High range resolution is achieved by an operating frequency between 220 GHz and 260 GHz in a stepped frequency continuous wave (SFCW) setup. A matched filter approach is used to simulate the reconstructed 3D image through the array. This allows the evaluation of the designed array geometry in regard of resolution and side lobe level. In contrast to earlier demonstrations, in which synthetic reconstruction is only performed in a 2D plane, an optics-free full 3D recon- struction has been implemented in our concept. Based on this simulation we designed an array geometry that enables to resolve objects with a resolution smaller than 1mm and moderate side lobe level.

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 costs, hardware complexity and potentially the required measurement time of the system, while providing a comparable imaging quality to full array based approaches. But in the presence of dominant specular reflections, sparse arrays can lead to an inhomogeneous illumination which impairs images and hampers their correct interpretation. We propose an experimentally verified method to simulative asses the design of sparse line arrays, taking in consideration the scattering characteristics of the object under test.

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 imaging applications. The realized system employs a sparse array consisting of 12 emitters and 12 heterodyne receivers and operates in the W frequency band between 75 GHz and 110 GHz. The applied stepped-frequency continuous-wave radar concept provides 35 GHz modulation bandwidth for fast volume inspections of large test objects in combination with a band-conveyor.

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 process control, we report on the realization of a stepped-frequency modulated sparse array with 12 emitters and 12 heterodyne receivers operating in the frequency range from 75 GHz to 110 GHz. The system is ready to perform imaging tasks and will be in a final step combined with a band-conveyor for fast terahertz image acquisition.