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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.

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.

An active frequency-modulated continuous wave (FMCW) terahertz (THz) system has been developed to image objects in millimeter-scale resolution in three dimensions. The two-dimensional (2-D) aperture synthesis enables the improvement of cross range resolution, and its high range resolution is achieved through the use of the broadband sweep signal, whose frequency ranges from 514 to 565 GHz. The 3-D data are sufficiently focused by the wavenumber domain approach derived for the dechirped data. We present the THz 3-D imaging strategy using synthetic aperture radar (SAR) techniques, the related theoretical background and the experimental results in this paper.

We present a simulation and data-processing framework that incorporates full 3-D configuration, simulation, and reconstruction functionalities for hybrid synthetic aperture THz systems, which combine mechanically scanned real imaging projection (1-D) and synthetic imaging reconstruction (2-D). The simulation is based on a ray-casting approach including models for reflections at rough surfaces and angular antenna sensitivity. A near real-time GPU reconstruction is introduced to process large amounts of data in parallel. Techniques for fast fusion of reconstructed data from several acquisition directions are presented. They serve as basis to visualize a fully consistent 3-D object. The modular framework allows a fast, easy, and efficient evaluation and prediction of system properties including scattering behavior for these types of scanner setups.

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.

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.

We report on the development of an active stand-off imaging system operating in the 80 GHz - 110 GHz frequency range. 3D real-time imaging is enabled by a combination of a mechanically scanned one-dimensional conventional imaging projection with a rotating metallic reflector and a two-dimensional synthetic imaging reconstruction with a linear array of transmitter (Tx) and receive r (Rx) elements. The system is conceived, in order to allow a resolution better than 1cm both in lateral, as well as in range directions by using a multi-view imaging geomet ry with an aperture larger than 2 m x 2 m. The operation distance is 8.5 - 9 m. The 2D synthetically reconstructed imaging planes are derived from the correlation of 20 sources and 24 coherent detectors. Range information is obtained by operating in a frequency modulated continuous wave (FMCW) mode. Real-time imaging is enabled by implementing the synthetic image reconstruction algorithms on a general purpose graphics processing unit (GPGPU) system. A multi-view imaging geometry is implemented, in order to enhance the imaging resolution and to reduce the influence of specular reflections. Keywords: THz, millimeter-wave, imaging, stand-off, FMCW, deramping, synthetic reconstruction. *matthias.kahl@uni-siegen.de