Abstract
Imaging in the terahertz frequency range has attracted growing interests since the first image of a leaf more than 20 years ago, due to its countless applications in basic and applied research, medical imaging, and nondestructive testing. However, most terahertz imaging approaches rely on focusing optics which require knowledge about the imaging scene before the actual imaging takes place. Further, imaging is mostly restricted to short distances and high resolution is only achieved for systems with a high bandwidth. Here, we present a method that enables high-resolution imaging of small metallic and dielectric objects at distances up to 2 m based on a synthetic aperture. We derive a simple approximation for the resolution of partial circular synthetic apertures with limited bandwidth. The bandwidth limitation is encountered by replacing the measured signals with replica signals of high bandwidth and equal round-trip time so that the resolution is only limited by the carrier frequency and signal-to-noise ratio of the measurement system.
Highlights
Due to its unique properties, i.e., nonionizing radiation and high transmission through many dielectric materials, terahertz waves have been subject to intense research for different imaging applications during the last 20 years [1]
In [17], we demonstrated an extended method for lensless imaging for broadband THz time-domain spectroscopy (TDS) systems which enabled the reconstruction of high-resolution images of samples with mm and sub-mm features without a priori information about the imaging scene
We present a promising imaging approach for long-range terahertz imaging with resolution in the submm range based on a vector network analyzer (VNA) with two extenders for the THz range
Summary
Due to its unique properties, i.e., nonionizing radiation and high transmission through many dielectric materials, terahertz waves have been subject to intense research for different imaging applications during the last 20 years [1]. Other areas of interest are non-destructive testing [6], hydration monitoring [7] and spatially resolved thickness measurements, e.g. of tablet coatings [8], [9], paint layer thickness [10] and in the field of art and heritage conservation [11], [12] For these applications, the object under test (OUT) is typically located at the focus of a THz time-domain spectroscopy (TDS) system either in reflection [13] or in transmission [14] mode. The algorithm is based on the image reconstruction with a broadband replica signal with equal round-trip time replacing the narrowband measured signal In this case, the resolution is only limited by the carrier frequency and the SNR of the system. We demonstrate that a clear image reconstruction of a 5 mm hex wrench and a wooden lead pencil is possible at a distance of 2 m with a bandwidth of just 20 GHz
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