High-Resolution Long-Range THz Imaging for Tunable Continuous-Wave Systems

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. INDEX TERMS long-range terahertz imaging, lensless terahertz imaging, VNA, terahertz image migration algorithms

Short-Range SAR Imaging From GHz to THz Waves

“…However, in reality the resolution is limited by the nonisotropy of the radar cross section of the object under test and angles at which each scatterer is observed. Hence, for further information in [16] and in [26] new approaches for the calculation of the resolution for circular synthetic aperture radar are presented. Although, the described imaging method is for a moving transceiver, the same principle holds true when the movement is performed using the object under test, known as inverse SAR (ISAR) [1].…”

“…Based on SAR technique, imaging at THz frequencies can also be realized with the photonics system such as timedomain spectroscopy [4], [18]. It provides very high spatial resolution due to much larger bandwidth (>1 THz) and shorter wavelength (<0.1 mm) but the systems are limited to very short-range (<30 cm) and low signal to noise ratio [16], [18]. The photonic approach is not of focus in this paper, even though similar algorithms can be applied.…”

High-Power Lensless THz Imaging of Hidden Objects

“…Nevertheless, the high-average power source outperforms the commercial system in terms of image quality as we will see in the following section, indicating the potential of this source for through-wall imaging application and already setting possible improvements for future developments of our high-average power THz-TDS. For example, the high dynamic range allows the application of replica waveforms with a higher bandwidth to increase the axial image resolution significantly [35].…”

Towards Practical Terahertz Imaging System With Compact Continuous Wave Transceiver

“…On the other hand, since a fixed transceiver pair yields only a single pixel, spatial sampling is another key issue for THz imaging systems; the methods used for spatial sampling significantly affect the size, data acquisition time, and cost of the system. In general, both beam-focusing with quasi-optic components and digital focusing techniques such as holography [27], [28] or synthetic aperture radar (SAR) [1], [29]- [31] can be applied to 2D and/or 3D imaging with sufficient spatial sampling. Quasi-optic systems provide high resolution defined by diffraction-limit [32], [33], while the imaging distance is fixed by the focal length of the lens.…”

“…In general, electronic and photonic devices can be configured to generate CW signals below and above the 300-GHz band, respectively. Multiplication of millimeter waves (MMW) is commonly performed in THz imaging experiments; although the equipment involved is expensive, the refined technique offers better bandwidth and increases power output by the order of a few milliwatts [18], [21], [23], [24], [29], [43]. A wellcalibrated system can be directly applied for 3D imaging, as an example, a vector network analyzer operated at the 300-GHz band demonstrated sub-millimeter resolution at a distance of ~2 m by lens-free digital focusing [29].…”