Current active millimeter-wave (MMW) imaging system applied to personnel surveillance uses shape recognition in the reconstructed image to identify dangerous goods. This method relies on a single kind of information namely pixel value to distinguish potential threats. It can be susceptible to interference such as noise and multiple scattering and it is difficult to distinguish fine structures. In view of this situation, this paper presents a feature acquiring through multi-angle imaging to characterize target’s scattering property. This feature can add one more dimensional information of targets except for the original reconstructed image, and it can distinguish objects more accurately to assist the realization of target classification. Electromagnetic simulation has been conducted and the reconstructed image results as well as quantitative studies show the effectiveness of this feature in assisting target distinction.
Terahertz (THz) time-domain spectroscopy is considered as an attractive tool for the analysis of chemical composition. The traditional methods for identification and quantitative analysis of chemical compounds by THz spectroscopy are all based on full-spectrum data. However, intrinsic features of the THz spectrum only lie in absorption peaks due to existence of disturbances, such as unexpected components, scattering effects, and barrier materials. We propose a strategy that utilizes Lorentzian parameters of THz absorption peaks, extracted by a multiscale linear fitting method, for both identification of pure chemicals and quantitative analysis of mixtures. The multiscale linear fitting method can automatically remove background content and accurately determine Lorentzian parameters of the absorption peaks. The high recognition rate for 16 pure chemical compounds and the accurate predicted concentrations for theophylline-lactose mixtures demonstrate the practicability of our approach.
KEYWORDS: Spectroscopy, Terahertz radiation, Control systems, Field programmable gate arrays, Signal to noise ratio, Data acquisition, Sensors, Modulation, Data communications, Laser beam diagnostics
This paper discusses the development of data acquisition and control system (DACS) for a portable terahertz time-domain spectrometer (THz-TDS). In this system, field programmable gate array (FPGA) severed as main control unit (MCU), which controls and harmonizes other functional modules of the spectrometer, including the linear delay stage, high voltage modulation and the AD converter. A digital lock-in amplifier implemented within the FPGA is employed to restore the weak THz signal. USB is used as the communication interface between the FPGA and the computer, which transfers commands and THz waveform data. The spectrometer can scan a waveform within a 30-ps time window in about 10 seconds, which has a spectral resolution better than 50GHz and a dynamic range up to 49dB.
Continuous-wave (CW) terahertz (THz) imaging system has advantages of high power, compact structure and low cost, thus having been investigated for widespread applications. In typical reflection mode of CW imaging, the obtained image is usually degraded by repeated fringes, which is caused by interference phenomenon. The undesired interference signal originates from the reflection of surfaces of samples and lenses. When the samples are titled placed or their surfaces are uneven, the detected signal intensity is fluctuant even if the same sample lies in different positions. Therefore, small-sized or weekly absorbing objects are hard to be distinguished. Based on cartoon-texture decomposition, we propose a practical method to restore CW THz reflection images. After decomposition, the fringes and the objects are separated. In order to preserve edges, sharpening and fusion steps are employed respectively. The object in the final image is obvious with little loss of information.
The wavelength of terahertz radiation is typically on the order of millimeters and submillimeters, and not negligible
compared to the sizes of the optical components and the sample structure. Diffraction and scattering effects
will be prominent in this situation, yielding spatiotemporal reshaping of terahertz pulse during its propagation. One representative example involves the propagation of a focused terahertz beam through a dielectric edge, which is frequently encountered in terahertz pulsed imaging. In this paper, we present a theoretical and experimental investigation on this edge diffraction problem. Benefiting from the presence of edge diffraction, high contrast image is likely to be obtained, which enables a weak-absorption (or transparent) object to be clearly resolved and reveals potential applications of terahertz imaging in nondestructive detection and biomedical diagnosis.
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