Experimental results of homodyne terahertz interferometric 2-D imaging of RDX are presented. Continuous waves at
0.25-0.6 THz are used to obtain images of a C-4 sample at several THz frequencies. The performance of an N element
detector array is imitated by only one detector placed at N positions. The distance between the C-4 sample and the
detector array is ~30 cm. By taking interferometric images at several THz frequencies RDX can be recognized by the
spectral peak at 0.82 THz. Simulations of interferometric images of two point sources of spherical waves are presented.
The terahertz interferometric imaging method can be used in defense and security applications to detect concealed
weapons, explosives as well as chemical and biological agents.
Experimental results of two-dimensional homodyne terahertz interferometric imaging are presented. The
performance of an N element detector array is imitated by only one detector placed at N positions.
Continuous waves at 0.25-0.3 THz are used to detect concealed objects: a metal object and an RDX
sample. The terahertz interferometric imaging method can be used in defense and security applications to
detect concealed weapons, explosives as well as chemical and biological agents.
Experimental results of homodyne terahertz interferometric 1-D and 2-D imaging are presented. Continuous waves at 0.25-0.3 THz are used to detect a metal object behind a barrier. The performance of an N element detector array is imitated by only one detector placed at N positions. The reconstructed images are in a good agreement with theoretical predictions. The terahertz interferometric imaging method can be used in defense and security applications to detect concealed weapons, explosives as well as chemical and biological agents.
In recent times, the far infrared or the terahertz (1 THz = 1012 cycles/sec and 300μm in wavelength) region of electromagnetic spectrum has become a promising radiation for spectroscopic identification of different types of biomaterials. The present work investigates the effect of grain size on the THz spectra of chalk, salt, sugar and flour using THz time-domain spectroscopy. It has been observed that at lower frequencies, solids of small grain sizes of nonabsorbing materials show rising trends in their extinction spectra. Here, we obtain extinction spectra of granular salt, chalk, sugar and flour between 0.2 to 1.2 THz and show that the experimentally obtained extinction can be predicted on the basis of the Mie Scattering model for small grain sizes. The current study is an attempt to understand the absorption spectrum of a few such materials having no significant intrinsic absorption in the THz region by separating the independent contributions of true absorption of the material and scattering losses due to its morphology in the extinction of the material. This would help in distinguishing these materials based on their rising trend of the extinction spectra at lower frequencies.
In recent times, Terahertz (1 THz = 1012 cycles/sec and 300 μm in wavelength) spectroscopy has become a promising technique for spectroscopic identification of different materials having contemporary interest. In this study we report a direct measurement of reflection spectra of the explosive C-4, which shows significant absorption around 0.8 THz, using THz time domain spectroscopic techniques. A contrast in reflection of around 8% has also been observed between the neighboring frequencies of 0.7 THz and 0.9 THz. The spectral data have been used to create realistic synthetic images for use in simulations of interferometric detection in a stand-off THz imaging system. The results obtained are analyzed using Artificial Neural Networks for positive identification of the agents with an interferometric array of few linear detectors in near field mode.
Recently, there has been a significant interest in employing Terahertz (THz) technology, spectroscopy and imaging for standoff detection applications. There are three prime motivations for this interest: (a) THz radiation can detect concealed weapons since many non-metallic, non-polar materials are transparent to THz radiation, (b) target compounds such as explosives, and bio/chemical weapons have characteristic THz spectra that can be used to identify these compounds and (c) THz radiation poses no health risk for scanning of people. This paper will provide an overview of THz standoff detection of explosives and weapons including discussions of effective range, spatial resolution, and other limitations. The THz approach will be compared to alternative detection modalities such as x-ray and millimeter wave imaging.
It has been suggested that interferometric/ synthetic aperture imaging techniques, when applied to the THz regime, can provide sufficient imaging resolution and spectral content to detect concealed explosives and other weapons from a standoff distance. The interferometric imaging method is demonstrated using CW THz generation and detection. Using this hardware, the reconstruction of THz images from a point source is emphasized and compared to theoretical predictions.
The application of near-field interferometric imaging to the Terahertz frequency range for detection of concealed objects is discussed. A circular array architecture can be employed to compensate for near-field distortions and increase the field of view and depth of focus. The lateral and focusing errors of this imaging method are discussed as well as the trade-offs of interferometric imaging compared to a focal plane array architecture.
Ultrafast laser techniques have opened up a tremendous research opportunity in studying the interaction of short pulses of light with matter. With discovering of the picosecond photoconducting hertzian dipoles and high-brightness THz beams characterized with an ultrafast detector, we have seen more and more applications of ultrafast light in non-invasive imaging. Standard methods, when applied to the measurement of thin optical materials, doesnot independently determine the material's thickness and index of refraction. The proposed method is fundamentally different from other imaging such as contrast difference in optical coherent tomography (OCT) or the peak-to-peak intensity ratio as in THz imaging to determine index of refraction and thickness. We show that the application of ultrafast techniques allows simultaneous measurements of material thickness and optical constants in optical precision from transmission measurements. Such finding invites new perspectives in imaging and other applicable disciplines such as imaging processing after recording of the THz waveform of biological samples.
This article shows the applications of ultrafast light in studying material optical properties and its application for rudimental imaging. Standard methods, when applied to the imaging, can not independently determine the material's thickness and index of refraction. The proposed method is fundamentally different from other imaging such as contrast difference in optical coherent tomography (OCT) or the peak-to-peak intensity ratio as in THz imaging to determine index of refraction and thickness. We show that the application of ultrafast techniques allows simultaneous measurements of material thickness and optical constants in optical precision from transmission measurements. Such finding invites new perspectives in imaging and other applicable disciplines.
Terahertz Time domain spectroscopy (THz-TDS) can provide the optical response of a medium in both amplitude and phase. We show that such capability can enable a detail analysis of optical properties of biological sample. Such study is important for standoff detection of presence of biological sample, where a detail analysis is difficult if not possible due to a complicated system involved and multiple effects involved. We proposed a transfer function study of the response of such system.
Terahertz Time domain spectroscopy (THz -TDS ) can provide the optical response of a medium in both amplitude and phase. We show that such capability can enable a detail analysis of optical properties around a resonance regime. Such study is important for standoff detection of explosive material where numerous absorption peaks exist. We proposed a model where one can synthesize the optical properties with THz-TDS around the resonance regimes.
Terahertz Time domain spectroscopy (THz -TDS ) can provide the optical response of a medium in both amplitude and phase. We show that such capability can enable a detail analysis of optical properties of RDX sample. Such study is important for standoff detection of presence of RDX sample, where a detail analysis is difficult if not possible due to a complicated system involved and multiple effects involved. We proposed a match filter method for detection of RDX inside or behind a barrier.
A non-invasive means to detect and characterize concealed agents of mass destruction in near real-time with a wide field-of-view is under development. The method employs spatial interferometric imaging of the characteristic transmission or reflection frequency spectrum in the Terahertz range. However, the successful (i.e. low false alarm rate) analysis of such images will depend on correct distinction of the true agent from non-lethal background signals. Neural networks are being trained to successfully distinguish images of explosives and bioagents from images of harmless items. Artificial neural networks are mathematical devices for modeling complex, non-linear relationships. Both multilayer perceptron and radial basis function neural network architectures are used to analyze these spectral images. Positive identifications are generally made, though, neural network performance does deteriorate with reduction in frequency information. Internal tolerances within the identification process can affect the outcome.
The imaging properties of planar, spherical, and circular interferometric imaging arrays are examined in the near-field region limit. In this region, spherical and circular array architectures can compensate for near-field distortions and increase the field of view and depth of focus. The application of near-field interferometric imaging to the Terahertz frequency range for detection of concealed objects is emphasized.
A proposed, non-invasive, means to detect and characterize concealed biological and explosive agents in near real-time with a wide field-of-view uses spatial imaging of their characteristic transmission or reflectivity wavelength spectrum in the Terahertz (THz) electro-magnetic range (0.1-3 THz). Neural network analyses of the THz spectra and images will provide the specificity of agent detection and reduce the frequency of false alarms. Artificial neural networks are mathematical devices for modeling complex, non-linear functionalities. The key to a successful neural network is adequate training with known input-output data. Important challenges in the research include identification of the preferred network structure (e.g. multi-layer perceptron), number of hidden nodes, training algorithm (e.g. back propagation), and determination of what type of THz spectral image pre-processing is needed prior to application of the network. Detector array images containing both spectral and spatial information are analyzed with the aid of the Neurosolutions(TM) commercial neural network software package.
The Frequency Agile Solar Radiotelescope (FASR) will be a broadband synthesis-imaging array with 3-km or larger baselines, operating over a broad frequency range of 0.1-30 GHz. The instrument demands a site with low levels of Radio Frequency Interference (RFI) over this entire band. The site also must be large enough to accommodate the expected size of the array configuration and ideally would provide room to grow with future upgrades. The site must have a benign environment in which at least 100 separate elements will operate with little degradation and weather-related downtime. Several sites in the U.S. are being considered. We discuss what criteria are being used to assess the sites, and give some initial results of testing some of the sites.
For many years, ground-based radio observations of the Sun have proceeded into two directions: (1) high resolution imaging at a few discrete wavelengths; (2) spectroscopy with limited or no spatial resolution at centimeter, decimeter, and meter wavelengths. Full exploitation of the radio spectrum to measure coronal magnetic fields in both quiescent active regions and flares, to probe the thermal structure of the solar atmosphere, and to study energy release and particle energization in transient events, requires a solar-dedicated, frequency-agile solar radiotelescope, capable of high-time, - spatial, and -spectral resolution imaging spectroscopy. In this paper we summarize the science program and instrument requirements for such a telescope, and present a strawman interferometric array composed of many (greater than 40), small (2 m) antenna elements, each equipped with a frequency- agile receiver operating over the range 1 - 26.5 GHz.
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