KEYWORDS: Imaging systems, Sensors, Signal to noise ratio, Performance modeling, Target detection, Signal attenuation, Contrast transfer function, Atmospheric modeling, Extremely high frequency, Backscatter
The U.S. Army Research Laboratory (ARL) has continued to develop and enhance a millimeter-wave (MMW) and submillimeter- wave (SMMW)/terahertz (THz)-band imaging system performance prediction and analysis tool for both the detection and identification of concealed weaponry, and for pilotage obstacle avoidance. The details of the MATLAB-based model which accounts for the effects of all critical sensor and display components, for the effects of atmospheric attenuation, concealment material attenuation, and active illumination, were reported on at the 2005 SPIE Europe Security and Defence Symposium (Brugge). An advanced version of the base model that accounts for both the dramatic impact that target and background orientation can have on target observability as related to specular and Lambertian reflections captured by an active-illumination-based imaging system, and for the impact of target and background thermal emission, was reported on at the 2007 SPIE Defense and Security Symposium (Orlando). Further development of this tool that includes a MODTRAN-based atmospheric attenuation calculator and advanced system architecture configuration inputs that allow for straightforward performance analysis of active or passive systems based on scanning (single- or line-array detector element(s)) or staring (focal-plane-array detector elements) imaging architectures was reported on at the 2011 SPIE Europe Security and Defence Symposium (Prague). This paper provides a comprehensive review of a newly enhanced MMW and SMMW/THz imaging system analysis and design tool that now includes an improved noise sub-model for more accurate and reliable performance predictions, the capability to account for postcapture image contrast enhancement, and the capability to account for concealment material backscatter with active-illumination- based systems. Present plans for additional expansion of the model’s predictive capabilities are also outlined.
It is shown that with appropriate multimode illumination and modulation strategies, it is possible to achieve the high sensitivity of active illumination, with the elimination of the need for "strategic" angular orientation of the target and to do so while minimizing the impact of coherent effects such as speckle. It is also shown that very modest terahertz (THz) power levels correspond to very high brightness temperatures, even when this power is divided among the many modes of large enclosures. We also consider how technical advances in the THz will continue to expand the scenarios of applicability for these approaches.
The U.S. Army Research Laboratory (ARL) and the U.S. Army Night Vision and Electronic Sensors Directorate
(NVESD) have developed a terahertz-band imaging system performance model/tool for detection and identification of
concealed weaponry. The details of the MATLAB-based model which accounts for the effects of all critical sensor and
display components, and for the effects of atmospheric attenuation, concealment material attenuation, and active
illumination, were reported on at the 2005 SPIE Europe Security & Defence Symposium (Brugge). An advanced
version of the base model that accounts for both the dramatic impact that target and background orientation can have on
target observability as related to specular and Lambertian reflections captured by an active-illumination-based imaging
system, and for the impact of target and background thermal emission, was reported on at the 2007 SPIE Defense and
Security Symposium (Orlando). This paper will provide a comprehensive review of an enhanced, user-friendly,
Windows-executable, terahertz-band imaging system performance analysis and design tool that now includes additional
features such as a MODTRAN-based atmospheric attenuation calculator and advanced system architecture
configuration inputs that allow for straightforward performance analysis of active or passive systems based on scanning
(single- or line-array detector element(s)) or staring (focal-plane-array detector elements) imaging architectures. This
newly enhanced THz imaging system design tool is an extension of the advanced THz imaging system performance
model that was developed under the Defense Advanced Research Project Agency's (DARPA) Terahertz Imaging
Focal-Plane Technology (TIFT) program. This paper will also provide example system component (active-illumination
source and detector) trade-study analyses using the new features of this user-friendly THz imaging system performance
analysis and design tool.
Single resonance chemical remote sensing, such as Fourier-transform infrared spectroscopy, has limited recognition
specificity because of atmospheric pressure broadening. Active interrogation techniques promise much greater
chemical recognition that can overcome the limits imposed by atmospheric pressure broadening. Here we introduce
infrared - terahertz (IR/THz) double resonance spectroscopy as an active means of chemical remote sensing that
retains recognition specificity through rare, molecule-unique coincidences between IR molecular absorption and a
line-tunable CO2 excitation laser. The laser-induced double resonance is observed as a modulated THz spectrum
monitored by a THz transceiver. As an example, our analysis indicates that a 1 ppm cloud of CH3F 100 m thick can
be detected at distances up to 1 km using this technique.
The U.S. Army Night Vision and Electronic Sensors Directorate (NVESD) and the U.S. Army Research Laboratory
(ARL) have developed a terahertz-band imaging system performance model for detection and identification of
concealed weaponry. The details of this MATLAB-based model which accounts for the effects of all critical sensor and
display components, and for the effects of atmospheric attenuation, concealment material attenuation, and active
illumination, were reported on at the 2005 SPIE Europe Security and Defence Symposium. The focus of this paper is to
report on recent advances to the base model which have been designed to more realistically account for the dramatic
impact that target and background orientation can have on target observability as related to specular and Lambertian
reflections captured by an active-illumination-based imaging system. The advanced terahertz-band imaging system
performance model now also accounts for target and background thermal emission, and has been recast into a user-friendly,
Windows-executable tool. This advanced THz model has been developed in support of the Defense Advanced
Research Project Agency's (DARPA) Terahertz Imaging Focal-Plane Technology (TIFT) program. This paper will
describe the advanced THz model and its new radiometric sub-model in detail, and provide modeling and experimental
results on target observability as a function of target and background orientation.
There has been enormous interest and considerable investment in recent years devoted to the development of the
Terahertz (aka Submillimeter) spectral region for imaging and sensing. While a number of 'one of a kind' scientific
applications (astrophysics, upper atmospheric processes, physical chemistry, molecular physics, etc.) in this spectral
region have been highly successful, there are no 'public' applications of the THz. It is the central thesis of this paper that
for most of the well publicized applications the lack of signature, clutter, and phenomenolgical knowledge - Signature
Science - impedes progress at least as much as technical shortcomings. Thus, Signature Science represents a major
scientific opportunity and a technological necessity. An important goal of Signature Science should be to provide a
scientifically based body of knowledge that will make end-to-end system analyses possible at an early stage of system
and application conceptualization and design. In this paper we will focus on three THz applications that have the
potential to make a near term impact: 1) Imaging through obscurants, 2) Spectral signatures of gas phase chemicals, and
3) Signature quantification and analysis methods for optimal use of the information content of the spectra of solids,
especially explosives.
Terahertz imaging sensors are being considered for providing a concealed weapon identification capability for military and security applications. In this paper the difficulty of this task is assessed in a systematic way. Using imaging systems operating at 640 GHz, high resolution imagery of possible concealed weapons has been collected. Information in this imagery is removed in a controlled and systematic way and then used in a human observer perception experiment. From the perception data, a calibration factor describing the overall difficulty of this task was derived. This calibration factor is used with a general model of human observer performance developed at the US Army Night Vision and Electronic Sensors Directorate to predict the task performance of observers using terahertz imaging sensors. Example performance calculations for a representative imaging sensor are shown.
We have developed several millimeter/submillimeter/terahertz systems to study active and passive imaging and associated phenomenology. For measuring the transmission and scattering properties of materials, we have developed a dual rotary stage scattering system with active illumination and a Fourier Transform spectrometer. For imaging studies, we have developed a system based on a 12-inch diameter raster-scanned mirror. By interchange of active sources and both heterodyne and bolometric detectors, this system can be used in a variety of active and passive configurations. The laboratory measurements are used as inputs for, and model calibration and validation of, a terahertz imaging system performance model used to evaluate different imaging modalities for concealed weapon identification. In this paper, we will present examples of transmission and scattering measurements for common clothing as well as active imaging results that used a 640 GHz source and receiver.
The U.S. Army Night Vision and Electronic Sensors Directorate and the U.S. Army Research Laboratory have developed a terahertz-band imaging system performance model for detection and identification of concealed weaponry. The MATLAB-based model accounts for the effects of all critical sensor and display components, and for the effects of atmospheric attenuation, concealment material attenuation, and active illumination. The model is based on recent U.S. Army NVESD sensor performance models that couple system design parameters to observer-sensor field performance using the acquire methodology for weapon identification performance predictions. This THz model has been developed in support of the Defense Advanced Research Project Agencies' Terahertz Imaging Focal-Plane-Array Technology (TIFT) program and is presently being used to guide the design and development of a 0.650 THz active/passive imaging system. This paper will describe the THz model in detail, provide and discuss initial modeling results for a prototype THz imaging system, and outline plans to validate and calibrate the model through human perception testing.
Important applications of the Terahertz/Submillimeter/Nearmillimeter/Millimeter/Far Infrared have been known for many years and a number of scientific laboratory and field instruments that approach fundamental limits have been developed. More recently, a number of 'public' applications for the non-specialist have been heavily promoted. In spite of this, no 'public' application has come to fruition and been widely adopted. With specific examples, we will show that advances in technology and scientific understanding are poised to change this. A particular emphasis will be placed on distinguishing between those opportunities for which there is a clear path to a 'public' application and those for which fundamentally unknown phenomenology or technological breakthroughs will be required.
There has been considerable interest in the use of the Submillimeter/THz (SMM/THz) spectral region for gas analysis and detection. This has been driven both by the importance of the application and the THz-TDS community. In this paper we will discuss and compare the attributes of an attractive alternative: cw submillimeter spectroscopy. Particular attention will be paid to sensitivity, specificity, and the investigations of harsh environments. A particularly simple system approach, the FAst Scan Submillimeter Spectroscopy Technique (FASSST), will be discussed and a compact and potentially very low cost implementation described. Results will be presented which include the analyses of complex mixtures of gases with absolute specificity.
The THz is unique among spectral regions because of the relative infancy of its commercial applications. Much of this infancy has been due to the well known difficulties of generating and detecting radiation. However, the enormous number of important applications in each of the other spectral regions has resulted at least as much from their large in-vestment in systems and applications development - an 'X' factor - as from the technological maturity of the spectral region. Examples in the radio region include magnetic resonance imaging (rf + 'X' = shaped magnetic fields, rf pulse sequences, and signal processing) and cruise missiles (rf = 'X' = rocket and guidance system). In the visible, Night Vision (light = 'X' = electron multiplication and fluorescence) serves as an example.
To grow to maturity, the THz needs not only to optimize its technology for native applications (imaging through ob-scuration, chemical sensing, etc.), but to integrate its attributes with other technologies to address a broader range of challenges. In this paper we will discuss the underlying physics of interactions in the THz to see how they lead to both the attractive and limiting features of the spectral region, while at the same time providing hints about how to overcome these limitations by considering 'X'. Specific examples of 'X' will be provided and the authors will welcome comments, suggestions, and ideas from the audience.
Terahertz imaging is becoming more viable for many applications due to advances in detector and emitter technologies. One of the applications for THz imaging is the detection and identification of concealed weapons (e.g., in airport security screening lines). The path described here provides an imaging performance model for the application of concealed weapon identification. The approach is the typical U.S. Army target acquisition model for sensor performance prediction coupled to the acquire methodology for weapon identification performance prediction.
Laser Induced Breakdown Spectroscopy (LIBS) is an atomic emission spectroscopic technique that utilizes a pulsed laser to create a microplasma on the target together with an array spectrometer to capture the transient light for elemental identification and quantification. LIBS has certain important characteristics that make it a very attractive sensor technology for military uses. Such attributes include that facts that LIBS (1) is relatively simple and straightforward, (2) requires no sample preparation, (3) generates a real-time response, and (4) only engages a very small sample (pg-ng) of matter in each laser shot and microplasma event, (5) has inherent high sensitivity, and (6) responds to all forms of unknowns, and, therefore, is particularly suited for the sensing of dangerous materials. Additionally, a LIBS sensor system can be inexpensive, configured to be man-portable, and designed for both in-situ point sensing and remote stand-off detection with distances of up to 20-25 meters. Broadband LIBS results covering the spectral region from 200-970 nm acquired at the Army Research Laboratory (ARL) under laboratory conditions for a variety of landmine casings and explosive materials. This data will illustrate the potential that LIBS has to be developed into a hand-deployable device that could be utilized as a confirmatory sensor in landmine detection. The concept envisioned is a backpack-size system in which an eyesafe micro-laser is contained in the handle of a deminer's probe and light is delivered and collected through an optical fiber in the tapered tip of the probe. In such a configuration, analyses can be made readily by touching the buried object that one is interested in identifying.
A new FAst Scan Submillimeter Spectroscopic Technique (FASSST) is described. It uses voltage tunable Backward Wave Oscillators (BWOs) as primary sources of radiation. In contrast to the more traditional phase or frequency lock techniques, it uses fast scan (approximately 105 Doppler limited resolution elements/sec) and optical calibration methods. Its attributes include (1) absolute frequency calibration to approximately 1/10 of a Doppler limited linewidth (less than 0.1 MHz), (2) high sensitivity, (3) the ability to measure many thousands of lines/sec, and (4) simplicity. This system is made possible by (1) the excellent short term spectral purity of the broadly (approximately 100 GHz) tunable BWOs, (2) a very low noise, rapidly scannable high voltage power supply, (3) fast data acquisition, and (4) software capable of automated calibration and spectral line measurement.
We fabricated and tested a low temperature cell which is mounted directly on the second stage of a CTI-Cryogenics Model 22C CRYODYNE CRYOCOOLER. The vacuum system consists of a room temperature vacuum shroud, a radiation shield maintained at 77K and the cell which is mounted directly to the second stage of the cryocooler. The ultimate cell temperature is 12.4 Kelvin, and the low temperature limit increases at a rate of 5.6 Kelvin/Watt. We achieve a cell temperature of 22 Kelvin under typical experimental conditions of approximately 29 milli Torr helium, slow flowing gas, and a heated injector. The absorption path length of the cell is 3.35 cm, and the window clear aperture is 1.27 cm. We preformed a series of experiments in which we determined the translational temperatures of vibration- rotation transitions in the band of CO for different cell temperatures. The results of our tests are discussed in this paper.
Molecular spectroscopy was the earliest application in the terahertz spectral region and remains one of the most important. With the development of modern technology, spectroscopy has expanded beyond the laboratory and is the basis for a number of important remote sensing systems, especially in atmospheric science and studies of the interstellar medium. Concurrently, these spectroscopic applications have been one of the prime motivators for the development of terahertz technology. This paper will review these issues in the context of the requirements placed on future technology developments by spectroscopic applications.
By use of techniques which we have developed over the past few years, it is now possible to experimentally measure pressure broadening parameters over a three decade range of temperature, including the very low temperature regime in which most spectroscopically active gasses have vanishingly small vapor pressures. As a result, collisional processes can be considered in a manner analogous, albeit at a lower resolution, to that of the more familiar energy level spectroscopy.
Remote sensing techniques based on millimeter and submillimeter spectroscopy have been shown to be powerful tools for the observation and measurement of many species of importance to the ozone cycle in the upper atmosphere. This paper discusses laboratory techniques and theoretical methods for the development of the data base which has been used as a basis for the interpretation of many of these remote sensing experiments. Recent work on HNO3, H2O, and HOOH is used as specific examples in this talk.
We have fabricated and characterized several GaAs/AlGaAs multiquantum well infrared detectors at
temperatures ranging from 6 K to 77 K. The detectors were designed to have a single bound state in the quantum
well and the first excited state in the continuum above the AIGaAs conduction band edge. The difference in
energy between the two levels, as determined by the quantum well width and aluminum mole fraction in the
barrier, was chosen such that absorption would occur in the 8-14 tm wavelength region. Each detector was
characterized by FTIR absorption, dark current, responsivity, spectral noise density, and thermal activation energy
measurements. The -maximum observed detectivity is 1 .8 x 1 012 cmIHz/W at ?= 8.3 jim and 6 K.
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