Assessment of Millimeter-Wave and Terahertz Technology for Detection and Identification of Concealed Explosives and Weapons (2007) / Chapter Skim
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3 Component Technology
3 Component Technology
Pages 27-40
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From page 27... ...
personnel in the future. One of the best reviews of component maturity in the millimeter-wavelength/ terahertz region is by Siegel,1 describing in detail the state of the art of numerous 1 P.H.
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From page 28... ...
The heterodyne receiver is generally more sensitive than a direct detecting receiver, as this down-conversion process allows the use of lower frequency and thus more sensitive detection schemes. The use of stable narrowband sources with a heterodyne receiver also provides an ability to measure spectral features rapidly with high resolution.
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From page 29... ...
NOTE: CW, continuous wave; TUNNETT, Tunnel Injection Transit Time; SLED, Super Luminescent Diode light source; RTD, Resonant Tunneling Diode; HG, harmonic generation; QC laser, Quantum Cascade laser; III-Vs, compounds from the periodic chart columns III and V (e.g., GaAs) ; IMPATT, Impact Ionization Avalanche Transit Time; and Gunn, electron drift velocity is decreasing as electric field in semiconductor is increasing above certain critical value.
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From page 30... ...
While in the past it was common to use whiskered Schottky diodes as multipliers, planar diodes have become readily available and offer suitable performance. Their output power levels can be limited by the maximum amount of input power that they can handle or by even the existence of a suitable source of RF that can be multiplied efficiently.
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From page 31... ...
COMPONENT TECHNOLOGY 31 FIGURE 3-3 Power levels available from the various devices produced by Virginia Diodes. SOURCE: Courtesy of Virginia Diodes.
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From page 32... ...
pulsed lasers that can illuminate semiconductor material that illuminates a photoconductor, such as gallium arsenide, to generate carriers. The carriers then form a current in the material, which is radiated through an antenna.
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From page 33... ...
Some applications will also require that the phase of the illumination function be preserved so that advanced imaging techniques such as holography or sparse aperture array synthesis can be performed. In the region of 30 GHz to 300 GHz, numerous types of receiver components are available commercially, so there can be systems trade-offs that include issues of cost and complexity as well as strict component performance.
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From page 34... ...
As was pointed out above, direct detectors have yet to achieve the sensitivity of heterodyne receivers, but they are more viable for combining into arrays to perform imaging using architectures similar to what is used in the visible and infrared region. Other than sensitivity, limitations to date for uncooled detectors have included long output time constants, which is a detriment for video frame rate imaging.
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From page 35... ...
They are usually simple enough to be produced in arrays but must be integrated with micro antennas to efficiently couple RF energy into them. Since detectors are less sensitive than heterodyne receivers, they also must operate over a wide RF bandwidth, which makes the design of the micro antenna coupling more difficult.
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From page 36... ...
SYSTEM PERFORMANCE Table 3-1 summarizes the performance trends of component technology as a function of increasing frequency or operating range that impact how well any millimeterwavelength/terahertz system would be expected to perform. While Table 3-1 only indicates trends, it does show how the phenomenological effects and component performance issues interact.
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From page 37... ...
As the most critical component in the atmosphere is water vapor, these tests provide a good baseline for potential real-world operations. TABLE 3-2 Parameters Used for Systems Analysis of Standoff Imaging Sensor Model Component Parameter Transmitter and receiver 0.5 m antenna diameter Transmitter power 10 mW Receiver noise equivalent 10-12 W/√(Hz)
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From page 38... ...
Analysis of performance in this millimeter-wavelength/terahertz spectral region would benefit greatly from the extensive characterization of the reflectivities and losses of materials pertinent to the problem as well as the variability with aspect angle, surface roughness of materials, and environmental conditions. It is important to note that the components defined in the model have not yet been realized but are being developed, at fixed frequencies, under the DARPA TIFT program.
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From page 39... ...
COMPONENT TECHNOLOGY 39 Conclusion: The technology base for millimeter-wavelength/terahertz security screening is expanding rapidly internationally, yet there is insufficient technology available to develop a system capable of identifying concealed explosives. Recommendation: To perform an accurate assessment of the applicability of millimeterwavelength/terahertz-based technology to explosive detection, the Transportation Security Administration will need to do the following: (1)
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