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The Optoelectronics Division of EEEL has developed and demonstrated an ultra-wideband, millimeter-wave/terahertz detector array for use in contraband detection and characterized it in a testbed using tools developed by EEEL specifically for terahertz metrology. The modular system of 8-element, linear detector arrays, scalable to a 128-channel THz camera, is based on Nb and NbN superconducting microbolometer detectors co-developed with VTT of Finland. Raster-scanned images of human subjects rank among the best in their class, clearly revealing handguns, ceramic knives, and other contraband hidden under clothing and distinguishing them from wallets, cellphones and other innocuous items.
The unique wideband (0.2-1.4 THz) feature of the imaging system enables a high spatial resolution with a reasonable optical system footprint (approximately 1 x 1 x 2 m). An optical design based on a Schmidt telescope and a periscopic conical scanner provides a wide field of view of 2 m x 4 m (14o x 28o) at a range of 8 m. Modularity is arguably the most important feature of the system, enabling reconfigurable focal plane sampling and lowered costs due to the robustness to lower-yield fabrication processes.
Tuning and characterization of the spatial resolution, noise-equivalent temperature difference, and spectral response of the imaging system was accomplished using standard targets, calibrated blackbodies, and variable filters specifically developed for terahertz frequency metrology by EEEL researchers Erich Grossman and Charles Dietlein (a graduate student at the University of Colorado). For example, the Aqueous Blackbody Calibration (ABC) source, developed in a DARPA-funded program, provides an accurately known radiometric temperature over most of the imager’s frequency band. It enabled the measurement of record low noise-equivalent temperature difference (a measure of detector sensitivity) of 0.2 K for the Nb detector elements.
The latest imaging results received significant recognition when they were presented at the SPIE Defense and Security symposium in March 2008. Although the commercial 94 GHz cameras are at a more advanced technology-readiness level, their performance is topping out. NIST’s ultrawideband and modular approach generated considerable excitement about potential future capabilities in terahertz imaging.
For more information contact: Erich Grossman, phone 303-497-5102
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