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RADIO/RADAR/OPTICAL TAGS 20 it could be used by ground-based friendly forces to communicate covertly with radar systems on aircraft. A simple message (e.g., 30 bits) could be coded into the PARD and read by a VESTA radar system passing overhead without the message being detectable to hostile forces. COVERT RADAR TAGS AND APPLICATIONS Bill Hurley discussed his company's technology, which involves incorporating a large number of radar reflectors in a substrate to form passive RFID tags. The radar reflectors used by Inkode are usually simple aluminum fibers that form half-wave resonators within the object to be tracked (e.g., a piece of paper). The radar- reflecting fibers are approximately the same diameter as paper fibers (typically 6.5 mm long and 1.5 µm in diameter). Randomly oriented radar-reflecting fibers provide a unique backscatter pattern that can be read and stored in a database for future identification. Ordered patterns can also be designed so that individual resonators are coupled or decoupled, whatever is likely to give the optimum backscatter pattern. Tagging an object with this technology costs about 1 cent. When illuminated with radar, the backscattered fields interact to create a unique interference pattern that enables one tagged object to be identified and differentiated from other tagged objects. In the near field (that is, where there is an interaction between the tag and antenna), the backscatter depends on the position of the tag in the substrate, and information is represented in a scalar waveform. Beyond the near field, the backscatter is like a traditional radar, with no coupling between tag and antenna. For nonmilitary applications, the reader is less than 1 meter from the tag. For military applications, the reader and tag could theoretically be separated by a kilometer or more. The tags are the same in these two cases, but the reader is different. The normal operating point is 10 mW at 24 GHz. The three most commonly used tags are free, individual resonators, continuous filaments, and photolithographically printed patterns of filaments. They consist of aluminum/glass fibers, either coaxial or side by side. These may be adapted for inclusion in a variety of objects, including paper, airline baggage tags, book bindings, clothing and other fabrics, and plastic sheet. The reflectance of the half-wave resonators is very efficient. In a typical 8½ by 11 inch piece of paper, there are over 8,000 radar reflecting fibers. This can theoretically be seen from a distance comparable to the distance from which a 1 m2 target can be seen. NANOSCALE DESIGN AND FABRICATION CONSIDERATIONS FOR PHOTONIC TAGS AND RADAR DEVICES Dennis Prather discussed applications of nanoscale photonics technology and some of the tools being developed to fabricate these devices. Applications include (1) diffractive (as opposed to refractive) lenses that can reduce the size and weight of millimeter wave imaging systems; (2) polarization-dependent tags or reflectors that can be read with a CO2 laser; (3) integrated sensors using components made from photonic crystal devicesâ that is, devices that guide light based on the scattering properties created by tailoring the material profileâsuch as beam splitters, optical switches, and couplers; and (4) motion microsensors, both rotational and linear. The beam routing in photonic crystal devices is strongly wavelength-dependent; for instance, such devices exhibit a sharp spike in reflectivity over a narrow frequency bandgap. This property can be exploited in photonic tags. Prather discussed the capabilities of various fabrication methods for creating nanoscale patterned structures, such as e-beam and interferometric lithography, methods for optical drilling of nanoscale cylindrical holes through silicon, and techniques for making tapered connectors to link microscale
