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RADIO/RADAR/OPTICAL TAGS 21 components to nanoscale components. He showed how silicon could be patterned on the nanoscale by E-beam lithography to create dramatically different images when viewed with horizontally (as opposed to vertically) polarized infrared laser light. Prather showed how a micro-ring laser patterned in silicon (~10 Âµm in diameter, with coupled output- sampling waveguides) could measure rotation rate and an optical waveguide can be inserted into a microcantilever MEMS device to measure linear motion. The small dimensions of nano-optical waveguides mean that relatively small electric fields (millivolts) can be used to modulate light transmission through the electro-optic effect, because these fields equate to kilovolts per meter. By coupling an RF antenna to a nano-optical waveguide fabricated from silicon and a few micrometers in width, for instance, one can impose millimeter-wave sidebands on the optical carrier using the electro-optic effect, separate the sidebands from the carrier with a dispersive element, and measure the sideband strength with a low-frequency detector, thus avoiding the need for any RF circuits. Future challenges include further development of nanoscale integration techniques and refining the compatibility of manufacturing processes for both micro- and nanostructures. DYNAMIC OPTICAL TAGS Stephen Griggs described a DARPA-funded program starting to develop small, retro-reflecting optical tags that can be attached to targets, assets, and precision special reference points. These tags would provide non-RF location and tracking, with covert, two-way data exchange in friendly and denied areas. The specifications are as follows: size, 25Ã25Ã5 mm (a small thickness is critical for covertness); operating temperature â¢40Â° to 70Â°C; data rate, >100 kbps; optically readable from a distance of 10 km (line of sight) by an airborne or handheld interrogator; operating time >2 months; acceptance angle >+/â¢60Â°; cost, <$100 per tag; and non-visually alerting. As the tagged object moves through a region, the Department of Transportation could record location information (via GPS) as well as other data (imagery, audio, etc.) that can provide vital information and decrease target ambiguity. The main technical challenges are to develop (1) thin, retro-reflecting optics that can be modulated and (2) tag-specific transceiver systems that are eye-safe at the tag (range of =1.3â2 Âµm) and that can search and interrogate quickly and automatically (both handheld and airborne versions). The power requirements for the tag are estimated to be about 75 mA-hours total over 2 months. Commercial lithium coin batteries can be used. They are available in the appropriate thickness and can provide up to 120 mA- hours at approximately 3 volts. The range requirement also appears achievable based on the sensitivity of present photomultiplier tubes relative to the expected return signal intensity. The program will be structured in three phases. The first, beginning in FY04, will develop various tag technologies and utilize a bench interrogator. The second, beginning in FY05 (assuming a positive go-no go decision), will focus on dynamic optical tag system design, including a handheld interrogator. If progress through FY05 is acceptable, prototype dynamic optical tag systems will be demonstrated using an airborne interrogator in FY 06 and 07. PANEL 5 DISCUSSION The discussion consisted largely of questions directed at individual presenters. Shellans was asked what materials are used in the PARD, given that one must modulate at rates greater than 100 kbps. He described the use of indium phosphide, which can be modulated at rates greater than 200 kbps (by
RADIO/RADAR/OPTICAL TAGS 22 moving just the electron cloud). In the future, it should theoretically be possible to transmit voice using PARD. Hurley was asked how one can read a specific passive RF tag at a random angle if the tag is made up of randomly oriented fibers. The answer is that the computer processing the return signal can calculate what the return signal from each tag in its library should look like as it is rotated with respect to the interrogating radar and match the pattern to the observed pattern. Instead of inserting fibers into the material to be tagged, it is also possible to print metal lines on the surface. Currently, the main applications of these passive tags are in security documents (e.g., passports) and currency. Prather was asked why, given the potential value of photonic band gap materials, the technology is not in use. For example, one can control the reflection spectra off various materials (though it is hard to make the reflection isotropic) and one can use the band gap to confine light effectively in waveguides and achieve a very high photonic âwiring densityâ and overlay capability with no crosstalk between waveguides. His answer was that satisfactory processes for integrating hybrid devices on silicon still need to be developed, and losses need to be reduced. Griggs was asked how small the optical retroreflectors can be made. With UV interrogating light, it is possible to make them with micrometer-scale dimensions. All of the corner cube reflectors being evaluated can modulate light at frequencies of at least 200 kHz. To make retroreflectors more covert, one can make them reflective at only a single frequency using photonic band gap materials. The reflection wavelength can be tuned to suppress the visible wavelength signal. Gratings can also be used as retroreflectors if they are oriented at the right angle, but of course the angle is wavelength-dependent. Griggs stressed that if one wants to make the tags very small and have reasonable data transmission rates, they must be optical. No RF device can match the size and data rate of optical devices. For example, using an optical device one can detect voice signals on the ground from a U2 aircraft using a 2-ft. by 2-ft. box. RF may be appropriate if lower data rate, power, and range are acceptable. The disadvantage of optical devices is that they must operate within the line of sight. Covert dynamic optical tags are expected to be manually placed on noncompliant targets.