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Smart materials and structures are part of a rapidly evolving, multidisciplinary approach to using a material's intrinsic properties or combining materials to achieve inherent intelligence (Rogers 1989; Ahmad, et al. 1990). Smart materials may be defined as materials that possess intrinsic properties capable of responding and adapting to external stimuli. The material' s intelligence may be the result of its composition, processing, microstructure, presence of defects, or conditioning. Smart structures may be comprised of integrated smart materials and/or more discrete components such as actuators or sensors that, in combination, provide the required intelligence. Optical fibers have been the basis of advanced polymer composites to prepare intelligent structures for the past ten years (Claus 199 1). Optical fibers are small, immune to electromagnetic interference, and lightweight. They can be embedded in other materials, have an adjustable composition, and can operate in harsh environmental conditions. Optical fiber-based "smart structures" are able, via embedded or attached optical fiber (the "smart material") and the associated electronic circuitry, to monitor the polymer's physical integrity and structural behavior during use. The unique ability of optical fiber to act as a signal transmitter as well as to modulate a propagating optical signal as a response to external stimuli has led to numerous applications of optical fiber-based smart structures. Although capable of detecting electrical and chemical phenomena, optical fiber sensors have been developed primarily for determining strain, thermal expansion, and vibration of structural components. Non-optical glass or polymer fibers are typically embedded in polymer structures to enhance strength and toughness, for example, panels for the automobile and aircraft industry. Replacing a portion of the structural fiber with optically-conducting fiber permits fabricating robust, optically-active structures such as tamper-indicating secure containers. Secure containers are optical fiber-based smart structures that offer the ability to continually or passively monitor the integrity of the container walls. Continually monitored secure containers monitor in real time, with a container breach activating the smart structure. Smart structure activation can produce numerous consequences within the container, depending on the specific application of the container, the size of the container and the complexity of the accompanying electronics. At a minimum, smart structures can be given the ability to recognize and record container breaching. Difficulty in defeating the tamper-indicating secure container depends on the smart materials' stealth and the smart structure's complexity. Complexity can be enhanced by incorporating the smart material into the container walls using additional, non-active decoy material. In addition to tamper indication, the combination of optical fiber embedded in a polymer matrix lends itself to various sensing capabilities. Either the fiber can act as a buried sensor, for example, detecting radiation or temperature changes, or the polymer matrix can be made sensitive to pressure or specific chemical(s), causing the polymer to react and create a signal in the optical fiber. For example, chemical sensors can be prepared by embedding an optical fiber array into a polymer sheet that is then coated with another polymer sensitive to the specific chemical. Similar to the manner in which discrete optical fibers function (light travelling down a high refractive index core a Pacific Northwest Laboratory is operated by Battelle Memorial Institute for the U.S. Department of Energy under Contract DE-ACO6-76RLO 1830. reflecting off a lower refractive index cladding), a continuous, linear region of high(er) refractive index can be created in a clear polymer such as poly(methylmethacrylate), creating an optical path or channel. These clear polymer "windows" are capable of tamper indication as well as providing sensing capabilities. Waveguide circuitry written into the polymer window can be designed so that, when the window experiences a stimulus such as a temperature or pressure change, the stimulus is manifested in an attenuation or phase shift between sensing and reference waveguide paths. The objective of this paper is to describe the design, construction, and potential applications of several optical-based smart structures. The properties of optical fiber/polymer matrix smart structures will be exemplified through the construction of a small, tamper-indicating secure container and tamper-indicating panels for a secure video system. Tamper-indicating and sensing capabilities of polymer windows containing channel waveguides, plus their integration into secure containers will be discussed. Problems associated with design, construction, and growth potential of optical-based smart structures to other technical areas will be addressed.
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