Easily encodable surface acoustic wave (saw) security devices

Abstract

This invention features a method for encoding surface acousting wave (SAW) delay lines. The method uses a split-electrode transducer as a programmable reflector. When shorted out, it does not reflect surface waves. When open-circuited, it partially reflects incident surface waves. As manufactured by conventional photolithographic methods, each of several such transducers is shorted out by a fusible link. In the as-manufactured state, the array of shorted transducers produces no reflection of surface waves. If a fusible link is destroyed by passage of electric current or other means, the corresponding transducer becomes a partial reflector. The status of these switchable reflectors may be read by exciting a separate launch transducer with a burst of electromagnetic energy containing substantial energy at the basis operating frequency of the SAW transducers or at suitable harmonics. SAW energy reflected by the switchable reflectors appears at the same transducer at delay times determined by the spacing of the elements. The presence of a pulse at a given delay time indicates that the corresponding fusible link has been destroyed; conversely, absence of the pulse indicates that the fusible link is intact. Thus, destruction of fusible links provides a method for encoding SAW devices for security and other applications, in situ.