Theoretical Analysis of Comb Transducers for Surface Acoustic Waves

A comb transducer is one of the surface acoustic wave (SAW) transducers utilizing bulk-to-surface mode conversion, and is capable of exciting SAW's very efficiently on the surface of any test material for NDE applications. However, no thorough analysis of the transducer is available in the literature, and many problems on the mode conversion characteristics remain unexplained. In this paper, the operation of a typical comb transducer has been theoretically analyzed on the basis of the leaky wave concept and the coupled mode theory. The answers to such questions as the required appropriate coupling length between the comb coupler and a test substrate in relation to the mode conversion efficiency (MCE), the effect of liquid couplant thickness on the MCE, and the value of MCE expected to be obtained in theory are given.