Direct measurement of SAW dispersion relation by laser probe

A unique technique for direct measurement of SAW dispersion relation using a phase-sensitive laser probe is presented. This technique is based on analysis of fixed-frequency longitudinal scans across a reflection grating structure. The first major step is purely experimental, in which the various contributing waves are identified by examining the respective spatial spectrum of the measured profiles. For a leaky wave grating, this yields directly the real part of the wavenumbers of both the counter-propagating leaky waves and the Rayleigh wave as functions of frequency. Standard signal processing techniques can then be applied to allow extraction and separate examination of the individual waves. The second major step is to compare the measured real part of the leaky wave wavenumbers and the extracted profiles with theoretical models for verification and/or reduction of the model parameters. By comparing individually extracted profiles at fixed frequency points, the velocity, reflectivity, as well as propagation loss, can be verified and/or obtained with high degree of accuracy. Results from leaky wave grating on 64° LiNbO₃ are reported. These include the first direct observation of frequency dependent propagation loss due to bulk scattering. Comparison with the latest Coupling-of-Modes modeling is also discussed. Using the same measurement and data reduction methodology with a long transducer, independent measurements of the transduction coupling strength and the bulk radiation loss appear feasible. Furthermore, this technique should be equally applicable to purely Rayleigh wave and surface transverse wave grating structures