Improved method of measuring surface acoustic wave velocity in anisotropic structures

The paper describes an original interference method of measuring the velocity of a surface acoustic wave (SAW) propagating in an anisotropic solid. The method is based on the optical probing of the SAW simultaneously at two points and new possibilities arising from the use in the measurements of a two-beam optical interferometer. In the experimental arrangement, two spatially separated optical beams are diffracted by the SAW and are then combined to form an interference pattern. From a measurement of the period of the oscillations of this pattern as the SAW frequency is varied, the phase and group velocities of the SAW are found and also their dispersion in the anisotropic semi-space. To do this, the inverse problem of the diffraction and interference of two beams of light reflected from the SAW is solved. The software of the experiment includes: a) accumulation of data with statistical treatment and with autocorrection of the phase drift of the interferometer caused by the change in the external conditions; and b) a Fourier analysis of the signal spectrum using generalized variables with allowance for the SAW dispersion; c) a computational algorithm for determining the phase and groups velocities of the SAW. The accuracy of the velocity determination depends on the experimental conditions and is of the order of 0.5 m/s for the phase and 3 m/s for the group velocity. Testing of the method was carried out for ST-quartz covered with a thin aluminum film.