Accurate optical measurement of surface acoustic wave phase velocity

A high-resolution technique for the optical detection of the phase and amplitude of high frequency surface acoustic waves (SAW´s) is presented. The test setup incorporates a mode-locked picosecond laser, harmonic mixing, and coherent detection, and it allows not only the measurement of the surface wave field but also the direct determination of the phase velocity. A measurement bandwidth in excess of 2 GHz is achieved. The maximum scan length was 4 cm. As a substrate, LiNbO₃ has been used for the test measurements. Minimum detectable surface displacement and dynamic range were 1 pm/Hz^1/2 and 40 dB, respectively. The method enables the determination of the phase velocity with a resolution of 1.5·10^-5 in dependence of crystal cut, temperature and frequency