Laser probing system for the accurate detection of surface acoustic wave phase velocities

Abstract

The present work demonstrates a high-resolution technique for the optical detection of the phase and amplitude of high frequency surface acoustic waves (SAWs). 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, LiNbO<SUB>3</SUB> has been used for the test measurements. Minimum detectable surface displacement and dynamic range were 1 pm/Hz<SUP>1/2</SUP> and 40 dB, respectively. The method enables the determination of the phase velocity with a resolution of 1.5 (DOT) 10<SUP>-5</SUP> in dependence of crystal cut, temperature, and frequency. An additional feature is that phase velocity values can be obtained with spatial resolution, i.e., velocity variation effects along the propagation path can be evaluated. We found that the assumption that the SAW velocity is constant across the whole device surface is not valid in general.