Direct and indirect dual-probe interferometers for accurate surface wave measurements

This paper describes the development and operation of two dual-probe interference systems for detection of surface acoustic waves. Both systems operate by comparing the surface displacement under each probe, thus enabling the surface wave velocity to be determined. In the so-called direct interference system the two probe beams are made to interfere with each other whereas in the indirect interference system each probe beam is made to interfere with a common reference. The relative merits of each system are compared in detail and experimental results showing operation of the systems are presented.