Sound velocity measurement based on optical heterodyne detection techniques using two successive ultrasonic waves

Abstract

A new optical method for measuring the velocity of sound has been investigated; it is based essentially on optical heterodyne detection techniques using a pair of ultrasonic light modulators of slightly different frequency. A light beating signal can be easily generated from the frequency difference between these modulators if signal and reference light waves obtained from the first order dual diffraction spectrum are photomixed with each other. The only optical components are the two modulators, and their alignment is not critical to the generation of a stable beating signal. Since the beating signal includes the sound velocity in each modulator in its phase factor, the sound velocity is immediately measured by making use of a conventional electronic phasemeter.