Optical remote measurement of ultrasound

During the last decade, significant advances in the field of remote detection of ultrasound have taken place. Optical systems with increasing sensitivity for monitoring ultrasound on samples with optically rough surfaces have been developed, so that some designs are now moving from the research laboratory into industrial environments. In this review, a range of optical techniques is presented. These techniques are analysed regarding their use for the measurement of ultrasonic waves. They include the use of optical self-mixing, phase modulators and photorefraction-based interferometers. For the case of established instruments such as two-beam interferometers and Fabry-Pérot interferometers, various newly developed configurations with associated ultrasonic transfer functions are described. Many interferometers have broadband frequency responses that extend up to several hundred megahertz. Some systems offer in-plane ultrasonic measurement, whilst most offer out-of-plane measurements. Emphasis is placed on their potential for industrial applications, taking into account the likely sample-surface roughness and environmental vibration.