A review of the various optical methods to detect ultrasound (bulk and surface waves) at the surface of opaque solids is presented. The most useful techniques are thoroughly analyzed. Their performance when nonideal conditions are encountered, such as vibrations, air turbulence, and rough light scattering surfaces is evaluated. This review includes a description of knife-edge techniques, optical heterodyning, differential interferometry, and velocity (time-delay) interferometry methods, plus a mention of various less-important tech-

Optical Detection of Ultrasound

Ultrasonic Generation by Pulsed Lasers

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-Perot 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

https://iopscience.iop.org/article/10.1088/0957-0233/10/11/201/pdf

Optical remote measurement of ultrasound

The characteristics of an interferometric system based on two-wave mixing at 1.06 µm in photorefractive InP:Fe under an applied field for the detection of ultrasonic motion of a scattering surface are described. A theoretical analysis of possible configurations for the detection of small phase modulation in the undepleted-pump approximation is presented. Experimental assessment of the device for both cw and pulse regimes is performed: The sensitivity, the etendue, the response time, and the behavior under ambient vibrations or moving inspected samples are provided. This adaptive device presents many features appropriate for industrial inspection and compares advantageously with the passive confocal Fabry–Perot device that is now widely used.

https://hal-iogs.archives-ouvertes.fr/hal-00673505/document

Detection of ultrasonic motion of a scattering surface by photorefractive InP:Fe under an applied dc field

The performance of an interferometric system based on two‐wave mixing at 1.06 μm in undoped GaAs crystal, for the remote detection of transient motion of a scattering surface, is described. In this system, the wave scattered by the surface is mixed inside the photorefractive crystal with a pump wave directly derived from the laser to provide the reference wave of the interferometer. The system shows several features appropriate to industrial applications, although its sensitivity is less than passive interferometric systems of the confocal Fabry–Perot type presently in use, except in the low frequency range (below 1 MHz).

R. Heon

Papers

Laser ultrasonic generation and optical detection with a confocal Fabry-Pérot interferometer