Optical Detection of Ultrasonic Disturbances

Abstract

Optical methods for detection of ultrasonic signals offer the potential for performing noncontact and remote sensing for ultrasonic inspection and acoustic emission testing of materials and structures. These capabilities are of particular advantage for sensing applications at elevated temperatures or in hostile environments. While a number of optical detection methods have been employed in the past, most efforts have involved the development of interferometric techniques of one form or another. Technological advances in conventional and holographic methods of optical interferometry have greatly improved the potential for noncontact and full field detection of ultrasonic signals. Through proper design, displacement sensitivities of 10−5 Angstroms per square root Hertz of system bandwidth (A/[Hz]½) have been achieved. With somewhat lower sensitivity, holographic methods have been shown to provide full field displays of acoustic displacements “frozen” in place on the surface test materials. These methods have no direct counterpart in conventional ultrasonic testing. Nevertheless, materials parameters such as ply orientation in composite materials and effective elastic modulus may be determined directly from the observed displacement pattern. Likewise, thickness variations and near surface defects can be detected as well. These optical methods have been demonstrated in the laboratory and are under development for industrial application. At the same time, new technological advances in the areas of fiber-optics, electronic (filmless) holography, and optical phase conjugation hold promise for improved applicability and sensitivity of optical methods for ultrasonic inspection.