Acoustooptical techniques for ultrasonic materials evaluation: optical fiber interferometry and pulse-echo systems

Abstract

The performance of two acoustooptical systems for ultrasonic materials characterization has been investigated. The first system uses an optical fiber interferometric technique to detect directly slowly varying residual stresses and both high frequency continuous and transient ultrasonic waves characteristic of acoustic emission events in composites. The potential for embedding optical fibers inside fiber composites during manufacturing makes this an attractive built-in alternative to conventional contact transducers for the nondestructive evaluation of composite materials. Experimental static and dynamic calibration as well as the frequency domain characterization of acoustic emission as detected by this system are discussed.

The second system is a pulse-echo transducer system that generates an electronically focusable ultrasonic field and detects the reflected field optically. The generating transducer consists of concentric ring electrodes etched on a single x-cut quartz crystal substrate with a 1 mm circular aperture at the center. By shaping the amplitude and phase profile of the high voltage pulse applied to each ring, a focused ultrasonic field results at an adjustable distance below the transducer. If the field is focused below the surface of a specimen, energy in the wave reflected from the focal point modulates the normal and parallel components of surface displacement. Furthermore, if the sample arm of an adaptively stabilized interferometer is focused on the free surface at the transducer center aperture, the normal component of surface motion may be detected. This system has potential applications in materials characterization and evaluation as well as biomedical imaging. The design of the system is discussed with particular emphasis on the necessary sub-system interfaces required for operational flexibility.