Interferometric pulse-echo ultrasonic transducer system

Abstract

The design, construction, and evaluation of a hybrid pulse-echo transducer system which uses an ultrasonic generating element and an optical detection technique is described. The transmitting transducer consists of a concentric aluminum-chromium ring pattern deposited on a 1.9-cm-diam circular x-cut quartz substrate with a circular 1.0-mm hole in the center. The rings are independently pulsed with a sequence of phased high-voltage signals in such a way that the ultrasonic waves resulting from the separate rings are focused at a controllable distance below the specimen surface. Energy in the focused wave reflected from this focus position is determined by the local reflection coefficient of the medium at the focal point. The reflected wave produces both normal and parallel components of particle displacement on the surface of the specimen at the location of the transmitting transducer and a wideband interferometric optical system insensitive to low-frequency background vibrations is used to measure the normal displacement through the hole. By processing the received signals for a range of ultrasonic transducer focal lengths, the system can be used to locate and size anomalies within solids and liquids. Applications in nondestructive evaluation and biomedical scanning are suggested. [Work partially supported by NSF and NASA.]