FIFTH CANADIAN CONFERENCE ON NONDESTRUCTIVE ... - IAEA

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OPTICAL DETECTION OF ULTRASOUND AT DISTANCE
Jean-Piz um Monchalin
Enzngu, Mines and Re souicc s, Ottawa, Ontario
+ Work performed in the laboratories of the Industrial Materials Research
Institute, Boucherville.
ABSTRACT
Various probes which are based on optical interferometry and are used to
detect ultrasound at distance (distances from less than 1 millimeter to
several meters are considered) are reviewed. Their properties and sensitivity
(detection limit) are critically discussed. We analyze a Michelson
interferometer probe which senses the surface displacement and another one,
working as an optical frequency discriminator, which senses the surface
velocity. A differential arrangement is also discussed. We present also
preliminary experimental results obtained by laser generation and optical
detection on steel at distances ranging from 1 to 2 meters.
1. INTRODUCTION
In recent years, considerable attention has been focused on the generation of
ultrasound using pulsed lasers and numerous results have been published (1, 2,
3, 4). This technology has the advantage to enable truly generation at
distance, so it enables to produce ultrasound in hot pieces (e.g. in the steel
industry) and at locations with difficult access. It is also worth noting
that very short ultrasonic pulses can be produced, which enables an excellent
resolution close to the surface. This technology, in order to be useful, has
however to be coupled with a detection means which can also operate at
distance. Optical interferometry has been recognized to be such a means.
Numerous interferometric systems for the detection of ultrasound have been
previously described, but if we except the one developped by a private company
(5), none can really be applied to an industrial environment. Industrial
applications require a system with a sufficiently high sensitivity to be able
to be used on pratical rough surfaces. These surfaces have not been specially
polished, as it can be done in the laboratory for more fundamental
experimentation, and they scatter incident light in various directions,
leaving a small fraction to be collected by the detection apparatus. This
system has also to be immune from ambient vibrations. Also, except for very