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A comparison of 3 optical systems for the detection of broadband ultrasound

Thursby, G.J. and McKee, Campbell and Culshaw, B. (2010) A comparison of 3 optical systems for the detection of broadband ultrasound. In: Smart sensor phenomena, technology networks and systems 2010. Proceedings of SPIE-The International Society for Optical Engineering . SPIE, Bellingham. ISBN 9780819480637

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Abstract

There are many applications of ultrasound in the field of material properties' evaluation and structural health monitoring. Here we will consider the detection of broadband laser generated ultrasound taking as an example acoustic emission as simulated by the pencil break test. In this paper three optical methods of detecting these ultrasound signals are compared; these are polarimetry, fibre Bragg gratings and vibrometery. Of these, the first two involve the bonding of a fibre sensor to the sample, whilst the vibrometer is a non-contact instrument that measures out-of-plane displacements. FBGs respond to the inplane strains associated with an ultrasound wave whilst the polarimeter detects birefringence produced by pressure waves acting normal to the fibre. The sensitivities of the systems are compared and their relative merits are discussed. It will also be shown that the polarimetric responses of symmetric and antisymmetric Lamb waves differ, which opens up the possibility of learning more about the nature of an acoustic signal using this technique than can be determined simply from the measurement of in-plane or out-of plane displacements alone.