Overview

A contact ultrasound sensing technology based upon the detection of acoustically-induced changes in the optical thickness of a Fabry Perot polymer film sensing interferometer has been developed as an alternative to piezoelectric based detection methods for ultrasonic measurement and imaging applications in medical and industrial fields. The technique provides an inherently broadband (>30MHz) response and excellent detection sensitivities (~1kPa) comparable to those of piezoelectric PVDF transducers. A key feature is that the sensing geometry is defined by the area of the polymer sensing film that is optically addressed. This provides high degree of flexibility in configuring the acoustically sensitive region enabling arbitrary aperture shapes, sizes and apodisations to be achieved. One consequence of this is that very small element sizes (in principle down to the optical diffraction limit of a few µms) can be obtained for low directional sensitivity without compromising detection sensitivity - a useful advantage over piezoelectric transducers in this respect. It also means that, by spatially sampling a relatively large aperture, a high density ultrasound array can readily be configured. Other advantages are that, the sensing element can be inexpensively batch fabricated using polymer film deposition techniques, has the ability to self-calibrate, and is electrically passive and immune to EMI. A range of devices using this type of sensor have now been developed. These include a miniature optical fibre hydrophone for medical and industrial ultrasound field characterisation and in situ measurements of medical ultrasound exposure. A photoacoustic-photothermal probe has also been developed as a diagnostic tool for the in vivo characterisation of biological tissues via measurement of their optical properties. A promising recent development has been demonstration of a high density 2D optical array with applications in photoacoustic imaging, medical and industrial transmission ultrasound and rapid transducer field mapping.