A 2D optical ultrasound array using a polymer film sensing interferometer

A 2D optical ultrasound receive array has been investigated. The transduction mechanism is based upon the detection of acoustically-induced changes in the optical thickness of a thin polymer film acting as a Fabry Perot interferometer (FPI). A sensor head employing a medium finesse polymer film FPI has been fabricated and characterised. By illuminating the interferometer with a large diameter (12 mm) laser beam and raster scanning a photodiode across the reflected output beam, a 2D ultrasound array was simulated. To demonstrate the concept, the ultrasound field distributions at various distances from the focus of a 5 MHz focussed ultrasound transducer were mapped. The system was also evaluated by performing transmission ultrasound imaging of several targets of known dimensions. The “array” aperture, defined by the dimensions of the incident optical field, was 12 mm in diameter and spatially sampled in 0.1 mm steps. Element sizes, defined by the photodiode aperture, were 0.4 mm for the focussed transducer measurements and 0.8 mm for the transmission ultrasound images. The wideband (30 MHz) noise-equivalent-pressure was 3 kPa and the acoustic bandwidth 12 MHz. It is considered that this approach has the potential to be used for transmission medical ultrasound, biomedical photoacoustic imaging and ultrasonic field characterisation applications