Characteristics of optimized fibre-optic ultrasound receivers for minimally invasive photoacoustic detection

A range of miniature (125 mu m o.d.) fibre optic ultrasound sensors based on the use of interferometric polymer optical cavities has been developed for minimally invasive photoacoustic imaging and sensing applications. It was observed that by careful selection of both the fibre tip and cavity geometry it is possible to achieve exceptional acoustic performance. Specifically, rounding the tip of the fibre to remove the presence of sharp diffractive boundaries enables a well behaved frequency response along with a near omnidirectional response at frequencies in the tens of MHz range to be achieved. The use of a plano-convex rather than a planar cavity provides high finesse and therefore detection sensitivity. Thus, by using a plano-convex cavity formed at the tip of radiused single mode fibre it was possible to realise a miniature ultrasound detector with a bandwidth of 80MHz, a noise-equivalent pressure of 40Pa (over a 20MHz measurement bandwidth) and a near omnidirectional response at frequencies as high as 30MHz. These characteristics suggest this type of sensor could find applications in interventional medicine for guiding needles or catheters, as mechanically scanned photoacoustic imaging probes or in laser scanning OR-PAM.