An L-band ultrasonic probe using polymer optical fibre

In recent years, Polymer optical fibre (POF) has been receiving increasing attention for sensing applications. For applications such as endoscopic ultrasound and photo-acoustics, PMMA polymer fibres deliver ever improving performance and more types of polymer are being trialled for these applications.

The fundamental properties of POF, when correctly leveraged, deliver at least an order of magnitude in improvements over silica fibres. POF delivers lower acoustic impedance, a reduced Young's Modulus and a higher acoustic sensitivity within the megahertz region. In contrast, existing piezo-electric transducers have an inherent narrow acoustic bandwidth and a proportionality to size that causes difficulties for applications such as endoscopy within the biomedical domain.

With the increasing take-up of POF, improvements have been made in fibre dopant distribution, the range of polymers available and connectorisation techniques. While newer polymer fibres are under preliminary study, PMMA has shown itself to be the most sensitive fibre to date despite historically higher levels of dopant inconsistency and an implementation around 830 nm that has a lower signal-to-noise ratio than possible.

The prior approach of edge filtering a Bragg grating with a high speed photodiode has been shown to function and is therefore maintained. We present a step index PMMA Bragg grating ultrasonic probe in the L-band for the first time. Detection is achieved using a Bragg grating less than 1cm in length in a fibre under 10cm long.

We examine the temporal and frequency response of the sensor over a 1-15 MHz range.