Non-Resonant Fibre-Optic Ultrasound Sensors

The small size, high sensitivity, broad detection bandwidth and immunity to electromagnetic interference of fibre-optic ultrasound sensors make them highly attractive for applications in biomedical imaging, metrology, non-destructive testing and structural health monitoring. Typically, such sensors derive their sensitivity from optically resonant structures, such as Fabry-Pérot cavities, which require specialist equipment for accurate fabrication. Here, a fibre-optic ultrasound sensor is presented that does not contain an optically or mechanically resonant sensing element, but instead comprises a simple deformable and reflective structure at its tip. This structure, which was fabricated manually using simple dip-coating methods, was interrogated by a fibre-coupled laser Doppler vibrometer that measured the absolute velocity of the reflective surface. This approach achieved a sensitivity, signal-to-noise ratio and noise-equivalent pressure approaching those obtained with Fabry-Pérot sensors, and substantially higher than those typically observed for piezoelectric hydrophones, whilst offering turn-key operation.