Comparison of a miniature, ultrasonic, optical fibre hydrophone with PVDF hydrophone technology

A miniature optical fibre hydrophone has been developed for the measurement of ultrasound in the range 1-30 MHz. The acoustically sensitive element comprises a 23 μm thick polymer film mounted at the end of an optical fibre. When illuminated by laser light launched into the fibre, the polymer film acts as a Fabry Perot interferometer. An incident acoustic wave modulates the optical thickness of the interferometer thereby producing a corresponding intensity modulation in the light reflected from the film. The system was characterised in terms of sensitivity, frequency response and directivity using a broadband (1-30 MHz) ultrasonic field produced by nonlinear propagation obtained by driving a 1 MHz PZT source with a high amplitude 1 MHz toneburst. PVDF needle and membrane reference hydrophones were used as comparisons. The minimum detectable acoustic pressure of the optical fibre hydrophone was found to be 10 kPa in a 25 MHz measurement bandwidth with a wideband response to 30 MHz. The -3dB beamwidth at 10 MHz was 60°. Such performance is comparable to that achieved with PVDF hydrophone technology, with additional advantages of immunity to EMI, small physical size, a flexible probe-type configuration, robustness and potentially low cost. Among the applications that might benefit from these advantages are single-use applications such as the measurement of industrial CW fields in hostile environments and in vivo measurements of medical ultrasound exposure