Moving towards an ideal frequency response with fibre-optic hydrophones

The frequency response of a polymer film Fabry-Perot fibre-optic hydrophone developed for the characterisation of ultrasound fields, has been investigated. The transduction mechanism of the hydrophone is based upon the detection of acoustically and thermally induced thickness changes in a polymer film Fabry-Perot interferometer deposited at the tip of a single mode optical fibre. The frequency response of the original sensor has been found to be significantly non-uniform over the 50 MHz operating bandwidth. A finite difference simulation of acoustic interactions with the sensor has successfully been used to predict the response and investigate the origin of the non-uniformities. Additionally, the model has been used to predict the response of a sensor with modified tip geometry in order to find a design capable of providing an improved response. Sensors with a hemispherical tip have now been fabricated, characterised experimentally and found to provide a significantly improved response. Measurements were made on a shockwave toneburst (f_c = 1 MHz), using both types of fibre-optic hydrophone and a 0.4 mm PVdF membrane hydrophone. Deconvolution was used (following IEC62127 - 1) in order to produce accurate pressure waveforms from the measured data. It was found that, in addition to providing an improved frequency response, the modification to the hydrophone geometry improved the efficacy of the deconvolution process.