Jerling, AE; (2015) Fabry-Pérot fibre optic hydrophones for determining the acoustic and thermal characteristics of high intensity and high pressure ultrasound fields. Doctoral thesis , UCL (University College London).

Abstract

This thesis reports the advancement of two forms of Fabry-Pérot fibre optic hydrophone for the characterisation of high intensity and high pressure ultrasound fields. The first form of fibre optic hydrophone was realised using a multi-layered hard dielectric Fabry-Pérot interferometer deposited at the tip of an optical fibre. The acoustic transduction mechanism is based on the detection of acoustically induced changes in the optical thickness of the spacer layer. Several hydrophone designs were realised and an extensive investigation into the acoustic performance of the hydrophones was conducted. Each hydrophone design is anticipated to be capable of characterising acoustic fields of high pressure, with working ranges of ~90MPa. In addition, the hydrophones were shown to be able to withstand acoustic fields with intensities up to 1833W.cm-2. The frequency response of the hydrophones was measured using substitution calibration and the features of the response were investigated using a finite difference model (AFiDS). The directional response was also measured, and the hydrophones were found to be most sensitive to acoustic fields of non-normal incidence. The sensors were shown to not be subject to self-heating when interrogated by light of optical power below 3.78mW and to have a noise equivalent pressure of 30.9kPa (at 3.5MHz over a 20MHz noise bandwidth). A 3MHz frequency component was observed during the acoustic characterisation and after extensive experimentation the transduction mechanism was found to be optically sensitive and not due to the Fabry-Pérot interferometer. A second form of Fabry-Pérot fibre optic hydrophone was introduced and has previously been reported to not be able to operate at temperatures in excess of 70°C. The cause for this limitation was investigated and found to be due to the annealing of the Parylene-C spacer within the Fabry-Pérot interferometer when elevated to temperatures exceeding 56°C. The effects of annealing on the interferometer transfer function, and acoustic response were investigated. Annealing was found to produce an irreversible shift in the reflectance minimum of the interferometer transfer function, as well as a change in the frequency response of the sensor, with the features moving to higher frequencies.

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