Aytac Kipergil, Esra; Martin, Eleanor; Mathews, Sunish; Treeby, Bradley; Alles, Erwin; Desjardins, Adrien; (2022) Fibre-optic hydrophones for high-intensity ultrasound detection: modelling and measurement study. Presented at: IEEE International Ultrasonics Symposium (IUS) 2022, Venice, Italy.

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Abstract

Background, Motivation and Objective: Fibre-optic hydrophones (FOHs) are widely used to detect and spatially characterise high-intensity focused ultrasound (HIFU) fields. In this context, the most common type of FOH consists of a fibre with a flat-cleaved uncoated tip. The ultrasound (US) field is detected by measuring changes in reflected light intensity due to pressure-induced modulations of the refractive index of the fluid. However, these sensors tend to have a low signalto-noise ratio (SNR) (with a high noise equivalent pressure [typically 2–3 MPa]), which imposes significant dynamic range constraints on field characterisation. In this study, we extend this bare FOH paradigm to include partially-reflective coatings on the fibre end faces, with a view to increase SNR whilst withstanding HIFU pressures. Previously, a limited number of studies have investigated this paradigm. Here, we present a comprehensive elasto-optic numerical model capable of predicting the sensitivity for arbitrary numbers of coatings, and use this model to design and fabricate an FOH comprising a single coating layer using a novel material. / Statement of Contribution/Methods: A simulation method based on the general transfer-matrix method was developed in MATLAB to compute the change of reflectance with respect to pressure (dR/dP, which is proportional to the FOH sensitivity). A single layer coated FOH comprising a quarter-wave layer (172 nm) of deposited TiO2 was fabricated. The FOH was placed in the focus of a HIFU source (diameter: 64 mm, focal length: 63.2 mm; H101, Sonic Concepts). The SNR gain observed experimentally was compared against numerical predictions. Furthermore, the potential of further increasing SNR using a multi-layer sensor configuration was investigated. Results/Discussion The SNR of the US measurement with the single-layer TiO2 coated sensor was found to be 21 dB higher than for an uncoated one (Fig. 1a), corresponding to a sensitivity gain of 11x. (c.f. 8.5x predicted with simulation). The difference between the measurements and the model can be attributed to the cleaving quality of the uncoated hydrophone or inaccuracies in the elasto-optic properties of the coating layer. The coated sensor endured pressures over 35 MPa (peak positive), and tests for higher pressures are underway. Moreover, simulations for configurations using multiple layers suggest the sensitivity could be significantly improved further. For instance, a 15-layer structure of alternating TiO2 and SiO2 coatings was predicted to achieve an increase in sensitivity of ca. 73×, while still being mechanically robust for HIFU applications.

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