Optimization of a 100 MHz fiber optic hydrophone for medical ultrasound applications using nanometer scale gold coating.

Abstract

This work describes a novel numerical model developed to optimize the design of a broadband, spatial averaging free fiber optic hydrophone probe (FOHP) for characterization of ultrasound fields in the frequency range 1–100 MHz. The design included minimization of the active sensor dimensions, so its cross-section is comparable with the half acoustic wavelength at the highest frequency of interest and improvement of voltage-to-pressure sensitivity of the probe measured in V/Pa. Initial simulations based on the assumption that bulk refractive index of sputtered material could be used indicated that a thin film gold coating (1–35 nm) could indeed enhance the voltage sensitivity of the FOHP by 16–30 dB; however, a follow up analysis revealed that determination of the coating thickness influence on the FOHP performance would require an introduction of the complex (as opposed to bulk) index of refraction of the sputtered film. The input parameters to the model, their selection criteria, and implementation of the coupled acousto-optic interaction of gold layer will be discussed. The measured prototypes produced unprecedented voltage sensitivity between −234 and −245 dB re 1 V per uPa or 2 and 560 mV/MPa, respectively. [The authors wish to acknowledge support of the NIH R01EB007117 grant.]