Optimization of wideband fiber optic hydrophone probe for ultrasound sensing applications

Acoustic characterization of medical ultrasound devices is needed for optimization of image quality in diagnostic applications and to ensure their safety and effectiveness in therapeutic applications. New generation of acoustic transducers operating at fundamental frequency 15 MHz are being developed and FDA requires that as many as 8 harmonics are considered for real-time pressure-time waveform measurements in dispersive nonlinear medium. Therefore, hydrophone probes are required to perform characterization of the acoustic output of these devices at least up to 100 MHz in terms of frequency response.The primary goal of this thesis is to develop a Fiber Optic Hydrophone Probe (FOHP) for spatial averaging free characterization of ultrasound field till 100 MHz. Spatial averaging free design of the sensor is based on optimization of fiber geometry to achieve an active fiber dimension of the order of 7 mum to be comparable with half of the acoustic wavelength at 100 MHz. An innovative aspect of this work includes the development of a semi-empirical model for extraction of complex refractive index at optical wavelengths of 980 nm, 1480 nm and 1550 nm and stress-strain relationship of thin film gold for thickness ranging from 2nm--35nm. These are indispensable in modeling and optimization of 100 MHz FOHP in calculation of optimized pressure to voltage responsivity.For responsivity optimization, a novel multi-physics numerical model based on the Finite Element Method (FEM) was employed to solve electromagnetic, mechanics, and acoustics performance of the FOHP. The model and the selection of its input parameters, including coupled acousto-optic interaction of thin film gold, are based on published physical parameters and accurate extraction of various fiber parameters.Optimization results indicate that cylindrically etched FOHP with 6 mum tip diameter and 5 nm gold coating provides the highest responsivity performance of 234 dB re 1V/ muPa. The experimental verification of the thin film coated FOHP using 1 MPa pressure amplitude, produced unprecedented voltage responsivity between -234 and -254 dB re 1V/muPa or 2V/MPa and 200 mV/MPa, respectively. The optimum detection sensitivity of around 0.3 kPa is achieved by Relative Intensity Noise cancellation of 10 dB using balanced optical photo-detector.