Hydrophone devices are capable of detecting pressure amplitude in immersion media such as liquids or gases and in solids.  Piezoelectric hydrophones are commonly used for measurement of large frequency bandwidths; however, problems arise from the generation of high temperatures and cavitation effects that are produced by High Intensity Focused Ultrasound (HIFU) fields, High Intensity Therapeutic Ultrasound (HITU) fields, lithotripter fields or the like. These problems can lead to device failure due to high pressure amplitudes. Also, such devices have large apertures which result in spatial averaging of certain acoustic fields. For example, existing hydrophone probes have aperture diameters on the order of about 500 micro or more which introduces spatial averaging of acoustic fields beyond 3 MHz. This spatial averaging can lead to errors in detection and faithful reproduction of the pressure-time waveform of the measured acoustic wave and result in a very poor spatial resolution.

A team of Drexel researchers have developed a fiber optic hydrophone sensor/probe that overcomes these limitations. This novel fiber optic hydrophone probe offers a flat frequency response up to 100 MHz with a sensitivity of -242dB re 1V/uPa making it a suitable replacement for many known hydrophones in terms of resolution, cost, flexibility, and dynamic range.  The realization of this fiber optic sensor is based on etching down single mode optical fibers to diameters of 5-8 um with gold coating thickness of about 3.5-7.5nm using a sputtering system for near infrared (700-1600nm) optical sources.

The performance of the sensor is optimized by proper etching the fiber for smooth surface conditions, careful selection of sputtering parameters (distance, separation, and flow current), use of organic primers (for improvement in adhesion of gold films to glass fibers), and with or without use of thermal annealing of gold films.  A cost effective manufacturing process has been developed. Achieved test results of the processed fiber optic hydrophone probes have shown much better performance than other optical and ultrasound sensors, making them commercially attractive for many applications, such as calibration of Piezoelectric hydrophones, imaging, medical diagnostic capabilities, and more.