Compact directional acoustic sensing using multi-fiber optical probes

A compact directional acoustic sensor concept is described, which uses an multi-optical fiber probe, a light emitting diode source, a photo-diode detector, and a short, slender cylindrical cantilever to the end of which is attached an optical reflector. A portion of the light exiting one fiber is collected by a second fiber after reflection from the mirror. Acoustically induced transverse displacement of the cantilever tip modulates the light collected by the second fiber, which then conveys the light to a photo-detector. Directional sensitivity is achieved by virtue of the dependence of the collected light on the cosine of the angle between the line connecting the probe fiber centers and the direction of displacement of the cantilever tip (the acoustic wave direction). An analytic model of the acoustic response of the cantilever tip is constructed, which is partially verified using a finite element-based model and experimentally validated using measurements of the acoustic response in air. The model is used to predict its acoustic response versus frequency, how that response depends upon damping near the cantilever resonance frequency, and to what extent and over what frequency band that response depends upon the acoustically generated flow force. [Work supported by ONR.]