Acoustic beam pattern for a fringe free laser interferometric sound sensor

Abstract

A fringe free laser interferometric sound sensor uses parallel, non-intersecting, modulated laser beams in a sound-filled fluid. Therefore, no interference fringes are produced or required in the fluid. Light from each laser beam is collected and combined in an envelope detector where the sound pressure signal appears as angle modulated sidebands. Since the laser beams do not intersect in the sound-filled fluid, we can use a beam separation distance that maximizes the optical path difference between the laser beams to increase sound detection sensitivity. We derive an equation that relates the optical path difference between a pair of laser beams and the peak change in the index of refraction of the fluid due a sound wave. We derive a three dimensional beam pattern and function for this sensor and compare it to that of a conventional line array.