Abstract
The electro‐optic hydrophone is a device that detects sound in water by scattering a laser beam off naturally occurring particles moving under the influence of acoustic signals. The theory of operation is the same as that of cw Doppler radar or (alternatively) of an optical heterodyne in which the laser beam is split into two beams to form a set of real (or virtual) fringes at the focal volume in the water [Rudd, J. Sci. Inst. 2, 55 (1969)]. Moving particles traverse these fringes and are intermittently illuminated. A photodetector converts the scattered light intensity into an intermittent electric current which is processed in a power spectrum analyzer. A laboratory experiment is described in which an acoustic signal at 200 Hz and peak amplitude of 4.35×10 (−8) m is detected by this principle. A discussion is then made of the minimum detectable signal of this device in the presence of noise from various origins. Methods are proposed to lower the threshold of detection from the level currently achieved.
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