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Passive Optical Detection of Underwater Sound

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Patent application, Filed 31 May 2005

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The invention, as embodied herein, comprises a passive acoustic sensor used for detecting sounds emanating from under the surface of a body of water. Specifically, the invention is designed to operate in an ocean environment, wherein turbulent waters are prevalent. Rather than employ standard hydrophones placed beneath the water to detect sound, the present invention detects sound pressure waves that occur when sound is emanated from within a body of water. Because the air-water interface at the surface of a body of water is a pressure release surface due to the difference in acoustic impedance between the air and the water environments, the sound pressure waves created by a sound emanating under the surface cause the surface to vibrate. This vibration can be detected optically using an acousto-optic sensor, such as a laser interferometer. Therefore, detection of sounds emanating from underwater may be accomplished via an optic sensor placed above the surface of the water. The source of such sounds include cetacean mammals or man-made objects that produce acoustic signals over a relatively broad range of frequencies. However, there are potential problems in using such a system in turbulent waters. Because the water acts as a specular reflector, the narrow laser beam employed by a sensor, such as a laser interferometer, must be almost perpendicular to the surface of the water in order to obtain a reflected beam to return to the sensor. Due to the turbulent nature of water in external environments, potential intermittent loss of the reflected beam is expected, and will increase as the slope of the wave surface changes relative to the laser beam angle. Therefore, a water surface with a poor reflective quality or that is highly turbulent will degrade optical sensor performance by increasing signal dropout rate. Various embodiments of the present invention have been developed to address these issues and are described in detail in this application.

Subject Categories:

  • Acoustooptic and Optoacoustic Devices
  • Optical Detection and Detectors
  • Physical and Dynamic Oceanography
  • Acoustic Detection and Detectors

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