Experimental demonstration of remote, passive acousto-optic sensing

Abstract

Passively detecting underwater sound from the air can allow aircraft and surface vessels to monitor the underwater acoustic environment. Experimental research into an optical hydrophone is being conducted for remote, aerial detection of underwater sound. A laser beam is directed onto the water surface to measure the velocity of the vibrations occurring as the underwater acoustic signal reaches the water surface. The acoustically generated surface vibrations modulate the phase of the laser beam. Sound detection occurs when the laser is reflected back towards the sensor. Therefore, laser alignment on the specularly reflecting water surface is critical. As the water surface moves, the laser beam is reflected away from the photodetector and no signal is obtained. One option to mitigate this problem is to continually steer the laser onto a spot on the water surface that provides a direct back-reflection. Results are presented from a laboratory test that investigates the feasibility of the acousto-optic sensor detection on hydrostatic and hydrodynamic surfaces using a laser Doppler vibrometer in combination with a laser-based, surface normal glint tracker for remotely detecting underwater sound. This paper outlines the acousto-optic sensor and tracker concepts and presents experimental results comparing sensor operation under various sea surface conditions.

A remote optical system for port and harbor defense

“…Initial tests of the acoustooptic sensor revealed high levels of signal dropout when operating on moving sea surfaces. Therefore, a glint tracking system has been devised and tested to enable detection by tracking and measuring at the locations of optical surface glints 6 , on moving water surfaces.…”

Spectra of laser acoustic signals in water

“…INTRODUCION LAS in water had been researched extensively recently [1,2,3,4], according to the absorption coefficient of water( =10.6 m, =870cm -1 ) CO 2 laser were usually chosen as exciting source of LAS, and the LAS could be used to detect the depth of ocean, to find and picture objects in water and to get relevant coefficients of water. LAS are used as ultrasound source and have many applications in ocean acoustics [5,6,7].…”

All-fiber heterodyne Laser Doppler Vibrometry for acousto-optic communication

“…The capability of measuring the velocity and displacement in a high-precision and non-intrusive remote scheme provides the Laser Doppler Vibrometry (LDV) a well-known application in Non-destructive testing (NDT) [1,2] , speech-signal acquirement [3] , and acousto-optic communication [4,5] . Depending on the monitoring of the Doppler shift of coherent light scatter generated by the glass or water vibration surface, LDV can reconstruct the sound information which frequency deviation is within ±0.5Hz.…”