Acoustic optic hybrid (AOH) sensor

Abstract

The ability of laser vibrometers to receive and process acoustic echoes from the water surface above a submerged target is established and evaluated. Sonar echoes from a submerged target are collected from the water surface by a laser vibrometer. Feasibility of this approach to sensing underwater sound is demonstrated. If the acoustic excitation at an otherwise undisturbed water surface is 195 to 168 dB re: 1 microPa, signal-to-noise ratio (SNR), at the vibrometer output, is shown to range from about 46 to 6 dB. Capillary waves and gravity waves at the water surface are expected and shown to have some destructive effect on the process of echo retrieval. A series of experiments to quantify the surface wave effects is described. The wave experiment results are reported. A successful attempt to acquire echoes from a submerged target over a grid of points for further processing into a three-dimensional image is made and described. The data acquisition and beamforming techniques constitute a three-dimensional, acoustic optic, synthetic aperture sonar (SAS). Beamformed images are included. For an aircraft towing acoustic sensors through the water with a mechanical link, this technique holds the promise of increased safety and improved fuel efficiency.

Optical remote sensing of sound in the ocean

“…[2][3][4] The motion of the surface induced by the acoustic signal can be much smaller than the motion induced by surface winds, and separating the two effects using this technique can be difficult. One technique to accomplish this is to measure the statistical properties of the scattered laser light, which are affected by the acoustic signal.…”

Experimental investigation of acousto-optic communications

“…The application of a laser Doppler vibrometer (LDV) to measure in-water acoustic signals by probing an airwater, pressure release surface has been more recently investigated [2][3][4][5][6]. The feasibility tests measured acoustooptic sensitivity values under a range of water surface conditions.…”

Empirical acousto-optic sonar performance versus water surface condition

“…Laboratory, water tank tests were previously conducted to demonstrate the feasibility of laser detection of sound on an air-water interface [3,4]. Baseline performance using a commercial Laser Doppler Vibrometer (LDV) resulted in detecting tonal signals having a Sound Pressure Level (SPL) of 120 dB relative to a microPasca1 (dB//uPa) at the water surface [3].…”