Laser interrogation of the air–water interface for in-water sound detection: Initial feasibility tests

Abstract

Proof of concept experimentation recently demonstrated a new laser-based acoustic sonar technique to measure the velocity of the air-water interface using laser Doppler velocimeter technology. The laser acoustic sonar concept is unique since it directs laser light from the air onto the water surface. The light scattered from the interface contains Doppler information from which the boundary velocity is obtained. Data storage and signal processing can then be performed on the detected signal and the acoustic pressure in the water calculated from the measured velocity. The laser-based system presents an alternative means for broadband sonar reception that does not interfere with the water environment. Acoustic pressure signals as low as 119 dB relative to 1 μPa, between 2 and 50 kHz have been detected in the laboratory using the laser velocity sensor on a static water surface. The methods of implementing the laser acoustic velocity sensor to measure pressure fluctuations on a hydrodynamic interface were also explored in a high-velocity water tunnel [sponsored by the Office of Naval Research]. The results from the initial proof of concept experimentation from both the hydrostatic and hydrodynamic air–water interfaces is presented, demonstrating the feasibility of this sensor concept.