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.

Experimental demonstration of remote, passive acousto-optic sensing

A system and method is provided for using acoustical pulses generated from an airborne laser source to locate objects under water. An array of acoustic sensors, such as passive sonobuoys at the ocean surface, is deployed in known or determinable locations by aircraft. Each area surrounded by acoustic sensors comprises a search cell, within which sonar scattering data can be used to locate objects. Following sonobuoy deployment, the aircraft uses the laser to rapidly generate many laser-acoustic pulse sources within each search cell, which in turn generate spherical acoustic pulses traveling through the water. The acoustic sensors receive the acoustic pulses, either on a direct path or on a scattered path after the pulse strikes an underwater object. The sensors record the acoustic signatures of the received pulses and transmit data of the recorded acoustic pulses to a processor such as processor on a nearby ship. The processor transforms the data of the acoustic signatures, together with data of the known locations of the acoustic sensors and the locations of the laser-acoustic pulse sources, into data of a location of the underwater objects.

Airborne Laser-Acoustic Mine Detection System

Covert communications between underwater and aerial platforms would increase the flexibility of surface and air vehicles engaged in undersea warfare by providing a new netcentric warfare communications capability and could have a variety of commercial and oceanographic applications. Research into an acousto-optic sensor shows promise as a means for detecting acoustic data projected toward the water surface from a submerged platform. The laser-based sensor probes the water surface to detect perturbations caused by an impinging acoustic pressure field. Experimental studies were conducted to demonstrate acousto-optic sensor feasibility for obtaining accurate phase preserved recordings of communication signals across the air-water interface. The recorded surface velocity signals were transferred to an acoustic communications receiver that used conventional acoustic telemetry algorithms such as adaptive equalization to decode the signal. The detected, equalized, and decoded bit error rate performance is presented for hydrostatic and more realistic, hydrodynamic water surface conditions

Experimental detection and reception performance for uplink underwater acoustic communication using a remote, in-air, acousto-optic sensor

An ultrasound probe including a backing layer provided with grooves in which a piezoelectric member is allowed to be installed and a manufacturing method thereof. The ultrasound probe includes the piezoelectric member, and the backing layer disposed on a rear-side surface of the piezoelectric member and provided, on a front-side surface thereof, with grooves in which the piezoelectric member is installed.

Ultrasound probe and manufacturing method thereof

A remote, aerial, laser-based sonar and communications concept has been demonstrated to direct laser beams from the air onto the water surface to remotely generate and detect underwater sound. This forms the basis for a portable, aerial sonar and communications system for both detecting underwater objects such as vehicles and mines as well as for bi-directional acoustic communication between an in-air platform and a submerged platform operating at speed and depth. Two laser systems are required. The first laser system is a high-energy laser, the opto-acoustic transmitter, whose optical energy is converted to acoustic energy at the water surface. The second laser system is a laser interferometer, the acousto-optic sensor that detects underwater sound by measuring the vibrations of the water surface caused by the incident underwater sound field. Historically, acoustic transmitters and detectors need to be submerged to interact with the underwater environment. This remote, aerial, laser-based sonar and communication system presents a revolutionary change in the way sonar and communications can be realized, by remotely activating and detecting underwater acoustics from the air.

Remote, aerial, opto-acoustic communications and sonar

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.

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

A vertical acoustic array system and method for acoustically imaging the tents of a partially or completely buried tank which can contain radioactive solids and may not be entirely filled. The transducer array is placed proximate a side of a tank in a vessel such as a tube filled with water. Dirt or other filler material located between the tube and the tank is saturated with water, allowing the acoustic signal to propagate through the side of the tank and into the tank. The vertical array consists of one or more transmitting units and receiving units set in a vertical array including dipole receiving elements which allow horizontal directionality. The signals are analyzed by a field unit which includes a portable self-powered trailer providing signal transmitting and analysis devices. The field unit can transmit signals to a second unit which provides further signal analysis.

System and method for acoustically imaging an underground tank

A composite transducer array comprises a piezoelectric polymer composite panel, a continuous electrode coupled to a first surface of the composite panel, and a plurality of electrically-isolated electrode segments coupled to a second surface of the composite panel. Each electrode segment is shaped as an angular segment of a circular ring. The electrode segments are arranged to define an array of concentric circular rings of electrode segments.

/pdf/broadband-and-wide-field-of-view-composite-transducer-array-517lkx8rgd.pdf

Broadband and wide field of view composite transducer array

A method of measuring the vapor-liquid boundary surrounding a supercavitational high speed underwater vehicle includes the steps of arranging a sensor on a surface of the vehicle. The sensor has a transmit source for emitting an optical signal and a plurality of optical detectors for receiving a reflected optical signal. The reflected optical signal is detected with one of the optical detectors, and a duration of time between the emitting of the optical signal and the receiving of the reflected optical signal is measured. The method determines a separation distance from the transmit source to the receiving optical detector. The duration of time and the separation distance are combined to compute a boundary distance.

Cavity wall measurement apparatus and method

An acoustic sensor used in underwater applications. The sensor includes a reflective material adhered to a structure, such as an outer submarine hull or any marine vessel hull. A laser interferometer is placed on the side of the structure with the reflective material. The laser interferometer sends a plurality of laser beams, in sequence or all at one time, to a plurality of points across the retro-reflective material. The laser beams reflect back to the interferometer, which captures the reflected beams using receiving optics. The phase modulation of the reflected laser beams is compared to a reference laser beam within the interferometer to obtain the vibration velocity characteristics of the hull surface structure. Since the reflective material is adhered to the structure, the structure vibration is the same as the vibration of the reflective material. From this vibration, the acoustic pressure associated with the structure may be calculated.

Kenneth M. Walsh

Papers

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

System and method for acoustically imaging an underground tank

Broadband and wide field of view composite transducer array

Cavity wall measurement apparatus and method

Optical underwater acoustic sensor