Lynn T. Antonelli

Buoyant device for bi-directional acousto-optic signal transfer across the air-water interface

A buoy system bi-directionally communicates in-air and underwater. A buoy having a shell to float on water has an upper portion in air and a lower portion in water. An array of acoustic transducers which is disposed in the lower portion receives acoustic signals and transmits acoustic signals. A dome-shaped retro-reflective coating on the upper portion is vibrated for retro-reflecting impinging laser illumination as data signals in air. An array of photo-detectors on the upper portion are responsive to impinging laser control signals and/or signals which may be transmitted as acoustic signals in water.

Experimental demonstration of remote, passive acousto-optic sensing

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.

Laser-based acousto-optic uplink communications technique

An apparatus for enabling acousto-optic communication comprising an in-water platform comprising means for emitting an acoustic signal to an acousto-optic interaction zone, an in-air platform comprising the ability for transmitting a first optical interrogation beam, the ability for receiving a portion of the first interrogation beam and a second laser beam formed from the reflection of the first interrogation beam off of the acousto-optic interaction zone, the ability for measuring and outputting a plurality of optical interferences between the portion of the first interrogation beam and the second reflected beam, and a signal converter receiving as input the plurality of optical interferences and outputting an electrical signal representing the received acoustic telemetry signal at the interrogation point at the air-water interface.

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

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 investigation of optical, remote, aerial sonar

Detecting underwater objects such as debris fields, submarines or mines for littoral area clearance, while in the air, would increase the autonomy and flexibility of subsurface, surface and air vehicles engaged in undersea warfare and could have a variety of commercial and oceanographic applications. Experimental research into a laser-based active sonar concept is being conducted for remote, aerial detection of submerged objects and for sonar mapping of an undersea area. An airborne high-energy, pulsed laser is used to remotely generate underwater acoustic energy. The acoustic wave propagate through the water and are reflected from underwater objects. The acoustic signals are then detected as they reach the water surface using a low-powered laser interferometer device. An array of surface detections of the acoustic field reflected from the underwater objects will then be analyzed using traditional time-delay beamforming techniques to locate underwater objects. The combination of these optical technologies provides a means for stealthy, remote, active as well as passive sonar that does not currently exist. Results are presented from a controlled laboratory test using commercial laser devices, which demonstrate the feasibility of the sonar concept for remotely searching for underwater objects. This paper outlines the aerial sonar concept and provides the initial experimental results of the underwater object illumination, detection and localization.

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

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 als...

Passive optical detection of underwater sound

A passive acoustic sensor that may be employed to detect sounds emanating from under the surface of a body of water. The sensor uses optics to determine vibration on the surface of a water body to detect sound pressure waves from underwater sound sources. The sensor is deployed above the surface and has no direct interaction with anything under the surface that may be emanating sounds. This allows the invention to operate without interfering with potential sound sources as well as allows for numerous deployment methods.

Experimental demonstration of multiple pulse nonlinear optoacoustic signal generation and control.

Generating underwater acoustic signals from a remote, aerial location by use of a high-energy pulsed infrared laser has been demonstrated. The laser beam is directed from the air and focused onto the water surface, where the optical energy was converted into a propagating acoustic wave. Sound pressure levels of 185 dB re microPa (decibel re microPa) were consistently recorded under freshwater laboratory conditions at laser-pulse repetition rates of up to 1000 pulses/s. The nonlinear optoacoustic transmission concept is outlined, and the experimental results from investigation of the time-domain and frequency-domain characteristics of the generated underwater sound are provided. A high repetition rate, high-energy per pulse laser was used in this test under freshwater laboratory conditions. A means of deterministically controlling the spectrum of the underwater acoustic signal was investigated and demonstrated by varying the laser-pulse repetition rate.

Remote, Aerial, Trans-Layer, Linear and Non-Linear Downlink Underwater Acoustic Communication

Both the linear mechanism for optical to acoustic energy conversion are explored for opto-acoustic communication from an in-air platform to a submerged vessel such as a submarine or unmanned undersea vehicle. This downlink communication can take the form of a bell ringer function for submerged platforms or for the transmission of text and/or data. The linear conversion mechanism, also known as the linear opto-acoustic regime where laser energy is converted to sound at the air-water interface, involves only the heating of the water medium. In this mode of operation, the acoustic pressure is also linearly proportional to the laser power. In contrast, the non-linear conversion mechanism, also known as the non-linear opto-acoustic regime where focused laser energy is converted to sound at the air-water interface, involves a phase change of the water medium through evaporation and vaporization which leads to the production of a plasma. In this mode of operation, the acoustic pressure is non-linearly related to the laser power. The non-linear conversion mechanism provides a more efficient, i.e. higher source level, yet less controllable method for producing underwater acoustic signals as compared to the linear mechanism. A number of conventional signals used in underwater acoustic telemetry applications as well as command and control applications are shown to be capable of being generated experimentally via the linear and nonlinear opto-acoustic regime conversion process. The communication range and data rates that can be achieved in both conversion regimes are addressed. The use of oblique laser beam incidence at the air-water interface to obtain considerable in-air range from the laser source to the in-water receiver is addressed. Also, the impact of oblique incidence on in-water range is examined. Optimum and sub-optimum linear opto-acoustic sound generation techniques for selecting the optical wavelength and signaling frequency for optimizing in-water range are addressed and discussed. Opto-acoustic communication techniques employing M-ary Frequency Shift Keying (FSK) and Multi-frequency Shift Keying (MFSK) are then compared with regard to communication parameters such as bandwidth, data rate, range coverage, and number of lasers employed. In the non-linear conversion regime, a means of deterministically controlling the spectrum of the underwater acoustic signal has been investigated and demonstrated by varying the laser-pulse repetition rate to provide M-ary Frequency Shift Keyed signaling. This physics-based conversion process provides a methodology for providing low probability of intercept signals whose information is embedded in noise-like signals. These laser generated signals can then be used in a frequency hopped spread spectrum technique with the use of the proper receiver structures to take advantage of the frequency diversity and periodicity inherent in this type of signal structure that could also be used to combat frequency selective fading in underwater acoustic channels

Bearing estimation error analysis using laser acoustic sensor measurements on a hydrodynamic surface

The basic operation of a novel laser-based sensor is investigated. A low power laser beam emanating from a laser Doppler velocimeter is used to interrogate an air-water interface for underwater sound detection. The sensor measures the normal velocity of the vibrations of the pressure release air-water interface. The objective of this paper is to discuss the feasibility and fundamental limitations on using the laser-based acoustic field velocity measurements of hydrodynamic air-water interface to determine acoustic field direction of arrival. This paper identifies the various contributions to bearing uncertainty using the Cramer-Rao lower bounds to establish the limitations of determining the bearing of the in-water sound.