Lee E. Estes

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.

A scanning laser Doppler vibrometer acoustic array

Experiments confirm that a laser Doppler vibrometer can be used to detect acoustic particle velocity on a fluid-loaded acoustically compliant, optically reflective surface. In these experiments, which were completed at the Acoustic Test Facility of the Naval Undersea Warfare Center, Scotchgard™ reflective tape was affixed to the interior surface of a standard acoustic window. The polyurethane array window had a thickness of 0.9525cm (0.375in.) and a material density of 1000kg∕m3. The surface velocity measured, using a commercial scanning laser vibrometer system (SLVS), was beamformed conventionally and flawlessly detected and localized acoustic signals. However, the laser Doppler vibrometer used in the experiments had relatively poor acoustic sensitivity, presumably due to high electronic noise in the photodetector, speckle noise, standoff distance, and drifting laser focus. An improved laser Doppler vibrometer, the simplified Michelson interferometer laser vibrometer sensor (SMIV), is described in brief....

Simulation Of A Shallow Water Channel Acoustic Telemetry

To be able to select appropriate signal architectures for various underwater acoustic communication channels a computer operable model that simulates the complete communications channel has been developed. The model utilizes time varying impulse responses and windowing techniques to determine the response to any given input signal. Statistical descriptors of the complex impulse and cw responses of the underwater acoustic environment of interest are used to develop the model impulse responses and to modify the input signal. This paper briefly outlines the model, and provides a comparison with some at sea experimental differential phase shift key (DPSK) data obtained in March of 1991 in Narragansett Bay, R. I. Doris Carvalho Naval Undersea Warfare Center Division Newport, RI 02840 11. MODEL DEVELOPMENT The general communication geometry of interest is depicted in Figure 1. As indicated, the source and receiver are in

Internal thermal noise in optical frequency converters

We present a detailed analysis of noise due to the frequency-converted internal thermal radiation in a proustite crystal pumped by a ruby laser and adjusted for noncritical phase matching with a 10.6-μm signal. A three-dimensional expression is developed for the wave-number mismatch Δk, and phase-matching surfaces are plotted, which make clear the relative importance of phase matching and system filters in determining the spectral and spatial characteristics of the frequency-converted noise. Comparison of theory and experiment establishes that a crystal absorption coefficient of 0.22 cm−1 at 10.6 μm will account for the observed noise level. This compares favorably with a direct measurement of 0.4 cm−1 for the crystal loss coefficient. Good agreement is found between experiment and predictions of the noise temperature dependence.

Laser beam propagation through the ocean's surface

The practicality of communication through the waters surface is explored using values from Buzzards Bay, Massachusetts. A series of measurements of absorption and attenuation as a function of frequency were taken in the bay. The resultant values were used to provide input to a numerical ray model of light propagating through the water column, a random surface and to a receiver in the air. Signal to noise ratios were calculated and show that an appropriately designed system is practical.

Description of and experimental results for a high data rate underwater acoustic telemetry link

A wideband system (40 kHz-50 kHz) whose coding is self-synchronous and is adjustable for variations in coherence and reverberation times to obtain a maximum reliable data transmission rate for any given set of conditions is presented. To maximize data rate the approach takes full advantage of system bandwidth. At its simplest level a spectral line is used to represent a bit. The major delimiters of multispectral transmission (within a given system bandwidth) in this environment are coherence time due to Doppler spreading and reverberation time due to multiple transmission path. Banks of different frequencies are sequentially transmitted, each bank being reused after the reverberation time. Theory, experimental setup, and preliminary results are presented.<<ETX>>

Temporal- and spatial-intensity-interferometer imaging through a random medium

We present experimental and theoretical results which show that temporal-intensity interferometry can be used to image coherently illuminated objects through a moving random medium. This system is compared with a spatial-intensity-interferometer imaging system (SIS) described by Beard and Barnoski. We show that the angular resolution of the temporal-intensity-interferometer imaging system (TIS) and the SIS is the average angular size of a scatterer, as seen from the detector.

Noise analysis of a simplified Michelson interferometer vibrometer hydrophone

The use of a Michelson interferometer to sense acoustic vibrations imposed on a water/plate/air boundary is investigated. To focus on fundamentals, a simplified interferometer is considered rather than the usual heterodyne or dual track homodyne configurations. Based on analytical models, the sensitivity limits due to photoelectron shot noise, laser light amplitude and phase noise, atmospheric turbulence, thermal vibration, and amplifier noise are predicted. Techniques for mitigation of turbulence and laser noise will be discussed.

A new instrument for optical forward scattering phase function surveys

In order to predict the performance of in the water and through the surface laser communication systems, it is necessary to know the inherent optical properties of the water environment. In particular, knowledge of the absorption and scattering coefficients and the forward scattering phase function (FSPF) are required. Analytical models and photon Monte Carlo simulations can then make use of these inherent optical properties to predict system performance. In this paper the authors describe the design of a new device that can be used to conduct FSPF surveys. The device makes use of a green helium-neon laser operating at a 543.5 nanometer wavelength and thermoelectrically cooled CCD camera to make real time measurements. The system is powered from the deploying surface vessel and the data is recorded at each depth by a shipboard portable computer. In this manner vertical profiles of the FSPF can be obtained. The authors present some early results.

Optical range for naval gunfire training

The Navy has a need to score Naval Gunfire Support exercises using a virtual land mass, positioned off the Pacific Missile Range Facility (PMRF) at Barking Sands, Kauai, Hawaii, as a target. The target area is approximately 15,000 yards from shore. As a part of this exercise, illumination rounds that detonate 1000 to 1500 feet above the surface and airburst high explosive rounds that detonate approximately 50 feet above the surface must be scored. The design requirements for the measuring system for the illumination rounds is detonation position accuracy in three dimensions of approximately +/- 15 yards and for the high explosive airburst rounds the determination of air as opposed to water detonation. Detection and measurement is to be automatic. The optical portion of the system consists of two shore based high resolution CCD cameras mounted on precision rotators, surveyed in via GPS and supported by appropriate hardware and software. One station is located 80 feet above sea level and the other is located 1500 feet above sea level. Data from both stations is telemetered to a central station for position computation and display. The base line along the shore is approximately 10,000 yards. Preliminary measurements, system design, error analysis, and calibration techniques will be discussed.