Directional acoustic density sensor
AssigneeThe United States of America as represented by the Secretary of the Navy
InventorsBenjamin A. Cray, Dimitri Donskoy
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Abstract
The invention as disclosed is a fiber optic interferometric directional acoustic density sensor that increases the directionality of a vector sensor that is much smaller in size than the wave length of an acoustic wave. This is accomplished through the use of second order directionality by measuring the acoustic fluctuations of fluid density at a point, wherein the acoustic density fluctuations are determined according to the principles of fluid compressibility and conservation of mass using a density fluctuation measuring apparatus that restricts two of the three vector components of the particle velocity of the acoustic wave and that employs a laser interferometer to measure the fluid density fluctuation along the remaining vector component.
Claims3
A directional acoustic density sensor for sensing acoustic waves comprising:
- a measuring cell with acoustically rigid walls that enclose a hollow section within said measuring cell, having apertures at opposite ends of the measuring cell, wherein said measuring cell is filled with a fluid medium, wherein the shape of said measuring cell restricts two of the spatial vector components of a particle velocity associated with the acoustic wave;
- an optical light source joined to said measuring cell and capable of emitting a beam of light in the hollow section within said measuring cell and through said fluid medium;
- a beam splitter joined to a wall within said hollow portion of said measuring cell that is capable of receiving the beam of light that is emitted by the optical light source and splitting said beam of light into a reference beam and a measurement beam, wherein the reference beam is directed perpendicular to the spatial vector component that is not restricted, wherein the measurement beam is directed parallel to the spatial vector component that is not restricted and therefore perpendicular to the reference beam, wherein the in-fluid lengths of both beams are equal;
- a plurality of optical fiber cable having a first end and a second end, said first end housed within the measuring cell and said second end located outside of the measuring cell, wherein the first end of a first optical fiber cable of said plurality of optical fiber cable is capable of receiving the reference beam and the first end of a second optical fiber cable of said plurality of optical fiber cable is capable of receiving the measurement beam at said first end;
- a light mixing cell joined to the second end of the plurality of optical fiber cable for combining the reference beam and the measurement beam, wherein the reference beam and the measurement beam travel along the plurality of optical fiber cable and enter the light mixing cell from the second end of the plurality of optical fiber cable, wherein the fiber lengths of said plurality of optical fiber cable are tuned to create constructive or destructive interference inside the light mixing cell;
- a photodiode joined to and contained in the light mixing cell capable of converting light into current and voltage;
- and
- a data acquisition and processing module that is programmable joined to said light mixing cell that receives the mixed beam composed of the reference beam and the measurement beam, and is able to detect any differences in fluid density fluctuations of the fluid medium in the measuring cell by measuring the phase difference, Δφ, between the reference beam and measuring beam according to the formula Δφ≅(2π/λlight)L(ΔCab/Cb) where λlight is the light wavelength in the fluid, L is the length of the either beam in the fluid, Cb is the speed of light in the fluid along the reference beam, ΔCab=Ca−Cb is the difference in speeds of lights along the measurement beam and reference beam determined by the difference in the refraction indexes, na and nb, along the respective beams according to the formula Ca,b=CV/na,b where Cv is the speed of light in a vacuum.
A method for measuring the acoustic fluctuations of fluid density for the purpose of determining vector sensor second order directionality comprising:
- containing a fluid medium in a measuring cell with acoustically rigid walls that enclose a hollow section within said measuring cell, having apertures at opposite ends of the measuring cell;
- restricting two of the spatial vector components of a particle velocity associated with an acoustic wave propagating through said contained fluid medium;
- emitting a beam of light in the hollow section within said measuring cell through said fluid medium;
- splitting said beam of light into a reference beam and a measurement beam, wherein the reference beam is directed perpendicular to the spatial vector component that is not restricted, wherein the measurement beam is directed parallel to the spatial vector component that is not restricted and therefore perpendicular to the reference beam, wherein the in-fluid lengths of both beams are equal;
- combining the reference beam and the measurement beam, to create constructive or destructive interference;
- and
- detecting any differences in fluid density fluctuations of the fluid medium through the use of a digital processing unit that measures the phase difference, Δφ, between the reference beam and the measuring beam to obtain a second order directionality.
Dependencies are inferred from the claim text.