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Sensor for optically sensing air borne acoustic wavesSensor for optically sensing air borne acoustic waves description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20060139653, Sensor for optically sensing air borne acoustic waves. Brief Patent Description - Full Patent Description - Patent Application Claims
RELATED APPLICATION [0001] The present invention is related to the application of Monsay, Penn, and Winfield, assigned to the Assignee of the present application, and entitled "Sensor And An Array of Sensors For Optically Sensing Water Borne Acoustic Waves" (35-HE-1514), filed concurrently herewith. BACKGROUND OF THE INVENTION [0002] 1. Field of the Invention [0003] The present invention relates to a novel acoustic wave sensor for sensing acoustic waves in a fluid typically air. The invention employs optical techniques, and more particularly depends upon the phase modulation which occurs when coherent light is passed through a fluid in which acoustic waves occur. The phase modulation may be imposed on a carrier provided by optical heterodyning and then detected electrically. The electrical signal recovered in this manner is representative of the acoustic waves. [0004] 2. Description of the Prior Art [0005] In the patent application of Gordon Jacobs, Serial No. 507,528 filed Sep. 19, 1974 entitled "Laser Hydrophone and Virtual Array of Laser Hydrophones", an acoustic sensor employing optical techniques was earlier proposed. The sensor, which was termed a "hydrophone", since it was designed for use in water, employed a laser beam which was focused on a small "focal" volume of water in which natural light scattering matter was suspended. The scattering matter, which vibrates in synchronism with any acoustic waves present, produces a phase modulation of the scattered light. The phase modulation was then recovered by optical heterodyne and sensitive phase demodulation techniques. The Jacobs arrangement contemplated both single hydrophones and arrays of hydrophones. In general, optical arrays, such as the Jacobs arrays, produce less hydrodynamic disturbance than the known large area piezoelectric arrays. [0006] The Jacob's arrangement was dependent upon a "doppler", type shift in frequency or phase, the doppler shift being caused by particle motion, toward and away from the sensing beam. Thus the maximum sensitivity was obtained by pointing the laser beam in a direction toward the source of acoustic waves and perpendicular to the wave fronts. [0007] The present invention shares certain of the objectives and in using optical techniques, certain of the means of the foregoing Jacobs' invention. [0008] The Jacobs invention is unlike the present invention which has application to the detection of acoustic waves in the air. The present invention has application to security systems, as for instance in the monitoring of sounds occurring along the perimeter of a property. SUMMARY OF THE INVENTION [0009] Accordingly it is an object of the present invention to provide an improved sensor of acoustic waves occurring in a fluid medium. [0010] It is another object of the present invention to provide an improved optical acoustic sensor for sensing air borne acoustic waves. [0011] These and other objects of the invention are achieved in an optical acoustic sensor comprising means for producing mutually coherent optical sampling and reference beams; optical means including an aperture at which the path of the sampling beam into the air is initiated and after reflection is terminated, and light reflective means arranged in the path of the sampling beam for reflecting significant sampling beam energy back via the aperture. [0012] The optical acoustic sensor further comprises an optical detector for coherently combining the reflected sampling beam with the reference beam to form an electrical heterodyne signal, phase modulated as result of the acoustic wave induced variation in the index of refraction, and finally a phase detector coupled to the output of the optical detector for detecting the acoustic wave induced phase variation of the sampling beam and thereby recovering an electrical signal representative of the acoustic waves. [0013] In accordance with the invention, the initial and reflected portions of the sampling beam path between aperture and the light reflective means are oriented with a substantial component parallel to the acoustic wavefronts of the acoustic waves. For maximum sensitivity, the sampling beam path is parallel in the farfield or tangential in the nearfield to the acoustic wave fronts. With a substantial component parallel to the acoustic wave front, the sampling beam is exposed to an acoustic wave induced density variation of like amplitude over a portion of the path of the sampling beam. As the acoustic waves traverse the path of the beam of light, the density variation produced by the acoustic wave, causes the index of refraction of the fluid (air) to vary, and thereupon phase modulates the sampling beam. The amount of phase modulation is in proportion to the accumulated variation in the index of refraction over the beam path. BRIEF DESCRIPTION OF THE DRAWINGS [0014] The inventive and distinctive features of the invention are set forth in the claims of the present application. The invention itself however together with further objects and advantages thereof may best be understood by reference to the following description and the following drawings in which [0015] FIG. 1 is an illustration, partially in block diagram format and partially in perspective, of a single optical acoustic sensor for optically sensing air borne acoustic waves; [0016] FIG. 2 is a perspective view of an optical acoustic sensor mounted upon a vehicle, for optically sensing air borne acoustic waves, as for instance conversations, the sensor using a non-specular optical reflector; and [0017] FIG. 3 is a perspective view of an arrangement employing a plurality of optical acoustic sensors mounted along consecutive line segments of a property line with optical transmitter/receivers and retro-reflectors at opposite segment ends, the arrangement being designed to optically sense air borne acoustic waves from near sources along the perimeter of a property for intrusion monitoring. DESCRIPTION OF THE PREFERRED EMBODIMENTS [0018] An optical acoustic sensor for optically sensing fluid borne acoustic waves is illustrated in FIG. 1. The sensor has as its principal optical, or light handling, components, the elements 10 to 23, and as its principal electrical or electrical signal handling components, the elements 24 to 27. The fluid, for instance air, in which the acoustic waves to be optically detected occur, is portrayed in the righthand portion of FIG. 1, with the acoustic wavefronts bearing the reference numeral 28. The source of acoustic wavefronts is not shown, being beyond the area of the illustration. (As will be explained, the features depicted in the righthand position of FIG. 1 are shown with substantial distortion in scale.) [0019] The optical elements produce a coherent optical beam which is projected into the fluid in which the acoustic waves occur, and recover a reflected optical beam which is phase modulated by its exposure to the acoustic waves. The process entails formation of an optical heterodyne in which a sampling "beam" is projected into the fluid and recovered by reflection, while a reference beam at an offset frequency is prepared to form an optical heterodyne with the returning sampling beam. The acoustic wave induced optical phase modulation is recoverable from this heterodyne. The optical heterodyne frequency is set for convenient recovery in an electrical format of the phase modulation at the photodetector 23. The heterodyne frequency may not exceed the bandwidth of the photodetector. The electrical components, which operate upon the modulated optical signal after conversion to an electrical format, prepare the signal for display, aural reproduction, or other utilization. Continue reading about Sensor for optically sensing air borne acoustic waves... Full patent description for Sensor for optically sensing air borne acoustic waves Brief Patent Description - Full Patent Description - Patent Application Claims Click on the above for other options relating to this Sensor for optically sensing air borne acoustic waves patent application. ###  How KEYWORD MONITOR works... a FREE service from FreshPatents1. Sign up (takes 30 seconds). 2. Fill in the keywords to be monitored. 3. Each week you receive an email with patent applications related to your keywords. Start now! - Receive info on patent apps like Sensor for optically sensing air borne acoustic waves or other areas of interest. ### Previous Patent Application: Fiber optic sensor system Next Patent Application: Displacement detector Industry Class: Optics: measuring and testing ### FreshPatents.com Support Thank you for viewing the Sensor for optically sensing air borne acoustic waves patent info. 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