Abstract: An acousto-optical vibration sensor, particularly for use in ultrasonic array transducers, such as for medical ultrasound, employs interferometric techniques. A receiver element has a light input port which receives laser light that exits via an output port only after undergoing a multitude of reflections between a static reflecting surface and a vibrating reflecting surface in a manner that enhances frequency-shifted light wave components. The receiver element includes a volumetric region including a layer of material which is both light transmissive and light scattering. In principle, ultrasonically shifted light wave components are enhanced by about six orders of magnitude, and vibration amplitudes significantly smaller than 0.01 A may be detected. Loss resulting from multiple reflections is compensated for by employing active lasing particles, sized so that lasing is induced by the frequency-shifted light.

Abstract: A fiber optic hydrophone is formed as a single interferometer having widely separated interferometer segments formed in each of a pair of optical fibers. The interferometer segments are designed to allow averaging of the effects of local noise sources over a broad area in order to increase the acoustic signal-to-noise ratio. The interferometer segments also allow deployment of the hydrophone around sharp bends. A first optical fiber is formed into a plurality of spatially separated outer coils. A second optical fiber is formed into a plurality of inner coils corresponding to the outer coils. The inner and outer coils are preferably arranged in concentric pairs to form a plurality of sensor segments. A plurality of spacers is arranged such that a spacer is between successive sensor segments.

Abstract: An opto-acoustic detector includes a fiber laser having a cavity which includes an optical fiber having an active medium portion doped with a gain material so that the detector may be used in high frequency, high density arrays. The cavity is bounded by a frequency selective reflector and a broadband or frequency selective reflector adapted to be acoustically coupled to a medium supporting ultrasound radiation (i.e. acoustic waves). In one embodiment, the frequency selective reflector is a diffraction grating placed external to the optical fiber. In another embodiment, the frequency selective reflector is a narrowband reflector placed directly in the optical fiber. An acoustic signal enhancing feature which increases sensitivity of the detector is provided by utilizing a reflective membrane having a thickness substantially less than the acoustic wavelength, .lambda., as the broadband optical reflector.

Abstract: A vibrationally and acoustically insulated structure includes a structure using enclosing a source of internally generated noise and vibrations. The structure further includes a vibration and acoustic insulating device positioned over a portion of the internal surface of the housing and having a vibration damping portion and acoustic absorbing portion. The vibration damping portion, such as a constrained damping layer having a continuous damping layer and a segmented constraining layer of individual rigid segments, reduces vibrations transmitted through the structure housing. The acoustic absorbing portion includes an acoustic barrier layer adjacent the constrained damping layer, an acoustic absorption layer adjacent to the acoustic barrier layer and one or more mounts for mounting the acoustic absorption layer and acoustic barrier layer to the constrained damping layer. The acoustic absorption device reduces the reverberating acoustic waves within the structure housing.

Abstract: A vibrationally damped structure includes a structure that is susceptible vibrations and a constrained damping layer positioned over at least a portion of the vibrating structure. The constrained damping layer includes a continuous damping layer bonded to the surface of the structure for providing the first reduction of vibrational energy transmitted through the structure. The constrained damping layer further includes a segmented constraining layer bonded to a portion of the continuous damping layer for providing a second reduction of vibrational energy transmitted through the structure and the continuous damping layer. The continuous damping layer preferably includes a layer of elastomeric material bonded to the structure with an epoxy bonding compound or the like. The segmented constraining layer includes a plurality of rigid segments, such as aluminum plates, bonded to the continuous damping layer with a bonding compound.

Abstract: A rigid plate with an aperture therethrough has a variable coupler fiber optic sensor encapsulated in the aperture by an encapsulant having an index of refraction that varies with applied stress. Incident light applied to an input fiber of the coupler is coupled to the output fibers thereof in a ratio dependent on the stress applied to the encapsulant. In an alternative embodiment, a well in a member superposed with respect to the plate is filled with a low bulk modulus material which abuts the encapsulant.

Abstract: A hydrophone group for shallow towed applications in less than 50 feet of ter. The hydrophone group has a series of hydrophones connected by relatively insensitive fiber optic interconnects. The individual hydrophones are sufficiently sensitive such that the interconnecting optical fiber does not introduce excessive noise. Each hydrophone is basically a sensing fiber wrapped around an air-backed mandrel. Each air-backed mandrel is formed of an extended solid frame substantially non-compliant along a longitudinal axis. The extended solid frame is provided with a channel around the periphery thereof. The channel extends substantially the entire length of the extended solid frame. A flexible outer covering surrounds the extended solid frame. The flexible outer covering is highly compliant in a radial direction extending from the longitudinal axis. The air-backed mandrel has a high frequency mechanical resonance.

Abstract: An ultrasound/vibration sensor consisting of a microchip laser or an array of microchip lasers constructed to oscillate at two different laser frequencies corresponding to two orthogonal polarizations is disclosed. The frequency difference between these two modes is chosen to be within the bandwidth of an electrical (as opposed to optical) signal processing system. When the microchip laser or microchip laser array is placed in an acoustic field, its cavity length is modulated which causes a frequency modulation of the frequency difference between the two modes. When the two laser output polarizations are mixed using a polarization scrambling device such as a polarizer at about 45 degrees to the polarization axes and then detected with a photodiode, one for each microchip laser, the resulting electrical signal contains the FM modulated beat frequency between the two polarization modes. This is then demodulated using an electrical signal processing system.

Abstract: A fiber optic bundle towed line array having a central strength member and plurality of longitudinal optical fibers spaced around the bundle near the array outer circumference. Each fiber has at least one in-line intensity modulated optical fiber hydrophone inserted at different preselected locations therealong. Patting material is molded around and embeds the optical fibers and the strength member so as to form a small diameter integral array unit.

Abstract: A fiber optic interferometric sensor for use in a hydrophone system at high pressures includes a reference fiber wrapped around a rigid inner cylinder. A solid layer of compliant material is applied over the reference fiber. The sensing arm of the interferometer is wound over the layer of material applied over the reference fiber. The outer material is sufficiently compliant to provide acoustic sensitivity comparable to that of air-backed hydrophones.

Abstract: Methods for ultrasonic/vibration detection include using a sensor consisting of a microchip laser or an array of microchip lasers constructed to oscillate at two different laser frequencies corresponding to two orthogonal polarizations is disclosed. The frequency difference between these two different frequencies is chosen to be within the bandwidth of an electrical (as opposed to optical) signal processing system. When the microchip laser or microchip laser array is placed in an acoustic field, its cavity length is modulated which causes a frequency modulation of the frequency difference between the two modes. When the two laser output polarizations are mixed using a polarization scrambling device such as a polarizer at about 45 degrees to the polarization axes and then detected with a photodiode, one for each microchip laser, the resulting electrical signal contains the FM modulated beat frequency between the two polarization modes. This is then demodulated using an electrical signal processing system.

Abstract: New apparatus comprise a optical fiber based RF signal train generator for storing transient RF pulses and regenerating the identical replicas for analysis. The apparatus further comprise RF receivers to process one stored pulse with a reference to other stored pulse. The present invention drastically increases our abilities to investigate acoustical, electromagnetic, and optical transient phenomena.

Abstract: An ultrasound/vibration detector array includes an electrically pumped vertical cavity surface emitting laser (VCSEL) array. The cavity length of each laser or pixel of the array is modulated by the acoustic field at the point where the acoustic field contacts the pixels. The resulting laser output is frequency modulated by the acoustic field. This modulation is converted to amplitude modulation at the detector head and then either detected with a charge-coupled-device (CCD) array with the information being electrically communicated to the signal processing assembly or sent directly by optical fiber to the signal processing assembly for processing. This ultrasound/vibration detector array provides high frequency bandwidth detection, fine spatial resolution, and minimal electric cabling.

Abstract: A small diameter towed acoustic line array comprising a plurality of miniature fiber optic lever hydrophones spaced at preselected locations along the array and held in place by fiber holders. The array hydrophones are enclosed within a tubular outer sheath which is filled with an acoustically transparent fluid. The hydrophones respond to either the pressure mode or the pressure gradient mode, each hydrophone further comprising at least one receiving and one transmitting optic fiber of the same diameter, a covering sheath, a clear elastomer and a small reflector. The information received is transmitted to and processed by a beamformer external to the array.

Abstract: An optical communication system for ultrasound imaging systems employs a light source and photodetector array in the imaging console. The light source and photodetector array are coupled to each other through an optical fiber passing through the probe. Information from the probe is communicated to the console by an optical modulator, situated in the optical path from source to detector. The ultrasound signals are converted into electrical signals by probe electronics, and these electrical signals serve to alter either the phase or amplitude of optical signals passing through the modulator. Since the light source and photodetectors reside in the imaging console, very little power is dissipated in the probe.

Abstract: A device for converting mechanical vibrations into a digital signal uses a iaphragm having a reflective surface with a plurality of reflective facets disposed on a face of the diaphragm. The diaphragm is mounted such that it will be displaced a distance proportional to the external stimulus, such as acoustic energy, it receives. A light source provides a continuous optical beam which is directed onto the reflective surface and reflected by the surface onto an optical detector, which in turn, produces an electrical signal identifying the position on the detector illuminated by the reflected beam. As the diaphragm vibrates in response to the external stimulus receive, the optical beam will be reflected in different directions thereby changing the position that the beam strikes the optical detector. A microprocessor receives the electrical signals from the optical detector and produces a digital signal corresponding to the displacement of the diaphragm.

Abstract: A method of transmitting the output signal from each pixel of an array of vertical cavity surface emitting lasers (VCSEL's) operatively positioned in an acoustic detector which minimizes the size of interconnects to the different pixels, which uses optical fibers to transport the output signal from each VCSEL pixel to the signal processing assembly. The VCSEL array is fabricated onto an acoustic matching layer which is designed to allow maximum acoustic coupling into the transducer assembly. An acoustic damping layer is employed after the array in order to extinguish the acoustic energy in order to avoid reflections within the transducer. Alternatively, an acoustic reflecting layer can be employed depending on the sensitivity and bandwidth requirements of the application. The frequency modulation of the laser output caused by the acoustic disturbance is then converted into amplitude modulation in a Fabry-Perot cavity array attached to the damping layer.

Abstract: A plurality of air-backed elongate mandrels are arranged in an planar array such that their longitudinal axes are parallel. A length of a first optical fiber is wound around portions of each mandrel in a first group of the mandrels for exposure to the parameter. The first optical fiber is arranged such that exposing it to the parameter to be sensed causes the length of the first optical fiber to increase and decrease in direct proportion as the parameter increases and decreases. A length of the second optical fiber is wound around a second group of the mandrels for exposure to the parameter. The second optical fiber preferably is arranged such that exposing it to the parameter to be sensed causes the length of the second optical fiber to increase and decrease in inverse proportion as the parameter increases and decreases.

Abstract: The present invention relates to a hydrophone and to a virtual array of hydrophones for sensing the amplitude, frequency, and in arrays, the direction of sonic waves in water. The hydrophone employs a laser beam which is focused upon a small "focal" volume of water in which natural light scattering matter is suspended and which matter vibrates in synchronism with any sonic waves present. The vibration produces a phase modulation of the scattered light which may be recovered by optical heterodyne and sensitive phase detection techniques. The sonic waves are sensed at locations displaced from the focusing lenses. Because of this remote sensing capability, the physical hardware of an array of hydrophones may be confined to a small area comparable to the dimensions of the lenses themselves while the sensing of the sonic waves virtually occurs at widely spaced, remote focal volumes.

Abstract: A fiber optic sensor is formed by providing a first mandrel section having a first longitudinal slot therein, placing a reference fiber within the slot and winding a sensing fiber around the first mandrel. The sensor includes a second mandrel section having a second longitudinal slot therein and a spacer between the first and second mandrel sections. The sensing fiber and the reference fibers are wound around the spacer, and then the reference fiber is placed in the second longitudinal slot. The sensing fiber is wound around the second mandrel section. The sensor may include additional mandrels and spacers to form additional sections. A plurality of sensing coils may be formed on each mandrel section.

Abstract: A hydrophone is formed of first and second optical fibers coupled together to form a fiber optic interferometric sensor for sensing an acoustic signal. The optical fibers are wrapped around a pair of concentric, thin-walled hollow cylinders. The fiber wrapped around the inner cylinder is the reference leg of the interferometer and the fiber wrapped around the outer cylinder is the signal leg. The reference leg is exposed to the hydrostatic pressure but isolated from the acoustic signal. The sensing leg is exposed to both the hydrostatic pressure and the acoustic wave signal. The signal output from the interferometer is indicative of changes in the acoustic wave signal.

Abstract: A sensing portion for an interferometric planar hydrophone is disclosed which comprises: a support assembly; a plurality of hollow compliant mandrels mounted to the support assembly in a fixed planar relationship with respect to each other to form a planar configuration; a sensing optical fiber sequentially wound around each of the plurality of mandrels to form a single, serial, optical sensing arm or path between an input position on a first one of the plurality of mandrels and an output position on a last one of the plurality of mandrels; and an acoustically transparent material encapsulating the sensing portion to form a sealed compliant planar hydrophone.

Abstract: An underwater probe for determining true acoustic intensity by the direct asurement of true acoustic velocity and true acoustic pressure in a neutrally buoyant package, utilizes a moving-coil geophone embedded in a casting of syntactic foam and a pair of hydrophones on the exterior of the casting.

Abstract: An element responsive to acoustic particle acceleration for sensing acoustic signals in a region of low acoustic pressure is disclosed. The element may be isolated from acoustic noise when positioned adjacent an acoustic noise generating high acoustic impedance structure by a baffle which provides isolation from radiated and evanescent acoustic signals and structure vibration.

Abstract: A distributed acoustic sensor utilizing a Fizeau type fiber optic transducer and a distributed acoustic collector to gather and return acoustic disturbances from remote areas in buried, underground, or underwater installations. A light beam from a laser or LED source is transmitted over a fiber to a remote transducer which is coupled to a distributed acoustic collector. The collector may be an extended pipe or tube system filled with an acoustic conducting medium such as air or an appropriate liquid which communicates with the sensing diaphragm in the transducer. An interference beam whose characteristics are a function of the diaphragm motion is created in the transducer and transmitted back over the common single fiber toward the source. An in-line optical coupler/splitter diverts a portion of the returning modulated beam to a photodetector which outputs a proportional electrical signal to an electronic circuit for amplification and analysis.

Abstract: An apparatus for continuously monitoring changes in a liquid level in accance with a differential pressure is provided. A sensor body immersed in the liquid supports two thin-filmed metallic diaphragms that are independently, axially responsive to pressure of the liquid. Each diaphragm has a different modulus of elasticity. Two fiber optic pressure sensors are mounted within an air space encased by the sensor body in combination with the two diaphragms. Each of the two fiber optic pressure sensors detects the axial response of one of the two diaphragms. The displacement difference between the two diaphragms is an indication of differential pressure related to the liquid level.

Abstract: An acceleration insensitive interferometric hydrophone sensor having increased sensitivity comprises a rigid cylindrical support mandrel that is coaxial with and between thin walled sense and reference mandrels. Flanges on the reference and support mandrels form air filled cavities above and below optical reference and sense fiber windings, respectively, that are symmetrically wound on associated mandrels. The walls of the sense and reference mandrels are thin so that they both respond to the same incident acoustic wave signal although 180.degree. out-of-phase. A central collar on the support mandrel provides an acceleration insensitive location for making attachment to the hydrophone. In another embodiment, the reference mandrel is much thicker than the sense mandrel and cavities supporting the sense fiber are caused to operate as acoustic impedance mismatches for rendering the reference fiber winding substantially insensitive to and acoustically decoupling it from an incident acoustic pressure wave signal.

Abstract: An optical transducer, such as used in an ultrasound system, includes a signal laser which generates an optical signal the frequency of which varies in correspondence with acoustic energy incident on the transducer. An optical cavity in the signal laser is disposed such that incident acoustic energy causes compression and rarefaction of the optical cavity, and this displacement varies optical frequency generated by the laser. A laser pump coupled to the lasing medium is adapted to apply selected levels of excitation energy appropriate to the generation and detection of acoustic pulses. The signal laser alternatively is adapted such that the refractive index of the optical cavity is varied in correspondence with the incident acoustic energy to modulate the optical frequency of the light generated by the signal laser.

Abstract: An optical lever acoustic and ultrasound sensor with increased sensitivity using an optical amplification means to amplify the motion of an incident acoustic wave and convert it to an electric signal for image processing. Three approaches to the optical amplification are disclosed. In the first approach, the vibrating mirror is part of a cantilever that increases the angular deflection of the incident light beam. In the second approach, a second, stationary mirror is positioned approximately parallel to the vibrating mirror surface. The reflected light beam is reflected back onto the vibrating mirror, and picks up a another increment of the acoustic signal with each reflection. In the third approach, the effective moment of the optical lever is increased within a small volume by the use of two stationary mirrors to increase the path length from the vibrating mirror to the position-sensitive detector.

Abstract: A hydrophone includes a plurality of hydrophone components separated by finite spacings and interconnected to provide a single output signal. Each hydrophone component is based upon a single-mandrel design in which a cylindrical body is apportioned into sensing and reference sections. The sensing sections comprise coaxial arrangements of pliant inner and outer cylinders separated by an annular airspace while the adjacent reference sections comprise solid-walled cylinders. Finite separation distances between the hydrophone components result in reduced flow noise occasioned by increased sensing area while detection sensitivity is maintained.

Abstract: A miniature fiber optic hydrophone based on the principles of a Fabry-Perot interferometer. The hydrophone, in one embodiment, includes a body having a shaped flexible bladder at one end which defines a volume containing air or suitable gas, and including a membrane disposed adjacent a vent. An optic fiber extends into the body with one end terminating in spaced relation to the membrane. Acoustic waves in the water that impinge on the bladder cause the pressure of the volume therein to vary causing the membrane to deflect and modulate the reflectivity of the Fabry-Perot cavity formed by the membrane surface and the cleaved end of the optical fiber disposed adjacent to the membrane. When the light is transmitted down the optical fiber, the reflected signal is amplitude modulated by the incident acoustic wave. Another embodiment utilizes a fluid filled volume within which the fiber optic extends.

Abstract: An element responsive to acoustic particle acceleration for sensing acoustic signals in a region of low acoustic pressure is disclosed. The element may be isolated from acoustic noise when positioned adjacent an acoustic noise generating high acoustic impedance structure by a baffle which provides isolation from radiated and evanescent acoustic signals and structure vibration.

Abstract: Optical fiber coils are formed on inner and outer mandrels. The inner mandrel is spaced apart from the outer mandrel to form a cavity between them with the inner fiber coil being inside the cavity. Water-tight seals between the inner and outer mandrels keep the inner coil isolated from underwater acoustic waves that may be incident upon the outer coil. The optical fibers are included in a dual optical path fiber optic interferometer that includes the inner fiber coil in a first optical path and the outer fiber coil in a second optical path. A central wall divides the inner region of the inner mandrel into two cavities. A tie rod has a first end connected to the wall and a second end extending out of the first cavity in the inner mandrel. A backshell is connected to the second end of the tie rod. The backshell includes a hollow portion formed as a housing for protecting fiber optic components of the interferometer.

Abstract: The inner surface affixation system and process is a method for affixing wound optical fibers on the inner surface of a hollow cylinder. It is useful, for example, in the manufacture of rigid mandrel-based fiber optic sensors. The optical fibers of a sensor wound on the inner surface are less susceptible to damage than are optical fibers wound on the outside of the mandrel.The process requires that the fibers be supported on a cylinder, which is coated with an adhesive and placed within the sensor mandrel cylinder, at which time the circumference of the supporting cylinder is expanded, causing the fibers to engage the inner surface of the sensor mandrel cylinder. The adhesive is then cured, holding the fibers in place on the inner surface of the sensor mandrel cylinder while the supporting cylinder is contracted and removed from the sensor mandrel cylinder.

Abstract: An omnidirectional fiber optic hydrophone includes a concentrically-arranged pair of ring-shaped mandrels mounted between planar upper and base members. Each of the rings is formed of inner and outer annular portions separated by an annular void. Optical fibers wound about the outer circumference of the outer annular portion of the outer ring and about the outer circumference of the inner annular portion of the inner ring communicate with a source of optical energy and with a photodetector to provide signals for measuring acoustic wave-induced deflections of the rings. A plurality of mandrels may be employed in a single hydrophone which may be potted with elastomeric material or free flooded.

Abstract: An acoustic sensor, suitable for use as an ultrasonic transducer, microphone or hydrophone, uses an optical lever to amplify the motion of the sensor surface and convert it to an electrical signal suitable for image processing. In the acoustic sensor, a beam of light from a laser is directed at an oblique angle onto a reflective surface coupled to a sensor membrane. The reflected light strikes a position-sensitive light detector (PSD) which generates an electrical signal indicative of the position of the spot of light on the PSD. When an incident acoustic wave strikes the sensor membrane, the small movements of the reflective surface result in large motions of the spot of light on the PSD, thereby amplifying the acoustic signal and converting it into an electrical signal. Also disclosed is a multi-element sensor array suitable for linear array or phased array imaging.

Abstract: There is described an optical acoustic sensor that is pressure-compensated for operation in high-pressure environments such as the deep sea and that may be employed in multi-sensor arrays. The sensor includes an acoustically responsive diaphragm that is illuminated by light conducted through an optical fiber and that reflects the light back into the fiber. Light waves reflected by the diaphragm and fiber end interfere, and the resulting interference is conducted by the optical fiber to a to a photodetector and associated signal processing electronics such as a phase generated carrier demodulator. Pressure compensation is provided to equalize the static pressure on both sides of the diaphragm. Additionally, an array of sensors is described that employs optical time division multiplexing.

Abstract: A hydrophone array employing a plurality of laser sources coupled onto a single optical fiber communications link. A matching plurality of optical fiber resonant rings, each having a resonance peak at a wavelength substantially equal to the center wavelength of one of the lasers, is selectively coupled to the optical fiber communication link so that substantially only optical energy in a narrow bandwidth around one of the laser center wavelengths is coupled to the corresponding one of the optical fiber resonant rings. Corresponding photodetectors are also wavelength selectively coupled to the optical fiber communications link so that each detector receives only optical energy corresponding to the wavelength of one of the lasers and its corresponding resonant ring, to provide an output signal indicating variations in acoustic pressure at individual resonant rings in the array.

Abstract: An omnidirectional hydrophone having an elastic shell in the form of an ote ellipsoid of revolution having the ratio of its major axis to is minor axis greater than about .sqroot.(2-.nu.) where .nu. is Poisson's ratio of the shell material, wherein the circular circumference of the shell (at different circular parallels of latitude) undergoes strains of opposite sign when the shell is subjected to a pressure change. The differential strains are advantageously measured by an optical fiber interferometer having one leg wound about the equatorial circumference of the shell and another leg spirally wound near one or both of the poles.

Abstract: A mismatched path length fiber optic interferometer is optically coupled to an optical fiber and configured to form an omnidirectional acoustic sensor. A second mismatched path length fiber optic interferometer is optically coupled to the optical fiber and configured as a first gradient sensor. A second fiber optic gradient sensor is also optically coupled to the optical fiber. A detector optically coupled to the omnidirectional acoustic sensor and to the gradient sensors converts optical signals output therefrom to electrical signals indicative of the magnitude and direction of changes in an acoustic field. The omnidirectional acoustic sensor may include a length of optical fiber wrapped around the housing while the gradient sensors are mounted inside the housing. The housing perferably has a volume that is adjustable for controlling the buoyancy thereof. Each gradient sensor preferably comprises a pair of mandrels formed to enclose chambers. Optical fiber coils are formed on the mandrels.

Abstract: An omni-directional hydrophone having a pair of fiber-optic windings wrapped around a resilient ball to form a spherical acoustic sensor is provided. The fiber-optic pair has a first fiber which has a bonded jacket and a second fiber which has an unbonded jacket. The fiber with the bonded jacket is sensitive to both vibration of the mounting structure and impinging acoustic signals. The fiber with the unbonded jacket is sensitive to vibration but insulated from the acoustic signals. By comparing the signals from the two fibers, the hydrophone detects acoustic signals by detecting the phase difference between the two fibers. As both fibers are equally affected by vibration, there is no phase difference caused by vibration. This mechanization permits the canceling of vibration induced signals while retaining full sensitivity to acoustic signals. The hydrophone uses a standard fiber-optic laser source and detection system to convert the acoustic generated phase change to amplitude dependent voltage signals.

Abstract: A fiber-optic hydrophone having a pair of jacketed fiber optic windings fed in a concentric planar spiral configuration in a layer of polyurethane is provided. One of the fiber optic windings has a fiber with a bonded acoustically sensitive jacket thereby increasing its sensitivity to acoustic energy. The second fiber optic winding, the reference winding, has a unbonded jacket enclosing the fiber resulting in reduced sensitivity to acoustic energy. Sensitivity to vibrational energy; however, is not reduced. The combining of signals from the pair of fibers provides a vibration-canceled acoustic signal.

Abstract: A method and apparatus (10) for sensing sound in a fluid medium. The apparatus (10) includes an element (12) for generating polarized light having a predetermined plane of polarization. In addition, the apparatus (10) further includes an element (14) for rotating the plane of polarized light in response to sound waves propagating in the fluid medium, as well as an element (32) for detecting the polarized light. Finally, the apparatus (10) includes an element (34) for generating a signal from the output of the detecting element (32) which is indicative of the amplitude and frequency of the sound waves.

Abstract: An expandable sonar array structure includes a central body with a plurality of radially extendable hydrophone support arm assemblies attached to said body and including a motor-driven drive member to which all the arm assemblies are attached through pushrods. Each arm assembly includes three substantially vertically aligned, radially extending arms carrying hydrophones with top and bottom arms attached to the pushrods through short arms with pivots chosen so that the arms move in opposite directions. The center arm is attached to the lower arm by means of a link which closes a parallelogram arrangement such that the center arm moves essentially in parallel with the lower arm. An additional arm is pivotally attached to the outboard end of the center arm and extends the center arm to essentially the diameter of the upper and lower arms, and its outboard end is attached through a link to the outside end of the upper arm.

Abstract: An acousto-optical seismic sensor array includes a distributed set of optical-fiber sensing coils. A light pulse is launched through the sensing coils in serial order. The light pulse is cumulatively data-modulated by the respective sensing coils and is returned as a time-division multiplexed pulse train. The pulse train is split into a first pulse train and a retarded second pulse train. The retardation time equals the travel-time delay of a light pulse between sensors. The retarded pulse train is compared with the first pulse train to determine the phase shift therebetween for consecutive pulses. The phase shift is an analog of the quantity being sensed.

Abstract: Inertially insensitive optical fibre hydrophones are described, utilizing materials and configurations to obtain acceleration balanced designs suitable for underwater applications where acceleration is a problem. Balancing and/or insensitivity is obtained both in longitudinal and axially perpendicular directions. Acoustic sensitivity of a coil formed by the fibre is maintained by the properties of materials used to provide a rigid former, encapsulant material and secondary material which form the hydrophone.

Abstract: A dual beam hydrophone wherein a reference laser beam and a signal laser m are both modulated simultaneosuly by the movement of reflecting surfaces responding to pressure variatrions due to an impinging acoustic wave. Each beam, travels the same path length within the hydrophone before being detected, thus eliminating any otherwise needed signal compensation. The reference beam and signal beam are acoustically modulated 180 degrees out of phase which reduces by one half the number of reflections normally required to produce the same sensitivity.

Abstract: A system for signal stabilizing in-phase modulated optical hydrophone arr employs interferometry with homodyne detection. Phase stabilization is accomplished by modulating the input laser signal in proportion to variations in the output of an optical transducer to balance the output phase so that the fringes are kept at optimum position. Additionally, fluctuations in light intensity are compensated for so that a photodetector responds only to phase shift variations. The technique used is to split the input beam into signal and reference beams using a beam divider, exposing the signal beam to the acoustic pressure of interest, recombining the signal beam with the reference beam, detecting the combined beams and filtering the resulting signal to separate out the acoustic information of interest from the phase shift and light intensity portions used to stabilize the input beam.

Abstract: A streamer-cable assembly is provided having a tubular outer jacket enclosing an open-cell and closed-cell foam core. The foam core provides an axial hole for retaining a plurality of pressure sensors and a plurality of off-axial holes providing passages for the transmission bundle. Longitudinal channels along the exterior of the foam inserts receive substantially rigid stress members which provide longitudinally strength to the streamer. Cable noise associated with the cable jerk and cable whip/vibration is radially reduced by the open-cell foam core and rigid stress members while the closed-cell foam core provides buoyancy to the streamer cable at the connectors between sections.

Abstract: An omnidirectional hydrophone having an elastic shell which is spheroidal so that the circumference of the shell about different axes changes differentially when the shell is subjected to pressure variations. The differences in circumference are advantageously measured by an optical fiber interferometer having one leg wound about the equatorial circumference of the shell and another leg wound about its meridional circumference. The shell may be an oblate spheroid having the ratio of its major axis to is minor axis greater than about (2-.nu.)1/4, where .nu. is Poisson's ratio of the shell material, so that the shell narrows along one axis and widens along the other when the shell is subjected to a pressure change.