USA As Represented by Secretary of the Navy

Anthony Dandridge

Alan D Kersey

Alan B Tveten

Sandeep T Vohra

Bruce A Danver

Aileen M Yurek

Frank Bucholtz

Michael J Marrone

Kee P Koo

Nicholas J Frigo

Allen R Davis

Brandon F Redding

Catherine H Bulmer

George H Sigel, Jr

Clay K Kirkendall

A method of interferometric optical sensing via spatial demodulation includes emitting a laser beam; splitting the laser beam into a reference beam and an interrogation beam; converting a desired signal into a change in the optical path of the interrogation beam via an optical sensor; and capturing the reference beam and the interrogation beam via a camera, wherein the interrogation beam is incident to the camera at a first angle and the reference beam is incident to the camera at a second angle different from the first angle, thereby causing an interference pattern at the camera.

Advantages of spatial demodulation in interferometric optical sensing applications

This invention pertains to a sensor comprising of glass optical fiber, a cellular pressure release material surrounding the fiber, and means for determining responsivity of the sensor, the sensor having responsivity of at least about &minus;160 dB re rad/m-&mgr;/Pa, which corresponds to sensitivity of about 50 times better than prior art. The sensor can be made part of an optical interferometer which is characterized by splitting of an entering light beam and measurement of an optical phase shift caused by light traversing different paths. The sensor can form a part of a fiber Bragg grating geometry characterized by a fiber Bragg grating in the glass fiber covered by the cellular pressure release material wherein shift of the Bragg wavelength induced by strain on to the cellular pressure release material is measured.

Coated fiber pressure sensors utilizing pressure release coating material

The interferometric fiber optic accelerometer is viewed as a mass-spring transducer housed in a sensor case. The sensor case is attached to a moving part whose motion is inferred from the relative motion between the mass and the sensor case. A flexural disk is housed in a sensor case which is accelerated in a direction normal to the plate surface. The plate undergoes displacement resulting in strains on the plate surface. A coil of optical fiber, made to be part of an optical interferometer, is attached to the flexural disk, the strain from the disk is transferred to the fiber thus changing the path length of the fiber interferometer. The interferometer output in demodulated providing the acceleration response. The design of the accelerometer housing is such that it is highly immune to extraneous signals, i.e., dynamic and hydrostatic pressure. Fabrication of the optical fiber coils is accomplished by winding the optical fiber, with a specially designed chuck to form reference and sensing fiber coils which are then bonded to the flexural disk to form the accelerometer.

Fiber optic accelerometer sensor and a method of constructing same

A fiber optic sensor array has multiple segments, each capable of detecting a physical condition such as an acoustic wave. The segments are separated by weak reflectors such as fiber optic Bragg gratings. Light from a light source is input into the input end of the array. Light reflected by each of the reflectors has a phase shift representing the effects of the physical condition on all of the segments from the input end to that reflector. To address a specific reflector, the return light is demultiplexed. This demultiplexing is done by modulating the light input into the input end of the array with a pseudo-random bit sequence and correlating the output with a time-shifted version of the pseudo-random bit sequence to isolate the part of the output caused by that reflector. To address a specific segment, the phase shifts from two adjacent reflectors are determined. The return light can be strengthened by mixing it with a portion of the light picked off from the light source.

Coherent reflectometric fiber Bragg grating sensor array

A forward-coupled array topology is used for a ladder-type fiber optic ser system in which each optical path in the system includes the same total number of input and output optical fiber sections and approximately the same total number of input and output couplers and splices so that the coupler splitting ratios for optimum optical performance are independent of light losses in the system.

Fiber optic sensor array system with forward coupled topology

The interferometric fiber optic accelerometer is viewed as a mass-spring nsducer housed in a sensor case. The sensor case is attached to a moving part whose motion is inferred from the relative motion between the mass and the sensor case. A flexural disk is housed in a sensor case which is accelerated in a direction normal to the plate surface. The plate undergoes displacement resulting in strains on the plate surface. A coil of optical fiber, made to be part of an optical interferometer, is attached to the flexural disk, the strain from the disk is transferred to the fiber thus changing the path length of the fiber interferometer. The interferometer output in demodulated providing the acceleration response. The design of the accelerometer housing is such that it is highly immune to extraneous signals, i.e., dynamic and hydrostatic pressure. Fabrication of the optical fiber coils is accomplished by winding the optical fiber, with a specially designed chuck to form reference and sensing fiber coils which are then bonded to the flexural disk to form the accelerometer.

Apparatus for winding optic fiber coils

A method and apparatus for overcoming polarization induced signal fading in both heterodyne communication and interferometric sensing. An adjustable birefringent element in series with a linear-polarization beam splitter forms an elliptical-polarization beam splitter. The birefringent element controllably evolves the states of polarization of two input signals thereby controlling the power contribution of each signal onto the orthogonal axes used by the linear beam splitter. When the states of polarization are evolved such that there are equal signal to reference power ratios on the beam splitter axes, subsequent detectors generate a constant, optimum amplitude signal without the need for weighting or decision circuits.

Adaptive polarization diversity detection scheme for coherent

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. Each hydrophone is connected in a chain by a plurality of interconnects having a substantially lower sensitivity than each hydrophone.

Hydrophone group design for shallow towed applications

A code-division multiplexed system comprising an optical source, a code generator for developing and applying a pseudo-random bit sequence code to the optical source to cause the optical source to develop a unipolar coded optical signal, a means for dividing the unipolar coded optical signal into N optical paths separated from each other by associated successive integer multiples of a bit period T of delay, N information signal sources responsive to the unipolar coded optical signals for selectively producing N differently-delayed optical information signals, a means for combining the N differently-delayed optical information signals into an output optical signal comprised of an intensity sum of the overlapping N differently-delayed optical unipolar information signals, a photodetector responsive to the output optical signal for developing an input electrical signal corresponding to the sum of N overlapping unipolar optical signals, an adjustable delay circuit responsive to the pseudo-random bit sequence code for producing a desired bipolar reference code corresponding to the delay associated with an associated desired one of the N differently-delayed optical information signals, and a correlation circuit responsive to the input electrical signal and to the desired bipolar reference code for synchronously correlating the desired bipolar reference code with the input electrical signal to extract a desired electrical signal corresponding to the desired one of the N differently-delayed optical information signals and to substantially suppress any other electrical signal that is asynchronously aligned with the desired bipolar reference code.

Fiber optic network system with low crosstalk using code-division

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. In an operational system, an interferometric planar hydrophone is disclosed which comprises: the sensing portion in which the sensing arm is operative to measure a predetermined measurand in a preselected environment and which sensing portion further includes a reference optical fiber environmentally isolated from the predetermined measurand; a light source for transmitting coherent light to both of the sensing and reference optical fibers; and circuitry responsive to light received from the sensing and reference optical fibers for developing an output signal indicative of phase shift differences between the sensing and reference optical fibers to determine the predetermined measurand.

Hydrophone transduction mechanism

An improved inline fiber optic sensor array is achieved by packaging a pllity of fiber optic sensor units such that each fiber optic sensor unit incorporates a complete functional sensor and adjacent fiber optic sensor units are separated by a delay element connected in series between each pair of fiber optic sensor units. Delay elements temporally separate measureand signals received from the individual fiber optic sensor units and permit decoupling of signals produced in response to environmental stress on the non-sensor unit portions of the array.

Inline fiber optic sensor arrays with delay elements coupled between

A system and method for minimizing polarization-induced phase noise in an interferometric fiber sensor is disclosed. The system includes an optical source of light; a single mode input fiber; means coupled between the optical source and the input fiber for scrambling the polarization state of the light from the optical source at a first frequency to effectively produce and pass depolarized light to the input fiber; an interferometric fiber sensor responsive to light from the input fiber for producing at its output an interference pattern proportional to a phase shift produced by a field being sensed by the interferometric fiber sensor; detection means responsive to the interference pattern for developing a photocurrent signal averaged over a period of time longer than the period of the first frequency; and demodulation means responsive to the photocurrent signal for producing an electrical output signal substantially proportional to the phase shift produced by the field being sensed by the interferometric sensor.

System and method for minimizing input polarization-induced phase noise

An integrated-optic, polarization-selective phase modulator is disclosed. a preferred embodiment of the invention a channel waveguide provides a path therethrough for polarized light having horizontal and vertical polarization components, and an electrode structure is disposed above the channel waveguide and is responsive to the application of a control voltage to the electrode structure for substantially phase modulating only the vertical component of the polarized light.

Integrated-optic, polarization-selective phase modulator

A new serial interferometric fiber-optic sensor array configuration is diosed. In a preferred embodiment of the invention pulsed light from a laser source is transmitted into a long input fiber which forms a series of N sensor elements, each of optical path length L. Each sensor element is responsive to any change in an associated predetermined physical parameter for changing its optical path length. A small portion of the optical power in the input fiber is tapped-off to a long output fiber at tap points between adjacent sensor elements and at tap points before the first and after the last sensor elements in order to produce at the output of the last sensor element a series of N+1 pulses separated in the time domain. This series of N+1 pulses from the output of the last sensor element is applied to each of two paths in a compensating interferometer of optical path imbalance L. The compensating interferometer coherently mixes pulses obtained from each adjacent pair of tap points to obtain a series of interferometric signals respectively indicative of any change in the physical parameters to which the sensor elements are respectively responsive.

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Burke, VA, United States of America

Anthony Dandridge