RF-photonic system for acoustic and/or vibrational sensing using optical fiber and method thereof

• US 10,031,246 B2
• Filed: 06/18/2013
• Issued: 07/24/2018
• Est. Priority Date: 11/15/2012
• Status: Active Grant

• Current Assignee: The United States Of America As Represented By The Secretary Of The Army
• Original Assignee: The United States Of America As Represented By The Secretary Of The Army
• Inventors: Zhou, Weimin, Okusaga, Olukayode, Cahill, James P
• Primary Examiner(s): Caputo, Lisa
• Assistant Examiner(s): Roberts, Herbert K
• Application Number:
  US13/920,570
• Publication Number:
  US 20140130601A1
• Time in Patent Office: 1,862 Days
• CPC Class Codes: G01H 9/004 using fibre optic sensors light guides per se G02B6/00, acousto-optical devices specially adapted for gating or modulating in optical wave guides G02F1/125
• CPC Class Codes: G01S 15/02 using reflection of acoustic waves G01S15/66 takes precedence
• CPC Class Codes: G01S 7/521 Constructional features
• CPC Class Codes: G01V 1/18 Receiving elements, e.g. seismometer, geophone or torque detectors, for localised single point measurements

Abstract

A method and system comprising:

• • an optoelectronic oscillator comprising a light generator, a modulator, and at least one optical fiber, the optoelectronic oscillator operating to send a modulated carrier signal through the optical fiber into a target area and receive the return signal;
  • a circuit for removal of the carrier frequency operatively connected to the oscillator comprising at least one phase shifter; at least one mixer operatively connected to the at least one phase shifter; at least control circuit operatively connected the at least one mixer and the at least one phase shifter for controlling the phase shifter and operating to cancel signals other than the sideband signals;
  • a signal processor for processing the sideband signals to detect acoustical and/or vibrational signals from the target area. The method comprises sensing vibrational and/or acoustical signals for detection of acoustical and/or vibrational signals in the target area.

Claims

1. A system for sensing vibrational and/or acoustical signals from a target area comprising:

an optoelectronic oscillator comprising a light generator, a modulator, and at least one optical fiber, the optoelectronic oscillator operating to send a modulated carrier signal having a carrier frequency through the optical fiber into a target area and receive a return signal comprising a carrier frequency component and sideband signals that resonate within the optoelectronic oscillator;

a circuit for removal of signals other than sideband signals operatively connected to the optoelectronic oscillator for receiving of the optical return signal from the target area and substantially removing the carrier frequency;

the circuit for removal of the carrier frequency comprising;

a phase shifter;

a mixer operatively connected to the phase shifter;

a control circuit operatively connected to the mixer and the phase shifter for controlling the phase shifter and operating to remove the carrier frequency;

a signal processor operatively connected to the circuit for removal of the carrier frequency, the signal processor operating to process the sideband signals to detect acoustical and/or vibrational signals from the target area.

2. The system of claim 1 wherein the circuit for removal of signals other than sideband signals comprises an interferometer circuit and wherein the carrier frequency signal is an RF carrier frequency range signal that is removed by interferometry;

and further comprising a sensor operatively connected to the at least one optical fiber, the at least one optical fiber being positioned in the target area to detect target signals comprising acoustical waves and/or vibrations, the acoustical waves and/or vibrations being created from sources comprising movement of vehicles, equipment and/or humans in the target area, digging in the target area, and/or seismic tremors, and wherein the return signal comprises a carrier frequency component and sideband signals of the carrier frequency that resonates within the optoelectronic oscillator, and wherein the a circuit for removal of the carrier frequency operates to remove the carrier frequency component and transfer the remaining sideband signals to the signal processor, and wherein the signal processor processes the sideband signals to classify the sources of the target signals.

3. The system of claim 1 further comprising a sensor operatively connected to the at least one optical fiber, the sensor being positioned in the target area to detect vibrational and/or acoustical signals and wherein the return signal comprises a carrier frequency component and a vibrational and/or acoustical signal component in the form of the sideband signals of the carrier frequency that resonate within the optoelectronic oscillator, and wherein the circuit for removal of the carrier frequency comprises an interferometer that operates to remove the carrier frequency and transfer the remaining sidebands signals to the signal processor.

4. The system of claim 3 wherein the return signal comprises a carrier frequency component and a target signal component in the form of sideband signals of the carrier frequency, and wherein the interferometer operates to remove the carrier frequency such that substantially only the sideband signals remain;

and wherein the mixer comprises a first input operatively connected to the phase shifter and a second input operatively connected to the at least one optical fiber through a delay line;

the delay line operating to delay the sideband signals to facilitate recognition thereof.

5. The system of claim 1 further comprising a delay line operatively connected to the at least one optical fiber;

wherein the circuit for removal of signals other than sideband signals is a homodyne interferometer circuit and wherein the carrier frequency signal is removed by interferometry; and

wherein the mixer comprises a first input operatively connected to the phase shifter and a second input operatively connected to the at least one optical fiber through the delay line.

6. The system of claim 1 wherein the light generator comprises a laser, and wherein the modulator modulates the light beam at microwave frequencies, and wherein the circuit for removal of the carrier frequency comprises a first input for receiving the return optical signal, a second input for receiving the modulated carrier signal, and wherein the mixer, the phase shifter and the control circuit comprise feedback circuitry, the feedback circuitry operating to provide a signal that is substantially 90°

out of phase with the carrier signal so as to cancel the carrier signal, and output the remaining sideband signals to the signal processor, the remaining sideband signals being processed to detect the presence of vibrational and/or acoustical signals; and

wherein the mixer comprises two inputs comprising a first input operatively connected to the phase shifter and a second input operatively connected to the at least one optical fiber through a delay line.

7. The system of claim 1 further comprising a sensor operatively connected to the at least one optical fiber, the sensor comprising a base, an elastic core operatively connected to the base, and a receiving surface movable by acoustical and/or vibrational waves operatively associated with the elastic core, the at least one optical fiber being wound on the elastic core, whereby acoustic and/or vibrations are transmitted by the surface to the at least one optical fiber causing stress and/or strain in the at least one optical fiber causing the round trip time to change, whereby the sideband signals are created by the vibrational and/or acoustical signals striking the receiving surface.

8. The system of claim 7 wherein the receiving surface is one of planar, conical or three-dimensional, and wherein the portion of the at least optical fiber wound on the elastic core comprises a length of at least five hundred meters.

9. A method for sensing acoustic and/or vibrations from a target area comprising:

providing an oscillating circuit comprising;

a light generator, a modulator operatively connected to the light generator,

an optical fiber operatively connected to the modulator,

a delay line operatively connected to the optical fiber;

a sensor operatively connected to the optical fiber, and

an RF coupler operatively connected to the optical fiber;

generating light utilizing the light generator;

modulating the light utilizing the modulator to create a radio frequency carrier signal;

transmitting the carrier signal through the optical fiber to the sensor;

the sensor operating to receive vibrational and/or acoustical signals to create sideband signals;

receiving the return carrier signal and sidebands through the optical fiber by the RF coupler for transfer of an electronic signal to the modulator to complete the oscillating circuit;

providing a circuit for removal of the carrier frequency operatively connected to the oscillating circuit to substantially remove the carrier frequency comprising

at least one phase shifter operatively connected to an output of the sensor;

at least one mixer having at least two inputs;

first input operatively connected to the at least one phase shifter and a second input operatively connected to the sensor through a delay line such that the signal received at the second input is a delayed output signal of the sensor;

at least control circuit operatively connected the at least one mixer and the at least one phase shifter for controlling the phase shifter and operating to cancel signals other than the sideband signals;

processing the sideband signals using a signal processor operatively connected the mixer to process the information contained in the sideband signals relating to the vibrational and/or acoustical signals.

10. The method of claim 9 wherein the sensor has at least one output and wherein the circuit for removal of the carrier frequency further comprises a splitter operatively connected to at least one output of the sensor, an optical intensity modulator operatively connected to the splitter, a second laser operatively connected to the optical intensity modulator, a photodetector, and amplifier;

the beam from the second laser being monitored by the optical intensity modulator and sent through the delay line to the photodetector;

the output of the photodetector being operatively connected to the amplifier;

the output of the amplifier being operatively connected to the second input of the mixer; and

wherein the signal processor operates to classify the nature of the information contained in the sidebands to identify the origin of the vibrational and/or acoustical signals.

11. The method of claim 10 wherein the sensor comprises a spool of optical fiber having a receiving surface adjacent to the optical fiber, the vibrational and/or acoustical signals from the target area striking the receiving surface to create vibrations in spool of optical fiber thereby causing changes in the round-trip time of the optoelectronic oscillator and frequency variations in the carrier signal passing through the optical fiber.

12. A system for sensing vibrational and/or acoustical signals from a target area comprising:

an optoelectronic oscillator comprising;

at least one light generator, at least one modulator operatively connected to the light generator, at least one optical fiber operatively connected to the light generator, a sensor operatively connected to the optical fiber, and at least one output, the at least one modulator operating to modulate the light from the at least one light generator to create a radio frequency carrier signal for passage through the optical fiber to the sensor, the sensor operating to sense vibrational and/or acoustical signals which create sidebands in the carrier signal, the optical fiber operating to return the carrier signal with sidebands to the modulator and to at least one output;

a circuit for removal of signals other than sideband signals operatively connected to the oscillating circuit to substantially remove frequencies other the sidebands comprising;

at least one phase shifter operatively connected to another of the at least one output;

at least one mixer operatively connected to the at least one phase shifter;

at least control circuit operatively connected the at least one mixer and the at least one phase shifter for controlling the phase shifter and operating to cancel or remove signals other than the sideband signals;

the at least one mixer operating to output the remaining sideband signals;

a signal processor operatively connected to the at least one mixer for processing the sideband signals to detect vibrational and/or acoustic signals from the target area.

13. The system of claim 12 wherein the at least one modulator is configured to modulate the intensity of the light from the at least one light source to form an RF frequency carrier wave and further comprising at least one fiber optic delay operatively connected to the second input of the mixer for delaying the carrier frequency and sideband signals and at least one RF converter operatively connected to the at least one fiber optic delay for converting the optical signal to a delayed electrical signal based upon the carrier signal for input into the mixer;

wherein the circuit for removal of signals other than sideband signal is an interferometric circuit that removes the carrier frequency by interferometry due to the phase difference between the inputs to the mixer.

14. The system of claim 12 wherein the second input to the mixer is operatively connected to an optical fiber delay and wherein the light beam modulated for input to the sensor passes through the optical fiber delay before input to the mixer such that the signal received at the second input of the mixer is delayed relative to the signal passing through the sensor to facilitate recognition of the sideband signals attributable to the vibrational and/or acoustical signals from the target area.

15. The system of claim 12 wherein the circuit for removal of signals other than sideband signals further comprises at least one laser and at least one laser modulator, the at least one laser generating light into the at least one laser modulator for input into a fiber optic delay.

16. The system of claim 12 wherein the circuit for removal of signals other than sideband signals comprises first and second lasers, first and second laser modulators, first and second fiber optic delays, first and second RF converters, each operatively connected to the first and second fiber optic delays, first and second phase shifters, each operatively connected to the at least one output of the optoelectronic oscillator;

first and second mixers, each of the first and second mixers being operatively connected to the first and second RF converters and first and second phase shifters, respectively;

first and second control circuits operatively connected to the first and second mixers and the first and second phase shifters, respectively;

the first and second control circuits each operating to cancel signals other than the sideband signals;

the first and second mixers operating to each output the sideband signals; and

wherein the signal processor is operatively connected to the first and second mixers for processing the sideband signals to detect vibrational and/or acoustic signals from the target area.

17. The system of claim 12 wherein the sensor comprises a base, an elastic core operatively connected to the base, and a receiving surface movable by acoustical and/or vibrational waves operatively associated with the elastic core, the at least one optical fiber being wound on the elastic core, whereby acoustic and/or vibrations are transmitted by the surface to the at least one optical fiber causing stress and/or strain in the at least one optical fiber causing the round trip time to change, whereby sidebands to the carrier signal are created by the vibrational and/or acoustical signals striking the receiving surface.

18. The system of claim 12 wherein the optical fiber operates to return the carrier signal substantially at the carrier frequency with sidebands comprising a vibrational and/or acoustical signal component, the vibrational and/or acoustical signal component forming side bands, and wherein the circuit for removal of signals other than sideband signals operates to remove signals attributable to the carrier frequency such that only signals attributable to the vibrational and/or acoustical signals remain.

19. The system of claim 12 wherein the sensor is adapted to be positioned in a target area to detect target signals comprising acoustical waves and/or vibrations, the acoustical waves and/or vibrations being created from sources comprising movement of vehicles, equipment and/or humans in the target area, digging in the target area, and/or seismic tremors, and wherein the return signal comprises a carrier frequency component and a side band component perpetrated by the acoustical waves and/or vibrations, and wherein the circuit for removal of signals other than sideband signals operates to transfer the remaining sidebands to the signal processor, and wherein the signal processor processes the sidebands to classify the sources of the target signals.

20. The system of claim 12 wherein the at least one control circuit comprises an electronic servo which controls the phase shifter causing the phase shifter to generate a signal which in effect cancels the signals substantially at the carrier frequency by generating a cancelling signal substantially ninety degrees out of phase such that the sideband signals remain, the at least one control circuit operatively connected the at least one mixer and the at least one phase shifter so as to form a feedback loop.