Coherent reflectometric fiber Bragg grating sensor array

• US 6,285,806 B1
• Filed: 05/31/1998
• Issued: 09/04/2001
• Est. Priority Date: 05/31/1998
• Status: Expired due to Term

• Current Assignee: the united states of america as represented by the secretary of the navy
• Original Assignee: the united states of america as represented by the secretary of the navy
• Inventors: Kersey, Alan D., Vohra, Sandeep T., Dandridge, Anthony
• Primary Examiner(s): Arroyo, Teresa M.
• Assistant Examiner(s): INZIRILLO, GIOACCHINO
• Application Number:
  US09/093,827
• Time in Patent Office: 1,192 Days
• Field of Search: 385/12, 385/13, 250/227.14, 250/227.23
• US Class Current: 385/12
• CPC Class Codes: G01D 5/35383 using multiple sensor devices using multiplexing techniques
• 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: G01M 11/3118 using coded light-pulse sequences

Abstract

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.

Claims

1. A fiber optic sensor array for detecting a physical condition, the fiber optic sensor array comprising:

bit sequence generating means for generating a pseudo-random bit sequence;

light source means, receiving the pseudo-random bit sequence, for emitting a light beam which is modulated in accordance with the pseudo-random bit sequence;

an optical fiber disposed to receive the light beam so that the light beam propagates in the optical fiber in a first direction, the optical fiber comprising a plurality of fiber segments disposed in series, each of the plurality of fiber segments having an optical characteristic which varies in accordance with the physical condition, the fiber segments being separated by means for weakly reflecting portions of the light beam to form return optical signals which propagate in the optical fiber in a second direction which is opposite to the first direction, each of the return optical signals representing an effect of the physical condition on the light beam;

photodetecting means, receiving the return optical signals, for producing a photodetector output which represents a sum of the return optical signals;

time delay means, receiving the pseudo-random bit sequence, for producing a time-delayed pseudo-random bit sequence, said time delay means comprising means for producing a plurality of time delayed pseudo-random bit sequences, each having a different time delay;

correlating means, receiving the time-delayed pseudo-random bit sequence and the photodetector output, for performing a correlation between the time-delayed pseudo-random bit sequence and the photodetector output to produce a correlation result representing one of the return optical signals, said correlating means comprising means for receiving a plurality of time-delayed pseudo-random bit sequences, for producing a plurality of correlations to produce a plurality of correlation results, each representing a different one of the return optical signals; and

subtracting means, receiving the plurality of correlation results, for calculating a difference between two of the return optical signals to determine an effect on the light beam by one of the fiber segments.

2. A fiber optic sensor array as in claim 1, wherein each of the reflecting means comprises a fiber optic Bragg grating formed in the optical fiber.

3. A fiber optic sensor array as in claim 1, wherein the light source means comprises:

a laser for emitting light; and

a modulator, receiving the light emitted by the laser and the pseudo-random bit sequence, for modulating the light emitted by the laser in accordance with the pseudo-random bit sequence to produce the light beam.

4. A fiber optic sensor array as in claim 3, wherein the modulator comprises an electro-optic switch.

5. A fiber optic sensor array as in claim 3, wherein:

the light source means further comprises a first coupler, disposed in a path of the light emitted by the laser between the laser and the modulator, for picking off a portion of the light emitted by the laser;

the optical fiber further comprises a second coupler for picking off portions of the return optical signals; and

the photodetecting means further comprises;

a third coupler for coherently mixing the portion of the light picked off by the first coupler with the portions of the return optical signals picked off by the second coupler; and

means for detecting a result of coherent mixing by the third coupler.

6. A fiber optic sensor array as in claim 5, wherein:

the result of coherent mixing comprises two outputs of the third coupler; and

the means for detecting comprises a pair of photodetectors, each detecting one of the two outputs of the third coupler.

7. A fiber optic sensor array as in claim 6, wherein the means for detecting further comprises a differential amplifier, receiving outputs of the pair of photodetectors, for producing a balanced detector output.

8. A method of detecting a physical condition, the method comprising:

(a) generating a pseudo-random bit sequence;

(b) emitting a light beam which is modulated in accordance with the pseudo-random bit sequence;

(c) causing the light beam to enter an optical fiber so that the light beam propagates in the optical fiber in a first direction, the optical fiber comprising a plurality of fiber segments disposed in series, each of the plurality of fiber segments having an optical characteristic which varies in accordance with the physical condition, the fiber segments being separated by means for weakly reflecting portions of the light beam to form return optical signals which propagate in the optical fiber in a second direction which is opposite to the first direction, each of the return optical signals representing an effect of the physical condition on the light beam;

(d) receiving the return optical signals and producing a photodetector output which represents a sum of the return optical signals;

(e) producing a time-delayed pseudo-random bit sequence by producing a plurality of time-delayed pseudo-random bit sequences, each having a different time delay;

(f) performing a correlation between the time-delayed pseudo-random bit sequence and the photodetector output to produce a correlation result representing one of the return optical signals by performing a plurality of correlations to produce a plurality of correlation results, each representing a different one of the return optical signals; and

(g) calculating a difference between two of the return optical signals to determine an effect on the light beam by one of the fiber segments.

9. A method as in claim 8, wherein each of the reflecting means comprises a fiber optic Bragg grating formed in the optical fiber.

10. A method as in claim 8, wherein step (b) comprises:

(i) emitting light; and

(ii) modulating the light emitted in step (b) (i) in accordance with the pseudo-random bit sequence to produce the light beam.

11. A method as in claim 10, wherein step (b) (ii) comprises modulating the light emitted in step (b)(i) with an electro-optic switch.

12. A method as in claim 10, wherein:

step (b) further comprises picking off a portion of the light emitted by the laser; and

step (d) further comprises;

(i) picking off portions of the return optical signals from the optical fiber;

(ii) coherently mixing the portion of the light picked off in step (b) with the portions of the return optical signals picked off in step (d)(i); and

(iii) detecting a result of the step of coherently mixing.

13. A method as in claim 12, wherein:

the result of the step of coherently mixing comprises two outputs; and

step (d)(iii) comprises detecting each of the two outputs to produce an output signal.

14. A method as in claim 13, wherein step (d) further comprises producing a balanced detector output in accordance with the output signals.

15. An optical monitoring system, comprising:

an optical coupler, said fiber comprising a plurality of partially reflective and partially transmissive elements;

an optical source disposed to launch an input signal into said fiber;

a coupler in said fiber disposed to receive signal reflected from said elements; and

a detector effective, responsive to said coupler, to determine the phase of at least a portion of said signal reflected from at least one of said elements, said detector comprising a correlator disposed to correlate said input signal with said at least a portion of said signal reflected from said at least one element.

16. The system of claim 15, wherein:

said optical source comprises;

means for generating an optical carrier; and

a modulator disposed to impose a modulation signal on said carrier;

wherein said detector comprises a demodulator disposed to remove said carrier from said at least a portion of said signal reflected from said at least one of said elements, effective to produce a demodulated signal;

wherein said correlator is disposed effective to correlate said demodulated signal and said modulation signal.

17. The system of claim 15, wherein said at least one of said elements is a member of the group consisting of:

Bragg gratings, Fresnel reflectors;

preselected anomalies in the refractive index of said fiber.

18. The system of claim 15, wherein said at least one of said elements is a Bragg grating.

19. An optical system comprising:

an optical fiber comprising a plurality of partially reflective elements;

an optical source disposed to launch an optical signal into said fiber;

a phase detector disposed effective to determine the phase between said optical signal and light reflected from a preselected one of said elements, said phase detector comprising a correlation means; and

a subtractor means for subtracting known phase shifts from a cumulative phase detected by said correlation means, thereby detecting a phase shift induced by a preselected one of said elements.

20. A fiber optic sensor array for detecting a physical condition, the fiber optic sensor array comprising:

bit sequence generation means for generating a pseudo-random bit sequence;

light source means, receiving the pseudo-random bit sequence, for emitting a light beam which is modulated in accordance with pseudo-random bit sequence, the light source comprising;

a laser for emitting light;

a modulator;

a first coupler, disposed in a path of the light emitted by the laser between the laser and the modulator, for picking off a portion of the light emitted by the laser;

an optical fiber disposed to receive the light beam so that the light beam propagates in the optical fiber in a first direction, the optical fiber comprising a plurality of fiber segments disposed in series, each of the plurality of fiber segments having an optical characteristic which varies in accordance with the physical condition, the fiber segments being separated by means of weakly reflecting portions of the light beam to form return optical signals which propagate in the optical fiber in a second direction which is opposite to the first direction, each of the return optical signals representing an effect of the physical condition on the light beam, the optical fiber further comprises a second coupler for picking off portions of the return optical signals;

photodetecting means for producing a photodetector output which represents a sum of the return optical signals, the photodetecting means further comprises;

a third coupler for coherently mixing the portion of the light picked off by the first coupler with the portions of the return optical signals picked off by the second coupler; and

means for detecting a result of coherent mixing by the third coupler;

time delay means for producing a time-delayed pseudo-random bit sequence; and

correlating means for performing a correlation between the time-delayed pseudo-random bit sequence and the photodetector output to produce a correlation result representing one of the return optical signals.

21. A method of detecting a physical condition, the method comprising:

(a) generating a pseudo-random bit sequence;

(b) emitting a light beam which is modulated in accordance with the pseudo-random bit sequence to produce the light beam, and wherein step(b) further comprises picking off a portion of the light emitted by the laser;

(c) causing the light beam to enter an optical fiber so that the light beam propagates in the optical fiber in a first direction, the optical fiber comprising a plurality of fiber segments disposed in series, each of the plurality of fiber segments having an optical characteristic which varies in accordance with the physical condition, the fiber segments being separated by means for weakly reflecting portions of the light beam to firm return optical signals which propagate in the optical fiber in a second direction which is opposite to the first direction, each of the return optical signals representing an effect of the physical condition on the light beam;

(d) receiving the return optical signals and producing a photodetector output which represents a sum of the return optical signals, and wherein step(d) further comprises

(i) picking off portions of the return optical signals;

(ii) coherently mixing the portion of the light picked off in step(b) with the portions of the return optical signals picked off in step(d)(i); and

(iii) detecting a result of the step of coherently mixing;

(e) producing a time-delayed pseudo-random bit sequence; and

(f) performing a correlation between the time-delayed pseudo-random bit sequence and the photodetector output to produce a correlation result representing one of the return optical signals.

22. An optical monitoring system, comprising

an optical fiber, said fiber comprising a plurality of partially reflective and partially transmissive elements;

an optical source disposed to launch an input signal into said fiber, said source further comprises;

a laser for emitting light;

a modulator;

first coupler, disposed in a path of the light emitted by the laser between the laser and the modulator, for picking off a portion of the light emitted by the laser

a second coupler for picking off portions of the return optical signals reflected from said elements; and

a detector effective, responsive to said second coupler, to determine the phase of at least a portion of said signal reflected from said at least one of said elements, wherein the detector further comprises a third coupler for coherently mixing the portion of the light picked off by the first coupler with the portions of the return optical signals picked off by the second coupler.