MULTIPLEXABLE FIBER-OPTIC STRAIN SENSOR SYSTEM WITH TEMPERATURE COMPENSATION CAPABILITY

• US 20040028311A1
• Filed: 04/24/2003
• Published: 02/12/2004
• Est. Priority Date: 04/02/1999
• Status: Active Grant

• Current Assignee: Intelligent Fiber Optic Systems Corporation
• Original Assignee: Intelligent Fiber Optic Systems, Inc.
• Inventors: Moslehi, Behzad, Black, Richard James, Toyama, Keiichiro, Shaw, Herbert John
• Granted Patent: US 6,895,132 B2
• Time in Patent Office: Days
• US Class Current: 385/12
• CPC Class Codes: G01B 11/16 for measuring the deformation in a solid, e.g. optical strain gauge
• CPC Class Codes: G01D 5/35316 using a Bragg gratings
• CPC Class Codes: G01D 5/35354 Sensor working in reflection
• CPC Class Codes: G01D 5/35387 using wavelength division multiplexing
• CPC Class Codes: G01L 1/246 using integrated gratings, e.g. Bragg gratings
• CPC Class Codes: G02B 6/02057 comprising gratings
• CPC Class Codes: G02B 6/022 using mechanical stress, e.g. tuning by compression or elongation, special geometrical shapes such as "dog-bone" or taper G02B6/02204 takes precedence; optical modulation using photo-elastic effect G02F1/0131
• CPC Class Codes: G02B 6/2932 comprising a directional router, e.g. directional coupler, circulator
• CPC Class Codes: G02B 6/29395 configurable, e.g. tunable or reconfigurable switching G02B6/35
• CPC Class Codes: G02B 6/29398 Temperature insensitivity

Abstract

A fiber optic sensor comprises two independent fibers having Bragg gratings which are coupled to commutating broadband optical sources through splitters and wavelength discriminators. The ratio of detected optical energy in each of two detectors examining the wave intensity returned to a wavelength discriminator coupled with the characteristic of the wavelength discriminator determines the wavelength returned by the grating. In another embodiment, tunable filters are utilized to detect minimum returned wave energy to extract a sensor wavelength Reference to the original grating wavelength indicates the application of either temperature or strain to the grating. In another embodiment, a plurality of Bragg grating sensor elements is coupled to sources and controllers wherein a dimensional change in a fiber having a Bragg grating is detected using a measurement system comprising broad-band sources, optical power splitters, a high-sensitivity wavelength discriminator, optical detectors, and a controller.

Claims

1. ) a fiber-optic sensor system comprising:

a first sensor grating and a second sensor grating;

a first optical splitter and a second optical splitter, each said splitter having a source port, a filter port, and a sensor port, wherein optical energy applied to said sensor port is divided between said source port and said filter port, and wherein optical energy applied to said source port travels to said sensor port, said first optical splitter sensor port coupled to said first sensor grating, and said second optical splitter sensor port coupled to said second sensor grating;

a wavelength discriminator having a first splitter input and a second splitter input, a first detector output, and a second detector output, wherein optical energy applied to said first splitter input divides between said first detector output and said second detector output in a manner related to the wavelength of said applied optical energy, and where the sum of said first detector output and said second detector remains constant over the range of said applied wavelengths;

said wavelength discriminator first splitter input coupled to said first splitter filter port, said wavelength discriminator second splitter input coupled to said second splitter filter port;

a first detector and a second detector, each of said detectors accepting an optical input and producing an electrical output, said first detector coupled to said wavelength discriminator first detector output, and said second detector coupled to said wavelength discriminator second detector output;

a first optical source and a second optical source, each said source having an enable input and producing an optical output when said enable input is active, said first optical source output coupled to said first splitter source port, and said second optical source output coupled to said second splitter source port;

a controller having a first enable output coupled to said first optical source, a second enable output coupled to said second optical source, a first input coupled to said first detector, and a second input coupled to said second detector, said controller producing said first enable and said second enable during mutually exclusive time intervals, and recording said first detector and said second detector responses.

2. ) The sensor of claim 1 wherein said controller determines the wavelength of said first sensor or said second sensor by comparing said first detector and said second detector response to a stored copy of said wavelength discriminator characteristic which relates wavelength to said detector response.

3. ) The sensor of claim 2 wherein said comparison uses a normalized power ratio formed by dividing the difference of said first detector output and said second detector output by the sum of said first detector output and said second detector output.

4. ) The sensor of claim 3 wherein said normalized power ratio to wavelength relationship is stored in a look-up table.

5. ) the sensor of claim 3 wherein said normalized power ratio to wavelength relationship is stored in the form of the coefficients of a power series.

6. ) The sensor of claim 4 or 5 wherein said normalized power ratio is modified though the removal of offset values for said first detector and said second detector.

7. ) The sensor of claim 4 or 5 wherein said first sensor or said second sensor measure temperature.

8. ) The sensor of claim 4 or 5 wherein one of said first or second sensor measures temperature and the remaining said sensor measures strain.

9. ) The sensor of claims 1, 2, or 3 wherein said first optical source and said second optical source are modulated in frequency or amplitude by a modulation function, and said first detector output and said second detector output are formed by demodulation of said detector input signal.

10. ) a fiber-optic sensor system comprising:

a plurality n of sensor gratings, each reflecting optical energy at a unique wavelength λ

n, and arranged in a series configuration having a single sensor port;

an optical splitter having a source port, a filter port, and a sensor port, wherein optical energy applied to said sensor port is divided between said source port and said filter port, and wherein optical energy applied to said source port travels to said sensor port, said splitter sensor port coupled to said single sensor port;

an optical source providing optical power which includes at least the spectral bandwidth of each of said n sensor wavelengths, said optical source coupled to said optical splitter source port, and said optical source having an enable input;

a wavelength measurement apparatus having an input and a plurality of complimentary detector output pairs, each of said complimentary detector output pairs responsive to one of said sensor grating wavelengths, said wavelength measurement apparatus input coupled to said optical splitter filter port;

a controller enabling said optical source and examining each of said wavelength measurement apparatus complimentary detector outputs, said controller having a stored relationship between a given complimentary detector output and a wavelength, and said controller using said stored relationship to determine said wavelength for each complimentary detector output pairs.

11. ) The sensor system of claim 10 wherein said wavelength measurement apparatus further comprises

a wavelength separator having an input and a plurality of n outputs, each of said n outputs responsive to one of said n sensor wavelengths, said wavelength separator input coupled to said wavelength measurement apparatus input;

a plurality n of wavelength discriminators, each having an input, a first detector output, and a second detector output, wherein optical energy applied to said input divides between said first detector output and said second detector output in a manner related to the wavelength of said optical energy, and where the sum of said first detector output and said second detector output remains constant over the range of said applied wavelengths;

each of said wavelength discriminator inputs coupled to each of said wavelength separator outputs, and each of said wavelength discriminator first and second outputs coupled to detectors and producing said wavelength measurement apparatus detector complimentary outputs.

12. ) The sensor of claim 11 wherein said controller determines the wavelength of each of said sensors by comparing said first detector and said second detector response to a stored copy of said wavelength discriminator characteristic which relates wavelength to said detector response.

13. ) The sensor of claim 12 wherein said comparison uses a normalized power ratio formed by dividing the difference of said first detector output and said second detector output by the sum of said first detector output and said second detector output.

14. ) The sensor of claim 13 wherein said normalized power ratio to wavelength relationship is stored in a look-up table.

15. ) the sensor of claim 13 wherein said normalized power ratio to wavelength relationship is stored in the form of the coefficients of a power series.

16. ) The sensor of claim 14 or 15 wherein said comparison values are modified though the removal of offset values for said first detector and said second detector.

17. ) The sensor of claim 14 or 15 wherein said sensor gratings measure temperature.

18. ) The sensor of claim 14 or 15 wherein at least one of said sensor gratings measures temperature and at least one of said sensor gratings measures strain.

19. ) The sensor system of claim 10 wherein said wavelength measurement apparatus further comprises

a wavelength discriminator having an input, a first output, and a second output, wherein optical energy applied to said input splits between said first output and said second output in a manner related to the wavelength of said optical energy, and where the sum of said wavelength discriminator first output and said wavelength discriminator second output remains constant over the range of said applied wavelengths;

a first and a second wavelength separator, each having an input and a plurality of n outputs, each of said n outputs responsive to one of said n sensor wavelengths;

said first wavelength separator input coupled to said first wavelength discriminator output, and said second wavelength separator input coupled to said second wavelength discriminator output;

each of said first wavelength separator outputs and said second wavelength separator outputs responsive to a single said sensor wavelength coupled to a detector responsive to said separator output wave energy, each pair of said detectors forming said wavelength measurement apparatus complimentary detector output pairs.

20. ) The sensor of claim 19 wherein said controller determines the wavelength of each of said sensors by comparing said complimentary detector output pair responses to a stored copy of said wavelength discriminator characteristic which includes the relationship between said complimentary output pair responses and a wavelength.

21. ) The sensor of claim 20 wherein said comparison uses a normalized power ratio formed by dividing the difference of said first detector output and said second detector output by the sum of said first detector output and said second detector output.

22. ) The sensor of claim 21 wherein said normalized power ratio to wavelength relationship is stored in a look-up table.

23. ) the sensor of claim 22 wherein said normalized power ratio to wavelength relationship is stored in the form of the coefficients of a power series.

24. ) The sensor of claims 22 or 23 wherein said comparison values are modified though the removal of offset values for said first detector and said second detector.

25. ) The sensor of claims 22 or 23 wherein at least one of said sensors measure temperature.

26. ) The sensor of claims 22 or 23 wherein at least one of said sensors measures strain.

27. ) a fiber-optic sensor system comprising:

a plurality n of sensor gratings, each reflecting optical energy at a unique wavelength λ

n, and arranged in a series configuration having a single sensor port;

an optical splitter having a source port, a filter port, and a sensor port, wherein optical energy applied to said sensor port is divided between said source port and said filter port, and wherein optical energy applied to said source port travels to said sensor port, said single sensor port coupled to said splitter sensor port;

an optical source providing optical power which includes at least the spectral bandwidth of said n sensor wavelengths, said optical source coupled to said optical splitter source port;

a wavelength detection apparatus having an input port coupled to an output port through a plurality n of tunable filters in series configuration, each of said tunable filters having a tuning range which is greater than the sensor wavelength range of each of said sensor gratings, and said wavelength detection input port coupled to said optical splitter filter port, said wavelength detection output port coupled to a detector having an output proportional to an input optical amplitude;

a controller measuring said detector output while varying said tunable filters until a minimum detector response is found.

28. ) The sensor system of claim 27 wherein each of said tunable filters is controlled by a tunable filter control voltage which is varied by said controller such that only one said tunable filter has a varying control voltage applied at a time, while the other said tunable filters have a fixed voltage applied during the measurement interval.

29. ) The sensor system of claim 27 wherein said tunable filters are controlled by a tunable filter control voltage which is varied by said controller such that more than one said tunable filter control voltage is varied during the measurement interval.

30. ) The sensor of claim 27 wherein said controller determines the wavelength of each of said sensors by comparing said tunable filter control voltage which produces said detector minimum response to a stored copy of the relationship between said tunable filter output voltage and said wavelength.

31. ) The sensor of claim 29 wherein said controller determines the wavelength of each of said sensors by comparing said detector minimum response when said tunable filter control voltage is applied to a stored copy of the relationship between said tunable filter control voltage and said wavelength.

32. ) The sensor of claim 30 wherein said relationship between said tunable filter control voltage and said wavelength is stored in a look-up table.

33. ) the sensor of claim 30 wherein said relationship between said tunable filter control voltage and said wavelength is stored in the form of the coefficients of a power series.

34. ) The sensor of claim 28 or 29 wherein at least one of said sensors measures temperature.

35. ) The sensor of claim 28 or 28 wherein at least one of said sensors measures strain.

36. ) a fiber-optic sensor system comprising:

a first sensor grating and a second sensor grating;

a first optical splitter and a second optical splitter, each said splitter having a source port, a filter port, and a sensor port, wherein optical energy applied to said sensor port is divided between said source port and said filter port, and wherein optical energy applied to said source port travels to said sensor port, said first optical splitter sensor port coupled to said first sensor grating, and said second optical splitter sensor port coupled to said second sensor grating;

a third splitter with a first and second input and a first and second output, wherein optical energy applied to either said first or said second input divides equally between said first and said second outputs;

a broadband grating having an input and an output;

a first detector and a second detector,

said third splitter first input coupled to said first splitter filter port and said third splitter second input coupled to second splitter filter port, said third splitter first output coupled to the input of said broadband grating and said third splitter second output coupled to said second detector, said broadband filter output coupled to said second detector;

wherein optical energy present in said first or said second input of said third splitter and measured at said first detector varies monotonically from a first discriminator wavelength to a final discriminator wavelength, while said second detector output remains constant, and said sensor operating range is between said first discriminator wavelength and said second discriminator wavelength;

said first optical source and said second optical source each having an enable input and producing an optical output when said enable input is active, said first optical source output coupled to said first splitter source port, and said second optical source output coupled to said second splitter source port;

a controller having a first enable output coupled to said first optical source, a second enable output coupled to said second optical source, a first input coupled to said first detector, and a second input coupled to said second detector, said controller producing said first enable and said second enable during mutually exclusive time intervals, and recording said first detector and said second detector responses.

37. ) The sensor of claim 36 wherein said controller determines the wavelength of said first sensor or said second sensor by comparing said first detector and said second detector response to a stored copy of said wavelength discriminator characteristic.

38. ) The sensor of claim 37 wherein said comparison uses a normalized power ratio formed by dividing said first detector output by said second detector output.

39. ) The sensor of claim 38 wherein the relationship between said normalized power ratio and wavelength is stored in a look-up table.

40. ) the sensor of claim 38 wherein the relationship between said normalized power ratio and wavelength is stored in the form of the coefficients of a power series.

41. ) The sensor of claim 39 or 40 wherein said normalized power ratio is modified though the removal of offset values for said first detector and said second detector.

42. ) The sensor of claim 39 or 40 wherein said first sensor or said second sensor measures temperature.

43. ) The sensor of claim 39 or 40 wherein said first sensor or said second sensor measures strain.

44. ) The sensor of claim 36 wherein said first optical source and said second optical source are modulated in frequency or amplitude by a modulation function, and said first detector output and said second detector output are formed by demodulation of said detector input signal.

45. ) a fiber-optic sensor system comprising:

a first sensor grating and a second sensor grating;

a first optical splitter and a second optical splitter, each said splitter having a source port, a filter port, and a sensor port, wherein optical energy applied to said sensor port is divided between said source port and said filter port, and wherein optical energy applied to said source port travels to said sensor port, said first optical splitter sensor port coupled to said first sensor grating, and said second optical splitter sensor port coupled to said second sensor grating;

a third splitter having an input and a first and second output wherein optical energy applied to said input divides equally between said first output and said second output;

a fourth splitter having an input and a first and second output wherein optical energy applied to said input divides equally between said first output and said second output;

a coarse wavelength discriminator having a first and a second input, and a first and a second output, said first coarse wavelength discriminator input coupled to said first output of said third splitter, said coarse wavelength discriminator second input coupled to said fourth splitter first output;

a fine wavelength discriminator having a first and a second input, and a first and a second output, said first fine wavelength discriminator input coupled to said second output of said third splitter, said fine wavelength discriminator second input coupled to said fourth splitter second output;

a first coarse detector converting an optical input to an electrical response, said optical input coupled to said first coarse wavelength discriminator output;

a second coarse detector converting an optical input to an electrical response, said optical input coupled to said second coarse discriminator output;

a first fine detector converting an optical input to an electrical response, said optical input coupled to said first fine wavelength discriminator output;

a second fine detector converting an optical input to an electrical response, said optical input coupled to said second fine discriminator output;

wherein optical energy present in said coarse wavelength discriminator first or second input produces said first detector and said second detector responses which vary monotonically from a first discriminator wavelength to a final discriminator wavelength, said monotonic relationship between said detector response and a wavelength stored in said controller;

said first optical source and said second optical source each said source having an enable input and producing an optical output when said enable input is active, said first optical source output coupled to said first splitter source port, and said second optical source output coupled to said second splitter source port;

a controller having a first enable output coupled to said first optical source, a second enable output coupled to said second optical source, a first coarse input coupled to said first coarse detector, and a second coarse input coupled to said second coarse detector, a first fine input coupled to said first fine detector, and a second fine input coupled to said second fine detector, said controller producing said first enable output and said second enable output during mutually exclusive time intervals, and recording said coarse detector responses and said fine detector responses.

46. ) The sensor of claim 45 wherein said controller determines the wavelength of said first sensor or said second sensor by comparing said coarse first detector response and said coarse detector second response to a stored copy of said coarse wavelength discriminator and said fine wavelength discriminator characteristic.

47. ) The sensor of claim 46 wherein said comparison uses a normalized power ratio formed from the ratio of the difference of said first coarse detector output and said second coarse detector output divided by the sum of said first coarse detector output and said second coarse detector output.

48. ) The sensor of claim 47 wherein the relationship between said normalized power ratio and wavelength is stored in a look-up table.

49. ) the sensor of claim 47 wherein said the relationship between said normalized power ratio and wavelength is stored in the form of the coefficients of a power series.

50. ) The sensor of claim 46 wherein said normalized power ratio is modified by the measurement of said first fine detector response and said second fine detector response.

51. ) The sensor of claim 50 wherein said modification of said normalized power ratio using said fine detector response is stored in a look-up table.

52. ) the sensor of claim 50 wherein said modification of said normalized power ratio using said fine detector response is stored in the form of the coefficients of a power series.

53. ) The sensor of claim 51 or 52 wherein said first sensor or said second sensor measures temperature.

54. ) The sensor of claim 51 or 52 wherein said first sensor or said second sensor measures strain.

55. ) The sensor of claims 51 or 52 wherein said first optical source and said second optical source are modulated in frequency or amplitude by a modulation function, and said coarse detector outputs and said fine detector outputs are formed by demodulation of said detector input signal.