Method and devices for time domain demultiplexing of serial fiber bragg grating sensor arrays

• US 20020025097A1
• Filed: 04/03/2001
• Published: 02/28/2002
• Est. Priority Date: 10/07/1999
• Status: Active Grant

• Current Assignee: Moog Insensys Ltd.
• Original Assignee: David Cooper, Peter W.E. Smith
• Inventors: Cooper, David J.F., Smith, Peter W.E.
• Granted Patent: US 6,571,027 B2
• Time in Patent Office: Days
• US Class Current: 385/12
• CPC Class Codes: G01D 5/35383 using multiple sensor devices using multiplexing techniques
• CPC Class Codes: G01L 1/246 using integrated gratings, e.g. Bragg gratings
• CPC Class Codes: G02B 6/29319 With a cascade of diffractive elements or of diffraction operations forming interferometer by splitting and recombining G02B6/29347 - G02B6/29358

Abstract

The present invention provides a method and devices for time division multiplexing of a fiber optic serial Bragg grating sensor array containing more than one Bragg grating. The device provides a pulse read-out system that allows for a reduction in system noise and an increase in sensor resolution and flexibility. In one aspect the optical signals reflected from the Bragg grating sensor array are gated by an electronically controlled optical modulator before any wavelength measurement is performed to determine the sensor information. This offers significant advantages since the sensor information is encoded into the wavelength of the optical signal and not its intensity. Therefore the sensor signal information is not distorted by the gating. Since the gating or switching of the optical modulator is performed on the optical signal, the speed of the electronic processing needs only to be performed at the speed of variation of the sensor information and the choice of methods of wavelength measurement is not influenced by the gating action. The device may include one or more optical amplifiers that provide the ability to demultiplex the reflected signals electronically.

Claims

1. A Bragg grating sensor device, comprising:

a) a broadband light source adapted to produce optical pulses;

b) a Bragg sensor array including at least two spaced apart Bragg gratings located in an optical waveguide, said Bragg sensor array being optically coupled to said light source; and

c) an optical modulator optically coupled to said Bragg sensor array for receiving optical signals reflected from said Bragg sensor array, wavelength detection means optically coupled to said optical modulator for analysing wavelength content of said optical signals, adjustable gating means connected to said optical modulator for gating said optical modulator for selectively transmitting optical signals reflected from a preselected Bragg grating in said Bragg sensor array to said wavelength detection means.

2. The device according to claim 1 wherein said optical waveguide is an optical fiber.

3. The device according to claim 2 wherein said optical fiber is a single mode optical fiber, and wherein said light source, said Bragg sensor array, said optical modulator and said wavelength detection means are optically coupled in an optical fiber network using single mode optical fibers.

4. The device according to claim 2 wherein said adjustable gating means includes a variable timer circuit controller connected to said optical modulator for switching open and closed said optical modulator at selectively adjustable times after production of said optical pulses.

5. The device according to claim 4 wherein said optical fiber is a single mode optical fiber, and wherein said at least two Bragg gratings is a plurality of Bragg gratings spaced an effective minimum distance apart from each other.

6. The device according to claim 5 wherein said light source is adapted to produce optical pulses having a pulse width shorter than a time required for an optical pulse to travel approximately twice a distance between any two spatially closest Bragg gratings in said at least two Bragg gratings.

7. The device according to claim 5 wherein said light source is adapted to produce optical pulses with a period between said light pulses being greater than a time for an optical pulse to travel approximately twice a distance from a first Bragg sensor closest to said light source to a last Bragg grating farthest from said light source.

8. The device according to claim 7 wherein said variable timer circuit controller holds said optical modulator open permitting optical signals to be transmitted to said wavelength detection means for a period of time longer than said pulse width.

9. The device according to claim 8 wherein said optical modulator is an electro-optic modulator, and wherein said variable timer circuit controller includes an electrical trigger pulse generator connected to said electro-optic modulator for applying trigger voltage pulses to said electro-optic modulator for switching to said electro-optic modulator to a transmission state in which said optical signals propagate to said wavelength detection means, said electrical pulse generator including adjustment means for adjusting a length of time said trigger voltage pulses are applied to said electro-optic modulator for controlling a length of time said electro-optic modulator remains in said transmission state.

10. The device according to claim 9 wherein said variable timer circuit controller includes a variable electrical delay generator connected to said electrical trigger pulse generator for controlling when said electrical trigger pulse generator applies said electrical trigger pulses to said electro-optic modulator.

11. The device according to claim 10 wherein said variable electrical delay generator is adapted to be triggered by production of optical pulses such that said electro-optic modulator is gated to transmit said optical signals an adjustable time after production of said optical pulses.

12. The device according to claim 3 wherein said optical fiber network includes a first optical fiber section connected between said light source and an optical coupler, a second optical fiber section connected at one end thereof to said optical coupler having said at least two spaced apart Bragg gratings located therein, and a third optical fiber section connected at one end thereof to said optical coupler optically coupled to said optical modulator.

13. The device according to claim 1 wherein said optical modulator is a semiconductor electro-absorption modulator.

14. The device according to claim 1 wherein said optical modulator is a Mach-Zehnder integrated optical modulator.

15. The device according to claim 1 wherein said light source is a mode locked laser.

16. The device according to claim 9 wherein said electro-optic modulator is a lithium niobate opto-electronic modulator.

17. The device according to claim 12 including a polarization control element in said optical fiber network between said Bragg sensor array and said electro-optical modulator for controlling a state of polarization of said optical pulses reflected by said Bragg gratings.

18. The device according to claim 1 including at least one optical amplifier optically coupled either between said light source and said Bragg sensor array or between said Bragg sensor array and said optical modulator.

19. The device according to claim 18 wherein said optical waveguide is an optical fiber, and wherein said light source, said Bragg sensor array, said optical modulator and said wavelength detection means are optically coupled in an optical fiber network using optical fibers.

20. The device according to claim 19 wherein said optical fiber network includes a first optical fiber section connected between said light source and an optical coupler, a second optical fiber section connected at one end thereof to said optical coupler having said at least two spaced apart Bragg gratings located therein, and a third optical fiber section connected at one end thereof to said optical coupler and the other end thereof being optically coupled to said optical modulator.

21. The device according to claim 20 wherein said at least one optical amplifier is a unidirectional optical amplifier optically coupled to said first optical fiber section between said light source and said optical coupler.

22. The device according to claim 21 wherein said optical amplifier is selected from the group consisting of rare-earth doped fiber amplifiers, fiber Raman amplifiers and semiconductor-based optical amplifiers.

23. The device according to claim 21 wherein said optical amplifier is an erbium-doped fiber amplifier pumped by a semiconductor laser.

24. The device according to claim 20 wherein said at least one optical amplifier is a unidirectional optical amplifier optically coupled to said third optical fiber section between said optical coupler and said optical modulator.

25. The device according to claim 24 wherein said unidirectional optical amplifier is selected from the group consisting of rare-earth doped fiber amplifiers, fiber Raman amplifiers and semiconductor-based optical amplifiers.

26. The device according to claim 24 wherein said unidirectional optical amplifier is an erbium-doped fiber amplifier pumped by a semiconductor laser.

27. The device according to claim 20 wherein said at least one optical amplifier is a bidirectional optical amplifier optically coupled to said second optical fiber section between said optical coupler and said Bragg sensor array.

28. The device according to claim 27 wherein said bi-directional optical amplifier is selected from the group consisting of rare-earth doped fiber amplifiers, fiber Raman amplifiers and semiconductor-based optical amplifiers.

29. The device according to claim 27 wherein said bi-directional optical amplifier is an erbium-doped fiber amplifier pumped by a semiconductor laser.

30. The device according to claim 18 wherein said broadband light source is one of a semiconductor laser diode, a light emitting diode, a super-luminescent light emitting diode, an edge-emitting light emitting diode, an amplified spontaneous emission light source and a mode-locked fiber laser.

31. The device according to claim 18 wherein said light source is a super-luminescent light emitting diode.

32. The device according to claim 20 wherein said at least one optical amplifier includes a first unidirectional optical amplifier optically coupled to said first optical fiber section between said light source and said optical coupler and a second unidirectional optical amplifier optically coupled to said third optical fiber section between said optical coupler and said optical modulator.

33. The device according to claim 20 wherein said at least one optical amplifier includes a unidirectional optical amplifier optically coupled to said third optical fiber section between said optical coupler and said optical modulator and a bidirectional optical amplifier optically coupled to said second optical fiber section between said optical coupler and said Bragg sensor array.

34. The device according to claim 20 wherein said at least one optical amplifier includes a unidirectional optical amplifier optically coupled to said first optical fiber section between said light source and said optical coupler and a bidirectional optical amplifier optically coupled to said second optical fiber section between said optical coupler and said Bragg sensor array.

35. The device according to claim 20 wherein said at least one optical amplifier includes a first unidirectional optical amplifier optically coupled to said first optical fiber section between said light source and said optical coupler and a second unidirectional optical amplifier optically coupled to said third optical fiber section between said optical coupler and said optical modulator, and a bidirectional optical amplifier optically coupled to said second optical fiber section between said optical coupler and said Bragg sensor array.

36. The device according to claim 18 including a signal generator connected to said light source and said wavelength detection means, and wherein said broadband light source adapted to produce optical pulses is modulated by a low frequency signal produced by said signal generator, wherein said wavelength detection means includes synchronous detection means, and wherein said modulation applied to said wavelength detection means provides a reference signal for said synchronous detection means.

37. The device according to claim 36 wherein said low frequency signal is in the kilohertz range.

38. The device according to claim 21 including a signal generator connected to said light source and said wavelength detection means, and wherein said broadband light source adapted to produce optical pulses is modulated by a low frequency signal produced by said signal generator, wherein said wavelength detection means includes synchronous detection means, and wherein said modulation applied to said wavelength detection means provides a reference signal for said synchronous detection means.

39. The device according to claim 24 including a signal generator connected to said light source and said wavelength detection means, and wherein said broadband light source adapted to produce optical pulses is modulated by a low frequency signal produced by said signal generator, wherein said wavelength detection means includes synchronous detection means, and wherein said modulation applied to said wavelength detection means provides a reference signal for said synchronous detection means.

40. The device according to claim 27 including a signal generator connected to said light source and said wavelength detection means, and wherein said broadband light source adapted to produce optical pulses is modulated by a low frequency signal produced by said signal generator, wherein said wavelength detection means includes synchronous detection means, and wherein said modulation applied to said wavelength detection means provides a reference signal for said synchronous detection means.

41. The device according to claim 18 including an optical tap optically coupled to said optical amplifier, and wherein said optical tap is used to monitor an average noise level originating in said optical amplifier added to the optical signals reflected from said Bragg sensor array, and wherein said noise level is used as a reference level for said wavelength detection means.

42. The device according to claim 41 wherein said optical tap couples a portion of backward traveling noise in said optical amplifier, wherein forward traveling noise in said optical amplifier is determined from a measurement of the backward traveling noise from said optical tap.

43. The device according to claim 18 wherein the signals from said Bragg sensor array are gated by said optical modulator at a time such that no signal originating from said light source is transmitted through said optical modulator to said wavelength modulator, and wherein the detection of the signal at said time gives an indication of the noise from said amplifier, and wherein the signal at said time is used as a reference for said wavelength detection means.

55. The device according to claim 3 wherein said plurality of Bragg gratings have substantially equal center wavelengths.

58. The method according to claim 57 wherein said optical signals reflected by a preselected Bragg grating are gated using an electronically gated wavelength detection means.

59. The method according to claim 56 wherein said light source is adapted to produce optical pulses having a pulse width shorter than a time required for a light pulse to travel approximately twice a distance between any two spatially closest Bragg gratings in said at least two Bragg gratings.

60. The method according to claim 59 wherein said light source is adapted to produce optical pulses with a period between said optical pulses being greater than a time for an optical pulse to travel approximately twice a distance from a first Bragg grating closest to said light source to a last Bragg grating farthest from said light source in said Bragg sensor array.

61. The method according to claim 56 including amplifying one of said light pulses from said broadband light source and optical signals reflected from said Bragg sensor array.

44. A Bragg grating sensor device, comprising:

a) a broadband light source adapted to produce optical pulses;

b) a Bragg sensor array including at least two spaced apart Bragg gratings located in an optical waveguide, said Bragg sensor array being optically coupled to said light source;

c) wavelength detection means optically coupled to said Bragg sensor array for analysing wavelength content of said optical signals reflected from said Bragg sensor array;

d) at least one optical amplifier optically coupled either between said light source and said Bragg sensor array or between said Bragg sensor array and said wavelength detection means; and

e) electronic gating means connected to said wavelength detection means for gating signals produced by said wavelength detection means for selectively analysing optical signals reflected from a preselected Bragg grating in said Bragg sensor array.

45. The device according to claim 44 wherein said optical waveguide is an optical fiber.

46. The device according to claim 45 wherein said broadband light source, said Bragg sensor array and said wavelength detection means are optically coupled in an optical fiber network using optical fibers.

47. The device according to claim 46 wherein said optical fibers are single mode optical fibers, and wherein said at least two Bragg gratings is a plurality of Bragg gratings spaced an effective minimum distance apart from each other.

48. The device according to claim 47 wherein said broadband light source is adapted to produce optical pulses having a pulse width shorter than a time required for an optical pulse to travel approximately twice a distance between any two spatially closest Bragg gratings in said at least two Bragg gratings.

49. The device according to claim 47 wherein said broadband light source is adapted to produce optical pulses with a period between said light pulses being greater than a time for an optical pulse to travel approximately twice a distance from a first Bragg sensor closest to said light source to a last Bragg grating farthest from said light source.

50. The device according to claim 46 wherein said optical fiber network includes a first single mode optical fiber section connected between said light source and an optical coupler, a second single mode optical fiber section containing said Bragg sensor array connected at one end thereof to said optical coupler, and a third single mode optical fiber section connected at one end thereof to said optical coupler optically coupled to said wavelength detection means.

51. The device according to claim 44 wherein said broadband light source is one of a semiconductor laser diode, a light emitting diode, a super-luminescent light emitting diode, an edge-emitting light emitting diode, an amplified spontaneous emission light source and a mode-locked fiber laser.

52. The device according to claim 46 including an optical tap optically coupled to said optical amplifier, and wherein said optical tap is used to monitor an average noise level originating in said optical amplifier added to the optical signals reflected from said Bragg sensor array, and wherein said noise level is used as a reference level for said wavelength detection means.

53. The device according to claim 52 wherein said optical tap couples a portion of backward traveling noise produced in said optical amplifier, wherein forward traveling noise produced in said optical amplifier is determined from a measurement of the backward traveling noise from said optical tap.

54. The device according to claim 44 wherein said electronic gating means is used to monitor the signal at a time when no signal originating from said light source is present, and wherein the monitored signal at said time gives an indication of the noise from said amplifier, and wherein the signal at said time is used as a reference for said wavelength detection means.

56. A method for time domain demultiplexing a serial fiber Bragg grating array comprising at least two Bragg gratings spaced apart from each other in a sensor network, comprising:

directing optical pulses from a broadband light along said sensor network toward said Bragg grating array; and

gating optical signals reflected by said Bragg sensor array to preselect optical signals reflected from a selected Bragg grating, said optical signals being gated using a gated optical modulator, and spectrally analyzing said preselected optical signals to determine a wavelength content of said reflected optical signals.

57. A method for time domain demultiplexing a serial fiber Bragg grating array comprising at least two Bragg gratings spaced apart from each other in a sensor network, comprising:

directing optical pulses from a broadband light along said sensor network toward said Bragg grating array;

amplifying one of said light pulses from said broadband light source and optical signals reflected from said Bragg sensor array; and

detecting optical signals reflected by said Bragg sensor array by a wavelength detection means and gating signals produced by said wavelength detection means to preselect optical signals reflected from a selected Bragg grating, and spectrally analyzing said preselected optical to determine a wavelength content of said reflected optical signals.