Fiber optic sensing device and method of making and operating the same

• US 20060215959A1
• Filed: 03/22/2005
• Published: 09/28/2006
• Est. Priority Date: 03/22/2005
• Status: Active Grant

• Current Assignee: General Electric Company
• Original Assignee: General Electric Company
• Inventors: Xia, Hua, McCarthy, Kevin Thomas, Krok, Michael Joseph, Taware, Avinash Vinayak, Deng, Kung-Li Justin
• Granted Patent: US 7,421,162 B2
• Time in Patent Office: Days
• US Class Current: 385/37
• CPC Class Codes: G01D 5/35303 using a reference fibre, e.g. interferometric devices
• CPC Class Codes: G01D 5/35358 using backscattering to detect the measured quantity
• CPC Class Codes: G01D 5/35383 using multiple sensor devices using multiplexing techniques
• CPC Class Codes: G01N 21/774 the reagent being on a grating or periodic structure
• CPC Class Codes: G02B 6/02085 characterised by the grating profile, e.g. chirped, apodised, tilted, helical
• CPC Class Codes: G02B 6/02123 characterised by the method of manufacture of the grating photolithography G03F7/0005
• CPC Class Codes: G02B 6/02147 Point by point fabrication, i.e. grating elements induced one step at a time along the fibre, e.g. by scanning a laser beam, arc discharge scanning G02B6/02133 and G02B6/02142 take precedence

Abstract

In accordance with one exemplary embodiment the invention provides a multi-parameter fiber optic sensing system with an aperiodic sapphire fiber grating as sensing element for simultaneous temperature, strain, NO_x, CO, O₂ and H₂ gas detection. The exemplary sensing system includes an aperiodic fiber grating with an alternative refractive index modulation for such multi-function sensing and determination. Fabrication of such quasiperiodic grating structures can be made with point-by-point UV laser inscribing, diamond saw micromachining, and phase mask-based coating and chemical etching methods. In the exemplary embodiment, simultaneous detections on multi-parameter can be distributed, but not limited, in gas/steam turbine exhaust, in combustion and compressor, and in coal fired boilers etc. Advantageously, the mapping of multiple parameters such as temperature, strain, and gas using sapphire aperiodic gratings improves control and optimization of such systems directed to improve efficiency and output and reduce emissions.

Claims

1. A fiber optic sensor cable, comprising:

a core having a first index of refraction and a plurality of grating elements each having an index of refraction different from the first index of refraction, the core comprising;

a first pair of grating elements configured to reflect a first wavelength of light in phase;

a second pair of grating elements configured to reflect a second wavelength of light in phase; and

a third pair of grating elements configured to reflect the first wavelength of light in phase, wherein at least one grating element of the second pair of grating elements is located between at least one grating element of the first pair and at least one grating element of the third pair; and

a cladding disposed circumferentially about the core.

2. The fiber optic sensor cable of claim 1, wherein the index of refraction of the plurality of grating elements is lower than the first index of refraction.

3. The fiber optic sensor cable of claim 1, wherein the core comprises a fused silica fiber.

4. The fiber optic sensor cable of claim 1, wherein the core comprises a sapphire fiber.

5. The fiber optic sensor cable of claim 1, wherein the distance between adjacent grating elements of the first pair is at a first distance and the distance between adjacent grating elements of the second pair is at a second distance.

6. The fiber optic sensor cable of claim 5, wherein the distances between adjacent gratings along a longitudinal axis of the core alternate between the first distance and the second distance.

7. The fiber optic sensor cable of claim 5, wherein distances between adjacent gratings have an aperiodic pattern.

8. The fiber optic sensor cable of claim 7, wherein a periodicity of the aperiodic pattern is a function of the first and second distances and wherein the first and second distances are defined by the equation S_j={S_j-1,S_j-2} for j values greater than two.

9. A fiber optic sensing device, comprising:

a core having a first index of refraction and a plurality of grating elements each having an index of refraction different from the first index of refraction, the core comprising;

a first portion, wherein the distance between adjacent grating elements is at a first distance;

a second portion, wherein the distance between adjacent grating elements is at a second distance different than the first distance; and

a third portion, wherein the distance between adjacent grating elements is at the first distance;

wherein the second portion is disposed between the first and third portions;

a light source configured to illuminate the core; and

a detector system configured to estimate at least one parameter based upon at least one diffraction peak generated from the plurality of grating elements.

10. The fiber optic sensing device of claim 9, wherein the detector system comprises an indium gallium arsenide based detector.

11. The fiber optic sensing device of claim 9, wherein the detector system is configured for measuring a temperature in an environment.

12. The fiber optic sensing device of claim 9, wherein the detector system is configured for measuring a strain, or a stress, or a pressure, or any combination thereof in an environment.

13. The fiber optic sensing device of claim 9, wherein the sensing device is configured for use in a narrow band reflector, or a broadband mirror, or a wavelength division multiplexing (WDM) filter, or a differential photonic sensor, or combinations thereof.

14. The fiber optic sensing device of claim 9, wherein the index of refraction of the plurality of grating elements is lower than the first index of refraction.

15. The fiber optic sensing device of claim 9, wherein the detector system is configured to process first and second diffraction peaks to estimate the first and second sensed parameters.

16. The fiber optic sensing device of claim 15, wherein the detector system is configured to process the first and second diffraction peaks to determine the presence of a gaseous environment.

17. A method of detecting a plurality of parameters in an environment, comprising:

providing a source of light to a fiber optic sensor cable having a plurality of grating elements and comprising first, second and third portions, wherein adjacent gratings in the first and third portions are at a first distance from one another and adjacent gratings in the second portion are at a second distance from one another, and wherein the second portion is located between the first and third portions; and

detecting light emitted from the fiber optic sensor cable.

18. The method of claim 17, wherein detecting comprises determining a first diffraction peak corresponding to a first wavelength of light and determining a second diffraction peak corresponding to a second wavelength of light different from the first wavelength of light.

19. The method of claim 18, comprising simultaneously measuring first and second parameters corresponding to changes in the environment based on the first and second diffraction peaks.

20. The method of claim 19, wherein measuring the first parameter comprises detecting a temperature in the environment.

21. The method of claim 19, wherein measuring the second parameter comprises detecting a strain, or a stress, or a pressure, or any combination thereof in the environment.

22. The method of claim 17, further comprising providing a thermal mapping in an exhaust system of a gas turbine, or a steam turbine, or a coal fired boiler, or a combustor, or a compressor of a jet engine.

23. The method of claim 17, further comprising detecting at least one parameter in a gas turbine, or a steam turbine, or a coal fired boiler.

24. A method, comprising:

providing a source of light to a fiber optic sensor cable having a plurality of grating elements;

reflecting a first wavelength of light via a first pair of grating elements such that waves of the first wavelength of light are in phase with one another;

reflecting a second wavelength of light via a second pair of grating elements such that waves of the second wavelength of light are in phase with one another; and

reflecting the first wavelength of light via a third pair of grating elements, wherein at least one grating element of the second pair of grating elements is located between at least one grating element of the first pair of grating elements and at least one grating element of the third pair of grating elements such that waves of the first wavelength of light are in phase with one another.

25. The method of claim 24, comprising reflecting a third wavelength of light via the third pair of grating elements in response to an environmental condition.

26. A method of sensing a plurality of parameters, comprising:

providing a light source to a fiber optic sensor cable having first and third pair of grating elements configured to reflect a first wavelength of light and a second pair of grating elements configured to reflect a second wavelength of light, wherein at least one grating element of the second pair of grating elements is located between at least one grating of the first and third pair of grating elements;

detecting light emitted from the fiber optic sensor cable;

detecting diffraction peaks from the light emitted from the fiber optic sensor cable; and

correlating the diffraction peaks to changes in monitored parameters.

27. The method of claim 26, comprising correlating the monitored parameters corresponding to an environmental condition.

28. The method of claim 27, comprising correlating the monitored parameters to a change in temperature.

29. The method of claim 27, comprising correlating the monitored parameters to the presence of a pre-selected gas.

30. The method of claim 27, comprising correlating the monitored parameters to a change in mechanical stresses.

31. A method of fabricating a fiber optic sensing device, comprising:

providing a core having a first index of refraction;

providing a first pair of grating elements configured to reflect a first wavelength of light in phase, wherein the distance between adjacent grating elements of the first pair is at a first distance;

providing a second pair of grating elements configured to reflect a second wavelength of light in phase, wherein the distance between adjacent grating elements of the second pair is at a second distance;

providing a third pair of grating elements configured to reflect the first wavelength of light in phase, wherein at least one grating element of the second pair of grating elements is located between at least one grating element of the first pair and at least one grating element of the third pair; and

disposing a cladding circumferentially about the core.

32. The method of claim 31, further comprising providing a light source for illuminating the core for generating diffraction peaks representative of sensed parameters via the first, second and third pair of grating elements.

33. The method of claim 31, further comprising coupling a detector system to the fiber optic sensing device for determining the sensed parameters based upon the diffraction peaks generated from the fiber optic sensing device.

34. The method of claim 31, comprising providing the first, second and third pair of grating elements disposed in a fused silica fiber.

35. The method of claim 31, comprising providing the first, second and third pair of grating elements disposed in a sapphire fiber.

36. The method of claim 31, comprising providing the first, second and third pair of grating elements through a ultraviolet light exposure laser inscribing technique.

37. The method of claim 31, comprising providing the grating elements such that alternating distances between adjacent gratings along a longitudinal axis of the core between the first distance and the second distance are present.

38. A method of fabricating a fiber optic sensor cable, comprising:

providing a core having a first index of refraction;

mechanically altering the core to form altered portions within the core, wherein an index of refraction of each of the altered portions is different than the first index of refraction; and

disposing a cladding circumferentially about the core.

39. The method of claim 38, wherein mechanically altering the core comprises providing a phase change in the core through an ultraviolet light exposure inscribing technique.

40. The method of claim 38, wherein mechanically altering comprises altering a diameter of the fiber optic sensor cable through a diamond saw cutting technique.

41. A fiber optic sensor cable for sensing a parameter in a harsh environment, comprising:

a core having a first index of refraction; and

a plurality of mechanically altered portions each having an index of refraction different than the first index of refraction.

42. The fiber optic sensor cable of claim 41, wherein the plurality of mechanically altered portions are aperiodically spaced.

43. The fiber optic sensor cable of claim 42, comprising;

a first pair of grating elements configured to reflect a first wavelength of light in phase;

a second pair of grating elements configured to reflect a second wavelength of light in phase; and

a third pair of grating elements configured to reflect the first wavelength of light in phase, wherein at least one grating element of the second pair of grating elements is located between at least one grating element of the first pair and at least one grating element of the third pair.

44. The fiber optic sensor cable of claim 42, comprising:

a first portion, wherein the distance between adjacent grating elements is at a first distance;

a second portion, wherein the distance between adjacent grating elements is at a second distance different than the first distance; and

a third portion, wherein the distance between adjacent grating elements is at the first distance;

wherein the second portion is disposed between the first and third portions.

45. A distributed sensor system for sensing a parameter in a harsh environment, comprising:

a plurality of sensors disposed on a distributed cable, wherein each of the plurality of sensor comprises;

a core having a first index of refraction; and

a plurality of mechanically altered portions each having an index of refraction different than the first index of refraction.

46. The sensor system of claim 45, wherein the distributed cable comprises sapphire.

47. The sensor system of claim 45, wherein the harsh environment comprises a gas turbine exhaust, or a steam turbine exhaust, or a coal-fired boiler, or an aircraft engine, or a downhole application.

48. The sensor system of claim 45, further comprising an illumination source configured to illuminate the core of each of the plurality of sensors.

49. The sensor system of claim 45, further comprising an optical spectral analyzer to measure a parameter based on a wavelength spectrum from the distributed sensor system.

50. A method of sensing a parameter in a harsh environment, comprising:

providing a source of light to a distributed sensor system having a plurality of sensors, wherein each of the plurality of sensors comprises a core having a first index of refraction and a plurality of mechanically altered portions each having an index of refraction different than the first index of refraction; and

detecting light emitted from the distributed sensor system.

51. The method of claim 50, wherein the harsh environment comprises a gas turbine exhaust, or a steam turbine exhaust, or a coal-fired boiler, or an aircraft engine, or a downhole application.

52. A fiber optic sensor cable, comprising:

a core; and

a cladding disposed circumferentially about the core;

wherein the core comprises first, second, and third portions, the second portion being located between the first and third portions, the first and third portions having a first index of refraction and the second portion having a second index of refraction different from the first index of refraction.