Methods and apparatus for mechanically enhancing the sensitivity of longitudinally loaded fiber optic sensors

• US 6,246,048 B1
• Filed: 05/18/1999
• Issued: 06/12/2001
• Est. Priority Date: 05/18/1999
• Status: Active Grant

• Current Assignee: Schlumberger Technology Corporation (Schlumberger NV)
• Original Assignee: Schlumberger Technology Corporation (Schlumberger NV)
• Inventors: Balkunas, Stephen C., Yamate, Tsutomu, Madhavan, Raghu, Schroeder, Robert J., Ramos, Rogerio T.
• Primary Examiner(s): Allen, Stephone B.
• Application Number:
  US09/313,603
• Time in Patent Office: 756 Days
• Field of Search: 250/227.14, 250/227.17, 250/227.18, 250/227.21, 356/32, 356/35.5, 385/77, 385/85
• US Class Current: 250/227.18
• CPC Class Codes: G01L 1/246 using integrated gratings, e.g. Bragg gratings

Abstract

An optical fiber is provided with a Bragg grating formed along a portion of its core and a mechanical structure arranged adjacent to the Bragg grating for amplifying longitudinal strain on the fiber in the vicinity of the grating. The mechanical structure is designed to convert ambient pressure into longitudinal strain on the fiber in the vicinity of the grating and to allow the fiber to pass through the structure so that several pressure measuring apparatus may be arranged along a single optical fiber. An intermediate structure is provided between the fiber and the mechanical structure for minimizing buckling of the fiber. The methods of the invention include converting pressure into longitudinal strain on an optical fiber, amplifying the effect of pressure on the longitudinal strain, measuring pressure by determining the spectral location related to peaks (or minimums) of light reflected from an optical grating subjected to longitudinal strain.

Claims

1. A fiber optic transducer, comprising:

a) a fiber optic having a core having at least one grating formed along at least one portion of said core;

b) a mechanical structure coupled to said fiber optic core which converts pressure and/or temperature on said mechanical structure to longitudinal strain on said fiber optic core at said grating; and

c) an intermediate structure between said fiber optic and said mechanical structure, wherein

said mechanical structure is adapted to allow said fiber optic to pass through and exit said mechanical structure, said fiber optic passes through and exits said mechanical structure, and said intermediate structure is adapted to minimize buckling of, said fiber optic.

2. A fiber optic transducer according to claim 1, wherein:

said mechanical structure includes a tube covering said fiber optic, said tube having two ends, a pair of sealing members, each end of said tube being sealed by one of said sealing members which physically couples said fiber optic to said tube.

3. A fiber optic transducer according to claim 2, wherein:

said intermediate structure includes a soft filling between said tube and said fiber optic.

4. A fiber optic transducer according to claim 3, wherein:

said soft filling has a Young'"'"'s modulus much lower than that of said tube.

5. A fiber optic transducer according to claim 4, wherein:

said soft filling is silicon rubber.

6. A fiber optic transducer according to claim 2, wherein:

said intermediate structure includes a filling rod coupled to said fiber optic in the vicinity of said grating, and

said mechanical structure includes a pair of rigid rods on either side of said filling rod.

7. A fiber optic transducer according to claim 6, wherein:

said rigid rods are made of a material having a coefficient of thermal expansion which compensates for the thermal expansion of said tube so that longitudinal strain on said fiber optic is only the result of changes in pressure and not the result of changes in temperature.

8. A fiber optic transducer according to claim 6, wherein:

said rigid rods are made of a material having a coefficient of thermal expansion which enhances the thermal expansion of said fiber so that longitudinal strain on said fiber optic is mainly the result of changes in temperature.

9. A fiber optic transducer according to claim 1, wherein:

said mechanical structure includes a housing having a stepped inner diameter defining two end cavities and a middle cavity, a diaphragm covering one of said end cavities, and a rigid rod coupled to said diaphragm and extending partially into said middle cavity, and

said intermediate structure includes a filling rod coupled to said fiber optic in the vicinity of said grating, said filling rod located in said middle cavity adjacent said rigid rod.

10. A fiber optic transducer according to claim 9, wherein:

said mechanical structure includes two diaphragms, one covering each end cavity, and two rigid rods, each rigid rod being coupled to a respective diaphragm and partially entering said middle cavity.

11. A fiber optic transducer according to claim 10, wherein:

each of said diaphragms defines a hole through which said fiber optic passes.

12. A fiber optic transducer according to claim 10, wherein:

each of said end cavities defines a side hole through which said fiber optic passes.

13. A fiber optic transducer according to claim 12, wherein:

each of said rigid rods defines a side hole through which said fiber optic passes.

14. A fiber optic transducer according to claim 9, wherein:

said diaphragm covers a first one of said two end cavities, a second one of said two end cavities being sealed with said fiber optic passing therethrough.

15. A fiber optic transducer according to claim 14, wherein:

said diaphragm defines a hole through which said fiber optic passes.

16. A fiber optic transducer according to claim 14, wherein:

said first one of said end cavities defines a side hole through which said fiber optic passes.

17. A fiber optic transducer according to claim 16, wherein:

said rigid rod defines a side hole through which said fiber optic passes.

18. A fiber optic sensing system, comprising:

a) a light source;

b) a spectral analyzer; and

c) a fiber optic transducer including

i) a fiber optic having a core with at least one grating formed along at least one portion thereof,

ii) pressure and/or temperature responsive means for generating longitudinal strain on said core at said grating, and

iii) an intermediate structure between said fiber optic and said mechanical structure, wherein

said light source is arranged to direct light into said core and said spectral analyzer is arranged to detect light exiting said core,

said pressure and/or temperature responsive means is arranged to allow said fiber optic to pass through and exit said pressure and/or temperature responsive means and said fiber optic passes through and exits said pressure and/or temperature responsive means, and

said intermediate structure is adapted to minimize buckling of said fiber optic.

19. A fiber optic sensing system according to claim 18, wherein:

said pressure and/or temperature responsive means includes a tube, said tube having two ends, a pair of sealing members, each end of said tube being sealed by one of said sealing members which physically couples said fiber optic to said tube.

20. A fiber optic sensing system according to claim 19, wherein:

said intermediate structure includes a soft filling between said tube and said fiber optic.

21. A fiber optic sensing system according to claim 20, wherein:

said soft filling has a Young'"'"'s modulus much lower than that of said tube.

22. A fiber optic sensing system according to claim 21, wherein:

said soft filling is silicon rubber.

23. A fiber optic sensing system according to claim 19, wherein:

said intermediate structure includes a filling rod coupled to said fiber optic in the vicinity of said grating, and

said pressure and/or temperature responsive means a pair of rigid rods on either side of said filling rod.

24. A fiber optic sensing system according to claim 23, wherein:

said rigid rods are made of a material having a coefficient of thermal expansion which compensates for the thermal expansion of said tube so that longitudinal strain on said fiber optic is substantially the result of changes in pressure only.

25. A fiber optic sensing system according to claim 23, wherein:

said rigid rods are made of a material having a coefficient of thermal expansion which enhances the thermal expansion of said fiber so that longitudinal strain on said fiber optic is substantially the result of changes in temperature only.

26. A fiber optic sensing system according to claim 18, wherein:

said pressure responsive means includes a substantially cylindrical housing having a stepped inner diameter defining two end cavities and a middle cavity, a diaphragm covering one of said end cavities, and a rigid rod coupled to said diaphragm and extending partially into said middle cavity, and

said intermediate structure includes a filling rod coupled to said fiber optic in the vicinity of said grating, said filling rod located in said middle cavity adjacent said rigid rod.

27. A fiber optic sensing system according to claim 26, wherein:

said pressure responsive means includes two diaphragms, one covering each end cavity and two rigid rods, each rigid rod being coupled to a respective diaphragm and partially entering said middle cavity.

28. A fiber optic sensing system according to claim 27, wherein:

each of said diaphragms defines a hole through which said fiber optic passes.

29. A fiber optic sensing system according to claim 27, wherein:

each of said end cavities defines a side hole through which said fiber optic passes.

30. A fiber optic sensing system according to claim 29, wherein:

each of said rigid rods defines a side hole through which said fiber optic passes.

31. A fiber optic sensing system according to claim 26, wherein:

said diaphragm covers a first one of said two end cavities, a second one of said two end cavities being sealed with said fiber optic passing therethrough.

32. A fiber optic sensing system according to claim 31, wherein:

said diaphragm defines a hole through which said fiber optic passes.

33. A fiber optic sensing system according to claim 31, wherein:

said first one of said end cavities defines a side hole through which said fiber optic passes.

34. A fiber optic sensing system according to claim 33, wherein:

said rigid rod defines a side hole through which said fiber optic passes.

35. A fiber optic sensing system, comprising:

a) a light source;

b) a spectral analyzer;

c) a fiber optic having a core with a plurality of spaced apart gratings formed along at least one portion thereof;

d) a plurality of pressure and/or temperature responsive means for generating longitudinal strain on said core at respective gratings;

e) a corresponding plurality of intermediate structures, each being arranged between said fiber optic and one of said plurality of pressure and/or temperature responsive means, wherein

said light source is arranged to direct light into said core and said spectral analyzer is arranged to detect light exiting said core,

said pressure and/or temperature responsive means are each adapted to allow said fiber optic to pass through and exit said pressure and/or temperature responsive means and said fiber optic passes through and exits said pressure responsive means, and

said intermediate structure is adapted to minimize buckling of said fiber optic.

36. A method of measuring pressure, comprising:

a) optically coupling a fiber optic grating transducer to a light source, the fiber optic grating transducer including a mechanical structure for converting pressure to longitudinal strain on the grating of the fiber optic, the mechanical structure being arranged to allow the fiber optic to pass through and exit the mechanical structure and the fiber optic passing through and exiting the mechanical structure, and an intermediate structure between the fiber optic and the mechanical structure, the intermediate structure being adapted to minimize buckling of the fiber optic.;

b) directing light from the light source into the core of the fiber optic grating transducer;

c) optically coupling a spectral analyzer to the fiber optic grating transducer; and

d) measuring the spectral location related to a spectral peak detected by the spectral analyzer to determine the pressure ambient to the fiber optic grating transducer.