Optical sensing apparatus and method

• US 4,342,907 A
• Filed: 02/01/1980
• Issued: 08/03/1982
• Est. Priority Date: 12/12/1977
• Status: Expired due to Term

• Current Assignee: Pedro B. Macedo, Theodore A. Litovitz
• Original Assignee: Pedro B. Macedo, Theodore A. Litovitz
• Inventors: Lagakos, Nicholas, Litovitz, Theodore A., Meister, Robert, Mohr, Robert K., Macedo, Pedro B.
• Primary Examiner(s): Nelms, David C.
• Assistant Examiner(s): Hostetter, Darwin R.
• Application Number:
  US06/117,637
• Time in Patent Office: 914 Days
• Field of Search: 250/227, 350/39.15, 350/39.29, 73/705
• US Class Current: 250/227.14
• CPC Class Codes: G01B 11/18 using photoelastic elements
• CPC Class Codes: G01D 5/35345 using Amplitude variations to detect the measured quantity
• CPC Class Codes: G01D 5/35377 Means for amplifying or modifying the measured quantity
• CPC Class Codes: G01K 1/024 for remote indication
• CPC Class Codes: G01L 11/025 using a pressure-sensitive optical fibre
• CPC Class Codes: G02B 6/14 Mode converters
• CPC Class Codes: G02B 6/2852 using tapping light guides arranged sidewardly, e.g. in a non-parallel relationship with respect to the bus light guides (light extraction or launching through cladding, with or without surface discontinuities, bent structures)
• CPC Class Codes: Y10S 177/06 Photoelectric

Abstract

A sensor for measuring stress, temperature, pressure, sound, etc. comprising an optical waveguide, preferably an optical fiber waveguide, a light source which injects light into one end of the waveguide, a deformer contacting and deforming the waveguide to cause light to couple from originally excited modes to other modes, and an optical detector to detect the change in light coupling caused by deformation of the waveguide.

Claims

1. A sensor comprising an optical waveguide having means for transmitting light in at least two groups of modes denoted A and B, an optical light source means for injecting light into said waveguide so that light transmitted at least initially along said waveguide will be substantially in one of said groups, deformer means responsive to an external stress for deforming a region of said waveguide with such deformation being effective to change the character of the transmission of at least a portion of the light in the vicinity of the deformation from said one group to the said other group of modes, the change being approximately a linear function of the change in the external stress, and an optical detector having means to detect a change in the light in said other group only and thereby monitor said external stress.

2. A sensor as claimed in claim 1 wherein said waveguide has two cores and modes A are in one core and modes B are in another core.

3. A sensing device as claimed in claim 1 wherein a plurality of deformer means are provided each spaced apart along spaced regions of said waveguide and wherein each said deformer means has associated therewith a connecting optical waveguide each connected to a common return optical waveguide, said optical detector'"'"'s means to detect a change in the light in said other group being associated with said common return waveguide so that a time delay introduced by the spatial separation between said plurality of deformer means will serve to differentiate the external stresses experienced by each said deformer means.

4. A sensor according to claim 1 wherein said A modes are lower order bound core modes and said B modes are higher order bound core modes.

5. A sensor according to claim 1 wherein said A modes are bound core modes and said B modes are leaky core modes.

6. A sensor according to claim 1 wherein said A modes are bound core modes and said B modes are clad modes.

7. A sensor according to claim 6 wherein said bound core modes are high order bound core modes.

8. A sensor according to claim 1 wherein said A modes are bound and leaky core modes and said B modes are clad modes.

9. A sensor according to claim 1 wherein said A modes are leaky core modes and said B modes are bound core modes.

10. A sensor according to claim 1 wherein said A modes are leaky core modes and said B modes are clad modes.

11. A sensor according to claim 1 wherein said A modes are clad modes and said B modes are bound core modes.

12. A sensor according to claim 1 wherein said A modes are clad modes and said B modes are leaky core modes.

13. A sensor according to claim 1 wherein said A modes are clad modes and said B modes are leaky and bound core modes.

14. A sensor according to claim 1 further comprising means to remove light from said B modes before entering said region so that the amount of light in said B modes in said region is less than 1% of that in said A modes.

15. A sensor according to claim 1 wherein said deformer means comprise at least two objects in contact with the waveguide.

16. A sensor according to claim 15 wherein at least one of said objects has a rough surface in contact with the waveguide.

17. A sensor according to claim 15 wherein at least one of said objects has at least one ridge on its surface in contact with the waveguide.

18. A sensor according to claim 17 in which each of said objects has a set of ridges and said sets interleave each other.

19. A sensor according to claim 1 wherein said deformer means is a deformable object attached to at least two points of the waveguide.

20. A sensor according to claim 1 further comprising a pressure sensitive enclosure connected to said deformer means so that a change in pressure at the enclosure causes a change in deformation of said region of said waveguide whereby said sensor measures pressure.

21. A sensor according to claim 20 further comprising a low frequency pressure transmitting device extending through the wall of the enclosure which allows the pressure inside the enclosure to be essentially equilibrated with low frequency components of the pressure external to the enclosure so that the change in deformation of the waveguide is determined only by high frequency components of said external pressure, whereby said sensor measures the high frequency component of the external pressure.

22. A sensor according to claim 21 wherein said low frequency components are less than 1 Hertz, and said high frequency components are greater than 20 Hertz.

23. A pressure sensor according to claim 20 wherein said sensor is a hydrophone.

24. A sensor according to claim 15 in further comprising means connected to said object such that deformation of said region is proportional to acceleration.

25. A sensor according to claim 1 wherein the deformer means comprises at least two materials having dissimilar thermal expansion coefficients and configured to cause a change in deformation of said region of said waveguide upon change in temperature, whereby said sensor is used to measure temperature.

26. A sensor according to claim 1 in which the optical waveguide is predeformed.

27. A sensor according to claim 1 which further comprises means to cause said light source to be amplitude modulated.

28. A sensor according to claim 1 wherein said optical light source is a semi-conductor laser.

29. A sensor according to claim 1 wherein said A modes in said region are bound core modes, and wherein said optical means maintains the numerical aperture of the light entering said region below the numerical aperture of said region.

30. A sensor according to claim 1 wherein said A modes in said region are low order bound core modes, and wherein said optical means maintains the numerical aperture of the light entering said region substantially below the numerical aperture of said region.

31. A sensor according to claim 1 wherein said waveguide is an optical fiber waveguide, said A modes in said region are bound and leaky core modes, and said optical means includes a reduced diameter portion of said optical fiber waveguide between said light source and said region.

32. A sensor according to claim 1 wherein said A modes in said region are bound and leaky core modes, said waveguide is an optical fiber waveguide, and said optical means include means causing the numerical aperture of the light to be greater than the numerical aperture of said optical fiber waveguide, and means for removal of clad modes.

33. A sensor according to claim 14 wherein the means to remove enough light from said B modes comprises a medium surrounding the waveguide, said medium having an index of refraction greater than or equal to the index of the outer surface of said waveguide.

34. A sensor according to claim 1 wherein said light source is separated from said region by an optical fiber waveguide at least 10 meters in length.

35. A sensor according to claim 1 wherein said optical detector is separated from said region by an optical fiber waveguide at least 10 meters in length.

36. A sensor according to claim 1 wherein said optical detector is a silicon diode optical detector.

37. A sensor according to claim 1 wherein said means to detect the light in said B modes allow light to radiate from the end of the waveguide, said optical detector being positioned in such a fashion as to detect only certain angular components of the light from the end of the waveguide.

38. A sensor according to claim 1 wherein said means to detect the light in said B modes comprise optical means for imaging in a plane the light from the end of said waveguide, and means to detect light only in certain regions of the image on said plane.

39. A sensor according to claim 1 wherein said B modes are clad modes, and said optical detector comprises a medium surrounding said waveguide and having an index of refraction equal to or greater than that of the outer surface of said waveguide providing means to detect light leaving said clad.

40. A sensor according to claim 1 further comprising a section of an optical fiber waveguide for transmitting light from said B modes connected to and extending between said waveguide and said optical detector.

41. A sensor according to claim 1 comprising one said optical light source, more than one said region, more than one said deformer means, and more than one said optical detector.

42. The sensor as claimed in claim 1 wherein said waveguide is an optical fiber having a core and at least one clad material completely encircling said core.

43. A sensor according to claim 3 wherein said waveguide comprises an optical fiber waveguide extending from said light source and including each of said light regions.

44. A sensor according to claim 3 wherein at least one said region and the respective deformer means is sensitive to pressure, and at least one said region and the respective deformer means is sensitive to temperature.

52. The sensing device as claimed in claim 3 wherein each said waveguide is an optical fiber having a core and at least one clad material completely encircling said core.

53. The sensing device as claimed in claim 1 wherein said waveguide is an optical fiber having a core and said core has a plurality of clads concentrically disposed to enclose said core.

54. The sensor as claimed in claim 3 wherein each said waveguide is an optical fiber having a core surrounded by a first clad material and a second clad material surrounding said first clad material.

45. A method comprising injecting light into an optical waveguide having at least two groups of modes A and B each grojup containing at least one mode, deforming a region of the waveguide by application of an external stress, the deformation producing a change in the coupling of light between A modes and B modes, causing light entering said region to be substantially in A modes, the deformation producing a change in the light power in B modes which change is a linear function of the change in said external stress, and optically detecting said light in B modes only and thereby monitoring said external stress.

46. A method as in claim 45 further comprising removing light from said B modes before entering said region so that the amount of light in said B modes in said region is less than 1% of that in said A modes.

47. A method as in claim 45 wherein stress is applied by compressing the waveguide between two objects at least one of which has a rough surface.

48. A method as in claim 45 further comprising enclosing said region of the wave guide with a pressure sensitive enclosure which allows the external pressure to cause the deformation of said region whereby pressure is measured.

49. A method as in claim 48 further comprising equilibrating the low frequency components of the external and internal pressure whereby only the high frequency components of the external pressure are measured.

50. A method as in claim 48 where the pressure is measured in water.

51. A method according to claim 45 wherein stress is applied to more than one said region of said waveguide for the purpose of making a plurality of measurements, passing the light from said regions to a common detector and utilizing time delay introduced by the spatial separation of said regions to monitor separately each of said regions.