Multi-fiber optic 2D-array device for sensing and localizing environment perturbation using speckle image processing

• US 6,590,194 B2
• Filed: 03/27/2001
• Issued: 07/08/2003
• Est. Priority Date: 03/27/2001
• Status: Expired due to Fees

• Current Assignee: Council of Scientific & Industrial Research
• Original Assignee: Council of Scientific & Industrial Research
• Inventors: Bandyopadhyaya, Somnath, Goel, Pramod Kumar, Chhabra, Jagdish Kumar, Sardana, Harish Kumar
• Primary Examiner(s): Epps, Georgia
• Assistant Examiner(s): HANIG, RICHARD E
• Application Number:
  US09/818,724
• Publication Number:
  US 20020139919A1
• Time in Patent Office: 833 Days
• Field of Search: 250/208.1, 250/227.14, 250/227.16, 250/227.19, 356/35.5
• US Class Current: 250/208.1
• CPC Class Codes: G01D 5/35383 using multiple sensor devices using multiplexing techniques
• CPC Class Codes: G01H 9/004 using fibre optic sensors light guides per se G02B6/00, acousto-optical devices specially adapted for gating or modulating in optical wave guides G02F1/125
• CPC Class Codes: G01L 1/24 by measuring variations of optical properties of material when it is stressed, e.g. by photoelastic stress analysis using infrared, visible light, ultraviolet
• CPC Class Codes: G02B 6/06 the relative position of the fibres being the same at both ends, e.g. for transporting images
• CPC Class Codes: G02B 6/3833 Details of mounting fibres in ferrules; Assembly methods; Manufacture
• CPC Class Codes: G02B 6/3834 Means for centering or aligning the light guide within the ferrule
• CPC Class Codes: G02B 6/4249 comprising arrays of active devices and fibres
• CPC Class Codes: G02B 6/4298 coupling with non-coherent light sources and/or radiation detectors, e.g. lamps, incandescent bulbs, scintillation chambers

Abstract

System for sensing perturbations in a distributed manner using an array of multimode fibers for localization (zone identification) of the perturbation, particularly, the present invention relates to a two dimensional array based speckle pattern sensing system having 4×3 (12) multimode fibers for sensing perturbation in multiple zones, said multimode fibers are imaged in parallel by a charge coupled device (CCD) camera and their speckle pattern analyzed by a conventional image processing hardware to determine the perturbations.

Claims

1. A system for sensing and localization (zone identification) of perturbations in a distributed manner using 2D-array of multimode fibres characterized in using a 2D multi-fiber adapter (MFA) in the sensing module where in the said system comprising a Source Module (source for optical energy), one or more multimode fibres passing through zones where perturbation sensing is desired, and a Receiving Module comprising of a Sensing Module, a charge coupled device (CCD) camera, and an image processing unit.

2. A system as claimed in claim 1, wherein the system can be used for sensing the perturbations selected from the parameters comprising of displacement, vibration, pressure, stress, strain, temperature, intrusion and acoustic wave.

3. A system as claimed in claim 1, wherein the Source Module comprises of 12 laser diodes with standard coupling optics and fiber pigtail.

4. A system as claimed in claim 3, wherein the laser diodes emit coherent monochromatic light having wavelength of 0.6328 μ

m.

5. A system as claimed in claim 1, wherein 12 multimode fibres having a fixed diameters between 50 to 125 μ

m, and Numerical Aperture (NA)=0.2 are used.

6. A system as claimed in claim 1, wherein one or more multimode fiber is used for each zone.

7. A system as claimed in claim 1, wherein the multimode optical fibers are made to pass through the zones where perturbation sensing is desired.

8. A system as claimed in claim 1, wherein the optical signals from the multimode fibres are guided to the Sensing Module using pigtails (pieces of fibres) having the same specification as that of the multimode fibres.

9. A system as claimed in claim 1, wherein the Sensing Module comprises of a Multi Fiber Adapter (MFA), a ground glass plate, and a lens assembly.

10. A system as claimed in claim 9, wherein the MFA has provisions for accommodating 12 multimode fibers.

11. A system as claimed in claim 9, wherein the fibres are mounted on to the MFA using ferrules of FC connectors.

12. A system as claimed in claim 9, wherein the fibres are arranged in the form of a 4×

3 array in the MFA.

13. A system as claimed in claim 9, wherein polished surfaces of the fibres are visible at the other end of the adapter.

14. A system as claimed in claim 9, wherein the other end of the MFA is a vertical plane surface with holes provided for emission of light from the 12 multimode fibres coming from each zone.

15. A system as claimed in claim 14, wherein the centers of the holes are 5 mm away.

16. A system as claimed in claim 9, wherein the glass plate is placed such that a speckle image generated at the output of each fiber is formed upon it.

17. A system as claimed in claim 9, wherein the glass plate is placed at a distance where the size of a speckle pattern becomes 5 mm in diameter.

18. A system as claimed in claim 9, wherein the glass plate is placed at a distance of 12.5 mm from the end of the multimode fiber having Numerical Aperture (NA)=0.2.

19. A system as claimed in claim 9, wherein the glass plate is round shaped, having a diameter of 32 mm and a thickness of 2 mm.

20. A system as claimed in claim 9, wherein the glass plate is made from a borosilicate crown glass precisely the BK7 optical glass.

21. A system as claimed in claim 9, wherein the bubble and inclusion content of the glass plate is very low and the cross section is lesser than 0.029 mm²/100 cm³.

22. A system as claimed in claim 9, wherein parallelism of the glass plate is of the order of 1′

of an arc and having with surface quality better than λ

/2.

23. A system as claimed in claim 9, wherein the ground surface of the glass plate is developed with fine grade emery.

24. A system as claimed in claim 9, wherein the ground surface of the glass plate is placed towards the camera objective.

25. A system as claimed in claim 9, wherein the glass plate will accommodate all 12 speckle images formed.

26. A system as claimed in claim 25, wherein the 12 speckle images formed on the glass plate have a diameter of 5 mm and will be just touching each other as they diverge linearly in the space.

27. A system as claimed in claim 26, wherein the speckle pattern formation occurs in conical shape with each fiber as center.

28. A system as claimed in claim 26, wherein the speckle pattern image when formed, does not overlap each other.

29. A system as claimed in claim 26, wherein the speckle image pattern covers a field of view (FOV) of 20×

15 mm.

30. A system as claimed in claim 9, wherein the lens assembly has multi-element lenses.

31. A system as claimed in claim 9, wherein the multi-element lens forms a single unified image of all the 12 speckle pattern images on the CCD array.

32. A system as claimed in claim 9, wherein the multi-element lens has all optical reduction factor of 0.3.

33. A system as claimed in claim 1, wherein the CCD camera senses perturbations from 12 zones simultaneously.

34. A system as claimed in claim 16, wherein each speckle corresponds to a zone in the field.

35. A system as claimed in claim 1, wherein the CCD camera has a sensing area of 6.4 mm×

4.8 mm, 756×

582 sensing elements and each pixel size of 8.6×

8.3 μ

m.

36. A system as claimed in claim 1, wherein the CCD camera is placed 95 mm from the glass.

37. A system as claimed in claim 1, wherein the CCD camera'"'"'s resolution is 768×

574 pixels (after image acquisition).

38. A system as claimed in claim 1, wherein the CCD image when viewed on a monitor is viewed full in analog form.

39. A system as claimed in claim 1, wherein the image processing unit digitalizes the output of the CCD camera.

40. A system as claimed in claim 1, wherein the digitalization of approximately square pixels in the resolution of 768×

574 pixels provides individual speckle patterns in 191×

191 square covering circular speckle pattern for each zone.

41. A system as claimed in claim 1, wherein the spatial resolution on the object plane is 26 μ

m (in 768×

574 resolution).

42. A system as claimed in claim 1, wherein the average diameter of individual grains is ≈

300 μ

m (with 5 mm per speckle pattern).

43. A multi-fibre 2D array based system useful for sensing environment perturbation that includes, but not limited to, displacement, vibration, pressure, stress, strain, temperature, intrusion, and acoustic wave, said system comprising a Source Module wherein the Source Module comprises of a set of Laser diodes (LD) for individually feeding the light to fibres spread over the fields of interest where zone based sensing is required, connected to a Receiving Unit comprising of a Sensing Module, a standard CCD camera to grab a speckle image;

and

an image processing unit for analyzing the environment perturbation arising in different zones by one or more multimode optical fibres spread over a field in the form of zones where perturbation sensing is desired.

44. A system as claimed in claim 43, wherein the Source Module consists of 12 laser diodes having a wavelength 0.6328 μ

m, and a conventional launching device for launching the light into optical fibres for each zone.

45. A system as claimed in claim 44, wherein the multimode fibres have a diameter ranging between 50 to 125 μ

m, and NA=0.2, suitable for speckle pattern formation at the other (receiving) end.

46. A system as claimed in claim 44, wherein the Sensing Module comprises of a Multi Fibre Adapter (MFA) for accommodating fibres corresponding to various zones of interest;

a diffusing glass plate placed at appropriate distance upon which the speckle images are formed; and

a lens assembly to image speckle pattern formed upon the glass plate on to the CCD plane.

47. A system as claimed in claim 46, wherein the Multi Fibre Adapter (MFA) has provisions to place 12 multimode optical fibres in the form of 4×

3 array, with their polished surfaces visible at the other end of the adapter.

48. A system as claimed in claim 47, wherein the circular ground glass plate is placed at an appropriate distance from the illuminating surface of the fiber, said glass plate is made up of a borosilicate crown glass, more particularly of BK7 glass having very low bubble and inclusion content and cross section <

0.029 mm²/100 cm³, which is relatively a hard glass and does not get scratched easily.

49. A system as claimed in claim 47, wherein the lens assembly has a reduction factor of 0.3 to image the 12 speckle patterns formed upon the glass plate on to the CCD camera.

50. A system as claimed in claim 47, wherein the standard C-mount CCD camera is a ½

″

CCD camera for capturing the image comprising of 12 speckle patterns.

51. A system as claimed in claim 44, wherein the image processing unit extracts the amplitude of the perturbation as well as performs zone identification.

52. A system as claimed in claim 44, wherein the Source Module consists of 12 laser diodes with standard coupling optics and fiber pigtail for launching monochromatic light into the 12 multimode fibres.

53. A system as claimed in claim 44, wherein the fibre emanate from the Source Module onwards in the field into the perturbing zone and the other end of the fibre reaches the Receiving Unit using standard SMA connector.

54. A system as claimed in claim 44, wherein the optical signals are guided to the Sensing Module using 12 pieces of fibres (12 tails) having the same specification as that of the multimode fibres, the fibers are mounted on the Multi-Fibre Adapter (MFA) using ferrules of FC connectors, and the MFA has the other end as vertical plane surface with holes provided for emission of light from all the 12 multimode fibres coming from the perturbing zone.

55. A system as claimed in claim 44, wherein the speckle pattern formation occurs in a conical space with each fibre as center, the ground glass plate at an appropriate distance is able to accommodate all the speckles just touching each other as they diverge linearly in the space, the image falling on the glass plate is capable of being sensed by a ½

″

CCD camera with an appropriate lens placed at an appropriate distance and the video output of the CCD camera can be directly interfaced with an image processing unit residing in the computer for further analysis.

56. A system as claimed in claim 44, wherein the multimode fibres are placed at a central spacing of 5 mm to use standard connectors as well as forming a unified image consisting of 12 speckle patterns closely touching each other at an appropriate distance after diverging in space, the designed spacing is kept as 5 mm so that no two speckle patterns overlap at a later distance where ground glass plate is kept, and the diameter of the light field formed by an optical fibre in a plane placed at a distance R is given by 2.NA.R where NA is the numerical aperture of the fibre.

57. A system as claimed in claim 44, wherein the glass plate is placed at a distance calculated in a manner that each speckle pattern size becomes 5 mm diameter again by the above formula, this distance comes out to be 12.5 mm for a multimode fibre of NA=0.2.

58. A system as claimed in claim 47, wherein the ground glass plate is made of BK7 glass, having 32 mm diameter and 2 mm thickness, the parallelism of the glass plate is of the order of 1′

of an arc with surface quality better than λ

/2, the ground surface is developed with a fine grade emery and it is placed towards the camera objective.

59. A system as claimed in claim 44, wherein the CCD camera has a multi-element camera objective lens, designed for keeping the field of view of 20×

15 mm on a ½

″

CCD with a working distance of 95 mm from the glass plate, and with an optical reduction of 0.3 for a ½

″

CCD size camera.

60. A system as claimed in claim 44, wherein the CCD camera resolution required is fairly standard with 768×

574 pixels (after image acquisition) because the average size of the speckles can be calculated as 2R(λ

/D), where D is the diameter of the source of light (optical fibre core), and R is the distance between the source and plane of registration, the diameter of a light field is 2.NA.R, the entire number of light and dark speckles in a single pattern can be calculated as NA².D²/λ

², if formed by a standard multimode fibre with optical parameters NA=0.2, and D=50 μ

m, and using a Laser diode (λ

=0.6328 μ

m), and the number of such speckles in a single circular pattern is approx. 252.

61. A system as claimed in claim 44, wherein the spatial resolution available in the system comes out to be 26 μ

m (with a FOV of 20×

15 mm in 768×

574 resolution) while the average diameter of the individual grains comes out to be 300 μ

m (with 5 mm dia for each circular speckle pattern), which can be easily resolved with a standard CCD camera.

62. A system as claimed in claim 44, wherein the image digitized at 768×

574 resolution provides 12 segments of size 191×

191 pixels for each speckle pattern in a square, each segment in the image is corresponding to a zone, and identification in addition to measurement of perturbation amplitude and processing of the speckle pattern for quantification of the perturbation depends upon user application and requirement.

63. A system as claimed in claim 44, where the standard CCD camera with the above resolution easily senses the perturbation from 12 zones simultaneously where each speckle corresponds to a zone in the field.

64. A system as claimed in claim 44, wherein three such devices can be used together with image processing systems which allow three simultaneous camera video signals to be digitalized, thereby increasing the number of zones by three times.

65. A system as claimed in claim 44, wherein the sensing elements:

the multimode fibres are arranged as minimum 4×

3 form allowing 12 zones using a standard CCD camera for image acquisition.

66. A system as claimed in claim 44, wherein the resolution is decided by the fibre zone length and layout.

67. A system as claimed in claim 44, where in the system allows standard CCD camera for image acquisition and allows parallel and concurrent sensing of environment from at least 12 different zones.

68. A system as claimed in claim 44, wherein the system allows sensing of many different environment perturbation parameters at a time, very suitable for monitoring and controlling of various perturbation factors multiplexed using a single device.

69. A system as claimed in claim 44, wherein the total field of view is 20×

14 mm for 4×

3 multimode fibres and the resolution for each of 12 circular speckle pattern is 191×

191 pixels.

70. A system as claimed in claim 44, wherein the number of array elements given here are 4×

3=12, which can be increased by further reducing the spacing to the physical limits of the standard connectors and appropriately changing the spacing of glass plate.

71. A system as claimed in claim 44, wherein the more than one sensing modules can be used together for further increase of number of zones, this is possible by using typically acquiring three simultaneous video channels for three such sensing modules.