Fiber optic detection system using a Sagnac interferometer

• US 5,046,848 A
• Filed: 09/08/1989
• Issued: 09/10/1991
• Est. Priority Date: 09/08/1989
• Status: Expired due to Term

• Current Assignee: Mcdonnell Douglas Corporation (The Boeing Co.)
• Original Assignee: Mcdonnell Douglas Corporation (The Boeing Co.)
• Inventors: Udd, Eric
• Primary Examiner(s): TURNER, SAMUEL A
• Application Number:
  US07/405,876
• Time in Patent Office: 732 Days
• Field of Search: 356/345, 356/350, 356/351, 356/349, 250/227.19
• US Class Current: 356/483
• CPC Class Codes: G01D 5/35303 using a reference fibre, e.g. interferometric devices

Abstract

The process and apparatus of at least two sensor configurations enable both the location and severity of an environmental disturbance on an optic path to be measured.

Claims

1. A fiber optic sensor, comprising:

optical path means for facilitating the propagation of light, said optical path means having first and second ends;

light emitting means for producing light energy;

first beam splitting means, having first, second and third ports, for directionally transporting light energy incident on said first port to said second port, and for directionally transporting light energy incident on said second port to said third port, said first port coupled to said light emitting means;

second beam splitting means having first, second, and third ports for directionally transporting light energy incident on said first port to said second and third ports, and for directionally transporting light energy incident on said second and third ports to said first port, said first port of said second beam splitting means coupled to said second port of said first beam splitting means said second port of said second beam splitting means being connected to said first end of said optical path means, said third port of said second beam splitting means being connected to said second end of said optical path means;

third beam splitting means having first, second and third ports, for directionally transporting light energy incident on said first port to said second port, and for directionally transporting light energy incident on said second port to said first and third ports, said first port of said third beam splitting means being coupled to said second port of said second beam splitting means;

phase modulation means connected along said optical path means and being coupled to said second port of said third beamsplitting means, for impressing a modulation frequency on light propagating in both directions along said optical path means, said phase modulation means being at least slightly offset from the center of said optical path means with respect to said second beam splitting means;

first phase sensitive detection means, optically connected to said third port of said first beam splitting means, and also connected to said phase modulation means, for detecting light from said first beam splitting means;

second phase sensitive detection means, optically connected to said third port of said third beam splitting means and to said light emitting means, for detecting light from said third beam splitting means, said second phase sensitive detection means comprising;

a detector, optically connected to said third port of said third beam splitting means, for detecting light from said third beam splitting means and producing a signal indicative of the characteristics of said second beam of light;

a phase sensitive detector, electrically connected to said detector, for demodulating said phase modulated frequency impressed upon said light propagating along said optical path means;

an integrator having an input connected to said phase sensitive detector and an output; and

variable oscillation means, having an input coupled to said output of said integrator and a first output coupled to said phase sensitive detector of said second phase sensitive detection means, a second output coupled to said light emitting means, and a third output connected to a signal processor means, said processor means being connected to said first and said second phase sensitive detection means for providing a position output and an amplitude output for the location and severity, respectively, of an environmental disturbance along said optical path means.

2. The fiber optic sensor of claim 1 wherein said light emitting means further comprises a superradiant diode.

3. The fiber optic sensor of claim 1 wherein said light emitting means further comprises a light emitting diode.

4. The fiber optic sensor of claim 1 wherein said optical path means further comprises an optical fiber.

5. The fiber optic sensor of claim 1 wherein said phase modulation means further comprises an integrated optic modulator.

6. The fiber optic sensor of claim 1 wherein said first phase sensitive detection means further comprises:

an oscillator connected to said phase modulation means;

a phase sensitive detector connected to said oscillator; and

,

a detector electrically connected to said phase sensitive detector of said first phase sensitive detection means and optically coupled to said third port of said first beam splitting means.

7. The fiber optic sensor of claim 6 wherein said phase modulator is an integrated optic modulator.

8. The fiber optic sensor of claim 1 wherein said first, second and third beam splitting means further comprise fiber optic beamsplitters.

9. The fiber optic sensor of claim 6 wherein said first and said second beam splitting means each further comprises a fiber optic beamsplitter.

10. The fiber optic sensor of claim 1 wherein said optical path means further comprises at least one optical fiber.

11. The optic sensor of claim 1 further comprising:

polarization means, optically coupled to said second port of said first beam splitting means, for polarizing said light produced from said light emitting means; and

,

intensity maintenance means, optically coupled to said polarization means and to said first port of said second beam splitting means, for maintaining the power of said light emanating from said light emitting means.

12. The optic sensor of claim 1 wherein said light emitting means further comprises:

a light source current driver having an input coupled to said second phase sensitive detection means, and an output; and

,

a light source having an input coupled to said output of said light source current driver, and an output coupled to said first port of said first beam splitting means.

13. A fiber optic sensor, comprising:

optical path means for facilitation the propagation of light, said optical path means having first and second ends;

light emitting means for producing light energy;

first beam splitting means, having first, second and third ports, for directionally transporting light energy incident on said first port to said second port, and for directionally transporting light energy incident on said second port to said third port, said first port coupled to said light emitting means;

second beam splitting means having first, second, and third ports for directionally transporting light energy incident on said first port to said second and third ports, and for directionally transporting light energy incident on said second and third ports to said first port, said first port of said second beam splitting means coupled to said second port of said first beam splitting means, said second port of said second beam splitting means being connected to said first end of said optical path means, said third port of said second beam splitting means being connected to said second end of said optical path means;

third beam splitting means having first, second and third ports, for directionally transporting light energy incident on said first port to said second port, and for directionally transporting light energy incident on said second port to said first and third ports, said first port of said third beam splitting means being coupled to said second port of said second beam splitting means;

phase modulation means connected along said optical path means and being coupled to said second port of said third beamsplitting means, for impressing a modulation frequency on light propagating in both directions along said optical path means, said phase modulation means being at least slightly offset from the center of said optical path means with respect to said second beam splitting means;

first phase sensitive detection means, optically connected to said third port of said first beam splitting means, and also connected to said phase modulation means, for detecting light from said first beam splitting means;

second phase sensitive detection means, optically connected to said third port of said third beam splitting means and to said light emitting means, for detecting light from said third beam splitting means, and

processor means, connected to said first and said second phase sensitive detection means, for providing a position output and an amplitude output for the location and severity, respectively, of an environmental disturbance along said optical path means;

said light emitting means further comprising;

a light source current driver having an input coupled to said second phase sensitive detection means, and an output; and

a light source having an input coupled to said output of said light source current driver, and an output coupled to said first port of said first beam splitting means.

14. A fiber optic sensor, comprising:

light emitting means for producing a first beam of light;

first beam splitting means, optically connected to said light emitting means, for receiving light and transmitting said first beam of light from said light emitting means and for coupling said received light in at least one other direction;

second beam splitting means, having a first port optically connected to said first beam splitting means and a second and third port, for receiving said first beam of light into said first port from said first beam splitting means and splitting said first beam of light into a second and a third beam of light into said second and said third ports, and for recombining said second and said third beams of light into a fourth beam of light;

third beam splitting means optically connected to said second port of said second beam splitting means for coupling a portion of said second beam of light away from said second beam splitting means;

optical path means, connected to said second beam splitting means, for directing said second and said third beams of light in counterpropagating directions;

phase modulation means, optically connected to said optical path means and to said third port of said second beam splitting means, for impressing a phase difference between said second and said third beams of light

oscillating means, connected to said phase modulation means, for generating a frequency signal and transmitting said frequency signal to said phase modulating means;

first detector means, connected to said first beam splitting means, for sensing said fourth beam of light;

first phase sensitive detection means, connected to said oscillating means and first detector means, for demodulating an environmental signal detected by said first detector means;

second detector means, connected to said third beam splitting means, for sensing said third beam of light;

second phase sensitive detection means, connected to said second detector means, for demodulating an environmental signal detected by said second detector means;

variable oscillating means, connected to said second phase sensitive detection means and said light emitting means, for generating and transmitting a variable second frequency signal to said light source means;

integration means, having an input connected to said second phase sensitive detection means and having an output connected to said variable oscillating means, for generating a signal, said signal being the integral of the output of said second phase sensitive detection means and for providing said signal to said variable oscillating means; and

,

signal processing means, having inputs connected to said variable oscillating means and said first phase sensitive detecting means, for generating a position signal and an amplitude signal indicative of the location and severity of an environmental disturbance along said optical path means.

15. The fiber optic sensor of claim 14 wherein said first, said second and said third beam splitting means further comprises a fiber optic beamsplitter.

16. The fiber optic sensor of claim 15 wherein said light emitting means further comprises a light emitting diode.

17. The fiber optic sensor of claim 15 wherein said light emitting means further comprises a superradiant diode.

18. The fiber optic sensor of claim 17 wherein said optical path means further comprises at least one optical fiber.

19. The fiber optic sensor of claim 18 wherein said phase modulation means further comprises:

a piezoelectric core; and

an optical fiber surrounding said piezoelectric core.

20. The optic sensor of claim 14 further comprising polarizing means, optically connected between said first beam splitting means and said second beam splitting means, for polarizing said light produced from said light emitting means.

21. The fiber optic sensor of claim 20 wherein said first, said second, and said third beam splitting means further comprises fiber optic beamsplitters.

22. The fiber optic sensor of claim 21 wherein said optical path means further comprises at least one optical fiber.

23. The fiber optic sensor of claim 22 wherein said phase modulation means further comprises:

a piezoelectric core; and

an optical fiber surrounding said piezoelectric core.

24. The fiber optic sensor of claim 23 wherein said light emitting means further comprises a light emitting diode.

25. The fiber optic sensor of claim 23 wherein said light emitting means further comprises a superradiant diode.

26. The optic sensor of claim 20 further comprising intensity maintenance means, optically connected between said polarizing means and said second beam splitting means, for maintaining the intensity of said first beam of light produced from said light emitting means.

27. The fiber optic sensor of claim 26 wherein said first and said second beam splitting means further comprises fiber optic beamsplitters.

28. The fiber optic sensor of claim 27 wherein said optical path means further comprises at least one optical fiber.

29. The fiber optic sensor of claim 28 wherein said phase modulation means further comprises:

a piezoelectric core; and

,

an optical fiber surrounding and piezoelectric core.

30. The fiber optic sensor of claim 29 wherein said light emitting means further comprises a light emitting diode.

31. The fiber optic sensor of claim 29 wherein said light emitting means further comprises a superradiant diode.

32. A fiber optic sensor, comprising:

first light emitting means for producing a first beam of light;

first beam splitting means, optically connected to said first light emitting means, for receiving light from said first light emitting means, and for directing light in at least two directions other than said first light emitting means;

second beam splitting means, optically connected to said first beam splitting means, for receiving said first beam of light from said first light emitting means through said first beam splitting means, and for splitting said received light into a second and a third beam of light, and for recombining said first and said second beams into a fourth beam of light;

fiber optic path means, optically connected to said second beam splitting means, for directing said first and said second beams in counterpropagating directions along a common path;

third beam splitting means, optically connected along and near one end of said fiber optic path;

phase modulation means, having an input and optically connected along and near one end of said fiber optic path means, for impressing a first modulation frequency on one of said second and third beams of light as it enters said fiber optic path means and upon the other of said second and third beams of light as it leaves said fiber optic path means;

first phase sensitive detection means, optically connected to said first beam splitting means and electrically connected to said phase modulation means, for detecting light from said first beam splitting means;

second phase sensitive detection means, optically connected to said second beam splitting means, for detecting said fourth beam of light from said second beam splitting means;

light modulation means, connected to said second phase sensitive detection means and optically connected to said third beam splitting means, for coupling an amplitude modulated beam of light into said fiber optic path means;

processor means, connected to said first and said second phase sensitive detection means, for providing a position output and an amplitude output.

33. A fiber optic sensor, comprising:

Optical path means for facilitating the propagation of light said optical path means having first and second ends;

first light emitting means for producing light energy;

first beam splitting means, having first, second and third ports, for directionally transporting light energy incident on said first port to said second port, and for directionally transporting light energy incident on said second port to said third port, said first port coupled to said first light emitting means;

Second beam splitting means having first, second, and third ports for directionally transporting light energy incident on said first port to said second and third ports, and for directionally transporting light energy incident on said second and third ports to said first port, said first port of said second beam splitting means coupled to said second port of said first beam splitting means, and wherein said second port of said second beam splitting means is coupled to said first end of said optical path means;

Third beam splitting means having first, second and third ports, for directionally transporting light energy incident on said first port to said second port, and for directionally transporting light energy incident on said second port to said first port, and for directionally transporting light energy incident on said third port to said second port, said second port of said third beam splitting means coupled to said second end of said optical path means;

phase modulation means connected to said first port of said third beam splitting means and to said third port of said second beam splitting means, for impressing a modulation frequency on light as it exits said second end of said optical path means and on light as it enters said second end of said optical path means, said phase modulation means at least slightly offset from the center of said optical path means with respect to said second beam splitting means;

second light emitting means, coupled to said third port of said third beam splitting means, for producing light energy;

first phase sensitive detection means, optically connected to said third port of said first beam splitting means, and also connected to said phase modulation means, for detecting light from said first beam splitting means;

second phase sensitive detection means, optically connected to said second port of said second beam splitting means and to said second light emitting means, for detecting light from said third beam splitting means and modulating said second light emitting means; and

,

processor means, connected to said first and said second phase sensitive detection means, for providing a position output and an amplitude output for the location and severity, respectively, of an environmental disturbance along said optical path means.

34. The fiber optic sensor of claim 33 wherein said second phase sensitive detection means further comprises:

a detector having an input connected to said second port of said second beam splitting means, and an output;

a phase sensitive detector having a first input connected to the output of said detector, a second input, a first output connected to said signal processing means, and a second output;

an integrator having an input connected to said second outPut of said phase sensitive detector, and an output; and

,

a light modulation variable frequency oscillator having an input connected to said output of said integrator, a first output connected to said second input of said phase sensitive detector, and a second output connected to said second light emitting means.

35. The fiber optic sensor of claim 33 wherein said first phase sensitive detection means further comprises:

a detector connected to said third port of said first beam splitting means; and

,

a phase sensitive detector having a first input electrically connected to said detector, a second input, and an output electrically connected to said signal processing means; and

,

an oscillator having a first output connected to said second input of said phase sensitive detector, and a second output connected to said input of said phase modulation means.

36. The fiber optic sensor of claim 33, wherein said phase modulation means is an integrated optic modulator.

37. The fiber optic sensor of claim 34, wherein said phase modulation means is an integrated optic modulator.

38. The fiber optic sensor of claim 33 wherein first, second and third beam splitting means further comprises a fiber optic beamsplitters.

39. The fiber optic sensor of claim 33 wherein said fiber optic path means further comprises at least one optical fiber.

40. The fiber optic sensor of claim 33 wherein said first and second light emitting means further comprises a light emitting diode.

41. The fiber optic sensor of claim 36 wherein said first and second light emitting means further comprises a superradiant diode.

42. The optic sensor of claim 36 further comprising:

polarization means, optically coupled to said second port of said first beam splitting means, for polarizing said light produced from said light emitting means; and

,

intensity maintenance means, optically coupled to said polarization means and to said first port of said second beam splitting means, for maintaining the power of said light emanating from said light emitting means.

43. The fiber optic sensor of claim 42 wherein said polarization means further comprises a fiber optic polarizer.

44. The fiber optic sensor of claim 42 wherein said intensity maintenance means further comprises a fiber optic intensity maintenance element.

45. The process of detecting an environmental disturbance along a fiber optic path comprising the steps of:

generating a beam of light modulated with a first frequency;

splitting said modulated beam of light into a first beam of light and into a second beam of light;

splitting said first beam of light into a third clockwise propagating beam of light and into a fourth counterclockwise propagating beam of light, both along said fiber optic path;

phase modulating said third and said fourth beams of light with a second frequency near one end of their counterpropagating fiber optic path;

recombining said third and said fourth beams of light into a fifth beam of light;

detecting said phase of said first frequency of modulation within said fifth beam of light;

detecting said phase of said second frequency of modulation within said fourth beam of light; and

,

computing the intensity and location of an environmental disturbance along said fiber optic path utilizing said detected phases of said first and said second frequencies of modulation.

46. The process of detecting an environmental disturbance along a fiber optic path as recited in claim 45, wherein said detecting of said phase of said first frequency of modulation step and said detecting of said phase of said second frequency of modulation step are performed simultaneously.