Wavelength coded resonant optical sensor

• US 4,775,214 A
• Filed: 10/01/1986
• Issued: 10/04/1988
• Est. Priority Date: 12/21/1983
• Status: Expired due to Term

• Current Assignee: Rosemount Incorporated (Emerson Electric Company)
• Original Assignee: Rosemount Incorporated (Emerson Electric Company)
• Inventors: Johnson, Lawrence A.
• Primary Examiner(s): Sikes, William L.
• Assistant Examiner(s): HEALY, BRIAN
• Application Number:
  US06/914,561
• Time in Patent Office: 734 Days
• Field of Search: 350/96.10, 350/96.11, 350/96.15, 350/96.12, 350/96.16, 350/96.13, 350/96.29, 350/96.14, 350/96.30, 350/370, 350/371, 350/380, 350/381, 350/382 , 350/384, 350/385, 250/227, 250/231 R, 250/231 P, 73/800
• US Class Current: 385/12
• CPC Class Codes: G01D 5/344 using polarisation G01D5/35303 takes precedence
• CPC Class Codes: G02B 6/29338 Loop resonators

Abstract

An optical sensor for providing an output signal representative of a sensed parameter such as pressure or temperature which includes an input/output optical waveguide and an optical resonator. The input/output waveguide is coupled to the resonator so that a significant change in light intensity at the output of the input/output waveguide occurs at resonance of the optical resonator. The optical resonator exhibits first and second independent resonant modes which differ from one another as a function of the parameter to be sensed. A narrow bandwidth variable frequency light source provides a frequency swept light beam to the input/output waveguide, and one or more photodetectors sense the intensity of the light beam at the output of the input/output waveguide to produce an output signal representative of the parameter based upon the time difference between occurrence of the first and second resonant modes.

Claims

1. An optical sensor system for providing an output signal representative of a sensed parameter, the optical sensor system comprising:

optical waveguide means for guiding a light beam, the optical waveguide means having an input and an output;

optical resonator means optically coupled to the optical waveguide means between the input and the output, the optical resonator means defining first and second resonant cavities which exhibit first and second sets of resonant frequencies, respectively;

the first and second cavities having characeteristics which are affected differently by changes in the parameter and which cause the first and second sets of resonant frequencies to vary differently in response to changes in the parameter, whereby a differential frequency shift occurs between resonant frequencies of the first and second sets which is a function of the parameter;

means for providing a light beam to the input which has a frequency which varies with time through a frequency range which includes at least one resonant frequency of each of the first and second sets; and

means responsive to the light beam from the output for providing the output signal as a function of a difference between resonant frequencies of the first and second sets.

2. The optical sensor system of claim 1 wherein the means responsive to the light beam provides the output signal as a function of a time difference between occurrence of resonant frequencies of the first and second sets.

3. The optical sensor system of claim 2 wherein the means for providing a light beam periodically varies the frequency of the light beam with time through the frequency range.

4. The optical sensor system of claim 3 wherein the means responsive to the light beam produces the output signal as a function of time between occurrences of resonant frequencies of the first and second sets and time between successive occurrences of the resonant frequencies of the first set.

5. The optical sensor system of claim 2 wherein the means responsive to the light beam comprises:

means for providing a first signal when an extremum in light intensity of the light beam corresponding to a resonant frequency of the first set occurs;

means for providing a second signal when an extremum in light intensity of the light beam corresponding to a second frequency of the second set occurs; and

means for providing the output signal as a function of the first and second signals.

6. The optical sensor system of claim 1 wherein the means for providing a light beam causes the light beam to contain portions having a first polarization and second polarization, wherein the optical resonator means has an effective birefringence which varies as a function of the parameter so that the optical resonator means exhibits the first and second sets of resonant frequencies for the portions of the light beam having the first and the second polarization, respectively, and wherein a difference between the resonant frequencies of the first and second sets is a function of the effective birefringence of the optical resonator means.

7. The optical sensor system of claim 6 wherein the optical resonator means comprises:

an optical ring resonator; and

a substrate for supporting the optical ring resonator.

8. The optical sensor system of claim 7 wherein the substrate is a diaphragm.

9. The optical sensor system of claim 8 and further comprising:

support means for supporting the diaphgram at an outer edge.

10. The optical sensor system of claim 9 wherein the first polarization corresponds to a TE polarization mode, and wherein the second polarization corresponds to a TM polarization mode.

11. The optical sensor system of claim 6 wherein the means responsive to the light beam comprises:

means for separating the light beam into a first beam having the first polarization and a second beam having the second polarization;

means for providing a first signal in response to a change in intensity of the first beam corresponding to an occurrence of resonance;

means for providing a second signal in response to a change in intensity of the second beam corresponding to an occurrence of resonance; and

means for providing the output signal as a function of a time difference between occurrence of the first and second signals.

12. The optical sensor system of claim 1 wherein the optical resonator means comprises:

a first ring resonator having a first perimeter and exhibiting the first set of resonant frequencies; and

a second ring resonator having a second, different perimeter and exhibiting the second set of resonant frequencies.

13. The optical sensor system of claim 12 and further comprising a substrate for supporting the first and second ring resonators proximate one another.

14. The optical sensor system of claim 13 wherein the first and second ring resonators are concentrically arranged.

15. The optical sensor system of claim 12 wherein the first and second ring resonators have different efficiencies so that intensity of the light beam at the output decreases by different amounts when resonant frequencies of the first and second sets occur.

16. The optical sensor system of claim 15 wherein the means responsive to the light beam comprises:

means for providing a first signal when the light beam exhibits a decrease in intensity at the output corresponding to occurrence of a resonant frequency of the first set;

means for providing a second signal when the light beam exhibits a decrease in intensity at the output corresponding to occurrence of a resonant frequency of the second set; and

means for providing the output signals as a function of a time difference between occurrence of the first and second signals.

17. The optical sensor system of claim 1 wherein the optical resonator means comprises an optical cube resonator.

18. The optical sensor system of claim 1 wherein the optical resonator means comprises a solid plate etalon resonator.

19. The optical sensor system of claim 1 wherein the optical resonator means comprises an integrated optic linear resonator.

20. The optical sensor system of claim 1 wherein the optical resonator means comprises an integrated optic ring resonator.

21. An optical sensor system for providing an output signal representative of a sensed parameter, the optical sensor comprising:

light source means for providing a light beam having a frequency which varies with time through a frequency range;

waveguide means for guiding the light beam between an input and an output;

first optical resonator means coupled to the waveguide means between the input and the output for exhibiting a first resonant condition in which a portion of the light beam is absorbed when the light beam has one of a first set of resonant frequencies;

second optical resonator means for exhibiting a second resonant condition in which a portion of the light beam is absorbed when the light beam has one of a second set of resonant frequencies, where the first and second optical resonator means have characteristics which are affected differently by the changes in the parameter which cause the first set and second sets to vary differently in response to a change in the parameter so that a difference between the resonant frequencies of the first and second sets of resonant frequencies varies as a function of the parameters; and

means for receiving the light beam from the output and providing the signal as a function of the difference between the resonant frequencies of the first and second sets.

22. The optical sensor system of claim 21 wherein the means for receiving the light beam comprises:

means for producing a first signal indicative of occurrence of the first resonant condition;

means for producing a second signal indicative of occurrence of the second resonant condition; and

means for providing the output signal as a function of a time difference between occurrence of the first and second output signals.

23. The optical sensor system of claim 21 wherein the first and second resonator means comprise:

a substrate; and

an optical ring resonator supported by the substrate, the optical ring resonator having an effective birefringence which varies as a function of the parameter, so that the optical ring resonator exhibits the first resonant condition for a first polarization of the light beam and exhibits the second resonant condition for a second polarization of the light beam.

24. The optical sensor system of claim 23 wherein the substrate is a pressure responsive diaphragm.

25. The optical sensor system of claim 24 and further comprising:

support means for supporting the diaphragm at an outer edge of the diaphragm.

26. The optical sensor system of claim 21 wherein the first and second optical resonator means are first and second optical ring resonators having first and second different perimeters, respectively.

27. The optical sensor system of claim 21 wherein the light beam has first and second polarization components, and wherein the first and second optical resonator means comprise an optical cube resonator which exhibits first and second indices of refraction which vary differently as a function of the sensed parameter.

28. The optical sensor system of claim 21 wherein the light beam has first and second polarization components, and wherein the first and second optical resonator means comprise a solid plate etalon resonator which exhibits first and second indices of refraction which vary differently as a function of the sensed parameter.

29. The optical sensor system of claim 21 wherein the light beam has first and second polarization components, and wherein the first and second optical resonator means comprise an integrated optic linear resonator which exhibits first and second effective indices of refraction which vary differently as a function of the sensed parameter.

30. An optical sensor system for providing an output signal representative of a parameter, the optical sensor comprising:

a variable frequency light source for providing a light beam which periodically varies in frequency within a frequency range in a predetermined manner with time;

optical resonator means defining first and second resonant cavities for exhibiting first and second sets of resonant frequencies, respectively, the first and second resonant cavities having characteristics which are affected differently by changes in the parameter and which cause the first and second sets of resonant frequencies to vary differently as a function of the parameter, whereby a differential frequency shift occurs between resonant frequencies of the first and second sets which is a function of the parameter;

input/output waveguide means for guiding the light beam from an input to an output, the input/output waveguide means being coupled to the optical resonator means so that intensity of the light beam at the output changes when the light beam has a resonant frequency of the first set and when the light beam has a resonant frequency of the second set; and

output means responsive to changes in intensity of the light beam at the output for providing the output signal as a function of a time difference between occurrence of the resonant frequencies of the first and second sets.

31. An optical sensor for sensing a parameter, the optical sensor comprising:

optical resonator means defining first and second resonant cavities for exhibiting first and second sets of resonant frequencies, respectively, the first and second resonant cavities having characteristics which are affected differently by changes in the parameter and which cause the first and second sets of resonant frequencies to vary differently as a function of the parameter, whereby a differential frequency shift occurs between resonant frequencies of the first and second sets which is a function of the parameter;

input/output waveguide means for guiding a light beam from an input to an output, the input/output waveguide means being coupled to the optical resonator means so that intensity of the light beam at the output changes when the light beam has a frequency approximately equal to a resonant frequency of the first set and when the light beam has a frequency approximately equal to a resonant frequency of the second set; and

output means responsive to changes in intensity of the light beam at the output for providing an output signal representative of the parameter based upon a difference in the resonant frequencies of the first and second set.

32. The optical sensor of claim 31 wherein the optical resonator means has an effective birefringence which varies as a function of the parameter so that the optical resonator means exhibits the first and second sets of resonant frequencies for first and second polarization components of the light beam, respectively, and wherein a difference between resonant frequencies of the first and second sets is a function of the effective birefringence of the optical resonator means.

33. The optical sensor of claim 32 wherein the optical resonator means comprises:

an optical ring resonator; and

a substrate for supporting the optical ring resonator.

34. The optical sensor of claim 33 wherein the substrate is a diaphragm.

35. The optical sensor system of claim 34 and further comprising:

support means for supporting the diaphragm at an outer edge.

36. The optical sensor of claim 31 wherein the optical resonator means comprises:

a first optical ring resonator having a first perimeter and exhibiting the first set of resonant frequencies; and

a second optical ring resonator having a second, different perimeter and exhibiting the second set of resonant frequencies.

37. The optical sensor of claim 36 and further comprising a substrate for supporting the first and second optical ring resonators proximate one another.

38. The optical sensor of claim 37 wherein the first and second optical ring resonators are concentrically arranged.

39. The optical sensor of claim 36 wherein the first and second optical ring resonators have different efficiencies so that intensity of the light beam at the output decreases by different amounts when resonant frequencies of the first and second sets occur.

40. The optical sensor of claim 31 wherein the optical resonator means comprises an optical cube resonator.

41. The optical sensor of claim 31 wherein the optical resonator means comprises a solid plate etalon resonator.

42. The optical sensor of claim 31 wherein the optical resonator means comprises an integrated optic linear resonator.

43. The optical sensor of claim 31 wherein the optical resonator means comprises an integrated optic ring resonator.

44. A pressure monitor, comprising:

pressure responsive means including a diaphragm having a waveguide loop located on one surface thereof, said diaphragm deflecting under pressure to cause stress-induced birefringence in said waveguide loop;

the waveguide loop defining first and second resonant cavities exhibiting first and second sets of resonant frequencies which vary depending on the amount of stress-induced birefringence in said waveguide loop;

optical input means positioned adjacent said waveguide loop for coupling light energy into said waveguide loop;

output means also positioned adjacent said waveguide loop for coupling a portion of light energy out of said waveguide loop; and

whereby a differential frequency shift between resonant frequencies of the first and second sets occurs in the spectral response of light energy resonating in said first and second resonant cavities of said waveguide loop as a result of said stress-induced birefringence which is proportional to pressure acting on said diaphragm.

45. A pressure monitor according to claim 44, wherein said input means includes an input waveguide formed along said one surface of said pressure responsive means, and wherein said input waveguide has a point which is tangentially located with said waveguide loop through which optical energy is coupled into said waveguide loop.

46. A pressure monitor according to claim 44, wherein said output means further includes an output waveguide formed along said one surface of said pressure responsive means, and includes a point tangentially located with respect to said waveguide loop through which a portion of optical energy therein is coupled out of said waveguide loop.

47. A pressure monitor according to claim 44, wherein said pressure responsive means includes a rigid rim portion which rigidly holds the cicumference of said diaphragm.

48. A pressure measuring system comprising:

pressure responsive means including a diaphragm having a waveguide loop located on one surface thereof, said diaphragm causing a stress-induced birefringence in said waveguide loop in response to pressure acting on a surface thereof;

the waveguide loop defining first and second resonant cavities exhibiting first and second sets of resonant frequencies which vary depending on the amount of stress-induced birefringence in said waveguide loop;

source means positioned to couple optical energy into said waveguide loop; and

frequency detector means positioned to receive light energy coupled out of said waveguide loop; and

whereby a differential frequency shift between resonant frequencies of the first and second sets occurs in the spectral response of light energy resonating in said first and second resonant cavities of said waveguide loop as a result of said stress-induced birefringence which is proportional to pressure acting on said diaphragm.

49. An optical sensor for sensing a parameter, the optical sensor comprising:

optical resonator means defining first and second resonant cavities for exhibiting first and second sets of resonant frequencies, respectively, the first and second cavities having characteristics which are affected differently by changes in the parameter and which cause the first and second sets of resonant frequencies to vary differently as a function of the parameter, whereby a differential frequency shift occurs between resonant frequencies of the first and second sets which is a function of the parameter;

means for coupling light energy into the optical resonator means; and

means responsive to light energy coupled out of the optical resonator means for providing an output signal representative of the parameter based upon the differential frequency shift.

50. The optical sensor of claim 49 wherein the optical resonator means comprises:

an optical ring resonator; and

a substrate for supporting the optical ring resonator.

51. The optical sensor of claim 50 wherein the parameter is pressure and the substrate is a diaphragm.

52. The optical sensor system of claim 51 and further comprising:

support means for supporting the diaphragm at an outer edge.

53. The optical sensor of claim 49 wherein the optical resonator means has an effective birefringence which varies as a function of the parameter so that the optical resonator means exhibits the first and second sets of resonant frequencies for the portions of the light energy having the first and the second polarization, respectively, and wherein the differential frequency shift is a function of the effective birefringence of the optical resonator means.