Magneto-optical sensing employing phase-shifted transmission bragg gratings

• US 20030133657A1
• Filed: 12/11/2002
• Published: 07/17/2003
• Est. Priority Date: 12/11/2001
• Status: Active Grant

• Current Assignee: Kyton, LLC
• Original Assignee: Lake Shore Cryotronics, Inc.
• Inventors: Kochergin, Vladimir, Swinehart, Philip
• Granted Patent: US 7,106,919 B2
• Time in Patent Office: Days
• US Class Current: 385/37
• CPC Class Codes: G01N 2021/1727 Magnetomodulation
• CPC Class Codes: G01N 21/1717 with a modulation of one or more physical properties of the sample during the optical investigation, e.g. electro-reflectance
• CPC Class Codes: G01R 15/246 based on the Faraday, i.e. linear magneto-optic, effect
• CPC Class Codes: G01R 33/0322 using the Faraday or Voigt effect
• CPC Class Codes: G02B 6/02109 having polarization sensitive features, e.g. reduced photo-induced birefringence
• CPC Class Codes: G02B 6/124 Geodesic lenses or integrated gratings
• CPC Class Codes: G02B 6/4202 for coupling an active element with fibres without intermediate optical elements, e.g. fibres with plane ends, fibres with shaped ends, bundles
• CPC Class Codes: G02B 6/4246 Bidirectionally operating package structures
• CPC Class Codes: H01S 5/18366 Membrane DBR, i.e. a movable DBR on top of the VCSEL

Abstract

A new fiber-optic magnetic field or current sensor design can increase the accuracy, resolution and environmental stability of the subject sensor. The design is based on phase-shifted fiber or planar waveguide Bragg grating, in which a Fabry-Perot resonator is formed around the phase shift. When the wavelength of incident light coincides with the wavelength of FP resonator mode, the magnetic field induced polarization rotation of the waveguided light will be strongly enhanced.

Claims

1. Apparatus for detecting a magnitude of a physical parameter, the apparatus comprising:

a light source;

a magneto-optic Faraday effect sensing element comprising a fiber phase shifted Bragg grating;

a first polarizer disposed between light source and said magneto-optic Faraday effect sensing element;

a second polarizer (analyzer) optically coupled to the magneto-optic Faraday effect sensing element to detect the appearance of a polarization state different from the one transmitted through the first polarizer; and

an optical detector optically coupled to the second polarizer.

2. An apparatus as in claim 1 wherein the light source comprises a tunable laser.

3. An apparatus as in claim 2 wherein the light source comprises a tunable VCSEL.

4. An apparatus as in claim 1 wherein the light source comprises a broad band light source, and the apparatus may or may not include a circulator and fiber Bragg gratings or other wavelength filters, the filtering reflection characteristics of which coincide with the Bragg grating optical feature of the sensing element.

5. An apparatus as in claim 1 wherein the light source comprises a broadband light source chosen from the group, consisting of a light emitting diode (LED), a Superluminescent diode (SLD) or a lamp.

6. An apparatus as in claim 1 wherein the detector is chosen from the group consisting of a semiconductor photodiode or a balanced photodetector employing two photodiodes and a polarizing beam splitter.

7. An apparatus as in claim 6 wherein the detector comprises plural photodiodes with a polarization splitter to detect orthogonal polarization components transmitted through the sensing element light.

8. An apparatus as in claim 1 wherein the physical parameter is at least one selected from the group consisting of a magnetic field or the magnetic field generated by an electrical current, utilized to determine the electrical current characteristics.

9. An apparatus as in claim 1 wherein the phase-shifted fiber Bragg grating is written into fiber and is selected from the group consisting of communication single-mode fiber with symmetrical core (low birefringence), a polarization-maintaining single-mode fiber with highly asymmetric core (high-birefringence), or a specifically formulated high Verdet constant glass fiber.

10. An apparatus as in claim 1 wherein the phase-shifted fiber Bragg grating comprises a constant-period Bragg grating.

11. An apparatus as in claim 1 is constructed from two or more superimposed phase-shifted Bragg gratings to compensate for birefringence.

12. An apparatus as in claim 1 wherein the phase-shifted fiber Bragg grating uses at least one phase shift.

13. An apparatus as in claim 1 further including an environmental effects compensation feedback arrangement together with a tunable laser

14. An apparatus as in claim 1 wherein the magneto-optic Faraday effect sensing element includes flux concentrators that increase the magnitude of the magnetic field in the positions of the phase shift(s).

15. An apparatus for detecting a magnitude of a physical parameter comprising:

a light source;

a planar waveguide phase shifted Bragg grating operating as a magneto-optic Faraday effect sensing element,

a first polarizer disposed between light source and said magneto-optic Faraday effect sensing element;

a second polarizer (analyzer) optically coupled to the magneto-optic Faraday effect sensing element to detect the appearance of a polarization state different from the one transmitted through the first polarizer; and

an optical detector optically coupled to the second polarizer.

16. The apparatus as in claim 15 wherein the light source comprises a tunable laser.

17. The apparatus as in claim 16 wherein the light source comprises a tunable VCSEL.

18. The apparatus as in claim 15 wherein the light source comprises a broad band light source, circulator and fiber Bragg gratings (or other wavelength filters such as Fabry-Perot filters) the filtering reflection characteristics of which coincides with the Bragg grating characteristics of the sensing element.

19. The apparatus as in claim 15 wherein the broadband light source is chosen from the group, consisting of:

a light emitting diode (LED);

a Superluminescent diode (SLD);

a lamp.

20. The apparatus as in claim 15 wherein the detector is chosen from the group consisting of a semiconductor photodiode;

a balanced photodetector (two photodiodes with polarization splitter to detect opposite polarization components of transmitted through the sensing element light).

21. The apparatus as in claim 15 wherein the physical parameter is at least one parameter selected from the group consisting of a magnetic field;

and a current flowing though an electric conductor through the magnetic field generating by said current.

22. The apparatus as in claim 15 wherein the phase-shifted waveguide Bragg grating is written into a planar waveguide.

23. The apparatus as in claim 15 wherein the phase-shifted waveguide Bragg grating is selected from the group consisted of constant-period Bragg grating, or form two or more superimposed phase shifted Bragg gratings to compensate birefringence.

24. The apparatus as in claim 15 wherein the phase-shifted fiber Bragg grating wherein at least one phase shift is used.

25. The apparatus as in claim 15 wherein an environmental effects compensation feedback method is used together with a tunable laser.

26. The apparatus as in claim 15 wherein a magneto-optic Faraday effect sensing element incorporating flux concentrators is utilized in order to increase the value of magnetic field in the position of phase shift (or phase shifts).

27. The apparatus as in claim 15 wherein the waveguide is constructed such as that the waveguide optical mode is at least partially localized in the magneto-optically active material.

28. The apparatus as in claim 15 wherein the magneto-optically active magnetic material has an in-plane magnetization anisotropy.

29. The apparatus as in claim 15 wherein the magneto-optically active magnetic material has well-defined easy axis anisotropy, wherein said second hard magnetic material layer is disposed such that the in-plane hard axis of said layer of hard magnetic material lies in the plane of light incidence and collinear to an external magnetic field to be measured;

said hard magnetic material layer producing a uniform bias magnetic field in the plane of said Faraday-active magnetic material.

30. The apparatus as in claim 29 wherein said hard magnet is placed in the vicinity of the sensing element such that the magnetic field produced by said hard magnet is uniform in the Faraday-active magnetic material and lies in the plane of said Faraday-active magnetic material and the direction of the magnetic field produced by the at least one hard magnet is perpendicular to an external magnetic field to be measured;

said hard magnet producing a uniform bias magnetic field in the plane of said Faraday-active magnetic material.

31. The apparatus as in claim 15 wherein said Faraday-active magnetic material is magnetic garnet single crystal thin film.

32. The apparatus as in claim 15 wherein said Faraday-active magnetic material is constructed from two or more layers of magnetic garnet single crystal thin film with different compositions and magneto-optical properties;

33. The apparatus as in claim 15 wherein said Faraday-active magnetic material is any other transparent material with adequately high Faraday activity for its purpose.

34. An apparatus for detecting a magnitude of a physical parameter, the apparatus comprising:

a polarization maintaining optic fiber forming an optical path having a midpoint;

two linearly polarized light waves traveling in said polarization maintaining optic fiber over said optical path;

at least one fiber quarter wave plate coupled to said optic fiber at substantially said mid-point for converting said two linearly polarized light waves into two opposingly circularly polarized light waves traveling on said optical path toward a magneto-optic Faraday effect sensing element, said fiber quarter wave plate being constructed from at least one short section of long beat length polarization maintaining fiber;

said magneto-optic Faraday effect sensing element including a fiber phase shifted Bragg grating sensitive to a physical condition coupled to said optic fiber at generally said mid-point in said optical path, said opposingly circularly polarized light waves passing through said fiber phase shifted Bragg grating, and said physical condition causing a differential phase shift in said opposingly circularly polarized light waves; and

a detector coupled to said optic fiber detecting said differential phase shift and producing an output in response thereto.

35. An apparatus as in claim 34, wherein said phase-shifted fiber Bragg grating is written into an optical fiber and is selected from the group consisting of a communication single-mode fiber with symmetrical core (low birefringence) or a polarization-maintaining single-mode fiber with highly asymmetric core (high-birefringence).

36. An apparatus as in claim 34, wherein said phase-shifted fiber Bragg grating incorporates at least one phase shift.

37. An apparatus as in claim 34, wherein said polarization maintaining optic fiber forms a loop, said phase-shifted fiber Bragg grating being located substantially mid-point of said optic fiber loop, said optic fiber loop having two optic fiber portions extending from said phase-shifted fiber Bragg grating and being co-located with one another.

38. An apparatus as in claim 37, comprising first and second fiber quarter wave plates disposed in said optical path substantially adjacent to and in opposite sides of said phase-shifted fiber Bragg grating, converting two counter-propagating linearly polarized light waves into two counter-propagating circularly polarized light waves prior to entry into said phase-shifted fiber Bragg grating and converting said circularly polarized light waves into linearly polarized light waves after exiting said phase-shifted fiber Bragg grating.

39. An apparatus as in claim 37, further comprising a phase modulator coupled to said optic fiber and phase modulating said two counter-propagating linearly polarized light waves by changing a state of birefringence during travel time of said light waves.

40. An apparatus as in claim 34, further comprising a single-ended light path, not a loop, a reflector disposed in said optical path substantially adjacent to said phase-shifted fiber Bragg grating, said reflector reflecting said circularly polarized light waves back through said phase-shifted fiber Bragg grating, said reflector further reversing the sense of circular polarization of said light waves.

41. An apparatus as in claim 40, wherein said fiber quarter wave plate is disposed in said optical path substantially adjacent to said phase-shifted fiber Bragg grating, and converting two co-propagating linearly polarized light waves traveling along two axes of said optical path into co-propagating right and left hand circularly polarized light waves.

42. An apparatus as in claim 40, further comprising:

a birefringence modulator coupled to said optic fiber and phase modulating two co-propagating linearly polarized light waves; and

a fiber quarter wave plate disposed in said optical path adjacent to said phase-shifted fiber Bragg grating circularly polarizing said linearly polarized light waves.

43. An apparatus as in claim 42, wherein said birefringence modulator comprises an integrated optics waveguide with electro-optic modulation.

44. An apparatus as in claim 42, wherein said birefringence modulator comprises a polarization maintaining, fiber-wound piezoelectric modulator.

45. An apparatus as in claim 42, wherein said birefringence modulator operates at a fundamental dither frequency of ½

τ

, where τ

is the round trip propagation time between said birefringence modulator and said phase-shifted fiber Bragg grating.

46. An apparatus as in claim 40, further comprising:

a linear polarizer linearly polarizing said light; and

said fiber quarter wave plate positioned at 45°

with respect to said polarization maintaining optical fiber, thus circularly polarizing said linearly polarized light waves.

47. An apparatus as in claim 34 wherein the physical parameter is at least one kind selected from the group consisting of a magnetic field or an electrical current flowing though conductor, by means of the magnetic field generated by said current.

48. An apparatus for detecting a magnitude of a physical parameter comprising:

a polarization maintaining optic fiber forming an optical loop;

a phase-shifted fiber Bragg grating sensitive to a physical parameter coupled to said optic fiber and disposed about midway in said optical loop;

first and second quarter wave plates coupled to said optic fiber in close proximity to said phase-shifted fiber Bragg grating for converting two counter-propagating linearly polarized light waves into two counter-propagating circularly polarized light waves prior to passing through said phase-shifted fiber Bragg grating, and converting said circularly polarized light waves into linearly polarized light waves after exiting said phase-shifted fiber Bragg grating, said magnetic field inducing a differential phase shift in said circularly polarized light waves passing through said phase-shifted fiber Bragg grating, said first and second quarter wave plates being constructed from short sections of long beat length polarization maintaining fibers; and

a detector coupled to said optic fiber detecting said differential phase shift and producing an output in response thereto.

49. The apparatus as in claim 48, wherein said phase-shifted fiber Bragg grating is written into an optical fiber and is selected from the group, consisting of:

communication single-mode fiber with symmetrical core (low birefringent);

polarization-maintaining single-mode fiber with highly asymmetric core (high-birefringent).

50. The apparatus as in claim 48, wherein said polarization maintaining optic fiber extending from said phase-shifted fiber Bragg grating being co-located with one another.

51. The apparatus as in claim 48, further comprising:

a light source injecting a light into said optic fiber; and

a polarizer coupled to said optical fiber linearly polarizing said light.

52. The apparatus as in claim 48, further comprising a phase modulator disposed in said optical loop phase modulating said linearly polarized light waves.

53. An apparatus as in claim 48 wherein the physical condition is at least one kind selected from the group consisting of a magnetic field or electrical current flowing though a conductor determined by means of the magnetic field generated by said current.

54. A method for detecting a magnitude of a physical condition, comprising:

applying polarized light to a magneto-optic Faraday effect sensing element comprising a fiber phase-shifted Bragg grating; and

analyzing light received from said fiber phase shifted Bragg grating to detect the appearance of a polarization state different from the first one.

55. A method for detecting a magnitude of a physical condition, comprising:

propagating plural linearly polarized light waves over an optical path formed by a polarization-maintaining optical fiber;

converting, substantially at the midpoint of said optical path, said plural linearly polarized light waves into opposingly circularly polarized light waves traveling over said optical path;

passing said opposingly circularly polarized light waves through a fiber phase shifted Bragg grating, said physical condition causing a differential phase shift in said opposingly circularly polarized light waves at said grating; and

detecting said differential phase shift.