All-optical fiber faraday rotation current sensor with heterodyne detection technique

Claim 2

A sensor system for detecting an electric current, said system comprising: source means for providing an input beam of frequency-modulated linearly-polarized light; polarization modulation means for converting said input beam of frequency-modulated light into polarization-modulated light; current sensing means for imparting a Faraday rotation to said polarization-modulated light as a function of the magnitude of the electric current to be detected; and means for measuring said Faraday rotation of said polarization-modulated light to determine the magnitude of said electric current.

Claim 3

The sensor system of claim 1 wherein said current sensing means comprises: a Faraday rotation current sensor for passing said polarization-modulated light through a magnetic field produced by application of the electric current to be detected thereto, the lines of force of said magnetic field being generally parallel to the direction of propagation of said polarization-modulated light so as to thereby impart a Faraday rotation to said polarization-modulated light.

Claim 4

The sensor system of claim 1, wherein said source means comprises: means for developing a modulation current; laser means for producing an input beam of frequency-modulated light in response to said modulation current; means for linearly polarizing said input beam of frequency-modulated light; and high-birefringence optical fiber for conveying said input beam of linearly-polarized frequency-modulated light to said modulation means.

Claim 5

The sensor system claim 3 wherein said developing means comprises a ramp current modulator.

Claim 6

The sensor system of claim 3 wherein said laser means comprises a diode laser.

Claim 7

The sensor system of claim wherein said polarization modulation means comprises: a high birefringence optical fiber for receiving one polarization mode of said frequency-modulated light and for passing said one mode of frequency-modulated light therethrough; means for splitting said one polarization mode of frequency-modulated light into two modes of frequency-modulated light; means for providing a net polarization phase delay between said two modes of frequency-modulated light; and means for combining said two modes of frequency-modulated light after said delay is provided into one beam of polarization-modulated light linearly polarized at an azimuth angle which depends upon the net polarization phase delay of said phase delay means.

Claim 8

The sensor system of claim 6 wherein said splitting means comprises a 45° splice within said high birefringence optical fiber.

Claim 9

The sensor system of claim 6 wherein said high birefringence optical fiber has a polarization axis and wherein said combining means comprises an optical fiber λ/4 plate having an axis oriented +45° to said polarization axis of said high birefringence optical fiber, wherein λ is the average wavelength of said frequency-modulated light.

Claim 10

The sensor system of claim 5 wherein said means for providing a net polarization phase delay comprises a length of high birefringence optical fiber having two different effective optical paths corresponding to said two modes of said frequency-modulated light.

Claim 11

The sensor system of claim 2 wherein said current sensing means comprises a coil of ultra-low birefringence optical fiber for repeatedly passing said polarization-modulated light through said magnetic field.

Claim 12

The sensor system of claim 10 wherein the electric current to be detected is passed through a wire coil wrapped around at least a portion of said ultra-low birefringence optical fiber coil.

Claim 13

The sensor system of claim 1 wherein said measuring means comprises: an output fiber polarizer for receiving said Faraday rotated polarization-modulated light and for allowing only a linearly polarized portion to pass therethrough as an intensity-modulated output beam; and analyzer means for measuring the light intensity of said output beam and for determining the phase shift of said intensity modulated output beam to determine the magnitude of said electric current.

Claim 14

The sensor system of claim 12 wherein said intensity-modulated output beam forms a heterodyne carrier signal containing a linear phase modulation placed on said heterodyne carrier signal by said electric current, and wherein said analyzer means comprises: means for generating said heterodyne carrier signal from said intensity-modulated output beam; and means for measuring the phase modulation of said heterodyne carrier signal to determine the magnitude of said electric current.

Claim 17

The method of claim 14 wherein said imparting step includes the step of: passing said polarization-modulated light through a magnetic field, generated by the electric current to be detected, and comprising lines of force generally parallel to the direction of propagation of said polarization-modulated light to thereby impart a Faraday rotation to said polarization-modulated light.

Claim 18

A sensor system for detecting an electric current, said system comprising: source means for providing an input beam of frequency-modulated linearly-polarized light; polarization modulation means for converting said input beam of frequency-modulated light into polarization-modulated light; Faraday rotation current sensing means for passing said polarization-modulated light through a magnetic field, produced by application of the electric current to be detected to sensing means, the lines of force of said magnetic field being generally parallel to the direction of propagation of said polarization-modulated light, so as to thereby impart a Faraday rotation to said polarization-modulated light; and measurement means for measuring said Faraday rotation of said polarization-modulated light to thereby determine the magnitude of said current, said source means comprising: current generator means for providing modulation current; laser means for receiving said modulation current and producing an input beam of frequency-modulated light in response to said modulation current, linear polarizer means for linearly polarizing said input beam of frequency-modulated light; and high-birefringence optical fiber for receiving said input beam of linearly-polarized frequency-modulated light and conveying said input beam of linearly-polarized frequency-modulated light to said modulator means; said polarization modulation means comprising: a high birefringence optical fiber for receiving one mode of said frequency-modulated light and for passing said one mode of frequency-modulated light therethrough; splitting means for splitting said one mode of frequency-modulated light into two modes of frequency-modulated light; phase delay means for providing a net polarization phase delay between said two modes of frequency-modulated light; combining means for combining said two modes of frequency-modulated light after said delay is provided into one beam of polarization-modulated light linearly polarized at an azimuth angle which depends upon the net polarization phase delay of said phase delay means and the emission frequency of said laser means, said high birefringence optical fiber having a polarization axis and said combining means comprising a an optical fiber λ/4 plate having an axis oriented +45° to said polarization axis of said high birefringence optical fiber, wherein λ is the average wavelength; said phase delay means comprising a length of high birefringence optical fiber having two different effective optical paths corresponding to said two modes of said frequency modulated light, and said Faraday rotation current sensing means comprising a coil of ultra-low birefringence optical fiber for repeatedly passing said polarization-modulated light through said magnetic field; and said measurement means comprising: an output fiber polarizer for receiving said Faraday rotated polarization-modulated light and for allowing only linearly polarized portion to pass therethrough as an intensity-modulated output beam; and analyzer means for measuring the light intensity of said intensity-modulated output beam and determining the phase shift of said intensity-modulated output beam to determine the magnitude of said electric current.

Claim 19

A sensor system for detecting an electric current, said system comprising: source means for providing an input beam of frequency-modulated linearly-polarized light; polarization modulation means for converting said input beam of frequency-modulated light into polarization-modulated light; Faraday rotation current sensing means for passing said polarization-modulated light through a magnetic field, produced by application of the electric current to be detected to sensing means, the lines of force of said magnetic field being generally parallel to the direction of propagation of said polarization-modulated light, so as to thereby impart a Faraday rotation to said polarization-modulated light; and measurement means for measuring said Faraday rotation of said polarization-modulated light to thereby determine the magnitude of said current, said source means comprising: current generator means for providing modulation current; laser means for receiving said modulation current and producing an input beam of frequency-modulated light in response to said modulation current, linear polarizer means for linearly polarizing said input beam of frequency-modulated light; and high-birefringence optical fiber for receiving said input beam of linearly-polarized frequency-modulated light and conveying said input beam of linearly-polarized frequency-modulated light to said modulator means; said polarization modulation means comprising: a high birefringence optical fiber for receiving one mode of said frequency-modulated light and for passing said one mode of frequency-modulated light therethrough; splitting means for splitting said one mode of frequency-modulated light into two modes of frequency-modulated light; phase delay means for providing a net polarization phase delay between said two modes of frequency-modulated light; combining means for combining said two modes of frequency-modulated light after said delay is provided into one beam of polarization-modulated light linearly polarized at an azimuth angle which depends upon the net polarization phase delay of said phase delay means and the emission frequency of said laser means, said high birefringence optical fiber having a polarization axis and said combining means comprising a an optical fiber λ/4 plate having an axis oriented +45° to said polarization axis of said high birefringence optical fiber, wherein λ is the average wavelength; said phase delay means comprising a length of high birefringence optical fiber having two different effective optical paths corresponding to said two modes of said frequency modulated light, and said Faraday rotation current sensing means comprising a coil of ultra-low birefringence optical fiber for repeatedly passing said polarization-modulated light through said magnetic field; and said measurement means comprising: an output fiber polarizer for receiving said Faraday rotated polarization-modulated light and for allowing only linearly polarized portion to pass therethrough as an intensity-modulated output beam; and analyzer means for measuring the light intensity of said intensity-modulated output beam and determining the phase shift of said intensity-modulated output beam to determine the magnitude of said electric current.