Technique of reducing the Kerr effect and extending the dynamic range in a Brillouin fiber optic gyroscope

• US 5,351,252 A
• Filed: 02/26/1993
• Issued: 09/27/1994
• Est. Priority Date: 02/26/1993
• Status: Expired due to Term

• Current Assignee: Board of Trustees of the Leland Stanford Junior University (Stanford University)
• Original Assignee: Board of Trustees of the Leland Stanford Junior University (Stanford University)
• Inventors: Huang, Shangyuan, Toyama, Keiichiro, Shaw, Herbert J., Kim, Byoung Y.
• Primary Examiner(s): Lee, John D.
• Application Number:
  US08/023,672
• Time in Patent Office: 578 Days
• Field of Search: 356/349, 356/350, 372/3, 372/6, 372/9, 372/26, 372/29-31, 372/33, 372/94
• US Class Current: 372/29.021
• CPC Class Codes: G01C 19/727 using a passive ring resonator
• CPC Class Codes: H01S 3/302 in an optical fibre

Abstract

A Brillouin fiber optic gyroscope having a feedback system which monitors the difference between counterpropagating Brillouin intensities and utilizes this difference in the form of a correction signal to control one of the circulating pump intensities so as to equalize the circulating pump intensities. The Brillouin fiber optic gyroscope further includes a second feedback system which detects electrical signals proportional to the phase-modulated, counterpropagating intensities in the gyroscope, and utilizes a combination of the electrical signals as an error signal to stabilize the resonant cavity at a length substantially equal to a length midway between the resonant lengths of the counterpropagating pump signals. The Brillouin fiber optic gyroscope of the present invention also provides a dynamic range of the gyroscope rotation rate that is twice the dynamic range of existing gyroscopes.

Claims

1. An apparatus for controlling the clockwise and counterclockwise circulating Brillouin intensities in a Brillouin fiber optic gyroscope, comprising:

means for detecting a difference between said clockwise circulating Brillouin intensity and said counterclockwise circulating Brillouin intensity; and

means responsive to said difference for controlling said clockwise and counterclockwise circulating Brillouin intensities.

2. An apparatus for controlling the difference between counterpropagating Brillouin intensities in a Brillouin fiber optic gyroscope responsive to counterpropagating pump signals in a cavity, comprising:

a coupler that taps light from said cavity;

a first detector that detects a portion of the circulating Brillouin intensities traveling in a first direction in said cavity;

a second detector that detects a portion of the circulating Brillouin intensities traveling in a second direction in said cavity;

a differential amplifier that provides an output signal responsive to a difference in intensities detected by said first and second detectors;

an integrator that integrates the output signal from said differential amplifier and provides an integrated output signal; and

an intensity attenuator responsive to said integrated output signal that selectively attenuates the pump light input to propagate in one direction in said cavity.

3. An apparatus for stabilizing the intensities of counterpropagating signals within the cavity of a Brillouin fiber optic gyroscope, comprising:

means for introducing counterpropagating pump signals into said cavity, said counterpropagating pump signals having a predetermined frequency selected so that said cavity is resonant to said predetermined frequency when said cavity is at rest, said counterpropagating pump signals causing the generation of counterpropagating Brillouin signals within said cavity, said counterpropagating Brillouin signals having first and second frequencies, said first and second frequencies having a frequency difference responsive to rotation of said cavity; and

means for actively adjusting the length of said cavity when said cavity is rotated to maintain said cavity at a length midway between a resonant length for said first frequency and a resonant length for said second frequency.

4. An apparatus for stabilizing the intensities of counterpropagating signals within the resonant cavity of a Brillouin fiber optic gyroscope, comprising:

means for introducing counterpropagating pump signals into said resonant cavity, said counterpropagating pump signals having a predetermined frequency, said resonant cavity having a first length when said resonant cavity is at rest, said first length selected to be substantially equal to a resonant length corresponding to said predetermined frequency, said resonant cavity having first and second effective lengths for said counterpropagating pump signals when said resonant cavity is rotating, said first effective length being greater than said first length and said second effective length being less than said first length;

means for detecting intensities of said counterpropagating pump signals and generating detected signals responsive thereto, said intensities varying as said resonant cavity is rotated and said first and second effective lengths change such that said predetermined frequency of said counterpropagating pump signals is no longer at resonance for either of said counterpropagating pump signals; and

means responsive to said detected signals for generating a stabilization signal to adjust said length of said resonant cavity to a second length and thereby adjust said first and second effective lengths of said resonant cavity such that said first and second effective lengths differ from said first length by a substantially equal amount, said second length being substantially midway between said first and second effective lengths.

5. An apparatus for stabilizing the intensities of counterpropagating pump signals within a resonant cavity of a Brillouin fiber optic gyroscope, comprising:

a laser source having an output for providing input light to said Brillouin fiber optic gyroscope;

a phase modulator connected to modulate said output of said laser source, said phase modulator comprising a signal generator and a piezoelectric cylinder;

a first coupler that taps a first portion of pump intensity traveling in a first direction in said resonant cavity;

a second coupler that taps a second portion of pump intensity traveling in a second direction in said resonant cavity;

a first detector that detects said first portion and that generates a first electric signal proportional to said detected first portion;

a second detector that detects said second portion and that generates a second electric signal proportional to said detected second portion;

an adder and a synchronous demodulator that receive said first and second electrical signals and that provides a demodulated sum output signal;

an integrator that integrates said demodulated sum output signal and provides an integrated output signal; and

a piezoelectric cylinder driven by said integrated output signal that varies the length of said resonant cavity to stabilize said pump intensities.

7. The apparatus of claim 5, wherein said phase modulator is connected to the resonant cavity loop of the Brillouin fiber optic gyroscope.

6. An apparatus for stabilizing the intensities of counterpropagating pump signals within a resonant cavity of a Brillouin fiber optic gyroscope, comprising:

a phase modulator comprising a signal generator and a piezoelectric cylinder;

a first coupler that taps a first portion of pump intensity traveling in a first direction in said resonant cavity;

a second coupler that taps a second portion of pump intensity traveling in a second direction in said resonant cavity;

a first detector that detects said first portion and that generates a first electric signal proportional to said detected first portion;

a second detector that detects said second portion and that generates a second electric signal proportional to said detected second portion;

an adder and a synchronous demodulator that receive said first and second electrical signals and that provides a demodulated sum output signal responsive to said first and second electrical signals;

an integrator that integrates said demodulated sum output signal and provides an integrated output signal; and

a piezoelectric cylinder driven by said integrated output signal that varies the length of said resonant cavity to stabilize said pump intensities.

8. An apparatus for controlling the circulating intensities in a Brillouin fiber optic gyroscope, and for stabilizing the intensities of counterpropagating Brillouin waves within the resonant cavity of a Brillouin fiber optic gyroscope, comprising:

means for detecting a difference in circulating Brillouin intensities;

means responsive to said difference in circulating Brillouin intensities for controlling the circulating intensities;

means for detecting counterpropagating pump intensities; and

means for combining said counterpropagating pump intensities and for generating a stabilization signal to stabilize the pump intensities at the midpoint between the resonant peaks of the counterpropagating pump intensities.

9. An apparatus for measuring rotation, comprising:

a loop comprised of optical fiber, said loop configured to form a resonant cavity for propagation of light in opposite directions therein;

a source of input pump light waves optically coupled to produce a pair of counterpropagating pump light waves in said loop, said pump light waves resonating in said loop at an intensity sufficient to generate a pair of rotation sensing waves in said loop, each of said pump light waves having an effective path length through said loop, said effective path length varying in accordance with the Sagnac effect when said loop is rotating such that the effective path length for one of said pump light waves is lengthened while the effective path length for the other of said pump light waves is shortened, the intensities of said rotation sensing waves being dependent upon said variations in the effective path length through said loop; and

a feedback circuit for controlling the length of said loop to thereby control the effective path lengths for said pump light waves during rotation of said loop, said feedback circuit connected to sense light that has travelled in both directions in said loop and to provide a feedback signal representing a differential intensity between the intensity of one of said pair of rotation sensing waves and the intensity of the other of said pair of rotation sensing waves, said circuit altering the length of said loop in response to said feedback signal such that said differential intensity of said rotation sensing waves is substantially independent of said rotation.

10. The apparatus of claim 9, wherein said feedback circuit comprises a pair of detectors which are optically coupled to detect portions of respective counterpropagating pump light waves.

11. The apparatus of claim 9, wherein said feedback circuit comprises a pair of detectors which are optically coupled to detect portions of respective counterpropagating rotation sensing waves.

12. An apparatus for measuring rotation, comprising:

a loop comprising optical fibers, said loop configured to form a resonant cavity for propagation of light in opposite directions therein;

a source of input pump light waves optically coupled to produce a pair of counterpropagating pump light waves in said loop, said pump light waves resonating in said loop at an intensity sufficient to generate a pair of rotation sensing waves in said loop, each of said pump light waves having an effective path length through said loop, said effective path length varying in accordance with the Sagnac effect when said loop is rotating such that the effective path length for one of said pump light waves is lengthened while the effective path length for the other of said pump light waves is shortened, the intensities of said rotation sensing waves being dependent upon said variations in the effective path length through said loop; and

a feedback circuit for controlling the length of said loop to thereby control the effective path lengths for said pump light waves during rotation of said loop, said feedback circuit connected to sense light that has travelled in both directions in said loop, said circuit responsively altering the length of said loop such that the differential intensity of said rotation sensing waves is substantially independent of said rotation, wherein said feedback circuit is additionally connected to alter the intensity of the input pump light waves such that the counterpropagating rotation sensing waves are substantially equalized in intensity.