System and method of stabilizing the scale factor shift in fiber optic gyroscopes using a spectral monitor array

• US 6,108,086 A
• Filed: 03/04/1999
• Issued: 08/22/2000
• Est. Priority Date: 03/04/1999
• Status: Expired due to Term

• Current Assignee: Northrop Grumman Systems Corporation (Northrop Grumman Corporation)
• Original Assignee: Litton Systems Incorporated
• Inventors: Rozelle, David M., Rahn, John P., Michal, Ronald J.
• Primary Examiner(s): Turner, Samuel A.
• Application Number:
  US09/262,795
• Time in Patent Office: 537 Days
• Field of Search: 356/350, 356/345, 385/12, 385/14, 250/227.19, 250/227.27
• US Class Current: 356/460
• CPC Class Codes: G01C 19/72 with counter-rotating light beams in a passive ring, e.g. fibre laser gyrometers

Abstract

A system and method for providing scale factor stabilization of a broadband light source used in fiber optic gyroscopes is provided. A primary bandpass filter is positioned in the propagation path of the light emitted by the broadband light source to narrow the spectral width of the optical signal transmitted to the fiber optic gyroscope, which reduces the centroid wavelength shift resulting when the broadband light source is exposed to ionizing radiation and other harsh environmental conditions. The filtered optical signal is then passed through the fiber optic gyroscope, where the filter optical signal is processed to measure the amount of rotation of the fiber optic gyroscope. The scale factor stabilization system further includes the spectral monitor array arranged to perform direct optical wavelength measurements of the filtered optical signal to determine whether a scale factor shift in the optical signal has occurred. When a scale factor shift is detected, the detected scale factor shift is used in a compensation algorithm to correct the scale factor of the optical signal. By monitoring the scale factor shift and adjusting the values measured by the fiber optic gyroscope in accordance with the detected scale factor shift, the accuracy of the fiber optic gyro can be improved to a fraction of a part-per-million.

Claims

1. A system for stabilizing the scale factor of a broadband light source used in fiber optic rotation sensors, comprising:

a broadband light source which emits a broadband optical signal;

a fiber optic rotation sensor arranged to receive the optical signal from the broadband light source;

the fiber optic rotation sensor including a detector which measures the rotation of the rotation sensor by processing the optical signal and analyzing a scale factor of the optical signal;

a primary bandpass filter connected in the propagation path of the optical signal between the broadband light source and the fiber optic rotation sensor, the primary bandpass filter narrowing the spectral width of the optical signal to a bandpass of the primary bandpass filter; and

a spectral monitor array connected to receive at least a portion of the optical signal processed by the fiber optic rotation sensor, wherein the spectral monitor array monitors the spectrum of the optical signal to determine whether a shift in the scale factor of the optical signal has occurred,

wherein the measured rotation of the rotation sensor is corrected in accordance with the scale factor shift determined by the spectral monitor array.

2. The system for stabilizing the scale factor of a broadband light source of claim 1, wherein the spectral monitor array includes a plurality of secondary bandpass filters each connected to receive at least a portion of the optical signal processed by the fiber optic rotation sensor and narrow the spectral width of the optical signal, wherein each of the secondary bandpass filters has a bandpass located within the bandpass of the primary filter.

3. The system for stabilizing the scale factor of a broadband light source of claim 2, wherein the aggregate bandpass of all of the secondary bandpass filters is substantially equal to the bandpass of the primary bandpass filter.

4. The system for stabilizing the scale factor of a broadband light source of claim 2, wherein the spectral monitor array further comprises a fiber optic coupler connected to receive the optical signal processed by the fiber optic rotation sensor, wherein the fiber optic coupler includes a plurality of output legs for directing at least a portion of the processed optical signal to each of the secondary bandpass filters.

5. The system for stabilizing the scale factor of a broadband light source of claim 4, where each one of the plurality of output legs is connected to a respective secondary bandpass filter.

6. The system for stabilizing the scale factor of a broadband light source of claim 2, wherein each of the plurality of secondary bandpass filters has a respective detector connected thereto for measuring the intensity of the optical signals output by the secondary bandpass filters.

7. The system for stabilizing the scale factor of a broadband light source of claim 6, wherein the measured intensities of the optical signals output by the secondary bandpass filters are utilized to determine the scale factor shift of the optical signal.

8. The system for stabilizing the scale factor of a broadband light source of claim 7, wherein the scale factor shift is reduced to a fraction of a part-per-million after the measured rotation is corrected in accordance with the determined scale factor shift.

9. The system for stabilizing the scale factor of a broadband light source of claim 2, wherein the secondary bandpass filters comprise Bragg Grating optical filters.

10. The system for stabilizing the scale factor of a broadband light source of claim 1, wherein the primary bandpass filter comprises a fiber optic bandpass filter which allows the propagation of wavelengths of light within its operating bandwidth while attenuating all wavelengths of light outside its operating bandwidth.

11. The system for stabilizing the scale factor of a broadband light source of claim 1, wherein the primary bandpass filter comprises an optical fiber situated between two fiber optic gratings, each of the gratings having a different periodicity so as to allow only wavelengths of light in a bandwidth between the periodicity of the gratings to propagate while attenuating all other wavelengths of light outside this bandwidth.

12. The system for stabilizing the scale factor of a broadband light source of claim 2, wherein the fiber optic rotation sensor comprises:

a multiplexer arranged to receive the optical signal from the primary bandpass filter;

an integrated optics device connected to the multiplexer for forming and processing counter-propagating waves from the optical signal;

a fiber optic sensing coil connected to the integrated optics device for generating a phase difference in the counter-propagating waves in relation to the rotation of the fiber optic rotation sensor,

wherein the counter-propagating waves are combined in the integrated optics device to generate an interference pattern based upon the phase difference between the combined counter-propagating waves, the multiplexer further arranged to receive the interference pattern generated by the integrated optics device;

a first multiplexer output optical fiber connecting the multiplexer to the detector and arranged to guide the interference pattern to the detector; and

a second multiplexer output optical fiber connecting the multiplexer to the spectral monitor array and arranged to guide the interference pattern to the spectral monitor array.

13. A method for stabilizing the scale factor of a broadband optical fiber source used in fiber optic rotation sensors, comprising the steps of:

providing a broadband optical signal from a broadband light source;

passing the broadband light signal through a primary bandpass filter to narrow the spectral width of the broadband optical signal;

outputting the filtered optical signal to a fiber optic rotation sensor;

processing the optical signal in the fiber optic rotation sensor to measure the amount of rotation of the fiber optic rotation sensor;

passing the processed optical signal through a spectral monitor array where the processed optical signal is analyzed to determine whether a scale factor shift has developed in the optical signal; and

correcting the measured amount of rotation of the fiber optic rotation sensor in accordance with the determined scale factor shift.

14. The method of stabilizing the scale factor of a broadband optical fiber source of claim 13, wherein the spectral monitor array includes a plurality of the secondary bandpass filters, each of the secondary bandpass filters having a respective bandpass located within the bandpass of the primary filter, wherein the step of analyzing the processed optical signals comprises the steps of:

directing at least a portion of the processed optical signal to each of the secondary bandpass filters;

generating a plurality of secondarily filtered optical signals, wherein each of the plurality of secondarily filtered optical signals has a respective wavelength of the bandpass its associated secondary bandpass filter;

measuring the intensity of the plurality of secondarily filtered optical signals; and

determining the scale factor shift of the processed optical signal by mathematically processing the measured intensities.

15. The method of stabilizing the scale factor of a broadband optical fiber source of claim 14, wherein the spectral monitor array includes a fiber optic coupler connected before the secondary bandpass filters for splitting the processed optical signal received by the spectral monitor array into a plurality of output signals and directing each of the plurality of output signals to a respective secondary bandpass filter.

16. The method of claim 14, wherein the primary bandpass filter and the secondary bandpass filters allow the propagation of wavelengths of light within their respective operating bandwidth while attenuating all wavelengths of light outside their respective operating bandwidth.

17. The method of claim 13, wherein the step of processing the optical signal in the fiber optic rotation sensor further comprises the steps of:

passing the filtered optical signal through a multiplexer connected to the primary bandpass filter;

forming and processing counter-propagating waves from the optical signal in an integrated optics device connected to the multiplexer;

generating a phase difference in the counter-propagating waves in relation to the rotation of the fiber optic rotation sensor in a fiber optic sensing coil connected to the integrated optics device;

combining the counter-propagating waves in the integrated optics device to generate an interference pattern based upon the phase difference generated between the combined counter-propagating waves;

guiding the interference pattern through the multiplexer to a detector connected to the multiplexer for measuring the rotation of the fiber optic rotation sensor.

18. The method of claim 14, wherein the aggregate bandpass of all of the secondary bandpass filters is substantially equal to the bandpass of the primary bandpass filter.

19. The method of claim 13, wherein the scale factor shift of the optical signal is reduced to a fraction of a part-per-million after the measured rotation is corrected in accordance with the determined scale factor shift.