Optics: measuring and testing – By dispersed light spectroscopy – Utilizing a spectrometer
Reexamination Certificate
1998-12-17
2001-01-16
Turner, Samuel A. (Department: 2877)
Optics: measuring and testing
By dispersed light spectroscopy
Utilizing a spectrometer
Reexamination Certificate
active
06175410
ABSTRACT:
BACKGROUND
In a typical fiber optic gyroscope, there are polarization phenomena that cause erroneous rotation detection by the gyroscope. Some polarization errors are caused by light being cross-coupled from one polarization state to another. In gyroscopes having, for example, a proton exchange integrated optical circuit, cross-coupling occurs at the coupling of integrated optical circuit with a light source and an optical fiber sensing loop. A phase modulator on such integrated optical circuit affects one polarization state of light differently than another for a given signal applied to the modulator. Certain origins and characteristics of various polarization errors appear to have not been known in the art prior to the present invention. Discoveries of such origins and characteristics and ensuing solutions are presented here.
SUMMARY OF THE INVENTION
A design for suppressing amplitude and intensity type polarization errors in fiber optic gyroscopes uses sophisticated modulation signals. The phase modulator or modulators within the sensing loop of the gyroscope act upon the light polarized along the pass axis of the polarizer differently than upon the small amount of light polarized along the reject axis of the polarizer. This situation exists in the case of integrated optical circuit modulators. For a proton-exchange lithium niobate modulator, the remaining reject axis light is essentially unmodulated in phase since it takes a different physical path through the integrated optical circuit. The waveguide only guides one polarization of light. The leakage of the other polarization state of the light through the chip is due to scattered light, which bypasses the phase modulator. The present invention, which eliminates the resulting polarization errors, applies to both polarization maintaining (PM) and depolarized (SM) type fiber optic gyroscopes.
There are several implementations for achieving this polarization error suppression. One is an open-loop signal processing system that suppresses some polarization errors by bias modulation, and suppresses remaining errors by decorrelation techniques. Such techniques are mentioned in U.S. Pat. No. 5,327,213 by Blake et al., issued Jul. 5, 1994, and entitled “Configuration Control of Mode Coupling Errors,” which is incorporated by reference in the present description. Also, U.S. Pat. No. 5,377,283 by Blake et al., issued Dec. 27, 1994, and entitled “Configuration Control of Mode Coupling Errors,” is incorporated by reference in the present description. Another open-loop processing system suppresses some polarization errors with bias modulation, and suppresses other polarization errors with a modulation signal meeting specific criteria supplied to a second phase modulator located on the opposite side of the sensing loop. A closed-looped signal processing system having ramp-like waveforms meeting certain criteria are supplied to modulators located on both sides of the loop, while the bias modulation signal is supplied to a modulator on either one or both sides of the loop.
General equations have been developed to describe the amplitude and intensity type polarization errors. These equations incorporate the fact that the phase modulators do not affect the two polarization states in the same way. In this case, there is negligible a.c. (alternating current) interference between the primary and spurious co-propagating waves in the loop. The primary waves are of the preferred polarization and the spurious or secondary waves are of the rejected polarization. Such is the situation in all-fiber gyroscopes. All significant polarization errors arise due to interferences between the various counter-propagating waves in the loop.
The present invention solves the polarization error problem in the situation wherein the phase modulator or modulators do not affect the primary and secondary polarization states in the same way, co-propagating waves also yield a.c. interference terms that fall within the demodulation bandwidth of the gyroscope. In this case, four types of amplitude polarization errors and one type of intensity polarization errors result. The different types of errors are distinguished and have various modulations applied to eliminate them. The modulations applied to the various error interferences are also different from the modulation applied to the main signal. This fact allows for the possibility that errors can be suppressed by modulation techniques, while maintaining good signal sensitivity. The present invention suppresses these errors.
REFERENCES:
patent: 4645345 (1987-02-01), Domann
patent: 5009480 (1991-04-01), Okada et al.
patent: 5018859 (1991-05-01), Chang et al.
patent: 5327213 (1994-07-01), Blake et al.
patent: 5377283 (1994-12-01), Blake et al.
patent: 5469257 (1995-11-01), Blake et al.
patent: 5473430 (1995-12-01), Kemmler
patent: 5781300 (1998-07-01), Strandjord et al.
Blake James N.
Lange Charles H.
Strandjord Lee K.
Szafraniec Bogdan
Fredrick Kris T.
Honeywell Inc.
Turner Samuel A.
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