US 4,735,506 A · Phase nulling optical gyroscope

Abstract

A gyroscope system includes a coherent light source that supplies counter propagating waves to a sensing loop through a pair of directional couplers. The polarizations of the waves are controlled so that they traverse identical optical paths before recombining in one of the couplers to form an interference pattern. Rotation of the sensing loop, a phase modulator and a frequency shifter cause phase changes in the counter propagating waves. A detector monitors the interference pattern of the combined waves and provides a signal to a coherent demodulator that controls the phase modulator. The output of the coherent demodulator is input to a servo-loop circuit that drives a voltage controlled oscillator. The output of the voltage controlled oscillator is an oscillatory signal having a frequency equal to the shift in frequency that the counter propagating waves experience in traversing the frequency shifter. The feedback circuitry adjusts the frequency shift to null the phase difference between the counter propagating waves. The frequency shift is linearly related to the frequency of the signal output from the voltage controlled oscillator. Each cycle of the output of the voltage controlled oscillator coresponds to a fixed angular increment of displacement of the sensing loop. The rotation rate of the sensing loop is a function of the frequency shift and the transit time of the waves through the sensing loop. The gyroscope system determines rotation rates and angular displacements over a wide dynamic range by measuring the frequency and zero crossings of the oscillatory output of the voltage controlled oscillator.

Claims16

Dependent claims

A fiber optic rotation sensor, comprising:

a length of optical fiber including a sensing coil formed therein;
means for introducing a pair of counterpropagating light waves into said sensing coil such that rotation of said sensing coil causes a phase shift between the counterpropagating light waves, said phase shift being indicative of the rotation rate of said sensing coil;
fiber optic frequency shifting means formed to interact with said length of optical fiber for producing a frequency shift in each of said counterpropagating waves to compensate for phase shifts caused by rotation of said sensing coil;
means for producing a frequency shift signal indicative of the frequency shift produced by said fiber optic frequency shifting means to indicate the rotation rate of said sensing coil, the frequency shift signal being an oscillatory signal having a frequency indicative of the rotation rate of the sensing coil, each oscillation of the frequency shift signal being indicative of a predetermined incremental angular displacement of said sensing coil;
and
a phase modulator formed in said optical fiber for modulating the phase of a first one of said counterpropagating waves before said first wave impinges upon said sensing coil and for modulating the phase of the second wave after said second wave has propagated through said sensing coil.

3 more elements

A fiber optic rotation sensor, comprising:

a length of optical fiber formed to have a sensing coil therein;
means for providing low coherence light of a first predetermined polarization state and a second predetermined polarization state for input to said sensing coil, said first and second predetermined polarization states being orthogonal;
a fiber optic directional coupler formed in said optical fiber for receiving said optical signal and forming a clockwise wave and a counterclockwise wave in said sensing coil, said optical fiber being formed to have a first portion extending between said directional coupler and said sensing coil and a second portion extending between said directional coupler and said sensing coil;
a phase modulator formed in said first portion for modulating the phase of one of said counterpropagating waves before input to said sensing coil and for modulating the phase of the other of said counterpropagating waves after it has traversed said sensing coils;
first means for maintaining the first signal in the first predetermined polarization state;
second means for maintaining said second signal in said second predetermined polarization state;
a frequency shifter formed in the optical fiber between said first and second polarization maintaining means, said frequency shifter producing a change in the frequency of said counterpropagating waves to compensate for the change in phase induced by rotation of said sensing coil;
means for detecting the recombined signals output from said directional coupler to produce a signal indicative of the phase difference between said first and second signals;
demodulator means for processing the output of said detector means to produce an error signal;
means for processing the error signal to produce a control signal;
means responsive to the control signal for providing a driving signal to said frequency shifter to maintain said error signal below a predetermined threshold;
and
means for processing said control signal to determine the rotation rate of said sensing coil.

10 more elements

Dependencies are inferred from the claim text.

Phase nulling optical gyroscope

Life of this patent

Phase nulling optical gyroscope

Life of this patent

Abstract

Claims16