Method and apparatus of monitoring and tracking optical frequency differences of modulated beams

Patent metadata

Field	Value
Publication number	US-9267799-B2
Application number	US-201414187589-A
Country	US
Kind code	B2
Filing date	Feb 24, 2014
Priority date	Feb 24, 2014
Publication date	Feb 23, 2016
Grant date	Feb 23, 2016

Abstract

A method of measuring beat frequency comprises modulating a first optical signal and a second optical signal, wherein the first modulated optical signal includes a first carrier frequency and a first plurality of sideband frequencies and the second modulated optical signal includes a second carrier frequency and a second plurality of sideband frequencies. The method also comprises combining a fraction of the first modulated optical signal with a fraction of the second modulated optical signal into a combined signal and determining a carrier beat frequency. The method further comprises selecting a frequency range from the combined signal; performing a fast Fourier transform (FFT) on an electrical signal representing the selected frequency range; tracking the carrier beat frequency based on the FFT; and outputting a rate signal based on the tracked carrier beat frequency, the rate signal indicating a rotation rate of the resonator fiber optic gyroscope.

First claim

What is claimed is:

1. A resonator fiber optic gyroscope comprising:
a first light source configured to produce a first laser beam having a first carrier frequency;
a second light source configured to produce a second laser beam having a second carrier frequency;
a first modulator coupled to the first light source and configured to modulate the first laser beam, the modulated first laser beam comprising the first carrier frequency and a first plurality of sideband frequencies;
a second modulator coupled to the second light source and configured to modulate the second laser beam, the modulated second laser beam comprising the second carrier frequency and a second plurality of sideband frequencies;
a first optical coupler coupled to an output of the first modulator and configured to split off a portion of the modulated first laser beam;
a second optical coupler coupled to an output of the second modulator and configured to split off a portion of the modulated second laser beam;
a beam combiner coupled to the first optical coupler and the second optical coupler; the beam combiner configured to combine the portion split off the modulated first laser beam with the portion split off the modulated second laser beam to produce a combined signal containing a carrier beat frequency between the first carrier frequency and the second carrier frequency;
a filter configured to select a range of frequencies in the combined signal;
a photodetector configured to convert the combined signal to an electrical signal; and
a processor configured to perform a fast Fourier transform (FFT) on the filtered electrical signal, to track the carrier beat frequency based on the FFT, and to output a rate signal based on the carrier beat frequency which indicates a rotation rate of the resonator fiber optic gyroscopes;
wherein the processor is further configured to identify the carrier beat frequency by outputting a command to the first modulator and to the second modulator to turn off modulation of the first laser beam and the second laser beam such that the first laser beam and the second laser beam are unmodulated;
wherein the beam combiner combines a portion of the unmodulated first laser beam with a portion of the unmodulated second laser beam to produce a combined signal containing the carrier beat frequency without sideband beat frequencies; and
wherein the processor is further configured to perform the FFT on the combined signal containing the carrier beat frequency without sideband beat frequencies, to identify the carrier beat frequency based on the FFT of the combined signal containing the carrier beat frequency without sideband beat frequencies, and to output a command to the first modulator and to the second modulator to turn on modulation of the first laser beam and the second laser beam after identifying the carrier beat frequency.

2. The resonator fiber optic gyroscope of claim 1 , wherein the filter is an optical filter coupled between an output of the beam combiner and an input of the photodetector such that the photodetector converts a filtered optical signal containing the selected range of frequencies into an electrical signal.

3. The resonator fiber optic gyroscope of claim 1 , wherein the photodetector is coupled between an output of the beam combiner and an input of the filter;
wherein the filter is an electrical bandpass filter configured to filter an electrical signal output from the photodetector to select the range of frequencies.

4. The resonator fiber optic gyroscope of claim 1 , wherein when the processor loses track of the carrier beat frequency, the processor is configured to reacquire the carrier beat frequency by outputting a command to the first modulator and to the second modulator to turn off modulation of the first laser beam and the second laser beam such that the first laser beam and the second laser beam are unmodulated;
wherein the beam combiner combines a portion of the unmodulated first laser beam with a portion of the unmodulated second laser beam to produce a combined signal containing the carrier beat frequency without sideband beat frequencies;
wherein the processor is further configured to perform an FFT on the combined signal containing the carrier beat frequency without sideband beat frequencies; to identify the carrier beat frequency based on the FFT of the combined signal containing the carrier beat frequency without sideband beat frequencies, and to output a command to the first modulator and to the second modulator to turn on modulation of the first laser beam and the second laser beam after identifying the carrier beat frequency.

5. The resonator fiber optic gyroscope of claim 1 , wherein the processor is configured to output the rate signal to one of a navigation unit or a display unit.

6. The resonator fiber optic gyroscope of claim 1 , wherein the processor is further configured to adjust a time series size and time resolution of the FFT according to the numerical stability of the FFT.

7. A navigation system comprising:
a resonator fiber optic gyroscope (RFOG) configured to measure rotation rate; and
a navigation unit configured to receive the measurements of rotation rate from the RFOG and to calculate a navigation solution based on the measured rotation rate;
wherein the RFOG comprises:
a first light source configured to produce a first laser beam having a first carrier frequency;
a second light source configured to produce a second laser beam having a second carrier frequency;
a first modulator coupled to the first light source and configured to modulate the first laser beam, the modulated first laser beam comprising the first carrier frequency and a first plurality of sideband frequencies;
a second modulator coupled to the second light source and configured to modulate the second laser beam, the modulated second laser beam comprising the second carrier frequency and a second plurality of sideband frequencies;
a first optical coupler coupled to an output of the first modulator and configured to split off a portion of the modulated first laser beam;
a second optical coupler coupled to an output of the second modulator and configured to split off a portion of the modulated second laser beam;
a beam combiner coupled to the first optical coupler and the second optical coupler; the beam combiner configured to combine the portion split off the modulated first laser beam with the portion split off the modulated second laser beam to produce a combined signal containing a carrier beat frequency between the first carrier frequency and the second carrier frequency;
a filter configured to select a range of frequencies in the combined signal;
a photodetector configured to convert the combined signal to an electrical signal; and
a processor configured to perform a fast Fourier transform (FFT) on the filtered electrical signal, to track the carrier beat frequency based on the FFT, and to output a rate signal to the navigation unit, wherein the rate signal is based on the carrier beat frequency and indicates the measured rotation rate of the resonator fiber optic gyroscope;
wherein the processor is further configured to identify the carrier beat frequency by outputting a command to the first modulator and to the second modulator to turn off modulation of the first laser beam and the second laser beam such that the first laser beam and the second laser beam are unmodulated;
wherein the beam combiner combines a portion of the unmodulated first laser beam with a portion of the unmodulated second laser beam to produce a combined signal containing the carrier beat frequency without sideband beat frequencies; and
wherein the processor is further configured to perform the FFT on the combined signal containing the carrier beat frequency wit

Assignees

• Honeywell Int Inc

Inventors

• Qiu Tiequn
• Ho Waymon

Classifications

• G01C19/727 Primary

  using a passive ring resonator · CPC title

• G01C21/00

  Navigation; Navigational instruments not provided for in groups G01C1/00 - G01C19/00 (measuring distance traversed on the ground by a vehicle G01C22/00; control of position, course, altitude or attitude of vehicles G05D1/00; traffic control systems for road vehicles involving transmission of navigation instructions to the vehicle G08G1/0968) · CPC title

• G01C19/72

  with counter-rotating light beams in a passive ring, e.g. fibre laser gyrometers · CPC title

• G02F2/00

  Demodulating light; Transferring the modulation of modulated light; Frequency-changing of light (G02F1/35 takes precedence) · CPC title

• G01C19/722 Primary

  of the mechanical construction · CPC title