Resonator fiber optic gyroscope employing common cavity length modulation along with high bandwidth laser stabilization
A resonator fiber optic gyroscope comprises a master laser device that emits a reference optical signal, a first slave laser device that emits a clockwise optical signal, and a second slave laser device that emits a counter-clockwise optical signal. A resonator ring cavity is in communication with the master laser device and the slave laser devices. A sine wave generator is coupled to the resonator ring cavity and outputs a common cavity modulation frequency comprising in-phase and quadrature signals. A laser stabilization servo receives a clockwise reflection signal that includes the common cavity modulation frequency from the resonator ring cavity. A modulation stripper coupled to the servo receives the in-phase and quadrature signals, receives a net error signal from the servo, demodulates the net error signal at the common cavity modulation frequency, and transmits a stripper signal to the servo to remove the signal at the common cavity modulation frequency.
1. A resonator fiber optic gyroscope (RFOG), comprising:
a master laser assembly including a master laser device configured to emit a reference optical signal;
a first slave laser assembly including a first slave laser device configured to emit a clockwise (CW) optical signal, the first slave laser device responsive to the reference optical signal through a CW optical phase lock loop;
a second slave laser assembly including a second slave laser device configured to emit a first counter-clockwise (CCW) optical signal, the second slave laser device responsive to the reference optical signal through a first CCW optical phase lock loop;
an optical resonator ring cavity in optical communication with the master laser device, the first slave laser device, and the second slave laser device;
a sine wave generator operatively coupled to the resonator ring cavity and configured to output a common cavity modulation frequency comprising an in-phase (I) signal component and a quadrature (Q) signal component;
a laser stabilization servo module configured to receive a CW reflection signal that includes the common cavity modulation frequency from the resonator ring cavity;
a modulation stripper operatively coupled to the laser stabilization servo module, the modulation stripper configured to:
receive the I and Q signal components of the common cavity modulation frequency from the sine wave generator;
receive a net error signal from the laser stabilization servo module;
demodulate the net error signal at the common cavity modulation frequency; and
transmit a stripper signal to the laser stabilization servo module to remove the signal at the common cavity modulation frequency; and
a resonance tracking electronics module operatively coupled to the resonator ring cavity and configured to output resonance tracking error signals to the first and second slave laser assemblies;
wherein the laser stabilization servo module is configured to send a tuning signal to the master laser device as part of a feedback stabilization loop for the master laser device.
2. The RFOG of claim 1 , wherein the laser stabilization servo module comprises:
a mixer configured to receive the CW reflection signal and a laser stabilization modulation frequency;
a summer configured to receive a mixed signal output from the mixer and the stripper signal from the modulation stripper, the summer configured to output the net error signal; and
an integrator configured to receive the net error signal output from the summer and send the tuning signal to the master laser device.
3. The RFOG of claim 1 , wherein the modulation stripper comprises:
a first mixer that receives the I signal component from the sine wave generator and the net error signal from the laser stabilization servo module;
a second mixer that receives the Q signal component from the sine wave generator and the net error signal;
a first integrator that receives a mixed signal from the first mixer;
a second integrator that receives a mixed signal from the second mixer;
a first multiplier that receives a control signal output from the first integrator and the I signal component;
a second multiplier that receives a control signal output from the second integrator and the Q signal component; and
a summer that combines multiplied signals output from the first and second multipliers, and transmits the stripper signal to the laser stabilization servo module.
4. The RFOG of claim 1 , wherein the sine wave generator and the modulation stripper are implemented together in a signal processing device.
5. The RFOG of claim 4 , wherein the signal processing device comprises an application-specific integrated circuit, or a field programmable gate array.
6. The RFOG of claim 4 , wherein the signal processing device comprises:
a first demodulator that receives the I signal component from the sine wave generator;
a second demodulator that receives the Q signal component from the sine wave generator;
a first accumulator that receives a demodulated signal from the first demodulator;
a second accumulator that receives a demodulated signal from the second demodulator;
a first multiplier that receives a control signal from the first accumulator and the I signal component;
a second multiplier that receives a control signal from the second accumulator and the Q signal component; and
a summer that combines multiplied signals output from the first and second multipliers, and outputs a digital stripper signal.
7. The RFOG of claim 6 , further comprising:
an analog to digital converter that receives the net error signal from the laser stabilization servo module and outputs a digital error signal to the first and second demodulators;
a first digital to analog converter that receives the I signal component from the sine wave generator and outputs an analog I signal to the resonator ring cavity through a first filter; and
a second digital to analog converter that receives the digital stripper signal from the summer and outputs an analog stripper signal to the laser stabilization servo module through a second filter.
8. The RFOG of claim 1 , wherein the laser stabilization servo module comprises a Pound-Drever-Hall servo module.
9. The RFOG of claim 1 , wherein the sine wave generator outputs a common cavity modulation frequency in a range from about 7 kHz to about 20 kHz.
10. The RFOG of claim 1 , further comprising a first phase modulator in optical communication with the resonator ring cavity, the first phase modulator configured to apply a laser stabilization modulation frequency to a portion of the reference optical signal from the master laser device.
11. The RFOG of claim 10 , further comprising a second phase modulator in optical communication with the resonator ring cavity, the second phase modulator configured to apply a first high frequency modulation to a portion of the CW optical signal from the first slave laser device.
12. The RFOG of claim 11 , wherein a portion of the CW optical signal from the resonator ring cavity is directed to a first photodetector that converts the CW optical signal to an electrical CW transmission signal.
13. The RFOG of claim 12 , wherein the resonance tracking electronics module comprises:
a first mixer that receives the CW transmission signal, and the first high frequency modulation that is doubled;
a first analog to digital converter (ADC) that receives a mixed signal from the first mixer, and converts the mixed signal to a digital signal;
a first demodulator that receives the digital signal from the first ADC, and the common cavity modulation frequency from the sine wave generator;
a first integrator that receives a demodulated signal from the first demodulator;
a first summer that receives an integrated signal from the first integrator, and a digital word; and
a first direct digital synthesizer that receives a control word signal from the first summer, and sends a first radio frequency (RF) tuning signal to the first slave laser assembly.
14. The RFOG of claim 13 , further comprising a third phase modulator in optical communication with the resonator ring cavity, the third phase modulator configured to apply a second high frequency modulation to a portion of the CCW optical signal from the second slave laser device.
15. The RFOG of claim 14 , wherein a portion of the CCW optical signal from the resonator ring cavity is directed to a second photodetector that converts the CCW optical signal to an electrical CCW transmission signal.
16. The RFOG of claim 15 , wherein the resonance tracking electronics module further comprises:
a second mixer that receives the CCW transmission signal, and the second high frequency modulation that is doubled;
a second ADC that receives a mixed signal from the second mixer, and converts the mixed signal from the second mixer to a digital signal;
a second demodulator that receives the digital signal from the second ADC, and the common cavity modulation frequency from the sine wave generator;
a second integrator that receives a demodulated signal from the second demodulator;
a second summer that receives an integrated signal from the second integrator, and a digital word; and
a second direct digital synthesizer that receives a control word signal from the second summer, and sends a second RF tuning signal to the second slave laser assembly.
17. The RFOG of claim 16 , further comprising a third slave laser assembly including a third slave laser device configured to emit a second CCW optical signal, the third slave laser device responsive to the reference optical signal through a second CCW optical phase lock loop.
18. The RFOG of claim 17 , further comprising a fourth phase modulator in optical communication with the resonator ring cavity, the fourth phase modulator configured to apply a third high frequency modulation to a portion of the CCW optical signal from the third slave laser device.
19. The RFOG of claim 18 , wherein the resonance tracking electronics module further comprises:
a third mixer that receives the CCW transmission signal, and the third high frequency modulation that is doubled;
a third ADC that receives a mixed signal from the third mixer, and converts the mixed signal from the third mixer to a digital signal;
a third demodulator that receives the digital signal from the third ADC, and the common cavity modulation frequency from the sine wave generator;
a third integrator that receives a demodulated signal from the third demodulator;
a third summer that receives an integrated signal from the third integrator, and a digital word; and
a third direct digital synthesizer that receives a control word signal from the third summer, and sends a third RF tuning signal to the third slave laser assembly.
20. A modulation generator and stripper for a resonator fiber optic gyroscope (RFOG), comprising:
a signal processing device comprising:
a sine wave generator configured to output a digital sine wave that includes digital in-phase (I) and quadrature (Q) signal components at a modulation frequency;
a first demodulator configured to receive the digital I signal component from the sine wave generator;
a second demodulator configured to receive the digital Q signal component from the sine wave generator;
a first accumulator configured to receive a demodulated signal from the first demodulator;
a second accumulator configured to receive a demodulated signal from the second demodulator;
a first multiplier configured to receive a control signal from the first accumulator, and the digital I signal component;
a second multiplier configured to receive a control signal from the second accumulator, and the digital Q signal component; and
a summer configured to combine multiplied signals from the first and second multipliers, and output a digital stripper signal;
an analog to digital converter configured to receive a net error signal from a laser stabilization servo of the RFOG, and output a digital error signal to the first and second demodulators;
a first digital to analog converter configured to receive the digital I signal component from the sine wave generator, and output an analog I signal to an optical resonator ring cavity of the RFOG; and
a second digital to analog converter configured to receive the digital stripper signal from the summer, and output an analog stripper signal to the laser stabilization servo.