A method of measuring beat frequency in a resonator fiber optic gyroscope, the method comprising: modulating a first optical signal from a first light source and a second optical signal from a second light source, 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; combining a fraction of the first modulated optical signal with a fraction of the second modulated optical signal into a combined signal; determining a carrier beat frequency between the first carrier frequency and the second carrier frequency; characterized in that the method further comprises: selecting a frequency range from the combined signal that is close to and includes the carrier beat frequency; performing a fast Fourier transform 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, wherein determining the carrier beat frequency comprises: temporarily turning off modulation of the first optical signal and the second optical signal such that the first and second optical signals are unmodulated; combining a fraction of the unmodulated first optical signal with a fraction of the unmodulated second optical signal to produce a signal containing the carrier beat frequency without sideband beat frequencies; performing an FFT on the signal containing the carrier beat frequency without sideband beat frequencies; identifying the carrier beat frequency based on the FFT; and turning on modulation of the first optical signal and the second optical signal after identifying the carrier beat frequency. modulating a first optical signal from a first light source and a second optical signal from a second light source, 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; combining a fraction of the first modulated optical signal with a fraction of the second modulated optical signal into a combined signal; determining a carrier beat frequency between the first carrier frequency and the second carrier frequency; selecting a frequency range from the combined signal that is close to and includes the carrier beat frequency; performing a fast Fourier transform 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, temporarily turning off modulation of the first optical signal and the second optical signal such that the first and second optical signals are unmodulated; combining a fraction of the unmodulated first optical signal with a fraction of the unmodulated second optical signal to produce a signal containing the carrier beat frequency without sideband beat frequencies; performing an FFT on the signal containing the carrier beat frequency without sideband beat frequencies; identifying the carrier beat frequency based on the FFT; and turning on modulation of the first optical signal and the second optical signal after identifying the carrier beat frequency.