The oscillation frequency in a Brillouin Fiber Ring Laser (BFRL) is determined mainly by the axial mode of the cold resonator located under the Brillouin gain curve that experiences the highest gain. This oscillation frequency is therefore temperature dependent since both the Free Spectral Range (FSR) and the gain curve center depend on the temperature.
Intracavity phase modulation in a fiber-optic ring laser gyro can provide 'optical dithering' to reduce the effects of frequency locking while retaining optical reciprocity in the cavity. We show that the use of two antiphased phase modulators placed symmetrically within the fiber cavity can provide uniformly distributed dithering. A modulation index of 2.4 theoretically eliminates the zero-order lock-in band around zero frequency, while use of a high modulation frequency puts higher-order lock-in bands outside the beat frequency dynamic range. Push-pull modulation allows for high modulation frequency with minimum dynamic perturbation of the cavity resonant behavior. We describe results with an experimental Brillouin fiber optic gyro operating at 1.3-micron wavelength using push-pull modulation together with a novel synthetic heterodyne detection scheme for sensing rotation rate and direction. A ten-fold reduction of the width of the zero-order lock-in band is observed. We also demonstrate that the observed frequency bias at zero rotation rate is caused by the Kerr effect due to the power imbalance between the two oppositely directed circulating lasers.
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