Abstract
We demonstrate a homologous heterodyne optical phase-locked loop for free spectral range measurement of a fiber ring resonator. In this loop, the frequency noise within the 10Hz bandwidth is reduced by more than 40dB from 147.350 to 0.014Hz, and the power spectral density of the frequency noise reaches 7.69×10-8 Hz2/Hz at 10Hz. Finally, the relative measurement accuracy of 1.39×10-9 is achieved by this loop and the free spectral range coefficient of thermal expansion is measured as -174.1±0.2 Hz/°C with a cavity finesse of 26.65. This work provides a method to measure free spectral range by tracking the resonance modes of the resonator and reducing frequency noise, especially for two signals with frequency offset.
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