Measuring the Cavity-Pulling Coefficient of Active Optical Clock via Pump Laser Modulation

The active optical clock (AOC) takes advantage of the narrowed linewidth and cavity pulling suppression of the bad cavity laser, making it a candidate for the next generation of optical clock. We propose a new experimental method to measure the cavity-pulling effect of a four-level AOC based on thermal cesium atoms. After modulating the frequency of the 459 nm pumping laser, atoms in different velocity groups are pumped to the 7P 1/2 energy level. Therefore, population inversion is achieved in separate velocity groups, and sidebands are created in the gain profile. By measuring the central frequency of the beat spectrum between the zeroth order AOC output and the sidebands, the cavity-pulling coefficient can be calculated. This method has the advantage of not requiring any additional optical frequency reference, and can be adopted in AOC systems other than those using thermal atoms. Furthermore, our proposed method for cavity-pulling measurement can serve as a basis for the characterization of AOCs.