Optimization of Pumping Light for Cs Four-level Active Optical clock

In this work, we present the optimization of pumping light for Cs four-level active optical clock in detail. The relationships between the power stability of 1470 nm clock-transition laser and the power stability, as well as frequency stability, of 459 nm pumping laser are demonstrated, respectively. Through building an external cavity diode laser with power stability of $1\times 10^{-4}@ 1\ \mathrm{s}$, the linewidth broadening of 1470 nm laser caused by power fluctuation of pumping laser is reduced to 43 Hz. Moreover, utilizing modulation transfer spectroscopy technique, the frequency stability of pumping laser is optimized by two orders of magnitude. Ultimately, the linewidth of 1470 nm active optical frequency standard is reduced to 120 Hz.