A Compact Pumping Laser at 459 nm with 10 −15 Stability for Cs Four-Level Active Optical Clock

Utilizing the modulation transfer spectroscopy (MTS) technique, we realize a compact pumping laser at 459 nm based on cesium (Cs) 6 2 S 1/2 F=4 to 72p 1/2 F'=3 transition with a stability of 1. 8 $\times 10^{-14}/\sqrt{\tau}$ and decreasing to $8\times 10^{-15}$ at 60 s. The 459 nm pumping laser stabilized by MTS technique, achieves the stability by about 2 orders of magnitude better than that of the saturation absorption spectroscopy (SAS), which will contribute to the linewidth reduction of 1470 nm ultimate laser in Cs four-level active optical clock. The 1470 nm stimulated radiation can reach a power of $100 \ \mu\mathbf{W}$ with the stabilized 459 nm pumping laser power being 6 mW. Such a stable laser not only can be used as the pumping laser in active optical clock system, but also a novel optical frequency standard for length and other applications.