Stabilizing 459 nm passive optical clock for pumping 1470 nm active optical clock

Optical clocks with thermal atoms are characterized by compact size, simple structure, reduced weight, and low power consumption and have the potential for broad out-of-the-lab and commercial applications. Here, we demonstrate a 459 nm optical clock based on the 6S_1/2–7P_1/2 transition in thermal ¹³³Cs atoms. Two methods, modulation transfer spectroscopy (MTS) and frequency modulation spectroscopy (FMS), are employed to stabilize the frequency of a 459 nm commercial laser to the atomic transition. The MTS-MTS and MTS-FMS beat-note measurements show short-term frequency stabilities of $3.7\times {10}^{-13}/\sqrt{\tau }$ and $6.4\times {10}^{-13}/\sqrt{\tau }$, respectively, at the averaging time τ. The 459 nm passive optical clock further serves as the pump for an active 1470 nm optical clock based on the cavityless lasing. The resultant 1470 nm output power reaches over 10 μW and the pump-beam-induced light shift is estimated to be 2π × 11 Hz with a fractional uncertainty of 2.4 × 10⁻¹⁸. These results demonstrate the feasibility of hybridizing passive and active optical clocks, providing a promising route toward compact multi-wavelength optical frequency standards.