420 nm Rb optical frequency standard with short-term frequency stability below 2 × 10^-13

We experimentally study the character of saturated absorption spectroscopy (SAS) of the ⁸⁷Rb 5S_1/2 → 6P_3/2 transition at 420 nm in a miniature cubic glass cell (3 × 3 × 3 mm³). The influences of the laser intensity and cell temperature on the saturated absorption resonance are comprehensively investigated to determine the optimal parameters for a compact stabilized frequency standard. By stabilizing the laser frequency to the optimized SAS signal, a beatnote stability of 2.4 × 10^-13 at 1 s is measured between two identical SAS systems, corresponding to a single-laser stability of 1.7 × 10^-13 at 1 s, assuming equal contribution from each laser. A further analysis of the noise budget reveals that the main contributions of short-term stability arise from the intermodulation effect and the laser frequency noise through the frequency modulation to amplitude modulation (FM-AM) conversion process. Our work provides a promising route toward high-performance miniaturized optical frequency standards based on high-lying excited-state transitions.