Cs microcell optical reference at 459 nm with short-term frequency stability below 2 $\times$ 10$^{-13}$

We describe the short-term frequency stability characterization of external-cavity diode lasers stabilized onto the 6S$_{1/2}$-7P$_{1/2}$ transition of Cs atom at 459 nm, using a microfabricated vapor cell. The laser beatnote between two nearly-identical systems, each using saturated absorption spectroscopy in a simple retroreflected configuration, exhibits an instability of $2.5\times10^{-13}$ at 1 s, consistent with phase noise analysis, and $3\times 10^{-14}$ at 200 s. The primary contributors to the stability budget at one second are the FM-AM noise conversion and the intermodulation effect, both emerging from laser frequency noise. These results highlight the potential of microcell-based optical references to achieve stability performances comparable to that of an active hydrogen maser in a remarkably simple architecture.