  We report on the development and short-term stability characterization of an
optical frequency reference based on the spectroscopy of the rubidium
two-photon transition at 778 nm in a microfabricated vapor cell. When compared
against a 778 nm reference signal extracted from a frequency-doubled
cavity-stabilized telecom laser, the short-term stability of the microcell
frequency standard is 3.5 $\times$ 10$^{-13}$ $\tau^{-1/2}$ until 200~s, in
good agreement with a phase noise level of $+$ 43 dBrad$^2$/Hz at 1~Hz offset
frequency. The two main contributions to the short-term stability of the
microcell reference are currently the photon shot noise and the intermodulation
effect induced by the laser frequency noise. With still a relevant margin of
progress, these results show the interest of this spectroscopic approach for
the demonstration of high-stability miniaturized optical vapor cell clocks.
Such clocks are poised to be highly beneficial for applications in navigation,
communications, and metrology.
