  Compactness, robustness and autonomy of optical frequency references are
prerequisites for reliable operation in mobile systems, on ground as well as in
space. We present a standalone plug and play optical frequency reference device
based on frequency modulation spectroscopy of the D2-transition in rubidium at
780 nm. After a single button press the hand-sized laser module, based on the
micro-integrated laser-optical bench described in [J. Opt. Soc. Am. B 38,
1885-1891 (2021)], works fully autonomous and generates 6 mW of frequency
stabilized light with a relative frequency instability of 1.4e-12 at 1 s and
below 1e-11 at 1e5 s averaging time. We describe the design of the device,
investigate the thermal characteristics affecting the output frequency and
demonstrate short-term frequency stability improvement by a Bayesian optimizer
varying the modulation parameters.
