  Optical frequency standards, lasers stabilized to atomic or molecular
transitions, are widely used in length metrology and laser ranging, provide a
backbone for optical communications and lie at the heart of next-generation
optical atomic clocks. Here we demonstrate a compact, low-power optical
frequency standard based on the Doppler-free, two-photon transition in
rubidium-87 at 778 nm implemented on a micro-optics breadboard. The optical
standard achieves a fractional frequency stability of
2.9x10$^{-12}$/$\sqrt{\tau}$ for averaging times $\tau$ less than 10$^{3}$ s,
has a volume of $\approx$35 cm$^3$ and operates on $\approx$450 mW of
electrical power. These results demonstrate a key step towards the development
of compact optical clocks and the broad dissemination of SI-traceable
wavelength references.
