  Low phase noise microwave oscillators are at the center of a multitude of
applications that span the gamut of photonics and electronics. Within this
space, optically-derived approaches to microwave frequency synthesis are
particularly compelling owing to their unique combination of ultrawideband
frequency access and the potential for resiliency to temperature and
environmental perturbation via common-mode noise rejection. We demonstrate here
an optical frequency divider that uses the 30 terahertz frequency gap between
two stimulated Brillouin scattering (SBS) lasers as the basis for frequency
division. The resulting microwave signal, centered at 10 GHz frequency,
exhibits exceptionally low phase noise levels of -95 dBc/Hz and -110 dBc/Hz at
10 Hz and 100 Hz frequency offset, respectively. Moreover, the two SBS lasers,
generated from a common fiber resonator, exhibits a high degree of correlated
noise cancellation in their frequency difference. We measure 16.1 dB of noise
rejection against intentionally applied vibrations, thus showcasing a promising
pathway towards portable and robust ultralow noise photonic-microwave
synthesis.
