A compact ultrastable photonic microwave oscillator

Frequency comb synthesized microwaves have been so far realized with tabletop systems, operated in well-controlled environments. Here, we demonstrate state-of-the-art ultrastable microwave synthesis with a compact rack-mountable apparatus. We present absolute phase noise characterization of a 12 GHz signal using an ultrastable laser at $\sim{194}\;{\rm THz}$∼194THz and an Er:fiber comb divider, obtaining $ - {83}\;{\rm dBc/Hz}$−83dBc/Hz at 1 Hz and $ \lt - {166}\;{\rm dBc/Hz}$<−166dBc/Hz for offsets greater than 5 kHz. Employing semiconductor coating mirrors for the same type of transportable optical frequency reference, we show that $ - {105}\;{\rm dBc/Hz}$−105dBc/Hz at 1 Hz is supported by demonstrating a residual noise limit of division and detection process of $ - {115}\;{\rm dBc/Hz}$−115dBc/Hz at 1 Hz. This level of fidelity paves the way for the deployment of ultrastable photonic microwave oscillators and for operating transportable optical clocks.