Compact and 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 ∼ 194 T H z and an Er:fiber comb divider, obtaining − 83 d B c / H z at 1 Hz and < − 166 d B c / H z for offsets greater than 5 kHz. Employing semiconductor coating mirrors for the same type of transportable optical frequency reference, we show that − 105 d B c / H z at 1 Hz is supported by demonstrating a residual noise limit of division and detection process of − 115 d B c / H z at 1 Hz. This level of fidelity paves the way for the deployment of ultrastable photonic microwave oscillators and for operating transportable optical clocks.