X-Band photonic microwaves with phase noise below -180 dBc/Hz using a free-running monolithic comb.

Free-running mode-locked monolithic optical frequency combs offer a compact and simple alternative to complicated optical frequency division schemes. Ultra-low free-running noise performance of these oscillators removes the necessity of external phase stabilization, making the microwave systems uncomplicated and compact with lower power consumption while liberating the sidebands of the carrier from servo bumps typically present around hundreds of kilohertz offsets. Here we present a free-running monolithic laser-based 8 GHz photonic microwaves generation and characterization with a cryogenically cooled power splitter to demonstrate a state-of-the-art phase noise floor of less than -180 dBc/Hz below 1 MHz offset from the carrier.