Characterization of long-term stable photonic microwaves based on a difference frequency comb

We report on a novel method for optical microwave generation using a frequency comb based on difference-frequency generation, which passively eliminates the carrier–envelope offset frequency (fceo), with the repetition rate (frep) locked to an optical reference. We demonstrate the generation of ultra-low phase noise microwave signals by transferring the stability of the optical reference to 9.6 GHz, reaching noise levels of −147 dBc/Hz at 1 kHz offset. The optimization of pulse timing after interleaving and a scheme for additional long-term stabilization of the microwave signal to global positioning system standards are discussed. This work presents a new variant of highly stable radio frequency signal generation for precision applications, such as radar, atomic clock local oscillators, and optical quantum technologies.