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 ($f_{\mathrm{ceo}}$), with the repetition rate ($f_{\mathrm{rep}}$) 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 GPS standards are discussed. This work presents a new variant of highly stable RF signal generation for precision applications, such as radar, atomic clock local oscillators and optical quantum technologies.