Frequency comb-based microwave transfer over fiber with 7×10(-19) instability using fiber-loop optical-microwave phase detectors.

We demonstrate a remote microwave/radio frequency (RF) transfer technique based on the stabilization of a fiber link using a fiber-loop optical-microwave phase detector (FLOM-PD). This method compensates for the excess phase fluctuations introduced in fiber transfer by direct phase comparison between the optical pulse train reflected from the remote site and the local microwave/RF signal using the FLOM-PD. This enables sub-fs resolution and long-term stable link stabilization while having a wide timing detection range and less of a demand in fiber dispersion compensation. The demonstrated fractional frequency instability between 2.856 GHz RF oscillators separated by a 2.3 km fiber link is 7.6×10(-18) and 6.5×10(-19) at 1000 and 82,500 s averaging times, respectively.