Free-space transfer of comb-rooted optical frequencies over an 18 km open-air link

Phase-coherent transfer of optical frequencies over a long distance is required for diverse photonic applications, including optical clock signal dissemination and physical constants measurement. Several demonstrations have been made successfully over fiber networks, but not much work has been done yet through the open air where atmospheric turbulence prevails. Here, via an 18 km outdoor link, we transmit multiple optical carriers that are extracted directly from a near-infrared frequency comb over a 4.2 THz spectral range in stabilization to a high-finesse cavity with a 1.5 Hz linewidth. Proof-of-concept experiments show that the comb-rooted optical carriers are transferred in parallel with collective suppression of atmospheric phase noise to -80 dBc/Hz. In addition, microwaves can also be delivered by pairing two separate optical carriers bound with inter-comb-mode coherence, e.g. a 10 GHz signal with phase noise of -105 dBc/Hz at 1 Hz offset. Further, an add-on demonstration is made on multi-channel coherent optical communications with the potential of multi-Tbps data transmission in free space.