Phase-stabilized free-space link for optical frequency transfer

Optical frequency transfer through the atmosphere is needed for diverse free-space applications, e.g. coherent communication, optical clock comparison and relativity experiments. The task requires an appropriate optical link system that provides the multimodal abilities of not only pointing, acquisition, and tracking (PAT) but also phase noise suppression in the presence of air turbulence. Here, we describe a free-space optical (FSO) link system built over a 1.4 km outdoor path to transmit optical frequencies with a fast beam tracking capability together with an active feedback compensation scheme of the atmospheric phase noise. The FSO link is able to offer a frequency transfer stability of 3.51 × 10−19 at 100 s averaging, which suffices to deliver the current state-of-the-art optical clock signals. •A free-space optical (FSO) link system was built over a 1.4 km outdoor path.•A tilt-tip control is applied for reducing beam wander induced by atmosphere.•The received beam is controlled by a fast steering mirror to keep power stably.•The FSO link is able to offer a frequency transfer instability of 3.51 × 10−19 at 100 s averaging time.