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

## TL;DR

This study demonstrates the successful transfer of phase-coherent optical frequencies over an 18 km open-air link, showcasing potential for high-precision applications and free-space optical communications.

## Contribution

It is the first to achieve stable, phase-coherent optical frequency transfer over an outdoor open-air link with atmospheric turbulence suppression.

## Key findings

- Optical carriers extracted from a frequency comb were transferred with -80 dBc/Hz phase noise.
- Microwave signals derived from optical carriers achieved -105 dBc/Hz phase noise at 1 Hz offset.
- Multi-channel coherent optical communication potential demonstrated for high-data-rate free-space transmission.

## Abstract

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.

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Source: https://tomesphere.com/paper/1908.02839