Free-space optical-frequency comparison over rapidly moving links

The comparison of optical reference frequency signals over free-space optical links is limited by the relative motion between local and remote sites. For ground to low-Earth-orbit comparison, the expected Doppler shift and Doppler rate typically reach ±4 GHz at 100 MHz s-1, which prevents the narrow-band detection required to compare optical frequencies at the highest levels of stability. As an important step to achieve these goals, we demonstrate a system capable of optical-frequency comparison in the presence of significant Doppler shifts. This system is demonstrated over a retroreflected drone link, with a maximum line-of-sight velocity of 15 m s-1 and Doppler shift of 19 MHz at a Doppler rate of 1 MHz s-1. The best fractional frequency stability obtained is 7×10-18 at an integration time of 5 s. These results are an important step toward ground to low-Earth-orbit optical frequency comparison, providing a scalable terrestrial testbed.