Nearly-Continuous Kilometer-Scale Free-Space Optical Frequency Comparison in the Presence of Doppler Shift

Unprecedented ultrastable free-space optical clock networks will promote widespread applications such as navigation, gravitational sensing, and spectroscopy. However, for optical frequency comparison, there inevitably occurs nonreciprocal phase noise due to atmospheric turbulence and the Doppler shift caused by platform motion. In this article, we propose a novel Doppler shift suppression technique, enabling nearly continuous optical frequency comparison for a kilometer-level moving platform. The Doppler shift-introduced phase deviation in the two-way optical frequency comparison (TWC) system can be passively eliminated by adding a time delay before the measurement site to match the link propagation delay. Moreover, the cycle slips and data interruptions can be effectively suppressed with the help of the phase modification algorithm based on the Kalman filtering process, resulting in nearly continuous operation of the system. The experimental results demonstrate the Allan deviation (ADEV) result can reach to
$3.45 \times 10^{-19}$
at 1000 s after time delay compensation, which is one order of magnitude lower than that without any time delay, for the 1.2-km free-space link with a simulated Doppler shift of 1 MHz. Our proposed technique provides an alternative solution for wide-scale free-space optical frequency transfer in the presence of Doppler shift.