Robust free-space optical frequency transfer in time-varying link distances conditions

Future inter-satellite clock comparison on high orbit will require optical time and frequency transmission technology between moving objects. Here, we demonstrate robust optical frequency transmission under the condition of variable link distance. This variable link is accomplished by the relative motion of a single telescope fixed on the experimental platform to a corner-cube reflector (CCR) installed on a sliding guide. Two acousto–optic modulators with different frequencies are used to separate forward signal from backward signal. With active phase noise suppression, when the CCR moves back and forth at a constant velocity of 20 cm/s and an acceleration of 20 cm/s², we achieve the best frequency stability of 1.9 × 10⁻¹⁶ at 1 s and 7.9 × 10⁻¹⁹ at 1000 s indoors. This work paves the way for future studying optical frequency transfer between ultra-high-orbit satellites.