Active feedback stabilization of a clock-transfer link for space-based gravitational-wave detection

In space-based gravitational wave detection, ultra-stable oscillators are employed as the primary frequency reference; however, the detection sensitivity is fundamentally constrained by the inherent clock jitter of these oscillators. To mitigate such jitter-induced noise, a clock transfer link, consisting of a frequency distribution module and an electro-optic modulator, is required to be established. In this work, an active feedback control scheme is proposed for the suppression of noise within this transfer link. By extracting the GHz-order sidebands generated from the electro-optic modulator, an error signal is constructed that is then utilized to regulate the voltage-controlled oscillator, thereby enabling closed-loop noise suppression. Experimentally, a phase noise of 1×10^-6rad/Hz^1/2 at 1 mHz is achieved for the inner loop, which exceeds the detection requirement by more than one order of magnitude. For the outer loop, a phase noise of 1×10^-5rad/Hz^1/2 at 1 mHz is obtained, also meeting the mission specification. The proposed scheme is therefore demonstrated to offer a feasible technical solution for high-precision clock transfer in future space-based gravitational wave missions.