Realization of highly reliable 10^-20-level instability optical phase transmission over a 1,402-kilometer commercial fiber-optic network

Optical phase transfer via fiber optics is the most effective method for optical frequency standard comparison on the scale below thousands of kilometers. However, the monotonic phase discrimination range of conventional optical phase-locked loops is limited, and link delays restrict the control bandwidth, which makes it a challenge to achieve a continuously reliable optical link. This paper presents an event-timing-based phase detection method that overcomes the monotonic phase discrimination range limitation of conventional phase-locked loops through dual-edge timestamp recording, achieving an optical phase measurement resolution on the order of 10 attoseconds. With such a technique, we established a 7-segment-cascaded optical link over 1402 km of commercial fiber, while sharing dense wavelength division multiplexing (DWDM) channels with live telecom traffic. The system maintained continuous operation for 11.7 days without phase cycle slips, despite encountered 15 km aerial fiber noise up to 21,000 rad²Hz^-1km^-1@1Hz. Relative instabilities of the link are 3.7×10^-15 at 1 s and 3.9×10^-20 at 100,000 s.