Implementation of ultrastable microwave frequency transfer with optical–electrical–optical relay stations over long-distance urban fiber link

The frequency transfer over long-distance optical fiber link is one of the important research subjects in the field of time and frequency. In this paper, the implementation of ultrastable microwave frequency transfer with optical–electrical–optical (OEO) relay stations are investigated over long-distance urban fiber link, and the experimental results show that the method is effective. In an effort to establish a full urban fiber link of long distance, a 106 km urban fiber is split into three sub-links by two customized dense wavelength division multiplexing components, then extended to 212 and 318 km with one and two OEO relay stations, respectively. The achieved frequency instability of the 212 and 318 km compensated link is 5.7 × 10⁻¹⁵ at 1 s, 4.0 × 10⁻¹⁸ at 4 × 10⁴ s and 1.3 × 10⁻¹⁴ at 1 s, 8.3 × 10⁻¹⁸ at 10⁵ s, respectively. In addition, a frequency signal coherent with the reference signal of the transfer system is extracted at each relay station along the 318 km compensated link. It shows that the frequency instability of the signal extracted at the first and second relay stations relative to the reference signal is 1.4 × 10⁻¹⁴ at 1 s, 4.6 × 10⁻¹⁷ at 10⁴ s and 2.7 × 10⁻¹⁴ at 1 s, 8.0 × 10⁻¹⁷ at 10⁴ s, respectively. The proposed scheme can potentially be used to further extend the length of fiber link by adding relay stations, which provides a novel solution for multiple-access ultrastable microwave frequency transfer over long-distance urban fiber link.