Remote Sr optical clock comparison with 10−17 fractional instability through a 54 km urban fiber link

Abstract

We present a frequency instability comparison between two Strontium optical lattice clocks situated at the two campuses of National Institute of Metrology of China (NIM) over 54-km-long optical fiber link. Two clock lasers are locked to the clock transitions of strontium atoms without corrections for the optical lattice clocks’ systematic frequency shifts and relativistic red shift, etc. We utilize the 1542 nm (ITU grid, Channel 44) ECDL laser for the transmission of optical frequencies and assess the fractional instability of the transmitted out-of-loop signal using a 108 km loop-back fiber. The fractional instability of the fiber link attains 1.2 × 10⁻¹⁶ at 1 s, which closely matches the theoretical limit calculated from the Power Spectral Density (PSD) of the fiber link noise. A modified transfer oscillator (TO) was employed to perform the frequency comparison between remote clock laser (698 nm) and transfer laser (1542 nm). Ultimately, the inter-site comparison of the two clocks achieved a fractional instability better than 5.6 × 10⁻¹⁶ at 1 s and 3.5 × 10⁻¹⁷ at 10,000 s.