Coherent optical phase transfer over a 32-km fiber with 1-s instability at $10^{-17}$

TL;DR

This paper demonstrates the stable transfer of ultrastable optical frequencies over fiber links exceeding 30 km, achieving extremely low instability and phase noise, enabling high-precision frequency comparisons.

Contribution

It presents a method for maintaining phase coherence over long fiber links with record low instability and phase noise, using active stabilization and frequency combs.

Findings

Transfer instability of 6×10^{-18} at 1 s for 7 km link

Total timing jitter of 0.085 fs over the fiber

Achieved 1-Hz linewidth in heterodyne beat between independent lasers

Abstract

The phase coherence of an ultrastable optical frequency reference is fully maintained over actively stabilized fiber networks of lengths exceeding 30 km. For a 7-km link installed in an urban environment, the transfer instability is $6 \times 10^{-18}$ at 1-s. The excess phase noise of 0.15 rad, integrated from 8 mHz to 25 MHz, yields a total timing jitter of 0.085 fs. A 32-km link achieves similar performance. Using frequency combs at each end of the coherent-transfer fiber link, a heterodyne beat between two independent ultrastable lasers, separated by 3.5 km and 163 THz, achieves a 1-Hz linewidth.