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 6×10−18 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.
@article{arxiv.0707.0096,
title = {Coherent optical phase transfer over a 32-km fiber with 1-s instability at $10^{-17}$},
author = {Seth M. Foreman and Andrew D. Ludlow and Marcio H. G. de Miranda and Jason E. Stalnaker and Scott A. Diddams and Jun Ye},
journal= {arXiv preprint arXiv:0707.0096},
year = {2009}
}