Long-distance frequency transfer over an urban fiber link using optical phase stabilization

We transferred the frequency of an ultrastable laser over $86\mathrm{km}$ of urban fiber. The link is composed of two cascaded $43\mathrm{km}$ fibers connecting two laboratories, Laboratoire National de Métrologie et d'Essais-Systèmes de Référence Temps-Espace (LNE-SYRTE) and Laboratoire de Physique des Lasers (LPL), in the Paris area. In an effort to realistically demonstrate a link of $172\mathrm{km}$ without using spooled fiber extensions, we implemented a recirculation loop to double the length of the urban fiber link. The link is fed with a $1542\mathrm{nm}$ cavity-stabilized fiber laser having a sub-Hz linewidth. The fiber-induced phase noise is measured and cancelled with an all fiber-based interferometer using commercial off-the-shelf pigtailed telecommunication components. The compensated link shows an Allan deviation of a few ${10}^{-16}$ at one second and a few ${10}^{-19}$ at $\mathrm{10,000}\text{\hspace{0.17em} seconds}$.