In-line extraction of an ultrastable frequency signal over an optical fiber link

We demonstrate in-line extraction of an ultrastable frequency signal over an optical link of 92 km of installed telecommunication fibers, following the proposition of Grosche in 2010 [German patent application DE 10.2008.062.139]. We show that the residual frequency noise at the extraction end is noticeably below that at the main link output when the extraction is near the input end, as expected from a simple model of the noise compensation. We obtain relative frequency instabilities, expressed as overlapping Allan deviation, of 8*10^-16 at 1s averaging time and a few 10^-19 at one day. These results are state-of-the-art for a link using urban telecommunication fibers. We also propose an improved scheme, which delivers an ultrastable signal of higher power, in order to feed a secondary link. In-line extraction opens the way to a broad distribution of an ultrastable frequency reference, enabling a wide range of applications beyond metrology.