Unidirectional two-way optical frequency comparison and its fundamental limitations

High-quality frequency transfer based on existing telecommunication fiber links allows state-of-the-art microwave and optical clocks to be compared on a continental and potentially even intercontinental scale. We present a half-a-year-long data set of unidirectional optical frequency transfer over a 2×43km urban fiber link. We observe a relative frequency instability of 8.0×10^-16 at 1 s and at best 6.0×10^-18 at 10⁶s integration time. Our results show that the unidirectional two-way method gives the possibility to perform comparisons of international primary standards over fiber links faster than over satellite links. Moreover, we investigate the major limiting factors of such a unidirectional setup.