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Joint Conference of the European Frequency and Time Forum and IEEE International Frequency Control Symposium (EFTF/IFCS)

Optical frequency transfer via 1840 km fiber link with superior stability

Stefan Droste, Filip Ozimek, Thomas Udem, Katharina Predehl, Theodor W. Hänsch, H. Schnatz, Gesine Grosche, Ronald Holzwarth

Published May 1, 2014• 1 citation
Publisher

Abstract

Optical frequency transfer over a 1840 km fiber link is presented. The link is actively stabilized to deliver a frequency with superior stability and accuracy. The spectral noise distribution of the stabilized link leads to an unprece-dented fast averaging in the modified Allan deviation.
DOI
10.1109/fcs.2014.6859987

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To compare the increasing number of optical frequency standards, highly stable optical signals have to be transferred over continental distances. We demonstrate optical-frequency transfer over a 1840-km underground optical fiber link using a single-span stabilization. The low inherent noise introduced by the fiber allows us to reach short term instabilities expressed as the modified Allan deviation of $2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}15}$ for a gate time $\ensuremath{\tau}$ of 1 s reaching $4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}19}$ in just 100 s. We find no systematic offset between the sent and transferred frequencies within the statistical uncertainty of about $3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}19}$. The spectral noise distribution of our fiber link at low Fourier frequencies leads to a ${\ensuremath{\tau}}^{\ensuremath{-}2}$ slope in the modified Allan deviation, which is also derived theoretically.
We investigated the transfer of a 194 THz optical frequency along a 1840 km fiber link. The fractional frequency instability expressed as the modified Allan deviation is 2.7 × 10−15 at 1 s with 4 × 10−19 after 100 s. The transferred frequency shows no systematic offset within an uncertainty of 3 × 10−19. Analyzing the underlying noise structure reveals a τ−2 response in the modified Allan deviation, which is confirmed by calculations of the frequency instability from frequency counter data as well as from phase noise data.
We demonstrate optical frequency transfer over an 1840 km underground optical fiber link using a single-span stabilization. To compensate for more than 420 dB of optical attenuation of the light we use twenty Erbium doped fiber amplifiers along the entire link and two additional fiber Brillouin amplifiers. The good passive stability of our fiber link allows us to reach short term instabilities expressed as the modified Allan deviation of 3 × 10-15for a gate time τ of 1 s reaching 4 × 10-19in just 100 s. By comparing the sent and transferred frequencies we find no systematic offset within the statistical uncertainty of about 3 × 10-19. The spectral noise distribution of our fiber link at low Fourier frequencies leads to a τ-2slope in the modified Allan deviation.
We demonstrate the long-distance transmission of an ultrastable optical frequency derived directly from a state-of-the-art optical frequency standard. Using an active stabilization system we deliver the frequency via a 146-km-long underground fiber link with a fractional instability of 3 x 10(-15) at 1 s, which is close to the theoretical limit for our transfer experiment. After 30,000 s, the relative uncertainty for the transfer is at the level of 1 x 10(-19). Tests with a very short fiber show that noise in our stabilization system contributes fluctuations that are 2 orders of magnitude lower, namely, 3 x 10(-17) at 1 s, reaching 10(-20) after 4,000 s.