A hybrid solution for simultaneous transfer of ultrastable optical frequency, RF frequency and UTC time-tags over optical fiber

Abstract— We describe a fiber-optic solution for simultaneous distribution of all signals generated at today’s most advanced time and frequency laboratories, i.e. an ultrastable optical reference frequency derived from an optical atomic clock, a radio frequency precisely linked to a realization of the SI-Second, and a realization of an atomic timescale, being the local representation of the virtual, global UTC timescale. In our solution both the phase of the optical carrier and the delay of electrical signals (10 MHz frequency reference and one pulse-per- second time tags) are stabilized against environmental perturbations influencing the fiber link instability and accuracy. We experimentally demonstrate optical transfer stabilities of 5×10-18 and 2×10-15 for 100 s averaging period, for optical carrier and 10 MHz signals, respectively.

Index Terms —fiber-optics, time and frequency transfer, atomic timescale, atomic clocks

I. INTRODUCTION OR today’s time and frequency (T&F) metrology the generation of precise and accurate frequency references or timescales are important for the most advanced scientific and industrial applications such as the search for a temporal variation of fundamental constants, geodesy, or mobile telecommunication.
The references or timescales discussed here are derived from either primary clocks such as Cs or Rb fountain clocks operating in the microwave domain [1], or more recently from optical atomic clocks, such as lattice clocks or single ion clocks [2]-[6]. Primary clocks developed over the last six decades have reached an uncertainty level in the low 10-16 [7], but optical atomic clocks supersede their microwave counterparts by two orders of magnitude reaching down to a few times 10-18. Considering this and the fact that optical

This work was supported by the EMPIR initiative co-funded by the European Union’s Horizon 2020 research and innovation programme and the Participating States via project 15SIB05. Funding was received from the Faculty of Computer Science, Electronics and Telecommunications, AGH University of Science and Technology. We would like to thank Thomas Legero for the help with providing ultrastable optical carrier used in experiments performed at PTB. Przemysław Krehlik and Łukasz Śliwczyński are with Faculty of Computer Science, Electronics, and Telecommunications, AGH University of Science and Technology, Krakow, Poland (e-mail: krehlik@agh.edu.pl, sliwczyn@agh.edu.pl). Harald Schnatz is with the Physikalisch-Technische Bundesanstalt, Braunschweig, Germany (e-mail: Harald.Schnatz@ptb.de). clocks are becoming operational in many laboratories worldwide, the generation of a superior optical timescale has been discussed and has recently been demonstrated in [7]. However, the generation of such accurate frequency references and timescales requires substantial effort in terms of cost and personnel, thus is realized only in a few dedicated institutions over the world - usually National Metrology Institutes (NMIs). In this situation the distribution of locally generated reference signals to remote users such as universities, scientific or navigation centers, telecommunication institutions, potentially other (smaller) NMIs, and other demanding customers is crucial for their widespread use. The traditional solutions for long-haul T&F transfer are based on satellite techniques, that use either GNSS or geostationary satellites [8]-[10]. The satellite T&F transfer is widely accessible, but does not allow to exploit the actual accuracy and stability of the best atomic clocks and timescales at the remote end. Additionally, even sophisticated satellite techniques allow only to compare two distant clocks by long- term data acquisition and post-processing due to the inherent noise of the T&F satellite link. This hampers real-time applications that require ultra-low noise at short time scales.
Various techniques based on optical fibers as a transmission medium are a rapidly developing alternative for the distribution of time and/or frequency signals [11]-[14]. However, the main problem which has to be addressed in any solution are the phase (delay) fluctuations, induced in the fiber by environmental perturbations, such as temperature variations, vibrations, and mechanical stress. Additionally, for a transfer of a timescale (e.g. a UTC realization produced at a NMI), the signal propagation delay of the entire transmission system has to be stabilized and calibrated.
The common idea used in practically all fiber-optic T&F distribution systems is to arrange a bi-directional transmission in a single fiber, to take advantage of the high symmetry of the phase fluctuations occurring in both directions. The system may operate either as a simple two-way link or in a closed- loop phase-stabilizing scheme.
For transferring a radio-frequency (RF) reference signal (usually 10 MHz

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