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European Frequency and Time Forum

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

Anthony Bercy, Christian Chardonnet, Olivier Lopez, Anne Amy‐Klein, Giorgio Santarelli, Fabio Stefani, Paul-Éric Pottie, Saïda Guellati-Khélifa

Published Jun 1, 2014• 1 citation
Publisher

Abstract

In the frame of the REFIMEVE+ project, aiming at disseminating an optical frequency standard to more than 20 laboratories in France, we present the progress made towards a metrological fibre wide-area network. We specifically address the key issue of multi-user dissemination and demonstrate in-line extraction on a main link, as first proposed by G. Grosche in 2010 [4].

DOI
10.1109/eftf.2014.7331479

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arXiv:1909.08055•Applied Optics•Aug 23, 2018•53 citations
In the frame of the REFIMEVE+ project, aiming at disseminating an optical frequency standard to more than 20 laboratories in France, we present the progress made towards a metrological fibre wide-area network. We specifically address the key issue of multi-user dissemination and demonstrate in-line extraction on a main link, as first proposed by G. Grosche in 2010 [4].
High-resolution time and frequency transfer between remote laboratories are of major interest for many fundamentals applications, such as tests of general relativity or temporal variation of fundamental constants performed by comparing clocks signals. We report on the progress of a metrological fiber network on the French territory, with extensions to Europe, which aims to provide an ultra-stable frequency signal to around 20 laboratories. This network is based on the dissemination of an ultra-stable signal using optical links through the academic fiber internet network. As a first step, we have demonstrated robust links of up to 1480 km with stability at 104s and accuracy below 10−19.
SummaryWe report the performance of REFIMEVE, a national metrological network of optical fiber links, using the academic fiber network. It enables the coherent dissemination of time and/or frequency reference signals from LNE-SYRTE to around 15 labs and more than 30 in the future. We will show the latest extensions of the network in the Paris urban area and all over France, the architecture required for such al network, and the progress in terms of robustness and uptime. We will also present some applications to precision measurements, in particular ultra-high-resolution molecular spectroscopy in the mid-infrared spectral range.
The transfer of ultra-stable frequencies between distant laboratories is required by many applications from fundamental metrology to high-precision measurements. For that purpose, optical fibre links have been intensively studied over the last decade. They have demonstrated impressive results far beyond the GPS capabilities on distances up to 2000 km, thanks to an active compensation of the fiber propagation noise.Up to now, optical links has been mainly implemented between two labs, when future applications will require the development of metrological fiber networks. Towards this goal, we have developed a few techniques which allow us to distribute the ultrastable optical signal to many users simultaneously. We have designed repeater laser stations, which can be used to build a cascaded link or to distribute the ultrastable signal among two optical links [1]. These stations allow us also to compare the output end signals of two optical links. These versatile stations will be a key-component of the national metrological network currently being developed in France within the Refimeve+ project. To complete the network, we have demonstrated the extraction of the ultrastable signal from a main link and its seeding to a secondary link [2].Optical links are already been successfully used for clocks comparison. Recently we were able to compare the atomic clocks of the French and German National Metrology Institutes through two connected optical links of total length 1415-km [3]. We found that the two fully independent Sr lattice optical clocks at the two sites agreed to better than 5x10-17 limited only by the clocks uncertainties. A fractional statistical uncertainty of 3x10-17 was reached after only 1000 s averaging time, which is 10 times better, and more than four orders of magnitude faster than with any other existing frequency transfer method. The atomic fountain primary frequency standards from the two institutes were also compared and they were found to agree well within their combined uncertainties of a few 10-16 [4].
Summary form only given. The transfer of ultra-stable frequencies between distant laboratories is a fundamental issue for a wide range of high-sensitivity experiments in advanced metrology and fundamental physics. Since a few years, several experiments of optical frequency transfer were reported over dedicated fiber, with a record distance of 920 km [1] recently demonstrated in Germany. The challenge is now to build a wide frequency dissemination network connecting laboratories and research agencies in Europe.For that purpose we exploit the existing fine meshing of the Internet fiber network, connecting laboratories via the National Research Networks (NRENs). In that scheme, the ultra-stable frequency signal is copropagating simultaneously with the Internet traffic in the same fiber using a dedicated channel in a dense wavelength division multiplexing (DWDM) approach. Optical add-drop multiplexers enable to insert and extract the ultra-stable frequency signal in and from the network.We have demonstrated an ultra-stable optical link over 540 km, including 470 km of public fiber network of the French NREN (RENATER) simultaneously carrying Internet data traffic [2]. The signal bypasses four amplification stages and two Data Center Facilities using multiplexers and custom-made bidirectional erbium-doped fiber amplifiers. One specific point of this experiment is that with Internet fibers, we have a very limited control on the attenuation and noise. For instance the Erbium fiber doped bidirectional amplifiers gain is drastically limited to avoid lasing oscillations due to parasitic reflections due to connectors, splices and more fundamental Rayleigh backscattering. Nevertheless the results are comparable with those obtained with dedicated fibers. Figure 1 shows the measured fractional frequency stability, we obtain 6×10-15 at 1 s and below 10-18 after 10000 second integration time. Such optical link will enable a broad range of high-sensitivity measurements, including the search for fundamental constants variation and gravitational mapping. This concept of fiber link has been very recently used to implement a giant fiber gyroscope [3]