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New Journal of Physics

An accurate and robust metrological network for coherent optical frequency dissemination

Etienne Cantin, Mads Tønnes, Rodolphe Le Targat, Anne Amy‐Klein, Olivier Lopez, Paul-Éric Pottie

Published Feb 18, 2021• 59 citations
Publisher

Abstract

Abstract We introduce multi-branch repeater laser stations (MLSs) for the dissemination of an ultra-stable signal from one point to multiple users and the simultaneous evaluation of the stability and accuracy of multiple links. We perform the study of the noise floor of this new instrument. We present then an optical fiber network of 4800 km built with three MLSs and 13 repeater laser stations (RLSs). We show the multi-user optical frequency dissemination on four links totalizing 2198 km with uncertainties below 1.1 × 10 −19 . The robustness of the network over two years is presented and stability and accuracy at 10 7 seconds integration time are finally showed.

DOI
10.1088/1367-2630/abe79e

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Abstract We introduce multi-branch repeater laser stations (MLSs) for the dissemination of an ultra-stable signal from one point to multiple users and the simultaneous evaluation of the stability and accuracy of multiple links. We perform the study of the noise floor of this new instrument. We present then an optical fiber network of 4800 km built with three MLSs and 13 repeater laser stations (RLSs). We show the multi-user optical frequency dissemination on four links totalizing 2198 km with uncertainties below 1.1 × 10 −19 . The robustness of the network over two years is presented and stability and accuracy at 10 7 seconds integration time are finally showed.
We transfer the frequency of an ultra-stable laser over a cascaded optical link comprising two compensated links of 150 km and a repeater station. Each link passes through two important nodes of the telecommunication network and includes 114 km of Internet fiber simultaneously carrying data traffic, through a dense wavelength division multiplexing scheme. The metrological signal is inserted in and extracted from the communication network using bidirectional optical add-drop multiplexers. The repeater station is working independently without any remote control. The phase noise on the two links is compensated with the usual round-trip technique. The 300-km multiplexed cascaded link shows an Allan deviation of 3×10-15at one second and 7×10-20at 20 hours. This work paves the way to a wide dissemination of ultra stable optical clock signals between distant laboratories via the Internet network.
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].
Abstract We investigate optical frequency dissemination over a 1400 km long fiber link in looped configuration over a pair of underground fibers between Braunschweig and Strasbourg. This fiber link is the first to combine fiber Brillouin amplifiers with a repeater laser station. Phase-coherent operation over more than five days is demonstrated. We analyze the repeatability of the performance over four campaigns and present results of 65 d in total. The weighted mean of the fractional frequency offset of the transferred optical frequency over the complete data set is (−1.1 ± 0.4)×10–20. By analyzing the stabilization signals of the two individual fibers, the correlation of the phase noise on the two fibers is shown to be >98%.
The transfer of ultra-stable frequencies between distant laboratories is required by many applications in time and frequency metrology, fundamental physics, particle accelerators and astrophysics. Optical fiber links have been intensively studied for a decade and brought the potential to transfer frequency with a very high accuracy and stability thanks to an active compensation of the propagation noise. We are currently developing an optical metrological network using the fibers of the French National Research and Education Network. Using the so-called dark-channel approach, the ultrastable signal is copropagating with data traffic using wavelength division multiplexing. Due to significant reflections and losses along the fibers, which cannot be compensated with amplifiers, we have developed some repeater stations for the metrological signal. These remotely-operated stations amplify the ultrastable signal and compensate the propagation noise. The link is thus composed of a few cascaded spans. It gives the possibility to increase the noise correction bandwidth, which is proportional to the inverse of the fiber length for each span. These stations are a key element for the deployment of a reliable and large scale metrological network. We report here on the implementation of a two-spans cascaded link of 740 km reaching a relative stability of a few 10-20 after 103 s averaging time. Extension to longer links and alternative transfer methods will be discussed.
With the progress in the development of optical frequency standards grows the demand for the dissemination of stable optical frequencies. To date, optical fiber links constitute the most promising medium to bridge large geographical distances while still maintaining a high degree of frequency stability and accuracy. We investigated the transfer of an optical frequency along different fiber links during the past years and achieved a fractional instability and uncertainty at a level lower than 10 − 19 using fiber links with lengths of up to almost 2000 km. We give an overview of different techniques and methods that can be used in combination with optical fiber links to achieve a stable frequency transfer. The results of different fiber links are summarized and an outlook of future links is given.
We transferred the frequency of an ultrastable laser over a 108-km-long urban fiber link comprising 22 km of an optical communication network fiber simultaneously carrying Internet data traffic. The metrological signal and the digital data signal were transferred over two different frequency channels in a dense wavelength-division multiplexing scheme. The metrological signal was inserted in and extracted from the communication network using bidirectional off-the-shelf optical add-drop multiplexers. The link-induced phase noise was measured and canceled with a round-trip technique using an all-fiber-based interferometer. The compensated link showed an Allan deviation of a few 10−16at one second and below 10−19at 10,000 s This work paves the way to a wide dissemination of ultrastable optical clock signals between distant laboratories via the Internet network.