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CLONETS-DS Clock Network Services-Design Study Strategy for clock services over optical-fibre networks

Josef Vojtěch, Lada Altmannová, Vladimír Smotlacha, Radek Velc, Rudolf Vohnout, H. Schnatz, Tara Cubel Liebisch, Vincenzo Capone, Tryfon Chiotis, Guy F. Roberts, Domenico Vicinanza, Artur Binczewski, Wojbor Bogacki, Krzysztof Turza, Paul-Éric Pottie, Philip A. Tuckey, Davide Calonico, Cecilia Clivati, Vittorio Curri, Ronald Holzwarth, Benjamin Sprenger, Ondřej Čı́p, Lenka Pravdová, Šimon Řeřucha, Javier Díaz, Eduardo Ros Vidal, Benoit Rat, Jan Kodet, Ulrich K. Schreiber, Jürgen Kusche, Dieter Meschede, Stefan Schröder, Simon Stellmer, Paweł Nogaś, Robert Urbaniak, Przemysław Krehlik, Łukasz Śliwczyński, Anne Amy‐Klein, Christian Chardonnet, Nicolas Quintin, Alwyn J. Seeds, Bruno Desruelle, Jean Lautier‐Gaud, Vincent Ménoret, Martin Rabault

Published Jan 1, 2023• 1 citation
Publisher

Abstract

The precise time and frequency transmissions based on optical fibre have evolved recently. The time and frequency transfer methods by GSSN are becoming gradually obsolete, definitely providing lower measurements than we can do. Yet, we do struggle with the best effotrt to enable clock comparison, and to utilize the experience with the fibre. An optical clock, they will need to be connected to others, so the collaboration is essential. We present here the further development of infrastructure in mentioned research field. And the synergy of shared entities like QKD, relativistic geodesy, any industrial and societal applications within the shared fibre, where we have been focused on.

DOI
10.1364/cleo_at.2023.jth2a.92

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The precise time and frequency transmissions based on optical fibre have evolved recently. The time and frequency transfer methods by GSSN are becoming gradually obsolete, definitely providing lower measurements than we can do. Yet, we do struggle with the best effotrt to enable clock comparison, and to utilize the experience with the fibre. An optical clock, they will need to be connected to others, so the collaboration is essential. We present here the further development of infrastructure in mentioned research field. And the synergy of shared entities like QKD, relativistic geodesy, any industrial and societal applications within the shared fibre, where we have been focused on.
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Optical clocks are considered as promising candidates for redefining the second in the International System of Units. Compared with microwave clocks, optical clocks are powerful tools for the fundamental research such as the constancy of the fundamental constants, the validity of Einstein’s theory of general relativity, and the predictions of quantum electrodynamics. Recently two research groups have demonstrated the optical clocks with an unprecedented precision level of 10-18, which is two orders better than the present primary frequency standard. Using two Sr optical clocks and three Cs fountain clocks, SYRTE group has demonstrated the definition of second with optical clocks.#br#For redefining the second with optical clocks in the future, the optical clocks from the remote laboratories should have a high precision and the frequency of the optical clocks need to be transferred over a long distance, with extremely high precision. Unfortunately the conventional means of frequency transfer such as two-way satellite time and frequency transfer can reach a 10-16 level in one day which is far below the requirement for an optical clocks. Various methods have been developed to transfer optical frequency signal via optical fibers. Especially a research group from Germany has achieved a frequency transfer stability of 10-19 level in hundreds of seconds with a fiber length of 1840 km.#br#We demonstrate the recent development of optical frequency transfer over a 70-km fiber spool at National Time Service Center. The measurement shows that the compensation for the fiber noise is close to the limitation induced by the fiber delay for the Fourier frequency from 1 Hz to 250 Hz. The transfer stability (Allan deviation) of the fiber link is 1.2×10-15 in 1 s averaging time, and 1.4×10-18 in 10000 s. A preliminary test of the optical frequency transfer over a 100-km spooled fiber is achieved with a stability of roughly one order worse than the 71 km result, 5×10-15 in 1 s.#br#We demonstrate a new scheme of remote compensation for optical frequency transfer via fibers against conventional local compensation method. This new scheme has the advantage of great simplification of the local site, which can find applications in massive extension of star network. The key feature is that we transfer the mixture of the round-trip signal and local reference to the remote user’s end via an auxiliary fiber. At remote site, the fiber noise is measured and compensated by AOM2 accordingly.#br#Transfer stabilities of 13×10-15 in 1 s averaging time and 4.8×10-18 in 10000 s are achieved with the remote fiber noise compensation via a 25 km fiber spool. The demonstrated transfer stability is comparable to that obtained by the local fiber noise compensation method.#br#The future star fiber network of optical frequency transfer can benefit from this method, because the simpler local setup is required and even can be shared in the central site for multitudinous remote users.
The unprecedented accuracy of modern optical clocks has spurred the development of matching frequency comparison techniques, with optical frequency transfer over fibre emerging as the method of choice. However, the generation and dissemination of the international reference time scale, as well as many applications outside metrology, require reliable and economically sustainable time transfer in addition to frequency transfer capabilities. To address these needs, a new EMPIR-funded project TiFOON – Time and Frequency Over Optical Networks - has been set up with the aim of advancing fibre-based frequency transfer capabilities in Europe towards a universal tool for time and frequency metrology and other demanding applications.
Precise Time and Frequency dissemination and its essential infrastructure on national or even international level are being focused recently. We present here the situation in Czechia, our strategy and approach to contribute with experience of non–commercial solution, open activity utilizing optical networks shared with other data traffic. The cost efficient solution of accurate time and very stable frequency is realized, and operated within shared spectrum of CESNET network infrastructure; the costs are under control then. We are focused on future developments, plans, upgrades concerning wavelength bands and geographic solutions and extensions. Bidirectional dark channels on various wavebands, we do realize them to utilize shared leased fibers, bidirectional compensation of the fiber losses is the benefit or must for these solutions. When operating precise Time and Frequency, single path with bidirectional amplification performed by optical amplifiers is preferred, however it is sensitive to feedbacks by fiber line, caused mainly by back scattering, reflections, and potential to cause unwanted oscillations. The interference with parallel data transmissions is the issue being carefully solved. Within this paper, we also briefly mention CLONETS Design Study project where we share the experience, and the consortium is about to plan and realize the coherent Pan-European Time and Frequency dissemination network to interconnect national research network, and to provide various Time and Frequency services for a wide range of users, research, non-commercial and commercial as well.
In the ever-advancing realm of modern technology, the demand for unparalleled precision and stability in timekeeping and frequency control has surged to unprecedented heights. As our interconnected world rellies more than ever on intricate synchronization and seamless communication, the development of cutting-edge optical infrastructure has emerged as a cornerstone in meeting these exacting demands. There has been obvious increased continuous focus on precise time and frequency transmission dissemination at a national and international level recently. We would like to present the situation in the Czech Republic, our strategy, approach, and our experience with a non-commercial, costeffective solution that utilizes optical networks shared with other traffic. The presented solution provides accurate time and stable frequency at a lower operational cost, using the shared spectrum of the CESNET3 network infrastructure.