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arXiv:2505.10314 · Slovak-Czech-Polish Optical Conference on Wave and Quantum Aspects of Contemporary Optics

Country-wide shared fibre-based infrastructure for dissemination of precise time: coherent optical frequency with vibration sensing

Josef Vojtěch, Tomáš Novák, Elisabeth Andriantsarazo, Vladimír Smotlacha, Ondřej Havliš, Rudolf Vohnout, Michal Spacek, Martin Šlapák, Lada Altmannová, Radek Velc, Petr Pospisil, Jan Kundrát, Martin Čížek, Jan Hrabina, Šimon Řeřucha, Lenka Pravdová, Josef Lazar, Ondřej Čı́p, Jaroslav Roztočil

Published Feb 17, 2025• 0 citations
arXivHTMLPDF

Abstract

With the increasing demand for ultra-precise time synchronization and frequency dissemination across various scientific, industrial, and communication fields, the Czech Republic has developed an innovative, non-commercial fiber-based infrastructure. This infrastructure serves as a shared platform, utilizing optical fibers to enable high-precision timing, coherent frequency transfer, and a newly implemented vibrational sensing capability. The project also addresses challenges posed by classical communication noise—particularly from Raman scattering—on quantum channels, especially for Quantum Key Distribution (QKD). By strategically separating classical and quantum channels into distinct wavelength bands, such as the C-band and O-band, the infrastructure achieves minimal interference while enabling multiple concurrent applications over shared fiber lines.

DOI
10.1117/12.3056668

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•Jun 23, 2014•0 citations
With the increasing demand for ultra-precise time synchronization and frequency dissemination across various scientific, industrial, and communication fields, the Czech Republic has developed an innovative, non-commercial fiber-based infrastructure. This infrastructure serves as a shared platform, utilizing optical fibers to enable high-precision timing, coherent frequency transfer, and a newly implemented vibrational sensing capability. The project also addresses challenges posed by classical communication noise—particularly from Raman scattering—on quantum channels, especially for Quantum Key Distribution (QKD). By strategically separating classical and quantum channels into distinct wavelength bands, such as the C-band and O-band, the infrastructure achieves minimal interference while enabling multiple concurrent applications over shared fiber lines.
There has been an increased 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, cost-effective solution that utilizes shared optical networks. The presented solution provides accurate time and stable frequency at a lower operational cost, utilizing the shared spectrum of the CESNET3 network infrastructure. We are committed to future developments and upgrades that will include the next wavelength bands and geographic extensions. Additionally, we have implemented bidirectional dark channels on various wavebands, which utilize shared leased fibers and offer bidirectional compensation for fiber losses. However, operating precise time and frequency requires a single path with bidirectional amplification performed by optical amplifiers, which are sensitive to feedback from the fiber line induced by back-scattering, and reflections, and which can cause unwanted oscillations. We have addressed this issue by carefully solving the interference with parallel data transmissions. In summary, we have implemented a cost-effective solution for precise time and frequency dissemination in the Czech Republic, which utilizes shared optical networks. We are committed to future developments, and we are also part of a consortium that plans to realize a Pan-European network to offer time and frequency services to a broad range of users.
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.
The co-propagation of quantum and classical signals through shared optical fibers is crucial for scalable quantum networks. However, this coexistence is fundamentally limited by spontaneous Raman scattering (SpRS) from the bright classical light, which generates overwhelming noise that disrupts the single-photon-level quantum signals. Here, we overcome this long-standing challenge by leveraging the inherently ultralow nonlinearity of hollow-core fiber (HCF) to suppress SpRS noise. By operating both the quantum time synchronization (QTS) and classical optical frequency transfer (OFT) signals within the telecom C-band, separated by only ~10 nm, we successfully demonstrate their simultaneous transmission over a 122-km HCF link. With a classical OFT power of 1 mW, the QTS performance shows negligible degradation, maintaining sub-picosecond time stability at 2000 s, while the OFT achieves a fractional frequency instability of 10^-20. Near-sub-picosecond QTS stability is preserved even when the classical power is increased to 3 mW. Furthermore, simulations based on our experimental data indicate that with next-generation low-loss HCF, the platform can tolerate classical powers beyond 10 mW and extend the QTS range to over 500 km. By realizing a unified quantum-classical time-frequency distribution framework, this work establishes HCF as a highly capable and practical platform for future scalable quantum networks.
The development of clocks based on optical transitions during the past three decades culminates in the availability of optical clocks with unprecedented stability and uncertainty . Simultaneously, increasing requirements for accurate time and frequency signals, e.g. for tests of fundamental physics or novel applications in relativistic geodesy, put forward new challenges. Typically, such applications rely on the comparison of two remote clocks. Thus, major challenges are how to synchronize these clocks over long distances or how to get the time or frequency signal of a clock to the location where it is required. It is generally agreed that optical fiber links are an excellent alternative to established satellite based distant clock comparison and synchronization techniques. A European joint research project called Network for European Accurate Time and Frequency Transfer (NEAT-FT) has been initiated in 2011 to lay the foundations for a novel approach to disseminate high-precision timing and ultrastable frequency signals by using existing fiber infrastructure. Since Europe has a large number of modern ultra-precise clocks, special emphasis is put on the development of new techniques for time transfer and phase-coherent comparison of remotely located optical clocks and the feasibility of a European fibre network connecting optical clocks in Europe. This talk highlights recent achievements and discusses some applications and prospects.