Ing. Michal Špaček

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.

The dissemination of precise time and coherent optical frequency over dedicated fibers or „dark“ channels within Dense Wavelength Division Multiplexing (DWDM) networks is crucial for advanced scientific research, geophysical monitoring, and emerging industrial applications. By leveraging actively stabilized phase-coherent optical frequency transfer technique, we maintain ultra-low timing jitter and frequency stability over long-haul fiber links without disrupting of data channels. This contribution explores utilization of a shared dark spectrum within existing DWDM infrastructure to enable high-precision time transfer and stable optical frequency dissemination while simultaneously facilitating distributed vibration sensing. Furthermore, the integration of distributed fiber sensing within the same spectral allocation enables real-time detection of environmental disturbances such as seismic events and structural vibrations. Experimental results demonstrate the feasibility of this dual-purpose approach, showcasing its potential for enhancing geophysical monitoring, smart infrastructure, and next-generation metrology applications while optimizing the use of existing telecom networks.

This paper presents a comparison of the benefits of optically amplified time transfer using the White Rabbit (WR) protocol compared to regeneration-based methods utilizing White Rabbit Switches (WRS). We highlight the advantages in terms of simpler way of calibration, lower jitter, and cost efficiency compared to conventional regeneration methods.

Ultra-stable frequency transfer and highly accurate time transfer methods, using optical fibers, have made a significant progress in recent years, demonstrating exceptional performance. Optical fiber links (cables) are increasingly relevant for other modern and demanding applications and novel scientific research activities, such as metrology, relativistic geodesy, distributed fiber sensing, quantum key distribution (QKD), fundamental research (including high energy physics), redefinition of SI units, optical clocks comparison and other advanced topics and various industrial and societal applications (GPS, distant early warnings). Optical fibers can offer a valuable alternative to radio- and satellite-based methods (because of serious concerns related to failures of such services especially known for the last few years). While many countries worldwide (not limited to Europe) have been building and developing national time and frequency dissemination fiber networks and establishing international fiber connections, a generic pan-European fiber network connecting these national fiber networks, and research time and frequency facilities (optical fountains, frequency cavities) does not yet exist. In response to recognized need, the GÉANT Association has launched an ambitious initiative to connect these nowadays isolated fiber components and create a fiber-based infrastructure for widespread distribution of time and frequency throughout Europe. This initiative, to be implemented in the next phase of GÉANT's GN5-2 project, will enhance existing national fiber connections and support time and frequency transfers. The result will be a comprehensive, Europe-wide time and frequency fiber infrastructure for before mentioned fields.

GÉANT Association aims to establish a fibre infrastructure for the distribution of time and frequency throughout Europe, with the implementation planned within the Horizon Europe GN5-2 funding cycle. These new fibre links will complement existing connections performing predominantly coherent optical frequency transfers, forming a basis of comprehensive Europe-wide infrastructure. This presentation will explore how this emerging fibre network will facilitate novel scientific research initiatives in Europe. Moreover, the development of a pan-European fibre infrastructure will unlock opportunities for pioneering research in applied and fundamental science. This encompasses studies such as geodesy e.g. for underground water monitoring or unification of height systems across Europe, earthquake monitoring, the search for dark matter, and urban activity surveillance.

Achieving optimal synchronization accuracy between two White Rabbit devices hinges on the proper selection of transceivers, which act as electro-optical converters connecting WR devices to the optical network infrastructure. The correct choice of transceivers can significantly improve resilience to changes in the time offset between WR devices due to temperature fluctuations in the connecting optical fiber. To compare the performance of BiDi WDM and DWDM transceivers, an experimental setup was established under laboratory conditions to simulate a real optical network used for distributing precise time and frequency between two remote locations. The optical connection was emulated by integrating a 20 km G.652.D optical fiber into a climatic chamber, which provided variable environmental conditions similar to those experienced in real applications. The study compared BiDi WDM 1310/1550 nm transceivers with DWDM Ch33/Ch34 transceivers. Results showed that DWDM transceivers exhibited nearly thirteen times less sensitivity to temperature-induced changes in the optical connection, leading to a smaller time offset. Therefore, for achieving the highest accuracy in synchronizing WR devices in practical applications, DWDM transceiver technology is essential.

This work addresses the calibration of asymmetry in optical transmission paths for precise time and frequency distribution. Specifically, we focus on calibrating White Rabbit technology, where local calibration is not possible due to the considerable distance between synchronized nodes. We developed an automatic calibration system using micro-electromechanical optical switches, which we verified under laboratory conditions. The verification process utilizes a basic calibration method, employing an auxiliary communication channel to transmit the 1PPS signal from a remote synchronized White Rabbit node for local comparison. The remote 1PPS signal's transmission direction is time-multiplexed. Experimental results from a laboratory model of a real optical transmission system demonstrated the alignment between the automatic calibration system and our verification chain. The change in optical system asymmetry, simulated by adding additional optical fiber to the existing path, resulted in measured asymmetry changes of 101.5 ps by the automatic system and 102.6 ps by the verification chain. The overall difference between these two calibration methods was 1.1 ps. These findings confirm that the automatic system provides reliable results for calibrating asymmetry in optical transmission systems.

This paper is focused on the implementation of a Time-to-Digital Converter (TDC) inside an FPGA circuit aimed at a specific application in the field of comparison of two time scales maintained by primary time standards (atomic clocks). The design requirements of the TDC were tailored to meet the needs of this intended use. That means there is a need for a wide measuring range of hundreds of milliseconds with time resolution as best as possible (smaller than 10 ps). Commercially-available TDCs on the market do not fulfil above mentioned requirements [2]. The implemented TDC utilizes a well-known combination of a delay line and a counter, which provides excellent resolution and a wide measuring range. We have selected FPGA type Cyclone V with 28 nm manufacturing technology to develop the TDC. Thanks to the specialized usage of the FPGA adders as a delay line and manufacturing technology of the used FPGA, we obtained the TDC with a resulting resolution of 8.6 ps. The significant advantages of this solution are flexibility, scalability, simple utilization into any FPGA system and availability to tune measuring range and input control interface based on specific needs.

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 stable radio frequency dissemination is becoming standard application in optical networks. The White Rabbit system is commonly used for this purpose to support applications that require precise time and a stable frequency signal. Optical fibers are preferred for distributing the precise time and frequency signal in this system. To achieve best results, i.e. determine absolute offsets, it is necessary to know the asymmetry of the optical transmission path in which the system is deployed. We developed a device based on a MEMS optical switch that measures the delay of the optical path in both the forward and reverse directions. These measurements are used to continuously evaluate changes in the asymmetry of the transmission path, and the resulting asymmetry can be used to calibrate the time transfer system.

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 summing integration (SumInt) analog to digital conversion (ADC) technique combines and preserve many of excellent features of double integration and sigma-delta ADCs. It is well suited for application where integral of continuously sensed input signal carries information to acquire. It has been initially conceived at the PiKRON company for digitizing compounds responses measured by UV-VIS spectrophotometric detectors in high-performance liquid chromatography systems. The compound concentration in the sample is proportional to the integral/area under response peak. In a contrast to double integration ADC, the SumInt ADC integrates input signal continuously and does not require reset/idle interval control. In comparison to sigma-delta ADC, the frequency of reference switching is much lower (less charge leakage). The paid price is sampling interval floating in a range of up to one half of the fixed modulator interval. The actual ESA funded De-Risk project focuses on reuse of the technique for low analog components count conversion in radiation tolerant systems where FPGAs are already in use.

Článek popisuje inovovaný systém pro porovnávání stupnice UTC(FEL) generované cesiovými hodinami 5071A/001 v. č. 3519 v Laboratoři přesného času a frekvence FEL ČVUT (Praha 6, Dejvice) a národní časové stupnice UTC(TP) udržované v Laboratoři státního etalonu času a frekvence (Praha 8, Kobylisy). Původní systém využíval pro porovnávání metodu společných pozorování (Common-View) pomocí GNSS přijímačů GTR 51/55 a současně dvoucestnou metodu přenosu času po optických vláknech pomocí optoelektronických adaptérů MATRIX. Nová metoda porovnávání je založena na technologii White Rabbit, která je v časových laboratořích FEL ČVUT, ÚFE AV a CESNET předmětem výzkumu od roku 2019.