Integration of vibration sensing in time transfer infrastructure

Integration of vibration sensing in time transfer infrastructure Josef Vojtecha, Tomas Novaka,d, Elisabeth Andriantsarazoa,c, Vladimir Smotlachaa,, Jaromir Simaa, Ondrej Havlisa, Michal Spaceka,c, Martin Slapaka, Tomas Horvatha, Rudolf Vohnouta, Lada Altmannovaa, Radek Velca, Petr Pospisila, Jan Kundrata, Martin Cizekb, Jan Hrabinab, Ondrej Cipb, Jaroslav Roztocilc aCESNET z.s.p.o., Prague, Czechia; bInstitute of Scientific Instruments of the Czech Academy of Sciences, Brno, Czechia; cFaculty of Electrical Engineering, Czech Technical University, Prague Czechia; dFaculty of Nuclear Science and Physical Engineering, Czech Technical University, Prague Czechia ABSTRACT 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 realtime 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. Keywords: precise time; coherent optical frequency; shared fiber infrastructure; Raman scattering; vibrational sensing; quantum-safe communication 1. INTRODUCTION Precise time and coherent frequency transfer over optical fibers has become a mature technology for applications requiring ultra-high synchronization accuracy. By utilizing advanced modulation schemes, phase-stabilized links, and ultrastable optical carriers, this method enables the distribution of timing and frequency references with sub-nanosecond time deviation and fractional frequency instabilities below 10⁻¹⁸ over hundreds to thousands of kilometers. Such performance has been demonstrated in numerous studies, including the work by Predehl et al. [1], who achieved a frequency transfer over a 920 km stabilized fiber link with an instability of 10⁻¹⁸, and Lopez et al. [2], who demonstrated a 540 km time and frequency dissemination network in France with comparable precision. These capabilities are crucial for scientific and industrial applications, including optical clock comparisons [3], very long baseline interferometry (VLBI) [4], synchronization of large-scale research infrastructures such as particle accelerators, and time-critical services in telecommunications and finance. Fiber-based time and frequency transfer systems thus provide a superior alternative to satellite-based methods such as GNSS, ensuring robust, traceable, and interference-resilient synchronization even in complex environments. Sharing existing telecom fiber infrastructure for the transfer of ultrastable time and frequency signals offers significant economic advantages by avoiding the high capital and operational costs of deploying dedicated links. This approach leverages already-installed optical networks, enabling precise synchronization services for research, finance, energy, and telecommunications sectors without major additional investment. Multiple works indicate that coexisting data and metrology signals over the same fibers can be achieved with negligible degradation of performance, making it a cost-efficient and scalable solution for national and international time–frequency dissemination networks [5]. 2. COHERENT REFERENCE TRANSFERS AND FIBER SENSING Techniques such as phase-sensitive optical time-domain reflectometry (φ-OTDR) have been successfully used to detect seismic events, monitor structural health, and localize acoustic perturbations with high spatial and temporal resolution [6,7]. However, presence of high-power pulses into the fiber may be limiting for others mean of transport [11]. On other hand optical fibers deployed with coherent reference transfer techniques provide not only ultra-stable optical frequency dissemination, but have also emerged as powerful tools for sensing of vibrations and acoustic phenomena. Recent advancements show that coherent frequency transfer schemes used in metrology networks can be repurposed or combined with sensing modalities to perform simultaneous transmission of frequency references and detection of mechanical disturbances along the same fiber [8,9]. This dual-use capability is especially attractive for applications in critical infrastructure monitoring, geophysical research, and smart cities, where leveraging existing telecom fiber networks for both precision timing and distributed sensing reduces deployment costs while enhancing functionality [10]. 3. POLARIZATION CHANGES MONITORING Changes in the state of polarization (SOP) in optical fibers caused by external perturbations—such as vibrations, acoustic waves, or mechanical strain—can be exploited for sensing. When an external force induces micro-bending or stress, the birefringence in the fiber changes locally, resulting in measurable variations in the SOP of backscattered or transmitted light [12,13]. Unlike phase-sensitive systems, polarization-based sensing does not require interferometric stability and can operate effectively with simple setups, making it attractive for applications such as perimeter intrusion detection, infrastructure monitoring, and seismic sensing [14]. SOP changes in backscattered signal can be used for polarization optical time domain reflectometry (P-OTDR), detecting environmental disturbances by monitoring SOP fluctuations along the fiber link. However, reach of P-OTDR is quite limited and high/power pulses launched into the fiber are necessary [12]. On other hand SOP monitoring can be implemented as non-intrusive method allows it to be deployed alongside conventional optical communication systems without disrupting data transmission, thus potentially enabling dual-use capabilities in existing telecom infrastructures [15]. Figure 1. Four output signal values from Polaribox device recorded during service works in inline hut CESNET with partners develops time and frequency infrastructure CITAF. It also deploys White Rabbit (WR) technology, an advanced protocol that synchronizes clocks with sub-nanosecond precision over Ethernet. WR employs Synchronous Ethernet (SyncE) and IEEE 1588 Precision Time Protocol (PTP), utilizing two-way message exchanges to ensure precise clock phase and offset synchronization [16]. Contrary to [15] we decided not to use dedicated signal for SOP sensing, but to listen (via optical coupler) on WR transfer. On 322km field deployed line we deployed WR transfer using commercial Small Form Pluggable (SFP) transceivers, channels 8 and 9 of Dense Wavelength Division Multiplex (DWDM) (wavelengths 1571.24 and 1570.42 nm) featuring extended input sensitivity of -32 dBm and +1 dBm or higher output power. Loss of the line has been compensated by five bidirectional EDFAs CzechLight SDN bidi. In front of their receivers we placed 50/50 couplers and install SOP monitoring devices described in [15]. In Figure 1. are clearly illustrated recorded SOP changes during service works, rack door open/close, transmission system doors open/close, and also fiber touching by service staff. Installation of monitoring couplers caused decrease of received power but influence on time transfer stability reaching TDEV of 8.10-12s for 103s averaging has not been observed. 4. PROSPECTIVE APPLICATION OF VIBRATIONAL SENSING This infrastructure also includes a vibrational sensing option, which allows monitoring of environmental changes in realtime. This capability enables detection of physical disturbances or structural changes along the fiber route, enhancing the system's utility for applications beyond metrology. There are multiple possibilities, the most affordable ones are represented by utilizing Doppler noise canceling processes on lines with coherent frequency transfers [10] or polarimetry [11]. Such a capability would be valuable for applications like detecting seismic activities, and ensuring security by identifying physical disturbances or potential breaches along the fiber route. This addition would further enhance the CITAF infrastructure’s versatility, offering an extra layer of utility for scientific, security, and environmental monitoring purposes. 5. CONCLUSIONS AND FUTURE PROSPECTS We showed that precise time transfer using White Rabbit technology can also be utilized for non-intrusive tampering/vibration sensing even for very long (300+ km) lines with bidirectional transfer. 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