Development of Photonic Instrumentation for Long-Range Fiber Sensing on Ultra-Stable Optical Frequency Networks

The deployment of ultra-stable optical frequency dissemination networks over existing telecommunication infrastructure opens new perspectives for large-scale fiber sensing. In France, the REFIMEVE network distributes ultra-stable optical reference signals through more than 5000 km of actively operated telecom fibers. Initially developed for frequency metrology applications, such infrastructures also provide an opportunity to investigate fiber-based environmental sensing at unprecedented spatial scales. The objective is to exploit existing permanent telecommunication infrastructures as large-scale environmental sensing systems for geophysical applications.

The originality of our approach relies on the use of ultra-stable optical carriers exhibiting sub-hertz linewidths and coherence lengths exceeding terrestrial scales. These properties enable highly coherent interferometric measurements over very long distances. These motivate the development of new photonic instrumentation dedicated to long-range sensing, potentially extending from continental to transoceanic links.

Previous experiments performed on REFIMEVE demonstrated the capability of the network to detect large seismic events (magnitude > 5) through phase perturbations accumulated along the optical path. However, these first demonstrations relied on integrated measurements with sampling times on the order of one second, limiting temporal resolution and hence event localization capabilities. In contrast with conventional distributed dynamic fiber sensing (DDFS) techniques, the measured signal corresponds to the integrated phase noise of the entire optical link, preventing direct spatial discrimination of perturbations.

Within the European project SENSEI, we investigate new coherent sensing architectures intended to combine long-range operation with improved spatial resolution. In this work, we present ongoing developments of a photonic sensing instrument based on repeater laser stations, capable of phase-locking onto the disseminated ultra-stable optical carrier. Once locked, the repeater station can introduce controlled phase modulation onto the transmitted optical field using a phase modulator. The sensing information is retrieved through coherent demodulation at the laser station (for backscattered light). We discuss the main principles underlying the proposed architecture, including modulation and demodulation strategies compatible with long-haul coherent propagation on active telecom networks.

Particular attention is given to polarization-related effects. We discuss approaches under investigation to mitigate polarization diversity and associated fading mechanisms. In addition, we examine the possibility of exploiting the evolution of the optical state of polarization itself as an additional sensing observable in such systems, potentially providing complementary information on environmental perturbations affecting the fiber link and complementary to the observations made in coherent telecommunication networks.