Given the ubiquity of optical fiber networks in both terrestrial and submarine environments, leveraging these facilities for sensing anomalous conditions alongside telecommunications can provide significant added value. In this context, distributed acoustic sensing (DAS) systems have been widely employed and discussed due to their sensitivity and ability to locate events. However, integrating them within existing networks is complex and expensive. On the other hand, the received state of polarization (SOP) is also sensitive to external factors, and it can be used for sensing: in this case, no extra hardware would be required since the SOP is already estimated in coherent receivers for data demodulation. The sensing information is provided “for free” by the already installed hardware, potentially requiring only a software upgrade. In this work, we analyze the feasibility of using polarization-based sensing to detect anomalous conditions in metropolitan environments. A polarimeter was used to evaluate SOP noise induced by urban factors, while a commercial coherent transceiver was employed to assess SOP estimation noise. We propose two algorithms for processing polarization data: a time-based method called SOP angular speed (SOPAS) and an adaptive, frequency-based approach named SOP-power spectral density gap (SOP-PSDG). These algorithms were compared by processing Stokes vector samples from the polarimeter when different sinusoidal vibrations are applied to the fiber through a mechanical shaker. Results demonstrate that a sampling rate of just a few tens of Hz is sufficient to effectively identify various hazardous conditions, with SOP-PSDG consistently outperforming SOPAS. Additionally, preliminary findings on the performances of these algorithms using SOP samples from a commercial coherent receiver are discussed.

The mountain scenario is often subject to catastrophic events such as rockfalls and avalanches, potentially dangerous for both people and civil infrastructures. Early-warning alarm systems that in case of such events can immediately turn on a traffic light on a mountain road and send remote alarms to control rooms, have already been developed in the geotechnical engineering sector, but all have some limitations and/or very high costs. In this work we propose an immediate-warning monitoring system based on fiber polarization sensing. Anomalies such as rockfalls and avalanches would change the instantaneous birefringence of the fiber installed in the area to be monitored, and can be sensed by looking at the state of polarization at the fiber output. The novelty of our paper is the development of an algorithm that is able to detect dangerous events, which we experimentally emulated on a reduced scale physical model of a mountain slope, in which fibers have been buried in different configurations. Our findings show that the system can correctly sense all the experimentally generated rockfall phenomena and for all the installed fiber configurations, and it is also robust to false alarms, provided that the monitoring algorithm main parameters are properly set.

Naturally occurring mass transport events can endanger human life and infrastructure integrity, especially in mountain areas often affected by landslides, debris flows, and avalanches. Currently available hazard detection systems can greatly help with the timely warning (within a few seconds) of ongoing, potentially dangerous events, but can often be inaccurate, unreliable and, due to the need for in-situ power supply, difficult to operate. Modern optical fiber sensing techniques, on the other hand, can provide exceptional accuracy but require longer measurement time and increased complexity and cost, which makes them unsuitable for such time- and cost-sensitive applications. In this paper, we present an on-field demonstration of a previously developed polarization-based optical fiber early-warning system. A mountain gully in the Valle d'Aosta region, in Italy, is used as a validation testbed to show the reliability of our approach in detecting anomalous vibration phenomena, and its robustness against the background noise associated, for instance, with car traffic or animal wildlife in the monitored area. Our findings show that the rate of variation of the state of polarization induced by real-life, hazardous rockfall events on the optical fiber is orders of magnitudes higher than the average variation rate recorded in normal conditions, thus making the proposed system a promising alternative in the mountain hazards sensors world. Moreover, we show that the system has great potential for increased accuracy in the monitoring of event dynamics by using different optical fibers in separate sections of the site, which can also be a source of additional cost reduction.

We demonstrate the real-time applicability of SOP-based anomalous vibrations detection. The proposed demo will engage the audience by showing the time evolution of two proposed metrics, with user-set parameters and different fiber-induced mechanical vibrations.

Distributed acoustic sensing (DAS), implemented through phase-sensitive optical time-domain reflectometry ($\phi$OTDR), has gained attention in geophysical and telecommunications environments. By utilizing the existing telecom infrastructure, DAS facilitates self-monitoring of networks and external events observation, from human activities to large-scale geophysical phenomena. However, integrating it with data signals on the same fiber is challenging, as DAS employs higher power signals that can interact with data channels. Although increasing DAS performance with higher bandwidths is possible, practical applications have mostly been limited to 1 GHz due to cost and complexity constraints. This paper investigates the coexistence conditions of DAS, specifically direct detection (DD) chirped-pulse-$\phi$OTDR (CP-$\phi$OTDR), with intensity-modulated and DD (IM-DD) telecommunication systems, both operating at comparable complexities (5 GHz chirp and 10 Gbit/s), particularly interesting for metro/access contexts. We varied the frequency spacing of data and DAS channels, the DAS pulse peak power, and the receiver optical filter bandwidth, identifying the conditions for feasible coexistence. Our analysis shows that nonlinear Kerr-induced modulation instability (MI) is the main impairment affecting IM-DD channels co-propagating with DAS. In this scenario, MI cannot be approximated as originating from a single frequency source, and the observed nonlinear impairments vary along the pulse with its sweeping central frequency. Numerical simulations validate the experimental results, showing close alignment.

As integrated sensing and communication (ISAC) has become a topic of considerable interest, state of polarization (SOP) monitoring is increasingly being investigated across all optical network segments. In this work, we propose a solution for simultaneous SOP sensing in upstream passive optical networks (PONs) across all active optical network units (ONUs) using a single polarimeter placed in parallel with the optical line terminal (OLT) receiver. The proposal is experimentally demonstrated using commercial GPON OLT and ONU hardware, showing that the system can detect whether the abnormal vibrations occurred in the feeder fiber or in a specific distribution fiber, thus providing coarse event localization. Since our solution exploits the inherent TDMA burst-mode operation of upstream GPON, we discuss the requirements in terms of polarimeter sampling rate and synchronization. Moreover, we experimentally characterize the ultimate limitation of the proposed SOP-sensing system when operating at very low average received power, as required by an implementation using an asymmetric tap that directs most of the power to the OLT receiver (for a minimal additional insertion loss on the data path) and only a small fraction to the polarimeter. To this end, we discuss the (post-)processing algorithm employed to extract and enhance the SOP estimate from the polarimeter, based on proper averaging of Stokes vector samples related to the same ONU burst. A quantitative metric based on the angular deviation of the Stokes vector on the Poincaré sphere is used to characterize polarization noise and estimation accuracy under heterogeneous traffic conditions. Finally, we measure and discuss the resulting SNR when applying a mechanical vibration to the feeder fiber.