The optical fiber deployed in the metropolitan infrastructure carrying high-speed WDM traffic data is used also for network surveillance, identifying and localizing the onset of vibrations and dynamic stress events, potentially dangerous for the integrity of the fiber telecom asset. A coherent approach is adopted in a counter-propagating interferometer to develop a sensing system exploiting the ring layout of the metro net- work. With respect to usual dual Mach-Zehnder interferometric sensors, where typically the reference arm is kept isolated, in our arrangement we use as reference arm a fiber inside the same deployed cable, minimizing the impact of the city strong environmental noise. Preliminary assessments of the proposed sensing solution have been experimented in a metro ring of 32 km standard single-mode fiber deployed in the city of Turin, Italy, devoted to telecom applications. The sensor is demonstrated to operate preserving the coexistence with the high-speed WDM telecom traffic. With respect to complex and expensive sensing solutions based on pulsed phase-OTDR techniques, in the present case continuous-wave counter-propagating optical signals are exploited and the spatial resolution in dynamic event localization depends only on the receiver sampling rate: in our implementation, by adopting off-the shelf low-cost commercial 20MS/s sampling boards, 10-meter spatial resolution is experimentally achieved. The proposed sensing solution provides significant added value to the deployed metro fiber infrastructure and the same interferometric approach based on the counter-propagating layout can be easily exploited in metro networks even in case of presence of optical nodes equipped with reconfigurable optical add-drop multiplexers useful for WDM traffic routing.

We demonstrate the possibility of using a deployed Passive Optical Network (PON) infrastructure for structural vibration monitoring for smart city applications and simultaneously downstream 10Gbit NRZ transmission. The impact of PON trunk fibre backscattering noise on sensing system performance is experimentally assessed.

A coherent optical fiber sensor with adequate sensitivity for detecting the acoustic emission (AE) during the propagation of a crack in a ferrous material is presented. The proposed fiber optic sensor is successfully compared in terms of the SNR (Signal to Noise Ratio) and detectable AE energy levels to commercially available AE piezo-transducers sensors and is proven to be an effective and advantageous alternative for sensing and monitoring fatigue damage in structural applications.

A counter-propagating coherent vibration sensing approach is exploited in a 32km deployed fiber ring network, proving its feasibility in early detection of critical events that may damage and put out of service the optical infrastructure.