Experimental and Simulative Study on IM-DD Transmission and Large Chirped-Pulse DAS Coexistence

Distributed acoustic sensing (DAS), implemented through phase-sensitive optical time-domain reflectometry (φOTDR), 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-φOTDR (CP-φOTDR), 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.