Reaching Long-Term Stability in CP-phi OTDR

Distributed Acoustic Sensing (DAS) based on coherent reflectometry with chirped pulses (CP-phi OTDR) is a recent, yet widely recognized technique in providing spatially-resolved distributed measurements of certain physical quantities over the length of an optical fiber. Any given perturbation along the cable can be retrieved as a local temporal shift of the backscattered optical power trace, and recovered using sequential trace-to-trace correlations. However, as a technique that relies on a General Cross-correlation (GCC) estimation function, it displays a non-zero probability of obtaining anomalous estimations, which can fundamentally limit the long-term performance. Furthermore, some demonstrated strategies proposed to mitigate this effect can directly lead to the emergence of cumulative errors, which become as critical as the measurement time increases. Here, we analyze the errors affecting the long-term stability in CP-phi OTDR systems, and we propose several strategies and processing methods to mitigate their effect. We demonstrate a similar to 3 mK error after a one-month continuous temperature experiment performed to a SMF, reaching unprecedented temperature stability in CP-phi OTDR systems with important implications in emerging fields such as DAS seismology or oceanography.