Abstract

Phase-sensitive optical time domain reflectometry (phi OTDR) is a simple and effective tool that allows the distributed monitoring of vibrations along single-mode fibers. In this letter we study the effects of modulation instability (MI) in long-range phi OTDR over conventional optical fibers. We demonstrate both theoretically and experimentally that MI can induce a position-dependent signal fading which leads to a nearly complete visibility loss in the interference signal recorded at certain positions and therefore to a sensitivity loss at these positions. While this effect is detrimental in the context of distributed vibration analysis using phi OTDR, we also believe that the technique provides a clear and insightful way to evidence the Fermi-Pasta-Ulam (FPU) recurrence associated to the MI process.