Characterization and modelling of induced "virtual" perturbations in chirped pulse phi-OTDR

Any temperature change or strain acting on a section of the fiber induces a local variation of the refractive index. If the fiber is monitored by a chirped pulse phi-OTDR system, the variation of the refractive index causes a local shift of the backscattering trace and produces a change in the round trip time of the light coming from any further position of the fiber. While usually negligible, due to the high sensitivity of the chirped pulse phi-OTDR, in extreme occasions the distributed round trip time change may appear in the measurement as an undesired "virtual" perturbation. In this paper, we discuss and experimentally validate a mathematical model to account for (and eventually correct) the "virtual" perturbation.