Dual-pulse phase-OTDR response to propagating longitudinal disturbance

A numerical model describing the effect of an external longitudinal disturbance acting on a sensitive fiber of a dual-pulse phase-OTDR is considered. In the proposed theoretical description it is assumed that the external disturbance forms a region of the elastic deformation in the optical fiber and the corresponding strain amplitude changes along this region in accordance with the shape of the Ricker wavelet. The region with the externally induced strain propagates along the fiber axis and causes the changes of the phases of the optical fields backscattered by this fiber. The resulting measured response in each spatial channel of the phase-OTDR consists of linear and nonlinear random contributions with respect to the external disturbance. The linear contribution is formed due to the change of the optical path within the gauge length-the distance between the beginnings of two scattering segments of a spatial channel. The nonlinear random contributions are produced due to the responses of the scattering segments themselves. These contributions degrade the phase-OTDR response fidelity. In particular, they lead to an error in the registration of the first arrival time of the propagating external disturbance. To verify the results of the numerical modeling, an experiment on registration of the external disturbance propagating along the fiber using a dual-pulse phase-OTDR has been carried out. In the experimental setup, the strained fiber regions with the same spatial shape of the strain propagating along the fiber were repeatedly excited. As a result, the random nature of the response measured at the phase-OTDR output has been confirmed, the statistical parameters of this response are in good agreement with the simulation results.