Random jumps in the phase-OTDR response

In the paper, we present a qualitative analysis of the dual-pulse phase optical time domain reflectometry (phase-OTDR) response to uniform and nonuniform propagating fiber strain. It is found that on average over all realizations of scattering centers the response of the dual-pulse phase-OTDR is linear with respect to an external perturbation. Meanwhile, individual responses contain random phase jumps, which are an intrinsic property of phase-OTDR. These jumps are the result of nonlinear responses of the scattering fiber segments and arise due to interference of random backscattered fields varying in time. Two types of phase jumps are considered: π jumps and 2π jumps; the first type is caused by the fading in phase-OTDR spatial channel, while the second type occurs when a nonuniform perturbation propagates along the fiber. The origin of the phase jumps is explained by considering the simulated response on the complex plane. It is shown that the distribution of 2π jumps can be well described by the Gaussian probability mass function (PMF), provided the number of 2π jumps is large. The conducted experiments on the registration of uniform and nonuniform fiber strain confirm the presence of the jumps in the phase-OTDR response.