Acquiring the large phase jumping beyond pi with phase unwrapping assisted by auxiliary light in PHI-OTDR
The absolute difference (phase jumping) between two adjacent phases should be less than pi in order to accurately recover the phase signal with traditional phase unwrapping algorithm, which limits the capability of quantitative measurement for traditional PHI-OTDR. In this article, a new process of phase unwrapping with an auxiliary light in coherent PHI-OTDR is proposed, so the resolvable phase jumping is extended to a larger value beyond pi and the capability of phase unwrapping is improved. In this PHI-OTDR system, wrapped differential phases between a reference position and the subsequent positions of fiber are firstly acquired for all probe light pulses. And wrapped differential phases of interleaving pulse sequence are unwrapped using the traditional phase unwrapping algorithm. Then, positions where the differential phases are correctly unwrapped are obtained from the discontinuously linear profile of the phase change curves of the original probe light. Based on these positions, the information of perturbation can be correctly retrieved. In experiment, the new process of phase unwrapping with the auxiliary for PHI-OTDR is executed and a perturbation signal which induces a maximum phase jumping of 3.7154 rad is well reconstructed. More experimental results all demonstrate the validity of the proposed method.