Fading Noise Suppression in Φ-OTDR Based on Nearest Neighbor Analysis

Phase sensitive optical time domain reflectometry (Φ-OTDR) has been widely used in the precise acquisition of the vibration signal along the sensing optical fiber, and has attracted more and more interest. However, polarization fading and coherent fading usually lead to the phase hopping of the recovered signal, thus it is difficult to realize the continuous measurement of vibration, especially the low frequency vibration. The polarization diversity and multi-frequency detection schemes which are usually used to solve the above problems need additional hardware, thus increasing the cost and complexity of the system. In this article, we propose a statistical method based on nearest neighbor analysis. For each interrogation fiber separation, the phases of the sub-separations are evaluated and selected to reconstruct the vibration. In the experiment, the vibrations with frequencies as low as 0.2 Hz are loaded on the fiber and recovered. The results show that the method can effectively suppress the phase hopping, correct distorted vibration and improve the performance of low frequency vibration measurement of Φ-OTDR system on the basis of only numerical calculation.