Study Of Threshold Algorithm For Φ-OTDR Fiber-optic Distributed Sensor System

Fiber-optic distributed disturbance sensor (FDDS) is wildly used in many fields such as perimeter security, oil and gas pipelines, communication cable due to its distinct advantages such as high sensitivity, resistance to electromagnetic interference, long distance monitoring, no external supply power, simple structure, accurate positioning precision, need only one optical fiber sensing, and so on. Phase-sensitive optical time domain reflectrometer (Φ-OTDR) has become a new hot spot in the field of FDDS because it has simple structure, easy to implement, system stability, accurate positioning and have the ability to detect multi-point disturbance events occurring simultaneously, et al. Accurate threshold is the key to reduce the nuisance alarm and the false positives of the O-OTDR distributed optical fiber sensor system effectively. In this thesis, we focus on the method of threshold process for Φ-OTDR distributed optical fiber sensor system. Based on selective average, a threshold acquisition algorithm is proposed by adopting the template for matching threshold. The results show that the proposed method can set the threshold more effectively than the current methods and signal-to-noise ratio is improved and thus the nuisance alarm and the false positives are lowered for the Φ-OTDR distributed optical fiber sensing system.The main achievements are described as the following:1. Theory and characteristics of Φ-OTDR distributed optical fiber sensing system are introduced. Based on the theory of optical waveguide and elastic mechanics, mathematical model of the disturbance is established for Φ-OTDR fiber-optic distributed sensor system. It offers theoretical basis of the signal processing and threshold algorithm of the sensing signal in the following study.2. The progress of threshold algorithm and application of Φ-OTDR distributed optical fiber sensor system are summarized. The application of current threshold schemes are investigated experimentally for Φ-OTDR distributed optical fiber sensor system. In terms of the disadvantage of the current the Φ-OTDR distributed optical fiber sensing system, we proposed a threshold algorithm based on selective average by adopting the template for matching threshold. The results demonstrate that the threshold algorithm based on the selective average can set the threshold more effectively than the current threshold algorithms. The signal-to-noise ratio is improved and thus the nuisance alarm and the false positives are reduced in the Φ-OTDR distributed optical fiber sensing system.3. Experimental system of Φ-OTDR FDDS is set up and the proposed threshold algorithm based on selective average is demonstrated experimentally. Compared with the direct average and moving average threshold algorithm, false positives of the proposed threshold algorithm can be reduced by 5% and 3%, respectively. The nuisance alarm can be lowered by 2%. Signal-to-noise ratio were increased by the increment of 2.55 dB and 1.1 dB, respectively. Compared with the scheme of combining MZ interferometer, the non-response rates of the Φ-OTDR was reduced by 4%. Experimental results demonstrate that the proposed algorithm can improve the stability and reliability of the O-OTDR distributed optical fiber sensor system.