Research purposes: Optical fiber vibration sensing system has been widely used in the oil, gas, frontier defence, prison and power industries. But, there are few reports about the application in railway defence. That is because the surrounding environment is complicated and there are many challenges to be overcomed in the optical fiber vibration sensing system application. For example, how to eliminate the effects of vibration caused by train, the natural environments such as wind and rain and how to identify and classify the intrusion events. In order to solve these problems, the feature signals of these events should be extracted firstly. Research conclusions: (1) In optical fiber vibration sensing system based on Sagnac interferometer, the peak-to-peak value, peak-to-average ratio, standard deviation, zero-crossing rate, short-term energy and kurtosis may serve as feature signals. (2) The feature signals of resting state, climbing concrete fence, breaking barbed wire, knocking concrete fence and rainstorm have been extracted, which shows significant difference among each other. (3) The research conclusions can be used in the identification and classification of intrusion events.
In polarization optical time domain reflectometry (POTDR) system, the performance of polarimetric measurement is largely constrained by the low signal to noise ratio (SNR) due to the weak Rayleigh backscattering and the degradation of the degree of polarization (DOP) of signal light. It will be indispensable to improve the SNR without sacrificing the DOP of backscattered signal for a sufficient dynamic range. In this paper, a Simplex coded POTDR (sc-POTDR) system was proposed and demonstrated. The relationships between the signal’s DOP and coding length/bit width were studied. Both numerical simulations and experiments show that the signal’s DOP has nothing to do with the length of Simplex code and only reducing the bit width can suppress the temporal depolarization effect. Applying 511-bit Simplex codes, a coding gain of 10.125dB has been demonstrated.
A novel scheme of quasi-distributed vibration disturbances detection system based on incomplete Polarization optical time domain reflectometry was proposed. The system was enhanced by employing Fresnel-reflection caused by FC/PC connector, which can improve the signal’s SNR significantly, while the temporal depolarization effect can be almost completely suppressed. Without performing any data averaging, the intrusion event can be detected and located precisely/instantaneously with good stability. Also the frequency components of vibration events applying on sensing fiber can be obtained with large dynamic range. It shows a very good potential in intrusion detection, vibration frequency measuring, etc.