A Fading Tolerant Phase-sensitive Optical Time Domain Reflectometry Based On Phasing-locking Structure

Phase-sensitive optical time domain reflectometry(φ-OTDR)has the advantages of fast response speed,high sensitivity and continuous distributed monitoring.It has extremely broad applications and values in perimeter security,seismic wave monitoring and many other fields.The signal monitored by the φ-OTDR system is the Rayleigh Backscattering Light(RBS).The strength and phase of the RBS signal are random,which leads to the appearance of fading during the superposition of the RBS signal.Fading will lead to a sharp deterioration of the signal-to-noise ratio(SNR),which in turn will cause significant distortion in vibration signal reconstructing.In order to suppress the fading,the SNR of the system can be improved by averaging the RBS signals.However,in the conventional φ-OTDR system,the RBS signals cannot be averaged directly.This is because that there are generally three different clock sources in conventional φ-OTDR system,which are located in the driving source of acoustic optical modulator(AOM),the pulse generator and the data acquisition(DAQ)card.They are respectively used to generate the carrier signal inside the AOM driving source,the modulation pulse signal input from the pulse generator to the AOM drive source,and the trigger signal of the DAQ card.Because these three clock sources are independent,the initial phase of each RBS signal trace collected by the φ-OTDR system will drift over time,resulting in extremely poor correlation between the RBS signals at adjacent moments.If the RBS signals with different initial phase are averaged directly,the SNR will be greatly reduced.Therefore,in conventional φ-OTDR system,it is difficult to directly average several RBS signals for noise reduction to relieve fading before phase demodulation.This paper firstly establishes a mathematical model.Then it explains and verifies that the carrier signal and the modulation pulse signal in conventional φ-OTDR come from independent clock source through simulation.On this basis,it proves that averaging the clock homologous signals has a significant effect in improving SNR compared with the clock non-homologous signals.Then a novel,coherent φ-OTDR system based on a phase-locking structure is proposed,and a synchronous pulse signal generator for realizing the phase-locking structure is designed using FPGA.This structure provides a clock homologous carrier signal,a modulation pulse signal and a DAQ trigger signal.Every original RBS signal trace acquired has the same initial phase.Then the moving average method is proposed.The moving average method is taken on RBS signals before phase demodulating to reduce the overall noise floor of the system and eliminate the phase demodulation error caused by fading.When the moving average window width is 50 for 1k Hz repetition frequency of optical pulse,the lowfrequency sinusoidal vibration signal of 1Hz is better reconstructed,and the SNR is improved by about 25 d B.Then,the optimization of the moving window width is performed through experiments,which proves that there is a corresponding moving window width value that can obtain high fidelity of the reconstructed vibration signal for each repetition frequency of optical pulse.On this basis,high SNR measurements of multiple low-frequency vibrations at different frequencies have been achieved.Therefore,it is proved that the phase-locking coherent φ-OTDR system combined with the moving average method can be applied to low-frequency vibration measurement.This paper proposes a coherent φ-OTDR system based on phase-locking structure,which can reduce the negative effect of fading on vibration signal reconstruction.This novel system expands the low-frequency measurement capabilities and is expected to significantly expand the application range of φ-OTDR in engineering.