Research On The Positioning Accuracy And Demodulation Algorithm Of Interferometric Fiber Optic Vibration Sensor

Distributed optical fiber vibration sensors have attracted considerable attention in recent years for their many advantages,such as intrinsically safe,immunity to electromagnetic interference,corrosion resistance,large-scale monitoring.Nowadays,distributed vibration sensor is one of the most promising candidates among the numerous sensing technologies that are suitable for civil facilities perimeter security,oil/gas pipeline safety monitoring,bridge structural health monitoring,and military base invasion early warning,etc.The interferometric fiber optic vibration sensing technology can be divided into two types:forward-transmission interference vibration sensor and back-scattering interference vibration sensor.For the forward-transmission interference vibration sensor,such as Sagnac interferometer and Mach-Zehnder interferometer,the changes of measured are modulated into the phase changes of the transmission light.The phase signals are converted into intensity signals through the interferometric structure to realize the detection of vibration signals.This kind of system has relatively simple structure and fast system response,however,the positioning accuracy is low.The back-scattering interference vibration sensor,such as phase-sensitive optical time domain reflectometer(?-OTDR),utilizes the interference effect of Rayleigh back-scattering light produced by highly coherent optical pulses,and the vibration position is obtained by establishing the relationship between back-scattering signal and time.The positioning accuracy is high,but there still exist some problems such as complex demodulation of the vibration signal and relatively low real-time positioning performance.This paper focuses on the positioning accuracy and demodulation algorithm of interferometric fiber optic vibration sensor.The main contents include:(1)A high-precision Sagnac vibration sensing system based on chaotic laser is proposed.The external cavity feedback semiconductor chaotic laser is used as the light source.The high-order null-frequencies in the vibration spectrum are utilized to locate the vibration.The experimental results show that the vibration positioning error is 22 m on a total length of 12.101 km sensing fiber.(2)A pulse-modulated Mach-Zehnder interferometer is proposed for high-precision vibration location.In the system,the continuous wave light is modulated into periodic pulse sequences and injected into the interferometric structure.The vibration position can be located by analyzing the envelope characteristics of the interference pulse sequence.The experimental results demonstrate that the vibration location error is 9 m over 940 m fiber length under20 ns pulse width.(4)An envelope orthogonal demodulation method is proposed to characterize the relative amplitude of external vibration in?-OTDR system.The influence of vibration on coherent Rayleigh back-scattering light is simulated,and the relationship between the interference amplitude peak power and vibration amplitude is derived.The interference peak power is calibrated according to the envelope orthogonal demodulation results.The experimental results show that the calibrated interference peak power increases linearly with the vibration amplitude,and a high correlation coefficient(R~2=0.93071)for linear fitting is obtained.(5)A back-scattering signal data matrix matching demodulation method is proposed and employed in the?-OTDR system for improving real-time vibration detection performance.The original Rayleigh back-scattering signal is transformed into the data matrix form.Subsequently,and the effective sensing data is extracted from the data matrix and then transmitted to the upper system at high speed.The experimental results show that the average response time of the system to external vibration is 50.1 ms.