| Distributed optical fiber sensing has many unique advantages,including the integrating of transmission and sensing,the distributed detecting distance reaches hundreds of kilometers,and it can be used to measure in harsh and complex environments.It has unique application advantages in the power system and other fields,which have received extensive attention and research from the academic community.Coherent optical time domain reflectometer,as one of the main directions of fiber optical sensing,can realize long-distance vibration sensitive detection,locate and restore vibration signals,and is a preferred method for distributed vibration detection.However,in the process of detecting vibration signals,since the system uses a narrow line-width laser as the light source to enhance the Rayleigh optical signal,there is a phenomenon of optical interference attenuation,and the laser has a frequency drift.In addition,the correctness of the phase unwinding also needs to be improved.All above problems will cause the signalto-noise ratio decreasing of the system,limiting the system’s ability to detect low frequency vibrations,and affecting the system’s positioning and reduction of vibration signals.Aiming at the above problems,a low-frequency vibration signal detection method based on cross-correlation spectrum algorithm was proposed.In addition,in order to improving the signal-to-noise ratio of the system,improvements are made in terms of noise cancellation and signal enhancement.As for the noise cancellation,a pulsed light modulation cascade structure with high extinction ratio is proposed.In terms of signal enhancement,the vibration signal restoration method based on an envelope-Least Mean Square(LMS)error algorithm is proposed.The envelope algorithm is used to extract the characteristics of the vibration signal from the amplitude signal.The LMS adaptive filter is designed to restore the vibration signal in the phase signal simultaneously.This article conducts research from the following aspects:(1)The basic principle of optical fiber vibration sensing is analyzed theoretically,the main detection methods of Rayleigh signals are introduced,and the amplitude signal and phase signal characteristics of Rayleigh signals are analyzed.Compared with the traditional moving differential positioning algorithm,a vibration localization method based on cross-correlation spectrum analysis is proposed and demonstrated.The theoretical spatial resolution of 20 m is increased to 3 m under the pulse width of 200 ns.(2)The system design scheme is given,the main parameters of the system are introduced,and the important components are selected.A vibration sensing system with high extinction ratio was designed and constructed,experimental analysis of the extinction ratio of the system was been taken.A high-speed signal acquisition and synchronization scheme is determined by analyzing the characteristics of the sensing signal.(3)The amplitude and phase signal demodulation scheme of Rayleigh signal is introduced,and the vibration signal reduction method based on envelope-LMS algorithm is proposed.A vibration detection experiment was designed to restore a 1k Hz sinusoidal signal and a third harmonic sinusoidal signal with a fundamental frequency of 500 Hz.The frequency-domain signal-to-noise ratio was increased from 10.4d B to 42.2d B,demonstrating the effectiveness of the restoration algorithm.The above research shows that the proposed vibration localization method based on cross-correlation frequency algorithm can achieve high spatial resolution detection of vibration signals.In a complex noise environment,the vibration detection system can effectively restore the vibration signal according to the envelope-LMS algorithm’s vibration signal restoration method,improving the signal-to-noise ratio in the frequency domain,and provide a new method for the detection of vibration signals. |
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