Research And Improvement Of Phase Demodulation Method For Distributed Optical Fiber Vibration Sensing System

This paper studies the distributed optical fiber vibration sensing system based on ?-OTDR.It has many advantages such as electrical insulation,intrinsic safety,anti-electromagnetic interference,small size,light weight,easy deployment and installation,and easy reuse and formation of a large-scale distributed sensor network.It has great application value and potential in the military area,power plant power station,airport bank and other important locations perimeter security,oil and petrochemical transportation pipelines,high-speed rail,large-scale construction of structural health monitoring and other aspects.For the system,the phase demodulation technology of the system signal is very important in order to obtain accurate external physical quantity change information.Among them,quadrature demodulation is one of the most commonly used phase demodulation techniques.The demodulation process is implemented on a computer.The required system structure is simple,the hardware requirements are low,and it is easy to implement.Therefore,the quadrature demodulation algorithm has been widely studied and applied since it was proposed.In this paper,the orthogonal demodulation process will be studied in detail,pointing out the lagging phenomenon caused by the vibration,and the relocation method is proposed to correct it to ensure the accurate positioning of the vibration.The SNR is 23.5dB.Furthermore,the influencing factors of spatial resolution are further discussed.The theoretical basis and the experimental analysis are used to determine the judgment basis of the spatial resolution of the system under different conditions.In addition,an in-depth analysis of the phase demodulation shows that the phase extraction process requires two phase unwrapping to effectively avoid phase demodulation errors caused by phase ambiguity.In order to avoid phase disorder,amethod of reducing the vibration by using the phase change at the adjacent position after the phase difference change peak is proposed to ensure that the vibration energy can be accurately reduced through the phase,and the consistency of the response of the system to the disturbance intensity is improved.The experiment achieves the reduction of multiple frequency vibrations and the reduction of multiple frequency superimposed vibrations by this method.The linear relationship between the vibration intensity and the phase change amplitude has a good linear correlation coefficient of0.9921.The specific work content of this article is described as follows:1.This paper introduces the theoretical basis of distributed vibration sensing based on Rayleigh scattering light.Combining the theoretical model of backward Rayleigh scattered light,the theoretical basis for extracting its amplitude and phase was analyzed.Several ?-OTDR structures and demodulation methods are described,their advantages and disadvantages are analyzed,and the factors that can affect?-OTDR system performance are pointed out and briefly analyzed.2.This paper analyzes the principle and process of ?-OTDR orthogonal demodulation in detail,points out the influence of digital filters used in orthogonal decomposition modulation on the demodulation results and system performance and carries out simulation analysis.Discussed and proposed a relocation method.At the same time,the influence of orthogonal demodulation on the spatial resolution of the system was studied.The reasons and rules were analyzed,and the method for judging the spatial resolution of the system was further defined.3.This paper analyzes the method and process of phase extraction in detail,and carries out the phase expansion and phase unwrapping operations for the phase demodulation results of the quadrature demodulation.Theoretical analysis shows that two phase unwrapping processing are needed to obtain the correct phase demodulation result.4.This paper sets up a coherent detection ?-OTDR vibration sensor system,analyzes and introduces the selection basis and performance parameters of key components used in the system.In the experiment,piezoelectric ceramic transducer was used to simulate the external vibration,and the vibration positioning lag phenomenon was compared with the simulation results.The relocation method was used to successfully correct the positioning error.In addition,the effect of orthogonal demodulation on the spatial resolution of the system is experimentally studied,and the variation of the spatial resolution with the order of the digital filter in the orthogonal demodulation is obtained.Further analysis yields the spatial resolution estimates determined by orthogonal demodulation.5.In this paper,based on experiments,the phase demodulation and phase unwrapping are explained in detail.The validity and necessity of the secondary dewrapping are verified and verified.The reasons of phase disorder in the unwrapping process are analyzed and the phase difference is proposed.The method for reducing vibration by using a phase change at a nearby position after a phase difference change peak is proposed.The experiment restores the phase of different waveforms,different frequency vibrations and multi-frequency composite vibrations,and verifies the linear relationship between the amplitude and the amplitude of the phase changes by changing the amplitude of vibration,and proposes a simple discrimination method of interference fading points.