Research On Polarization Diversity Coherent Optical Time Domain Reflection System Technology

China’s infrastructure investment of 45 trillion in 2022,compared to the previous maintained at 8.5% growth,bridges,railroads,highways,electricity,heat,gas and water pipelines,etc.have been developed substantially.These infrastructures,which are closely related to people’s livelihood,have to meet durability and safety if they are to be used for decades or even centuries.However,in the process of use,it is impossible to predict the impact of infrastructure by adverse factors such as erection environment,material corrosion,natural disasters and human damage.Distributed optical fiber acoustic sensing technology based on Rayleigh scattering has been widely used in many fields due to its unique advantages of being able to achieve remote monitoring,high spatial resolution,and wide sensing bandwidth.Coherent Optical Time Domain Reflectometry(COTDR)technology has been widely studied by researchers because of its higher spatial resolution and signal-to-noise ratio,wider frequency band response range,and ability to accurately restore disturbed signals,etc.However,in specific application scenarios,the sensitivity of the system cannot meet On the one hand,because the polarization direction of the incident light incident to the fiber optic interference structure is randomly perturbed with changes in ambient temperature and other changes;the birefringence effect caused by the random bending and twisting of the fiber has an impact on the polarization state of the transmitted light,and these effects change with the ambient temperature,mechanical vibration and other external factors,the optical path has polarization fading phenomenon,resulting in signal recovery errors,affecting the demodulation On the other hand,armored communication optical fibers are still used as transmission devices in most application scenarios.Although the service life and durability of these cables are high,the sensitivity of the cables is sacrificed,resulting in the loss of weak signals.In this thesis,in order to solve the above problems,we start from the hardware design and construction of Distributed Acoustic Sensing(DAS)system,adopt the In-phase and Quadrature(IQ)demodulation algorithm and polarization diversity reception to eliminate the polarization fading,and then simulate the basic properties of optical fiber to create a new high performance optical fiber.The system sensitivity is improved by simulating and producing a new type of highly sensitive optical fiber,which is then experimentally verified as follows:(1)Analysis and classification of existing sensors and distributed optical sensing technologies,and a brief introduction to their working principles and development status,and a summary of the research progress of optical cables.(2)The one-dimensional backward Rayleigh scattering model and the optical phase modulation mechanism are derived,based on which the working principle of coherent optical time-domain reflection technology and the phase demodulation algorithm are outlined,and the workflow of the IQ demodulation algorithm is briefly described.This is followed by a description of the representation of the polarization state and the causes of polarization fading for the polarization problem.Finally,the performance index of the system is analyzed to provide theoretical guidance for the subsequent construction of the system.(3)A coherent optical time domain reflection system based on IQ demodulation algorithm and polarization diversity reception technique is proposed.The system is simulated for the coherent optical time domain system beat frequency signal,and then the system is built to perform coherent demodulation using a two-in eight-out optical mixer.The key devices used in the system are tested,and the key indicators are measured and compared with the phase-sensitive optical time-domain reflection system,and the polarization diversity coherent optical time-domain reflection system effectively suppresses the polarization fading problem.(4)The principle of optical cable stress transfer is studied,and the structure of the sensitized optical cable is designed.Firstly,a theoretical analysis is carried out for the stress transfer of optical cable,and it is concluded that the sensitization effect of optical cable is related to the stress transferred to the fiber,that is,each structure of optical cable must weaken the stress as little as possible.Secondly,the simulation of parameters affecting the performance of optical cable is carried out,and four different styles of optical cable are constructed using finite element simulation software,and the optical cable structure,material,size and fiber pitch are changed to analyze the optical cable stresses.Finally,the four types of optical cables are subjected to fiber hydrophone experiments to verify the simulation results.