Research On Polarization Dependence Of Distributed Vibration Sensing Based On (?)-OTDR

With the high-speed development of economy and technology,the demand in the fields of health monitoring on large engineering structure?underwater acoustic detection?perimeter security and oil exploration is increasing rapidly.To meet the requirements of these applications,it is essential to detect and analyze vibration events with a high quality.Phase-sensitive optical time-domain reflectometry((?)-OTDR)based on Rayleigh scattering gains extensive attention for its advantage on long sensing distance?distributed measurement and high sensitivity.Phase noise level determines the minimum detectable phase signal and makes it the key factor limiting distributed sensing performance of vibration events.Interference fading and polarization fading are crucial in affecting phase noise of the system,while their polarization dependence leads to the result that phase noise is associated with input polarization states.Conducting research and analysis on polarization dependence of phase noise of (?)-OTDR system benefits distributed vibration sensing capacity significantly in terms of measurement accuracy.This paper focuses on polarization dependence of Rayleigh signal and phase noise of (?)-OTDR system as well as orthogonal polarization switching method based on PGC demodulation.Main research contents include: an analysis of generation and sensing mechanism of Rayleigh signal based on the model of multiple Rayleigh scattering interference within pulse duration,verifying polarization characteristics of the intensity of Rayleigh signal;analyzing main phase noise sources and the necessity of research into interference fading and polarization fading induced phase noise,in-depth theoretical analysis and experimental verification on polarization characteristics of phase noise;building orthogonal polarization switching experimental setup based on PGC demodulation on the basis of theoretical analysis and implementing effective restrain of polarization fading while eliminating the effect of polarization disturbances on the system.The main innovative points of the paper and the research results are as follows:1?analyze polarization dependence of Rayleigh signal on the basis of the analysis of generation and sensing mechanism of Rayleigh signal accounting for the model of multiple Rayleigh interference within pulse duration,and the link of polarization changes with Rayleigh signal intensity as well as the relation between polarization and phase demodulation is obtained,laying the foundation for subsequent analysis of polarization characteristics of phase noise.2?put forward the idea of analyzing phase noise level based on intensity of the original interference signal on the basis of theoretical analysis of system phase noise.Interference fading and polarization fading cause low intensity and then induced intensity noise is converted into phase noise.When there is a large intensity,the corresponding signal to noise ratio is high,which in turn causing a low phase noise level.3?verify polarization characteristics of phase noise in distributed vibration sensing system.For different input polarizations,phase noise changes for the same sensing channel.When the input polarization remains constant,different sensing channels have different phase noise levels because of polarization change caused by fiber birefringence.4?propose orthogonal polarization switching method to be applied to distributed vibration sensing system in terms of restraining polarization fading.The scheme realize the declination of overall phase noise level and insensitiveness to polarization disturbances.Experimental results show a reduction of overall phase noise at a level of about 10 dB for sensing channels.