Researches On Novel Methods And Technologies Of Distributed Fiber Strain Sensors

With the development of society and the advancement of science and technology,mankind's thirst for exploration of deep sea,deep earth and space is growing stronger and stronger.In order to sense the areas that are inaccessible to existed technologies,the development of sensors with high stability,high signal quality,high sensitivity and high networking capability becomes an urgent requirement.Fiber-optic based sensing systems have incomparable advantages over electrical sensing systems in extreme or harsh conditions such as strong electromagnetic radiation,flammable and explosive,high temperature and high pressure,and wide field without power supply.Among various types of fiber optic sensing systems,distributed optical fiber sensing(DOFS)systems are widely used in industrial monitoring,energy exploitation,traffic management and so on,because of their advantages like cost-effectiveness,easy packaging,long sensing distance and large capacity.The development of distributed optical fiber strain sensing(DOFSS)technology with faster response speed,higher stability,longer sensing distance,higher sensitivity and higher cost performance will have a revolutionary impact on the fields including oil/gas exploration and undersea seismic monitoring.The existing DOFSS systems still have many points that needs to be improved.In order to solve the problems of short repeaterless sensing distance of DOFSS system and the inability to simultaneously provide intrusion warning and security monitoring,which were mentioned in the sub-project “Research on Key Devices and Technologies of Longdistance Distributed Fiber-optic Sensing Network” of the National Natural Science Foundation of China(NSFC),this doctoral dissertation makes some breakthroughs by applying the new distributed optical amplification technology and signal multiplexing technology to design and realize the Brillouin Optical Time Domain Analysis(BOTDA)with sensing distance over 175 km and the DOFSS system that has > 150 km sensing distance and is able to statically and dynamically sense simultaneously.In addition,a distributed amplification method based on weak erbium-doped fiber(WEDF)is proposed in this dissertation,which provides a new way to further extend the repeaterless distance of DOFSS systems.Meanwhile,this dissertation conducts some principle studies on the problems of low signal quality and insensitivity to radial strain that exists in the reported DOFSS systems,where are mentioned in the NSFC project “Novel geophone based on fiber-optic distributed acoustic sensor”.In detail,we analyze the signal harmonic distortion caused by the magnitude imbalance of coherent demodulation modules,and propose a real-time compensation method.Then,based on the reconstruction of phase and polarization of Rayleigh backscattering light,we propose a method that can detect the axial and radial strain at the same time through demodulating the birefringent vector distribution along the fiber.Finally,this dissertation explores a novel distributed strain sensing technique based on the property that forward stimulated Brillouin scattering(FSBS)of optical fiber is sensitive to both axial and radial strain.This part of work is supported by the China Scholarship Council.The main works are shown below:(1)Analyze the impact of magnitude imbalance on coherent detection module in phase-sensitive optical time domain reflectometry(?-OTDR),and propose a real-time compensation method.This research focuses on the magnitude imbalance of 90° optical hybrid.Based on the analysis,this dissertation proposes a real-time method to compensate the imbalance through the statistical regularity of Rayleigh backscattering light.(2)Propose a distributed hybrid amplifying method based on WEDF and distributed Raman amplification(DRA).The 2nd random fiber laser(RFL)produces the pump both for the WEDF and DRA.This dissertation studies the relationship between the doping concentration of WEDF and the on-off gain of the system,and calculates the optimized value of concentration,which provides the guides to the system design.(3)Propose a method to calculate the distribution of polarization transmission matrices along the fiber through the Rayleigh backscattering light,in order to sense the radial strain change.?-OTDR with 90° optical hybrid can demodulate the phase and polarization of Rayleigh backscattering light simultaneously,so the axial and radial strain change can be detected at the same time.(4)Propose a method to sense the axial and radial strain change simultaneously through the FSBS of multicore fiber(MCF).The FSBS with high-order modes is difficult to be stimulated in universal single-modes fiber,but is easier to be stimulated in MCF.The cutoff frequency of FSBS with a specific mode can be distributed obtained through BOTDA with some design.(5)Realize an ultra-long repeaterless BOTDA through the hybrid of 3rd order RFLA and 2nd order RFLA.Under the condition that the spatial resolution of BOTDA is 8 m,and the strain sensitivity is ±40.12 ??,the repeaterless distance can achieve 175 km,and the figure of merit(Fo M)is above 200,000.(6)Realize an ultra-long repeaterless DOFSS system,which is able to dynamically and statically sense the strain change simultaneously.This system uses the 1st order DRA,2nd order RFLA and distributed Brillouin amplification(DBA),as well as the signal multiplexing,making repeaterless sensing distance achieve 150 km.The spatial resolutions of the dynamic and static sensing subsystem are 30 m and 8.5 m,respectively,and the static strain sensitivity is ±16.4 ??.