Multi-parameter-integrated Synchronous Sensing System Of Multi-core Optical Fiber And Its Engineering Applications

Fiber optic sensors are widely used in civil engineering,aerospace,transportation,offshore platforms,energy engineering and other fields,due to their advantages of high sensitivity,good stability,anti-electromagnetic interference,low loss at optical frequencies,resistance to high temperature and corrosion,etc.However,there is a general problem with fiber optic sensors in practical structure detection,that is,the fiber optic sensor is cross-sensitive to multiple external parameters(strain,temperature,vibration,etc.),which leads to a fact that the data cannot be measured directly or the detected data are distorted.On the other hand,different optical fiber sensing technologies have their own independent acquisition systems,their demodulation data cannot be synchronized due to the different sampling frequency and trigger time of each system,which greatly affects data processing.Based on the research both in China and aboard,the multi-core optical fiber in telecommunications engineering is introduced in this thesis for the first time into the field of structural health monitoring.The synchronous sensing performance of seven-core optical fiber is studied,the packaging type of seven-core optical fiber is summarized and its strain transminssion mechanism after packaging is analysed in the thesis.For engineering applications,a series of self-sensing products and a multi-parameter synchronous demodulation device are developed.In addition,a quantitative vibration identification algorithm based on?-OTDR is proposed,which realizes a distributed quantitative vibration monitoring.At the end,three experiments with their engineering backgrounds are carried out to verify the effectiveness of the multi-parameter integrated synchronous sensing system with seven-core optical fiber.The main research and results are as follows:1.A functional layout method of the seven-core fiber cores is proposed,and its multi-parameter synchronous sensing performance is verified by cross-checking.In consideration of the requirement for the number of cores,low crosstalk between the cores,maturity of matched couplers,etc.,the trench-assisted seven-core optical fiber is chosen as study object in this thesis.Based on the principle of improving measurement precision,the functional layout method of the seven-core fiber cores is proposed,which eliminates the temperature-strain cross-sensitivity effect and the bending-strain sensitivity effect.Besides,some corresponding core function layout diagrams are provided for different engineering applications.The sensing performance experiments of seven-core optical fiber are designed and implemented,including: seven-core fiber single-parameter sensing test and multi-parameter simultaneous sensing test.In the former test,strain sensitivity coefficient and temperature sensitivity coefficient of the seven-core fiber bragg grating(FBG)and Brillouin optical time domain reflectometer(BOTDR)are calibrated,and the temperature constant coefficient of the Raman optical time domain reflectometer(ROTDR)is measured and the vibration measurement range of the polarization-sensitive optical fiber vibration sensing technology(POFVS)is obtained.On this basis,it is verified in the latter test that each core of the seven-core optical fiber can not only demodulate independently,but also complement and modify with each other,laying the foundation for subsequent research.2.The packaging protection of the seven-core bare fiber is realized,and the strain transmission mechanism of the fiber after packaging is discussed through theoretical analysis and finite element analysis.According to the single-core bare fiber packaging methods in China and abroad,two packaging protection approachs for the seven-core bare fiber are proposed.One is to encapsulate into an optical cable,and the other is to composite into a series of self-sensing products.It is worth noting that the Hytrel-material tight-sleeve packaged optical cable in the first packaging protection approach is also applicable to the seven-core bare fiber with gratings,which realizes point and distributed sensing simultaneously on an optical fiber sensor.In addition,through theoretical analysis and finite element simulation,the strain transmission mechanism of the seven-core optical fiber after packaging is explored.The result shows that the strain transmission of the sensor cable has an end-to-end effect,and the low transmission rate section is less than 200 mm.In the second packaging approach,seven-core optical fibers are implanted into structural reinforcement materials to develop self-sensing cloth,self-sensing carbon plate,self-sensing FRP bar,and self-sensing steel strand that have both mechanical and perceptual properties.The performance tests show that the mechanical properties of these products meet the standard requirements,and it has good coordinated deformation ability between the sensing fiber and the substrate.3.A multi-parameter synchronous acquisition and demodulation device for multi-core optical fibers is developed.In order to address the issue of existing devices for optical fiber sensing system,which are independent from each other and cannot be used for synchronous acquisition,a lightweight,highly integrated,portable synchronous demodulation device for multi-core optical fiber sensor is produced.The device integrates three commonly used optical fiber sensing modules,FBG,ROTDR and POFVS.It can directly demodulate the output point strain,ambient temperature,vibration frequency and other sensing parameters.4.A quantitative identification method of vibration is proposed based on multi-parameter optimization algorithm.For further expanding the versatility of the seven-core optical fiber and laying the groundwork for perimeter security engineering application experiment,the multi-group social group learning algorithm is introduced into the amplitude-discrimination-type ?-OTDR multi-parameter optimization algorithm,and based on this,a new quantitative identification method for vibration disturbance is developed.This method is to solve the approximate distribution of the fiber strain during disturbance according to light intensity signal spectrum type,and to calculate the maximum strain energy,time-averaged strain,and coefficient of variation.On the basis of theoretical research,through the PZT vibration test in the laboratory and the outdoor geotechnical disturbance test,it is verified that with this method different degrees of vibration disturbance can be successfully quantified.This method makes up for the shortcomings of optical fiber sensing technology,which can only detect the vibration position but cannot quantify the amplitude in practical engineering applications.5.The effectiveness and practicability of the seven-core sensing fiber and the multi-parameter synchronous acquisition device are verified.Three practical engineering application experiments with crane beam,pipelines as well as an experiment for perimeter security are designed and implemented.According to the characteristics of different experimental subjects,the function design of the seven-core optical fiber is optimized,different sensing parameters and fiber optic sensing projects are chosen.The results show that in the crane beam and pipeline experiments,the seven-core optical fiber monitoring project combined with FBG,ROTDR and POFVS has achieved the simultaneous measurement of strain,temperature and vibration frequency.In another application experiments of perimeter security,the seven-core optical fiber combination of ?-OTDR,FBG,and ROTDR is used to synchronously demodulate monitoring parameters such as disturbance location,disturbance degree,strain,temperature and duration,which effectively improves the recognition success rate and information utilization rate of the system.