Research On Large-Scale Multiplexing Methods Of Fiber Optic Sensors Based On Frequency-Shifted Interferometry

Fiber optic sensor technology has been applied extensively in each field of national economy because of distinguish advantages including high sensitivity,fast response,easily multiplexing systeming and anti-electromagnetic interference.Accompanied by the rapid development of global informationization and internet of things,the conventional signal-point or quasi-distributed fiber optic sensing systems become increasingly unable to meet the requirements of the development demands of large capacity,long distance and low cost in fiber optic sensor multiplexing systems.Therefore,developing large-scale fiber sensor multiplexing systems are particularly important and urgent.Many conventional multiplexing techniques including Time-multiplexed(TDM),wavelength-multiplexed(WDM)and hybrid multiplexed schemes have been used as the effective methods for expanding fiber sensor multiplexing system capacity,and they have been widely used in fiber optic sensor multiplexing system technology.However,these conventional techniques are restricted by the problems of low system signal-noise-ratio(SNR),high cost,device bandwidth and etc.,which result in small multiplexing capacity and difficulties in realizing large-scale development of fiber optic sensor multiplexing systems.Therefore,the research on large-scale fiber optic sensor multiplexing systems based on a new fiber sensing mechanism has great academic significances and practical application value.Aiming at problems of conventional multiplexing schemes,this thesis proposes new schemes of large-scale fiber optic sensor multiplexing systems based on frequency-shifted interferometry(FSI)and carry out researches in a large-scale fiber optic Fabry-Perot sensor multiplexing system and large-scale fiber Bragg grating(FBG)sensor multiplexing systems based on FSI.The main research contents and achievements are as follows:Firstly,a hybrid multiplexing scheme of fiber-optic Fabry-Perot(F-P)sensors based on FSI is studied.According to the characteristic of the multiplexing scheme,a light path of the F-P sensor multiplexing system is designed,and the corresponding real-time data acquisition/processing system based on Labview is developed.The simulation results show that the multiplexing capacity of a two-channel hybrid multiplexing system can reach 573 when the source power is 3 mW,F-P sensor reflectivity-30 dB and the sensor separation 40 m.Meanwhile,a experimental two-channel hybrid multiplexing F-P sensor system is constructed based on FSI.The reflection spectrum of each F-P is resolved by the reference location algorithm,and the result is consistent with the spectrum measured by a spectrometer.The research results verify that this multiplexing scheme provides a new feasible and low-cost method to multiplex F-P sensors in large scale.Secondly,a new multiplexing scheme of a large-scale FBG sensor multiplexing system based on FSI is proposed.A weak FBG sensor array suitable for large-scale multiplexing systeming is fabricated online.Sixty-five fabricated weak FBGs are multiplexed simultaneously.The location and peak reflection wavelength of each FBG is resolved by an algorithm of searching maximums twice.Through the system stability test and the temperature sensing experiment,the measured mean measurement accuracy of the peak wavelengths was ±3.9 pm,corresponding to a temperature resolution of ±0.4 ?.By simulation analysis,the research results show that the multiplexing capacity can reach 3000 when the peak reflectivity of each FBG is optimal i.e.-40 dB,the sensor separation 10 m and the source power 14 mW.Therefore,FSI has great application value and development potential in large-scale FBG sensor multiplexing systems.Thirdly,a large-scale WDM FBG sensor multiplexing system based on FSI is studied.A large-scale FBG sensor multiplexing system consisting of 121 sensing units and each unit multiplexing three weak FBGs with different peak reflection wavelengths is constructed.The locations and peak reflection wavelengths of 363 weak FBGs are demodulated through the improved algorithm of searching maximums twice.A test platform used for temperature sensing is constructed and the obtained temperature sensitivity is ±0.4 ?,which is consistent with the weak FBG multiplexing system based on pure FSI scheme.Under the same experimental parameters,the maximum multiplexing capacity of the hybrid system can reach up to 3207.The research results show that this multiplexing scheme can further improve multiplexing capacity and meet the requirements of distributed measurement,and it has a more promising application future in large-scale fiber optic sensor networks.