| As an extension and expansion of the conventional Internet,the Internet of Things has achieved rapid development and extensive attention in recent years.It has risen to a part of the national development strategy in the 12th and 13th Five-Year Plan.Perception is the cornerstone of the development of the Internet of Things(IoT)industry.Sensor networks,based on various types of sensor components,play an important role in the IoT perception layer.They are the basic infructracture for the realization of the Internet of Things.The optical fiber sensors bring much versatilities to the underlying sensor networks of the Internet of Things,as they possess the merits of small size,light weight,low cost,robustness,corrosion resistance,electromagnetic interference resistance,no need for power supply to the sensor head,applicability for multi-point-multiplexing and distributed sensing,etc..Interferometric fiber optic point sensors,such as Sagnac interferometric and multimode interferometric fiber optic sensors,has the advantages of simplicity,flexibility,and high sensitivity.They are widely used in the sensing of multiple parameters such as temperature,strain,refractive index and so on.Distributed fiber optic sensors based on stimulated Brillouin scattering have important research value and application prospects in the fields of infrastructure(such as dams,bridges,large buildings,etc.)monitoring and geological structure monitoring.This thesis studies the interferometric fiber optic sensors,the stimulated Brillouin scattering based fiber optic sensors and their applications in dual-parameter sensing.The research work is supported by the National 973 Project,the 863 Project and the National Natural Science Foundation of China.The main research work and achievements are listed as follows:1.A Sagnac interferometric sensor based on two sections of Panda-type polarization maintaining fiber is proposed.The sensor is fabricated by embedding into the Sagnac loop two sections of Panda-type polarization maintaining fiber with different lengths.Its temperature and strain sensitivity can respectively reach as high as-1.495 nm/? and 20.67 pm/??,while its temperature and strain measurement error can be as low as± 0.13 0C and ± 14.06??.The theoretical analyses and experimental verification of the sensor are detailedly demonstrated in the thesis.2.A hybrid fiber optic sensor combining Sagnac interference and multi-mode interference is proposed.It is fabricated by incorporating a Panda-type polarization-maintaining fiber and a claddingless fiber into a Sagnac loop to form a SMP structure based Hi-Bi fiber loop mirror.With the high temperature sensitivity of the Hi-Bi FLM and the high refractive index sensitivity of the claddingless fiber asissted SMS structure,the proposed sensor enables simultaneous measurement of temperature and refractive index both with high sensitivities.Detailed therotical analyses and simulation of the sensor is carried out with the help of matrix analysis theory and modal analysis theory.Experimental verificaiton is also implemented.Experimental results exhibit that the sensor can aciheve both a high temperature sensitivity of-1.929 nm/? and a high average refractive index sensitivity of 235.3 nm/RIU.3.A three-layered Ge-doped optical fiber,SMF-a,is proposed for simultaneous sensing of temperature and strain in stimulated Brillouin scattering based optical fiber sensing system.Utilizing the modal analysis theory and the stimulated Brillouin gain theory,the Brillouin frequency shift and the peak Brillouin gain efficiency,as well as the temperature and strain sensitivity of the first three acoustic modes in SMF-a are numerically calculated and detailedly analyzed under different doping concentrations and doping structural parameters.Theoretical results show that,by rationally designing its doping concentration and doping structure,SMF-a is applicable for stimulated Brillouin scattering based temperature and strain dual-parameter sensing.The temperature and strain errors of SMF-a are respectively-4? and?110 ??,which can meet the measurement error requirements of the general Brillouin dual-parameter sensing system.4.A three-layered Ge/F co-doped single-mode fiber,SMF-b,is proposed for simultaneous measurement of temperature and strain in stimulated Brillouin scattering based fiber-optic sensing system.The inner core of SMF-b is co-doped with Ge/F.Its outer core is doped with GeO2 only and the outer cladding is pure SiO2.Based on optical and acoustic mode analysis theory as well as stimulated Brillouin gain theory,using the numerical analysis method,the effective velocity,Brillouin frequency shift and the peak Brilloin gain efficiency of the first three acoustic modes in SMF-b are theoretically simulated and analyzed under different doping concentrations and doping radii.Based on the simulations results,three sets of suitable doping concentration and doping radius parameters are selected for simultaneous measurement of temperature and strain.With the chosen parameters,theoretical analyses show that the temperature and strain sensitivity of SMF-b can respectively reach 1.190 MHz/? and 0.0356 MHz/??,and its temperature and strain errors can be as low as 0.9 ? and 28.8 ??.Temperature and strain erros have been greatly improved over SMF-a,the Ge doped single mode fiber.They are as good as those of the F-HDF,IPGIF and SI-FMF,which are few mode fibers.5.Based on the former work contributed by research pioneers,the dependences on temperature,strain and Ge doping concentration of optical refractive index and longitudinanl velocity are concluded and summarized.They are also verified by comparing simulation results and the simulation results.The dependent relationships can be used to help calculate stimulated Brillouin scattering properties,such as Brillouin frequency shift and Brillouin gain spectrum,in Ge-doped optical fibers with different doping structural and'doping concentrations under different temperature and strain variations. |
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