Investigation On Singlemode-multimode-Singlemode Fibre Structure Based Composite Interference Optical Fibre Sensor

With the rapid development of optical fibre sensing technology,various optical fibre sensors based on different fibre structures have been developed and fabricated by researchers at home and abroad.Among these,the single-mode-multimode-single-mode(SMS)fibre structures have attracted a lot of research attention due to its inherent advantages of compact structure,low-cost,and easy of fabrication.SMS fibre structures rely on multimode interference(MMI)effect as its underlying operating principle.Depending on the performance advantages of MMI,SMS fibre structures have been demonstrated that it can provide good measurement sensitivity for multiple physical parameters,including temperature,humidity,refractive index(RI),curvature,strain,displacement,liquid level,vibration,and magnetic field.In order to further improve the sensing performance of traditional SMS fibre structure,the researchers have proposed a variety of post-processing methods,such as fibre tapering,fibre side polishing,chemical etching,and sensitive materials coating,etc.In this thesis,we propose the concept of constructing composite interference within the multimode fibre(MMF)section of an SMS fibre structure,treating it as one of the methods to improve the sensing performance of traditional SMS fibre structure.In order to establish an effective composite interference within MMF section,five different methods including twisting,bending,asymmetric tapering,quasi Fabry-Perot(F-P)cavity embedding,and refractive index modulation have been proposed.Based on the MMI theory,we have theoretically analyzed different forms of composite interference in MMF from the mode coupling theory,and the light field distributions in MMF have also been simulated numerically using Beam Propagation Method(BPM).Based on the theoretical analysis and numerical simulation,we experimentally completed the fabrication of five kinds of SMS fibre structures based composite interference optical fibre sensors and finished the corresponding sensing characteristics and temperature dependence analysis.The main research contents of this thesis are as follows:1.The development and applications of SMS fibre structures in the optical fiber sensing field are summarized.The MMI within the SMS fibre structure was theoretically modeled by guided mode analysis and approximated guided mode analysis,respectively,and the modeling results were analyzed and compared.Several post-processing methods for improving the sensing performance of traditional SMS fibre structure,including fibre tapering,fibre side-polishing,chemical etching,sensitive materials coating,and establishing composite interference are summarized in detail.2.Based on the fibre twist technology,fibre bending technology,and fibre taper technology,the MMF section of SMS fibre structures were subjected to twist,balloon bending,and asymmetric S-shaped taper processing respectively to introduce extra MZIs in the MMF section and to construct composite interference.The fabrication parameters were optimized experimentally,and the obtained twisted SMS fibre structure,balloon-shaped bent SMS fibre structure,and S-tapered SMS fibre structure were used for curvature,micro-displacement and strain measurement.The corresponding sensing characteristics and temperature dependences were investigated respectively.3.Using arc discharge method,the end of MMF was reshaped into a hemispherical shape,and the two hemispherical MMF tips were spliced together to form a gourd-shaped structure with the help of fibre welding technology.Therefore,an extra Fabry-Perot interferometer(FPI)was introduced in the MMF section of an SMS fibre structure to construct composite interference.The fabrication parameters were optimized experimentally,and the obtained gourd-shaped SMS fibre structure was used for strain measurement.The corresponding strain sensing characteristics and temperature dependence were investigated.The temperature-strain cross-sensitivity was calculated and the demodulation matrix was obtained for simultaneous measurement of strain and temperature.In addition,the compensation relationship between strain and temperature was also given.4.Based on the long period grating(LPG)theory,the refractive index of MMF section was successfully modulated by periodically embedding the no-core fibre(NCF)within the MMF section of an SMS fibre structure.Therefore,an LPG was introduced in the MMF section to construct composite interference.The optimum fabrication parameters were determined from the simulation results,and the experimentally obtained refractive index modulated SMS fibre structure was used for RI measurement.The corresponding RI sensing characteristics were investigated,and its temperature dependence was analyzed to draw the temperature immunity conclusion.