Refractive Index Sensor Design And Research Of Structure Based On Few-mode Microfiber

With the rapid development of fiber optic technology,fiber optic sensors received the attention of the majority of researchers in biochemistry,environmental monitoring,medical and other fields,due to their high sensitivity,compact size,and immunity to electromagnetic interference.Improving the performance of sensors through new technologies and designing new sensing structures using different sensing elements have become hot topics.In this dissertation,a high performance interferometric fiber optic sensor based on few-mode microfiber as sensing structure has been proposed,and investigated experimentally.Specific contents are as follows:Firstly,a high-sensitivity and low-temperature-sensitivity fiber-optic magnetic field sensor based on a tapered two-mode fiber?TTMF?has been demonstrated.By testing the spectrum of TTMF,the section of TTMF has a specifically designed transition region as an efficient tool to filter higher-order modes,where the uniform modal interferometer just involved with HE₁₁and HE₁₂modes is achieved.Then the magnetic field response and temperature response of the proposed few-mode microfiber sensor were investigated.The experimental results show that a maximum sensitivity of 98.2 pm?Oe within a linear magnetic field intensity ranging from 0 to 140 Oe can be achieved.Significantly,the temperature cross-sensitivity problem can be resolved owing to the lower thermal expansion coefficient of the TTMF.With its low insertion loss,compactness,and ease of fabrication,the proposed sensor would find potential applications in the measurement of a magnetic field.Secondly,the ultra-sensitive sensing characteristics of the proposed few-mode microfiber near the dispersion turning point?DTP?have been investigated.Theoretical calculations reveal that sensitivity to the refractive index?RI?of the external environment near DTP can reach the order of10000nm/RIU.And the position of the DTP is significantly dependent on the diameter of microfiber and the external medium.As a result,the TTMF is immersed into a liquid that possesses a highly thermal coefficient ofdnFluid dt?28?-3.86?10^-4/Cto investigate its temperature characteristics.An improved sensitivity of?49.4 nm/^°C is achieved,which is a result of superposition of positive refractive index and negative refractive index.In addition,In addition,such structures have huge application potential for the field of biosensor applications due to simple detection scheme,quick response time,ease of handling and miniaturation.Thirdly,the ultra-sensitive sensing characteristics of the proposed few-mode microfiber near DTP have been used for detection of human immunoglobulin IgG.An effective and suitable optical platform for label-free biosensing has been investigated by the implementation of antibody/antigen immunoassays.Thus,the ultrasensitive detection of IgG levels can be achieved by exploiting the dispersion turning point?DTP?existed in the tapered two-mode fibers?TTMFs?since the sensitivity will reach±?on either side of the DTP.Tracking the resonant wavelength shift it was found that the fabricated TTMF device exhibited limits of detection?LOD?down up to concentrations of10?fg/mL.Such immunosensors based on the DTP have great significance on trace detection of IgG due to simple detection scheme,quick response time,and miniaturation.