Research On The Open Fiber Mach-Zehnder Interferometer Refractive Index Sensor

Fiber-optic refractive index(RI)sensors have the inherent merits of small size,light quality,anti-electromagnetic disturbance,high temperature resistance,high sensitivity and good chemical stability,they are widely applied in chemical production,environmental monitoring and biomedicine.Owing to ultrahigh RI sensitivity and compact structure,the open-cavity Mach-Zehnder interferometer(MZI)refractive index sensor is paid high attention by industry participants.In order to decrease transmission loss and enlarge the RI measurement range of the open-cavity MZI,this thesis proposes a method of decreasing the transmission loss based on multimode interference coupling principle and a method of the RI measurement range extension,respectively.Some researchs have been carried out,such as the fabrication of the sensor,the extension of the RI measurement range and the application of the sensor on the concentration polarization in-situ monitoring.The contents mainly include three parts:(1)The theoretical basis of the open-cavity MZI refractive index sensor is researched.Some principles have been analyzed such as the multimode interference coupling,the Mach-Zehnder interference and the sensing.The effects on the transmission spectrum caused by the technological parameters of the structure have been analyzed by numerical simulation,and the optimized technological parameters of the structure fabrication are obtained.(2)The sensing characteristic of the open-cavity MZI sensor is researched experimentally.The RI measurement experimental platform is constructed.The experimental results show that the RI sensitivity is about-1360nm/RIU(1.333-1.3468),which realizes the high RI sensitivity in the RI measurement.Based on the relation between the free spectral range of interference spectrum and the RI,the method of measurement range extension is researched and the correctness of the method is researched experimentally.The experimental results show that the RI measurement range can be extended to 0.07RIU,and the error of RI measurement is±44.173×10-5 RIu.(3)An in-situ monitoring system of concentration polarization is designed based on this sensor.The in-situ monitoring method of the concentration polarization has been researched based on the RI sensing principle,the experimental platform of concentration polarization in-situ monitoring is constructed and the phenomenon of concentration polarization is researched experimentally.The experimental results show that the phenomenon of concentration polarization can be monitored by observing the concentration changes of the membrane surface,which verifies the feasibility of the open-cavity MZI on the application of the concentration polarization in-situ monitoring.The research results of the thesis show that the optimized open-cavity MZI achieves the purpose of reducing the transmission loss,and the method of the RI measurement range extension effectively enlarges the measurement range of the open-cavity MZI.Furthermore,the application of the sensor also provides novel technical method for researching concentration polarization phenomenon of the membrane surface and membrane-fouling mechanism.