Research On The Refractive Index Measurement Method Of The Microstructure Fiber Fabry-Perot Interferometer

In the fields of environmental monitoring,biochemical research and food safety,refractive index(RI)as an important measurement index affects the actual production and processing,so it has always been the focus of attention.Among all kinds of RI sensors,optical fiber RI sensor has become a research hotspot because of its compact structure,high measurement accuracy,long-distance transmission,high temperature resistance and corrosion resistance.With the development of social industrialization,the practical application for RI measurement is required to improve sensitivity,wider measurement range and lower cost.Based on the principle of Fabry-Perot interferometry(FPI),the RI measurement methods of open-cavity FPI sensor and in-line composite-cavity FPI sensor are studied,and the application of in-line composite-cavity FPI sensor in gas-liquid detection is explored.For open-cavity FPI sensors,a method to extend the measurement range of RI is proposed.For the in-line composite-cavity FPI sensor,this paper presents a practical application in gas detection of hollow fiber membrane.The main research contents are as follows:(1)A RI measurement method based on open-cavity FPI sensor to locate a certain order of interferometric valley using free spectral range(FSR)is studied.Through theoretical deduction and Matlab simulation,the correctness of the method is verified.Moreover,an open-cavity Fabry-Perot sensor is fabricated,an experimental platform is built and measured.The feasibility of this method is analyzed from the experimental measurement.The experimental results show that the RI measurement range is 1.333?1.4 RIU,which is about 7 times higher than the traditional single peak measurement method.The linear response of the sensor is good in the whole measurement range and the sensitivity is 1260.282nm/RIU.The RI measurement with wide range and high sensitivity is realized.(2)The RI measurement method based on the in-line composite-cavity FPI is studied.The principle of RI measurement of the in-line composite-cavity Fabry-Perot sensor is analyzed.The fabrication process of the sensor is studied and the experimental platform is built.The experimental results of the RI and temperature characteristics of the three samples show that in the RI range of 1.333?1.34RIU and 1.34?1.36RIU,the RI sensitivities of the three examples are-116.3dB/RIU and-52.7dB/RIU,-198.2dB/RIU and-63.4dB/RIU,-176.6dB/RIU and-102.5dB/RIU,respectively.In addition,the temperature sensitivity of the three sensors is 0.0075nm/?,0.0075 nm/? and 0.0073 nm/? in the range of 20? to 50 ?.The piecewise linear fitting method is employed to achieve the higher RI sensitivity measurement of the in-line composite-cavity FPI Interferometer in different RI ranges.(3)The application of the in-line composite-cavity FPI Interferometer in gas detection is studied.The gas detection sensor for hollow fiber membrane is designed based on the in-line composite Fabry structure.The gas detection principle of the sensor was analyzed.Then the gas detection sensor samples were fabricated.Finally,the hollow fiber membrane gas detection system was built.The test results show that the designed sensor can realize online detection of gas in the membrane,which fills up the technique blank of the in-situ gas detection in the hollow fiber membrane.