Research On Interferometric Refractive Index Sensor Based On No-core Fiber

Internet of Things(IOT),as an extension and expansion of Internet,is called “the Internet where everything is connected”.As one of the key technologies of IOT,the sensing layer needs to sense and collect differnet kinds of information through different kinds of sensors.The sensing network based on different kinds of sensors is the infrastructure to realize sensing in IOT.Compared with the traditional sensing technology,optical fiber sensor has the advantages of small volume,light weight,corrosion resistance,electromagnetic interference resistance,low cost,multi-point multiplexing,distributed measurement,and is of great significance to the underlying sensing network of IOT.Interferometric optical fiber sensors are widely used in the measurement of many parameters such as temperature,refractive index,humidity,strain,gas concentration,etc,owing to their advantages of simple structure,flexibility and high sensitivity.In this thesis,interference optical fiber sensors based on no-core fiber(NCF)with tapered structure and core-offset structure are investigated.Through making different sensing structures,the sensing parameters are characterized and applications are studied.The main research contents are as follows:(1)On the basis of single mode--no-core--single mode(SMF-NCF-SMF)structure,the sensing characteristics based on NCF tapered structure are studied.Simulation analysis of SNS structure shows that reducing the diameter of NCF can further improve the external refractive index(RI)sensitivity.The NCF is tapered by flame taper and its RI sensing characteristics are studied.The experimental results are consistent with the simulation results.In the refractive index range of 1.335?1.36,the RI sensitivity of tepered NCF with a waist diameter of 37.5?m is 481.31nm/RIU,which is 4.8 times higher than that of un-tapered NCF.(2)The interference sensing structure based on the offset fusion of NCFs is studied.The sensor with an open-cavity structure is formed by fusing NCFs with large offset.The sensor is very sensitive to the external refractive index and is suitable for liquid with small changes in refractive index,such as liquid concentration sensing.The salinity and temperature characteristics of the sensor are experimentally studied.The salinity sensitivity is 3.475nm/% in the salinity range of 0.5% to 5%,and the temperature sensitivity is 0.065nm/? in the temperature range of 20?40?.(3)Use moisture sensitive materials to extend the sensing function of the sensor.An optical fiber humidity sensor based on graphene oxide and polyvinyl alcohol(GOQDs-PVA)polymer is studied.The tapered NCF structure is coated with GOQDs-PVA film and cascated with a fiber Bragg grating(FBG).The temperature and humidity sensitivity of the sensor are experimentally studied.In the humidity range of 30%RH to 80%RH,the humidity sensitivity is 143.27pm/%RH,and in the temperature range of 25?75?,the temperature sensitivity is 9.21pm/?,and the problem of cross-sensitivity of two parameters is solved.