Research On Performance Of Optical Fiber Liquid Refractive Index Sensor Based On Three-segment Structure

With the development of optical fiber sensing technology,optical fiber sensors have been applied in many fields.One of the key parameters of the optical properties of the reaction liquid is the refractive index,which plays an important role in detecting and controlling the concentration of substances in the liquid.Optical fiber refractive index(RI)sensors have a very broad application prospect in the biological,food and chemical industries.RI sensors based on Mach-Zehnder interferometer(MZI)in fiber optics have been extensively studied,but the method of processing MZI usually requires a complex manufacturing process and it is relatively expensive.In recent years,MZI based on mismatched core has attracted more attention due to its ease of manufacturing and rugged structure.For example,single mode fiber(SMF)sandwiched between two thin-core optical fibers(TCF)and single mode–multimode–thin single mode fiber(SMTS)structure are combined for RI measurement.On the basis of reviewing the development,application and research status of optical fiber sensors,this paper mainly uses three-stage optical fiber structure for refractive index sensing research,such as SMS,STCS and MMM deposited gold nanoparticles.The details are as follows:Firstly,In terms of theoretical analysis,we derived the mathematical formula of the refractive index sensitivity of the SMS fiber sensor,and used BPM software to simulate the light field inside the SMS structure,and also simulated the effect of different diameter multimode fibers on the sensor sensitivity.On the reverse side of experimental verification,the single-mode fiber and multimode fiber are used to fabricate the single mode–multimode–single mode fiber structure,and the cladding of the multimode fiber is etched by hydrofluoric acid solution.The effect of the diameter and the length of the multimode fiber on the sensitivity of SMS fiber liquid refractive index sensor were investigated by using hydrofluoric acid solution to corrode the cladding of the multimode fiber.Secondly,In terms of theoretical analysis,we derived the mathematical formula of the refractive index sensitivity of the hetero-core fiber structure sensor,and used BPM software to simulate the light field inside the hetero-core fiber structure,and also simulated the effect of different diameter thin-core single mode fibers on sensitivity.On the reverse side of experimental verification,a single mode–thin-core single fiber–single mode fiber structure was fabricated by using a segment of thin-core single mode fiber and two segments of single mode fiber.The cladding of the thin-core single mode fiber was corroded by hydrofluoric acid solution to explore the effect of the diameter and length of thin-core single mode fiber on the sensitivity of liquid refractive index sensor of hetero-core fiber structure.Finally,the structure of 62.5 ?m multimode fiber–105 ?m multimode fiber–62.5 ?m multimode fiber was fabricated using 105 ?m multimode fiber and62.5 ?m multimode fiber.The cladding of 105 ?m multimode fiber was etched by hydrofluoric acid solution,and the layer-by-layer deposition method was used to deposit gold nanoparticles.The gold nanoparticles were deposited on the surface of105 ?m multimode optical fibers,and the effects of the number of gold nanoparticles layers,the length of 105 ?m multimode fibers,and the time of the layer-by-layer deposition on the sensitivity of the MMM optical fiber l iquid refractive index sensor were investigated.