SPR Modulation Mechanism Of TiO₂ On Metal Film-Coated Optical Fiber And Its Sensing Performances Investigation

Microstructured optical fibers（MOFs）are a novel type of optical fiber with a flexible and versatile structural design that provides a new pathway for the integration of functional materials.The combination of functional materials with MOFs can effectively enhance the sensing characteristics of the fibers.Fiber optic sensors that combine MOFs with Surface Plasmon Resonance（SPR）technology offer many advantages such as high sensitivity,wide detection range,and real-time remote sensing,demonstrating rich research value in the sensing field.In this paper,relying on the theory of optical waveguides and the principles of SPR,we designed and simulated two types of composite membrane MOF refractive index sensors based on SPR effects,and systematically studied their performance.We also used a homemade coreless optical fiber to experimentally investigate the refractive index sensing characteristics of composite membrane-coated coreless optical fibers based on the SPR principle.The main research content of this paper includes:Firstly,we designed an Ag/TiO₂ composite membrane-coated D-shaped microstructured optical fiber sensor.Using finite element analysis and numerical simulation software,we adjusted the thicknesses of two types of thin films and optimized the air hole size and position structure to obtain a simple structure that is sensitive to the refractive index of liquids.The simulation results showed that the TiO₂ film effectively prevents the oxidation of silver while improving the refractive index sensitivity of the sensor.This phenomenon is caused by the stable chemical properties of TiO₂ and its higher relative dielectric constant compared to metals.The measurement range of the sensor is 1.340-1.385,and the maximum wavelength sensitivity reaches 11000 nm/RIU.Secondly,to explore the performance improvement of TiO₂ film on SPR fiber optic refractive index sensors based on different metals,we designed and simulated an Au/TiO₂ composite membrane-coated D-shaped fiber optic sensor.Compared to silver film,the gold film is more sensitive to changes in the surrounding refractive index.After optimizing various parameters,the results showed that the measurement range of the sensor expanded to 1.20-1.38,and the maximum refractive index sensitivity reached 14600 nm/RIU.Compared to the sensors coated with a single metal film,the sensing performance of both D-shaped sensors was improved by more than 100%.Finally,we used magnetron sputtering technology to deposit a silver film on the surface of a homemade coreless optical fiber in the laboratory.Then,a non-metallic material TiO₂ film was deposited on the outside of the coreless optical fiber with the silver film deposited.A composite film was formed on the surface of the coreless optical fiber,and the coated coreless optical fiber was fused into the constructed optical path to perform refractive index sensing experiments.When light is transmitted to the coreless optical fiber,the evanescent wave formed interacts with the composite film on the surface of the coreless optical fiber to generate SPR effect.When liquids with different refractive indices are covered around the sensor,the loss peak in the transmission spectrum will shift and become very sensitive.By calculating the shift of the loss spectrum of the sensor,the change of the refractive index of the environment can be detected.The experimental results show that the combination of silver film and TiO₂ film has practical significance.When the refractive index increases from 1.330 to 1.4026,the average sensitivity is 3827 nm/RIU,and the maximum sensitivity is 7740 nm/RIU.Compared with the coreless optical fiber sensor with only a silver film,the performance has been improved by about 20%.This proves that the addition of TiO₂ film can effectively enhance the refractive index sensing characteristics.