Research On Refractive Index Sensing Characteristics Of Metal-Coated D-Shaped Few-Mode Fiber

The optical fiber sensor based on the surface plasmon resoance(SPR)not only has the characteristics of SPR technology detection in real time,high sensitivity,marker-free detection,but also has the advantages of small size,adaptability and corrosion resistance of optical fiber.With the development of optical fiber microfabrication and post-processing technology,SPR technology can be combined with optical fiber through D-shaped polishing and metal coating,which can further enhance or extend the performance of fiber optic sensors.Few-mode fibers that can transmit multiple modes,which provide more mode selectivity to improve sensor sensitivity.In this paper,two types of SPR effect-based D-shaped few-mode fiber sensors were designed,and the refractive index sensing characteristics of metal-coated D-shaped few-mode fibers were investigated and analyzed by D-shaped profiling and silver coating of laboratory homemade few-mode fibers.Firstly,a silver-graphene composite membrane D-shaped few-mode fiber refractive index(RI)sensor was designed.Based on the finite element method,the sensing performance of different modes of this sensor was numerically analyzed under the optimized structural parameters.The sensor was found to have a RI detection range of up to 1.35-1.43 and maximum wavelength sensitivity of up to 9000 nm/RIU and 14500 nm/RIU for LP01 mode and LP31 mode,respectively.As a few-mode fiber supporting multi-mode transmission,higherorder modes exhibit higher sensitivity due to the tendency of the mode field to interfere with the metal film on the D-shaped.Secondly,a D-shaped RI sensor with gold coating was proposed for D-shaped fewmode fiber.The RI sensing was achieved by coating the polished surface with a stable gold film to excite the SPR effect.With optimized structural parameters,the RI detection range was up to 1.37-1.43 with a maximum sensitivity of 5400 nm/RIU.In addition,polarization filtering could be achieved at the communication windows of 1310 and 1550 nm by varying the polishing depth;a filtering bandwidth of 560 nm could be obtained when the fiber length is 4 mm.This device combined RI sensing and polarization filtering functions,contributing to the multifunctional development of optical devices.Finally,the sensing performance of the D-shaped few-mode fiber coated with silver film was experimentally investigated using the laboratory homemade few-mode fiber.After polishing and fusion splicing the multimode fiber,the multimode fiber-few-mode fibermultimode fiber structure was formed,and then the silver mirror reaction was used to coat the polished area with silver.The above operation completes the fabrication of D-shaped few-mode fiber sensor.Experiments showed that the RI measurement range of the sensor was 1.3330-1.4026,and the maximum sensitivity of 5465 nm/RIU could be achieved when the RI of the object to be measured was 1.4026.After further experiments,the sensor was found to have good repeatability and stability.