Research On High Sensitivity Optical Fiber Sensors Based On Special Optical Fibers

With the rapid development of optical fiber technology,optical fiber sensors have received widespread attention due to their advantages such as distributed sensing,high sensitivity,and strong resistance to electromagnetic interference.With the improvement of high standards for environmental testing,increasing the sensitivity and precision of optical fiber sensors is very important.Special optical fibers have been widely used in the production of optical fiber sensors due to their unique advantages.In this paper,four types of highly sensitive optical fiber sensors have been designed based on special optical fibers and sensitization materials,and their sensing characteristics in temperature,pressure,bending,strain,and magnetic field have been studied.In this paper,the sensitivity of optical fiber sensors has been improved by using special optical fibers and sensitizing materials,and good experimental results have been achieved.The main content of this article is as follows:（1）A Mach Zender interferometer with a structure of"single mode optical fiber（SMF）-multimode（MMF）-multi-core optical fiber（MCF）-SMF"was fabricated by splicing SMF,MCF,and MMF with offset,between MCF and SMF.Its bending and strain characteristics were studied.By measuring the bending and strain characteristics of the sensor,the bending sensitivity is-16.88 nm/m^-1,with linearity of 0.992,and the strain sensitivity is-1.353 pm/με,with linearity of 0.995.If the dislocation direction is selected as 0°,of 0°and 180°of the bending can be preliminarily determined.（2）A Mach Zehnder interferometer with a structure of"SMF-fine core optical fiber（TCF）-SMF"was fabricated using SMF and TCF.In order to improve the temperature sensitivity of the interferometer,thermosensitive materials,PDMS and ultraviolet glue,were uniformly coated on the surface of the TCF.The temperature characteristics were measured,and the experimental results showed a good sensitization effect.When the thickness of PDMS is 5μm,the temperature sensitivity of PDMS-coated sensor increased three times,reaching up to 90.0 pm/℃,with linearity of 0.998.When the thickness of UV adhesive reaches 5μm,the temperature sensitivity of UV glue coated sensor increased six times up to 166.8 pm/℃,with a linearity of 0.986.（3）A reference Fabry-perot interferometer（FPI）and a sensing FPI with close free spectral range were fabricated using capillary tube and PDMS,respectively.These two FPIs connected in series to form a cascaded FPIs sensor.Using the Vernier effect,the sensor sensitivity was improved.By measuring the temperature and pressure characteristics of the sensor,it is found that the temperature and pressure sensitivities of the single sensing FPI are 0.65 pm/°C and 4.64 nm/MPa,respectively.The temperature and pressure sensitivities of the cascaded FPIs sensor are 16.63 nm/°C and102.84 nm/MPa,respectively.The temperature and pressure sensitivities are about 26and 22 times higher than that of the single sensing FPI respectively,which is very close to the theoretical amplification factor of 29 times.Its repeatability,stability,and response time were studied,and its dual parameter simultaneous measurement has been discussed.（4）A novel sensor based on"SMF-MMF-Taper MCF（TMCF）-MMF-SMF"structure is proposed using SMF,MMF and TMCF.The temperature response characteristics of the structure were performed,which shows a sensitivity of 453.80pm/℃and a linearity of 0.993.Another structure with the same configuration was placed in a PDMS-filled groove shows enhanced temperature sensitivity of-693.20pm/℃.The same configuration was placed in a groove filled with Fe₂O₃-Ti-PDMS and a groove capillary filled with magnetic fluid to achieve the magnetic field sensing measurements.