Fabrication And Application Of Thermo-induced Profiled Core Fiber Sensor

In recent years,fiber optic sensors have been widely used in various monitoring fields due to their advantages of high sensitivity,anti-electromagnetic interference,small size and high reusability.Structure-modulated optical fiber sensors has become a direction for researchers to further study because of its advantages such as high sensitivity,easy to reuse,small volume.Based on the research of fiber structure modulation,this paper proposes a preparation method that only modulates the fiber core structure.This method is mainly divided into two steps.The first step is the asymmetric etching of the cladding by high-frequency CO₂laser,and the second step is the deformation of the fiber core caused by the heating of hydrogen and oxygen flame.This method takes advantage of the fact that the fiber tends to be self-circular under the influence of surface tension in the molten state,and the fiber core is subjected to cladding force and moves towards the direction of etching by the CO₂laser,thus forming the change of the fiber core structure.Based on this method,we fabricated two kinds of fiber sensor with deformed core,namely,the Mach-Zehnder interferometer with micro bending core（MBC-MZI）and the S-Shaped core long-period fiber grating（SSC-LPFG）,respectively.And their sensing and spectral characteristics were simulated and experimentally studied.Firstly,the refractive index and temperature sensing characteristics of MBC-MZI are studied.MBC-MZI is composed of two micro bending cores（MBCs）and is prepared by high frequency CO₂laser polishing and hydrogen-oxygen flame heating.The unique deformation method enhances the interaction between the core and the external state and excites higher modes in the cladding,resulting in higher refractive index sensitivity.Two resonant dips were selected to measure the temperature and refractive index characteristics,and the highest refractive index sensitivity was-271.7 nm/RIU and-333.8 nm/RIU.At the same time,the refractive index and spectral characteristics are simulated by simulation software,which are in good agreement with the experimental results.The temperature characteristics of MBC-MZI are 0.121 nm/℃and 0.171 nm/℃.At the same time,the temperature and refractive index resolution of MBC-MZI can be obtained by demodulation of the multi-parameter measurement matrix,so the structure can also be used for simultaneous measurement of temperature and refractive index.Secondly,the asymmetrical etching of the upper and lower cladding layers by the laser makes the force of the fiber core uneven in the heating process of hydrogen and oxygen flame,so as to obtain the S-type fiber core,and prepare the SSC-LPFG with different periods.The experimental results show that the number of resonant peaks changes with the increase of the number of periods,and the bending characteristics are positively correlated with the number of periods.The maximum bending sensitivity can reach-27.36 nm/m^-1.Meanwhile,the bending properties of SSC-LPFG in the two directions are-24.01 nm/m^-1and-4.88 nm/m^-1,respectively.The fiber optic sensor fabricated by the core modulation method directly changes the core structure,thus inducing more core energy to be transmitted into the cladding.Therefore,the effective energy coupling can be formed in a smaller modulation region.Fiber optic sensor prepared by this method has the advantages of small volume,high mechanical strength and high sensitivity.Hence,this fabrication method has a good development prospect in fiber optic sensor manufacturing.