Research On Temperature Sensing Performances Of Optical Fiber Interferometric Microstructure And Sensitization Methods

As an essential part of sensor technology,optical fiber sensing has attracted widespread attention.Discrete optical fiber interferometric temperature sensors possess the advantages of microstructure,high-temperature resistance,anti-electromagnetic interference,wide measurement range,and fast response speed,which meet the application requirements of harsh high-temperature environments and biochemical research.They are significant to modern industrial manufacturing,and national defense security,and modern biotechnology industries.Focusing on the hindrances of complex structures and manufacturing methods,no packaging protection measures,cross sensing,poor high-temperature stabilities,and low temperature sensitivities,this thesis carried out theoretical and experimental research of high-temperature-resistant optical fiber interferometric microstructured temperature sensors,and explored temperature sensitivity enhancement methods.The thesis’ s contents are as follows:Theoretical analyses and numerical simulations of optical fiber inline interferometric temperature sensing structures were carried out,which provided theoretical supports and guidances for the designs of optical fiber MZI sensing structure,optical fiber FPI sensing structure,and optical fiber MMI sensing structure subsequently.Given the complex structures and manufacturing processes,and cross-sensing matters of most optical fiber temperature sensing structures,a high-temperature-resistant optical fiber MZI temperature sensor based on suspended-core fiber(SCF)was proposed.The working principle and transmission spectral characteristic of the SCF-based optical fiber MZI temperature sensing structure were analyzed.Furthermore,the temperature sensing characteristic and the cross-sensing characteristic were studiedTransmissive optical fiber inline MZI temperature sensors are not suitable for oneway entry measurement environments.We proposed to construct a high-temperatureresistant FPI temperature sensor with SCF by simple cutting and fused splicing.Combining the theory and the experiment,we analyzed the reflection spectral characteristic of FPI temperature sensor based on SCF.In addition,we studied the temperature sensing characteristic within the large dynamic range of 50-800 ℃.The maximal repetitive sensitivity errors were respectively 0.13 pm/℃and 0.36 pm/℃ within the range of 50-500 ℃ and the range of 500-800 ℃.The wavelength stability error was less than 62 pm.Furthermore,the protective effect of the end-face packaging was verified,which eliminated the impact of the cross sensing.The temperature sensing sensitivities of optical fiber sensing structures relying on the inherent thermo-optical effect and thermal expansion effect are relatively low.A method to enhance the temperature sensitivity using the optical vernier effect created by parallel optical fiber FPIs structure was proposed.The feasibility of tuning temperature sensitivity amplification of parallel optical fiber FPIs structure was theoretically analyzed.The temperature sensing characteristics were studied.The linear average temperature sensitivities obtained within the dynamic ranges of 50-500 ℃ and 500-800 ℃ were respectively 64.6 pm/℃ and 102.1 pm/℃,which improved the sensitivity of optical fiber FPI temperature sensor by about 6.7 times.In addition,intending to further increase the sensitivity of optical fiber temperature sensors,we proposed a temperature sensitization method by encapsulating a multimode microfiber structure in isopropanol within a small dynamic range.Experiment results indicated that the proposed sensitization method could improve the temperature sensitivity to several nanometers per centigrade in the range of 25-50 ℃.The temperature sensitivity increased with the smaller diameter of microfiber.The maximal temperature sensitivity was up to 4.12 nm/℃,which was 2 orders of magnitude for the optical fiber temperature sensors relying on the inherent thermo-optical and thermal expansion effects.This way of liquid packaging microfiber structure can improve the temperature sensitivity and also enhance the mechanical strength of the microfiber structure.SCF with a special air hole cladding structure and high-temperature resistance was used as the sensing component in this research,which simplified the structure and manufacturing processes of optical fiber interference temperature sensors.The temperature sensors not only avoided additional packaging but also realized the miniaturization of structure,the good temperature sensing repeatability,the reversibility,and the high-temperature stability in a large dynamic temperature range,which has potential significance for applications in harsh high-temperature environments.Moreover,the temperature sensitivities were effectively improved using optical vernier effect and temperature-sensitive materials enclosing microfiber structure,which makes up for some shortcomings of optical fiber interference temperature sensors to a certain extent.