Fabrication And Characteristics Of Low Cost Fiber Temperature Sensing Structure Based On Capillary Tube With A High Sensitivity

In recent years,the optical fiber temperature sensing structure based on the silica capillary tube(SCT)stands out from many optical fiber sensing structures own to its advantages of low material cost and easy fabrication.Since the liquid material has both high thermo optic coefficient(TOC)and thermal expansion coefficient(TEC)values,the temperature sensitivity of the optical fiber sensing structure can be greatly improved by combining the liquid material and the optical fiber sensing structure based on the SCT.In this paper,we proposed three reflective optical fiber temperature sensing structures based on SCT and integrated the liquid into the optical fiber sensing structure.The sensing characteristics of the optical fiber temperature sensing structure were theoretically analyzed and verified by experiments.The specific work is as follows:A reflective fiber high-temperature sensing structure based on the resonance in sidewall of the STC was proposed.Based on the sidewall resonance theory,we analyzed the temperature sensing characteristics of the optical fiber sensing structure and simulated the light field distribution and the reflection spectrum.Besides,we also studied the influence of the length and wall thickness of the SCT on the reflection spectrum and calculated the temperature sensitivity.The high-temperature and stability characteristics of the structure are investigated experimentally.The temperature sensitivity was 26pm/°C,which was in agreement with the theoretical results.The fiber temperature sensing structure can realize high temperature sensing in a special environment,up to 800°C,and the reflective structure is easy to use in a narrow space environment.A reflective temperature sensing structure with a ethanol-sealed Fabry-Perot interference(FPI)cavity was proposed by filling the ethanol into the thick-core STC through the thin-core STC.Based on the FPI theory,we analyzed the influence of the microcavity length of the fiber sensing structure on the reflection spectrum and the relationship between its temperature sensitivity and the TOC of the medium in the microcavity.The temperature experiment result shows the the sensitivity of the fiber sturcture is –497.6pm/°C.In addition,from the experiment we find that the boiling point of ethanol will increase with the increasing of the liquid-gas ratio in the structure.When the liquid-gas ratio reaches 2.89,ethanol can maintain the liquid state up to its supercritical temperature(243 °C).The relationship between the reflected power of the fiber sensing structure and the temperature in this process was experimentally studied,and the sensitivity reached 0.38dBm/°C.Based on the optical fiber temperature sensing structure,a rapid response under a wide range varietion of temperature can be realized,and the cost can be reduced through power demodulation.A reflective fiber temperature sensing structure based on the thermal displacement of the liquid-gas interface was proposed.Highly sensitivity temperature sensing is realized based on the free expansion of the liquid polymer in the SCT.Based on the three-beam FP interference theory,we simulated the reflection spectrum,and extracted the interference fringes induced by the liquid cavity and the gas cavity separately by the filtering method,and calculated their respective temperature sensitivities.The temperature sensitivity,stability and resolution of the fiber sensing structure were investigated experimentally,and a high temperature sensitivity of 1.99nm/°C was obtained by the wavelength shift difference method.