Research On Temperature And Strain Sensing Characteristics And Repeatability Of FBG In Low Temperature Environment

The development of cryogenic technology has been widely used in aerospace,refrigerant storage and transportation,superconducting magnet and particle physics experiment.For engineering structures,extreme temperature environment will affect the health status of structures.Therefore,it is necessary to monitor the temperature and strain of structures and materials at low temperature,so as to prevent and eliminate the human and economic losses caused by safety accidents.Fiber Bragg grating sensor has the characteristics of intrinsic safety,anti-corrosion,anti-electromagnetic interference,fast response,small size,light weight,easy to be embedded in the structure and realize the integration of sensing structure.Moreover,an optical fiber can connect multiple sensors in series to form a distributed sensor network.It shows high applicability in the field of low temperature measurement.Starting from the demand for structural temperature and strain monitoring in low temperature environment,this paper studies the temperature and strain sensing characteristics,sensor packaging technology and sensor repeatability of fiber Bragg grating through theory and experiment.The main research contents are as follows:(1)The research status of fiber Bragg grating temperature and strain sensors at home and abroad is analyzed.Based on the theory of fiber Bragg grating temperature and strain sensing,the relationship between the temperature and strain of fiber Bragg grating sensor and the central reflection wavelength of fiber Bragg grating is deduced.Through spectral simulation,the formation mechanism of chirp phenomenon at low temperature is explored,and the factors affecting the temperature and strain sensing characteristics of fiber Bragg grating in low temperature environment are analyzed.(2)According to the low temperature measurement environment,the packaging technology of fiber Bragg grating temperature and strain sensor is proposed.Based on the characteristics of fiber Bragg grating and encapsulated sensor,a temperature and strain calibration test of low temperature platform is built.Through temperature and strain experiments of low temperature,the temperature sensitivity and strain sensitivity of bare fiber Bragg grating and fiber Bragg grating with different packaging methods are calibrated and measured.The relationship between temperature and wavelength and the relationship between strain and wavelength at different temperatures are analyzed.It is found that the relationship between bare fiber grating temperature and central reflection wavelength is nonlinear,and the temperature sensitivity decreases with the decrease of temperature.The temperature sensitivity at 80 K is 1 / 4 of that at 300 K;the temperature and wavelength of fiber Bragg grating encapsulated in brass tube show a linear relationship,and its temperature sensitivity is greater than 25 pm / K;The relationship between the strain and the central reflection wavelength of bare fiber Bragg grating and fiber Bragg grating encapsulated with brass sheet is linear,and the temperature sensitivity under normal temperature and liquid nitrogen temperature is basically the same.(3)In order to evaluate the sensor performance of fiber Bragg grating,the repeatability index evaluation method of linear and nonlinear sensors is proposed.Firstly,based on the experimental calibration data,the relationship between temperature with wavelength and strain with wavelength of fiber Bragg grating is established.Then,fit the data to obtain the temperature and strain sensitivity of the sensor,and evaluate the repeatability of the sensor through sensitivity analysis.Finally,the repeatability of the sensor is further measured by the repeatability index,and the repeatability indexes of temperature and strain fiber Bragg grating sensors at room temperature and low temperature are compared and analyzed.