Research On Characteristics Of Fiber Gratings In Cryogenic Environment

Now,fiber gratings have been widely used in space,electric power and medical fields because of its small size,easy integration and strong ability of anti-electromagnetic interference.But there are still some unresolved problems which affect the grating sensing performance in the working environment.For example,in the space environment,the temperature measurement performance of fiber gratings will be seriously affected by the cryogenic temperature environment(less than-100 °C).In this paper,the performance of metal-coated gratings in cryogenic temperature environment is studied by means of theoretical simulation and making real object.The main work is shown as follows:(1)The application of fiber grating temperature sensor in many fields and the research progress of fiber grating temperature sensor enhancing package in cryogenic temperature environment are introduced.The development process and present situation of metal-coated gratings are studied,and other sensor enhancing methods and their shortcomings are introduced.Finally,the metal-coated grating is selected for the research.(2)The coupled-mode theory and the transfer matrix theory of fiber grating are deduced.Based on the coupled-mode theory,the characteristics of the reflected spectrum and the time delay of fiber Bragg grating(FBG)are studied,analyzing effect of the change of parameters such as the center wavelength and the refractive index modulation depth on spectrum.The theory of FBG temperature sensor is studied,and the mathematic model of metal-coated FBG is deduced based on the mechanics theory.The influence of the parameters such as linear thermal expansion coefficient and elastic modulus on the sensing performance is studied and simulated with MATLAB simulation software.As a result,the curve and the three-dimensional simulation diagram of these parameters and the temperature sensitivity are obtained to study the influence of materials on temperature sensing performance.The relation curves between the thickness and the temperature sensitivity of various metal coatings were studied by the model and the copper is selected as the coating metal in the experiment.(3)Simulating copper-coated FBG at cryogenic temperature by COMSOL,the influence of different coating thickness on the deformation of FBG is studied in cryogenic temperature environment,and the relationship between the shrinkage displacement and temperature is simulated to study the changing process of FBG stress and deformation with temperature,the static force and deformation at 153 K are analyzed in detail.The working principle in the process of temperature measurement is studied.By simulating the wavelength change of copper-coated FBG in the cryogenic temperature environment,calculating the data of the central wavelength changing with the temperature,and fitting the curve of the data,it is found that the temperature sensitivity will change as the temperature changes in the cryogenic temperature state.(4)The common metal plating methods for grating surface are studied,and chemical plating is chosen as the main experimental method.A metal plating process was designed,which was pretreated by removing protective layer,removing oil,sensitizing and activating,and then electroless plating was carried out.By controlling the reaction time,the coated FBG with thickness of about 10μm,20μm and 30μm was made.Testing the sensing performance of copper-coated FBG at room temperature,the temperature sensitivity of the copper-coated FBG is 14.69 pm/K,17.06 pm/K and 19.34 pm/K respectively,which is much higher than the temperature sensitivity of the bare FBG,and the error with the simulation value is less than 5%,which proves the feasibility of the model.