Research On Sensing Characteristics Of FBG Based On Graphene And Polyimide Modification

With the advancement of science and technology and the improvement of production and living needs,the rapid and high-precision detection of temperature and humidity has attracted more and more attention.Especially in aerospace,precision instrument manufacturing,storage of special items and many other industries,suitable temperature and humidity can keep instruments and equipment in good working condition.Fiber optic temperature and humidity sensors have the advantages of antielectromagnetic interference,corrosion resistance,and strong stability.Among them,the temperature and humidity sensor prepared with FBG as the key device can be integrated on the surface of various devices,expanding the application range.This paper proposes a graphene and polyimide modified FBG humidity sensor,which can improve the sensitivity and response speed of the humidity sensor while reducing the effect of humidity hysteresis.The effects of different packaging methods on the performance of the polyimide modified FBG high temperature sensor were investigated.The main research results are as follows:Based on the coupled mode theory of fiber gratings,a theoretical model of the effect of different grating lengths and different refractive index modulation depths on the reflection spectrum of FBGs was established,and carry out numerical simulations.The reflection spectra of FBG were simulated with grating lengths of 4 mm,6 mm,8mm and 10 mm,respectively.The results shows that the increase of grating length will make the reflection bandwidth of the spectrum smaller,the number of resonance peaks more,and the reflectivity larger.The reflection spectra of FBG were simulated with refractive index modulation depths of 0.0001,0.0002,0.0003,and 0.0004,respectively.The results shows that with the increase of the refractive index modulation depth,the number of resonance peaks and reflectivity on both sides will also increase.A new type of humidity sensor structure modified by graphene and polyimide was proposed,and the experimental study of coating graphene films on the outside of polyimide films with different thicknesses to test the performance of humidity sensors was carried out.This method reduces the effect of humidity sensor hysteresis,while improving sensitivity and response speed.Under the thicker polyimide film,the sensitivity of the FBG sensor was increased by 1.8 times to 19.7 pm/%RH;Under the thinner polyimide film,the sensitivity of the FBG sensor increased by 1.53 and 1.61 times when the relative humidity increased and decreased,reaching 1.52 pm/%RH and 1.43 pm/%RH,respectively.During the rising and falling process,the minimum standard deviation of the wavelength corresponding to the same relative humidity value decreased from 0.00158 to 0.00039,indicating that graphene weakened the effect of hysteresis on the sensor.When the environment changes from 50%RH to71%RH,the response speed is improved from 5.33 s/%RH to 4.38 s/%RH,and when the environment changes from 71%RH to 50%RH,the response speed is improved from 5.62 s/%RH to 4.67 s/%RH.The standard deviation of the center wavelength is still low in the stability test of the sensor,and the maximum deviation of the repeatability test is still 3 pm,indicating that the performance of the sensor is improved after coating the graphene film.A method to fabricate high temperature sensor by encapsulating polyimidecoated FBG with capillary tube and stainless steel tube was proposed,and the effects of the two encapsulation methods on the performance of high temperature sensor were explored.The sensing probe was fixed in the stainless steel tube,and then two kinds of high temperature sensors were prepared by unfilled encapsulation(type I)and full glue encapsulation(type II).The experimental results show that the sensitivity of type I and type II high temperature sensors are 13.3 pm/°C and 12.7 pm/°C,respectively,and the response speed can reach 0.28 s/°C and 0.42 s/°C,respectively.When tested for stability at 100°C,the sensor standard deviations are 0.00472 and 0.00786,respectively.We summarize and analyze the experimental results,and obtain the main reasons for the differences in the performance parameters of type I and type II sensors.