Research On Evanescent Wave Optical Fiber Humidity Sensor Based On PVA Fil

With the development of economy and society,the requirements for monitoring and measurement of humidity are increasing.The traditional electronic humidity sensor which are well known to many industries are easily affected by electromagnetic interference for their sensing mechanism.This disadvantage makes electronic humidity sensor no longer suitable for some special environment.Fiber optic humidity sensor attracts great attention for their advantages,for instance,lightweight,small size,high temperature resistance,corrosion resistance,anti-electromagnetic interference,simple production,low cost,high sensitivity.However,the response and recovery time of the current fiber optic humidity sensor is still long,and the sensitivity of the sensor is not high enough.In order to achieve higher sensitivity,the selection of humidity sensitive materials is particularly critical.As a polymer moisture sensitive material,hydrogel can improve the sensitivity of the sensor because of its good water absorption performance.However,hydrogels’ high water retention makes most hydrogels have poor ability to desorb water molecules: after absorbing water molecules,water molecules are difficult to desorb which greatly increases the recovery time of humidity sensors based on hydrogel when the humidity decreases.In response to the above problems,this research uses polyvinyl alcohol(PVA)hydrogel as a humidity sensitive material,and added glycerol and Tween 20 as cross-linking agents to design a fiber optic humidity sensor based on PVA/Glycerol and PVA/Tween 20 film evanescent wave fiber optic humidity sensor,the main research work of the paper is as follows:(1)The hydrophilic PVA film was prepared by the improved pulling method.The more hydrophilic PVA/Glycerol and PVA/Tween 20 film was prepared by the improved pulling method and chemical cross-linking method,using glycerol and Tween 20 as cross-linking agents.(2)Compared with the optical fiber humidity sensor prepared by pure PVA film,the sensitivity of the sensor was significantly improved after blending with glycerol.Under the condition of 50 ml PVA solution,when the concentration of PVA is 4wt% and 3ml glycerol is added,the fiber optic humidity sensor based on PVA/Glycerol film has the best sensing performance: the sensor exhibits exponential performance in the entire humidity range and shows higher sensitivity in high humidity,the fitting degree of fit is 0.9802,and the average sensitivity is 160.3 lux/%RH.The response time is 55 s and the recovery time is 51 s.The sensor shows good repeatability.(3)Similarly,after blending with Tween 20,the sensitivity of the sensor has been significantly improved Under the condition of 50 ml PVA solution,when the concentration of PVA is 4wt% and blended with 3ml Tween 20,the optical fiber humidity sensor based on PVA/Tween 20 film has the best sensing performance: the sensor exhibits an exponential change in the entire humidity range.The variation law is more sensitive in high humidity,the fitting degree is 0.9914,and the average sensitivity is 426.1lux/%RH.The response time is 11 s and the recovery time is 9s.The sensor shows good repeatability.(4)The output light intensity of the optical fiber humidity sensor based on PVA film decreases with the increase of humidity.The sensing mechanism is that when humidity increases,the absorption coefficient of the cladding increases and the refractive index decreases due to the swelling of PVA,which affects the transmission of evanescent waves.However,the output light intensity of the optical fiber humidity sensor based on PVA/Glycerol and PVA/Tween 20 films increases with the increase of humidity.The sensing mechanism is based on the swelling of film after adsorbing water molecules,which causes reduced scattering of evanescent wave in light transmission of fiber and increase of the output light intensity.