Study On Optical Fiber Grating Based Humidity Sensing

Humidity plays an important role in fields of industrial and agricultural production,aerospace,food storage,biochemistry,transportation and civil engineering etc.Fiber Bragg grating（FBG）has the characteristics of small size,light weight and simple manufacture.When it is used for humidity sensing,it can not only improve the sensitivity,but also realize the integration of the sensor structure and function.Therefore,the FBG based humidity sensing technology has attracted more attention.In this paper,three kinds of FBG based humidity sensor are designed and manufactured through humidity-sensitive material preparing and coating technology.The sensing characteristics of each sensor are studied theoretically and experimentally.The main research contents and conclusions of this paper are as follows:（1）A FBG humidity sensor based on Graphene Oxide（GO）-Polyvinyl Alcohol（PVA）solution is designed and fabricated,and the humidity response characteristics of the sensor are investigated experimentally.The results show that when the mixture ratio of GO and PVA solution is 9:1 and the coating thickness is 89μm,the humidity sensitivity is 2.3 pm/%RH in the range of 50%～60%RH,the humidity sensitivity is 7.7 pm/%RH in the range of 60%～70%RH,the humidity sensitivity is 47.2 pm/%RH in the range of 70%～80%RH.The temperature sensitivity of the sensor is 11.2 pm/℃in the temperature range of 30～90℃.The stability error is 0.0647%RH.The response time and recovery time are 1.33 s and 4.55 s,respectively.（2）A FBG humidity sensor based on Aluminum Oxide（Al₂O₃）-PVA is designed and manufactured,and the relationship between humidity sensitivity of the sensor and the thickness of humidity sensitive coating are studied experimentally.The experimental results show that when the mixing ratio of Al₂O₃and PVA is 1:9 and the coating thickness is 80μm,the humidity sensitivity is 5.4 pm/%RH in the range of 47%～62%RH,the humidity sensitivity is 51.7 pm/%RH in the range of 62%～74%RH.In the temperature range of 30～70℃,the temperature sensitivity of the sensor is 13.9 pm/℃.The stability error is 0.03%RH,and the response time and recovery time are 3.4 s and 11.68 s,respectively.（3）A FBG humidity sensor based on etching is designed and manufactured.The diameter of FBG is etched to 50μm by hydrofluoric acid,and then the FBG is coated with Al₂O₃and PVA film with a thickness of 60μm by dipping coating method.Finally,the humidity sensitivity of FBG humidity sensor is studied experimentally.The results show that when the mixture ratio of Al₂O₃and PVA solution is 1:9 and the coating thickness is 60μm,the humidity sensitivity is 6 pm/%RH in the range of 40%～58%RH,the humidity sensitivity is32.3 pm/%RH in the range of 58%～70%RH.The temperature sensitivity of the sensor is 14.2pm/℃in the temperature range of 25～70℃.The stability error is 0.0745%RH.The response time and recovery time are 1.94 s and 5.19 s,respectively.The above FBG humidity sensors have the advantages of simple preparation technology,compact structure,low cost,good repeatability and high humidity sensitivity,which has certain reference significance for the development and application of humidity sensor based on FBG.