Research On Polymer-decorated Interferometric Optical Fiber Humidity Sensor

Humidity is an important indicator for environmental monitoring.The environmental humidity monitoring has important applications in some fields,such as human life,industrial and agricultural production,intelligent monitoring,medical treatment,and Internet of things(IOT)technology.Traditional hair hygrometer,dry and wet bulb hygrometer,electronic type hygrometer,and mechanical type hygrometer are widely used because of its low price and simple structure.But they have some obvious disadvantages,such as low measurement accuracy,small measurement range and long response time.However,the optical fiber humidity sensors have many outstanding advantages,such as small size,anti-electromagnetic interference,high measurement precision,multiplexing available,easy distributed sensing and remote detection capacity,and strong environmental adaptability.So,they have some broad application prospects in national defense,science and technology fields,industrial and agricultural production and etc.It is worth mentioning in particular that the fiber optic temperature and humidity sensors have great potential in remote distributed temperature and humidity monitoring for agricultural greenhouses.Facing the requirement of fiber-optic humidity sensors in agricultural greenhouses,we carry out the principles and experimental studies of interferometric fiber-optic humidity sensors modified by polymer.The main research contents are listed as follows:(1)Research on fiber-optic humidity sensor with decoration of PMMAA composite humidity sensor made of polymethyl methacrylate(PMMA)-microsphere and single-mode fiber(SMF)was proposed and experimentally fabricated.Based on the Fabry-Pérot interferometer(FPI)theoretical.The results show that,when the external ambient relative humidity(RH)rises,the volume of PMMA-microsphere can expand after absorbing moisture from the surrounding.It causes the length of the Fabry-Pérot(F-P)microcavity to grow.Then,the valley of the sensor reflection spectrum will shift.Thus,the humidity sensing can be realized.The experimental results show that the sensitivity of the sensor is up to 173.36pm/%RH in the humidity range of 30%-80%,and the linearity can reach to 0.99226.In addition,the proposed sensor demonstrates good stability and repeatability.Comparing with other fiber optic humidity sensors,the PMMA-microsphere and SMF composite humidity sensor designed and fabricated has the advantages of high sensitivity,simple structure,no coating and easy fabrication.(2)Research on fiber Bragg grating(FBG)temperature and humidity dual-parameter sensor with PMMA modificationA temperature and humidity dual-parameter optical fiber sensor composed of PMMA-microsphere and FBG composite structure has been proposed and experimentally fabricated.Based on the FPI temperature and humidity sensitive and FBG humidity insensitive characteristics,monitoring of ambient temperature and RH simultaneously is achieved.The experimental results show that when the RH change from 55% to 85%,the response sensitivity of the F-P,which is formed between PMMA-microsphere and the fiber end-face,is about151.36 pm/%RH.However,the FBG in the structure is not sensitive to humidity.When temperature changes from 33℃ to 58℃,the temperature response sensitivity of the F-P and FBG are about 45.68 pm/℃ and 10 pm/℃,respectively.When temperature and RH change simultaneously,these two-parameters can be monitored simultaneously by using the dual-parameter measurement matrix method.The optical fiber dual-parameter sensor offers numerous advantages,such as low cost,high sensitivity,and easy fabrication.(3)Research on fiber optic FPI humidity sensor based on vernier effectA parallel FPI humidity sensor modified by Norland optical adhesive(NOA)was proposed and fabricated.Based on vernier effect theory,FPI1 is consisted of two SMFs filled with NOA in the middle.When the humidity changes,the NOA absorbs water molecules and causes the cavity length of F-P cavity to change.Therefore,FPI1 is sensitive to humidity.FPI2 is a “SMF-air-SMF” structure.Two FPIs are connected in parallel by using couplers to build the temperature and humidity sensing test system.The experimental results show that the maximum humidity sensitivity of a single FPI1 is 118.84 pm/%RH in the range of40 %RH-70 %RH.After the parallel connection of FPI1 and FPI2 by using coupler,the average humidity sensitivity of the sensor is up to 772.44 pm/%RH.Compared with a single FPI1 humidity sensor,the sensitivity of this fiber optic FPI humidity sensor based on vernier effect can be increased nearly 6.68 times.In this paper,the research on optical fiber humidity sensor modified by polymer,which combines fiber optic and polymer materials,provides a new research idea for the design and research of new humidity sensors.The fabricated sensor has the advantages,such as simple structure,high sensitivity,low cost and easy fabrication.It is expected to have a certain application prospect in remote temperature and humidity monitoring for the agricultural greenhouses.