Studies Of Fiber Relative Humidity Sensor Based On Modal Interference

In recent years,with the fast development of the related technology,optical fiber sensors have been used widely in industrial and agricultural production,social security,engineering construction,environmental monitoring and so on,because of their advantages such as anti-corrosion,lightweight,high sensitivity,compact size,immunity to electromagnetic interference,compatibility with optical fiber communication systems.Compared with the conventional optical fiber sensors,the newly-developed optical fiber sensors based on modular interference are more compact in structure which further simplify the optical path.Therefore,this kind of sensors has been studied widely in recent years.Studies indicate these sensors can be potentially used to monitor relative humidity(RH)due to their series of advantages.In this paper,optical fiber RH sensors based on modular interference are studied and several kinds of optical fiber RH sensors have been proposed,which are described as follows:(1)A new optical fiber RH sensor coated with porous CeO2 nanosheets based on modular interference has been proposed.Controlled by reaction temperature,porous CeO2 nanosheets have been synthesized on optical fiber via a simple hydrothermal process.Experiments show that fiber coated with CeO2 nanosheetsis sensitive to the change of RH.Michelson interferometer is fabricated by the misaligned splicing of two standard single mode silica fibers,then CeO2 nanosheets are grown on the sensing fiber.The Michelson fiber interferometer coated with CeO2 nanosheets has excellent property of sensitivity to RH and the reflection spectrum has a blue-shift with increasing RH.The wavelength of the dip decreases linearly and the achieved sensitivity is 17.7pm/%RH.(2)A new optical fiber RH sensor coated with ZnO nanorods based on modular interference has also been prepared.The growth process of ZnO nanorods on the surface of the fiber is studied.The two-step growth method is proposed as follow:firstly,ZnO seed layer with particles is grown on the fiber;secondly,ZnO nanorods are vertically grown on the surface of seeded fibers.The fiber coated with ZnO nanorods has good sensitivity to RH.RH sensitivity of the Michelson interferometer with ZnO nanorods grown on the sensing fiber surface is studied.The reflection spectrum has a blue-shift as RH rises,and the wavelength of the dip decreases.linearly.ZnO nanorods with different diameters can be obtained by controlling the growth time of nanorods,and the RH sensitivity of the fiber interferometer can be enhanced through reducing the diameters of the ZnO nanorods.The achieved maximum RH sensitivity is 30.2pm/%RH,when the diameter of ZnO nanorods is 40nm.(3)A new optical fiber RH sensor of the interferometer based on modal interference in a no-core fiber(NCF)is proposed.The NCF is coated with polyvinyl alcohol(PVA)thin film.In the interferometer,when higher-order modes in NCF are excited by the core mode of single-mode fiber(SMF),modular interference appears.Beam propagation method is used to simulate the waveguide properties and results show that the interferometer has good sensitivity to refractive index.Experiments also show that the interferometer has good sensitivity to RH.The reflection spectrum has a blue-shift as RH rises,and the wavelength shifts linearly to RH.The influence of the diameter and length of the NCF on RH sensitivity is also studied.When the diameter of NCF decreases or the length of NCF increases,RH sensitivity increases with the highest RH sensitivity as 95pm/%RH.(4)A fiber sensor for simultaneous measurement of relative humidity(RH)and temperature is proposed.Fiber Bragg grating(FBG)is used to be the first sensor probe and no core fiber(NCF)is used to be the second sensor probe.Due to the different responses of the NCF and the FBG to RH and temperature,the simultaneous measurement at the same fiber sensor is achieved.By constructing the matrix,the change of temperature(?T)and the change of RH(?RH)are given respectively which solves the problem of the cross-sensitivity of the two parameters in practice.