Research Of Optical Fiber SPR Humidity Sensing Based On PVA Film Coating

Optic fiber humidity sensor has attracted the attention of a large number of researchers as an important design for the application of humidity sensor in extreme harsh environment.At present,the design of optical fiber humidity sensor is mainly based on the fiber interference structures or fiber grating structures,but the humidity sensitivity of these structures is not very high.In this paper,we demonstrated a design of highly sensitive humidity sensor based on fiber surface Plasmon resonance(SPR)sensor coated with humidity-sensitive material.The main contents are listed as follows:The first chapter is the introduction.The optical fiber humidity sensor and SPR sensor are summarized;the existing types of optical fiber humidity sensors and their sensing characteristics are introduced;at the same time,the development history of fiber SPR sensor and the structural design of various fiber SPRs are also introduced.The secondary chapter introduces the related theories of SPR effect.Based on Maxwell's equations,the excitation of surface plasma wave(SPW)at the interface between metal and dielectric is discussed.The propagation characteristics of evanescent wave during the total reflection phenomenon are introduced.Finally,the enhancement of the vector of incident light caused by prism and fiber and the resonance condition in these structures are discussed.In chapter 3,the fabrication process of D-type fiber SPR sensor and its refractive index sensing characteristics are introduced.Firstly,the three-stage polishing method of D-type fiber with low insertion loss is introduced.Then,the advantages and disadvantages of SPR device with gold or silver material are introduced respectively.Next,the magnetron sputtering technique for gold coating on the flat surface of D-fiber is introduced.Finally,the refractive index sensing characteristics of the developed D-fiber SPR sensor are studied.In chapter 4,a SPR humidity sensor based on polyvinyl alcohol(PVA)coated side-polished single mode fiber is designed and fabricated.The sensitivity of the sensor is about 1.28nm/RH% over the range of 40%-90% RH.In addition,the samples coated with PVA films of different thicknesses were found to exhibit different humidity sensing characteristics.Finally,the application of the device in human breathing monitoring and its temperature cross-sensitivity are studied.In chapter 5,a highly sensitive D-type polymer optical fiber(POF)SPR humidity sensor based on PVA coating is designed and fabricated,the resonant wavelength of the sensor exhibits a rapid redshift as RH increases.Because of the low-index of the POF compared to silica fiber,the sensitivity can be enhanced to 4.98nm/RH% as the RH varies from 40–90%,which is four times larger than the highest humidity sensitivity ever reported.There are two competitive mechanisms in the sensor: the red-shift of the resonant wavelength caused by film swelling and the blue-shift caused by the decrease of refractive index of the film.The contributions of these two mechanisms to the resonant wavelength are simulated individually.Finally,it is found that the device is very sensitive to the response of the breathing by studying the respiratory response characteristics of the device,and the wavelength shift can reach to 248.94 nm and the response speed is less than 500 ms.The temperature cross-sensitivity is as low as 0.27RH%/??The device has the advantages of high sensitivity,simple structure and fast response,which makes it a high potential value in practical application.