Research On The Solution Concentration Sensing Based On The Fresnel Reflection Of The Optical Fiber End Face

Solution concentration(SC)measurement has always been a hot topic in the sensing field,which has extremely important significance in many fields such as biochemistry,environmental monitoring,life science,food science,and so on.Due to the attractive advantages of small size,high stability,electromagnetic interference,and so on,optical fiber sensors have become one of the widely studied and applied sensors.However,the optical fiber SC sensors that have been studied using grating or special optical fiber structures as sensing units have problems of complex structures,high costs,and easy damage;while some researchers have proposed optical fiber SC sensors based on Fresnel reflection at the end face of the fiber tip(Fresnel reflection optical fiber SC sensor),which can effectively avoid damage to the sensing units and reduce the production cost of the sensor.However,there are common drawbacks of low sensitivity and low response speed,which greatly limit their application in the SC measurement field.In addition,most of the optical fiber SC sensors currently studied are aimed at non-corrosive solutions,and there are few optical fiber SC sensors for corrosive solution,like hydrofluoric acid(HF).Only one plastic optical fiber sensor based on localized surface plasmon resonance has achieved HF SC measurement,but this sensor has problems of complex preparation and high cost.Aiming at the above problems,this paper studies the concentration sensing of non-corrosive and corrosive solutions based on the Fresnel reflection of the optical fiber end face.Firstly,aiming at the problems of low sensitivity and long response time in existing Fresnel reflection based optical fiber SC sensors,this paper improves the measurement method of the Fresnel reflection optical fiber SC sensors based on the Fresnel reflection of the optical fiber end face and temperature sensing characteristics of optical fiber grating,and proposes a method that uses the spectral information(wavelength,temperature)of one side mode of optical fiber grating(FBG: fiber Bragg grating,TFBG: tilted FBG)to realize the simultaneous measurement of refractive index(RI)and temperature.Based on the proposed method,this paper has developed an FBG side mode-based Fresnel reflection optical fiber sensor and explained its relevant sensing characteristics and mechanism.In addition,in view of the complex preparation and high-cost problems of the existing optical fiber HF SC sensor,this paper develops a dynamic corrosion process of the end face(DCPEF)based all-fiber HF sensor based on the Fresnel reflection of the optical fiber end face and the specific reaction between silica and HF,and proves the feasibility of the proposed sensor theoretically and experimentally.Firstly,the sensing characteristics of the FBG side mode have been studied,it is found that there are significant differences between the actual and theoretical FBG side mode fringes,the former is irregular and asymmetric.In order to understand the formation mechanism of irregular FBG side mode fringes,the possible reasons for the forming of irregular FBG side mode fringes are analyzed,and a grid transfer matrix method is proposed to reveal the formation mechanism of irregular FBG side mode fringes,which proves that the non-uniformity of the period causes the forming of irregular side mode fringes.the sensing characteristics of the FBG side modes in the optical fiber sensing system based on FBG side modes and Fresnel reflection are explored by RI and temperature measurement experiments;the results show that the maximum RI and SC sensitivity of the selected FBG side mode dip can reach-568.010 d B/RIU and 0.8218 d B/%,respectively,and the solution temperature sensitivity can reach 0.0104 d B/℃;the grating temperature sensitivity characterized by the side mode wavelength can reach 10.64 pm/℃.Through parameter estimation,the sensing system unknown parameters are obtained and the influence of them on the sensing performance of the FBG side mode-based Fresnel reflection optical fiber sensor is revealed.In addition,a remote sensing system is built to analyze the remote sensing capability of the proposed sensor.The results show that the RI sensitivity of the sensor in the remote sensing system is significantly reduced due to the backscattered light,with a maximum RI sensitivity of-140.389 d B/RIU,which is only 1/4 of the RI sensitivity of the sensor in the common sensing system.This paper also investigates the sensing characteristics of TFBG side modes.Theoretically,a segmentation simulation method based on the Opti-Grating software is proposed to reveal the formation mechanism of irregular TFBG side mode fringes,and the formation of irregular TFBG side mode fringes caused by the nonuniformity of the grating period is proved for the first time.Experimentally,the RI and grating temperature sensing characteristics of the TFBG side mode are investigated;furthermore,the influences of the outpower of the light source and the type of optical splitter(coupler,optical circulator)on the sensing system are studied.By increasing outpower of the light source and using an optical circulator,a maximum RI sensitivity of-345.500 d B/RIU is obtained for the selected TFBG side mode.Due to the influence of apodization processing,compared to the TFBG side mode,the dip intensity variation of the FBG side mode caused by the change in RI is more significant.The grating temperature sensitivity of the selected TFBG side mode is 10.51 pm/℃.In addition,this paper systematically explores the sensing principle,HF detection feasibility,and remote detection capability of the DCPEF-based all-fiber HF sensor,the results show that:(i)With the progress of end face corrosion,a stepped hole is formed at the end face of the fiber tip,and accordingly,the reflective intensity received by the optical spectrum analyzer(OSA)oscillates first and then stabilizes;(ii)The HF SC will only affect the variation speed of the end face morphology,but will not affect the variation process of it.Therefore,the variation trends of the reflective intensities received by the OSA under different SCs of HF are the same,which oscillate first and then stabilize;(iii)The lower the HF SC,the higher the sensitivity of using the first dip time to characterize the HF SC,when the HF SC is 1.12 %,the proposed sensor shows high sensitivity and low measurement deviation,which are-1532s/% and ±0.009 %,respectively;(iv)Temperature,sealing property,and initial morphology will affect the SC measurement of HF,so it is necessary to ensure that the temperature is constant,the sealing property is good and the end face morphology is consistent when measuring HF SC;(v)The proposed sensor has the ability of remote sensing,and its sensitivity in the remote sensing system is the same as that in common sensing system.