Design And Performance Of Fluoride Ion Sensor Based On Special Optical Fiber Integrated With Sensing Membrane

Optical fiber sensors have the advantages of anti-electromagnetic interference,light weight,small size,high sensitivity and so on,which have been widely used in the research fields of strain,temperature,refractive index,pressure,current,biology and chemistry.In this paper,three kinds of fluoride ion sensors based on the mode interference of special fiber were designed and fabricated.First,a Mach-Zehnder sensor was presented,which the thin-core fiber(TCF)was embedded between two sections of the multi-mode fiber(MMF),and the single-mode fiber(SMF)was used to connect the MMF.Finally,the sensitive film was coated on the surface of the TCF by layer-by-layer self-assembly technology.In addition,a thick taper Michelson interferometer based on the end face encapsulation of a TCF was also fabricated,which was a waist-enlarged taper drawn between the SMF and TCF with the technology of fusion taper.Besides,a layer of silver film was deposited on the other end of the TCF to enhance the end surface reflection.In order to prevent the silver film from being oxidized and damaged,the silver film was encapsulated and protected with ultraviolet optical curing glue.The functional integration was performed by coating a sensitive film on the surface of the TCF.Finally,a Michelson reflective fluoride ion sensor based on no-core fiber(NCF)and TCF was fabricated.A section of NCF is spliced between the SMF and the TCF,the silver film was deposited on the end of the TCF,then the silver film was also protected by UV optical curing glue,finally the sensor was functionally modified with sensitive materials.The sensing process is based on the selective adsorption of fluoride ions by the sensitive film,which leads to the changes of sensitive film's refractive index and output interference spectrum.The fluoride ions can be detected by analyzing the shift of the center wavelength.The main research work of this paper is as follows:(1)A Mach-Zehnder interferometer was presented using the TCF,the neodymium-modified chitosan/polymethyl methacrylate was coated on the surface of TCF through layer-by-layer self-assembly technology to form the sensitive film with the thickness of 690 nm.This fluoride ion sensor has the average sensitivity of 14.145 nm/ppm and the response time of 60 s with excellent temperature,pH and time stability.(2)In order to improve the convenience of detection process,a waist-enlarged taper Michelson interferometer was made with TCF.In the experiment of refractive index and temperature,the sensor has high refractive index sensitivity and poor temperature response,indicating that the sensor has strong resistance to temperature interference which can be seen as an ideal refractive index sensor.The aluminum-doped zinc oxide sensitive material was prepared by the sol-gel method,and the sensor was functionalized by the dip-coating method.Characterization and analysis of X-ray diffraction(XRD),X-ray photoelectron spectroscopy(XPS)and transmission electron microscope(TEM)proved that the material was successfully prepared.Scanning electron microscope(SEM)results show that the sensitive film with the thickness of 860 nm was formed on the surface of the TCF.In the trace detection(0.01-50 ppb)of fluoride ion,the average sensitivity of the sensor is 60.016 pm/ppb,and the response time is 40 s.The sensor has excellent selectivity and good stability.(3)In order to further reduce the size of the sensor,a Michelson interferometer based on end-face packaging was designed with NCF and TCF.The sensor also has higher refractive index sensitivity and lower temperature sensitivity.CeCO₃OH was dip-coated on the surface of the sensor probe.The average sensitivity of the sensor was 2.4266 nm/ppm,and its response time was 25 s.The performance of the sensor(selectivity,temperature and pH)was analyzed and discussed,showing excellent selectivity to fluoride ions and good stability.