Preparation And Sensing Performance Of Ammonia Sensor Based On Thin-core Fiber Modal Interference

Due to small size,resistance to electromagnetic interference,high sensitivity and stability,optical fiber sensor has gained widely attention and applied to clinical diagnosis,environmental protection and food security,etc.In this paper,an ammonia sensors based on thin-core optical fiber interferometer(TCFMI)is presented.A thin-core optical fiber was inserted into a standard single-mode fiber,the side surface of the interferometer was then deposited with sensitive film by Layer-by-Layer(LbL)electrostatic self-assembly technique.The sensing process is that the sensitive film could selectively adsorb ammonia,which will change the refractive index of film and interference spectrum.Through analyze the movement of central wavelength could realize the ammonia detection.The main research works are as follow:(1)Based on BeamPROP software,the TCFMI model was built.The transmission spectrums of different length of sensors were achieved through the simulation.Considering the cost and convenient operation,choose 2cm thin-core optical fiber to the sensor fabrication and simulation.Simulating the refraction index sensitivity of the sensor,and then compare the results of simulation and experiment,both of the center wavelength of the transmission spectrum red shift linearly with the increasing of refractive index.The temperature sensitivity of the sensor was tested,the results showed that the temperature sensitivity is only 17.47pm/?.It means that the sensor could resist the change of temperature and suitable for refractive index sensor.(2)The(PAH/PAA)and [(PVPMC + PAH)/PAA] multilayer non-porous membrane was prepared on quartz substrate via LbL electrostatic self-assembly technology.The scanning electron microscopy(SEM)results showed that the thickness of PAH/PAA bilayer was 40 nm,the thickness of(PVPMC+PAH)/PAA bilayer was 46 nm.The [(PVPMC + PAH)/PAA] film was immersed in sodium chloride solution could carried PVPMC out from the three component system.The nanopores was about 20–40 nm.(3)The sensors coated with(PAH/PAA)film was tested with different concentrationsof ammonia solutions.The transmission dip shifted to shorter wavelength with the increasing of ammonia concentration.The movement of central wavelength was 6.87 nm,which means the sensitivity was estimated to be 0.027 nm/ppm.The sensor showed a better selectivity to ammonia and excellent reversible performance.The dynamic response rising time(t)and falling time(t)was about 160 s and 200 s,respectively.Monitoring the refraction index of(PAH/PAA)film with different ammonia concentration using refractometer,the results showed that the refractive index decrease 0.006625.The theory refractive index sensitivity was 850 nm/R.I.U..The PAA/PAH porous film was assemble to the outer surface of the thin-core optical fiber,response time reduced from 160 s to 70 s,the recovery time is also reduced from 200 s to 80 s.(4)The(PAH/PAA)+ [PAH/(PAA + SWCNTs-COOH)] film deposited on the TCFMI was tested with different concentrations of ammonia gas.The transmission dip shifted to shorter wavelength with the increasing of ammonia concentration.The ammonia concentration range from 1 to 20,the sensor results showed a good linearity.The sensitivity was 0.031 nm/ppm with ammonia concentration range from 1 to 20 ppm.The sensor showed a better selectivity to ammonia and excellent reversible performance.The dynamic response t and t was about 30 s and 70 s,respectively.