Research On Sensing Characteristics Of Hollow Band Gap Photonic Crystal Fibers

Sensors for gas,liquid,and biochemistry are widely used in the fields of environmental protection,safety production,defense and aerospace.Compared with electrochemical materials,optical fiber is anti-electromagnetic interference with advantages of small transmission loss,strong corrosion resistance.Optical fiber can be applied in high temperature and pressure,large-scale mechanical and electrical,flammable and explosive and other dangerous environments.The structure of hollow band gap photonic crystal fiber is unique,which can be used as a sensing unit for gas,liquids and other substances to response,so hollow band gap photonic crystal fiber has been widely used in gas sensing,liquid sensing,bio-chemical sensing and other fields.In this work,sensors based on hollow-core band gap photonic crystal fiber are designed.The paper mainly focuses on the following four parts:1.Firstly,several commonly numerical calculation methods of photonic crystal fibers and their advantages and disadvantages were introduced,respectively.Then,the vector finite element method was emphatically analyzed.Finally,the calculation steps of the vector finite element method were introduced and the calculation formulas are deduced.2.Then a photonic crystal optical fiber liquid sensor based on surface plasmon resonance was designed.The parameters of sensitivity and effective refractive index were simulated by COMSOL software.The relationship between the resonant wavelength of the fiber and the real part of the effective refractive index was analyzed.Finally,the thickness of the metal film,the thickness of the graphene film,the size of the air hole in the core,and the change of the refractive index of the measured object were calculated to determine the sensing characteristics of the photonic crystal fiber.3.Afterwards an air-core photonic crystal fiber gas sensor system was designed based on infrared absorption spectroscopy.The system includes three parts:light source,optical path and signal detection circuit.The DFB laser was used as light source.The home-made temperature controller and current-driver were used to control the DFB laser;the optical path was made up of optical fibers.Considering that the mode field diameters of the hollow core photonic crystal fiber and the single mode fiber do not match,the collimating lens and the focusing mirror were used for coupling of the two kinds of fibers.The gas chamber was separated from the circuit to provide safety for gas measurement.A current-to-voltage circuit and an amplify circuit were designed to detect the back-end signal.A commercial lock-in amplifier7265,NI data acquisition card and an information acquisition platform based LABVIEW were used for signal processing to extract the second harmonic signal to characterize the gas concentration.4.Finally the methane gas concentration measurement experiment was conducted and the second harmonic was used to characterize the gas concentration.The results show that the detection system has good linearity and stability.With the analysis of the sensor's Allan variation,the minimum detection limit is⁴.7ppm with an averaging time of 1 s.When the averaging time increases to 83 s,the minimum detection limit is reduced to 0.632ppm.The innovation of this thesis:1.A photonic crystal fiber-optic liquid sensor based on surface plasmon resonance was designed.A gold layer as a plasmon material was coated on the inside of the analyte channel to accurately detect the analyte.And a thin layer of graphene was coated on the outside of the gold layer to prevent oxidation.The finite element method?FEM?was used for numerical study of optical fiber performance and sensing performance.By studying the effect of the fiber structure parameters on the plasma excitation characteristics,the sensing performance was optimized.The results show that in the refractive index detection range of 1.37 to 1.40,the maximum refractive index sensitivity of the sensor is 12,000 nm/RIU,and the maximum amplitude sensitivity is 318 RIU^-1,at the same time the resolution is 3.1·10^-55 RIU.The sensor was suitable for detecting various high refractive index chemicals,biochemical and organic chemical analytes.2.A photonic crystal fiber gas sensing system based on absorption spectroscopy was designed.Using the all-fiber optical path structure,a small gas chamber was designed to isolate the gas detection part from the circuit.The detection system has good linearity and stability.The minimum detection limit is⁴.7 ppm with an averaging time of 1 s.When the averaging time increases to 83 seconds,the minimum detection limit is reduced to 0.632 ppm.