Methane Sensing Characteristics Of Photonic Crystal Fiber Long Period Grating With Nano-bilayer Films

Methane is the main component of natural gas,biogas,oil field gas and mine gas.It is very important to ensure the safe production and reduce the safety accident in the coal mine production environment or during the exploitation,transportation and storage of natural gas.Photonic crystal fiber long-period grating as a new type of passive optical devices,in the field of optical fiber sensing research has been widely carried out,meanwhile it also provides a new way for gas detection.(1)The influence of grating parameters and sensitive film parameters on the sensing characteristics of PCF-LPG methane sensor was simulated by combining finite element method and coupled local mode theory.The finite element method is used to analyze the field distribution in the PCF-LPG grating region.The effective refractive index and the transmission constant of the core mode and cladding modes are solved.The coupled local mode theory is used to solve the relationship between the core mode and the cladding mode.The transmission spectra of PCF-LPG methane sensors were obtained by simulation.Simulation results show that the refractive index of the sensitive film coated in the air hole surface decreases from 1.55 to 1.53,and the resonant wavelength of the transmission spectrum will have a blue shift.When the sensitive film thickness is 200 nm,the sensitivity of the sensor can be improved.(2)The effect of methane concentration on the refractive index of sensitive films composed of cryptophane-A-6Me and carbon nanotubes was studied.The experimental results show that the refractive index of sensitive films decreases 0.0049 RIU with the increase of methane concentration from 0% to 3.5%.PCF-LPG methane sensor was fabricated by selecting the appropriate method to fabricate PCF-LPG,preparing sensitive film coating solution and using air pressure driving device.(3)The PCF-LPG methane sensor with different sensitive film thickness was tested in the concentration range of 0%-3.5% methane.The experimental results show that the increase of the methane concentration will lead to the decrease of the refractive index of the sensitive film,thus the resonance wavelength of the spectrum occurs blue shift.In the sensor,when the thickness of the sensitive film is 210 nm,the sensitivity of the sensor can reach-1.078 nm /% and the detection limit is 0.18%.The PCF-LPG methane sensor fabricated has a good choice characteristic for methane gas.