Near-infrared Laser Methane Sensing Technique

In recent years,gas sensor based on slow light effect of the photonic crystal fiber has many advantages compared with the traditional gas sensor,which has attracted a wide attention from scientific researchers.This gas sensor based on slow light effect can not only reduce the device size,but also reduce the speed of photon propagation,which increases the effective optical length of the interaction between the light in the optical fiber and the medium（such as gas molecules）,thereby enhancing gas absorption.Therefore,this work focuses on the slow light enhanced methane gas sensing system.Methane（CH₄）is the main component of natural gas and it is also one of the gases that cause greenhouse effect.More importantly,during industrial production,such as the coal mining industry and the liquefaction of CH₄,due to its flammable and explosive characteristics,it will not only cause safety hazards but also cause environmental pollution.Therefore,it is particularly important to monitor the concentration of CH₄ in the air in real time.In this dissertation,a methane sensing system based on a traditional multi-pass cell is designed.Second,in order to enhance the absorption of methane gas,a tunable hollow-core bandgap photonic crystal fiber structure was designed using COMSOL software,and a near-infrared methane sensing system model based on the slow light effect of photonic crystal fiber was established.The effect of light effects on the absorption of methane gas was analyzed.The main research contents of this paper are as follows:1.The research significance of this paper was explained.And the research progress of gas sensor technology based on traditional multi-cell gas chamber and photonic crystal fiber gas sensor technology based on slow light effect are introduced.Compared to the traditional multi-pass cell,the photonic crystal fiber chamber based on the slow light effect enhances gas absorption.2.A near-infrared methane sensing system based on a traditional multi-pass cell was designed.The effective absorption path of the multi-pass cell was 14.8m.A distributed feedback laser was used as the light source.According to the standard gas distribution method,10ppmv-100ppmv of methane gas is provided.LabVIEW-based data generation,acquisition,and processing programs were written to extract the second harmonic,calibration concentration,and stability tests.To increase the second harmonic signal,the modulation depth of the system is optimized to be approximately0.0015cm^-1.3.The classification of photonic crystal fibers and their analysis methods are introduced.A tunable hollow band gap photonic crystal fiber structure was designed using microfluidic technology.The dispersion curve and enhanced absorption factor of the fiber were calculated using COMSOL software.Under the condition that the fiber parameters are constant,filling the liquid with different refractive index can change the dispersion curve of the optical fiber.Select the absorption wavelength of NH₃ at 1.5304μm and C₂H₂ at 1.534μm,fill the liquid with specific refractive index,The slow light mode of the photonic crystal fiber can be adjusted to the absorption wavelength of the two gases,at which time the absorption of the two gases will reach a maximum.At the same time,the slow light effect of porous-filled photonic crystal fibers was analyzed.4.A slow light enhanced methane sensing system model was established.The enhanced absorption factor of methane gas was calculated,and the effect of slow light effect on methane gas absorption was studied.LabVIEW software was used to extract the second harmonic signal of methane gas with or without slow light.The innovation of this thesis:1、An empty band gap type photonic crystal fiber structure was designed.The structural parameters were:lattice constantΛ=3.88,core diameter D=2.52Λ,and cladding air hole diameter d=0.9Λ.The microfluidic technology is used to fill the optical fluids with different refractive indices,and the specific mode of the optical fiber is tuned to match the absorption spectrum of the gas,enhancing the reaction between light and gas,and increasing the lower limit of gas detection.2、A slow light enhanced methane sensing system model was established.It includes tunable lasers,two fiber couplers,air-gap photonic crystal fibers,infrared detectors,amplifiers,and lock-in amplifiers.The absorption signal model was established and the second harmonic signal of methane gas under the effect of slow light was extracted with LabVIEW.