Photonic Crystal Fiber Methane Sensor With Nano-Film Based On Mode Interference

With the rapid development of optical fiber sensing technology, optical fiber gas sensors have attracted widely attention. Methane is the main component of mine gas, which can easily explode in the air and is regarded as the "the leading killer" in coal mine accidents. Thus, monitoring of the methane concentration is of great significance. Owing to the porous structure and unique transmission characteristics, the sensor based on photonic crystal fiber is particularly suitable for gas sensing field.In this paper, a photonic crystal fiber(PCF) methane sensing method based on modal interference is presented. The light guide mechanism is analyzed by using modal interference principle. With the finite element method, the methane sensing process is simulated, and effect of methane concentration on refractive index of sensitive film is measured. In process of fabricating the PCF methane sensitive film sensor, the technologies of optical fiber splice offset and capillary dip coating have been applied. By means of the sensing experiment system, the sensor performance can be evaluated. The main contents include:(1) Based on modal interference principle, the influence of sensitive film refractive index, thickness and length of PCF on characteristic wavelength of interference spectra are analyzed theoretically. Using finite element method, the electric field distribution of the core mode and cladding mode, effective refractive indices of core mode and cladding mode, and the difference of effective refractive index(PCF characteristic wavelength) in different refractive index, thickness of nano thin film and sensing length are stuided. It's shown that the effective refractive index difference between the core mode and cladding mode decreaseslowly, characteristic wavelength of interference spectrais blue shift, with the increasing of the sensitive film thickness from 100 nm to 300 nm and the reducing of the film refractive index from 1.455 to 1.410.(2) The influence of methane concentration on the refractive index of the Cryptophane A / UV-curable fluorosilicone thin film is in situ measured. The results show that the refractive index of thin film decreases linearly with the increase of methane concentration, the correlation coefficient is 0.996. By optimizing the fiber splicing parameters, PCF collapse length, the film coating process, the viscosity of the sensitive film coating solution, fiber optic splice offset mthod, and capillary dip coating technology, the PCF methane sensitive thin film sensor can be fabricated, and the optimal PCF collapse length is about 240?m.(3) For the PCF methane sensitive thin film sensor, effect of sensitive thin film thickness, PCF sensing lengthon the characteristic wavelength of interference spectra are studied in range of 0~3.5% methane concentration experimentally. The results show that the characteristic wavelength of interference spectra is blue shift, accompanied with the increasing of methane concentration. When the thin film thickness of the sensor is 240 nm, there are high sensitivity and the correlation coefficient. Meanwhile, the influence of non-methane gases(such as hydrogen sulfide(H2S), carbon dioxide(CO2), oxygen(O2)and nitrogen(N2)on characteristic wavelength of the sensor are also studied, the selectivity is very good.