광자결정 광섬유를 이용한 혼합가스 검출에 관한 연구

Abstract

The optical sensors based on optical fiber can be built as small and cheap systems. In a harsh actual industrial field, the distributed sensor systems and the remote sensor systems detecting gas at several locations by using one light source at the same time can be constructed. Also the optical system that has a diode laser with narrow line width can get the resolution of 0.0001cm-1 and has an advantage to the highly sensitivity corresponds to detection limit of part per billion (ppb) unit. Typically, the harmful gases produced in living environment and working environment emits their own absorption spectrum at the specific wavelength region for the incident light source. In this study, using a photonic crystal optical fiber, we made the optical sensor measuring the absorption spectrum that occurs in specific regions of wavelength in real time, and measured the properties with variation of the type of gas and temperature. The gas absorption spectrums were measured by the specific wavelength of light which occurs when the lights pass the hollow core presented inside the optical fiber. The optical gas sensors which can detect various kinds of gas concentrations by processing the electrical signal were fabricated. At this point to produce high sensitivity gas sensors, optical communication single mode fiber (SMF) and interfaces of sensor was bonded by low loss fusion splice. The concentration measuring system which is composed with diode laser and fiber optics-based optical structure and tunable optical absorption measurement techniques was used to measure the concentration of mixed gases. By using a laser diode with different wavelength, the measuring concentrations of various gases has been verified experimentally. A theoretical analysis of photonic band gap fiber gas sensor was investigated by using the Beer-Lambert’s law. The absorption wavelength of the target gas was screened by using Voigt liner shape function and spectroscopic properties with variation of temperature and pressure were investigated. Using direct absorption spectroscopy (DAS) a wavelength absorption spectroscopy (WAS), the concentration of CO, CO2, C2H2, CH4 base gases were quantitatively measured by the detection of absorption signal and 2f signal. And direct absorption spectroscopy and wavelength modulation spectroscopy (Wavelength Absorption Spectroscopy) was evaluated using a quantitative measure of resolution. The optimal measurement techniques which can measure concentration of below several part per million (ppm) were investigated. Through the analysis of injection current and the output and wavelength according to the temperature, absorption lines at center wavelength of 1568.773 nm (CO gas), 1572.335 nm (CO2 gas) and 1535.39 nm (C2H2 gas) were selected for being the best. The base CO gas spectral absorption characteristics were decreased approximately 5.2 times, and the line width was increased with increasing temperature. The line width of CO gas was higher than that of CO2 gas and the absorption rate was increased 0.21% with increasing temperature. With increasing pressure of CO gas, the line width was linearly increased and absorption rate was nonlinearly increased. The errors of CO, CO2, C2H2, CH4 gases which were measured by using direct absorption spectroscopy method, were 3.2% and 1%, 2.5% and 0.2%, respectively. Under the condition of fuel combustion, the direct absorption spectroscopy method and wavelength absorption spectroscopy method by comparing to the solid electrolyte absorption method, respectively, 0.78% and 0.14% were improved From these results, we can verify that photonic crystal fiber gas sensor can be used as a device sensing in real-time detection of gas molecular.