Abstract
In this paper, we present and discuss two methane gas detection systems using a mid-infrared (MIR) ( $\lambda =3.3\,\,\mu \text{m}$ ) light emitting diode (LED). Although methane is a major greenhouse gas, it is widely used in modern daily life. The need to effectively monitor the methane concentration has always been emphasized. As a first of its kind, the direct absorption method was employed. It included a light source, an MIR LED, an infrared photodetector, and a 20-mm gas cell with a proper gas mixer system. The output signal could be amplified using a signal amplifier or a lock-in technique. The detection limit was estimated as 49 ppm using the signal amplifier and 2.3 ppm using the lock-in technique. The MIR light is invisible to the naked eye but can be seen with a thermal camera. The optical alignment is time consuming. A microstructured infrared multimode fiber was adopted instead of a gas cell. The fiber surface was machined by a high-power Q-switched laser, and the interaction of light and methane molecules occurred on this micromachined fiber surface. The detection limit was estimated as 6.1 ppm. The dynamic range was estimated as 26 dB.
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