Remote sensing system for near-infrared differential absorption of CH_4 gas using low-loss optical fiber link

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Abstract
An all-optical remote sensing system utilizing long-distance low-loss optical fiber links in conjunction with high-radiant InGaAsP light emitting diodes and dielectric interference filters was developed for the real-time absorption measurement of molecular concentration in the near-infrared region. The highly sensitive technique was achieved employing the power-balanced two-wavelength differential absorption method in the system, which enables direct detection of differential absorption signals for the specific molecule being monitored. Based on the laboratory study of the 2ν3 overtone band of CH4 molecules at 1.66 μm, the system was used for remote detection of low-level CH4 gas using a 2-km long low-loss silica optical fiber link incorporating an InGaAsP light emitting diode. Thus we demonstrated that this method is capable of a high detection sensitivity as low as ~400 ppm, i.e., ~0.8% of the lower explosion limit of CH4 density in air. Further extensive applications of the system as well as potential improvement of its sensitivity are also discussed.