Quantitative Remote Sensing of Methane Using an Intensity-Modulated Dual-Wavelength He-Ne Laser

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  • 2波長発振He-Neレーザーを用いたメタンの定量遠隔測定

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It is known that the 3.392-μm He-Ne laser emission (Ne 3s<sub>2</sub>-3p<sub>4</sub>) is strongly absorbed by methane while the 3.391-μm emission (Ne 3s<sub>2</sub>-3p<sub>2</sub>) is absorbed only weakly. This property of methane has been utilized in differential absorption laser radars (DIAL) for methane sensing. In conventional DIAL systems, a pair of lasers, one emitting the 3.392-μm line and the other 3.391μm, have been used as a light source. These systems need, however, not only complex optics but also its precise adjustment to align the two laser beams on the same optical path. These problems are solved by using a dual-wavelength He-Ne laser developed recently. This laser emits the two lines coaxially whose intensities are modulated with equal amplitudes and opposite phases to each other. In the present paper two simple and sensitive methods of quantitative remote sensing of methane using a dual-wavelength HeNe laser are demonstrated. In the first method, two detectors are used, one monitoring both wavelength components, and the other only the 3.391-μm component in the reflected radiation. In the second method, the double modulation technique makes it possible to use only one detector. Laboratory simulation shows that both methods are capable of measuring quantitatively density-pathlength products down to about 40ppm·m at a signal-averaging time of 3s with reflecting targets at a 2.5-m distance. When the reflectivity is high, a 100ppm·m leakage can be detected with a target as distant as 30m.

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