Abstract
An external-cavity diode laser (ECDL) was employed to acquire spectrally resolved absorption profiles of methane lines in the R (9) manifold of the 2ν3 band near 6106 cm-1. The profiles were applied to high sensitivity detection of methane by low-frequency wavelength modulation spectroscopy (WMS) and harmonic detection (HD). In this study, the absorption profile near 6105.625 cm-1 is adopted. By measuring variations of signal amplitude at low concentrations of gas and background noise with frequency modulation amplitude (FMA), a group of optimum FMAs was obtained for 2f, 4f and 6f harmonic signals. Under these optimum conditions, minimum detectable absorbences (MDAs) of 4.7×10-4, 1.6×10-4 and 6.5×10-5, respectively, were realized; however 1.4×10-3 was obtained for direct absorption (DA).
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