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Abstract
An instrument based on wavelength modulation off-axis integrated cavity output spectroscopy (WM-OA-ICOS) was developed for detection of trace methane (CH4) gas. The wavelength modulation technique was employed to obtain the second harmonic of the CH4 absorption signal. The modulation parameters were optimized to obtain a maximum second harmonic signal. A noise-equivalent absorption sensitivity of 9.4 × 10−11 cm−1 Hz−1/2 (corresponding to a detection limit of 28.9 ppbv for CH4 at this wavelength) was achieved using 2f detection. Compared with the traditional off-axis integrated cavity output spectroscopy (OA-ICOS) technique, WM-OA-ICOS provided an 8-fold improvement in detection sensitivity. CH4 concentration measurements were also achieved by normalization of second harmonic signal to first harmonic signal (2f/1f). The dynamic range and linearity of WM-OA-ICOS for both 2f method and 2f/1f method were investigated. The result showed that the 2f/1f method exhibited better linearity than 2f method.