Trace methane gas detection by wavelength modulated off-axis integrated cavity output spectroscopy

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.

Keywords:

• Off-axis integrated cavity output spectrometer
• wavelength modulation
• trace gas of methane
• second harmonic (2f) detection

Additional information

Funding

This experiment is supported by the graduate project of innovative special funds of Nanchang Hangkong University [grant number YC2016054].