Optical-feedback cavity-enhanced absorption: a compact spectrometer for real-time measurement of atmospheric methane

We report on the application of the new technique of optical-feedback cavity-enhanced absorption spectroscopy to the real-time quantitative measurement of tropospheric methane traces from an airplane using a compact, low cost instrument based on a telecommunication-type diode laser operating close to room temperature. Methane concentration is obtained by fitting the absorption line centered at 1658.96 nm (6026.23 cm(-1)) which belongs to the first overtone transition of the CH stretch vibration. The measurement rate is about 30 Hz, but the response time is limited to about 0.3 s by the gas flow in the measurement cell. The instrument provides the absolute ambient methane concentration accurate to +/- 1% (+/- 20 ppb) without need for a periodic calibration. This is demonstrated by a hands-off comparison with a self-calibrating chromatographic setup during 10 days. The observed measurement stability can be extrapolated to much longer time periods. With respect to the short-term performance (minutes) fast concentration changes at the level of 1 ppb can be detected, and we believe this performance can be extended to the long term. Finally, a laboratory comparison with a lead-salt mid-infrared diode laser multipass spectrometer (operating close to 3028 cm(-1) at liquid nitrogen temperature) demonstrates a similar performance.