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.