Infrared Dual-Gas CH4/C2H6 Sensor Using Two Continuous-Wave Interband Cascade Lasers

An infrared dual-gas sensor system for the simultaneous detection and monitoring of methane (CH
₄
) and ethane (C
₂
H
₆
) at parts-per-billion by volume (ppbv) concentration levels was developed using two room temperature, distributed feedback interband cascade lasers, and two miniature multipass cells with an effective absorption length of 54.6 m. Laser direct absorption spectroscopy was used to detect CH
₄
utilizing the 3038.5-cm
^-1
absorption line, and second-harmonic wavelength modulation spectroscopy method was used to detect C
₂
H
₆
using the 2996.88-cm
^-1
absorption line. The 1σ CH
₄
detection limit is ~2.7 ppbv with a 1-s averaging time and exhibits a minimum value of ~1.7 ppbv for a 9-s averaging time; the 1σ C
₂
H
₆
detection limit is ~2.6 ppbv with a 3.4-s averaging time and shows an optimum averaging time of 65 s corresponding to a stability of ~0.36 ppbv. Using the dualgas sensor system, 24 h monitoring of the two atmospheric gases was performed in the Greater Houston area, TX, USA.