Methane concentration and isotopic composition measurements with a mid-infrared quantum-cascade laser

A quantum-cascade laser operating at a wavelength of 8.1 mu m was used for high-sensitivity absorption spectroscopy of methane (CH4). The laser frequency was continuously scanned with current over more than 3 cm(-1), and absorption spectra of the CH4 nu(4) P branch were recorded. The measured laser linewidth was 50 MHz. A CH4 concentration of 15.6 parts in 10(6) (ppm) in 50 Torr of air was measured in a 43-cm path length with +/-0.5-ppm accuracy when the signal was averaged over 400 scans. The minimum detectable absorption in such direct absorption measurements is estimated to be 1.1 x 10(-4). The content of (CH4)-C-13 and CH3D species in a CH4 sample was determined. (C) 1999 Optical Society of America.