Abstract

A quantum-cascade laser operating at a wavelength of 8.1 µm 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 ν4 P branch were recorded. The measured laser linewidth was 50 MHz. A CH4 concentration of 15.6 parts in 106 ppm in 50 Torr of air was measured in a 43-cm path length with ±0.5ppm accuracy when the signal was averaged over 400 scans. The minimum detectable absorption in such direct absorption measurements is estimated to be 1.1×10-4. The content of  13CH4 and CH3D species in a CH4 sample was determined.

© 1999 Optical Society of America

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