Abstract

A quantum-cascade laser operating at a wavelength of 8.1μm was used for high-sensitivity absorption spectroscopy of methane (CH<sub>4</sub>). The laser frequency was continuously scanned with current over more than 3cm<sup>-1</sup>, and absorption spectra of the CH<sub>4</sub> ν<sub>4</sub> P branch were recorded. The measured laser linewidth was 50 MHz. A CH<sub>4</sub> concentration of 15.6 parts in 10<sup>6</sup>(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 × 10<sup>-4</sup>. The content of <sup>13</sup>CH<sub>4</sub> and CH<sub>3</sub>D species in a CH<sub>4</sub> sample was determined.

© 1999 Optical Society of America

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