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
(010.1280) Atmospheric and oceanic optics : Atmospheric composition
(140.5960) Lasers and laser optics : Semiconductor lasers
(300.1030) Spectroscopy : Absorption
(300.6320) Spectroscopy : Spectroscopy, high-resolution
A. A. Kosterev, R. F. Curl, F. K. Tittel, C. Gmachl, F. Capasso, D. L. Sivco, J. N. Baillargeon, A. L. Hutchinson, and A. Y. Cho, "Methane concentration and isotopic composition measurements with a mid-infrared quantum-cascade laser," Opt. Lett. 24, 1762-1764 (1999)