A variable duty cycle quasi-cw frequency scanning technique was applied to reduce thermal effects resulting from the high heat dissipation of type I quantum-cascade lasers. This technique was combined with a 100-m path-length multipass cell and a zero-air background-subtraction technique to enhance detection sensitivity to a parts-in-10<sup>9</sup> (ppb) concentration level for spectroscopic trace-gas detection of CH<sub>4</sub>, N<sub>2</sub>O, H<sub>2</sub>O, and C<sub>2</sub>H<sub>5</sub>OH in ambient air at 7.9 μm. A new technique for analysis of dense high-resolution absorption spectra was applied to detection of ethanol in ambient air, yielding a 125-ppb detection limit.
© 2000 Optical Society of America
(010.1280) Atmospheric and oceanic optics : Atmospheric composition
(140.5960) Lasers and laser optics : Semiconductor lasers
(280.3420) Remote sensing and sensors : Laser sensors
(300.6320) Spectroscopy : Spectroscopy, high-resolution
Anatoliy A. Kosterev, Robert F. Curl, Frank K. Tittel, Claire Gmachl, Federico Capasso, Deborah L. Sivco, James N. Baillargeon, Albert L. Hutchinson, and Alfred Y. Cho, "Effective Utilization of Quantum-Cascade Distributed-Feedback Lasers in Absorption Spectroscopy," Appl. Opt. 39, 4425-4430 (2000)