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C. Reidl-Leuthner, B. Lendl:
"Toward Stand-Off Open-Path Measurements of NO and NO₂ in the Sub-Parts Per Million Meter Range Using Quantum Cascade Lasers (QCLs) in the Intra-Pulse Absorption Mode";
Applied Spectroscopy, 67 (2013), 12; S. 1368 - 1375.

Two thermoelectrically cooled mid-infrared distributed feedback quantum cascade lasers operated in pulsed mode have been used for the quasi-simultaneous determination of NO and NO₂ in the sub-parts per million meter (sub-ppm-m) range. Using a beam splitter, the beams of the two lasers were combined and sent to a retro-reflector. The returned light was recorded with a thermoelectrically cooled mercury cadmium telluride detector with a rise time of 4 ns. Alternate operation of the lasers with pulse lengths of 300 ns and a repetition rate of 66 kHz allowed quasi-simultaneous measurements. During each pulse the laser temperature increased, causing a thermal chirp of the laser line of up to 1.3 cm^{−1}. These laser chirps were sufficient to scan rotational bands of NO centered at 1902 cm^{−1} and NO₂ located at 1632 cm^{−1}. In that way an absorption spectrum could be recorded from a single laser pulse. Currently achieved limits of detection are 600 parts per billion meter (ppb-m) for NO and 260 ppb-m for NO₂ using signal averaging over 1 min. This work presents the first steps toward a portable stand-off, open-path instrument that uses thermoelectrically cooled detector and lasers.

Quantum cascade laser; Intra-pulse; Open path; Nitric oxide; Nitrogen dioxide

http://dx.doi.org/10.1366/13-07104