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

• Published in: Applied spectroscopy 2013-12, Vol.67 (12), p.1368-1375
• Main Authors: Reidl-Leuthner, Christoph, Lendl, Bernhard
• Format: Article
• Language: English
• ISSN: 0003-7028; 1943-3530
• DOI: 10.1366/13-07104

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.