METHOD AND APPARATUS FOR TRACE GAS DETECTION USING OFF-AXIS CAVITY AND MULTIPLE LINE INTEGRATED SPECTORSCOPY

摘要

A sensor for NO₂ with ultrahigh sensitivity of detection is created by combining off-axis integrated cavity output spectroscopy (OA-ICOS) (which can provide large path lengths of the order of several km in a small volume cell) with multiple line integrated absorption spectroscopy (MLIAS) (where the absorption spectra is integrated over a large number of rotational-vibrational transitions of the molecular species to further improve the sensitivity). Employing an external cavity tunable quantum cascade laser operating in the 1601 - 1670 cm^-1 range and a high finesse optical cavity, the absorption spectra of NO₂ over 100 transitions in the R- band have been recorded. Based on the observed linear relationship between the integrated absorption vs. concentration of NO₂, the sensor has an effective sensitivity of detection of 28 ppt for NO₂, which is among the most sensitive levels of detection of NO₂ to date. This technique can be applied using any tunable laser source, such as diode lasers, capable of tuning over multiple lines of target species including complex molecules and explosive compounds.