Cavity ring-down spectroscopy for molecular trace gas detection using a pulsed DFB QCL emitting at 6.8 μm

A trace gas sensor based on pulsed cavity ring-down spectroscopy (CRDS) was developed for measurement of the ν4 fundamental vibrational band of ammonia (NH3) centered at 1468.898 cm-1. A pulsed distributed feedback quantum cascade laser (DFB-QCL) operating at 6.8 μm (1470.58 cm-1) quite well covered the absorption band of the ammonia and strong fundamental vibrational absorption bands of different molecular gases in this unexplored region. The cavity was partially evacuated down to 0.4 Atm by a turbo-molecular pump to reduce the partial interference between the NH3 spectra and water near the absorption peak of ammonia. A sensitivity of nine parts per billion was reached for a measurement time of 120 s as well as an optical path length of 226 m. The device demonstrated high spectral performance and versatility due to its wide tuning range, narrow linewidth, and comparatively high-energy mid-IR radiation in the relatively unexplored 6.8 μm region, which is very important for high-resolution spectroscopy of a variety of gases.