Characterization of a swept external cavity quantum cascade laser for rapid broadband spectroscopy and sensing

The performance of a rapidly swept external cavity quantum cascade laser (ECQCL) system combined with an open-path Herriott cell was evaluated for time-resolved measurements of chemical species with broad and narrow absorption spectra. A spectral window spanning 1278-1390 cm-1 was acquired at a 200 Hz acquisition rate, corresponding to a tuning rate of 2×10⁴ cm^-1/s, with a spectral resolution of 0.2 cm^-1. The capability of the ECQCL to measure < 100 ppbv changes in nitrous oxide (N₂O) and 1,1,1,2-tetrafluoroethane (F134A) concentrations on millisecond timescales was demonstrated in simulated plume studies with releases near the open-path Herriott cell. Absorbance spectra measured using the ECQCL system exhibited noise-equivalent absorption coefficients of 5×10^-9 cm^-1Hz^-1/2. For a spectrum acquisition time of 5 ms, noise-equivalent concentrations (NEC) for N₂O and F134A were measured to be 70 and 16 ppbv respectively, which improved to sub-ppbv levels with averaging to 100 s. Noise equivalent column densities of 0.64 and 0.25 ppmv × m in 1 sec are estimated for N₂O and F134A.