CW EC-QCL-based sensor for simultaneous detection of H2O, HDO, N2O and CH4 using multi-pass absorption spectroscopy.

A sensor system based on a continuous wave, external-cavity quantum-cascade laser (CW EC-QCL) was demonstrated for simultaneous detection of atmospheric H<sub>2</sub>O, HDO, N<sub>2</sub>O and CH<sub>4</sub> using a compact, dense pattern multi-pass gas cell with an effective path-length of 57.6 m. The EC-QCL with a mode-hop-free spectral range of 1225-1285 cm<sup>-1</sup> operating at ~7.8 µm was scanned covering four neighboring absorption lines, for H<sub>2</sub>O at 1281.161 cm<sup>-1</sup>, HDO at 1281.455 cm<sup>-1</sup>, N<sub>2</sub>O at 1281.53 cm<sup>-1</sup> and CH<sub>4</sub> at 1281.61 cm<sup>-1</sup>. A first-harmonic-normalized wavelength modulation spectroscopy with second-harmonic detection (WMS-2f/1f) strategy was employed for data processing. An Allan-Werle deviation analysis indicated that minimum detection limits of 1.77 ppmv for H<sub>2</sub>O, 3.92 ppbv for HDO, 1.43 ppbv for N<sub>2</sub>O, and 2.2 ppbv for CH<sub>4</sub> were achieved with integration times of 50-s, 50-s, 100-s and 129-s, respectively. Experimental measurements of ambient air are also reported.

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