CW EC-QCL-based sensor for simultaneous detection of HO, HDO, NO and CH 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 HO, HDO, NO and CH 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 operating at ~7.8 µm was scanned covering four neighboring absorption lines, for HO at 1281.161 cm, HDO at 1281.455 cm, NO at 1281.53 cm and CH at 1281.61 cm. 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 HO, 3.92 ppbv for HDO, 1.43 ppbv for NO, and 2.2 ppbv for CH were achieved with integration times of 50-s, 50-s, 100-s and 129-s, respectively. Experimental measurements of ambient air are also reported.