Comparison of cw and pulsed operation with a TE-cooled quantum cascade infrared laser for detection of nitric oxide at 1900 cm-1

A quantum cascade laser operating near room temperature with thermoelectric (TE) cooling has been used in both continuous-wave (cw) mode (-9 °C) and pulsed mode (+45 °C) to detect atmospheric nitric oxide using spectral lines at 1900.07 cm^-1 (5.3 μm). The totally non-cryogenic spectrometer integrates the laser with a 69-m astigmatic multi-pass cell and a TE-cooled infrared detector to enable operation for extended time periods without operator attention. The pattern of reflections on the astigmatic cell mirrors has been designed to minimize optical interference fringes, which are substantially greater with cw mode than with pulsed operation. The detection method uses direct absorption with rapid- scan sweep integration to achieve sub-second time response. Detection precision for NO in air of 0.5 parts in 10⁹ Hz^-1/2 (1σ) is obtained in pulsed mode with an Allan variance minimum corresponding to 0.1 parts in 10⁹ after 30-s averaging time. The precision in cw mode improves to 0.1 parts in 10⁹ Hz^-1/2 and 0.03 parts in 10⁹ after 30-s averaging, corresponding to an absorbance per unit path length of 2×10^-10 cm^-1. The advantages and disadvantages of cw compared to pulsed operation are discussed. PACS 07.88.+y; 42.62.Fi; 82.80.Gk; 92.60.Sz