We report the development of a trace gas sensor for the detection of atmospheric formaldehyde utilizing a thermoelectrically cooled distributed-feedback quantum cascade laser operating in continuous-wave mode at 5.68 μm. Wavelength modulation spectroscopy was combined with second harmonic detection and zero air based background subtraction techniques to enhance both detection sensitivity and precision to ∼2.5 ppbv for H2CO measurement with an integration time of less than 1 second and a 36 m optical path length. A novel analysis technique based on wavelet transform for noise reduction was successfully applied to improve the sensor performance, yielding sub-ppb measurement precision without reducing the fast temporal response.
