Open path spatially resolved detection of atmospheric compounds using pulsed quantum cascade laser spectroscopy

We have demonstrated a possibility for a spatial resolved open path spectroscopy using pulsed quantum cascade laser (QCL). Using a pulsed light source, as a QCL, and a few retroreflectors placed at different distances, allows splitting the distance of interest to a few parts. Each retroreflector reflects a fraction of the energy back and reflected signals reach the detector with different delays. Using the retroreflectors with increasing sizes allows keeping the amplitudes of the received signals almost constant, independently of the distance. The spatial resolution &Dgr;l is proportional to the pulse length &Dgr;t and is given from the same equation used in LIDAR techniques. The thermal chirp during the relatively long laser pulse is used for fast wavelength scanning. The latter has the advantage of not being affected by atmospheric turbulence, which is essential for long open path measurements. 200 ns pulse duration was used to achieve a 30 m spatial resolution. The latter is even better than urban areas mesoscale modeling requires. At the same time, the tuning range is about 0.8 cm-1, which is sufficient to scan one or even several absorption lines at normal atmospheric pressure.

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