Broadband spectroscopic lidar for SWIR/MWIR detection of gaseous pollutants in air

A broadband SWIR/MWIR spectroscopic lidar for detection of gaseous pollutants in air is presented for doing differential optical absorption spectroscopy (DOAS). One of the distinctive parts of the lidar is the use of a picosecond PPMgO:LN OPG (optical parametric generator) capable of generating broadband (10 to <100 nm FWHM) and tunable (1.5 to 3.9 μm) SWIR/MWIR light. The optical source layout and properties are presented, along with a description of the lidar breadboard. Results from indoor simulated typical operation of the lidar will be discussed, the operation consisting in emitting the broadband coherent light along a line of sight (LOS) and measuring the back-scattering returns from of a topographic feature or aerosols. A second distinctive part is the gated MCT-APD focal plane array used in the output plane of the grating spectrograph of the lidar system. The whole of the returned spectra is measured, within a very short time gate, at every pulse and at a resolution of a few tenths to a few nm. Light is collected by a telescope with variable focus for maximum coupling of the return to the spectrograph. Since all wavelengths are emitted and received simultaneously, the atmosphere is "frozen" during the path integrated measurement and hopefully reduces the baseline drift problem encountered in many broadband scanning approaches. The resulting path integrated gas concentrations are retrieved by fitting the molecular absorption features present in the measured spectra. The use of broadband pulses of light and of DOAS fitting procedures make it also possible to measure more than one gas at a time, including interferents. The OPG approach enables the generation of moderate FWHM continua with high spectral energy density and tunable to absorption features of a great number of molecules. Proposed follow-on work and applications will also be presented.