Frequency chirped differential absorption LIDAR

We present a novel concept design of a differential absorption LIDAR for open path trace gas sensing in the atmosphere. To perform a range-resolved gas sensing we propose to arrange a set of retroreflectors in the laser beam path to measure a differential absorption in adjacent sections. In validation experiments we used a pulsed DFB quantum cascade laser fabricated by Alpes Lasers. The laser was excited with 200-ns current pulses with a repetition rate of 10 kHz. The frequency chirp rate was found to increase from 7.7 to 1.0 cm(-1)/mu s as peak injection current was increased from 7.1, to 7.8 A. We utilized the frequency chirp at laser substrate temperature of 24.0 degrees C to scan the 967.0 - 968.5 cm(-1) spectral interval containing the absorption lines of CO2 and NH3. We detected similar to 0.25 ppmv of NH3 in nitrogen at atmospheric pressure using a double-pass gas cell with an effective absorption path of 2.4 m. Digital filtering of the spectra was shown to be effective in eliminating a high-frequency noise. To demonstrate range-resolved capabilities of the sensor we used two retroreflectors inserted into the laser beam. A differential absorption of CO2 at 967.7 cm(-1) was measured with the gas cell placed in one of the sections. Our experiments indicate that the frequency chirped LIDAR can be used for open path spectroscopy of NH3 over the ranges up to similar to 1 km with a spatial resolution of similar to 30 m and detection limit of similar to 20 ppbv per a 30-m section.