Rotational Raman spectroscopy for the remote sensing of carbon dioxide

Laser spectroscopy for remote sensing uses an optical signal from the pollutant molecules which must be identified and discriminated against signals from other molecules and aerosols. To test the lidar and the calculations on interferometers, the laser was focused into a cell of CO2, and light scattered at 90 deg was focused into the interferometer and detected by the lidar. Pulses of neon laser light with 2-sec pulse width were backscattered in the CO2 cell. The interferometer was scanned for Raman peaks while the neon laser was pulsed at 90 pps. A lidar output vs. time plot is presented. Since the interferometer spacing could not be set closer than plus or minus 10 microns, the rotational Raman lines were probably not overlapping closely enough to obtain sharp Raman peaks. However, an inexpensive lidar with chart-recorder output can be combined with a commercial Fabry-Perot interferometer and a pulsed single-frequency laser to perform remote spectroscopy for continuous analysis of trace molecules within tens of meters.