Multi-frequency differential absorption LIDAR system for remote sensing of CO2 and H2O near 1.6 μm

The specifications and performance of a ground-based differential absorption LIDAR (light detection and ranging) system (DIAL) using an optical parametric oscillator (OPO) are presented. The OPO is injection-seeded with the output of a confocal filter cavity at frequencies generated by an electro-optic phase modulator (EOM) from a fixed-frequency external cavity diode laser (ECDL). The number of seed frequencies, frequency spacings, and duration is controlled with an arbitrary waveform generator (AWG) driving the EOM. Range resolved data are acquired using both photon current and photon counts from a hybrid detection system. The DIAL measurements are performed using a repeating sequence of 10 frequencies spanning a range of 37.5 GHz near 1602.2 nm to sequentially sample CO2 and H2O at 10 Hz. Dry air mixing ratios of CO2 and H2O with a resolution of 250 in and an averaging time of 10 min resulted in uncertainties as low as 6 mu mol/mol (ppm) and 0.44 g/kg, respectively. Simultaneous measurements using an integrated path differential absorption (IPDA) LIDAR system and in situ point sensor calibrated to WMO (World Meteorological Organization) gas standards are conducted over two 10 hr nighttime periods to support traceability of the DIAL results. The column averaged DIAL mixing ratios agree with the IPDA LIDAR results to within the measured uncertainties for much of two measurement periods. Some of the discrepancies with the in situ point sensor results are revealed through trends observed in the gradients of the range resolved DIAL data.