The eye-safe transmitter of a ground-based, integrated path differential absorption (IPDA) LIDAR (light detection and ranging) system is described and specified in detail. The transmitter is based on an actively stabilized, continuous-wave, single-frequency external cavity diode laser (ECDL) operating from 1.60 microns to 1.65 microns. The ECDL is microwave (MW) sideband tuned using an electro-optical phase modulator (EOM) driven by an arbitrary waveform generator (AWG) and filtered using a confocal cavity to generate a sequence of 123 frequencies separated by 300 MHz. The scan sequence of single sideband frequencies of 600 ns duration covers a 37 GHz region across absorption lines of CO2, CH4, and H2O at a spectral scan rate of 10 kHz (100 ms/scan). Simultaneously, an eye-safe backscatter LIDAR system at 1.064 microns is used to monitor the atmospheric boundary layer. IPDA measurements of the CO2 and CH4 dry air mixing ratios are presented in comparison with those from a commercial cavity ring down (CRD) instrument. Differences between the IPDA and CRD concentrations are found in several cases to be well-correlated with the atmospheric aerosol structure from the LIDAR measurements. IPDA dry air mixing ratios of CO2 and CH4, are determined with fit uncertainties of 2.7 mmol/mol (ppm) for CO2 and 0.025 mmol/mol (ppm) for CH4 over 30 s measurement periods. For longer averaging times (up to 20 min.), improvements in these detection limits by up to 3-fold are estimated from Allan variance analyses. Two sources of systematic error are identified and methods to remove them are discussed including the decorrelated speckle noise arising from wavelength dependent interference and the seed power dependence of amplified spontaneous emission.
remote sensing, carbon dioxide, methane, LIDAR, integrated path DIAL