Methane concentration measurements with a mid-infrared optical parametric oscillator-based differential absorption lidar system

Methane plays an important role as one of the greenhouse gases in the atmosphere. The differential absorption lidar (DIAL) technique is widely used for range-resolved measurements of the concentration of atmospheric constituents. This technique uses two wavelengths to measure the difference in the absorption of the lidar return signal as the laser wavelength is varied between a on-line wavelength that is strongly absorbed by molecules and a off-line wavelength that is not absorbed. The advantage of the DIAL technique is that only differences or ratios in the various parameters need to be considered. Methane has several absorption bands accessible by lasers. The mid-infrared region of the spectrum is rich in absorption features for methane and has a weaker solar background than the near-IR region. These are the important advantages of a mid-infrared DIAL system for methane measurements. A ground-based lidar system in the 1.45-4 /spl mu/m region for the remote detection of methane is described and lidar measurements of methane concentration with the system are presented. The lidar system consists of four parts: the laser source, laser wavelength diagnostics, the receiver, and the data acquisition electronics.