Based on a set of the self-consistent Statz-de-Mars equations for a laser with coupled transitions (V- and L-type), a CO2-laser action is studied theoretically. Analytical solutions for the laser intensities are derived. The feasibility of enhancing the double-frequency laser transceiver sensitivity is shown for an operation on two transitions coupled through a common lower level. The results obtained from the experiments on the proximate atmospheric gas analysis using a coherent laser-reception lidar are discussed. The water vapour concentration measured by the lidar along the 500 m atmospheric path is compared with that detected by a commercial hygrometer to show good agreement for low turbulence. In the case of high turbulence the data are in reasonable agreement.