Nonlinear resonant absorption effects on the design of resonance fluorescence lidars and laser guide stars

Saturation effects may significantly reduce the backscattered signal expected for resonance fluorescence lidar systems. Saturation arises when the laser energy density within the layer is large enough to significantly alter the population densities of the atomic states within the layer. These altered state populations lead to nonlinear absorption of the laser energy resulting in reduced rate of fluorescence and increased rate of stimulated emission. The level of saturation is determined by the laser pulse length, pulse energy, beam-width, and linewidth. Analysis reveals that the saturation effects can be characterized by two parameters: the saturation time rs and the rms laser linewidth Acorms. The saturation time is the characteristic time of stimulated emission and is an indication of the level of saturation. The saturation time depends on pulse energy, pulse length, and beamwidth. The laser linewidth is important since it determines the effective absorption cross section of the atoms and also the percentage of atoms illuminated for an inhomogeneously broadened absorption line. The impact of saturation on lidar design is also examined. Design examples, including lidar systems for atmospheric research and laser guide stars for adaptive imaging applications in astronomy are studied in detail.