Propagation characteristics of collimated, pulsed laser beams through an absorbing atmosphere

Propagation of a collimated, pulsed laser beam in an absorbing medium has been investigated theoretically. The energy absorbed by the medium causes heating, leading to an index-of-refraction change that causes self-defocusing. The analysis is based upon the scalar wave equation in the Fresnel limit and the linearized equations of hydrodynamics. The governing equations are integrated numerically using a stable, explicit, finite-difference method. Numerical results for gaussian beams are presented in the form of irradiance profiles as functions of normalized range and time. An initially gaussian beam distribution is deformed into a relatively flat distribution with the peak irradiance near the edges of the beam.