Time-dependent propagation of high energy laser beams through the atmosphere

The computation of time-dependent three-space-dimensional laser beam propagation is described. The methods are applicable to the propagation of high energy laser beams through the atmosphere in the presence of a horizontal wind and turbulence for most situations of interest. Possible cases are propagation of cw beams through stagnation zones, multi-pulse propagation, including the self-consistent treatment of pulse self-blooming, and propagation involving transonic slewing. The solution of the Maxwell wave equation in Fresnel approximation is obtained by means of a discrete Fourier transform method, which, surprisingly, gives excellent results for diffraction problems. The latter provide a stringent test for the accuracy of any solution method. Considerable use is also made of discrete Fourier transform methods in solving the hydrodynamic equations. The treatment of turbulence is based on the generation of random phase screens at each calculation step along the propagation path. In a time-dependent calculation the random phase screens can be either made to move with the wind at a given propagation position or generated anew for each successive time. This work was done under contract to the Advanced Research Projects Agency of the Department of Defense, the Army Missile Command, Huntsville, Alabama, and the U.S. Energy Research and Development Administration.