Propagation of a Focused Laser Beam in a Turbulent Atmosphere.

by R. F. Lutomirski

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A method of calculating the performance of a laser system with beam truncated by focusing optics in a turbulent atmosphere. Previous analyses have been limited to vacuum calculations with untruncated beams, and even then have not conserved the irradiance. This approach separates the geometry of the problem (the complex aperture distribution) from the beam propagation. The turbulence parameters appear only in the mutual coherence function describing the reduction in lateral coherence with increasing distance. Based on proofs presented in RM-6055, this report shows that, while the vacuum focal point intensity for a given laser power output will increase with finer focusing, the effect of turbulence limits this increase. Turbulence can virtually eliminate the vacuum advantage of visible over infrared wave-lengths in focusing the beam at practical ranges. To predict beam patterns for design purposes, we need direct mutual coherence function measurements, now notably lacking. 32 pp. Ref. (MW)

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