Short-Term Average Optical Beam Spread in a Turbulent Medium.

On the basis of the extended Huygens-Fresnel principle, a general expression is derived for the short-term average optical beam spread, as measured with respect to the instantaneous center of energy of the beam, of an initially coherent optical beam wave propagating in a weakly inhomogeneous medium. The present analysis applies to the near field of the effective coherent transmitting aperture, where the beam wanders dances as a whole and does not break up into multiple patches or blobs. Central to the analysis is the short-term average mutual coherence function MCF of a spherical wave. This quantity is obtained from the corresponding long-term MCF by removing the random tilt of the wave front. Analytic expressions for the short-term beam spread are presented for the case of a Kolmogorov spectrum and the short-term average MCF derived by Fried. Author Modified Abstract