Intensity of a focused beam in a turbulent medium

The effect of the strength of turbulence on the intensity distribution in the focal plane of a focused laser beam is studied. The intensity in the focal region is found to be unaffected by the turbulence provided C
n
²
< 0.45 k
0
^-2
S
^-5/6
F
^-1
, where C
n
is the index of the refraction structure constant, k
0
is the signal wavenumber, S is the initial cross section of the beam, and F is the focal length.