The statistical properties associated with the intensity fluctuations of a He-Ne laser beam propagating through extended clear-air turbulence over a homogeneous range were measured and compared with existing theoretical models. The statistics were determined by measuring the first five normalized moments of the beam intensity over varying distances from 183 m to 3 km. The beam geometry was approximately that of a spherical wave. Calculated values of the refractive-index-structure parameter C<sub>n</sub> <sup>2</sup> varied from a low of approximately 10<sup>-14</sup> m<sup>-2/3</sup> to a high value of the order of 10<sup>-10</sup> m<sup>-2/3</sup>. Thus the data obtained correspond to many conditions of turbulence from weak to strong turbulence in the saturation regime. We found that the data generally support the lognormal model under conditions of weak turbulence and the negative-exponential distribution in the superstrong-turbulence regime. The K distribution fits some of, but not all, the data in the saturation regime. For conditions of strong turbulence before the superstrong regime, none of the above statistical models fits the data well. This is due primarily to the fact that a looping effect appeared in the data whereby the third- and higher-order normalized moments appear to follow a looped curve in the vicinity of their maximum values when they are plotted as functions of the second normalized moment. None of the theoretical curves for the normalized moments of the above-named distributions exhibits this looping effect (since they are essentially one-parameter distributions), and therefore they have limited applicability with respect to our data.
© 1981 Optical Society of AmericaPDF Article