Electromagnetic Field and Intensity Fluctuations in a Weakly Inhomogeneous Medium.

A self-consistent Green's function technique is used to obtain the electromagnetic field and its corresponding intensity to second order in the index of refraction fluctuations. It is found that for propagation distances less than a critical length, the perturbation method gives valid results. The field is primarily coherent since the fluctuations in the field are small. The solution obtained for the field is also shown to conserve energy. (Physically, the energy removed from the coherent average field is transferred to the incoherent fluctuating part of the field.) When the solution obtained here is compared with the results of the Rytov approximation, it is concluded that the Rytov approximation is not valid for propagation distances exceeding the critical length. For these distances, the perturbation method breaks down; the field is essentially incoherent since the coherent component of the field is exponentially small. For these range values, a statistical argument is given to obtain intensity statistics, and an approximate expression valid for all range values is derived for the intensity statistics. This expression is found to be in good agreement with experiment.