Photocount distribution for laser radiation scattered by the atmosphere

Aspects of calculating the photocount distribution P(n,T) allowing for the signal frequency spectrum are analyzed in relation to laser signab scattered in the atmosphere. The conversion to a semiclassical analysis is substantiated. A convenient recurrence formula for calculating P(n,T) using a series of eigenvalues of a certain integral Fredholm equation is given and a simple criterion for truncation of this series is presented. Attention is focused on the case of a Voigt spectrum of the received signal. The expression obtained for the correlation radius can be used to classify situations according to the ratio of the sampling time to the signal correlation radius. The small-perturbation method is developed and its range of validity is determined qualitatively. An effective approximation is given for the case of a "long" selection time. An expression is obtained for the photocount variance.