Spectra of temporal intensity fluctuations of laser radiation traveling in a turbulent atmosphere
The Huygens-Kirchhoff principle is generalized to continuously inhomogeneous media and used in a study of the temporal frequency spectra of the intensity fluctuations in laser beams traveling in a turbulent atmosphere. The propagation of an optical wave in a medium which is randomly inhomogeneous in time and space is considered in a quasistationary approximation using the frozen turbulence hypothesis. Approximate analytic expressions are obtained for the temporal correlation coefficient in the limiting cases of strong and weak intensity fluctuations. It is shown that, in the case of weak intensity fluctuations, the main contribution to the temporal spectrum is made by frequencies corresponding to the transit time of "frozen" inhomogeneities across the transmitting aperture. The spectrum shifts toward higher frequencies when the length of the path is increased or the intensity of turbulent pulsations of the refractive index becomes greater. In the case of strong fluctuations, the form of the spectrum is governed entirely by meteorological turbulent conditions and is independent of the diffraction size of the transmitting aperture or the focusing of the radiation. The results of the calculations are compared with the experimental data.