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High-power femtosecond laser radiation propagates nonlinearly in air, exhibiting pulse self-focusing and strong multiphoton medium ionization, which leads to the spatial fragmentation of laser pulses into highly localized light channels commonly called filaments. Filaments are characterized by high optical intensity and reduced (even zero) angular spreading and can contain laser plasma or be plasmaless (postfilaments). The presence of optical turbulence on the propagation path dramatically changes pulse filamentation dynamics and in some cases causes pulse fragmentation enhancement and collapse arrest. For the first time, to the best of our knowledge, we experimentally and theoretically investigate the transverse profile of Ti:sapphire femtosecond laser radiation nonlinearly propagating a 65 m air path to the region of postfilament evolution after passing through an artificial localized air turbulence. We show that when a turbulence layer is placed before the filamentation region, the average number of high-intensity local fluence maxima (“hot points”) in the beam profile and their sizes grow as the turbulence strength increases, and then saturates at some levels. On the contrary, the deposition of a turbulence screen within the filamentation region has almost no effect on either the number or the average diameter of postfilaments.

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Equations of the Markov approximation method for the problem of laser beam propagation in the turbulent atmosphere with precipitation have been obtained by replacing precipitation particles with spatially disordered finite black screens and by replacing turbulent inhomogeneities with analogous phase screens. Solution of the equation for the mutual coherence function is represented as a convolution of the three functions using the Wigner function. The results of simultaneous measurements of laser irradiance fluctuation along the 200-m, 2.5-km, and 5-km paths during rain are presented. A qualitative model of irradiance fluctuations at multiple scattering owing to shading of a receiver by an adjacent particle layer is suggested. A saturation phenomenon of the laser radiation scintillation index in the turbulent atmosphere with precipitation is suggested and observed.

The results of floodlight sounding of atmospheric layers with spatio-oriented nonspherical particles are represented in this paper. It is shown that components obeying the regularities of diffuse scattering and specular reflection occur in the scattered radiation as a result of interaction of the directional optical radiation with such a layer. Depending on observational conditions the intensity of specularly reflected component may be several times higher than the intensity of radiation diffusely scattered in the same direction.The results of interaction between electromagnetic radiation and nonspherical atmospheric particles with anisotropic properties are of both scientific and practical interest. Of special importance is the interaction of the directional optical radiation with spatio-oriented atmospheric crystals of ice, whose ratio of dimentions to radiation wavelength is much more than unity. Such partides are of frequent occurrence in the Earth's atmosphere, they are classified as large (gigantic and supergigantic), they are characterized by variety of shapes, and they have large reflectances.1 The laws of geometric optics, in addition to the wave theory, can be applied to them.2'3The specific property of the Earth's atmosphere is the temperature inversions observed in different climatic zones in all seasons not only near the Earth's surface but in the free atmosphere as well. In the atmospheric layers related to temperature inversions the turbulence and verti-. cal motions are substantially attenuated or absent.4 The Fig. 1 aspects of aerodynamics of atmospheric nonspherical partides are considered in detail in Ref. 5. It is shown that in a wide range of values of Reynolds numbersuch particles can be steadily oriented with their largest radius perpendicular to the incoming flow, so that maximum aerodynamic resistance is provided. Ice crystals entering the layers related to temperature inversions and moving under the action of gravity and resistance to incoming air flow can be steadily oriented in space, with the largest aerodynamic radius parallel with the planet surface, giving anisotropic properties to the atmosphere as a medium of radiation propagation.Taking into account what has been said above the hypothesis on specular properties of the layer of nonturbulent atmosphere containing many spatio-oriented ice crystals was formulated. In practice a specular reflection from this layer can be recorded in a rather simple and reliable way by the observer being in the same plane with optical radiation incident on the layer and reflected from it taking account of the law of equal angles of incidence and reflection. The temporal stability of specular reflection will depend on stability of spatial orientation of ice crystal clusters. In this case a virtual image of the light source must be observed against the background of diffuse scattering on molecules and aerosol. The \ p / 348 ISPIE Vol. 2578 Q-8194-1942-7/95/$6.OO Downloaded From: http://proceedings.spiedigitallibrary.org/ on 06/22/2016 Terms of Use: http...