Laser beam broadening and depolarization in dense fogs

The spatial profiles and polarization properties of a Gaussian laser beam transmitted through dense water-droplet fogs have been studied in the laboratory. A narrow, linearly polarized beam at 514 nm experiences various degrees of broadening on propagating through a 1 m thick layer of fog. The extent of the observed broadening has been found to exhibit a strong dependence on both the attenuation coefficient of the fog and on receiver paramaters. Even at optical thicknesses in excess of 5 the beam retains a Gaussian core which does not broaden appreciably. Measurements of the polarization state of forward-scattered light reveal that in dense fogs the linear polarization is strongly preserved. The linear depolization ratio δ increases with distance off the laser beam axis, although it never exceeds 0.01 in our experiment. The applicability of a single-scatteing model is describing the results is discussed.

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