<title>Wavelength and altitude dependence of laser beam propagation in dense fog</title>

Abstract

The availability of free-space laser communications systems critically depends upon estimating the accuracy of atmospheric attenuation in different weather conditions. Fog and extreme rain are the primary types of weather that can affect shorter wavelength laser communication links. Traditionally, designers and users of free-space laser communication equipment assumed that light suffers from less atmospheric attenuation at longer infrared wavelengths such as 1.55µm and 3.5µm compare to 0.85µm. Thought, as some experimental studies show, it is not safe to assume that the performance of detection systems in IR is always better in fogs than at visual wavelengths. In dense stages in long persisting fogs the attenuation at longer wavelengths often becomes large, or even larger, than that in the visible. To investigate performance of laser systems in the dense fog, we ran extensive MODTRAN calculations of atmospheric transmission for the different types of dense fog at visible-IR range of wavelengths. Our results show that an atmospheric attenuation in such fogs slightly increases with wavelength in the range 0.5-4 µm and then decreases with the longer wavelengths. This is in contrast with widely used parameterization that states decreasing of the fog attenuation beyond visible. To investigate this further, we used concentrations and size distributions data collected in dense persisting fogs at different altitudes, and performed Mie scattering calculations. We determined fog extinction in a range of altitudes (10-100m) and at four wavelengths used in free-space communication: 0.85µm, 1.55µm, 3.5µm and 10.6 µm. We found that atmospheric attenuation in dense fog sharply increases with altitude, slightly increases at 3.5µm compared to 0.85µm and then decreases at 10.6 µm.

Further Insight into the Nature of Ball-Lightning-Like Atmospheric Pressure Plasmoids

“…Also expected from gas-phase H 2 O is the bending band, located between 1500 and 1700 cm –1 , which appears to be present just above the noise floor. Interestingly, there is little evidence of the presence of water fog, which we would expect to see as a broad, low-resolution feature in this same region . It is possible, however, that such a spectrum is being masked by the strong H 2 O absorptions, and so its presence cannot be completely dismissed.…”

“…Interestingly, there is little evidence of the presence of water fog, which we would expect to see as a broad, lowresolution feature in this same region. 46 It is possible, however, that such a spectrum is being masked by the strong H 2 O absorptions, and so its presence cannot be completely dismissed.…”

Simulating Z–LWC Relations in Natural Fogs with Radiative Transfer Calculations for Future Application to a Cloud Radar Profiler

“…However, even more parameter sets were found, not all could be included because the level of information on fog occurrence was insufficient (e.g. KALASHNIKOVA et al 2002;KIM et al 2001) or because they were only recalculated from existing values (HESS et al 1998;AWAN et al 2008).…”

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