Optical communication under low-visibility conditions - Recent experimental results

Inclement weather, in the form of rain, snow, and fog, imposes a major constraint on the availability of atmospheric optical communication systems. The presence of particulates and aerosols in the optical path causes a variety of corruptive effects, including potentially severe scattering and some absorption. When forward scattering is dominant, some of the optical signal energy resident in the scattered field can be recovered by a properly designed receiver. Such a receiver has been implemented using variable-field-of-view optics. Preliminary tests on a 1.1 Km link, using an infrared LED source, indicate that the received signal can be increased by at least 10 db by opening the field-of-view from 1 milliradian to 4 milliradians, when the visibility is one-half the range. Experiments with a HeNe laser also reveal that a LED of comparable power is often the preferred source.