Multipath Dispersion in Low Visibility Optical Communication Channels

The potential for optical communication through low-visibility scattering atmospheres is the underlying concern of this investigation. To access that potential, the effects of the channel upon the optical signal propagating through it must be determined. The work reported here is primarily concerned with one of those effects the time dispersion, or multipath spread, of the transmitted signal. The investigation was primarily an experimental one, utilizing the existing propagation facility operating over a thirteen kilometer experimental path between the MIT campus and a field station located at an Air Force site in suburban Boston. Experiments were performed at visible and near infrared wavelengths, with highly collimated transmitting beams. Measurements of the multipath spread for various receiver fields of view and at various offaxis angles were made. Little multipath and angular spread was observed within the realm of atmospheric conditions for which a detectable signal could be obtained. For optical thicknesses less than ten, the on-axis measurements are consistent with the hypothesis that the received signal is dominated by unscattered radiation however, the off-axis measrements suggest that the scattered field itself may be only slightly spread in time and angle. Guided by the observed narrowness of the angular spectrum, an approximation to the linear transport equation was developed. The resulting equation is more amenable to solution than is the full transport equation. Here it is solved for isotropic scatter. For such scattering it is found that substantial spreading in both time and angle will occur when the optical thickness becomes appreciably greater than one.