Impacts of laser beam diverging angle on free-space optical communications

A free-space optical (FSO) communication system that does not implement compensation strategies for the effects of beam wander may experience deep long-term channel fades or even severe communication outage when the laser spot wanders away from the receiver aperture due to the effects of atmospheric turbulence. The impacts of laser beam diverging angles on channel fading probability density function (pdf) and on bit-error-rate (BER) performance of both uncoded and coded FSO communication systems are studied for different receiver aperture sizes and propagation distances. The optimal diverging angles are determined according to the average uncoded and coded BER for both uncoded and coded system, respectively. For a fast-tracked FSO communication system, a focused beam always provides better channel fading, coded BER, and uncoded BER than beams with larger diverging angles. For an untracked system, smaller beam diverging angels have better BER performance and lower channel fading for short distances when beam wander is small compared with the receiver aperture size, and larger beam diverging angles are preferred for longer distances when beam wander becomes significant. The optimal beam diverging angles of the coded FSO communication systems are smaller than those of the uncoded systems.