Adaptive optics model characterizing turbulence mitigation for free space optical communications link budgets

Free-space optical communications (FSOC) is becoming an important option for both atmospheric and space-based high data rate networks. Long-range, mobile FSOC links in the former environments must mitigate the effects of turbulence if they are to provide reliable, high link availability under cloud-free atmospheric conditions. Adaptive optics (AO) has been proposed as one means of reducing link degradation in turbulence, but field validated AO performance models are few and not definitive. These models are important if credible FSOC high performance links are expected to be deployed using AO systems. This paper reviews the Strehl ratio for the Andrews AO model and provides comparisons between predicted AO-based FSOC link budgets and measured link performance derived from several field trials. These results suggest that AO systems perform well under weak turbulence or short range under 10 km, but only offer limited tip/tilt gain in moderate turbulence and no gain under high/saturation regime turbulence at longer ranges. Long range links close to the ground in high moderate turbulence may degrade AO performance further.