Far-Field Laser Intensity Drop-Outs Caused by Turbulent Boundary Layers

Usually aero-optical effects are quantified in time-averaged manner, like time-averaged OPDrms or time-averaged Strehl Ratio (SR) on a target. However, for airborne free-space laser-based communication systems instantaneous SR should be studied as well. An attached transonic boundary layer, for example, provides a relatively-high time-average SR; however, experimentally it was discovered that it has many sharp intensity dropouts, which typically last for several milliseconds. Left untreated, these drop-outs lead to significant data loss, disrupting and slowing down airborne laserbased communications. This paper presents experimentally-measured instantaneous near-field wavefront statistics due to laser transmission through subsonic boundary layers. The resulting far-field SR for different flow conditions and apertures sizes are also presented. Using scaling laws for boundary layers, a simple relation between flight conditions and the relative amount of time when the SR drops below a prescribed threshold is developed, this model leads to the development of a method for predicting system performance for a free-space communication system. This method is discussed along possible approaches to using it for designing and optimizing current and future laser-based communication systems.