Abstract
One aspect of the propagation-physics challenge associated with airborne, free-space, optical communications
(FSOC), for example, is the characterization and mitigation of link losses due to aero-optic interactions. That
is, air-density gradients due to compressibility effects in turbulent boundary layers, separated flows, and freeshear
flows can disturb the wavefront in the near field of the transceiver. To better understand these aero-optical
mechanisms, a model of a nose-mounted, FSOC transceiver recently was placed in a compressible-flow wind tunnel,
and the resulting wavefront degradations, as a function of flow scenario, were recorded. High-speed, time-resolved
movies of the aero-optic disturbances have been realized, using a Schlieren-imaging technique, and a
very-highframe-rate camera. Discrete, vortical structures (amid otherwise-irregular shedding) were seen to emerge and
convect past the clear aperture. The frequencies of these disturbances have been estimated from the movies, and
these have been compared with high-speed, time-resolved wavefront reconstructions. Losses of -3.5 dB (for the
case of Mach - 0.45 at 10 kft, side view, and λ - 1.55 μm, for example), and disturbance frequencies of - 1200
Hz (and higher) were observed. The system-level impact of the resulting wavefront degradations will be discussed.
