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Free-space optical (FSO) communications systems are under development by NASA to support a wide variety of missions (e.g., long-distance deep-space links and near-Earth low-Earth orbiting (LEO) links) that propagate through the turbulent atmospheric channel to reach a ground station. In many cases, the ground receiver couples light into single-mode fiber (SMF) for subsequent detection. A communications signal coupled from free-space into receiver optics, and ultimately to SMF, after propagation through the atmosphere experiences time- and space-varying intensity and phase changes. Despite the sensitivity introduced by the need to couple into a single spatial mode, this approach often offers superior system performance because it enables the use of widely-available high-performance SMF components. The impact of the atmospheric channel for any particular link depends on several parameters including aperture size relative to the atmospheric coherence length, weather, elevation angle, and direction through the atmosphere (i.e., uplink vs. downlink). Verifying FSO communications terminals often requires characterizing hardware performance in several representative channel conditions (e.g., calm, medium, and strong) to determine realistic performance bounds. This paper discusses atmospheric scenario modeling, reports the design of a high-speed wide-dynamic-range fade emulator designed for laboratory testing of optical communications transceivers, and illustrates its use through testing on a commercially available transceiver. Results quantify power penalties for the transceiver under test that increase with severity of the fade profile. |
