Progress towards reliable free-space optical networks

Free-space optical communications (FSOC) links provide an appealing and complementary enhancement to current radio frequency (RF) systems because of their inherent benefits of high-bandwidth and directional communication. Although FSOC systems can be inoperable through clouds or thick fog, employing them in a hybrid RF/optical link configuration can yield a system that can operate under most weather conditions and provide high-bandwidth, secure, jam-resistant communications under most conditions. Beyond attenuation effects and line-of-sight limitations, FSOC link performance is primarily driven by optical turbulence along the beam path, which leads to severe fluctuation of the communications channel and distortion of the signal wavefront. Many methods have been either modeled or field-tested to reduce this fading with varying degrees of success. The approach taken in the DARPA Free Space Optical Experimental Network Experiment (FOENEX) program is a continuance of systems development work funded and developed by DARPA, the Air Force Research Laboratory (AFRL), and the Naval Research Laboratories (NRL). The use of QoS-based link-level techniques was successfully demonstrated under the AFRL Iron T2 and DARPA ORCA programs. The FOENEX program extends these methods via technology developments at the physical layer as well as implementing the network methods to ensure end-to-end high bandwidth connectivity. This paper focuses on progress to date in networking technologies that will support free space optical networks (FSON) out to 200 km ranges even when individual links are disrupted up to 5% of the time.