40-Gbps modem architecture for free-space optical communications

A 40-Gbps free-space optical communications (FSOC) link architecture incorporating high-sensitivity modems and a high-dynamic range optical automatic gain controller (OAGC) is presented. The architecture leverages 10-Gbps modems, each supporting 10-Gigabit Ethernet, to ease interfaces to commercial network systems. The 10-Gbps modems employ return-to-zero differential phase shift keying waveforms and a commercial Reed-Solomon forward error correction system for maximum channel sensitivity. For transmission over the FSOC link, the modem channels are wavelength multiplexed. When coupled with the OAGC, the communications system achieves a sensitivity of better than 10 photons per bit. Low-noise optical gain is provided by the OAGC with a noise figure of better than 3.1 dB and a dynamic range of greater than 60 dB. This paper discusses the development of the high-sensitivity modem architecture and high-dynamic range OAGC, addresses system level considerations with regards to operation over highly turbulent FSOC links, discusses metrics for characterizing their performance, and presents results of system tests against field-collected scintillation reproduced in the laboratory.