The possible limiting components on optical communication bandwidth are the source, the modulator, the propagation medium, and the detector. It is easy to show that the source bandwidth is the fundamental limit. The possibility of source bandwidth limited communication is demonstrated theoretically and experimentally. The basic principles involved are readily extendable to more practical partial approaches to this limit.
H. J. Caulfield and Tomas Hirschfeld, "Optical communication at the source bandwidth limit," Appl. Opt. 16, 1184-1186 (1977)