Speed and energy analysis of digital interconnections: comparison of on-chip, off-chip, and free-space technologies.

We model and compare on-chip (up to wafer scale) and off-chip(multichip module) high-speed electrical interconnections withfree-space optical interconnections in terms of speed performance andenergy requirements for digital transmission in large-scalesystems. For all technologies the interconnections are firstmodeled and optimized for minimum delay as functions of theinterconnection length for both one-to-one and fan-outconnections. Then energy requirements are derived as functions ofthe interconnection length. Free-space optical interconnectionsthat use multiple-quantum-well modulators or vertical-cavitysurface-emitting lasers as transmitters are shown to offer aspeed-energy product advantage as high as 30 over that of the electrical interconnection technologies.