Fundamental geometric advantages of free-space optical interconnects

The interconnection advantages of free-space optics are analyzed in terms of smart pixel I/O bandwidth density and the physical limitations of photonic 3-D and electronic planar interconnections. The total circuit substrate area required to implement parallel data interchange networks is the basis for comparison, and is extended to estimate important performance costs such as system volume. It is shown that 3-D optical interconnections provide a significant benefit in those problems with interconnection networks containing sub-elements with infernal or external bandwidths that exceed the capabilities of single VLSI chips. Furthermore, even larger benefits are obtained for networks whose subnetworks have internal bandwidths that exceed the capabilities of high-density packages such as MCMs. The results suggest that 3-D optical interconnections provide the highest benefits for applications in which globally interconnected networks are required to implement links across several smart pixel integrated circuit chips. In this case, optics is shown to provide an advantage of ~2 orders of magnitude or more in volume and signal latency. The analysis provides a basic guideline in determining whether and how strongly free-space optical interconnects can be applied to a given architecture