We address the problem of how to exploit optics for ultrascale High Performance Computing interconnect fabrics. We show that for high port counts these fabrics require multistage topologies regardless of whether electronic or optical switch components are used. Also, per stage electronic buffers remain indispensable for maintaining throughput, lossless-ness and packet sequence. Although the notion of true all-optical packet switching is not yet viable, we show that appropriate use of optical switching technology offers power and scaling advantages that can be leveraged economically, and propose a hybrid opto-electronic HPC interconnect fabric architecture that combines the strength of electronics in processing and storing information with t...