The continued Moore’s Law scaling of transistor density is outstripping the capacity of electrical communication lines both on and off high-performance CMOS chips. Off-chip communications, limited by pin-density, serialization speeds, the number of board layers, and ultimately the communications efficiency (energy/bit), presents the most pressing concern. Without increased inter-chip communications bandwidth at reduced power consumption, high-performance computers will become increasingly imbalanced (bytes/FLOPs<<1) resulting in substantially degraded real-world application performance.
Silicon microphotonics offers a solution to the communications bottlenecks in high-performance CMOS circuits enabling ultralow power terabit-per-second communication lines on-chip using wavelength- division-multiplexing (WDM). Recently, at Sandia, we have demonstrated the lowest energy-per-bit silicon modulators. At a measured energy-per-bit of 3.8fJ, these modulators offer the potential for a one-thousand-fold reduction in inter-chip communications power. Moreover, within the same process, we demonstrated the first high-speed silicon bandpass switches, devices that offer potential for low-power high-speed optical domain routing of WDM signals. These demonstrations among others in the microphotonics community combined with the limitations of electrical communications have convinced the high performance computing community at Sandia that exascale computers will utilize silicon microphotonic networks. The impact of microphotonic communication lines and networks extends well beyond high performance computing applications into the areas of telecom routers, intra-satellite communications, and even digital imagers. For example, large, high frame-rate digital imagers (i.e. focal plane arrays) have bandwidth requirements that extend into the terabits-per-second. With silicon microphotonic communication lines integrated into imagers, frame-rates, and pixel counts previously unimaginable will be possible leading to future applications of high-speed vision, image processing, and control systems.
Importantly, our recent advances in low power modulators and switches came about only through a close coupling of rigorous electromagnetic design with nanofabrication expertise. To achieve future advances, rigorous electromagnetic design must be coupled with the design of advanced CMOS circuitry to drive, monitor, and control the performance of the microphotonic circuits and networks. And, to implement high quality detectors, and ultimately, even sources, intricate crystal growth challenges remain. Therefore, while the impact of microphotonic inter- and intra-chip communication lines will be substantial, overcoming the substantial inter-disciplinary challenges will require close collaboration between CMOS designers, microphotonic chip designers, nanofabricators, and crystal growers.