Abstract
Electrical interconnects are becoming a bottleneck in the way towards meeting future performance requirements of integrated circuits. Moore’s law, which observes the doubling of the number of transistors in integrated circuits every couple of years, can no longer be maintained due to reaching a physical barrier for scaling down the transistor’s size lower than 5 nm. Heading towards multi-core and many-core chips, to mitigate such a barrier and maintain Moore’s law in the future, is the solution being pursued today. However, such distributed nature requires a large interconnect network that is found to consume more than 80% of the microprocessor power. Optical interconnects represent one of the viable future alternatives that can resolve many of the challenges faced by electrical interconnects. However, reaching a maturity level in optical interconnects that would allow for the transition from electrical to optical interconnects for intra-chip and inter-chip communication is still facing several challenges. A review study is required to compare the recent developments in the optical interconnects with the performance requirements needed to reach the required maturity level for the transition to happen. This review paper dissects the optical interconnect system into its components and explains the foundational concepts behind the various passive and active components along with the performance metrics. The performance of different types of on-chip lasers, grating and edge couplers, modulators, and photodetectors are compared. The potential of a slot waveguide is investigated as a new foundation since it allows for guiding and confining light into low index regions of a few tens of nanometers in cross-section. Additionally, it can be tuned to optimize transmissions over 90° bends. Hence, high-density opto-electronic integrated circuits with optical interconnects reaching the dimensions of their electrical counterparts are becoming a possibility. The latest complete optical interconnect systems realized so far are reviewed as well.
Highlights
The interconnect problem in the integrated circuits industry was brought up a couple of decades ago [1] and the search continues for a solution [2,3,4,5,6,7,8,9,10,11]
Researchers at IBM have highlighted the importance of optical interconnects in supercomputing [45]: “Interconnects of the future will be dominated by optics, as this offers the potential for a far better cost solution for all distances
The theoretical limits associated with optical interconnects mean that it will eventually bring in endless possibilities and will continue to advance with technology scaling
Summary
Signaling using a low voltage swing was proposed [25,26] as a possible solution in metal interconnects, but it has many problems in practice noisewire sensitivity and theoretical limits, whether thisbecause be offor density associated sensitive analog circuitry. Many-core chips using state-of-the-art metal wire technology, including repeaters and low voltage swings, will still find their whole power budget (150 W to 200 W) taken up by interconnects [5]. Another power management technique is to turn off parts of the chip to avoid overheating. Progress has been very rapid to overcome these challenges over the past few years, so we can be optimistic
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