Numerous challenges present themselves when scaling traditional on-chip electrical networks to large manycore processors. Some of these challenges include high latency, limitations on bandwidth, and power consumption. Researchers have therefore been looking for alternatives. As a result, on-chip nanophotonics has emerged as a strong substitute for traditional electrical NoCs. As of 2017, on-chip optical networks have moved out of textbooks and found commercial applicability in short-haul networks such as links between servers on the same rack or between two components on the motherboard. It is widely acknowledged that in the near future, optical technologies will move beyond research prototypes and find their way into the chip. Optical networks already feature in the roadmaps of major processor manufacturers and most on-chip optical devices are beginning to show signs of maturity. This article is designed to provide a survey of on-chip optical technologies covering the basic physics underlying the operation of optical technologies, optical devices, popular architectures, power reduction techniques, and applications. The aim of this survey article is to start from the fundamental concepts and move on to the latest in the field of on-chip optical interconnects.

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In this paper, we propose to use optical NOCs to design cache access protocols for large shared L2 caches. We observe that the problem is unique because optical networks have very low latency, and in principle all the cache banks are very close to each other. A naive approach is to broadcast a request to a set of banks that might possibly contain the copy of a block. However, this approach is wasteful in terms of energy and bandwidth. Hence, we propose a novel scheme in this paper, TSI , which proposes to create a set of virtual networks (overlays) of cache banks over a physical optical NOC. We search for a block inside each overlay using a combination of multicast, and unicast messages. We additionally create support for our overlay networks by proposing optimizations to the previously proposed R-SWMR network. We also propose a set of novel hardware structures for creating and managing overlays, and for efficiently locating blocks in the overlay. The performance of the TSI scheme is within 2-3% of a broadcast scheme, and it is faster than traditional static NUCA schemes by 50%. As compared to the broadcast scheme it reduces the number of accesses, and consequently the dynamic energy by 20-30%.