Due to the rise of 5G, IoT, AI, and high-performance computing applications, datacenter traffic has grown at a compound annual growth rate of nearly 30%. Furthermore, nearly three-fourths of the datacenter traffic resides within datacenters. The conventional pluggable optics increases at a much slower rate than that of datacenter traffic. The gap between application requirements and the capability of conventional pluggable optics keeps increasing, a trend that is unsustainable. Co-packaged optics (CPO) is a disruptive approach to increasing the interconnecting bandwidth density and energy efficiency by dramatically shortening the electrical link length through advanced packaging and co-optimization of electronics and photonics. CPO is widely regarded as a promising solution for future datacenter interconnections, and silicon platform is the most promising platform for large-scale integration. Leading international companies (e.g., Intel, Broadcom and IBM) have heavily investigated in CPO technology, an inter-disciplinary research field that involves photonic devices, integrated circuits design, packaging, photonic device modeling, electronic-photonic co-simulation, applications, and standardization. This review aims to provide the readers a comprehensive overview of the state-of-the-art progress of CPO in silicon platform, identify the key challenges, and point out the potential solutions, hoping to encourage collaboration between different research fields to accelerate the development of CPO technology. Graphical Abstract

Networks-on-chip (NoCs) can improve the communication bandwidth and power efficiency of multiprocessor systems-on-chip (MPSoC). However, traditional metallic interconnects consume significant amount of power to deliver even higher communication bandwidth required in the near future. Optical NoCs are based on optical interconnects and optical routers, and have significant bandwidth and power advantages. This paper proposed a high-performance low-power low-cost optical router, Cygnus, for optical NoCs. Cygnus is non-blocking and based on silicon microresonators. We compared Cygnus with other microresonator-based routers, and analyzed their power consumption, optical power insertion loss, and the number of microresonators used in detail. The results show that Cygnus has the lowest power consumption and losses, and requires the lowest number of microresonators. For example, Cygnus has 50% less power consumption, 51% less optical power insertion loss, and 20% less microresonators than the optimized traditional optical crossbar router. Comparing to a high-performance 45nm electronic router, Cygnus consumes 96% less power. Moreover, the passive routing feature of Cygnus guarantees that, while using dimension order routing algorithm, the maximum power consumption to route a packet through a network is a small constant number, regardless of the network size. For example, the maximum power consumption is 4.80fJ/bit under current technologies. We simulated and analyzed an 8 times 8 2D mesh NoC built from Cygnus and showed the end-to-end delay and network throughput under different offered loads and packet sizes.

Optical networks-on-chip (ONoCs) are emerging communication architectures that can potentially offer ultrahigh communication bandwidth and low latency to multiprocessor systems-on-chip (MPSoCs). In addition to ONoC architectures, 3-D integrated technologies offer an opportunity to continue performance improvements with higher integration densities. In this paper, we present a 3-D mesh-based ONoC for MPSoCs, and new low-cost nonblocking 4 × 4, 5 × 5, 6 × 6, and 7 × 7 optical routers for dimension-order routing in the 3-D mesh-based ONoC. Besides, we propose an optimized floorplan for the 3-D mesh-based ONoC. The floorplan follows the regular 3-D mesh topology but implements all optical routers in a single optical layer. The floorplan is optimized to minimize the number of extra waveguide crossings caused when merging the 3-D ONoC to one optical layer. Based on a set of real applications and uniform traffic pattern, we develop a SystemC-based cycle-accurate NoC simulator and compare the 3-D mesh-based ONoC with the matched 2-D mesh-based ONoC and 2-D electronic NoC for performance and energy efficiency. Additionally, we quantitatively analyze thermal effects on the 3-D 8 × 8 × 2 mesh-based ONoC.

Crosstalk noise is an intrinsic characteristic of photonic devices used by optical networks-on-chip (ONoCs) as well as a potential issue. For the first time, this paper analyzed and modeled the crosstalk noise, signal-to-noise ratio (SNR), and bit error rate (BER) of optical routers and ONoCs. The analytical models for crosstalk noise, minimum SNR, and maximum BER in meshbased ONoCs are presented. An automated crosstalk analyzer for optical routers is developed. We find that crosstalk noise significantly limits the scalability of ONoCs. For example, due to crosstalk noise, the maximum BER is 10-3 on the 8×8 meshbased ONoC using an optimized crossbar-based optical router. To achieve the BER of 10-9 for reliable transmissions, the maximum ONoC size is 6×6. A novel compact high-SNR optical router is proposed to improve the maximum ONoC size to 8×8. Copyright 2010 ACM

As benchmark programs for microprocessor architectures, network-on-chip (NoC) traffic patterns are essential tools for NoC performance assessments and architecture explorations. The fidelity of NoC traffic patterns has profound influence on NoC studies. For the first time, this paper presents a realistic traffic benchmark suite, called MCSL, and the methodology used to generate it. The publicly released MCSL benchmark suite includes a set of realistic traffic patterns for 8 real applications and covers popular NoC architectures. It captures not only the communication behaviors in NoCs but also the temporal dependencies among them. MCSL benchmark suite can be easily incorporated into existing NoC simulators and significantly improve NoC simulation accuracy. We developed a systematic traffic generation methodology to create MCSL based on real applications. The methodology uses formal computational models to capture both communication and computation requirements of applications. It optimizes application mapping and scheduling to faithfully maximize overall system performance and utilization before extracting realistic traffic patterns through cycle-accurate simulations. Experiment results show that MCSL benchmark suite can be used to study NoC characteristics more accurately than traditional random traffic patterns.

Crosstalk noise is an intrinsic characteristic as well as a potential issue of photonic devices. In large scale optical networks-on-chips (ONoCs), crosstalk noise could cause severe performance degradation and prevent ONoC from communicating properly. The novel contribution of this paper is the systematical modeling and analysis of the crosstalk noise and the signal-to-noise ratio (SNR) of optical routers and mesh-based ONoCs using a formal method. Formal analytical models for the worst-case crosstalk noise and minimum SNR in mesh-based ONoCs are presented. The crosstalk analysis is performed at device, router, and network levels. A general 5 × 5 optical router model is proposed for router level analysis. The minimum SNR optical link candidates, which constrain the scalability of mesh-based ONoCs, are identified. It is also shown that symmetric mesh-based ONoCs have the best SNR performance. The presented formal analyses can be easily applied to other optical routers and mesh-based ONoCs. Finally, we present case studies of mesh-based ONoCs using the optimized crossbar and Crux optical routers to evaluate the proposed formal method. We find that crosstalk noise can significantly limit the scalability of mesh-based ONoCs. For example, when the mesh-based ONoC size, using optimized crossbar, is larger than 8 × 8, the optical signal power is smaller than the crosstalk noise power; when the network size is 16 × 16 and the input power is 0 dBm, in the worst-case, the signal power is -24.9 dBm and the crosstalk noise power is -11 dBm.