Digital Infrastructure Pivots on Silicon Photonics: An Overview

Introduction

The exponential growth of global data traffic has been driving the development of optical fiber communication technology for several decades. Optical fiber communication was first widely commercialized in the early 1980s for telephone networks. The advent of the Internet dramatically increased the demand for higher bandwidth, leading to the telecom bubble burst around 2000. However, the need for greater bandwidth has continued unabated with the emergence of affordable broadband, mobile services, and smartphones in the late 2000s. More recently, hyperscale data centers have been built to support the digital infrastructure powering our modern internet services.

The Evolution of Optical Communication

The historical evolution of optical fiber communication technology can be visualized by plotting bandwidth versus distance, as shown in Figure 1. The empirical line divides the preference between electrical and optical transmissions, with optical being favored for large bandwidth-distance products. As bandwidth requirements increased over time, more optics were used at shorter distances. In the 2010s, 100-Gbps optical transceivers were widely deployed in hyperscale data centers. Now, with terabit-class I/Os required within servers and switches, the divide is crossing from electrical to optical signals.

Figure 1: Historical evolution of optical communication technology, showing the increasing use of optics for shorter distances as bandwidth demands grow.

The Post-Moore's Law Era

As we enter the post-Moore's law era, various accelerators like GPUs, DPUs, and TPUs have emerged to boost performance for applications like generative AI with large language models. Composable disaggregated computing aims to increase efficiency by reconfiguring specialized computing resources on-demand. However, this requires high bandwidth interconnects over long distances, where optical interconnects have a key advantage.

The energy efficiency of optical transceivers, shown in Figure 2, has been improving at a rate nearly matching Moore's law. With expected efficiencies below 10 pJ/bit, comparable to memory I/Os, the remaining barriers are bandwidth density and cost reduction.

Figure 2: Trends of energy efficiency in pJ/bit for optical transceivers and switch ASICs, with co-packaged optics (CPO) providing a path to further improvements.

Enabling Technologies

Silicon photonics technology has been extensively researched due to its high-density integrability, manufacturability, and performance. Integrated silicon photonics optical I/Os are being co-packaged with high-performance logic chips to leverage their energy efficiency, small footprint, and low cost potential.

However, systems cannot scale without scaling switches, and electrical switch scaling lags behind I/O scaling. Optical circuit switching (OCS) is being commercially adopted to reduce the burden on electrical switches in data centers and AI systems. While OCS can only switch "circuits", not packets, its vastly superior energy efficiency means it will eventually be essential. A key challenge is the effective "softwarization" of OCS to make it as controllable as other virtualized network functions.

Research at AIST

To address these challenges, AIST (National Institute of Advanced Industrial Science and Technology) is conducting R&D in several areas:

1. Silicon photonics using its in-house 300-mm wafer 45-nm CMOS process line.

2. Next-generation co-packaged optics using polymer waveguide technologies.

3. Large-scale silicon photonic optical switches and control circuits.

4. Precise mathematical modeling for virtualizing and automating optically switched networks.

Future Outlook

In the post-Moore's law era, a domain-specific approach is crucial for scaling performance without increasing energy usage. However, this places greater demands on interconnects and networks. Photonics-electronics convergence through technologies like silicon photonics and co-packaging can help resolve these bottlenecks.

While optical switching promises high efficiency, a holistic control architecture optimized across the entire system is needed. This requires precisely describing the optical layer in a machine-readable way to enable co-design and integration of software/hardware including optics.

As digital infrastructure continues evolving, sustainable scaling will hinge on this photonics-electronics convergence. Silicon photonics and associated technologies pave the way for the next generation of energy-efficient, high-bandwidth computing systems and networks.

Reference

S. Namiki, "Digital Infrastructure Pivots on Silicon Photonics: An Aspect from the Past, Present, and Future of Optical Communications," presented at the National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Japan, 2024.