Optical Interconnect Technologies for AI Networks

The use of optical interconnects in AI networks today has been mainly limited to scale-out and front-end networks. While active optic cables (AOCs) have made some good market penetration for server-to-top of rack links, this segment and scale-up networks are dominated by copper interconnects. 

Scale-out and front-end networks require longer reaches where copper reach is limited. Optical interconnects with multi-mode and single-mode devices serve this space. Cost and power are often the main drivers for the choice between multi-mode and single-mode links. 

Vertical cavity surface emitting lasers (VCSELs) used in links below 100m have a good track record for large scale manufacturing driving costs down. There are strong collaborations across the industry to extend this advantage to support 200Gbps links. For longer reaches, they will be supported by external modulated lasers (EMLs) or silicon-photonics based solutions. To meet higher optical density in applications such as network switches, silicon photonics-based co-package optics (CPO) are gaining manufacturing maturity. Power benefits with CPO have also been demonstrated which can accelerate adoption. 

The trend for larger AI scale-up clusters has been growing over several generations. Cluster sizes beyond a rack are around the horizon and this would require optical reaches. To transition from copper interconnects, which are widely used today, to optical interconnects, key factors such as cost, power, escape bandwidth density, and reliability must be considered. Optics have to be placed close to or on a xPU substrate to support the bandwidth escape and performance requirement

VCSEL-based multi-mode near-package optics (NPO) can offer a low-cost, reliable and low power solution for scale-up networks. This approach leverages available 100G multi-mode components (VCSELs, ICs) and proven large-scale reliable manufacturing to fast-track the development cycle. 

In parallel, CPO technologies have been demonstrated to achieve very high escape density suitable for xPU scale-up needs. As the CPO industry matures and with more scale-out adoption, it will pave the way for adoption in scale-up networks. With further CPO innovation and process node scaling, CPO power will approach copper interconnects levels. 

Key Technologies Covered

• VCSEL NPO
• Co-package optics (CPO)
• Scale-Up networks with optics