Co-packaged Optics (CPO) represents the next evolution in server-to-server connections within data center racks, building upon the advancements made by Silicon Photonics-based Active Optical Cables (AOCs). While Silicon Photonics-based AOCs have significantly improved rack-to-rack connections, CPO focuses on optimizing connections within the rack itself. Current implementations for in-rack connections rely on high-speed copper cables, predominantly employing SFP-DD and QSFP-DD form factors, which deliver up to 400 Gbps with a power consumption of 1.5W per 100 Gbps. However, the limitations of these copper-based solutions are becoming increasingly apparent as data centers demand higher bandwidth and improved energy efficiency.

CPO technology offers a paradigm shift in interconnect design by integrating optical engines directly with switch ASICs, in contrast to Silicon Photonics-based AOCs which maintain a separation between the optical and electrical domains. This approach leverages advanced packaging technologies, including silicon interposers and 2.5D integration, to create a highly compact and efficient optical interface. The primary objective of CPO is to deliver unprecedented bandwidth, with projections suggesting the potential to exceed 3.2 Tbps per switch port, surpassing the current capabilities of Silicon Photonics-based AOCs.

A key advantage of CPO lies in its ability to minimize electrical trace lengths and optimize the electro-optical interface. By bringing the optical components in close proximity to the switch ASIC, CPO significantly reduces signal loss and power consumption. Current estimates indicate that CPO implementations may achieve power consumption levels below 1W per 100 Gbps, representing a substantial improvement over existing copper-based solutions and potentially surpassing the efficiency of Silicon Photonics-based AOCs.

Industry leaders such as Broadcom and Cisco are at the forefront of CPO development, focusing on solutions that leverage silicon photonics technology, similar to the approach used in advanced AOCs. These established firms are complemented by a range of startups exploring novel approaches to further enhance CPO performance and reduce costs. Some of these innovative techniques include the use of polymer waveguides and plasmonic devices, which offer potential advantages in terms of manufacturing scalability and optical efficiency beyond what is currently achievable with Silicon Photonics-based AOCs.

The implementation of CPO technology presents several technical challenges that researchers and engineers are actively addressing. These include thermal management issues arising from the close integration of optical and electrical components, the need for precise alignment between optical elements and ASICs, and the development of cost-effective manufacturing processes for high-volume production. Many of these challenges are similar to those faced in the development of Silicon Photonics-based AOCs, but are amplified by the tighter integration requirements of CPO.

Despite these challenges, the potential benefits of CPO are driving rapid advancements in the field. The technology promises to enable new architectures for data center networks, potentially revolutionizing the way servers and switches are interconnected within racks. As CPO matures, it is expected to complement and potentially supersede Silicon Photonics-based AOCs in certain applications, particularly for ultra-high-bandwidth, short-reach connections within data center racks.

The transition to CPO represents a significant shift in data center interconnect technology, requiring substantial investment in research, development, and infrastructure. However, the long-term benefits in terms of increased bandwidth, reduced power consumption, and improved system density are likely to justify these investments for many data center operators and equipment manufacturers. As both CPO and Silicon Photonics-based AOCs continue to evolve, they are poised to collectively reshape the landscape of data center networking, each technology finding its optimal niche in the complex ecosystem of modern data centers.

Worth watching:

• Broadcom (USA): Leads in developing switch chips with integrated optics, offering solutions that significantly reduce power consumption and increase bandwidth density.
• Cisco Systems (USA): Invests in CPO technology to create next-generation switches with tightly integrated optical interfaces, aiming for multi-terabit performance.
• Marvell Technology (USA): Develops CPO solutions that integrate closely with their networking chips, targeting cloud and enterprise data centers with high-efficiency designs.
• Ranovus (Canada): Specializes in multi-terabit co-packaged optics and advanced digital silicon, offering scalable solutions for next-generation data center interconnects.
• Aurrion (acquired by Juniper Networks) (USA): Pioneers silicon photonics technology for CPO applications, focusing on integrating III-V materials with silicon for improved performance.