Unlock the Future of AI | Co-Packaged Optics (CPO) Optical Fiber Infrastructure Design | Corning

Benoit Fleury
Published: October 11, 2024

While most are amazed by the possibilities AI enables, what is often not fully appreciated is the data processing capacity and componentry that brings AI to life. In this blog, I explain elements that should be taken into consideration when designing and deploying the fiber infrastructure for co-packaged optics.

What is co-packaged optics?

Co-packaged optics (CPO) is a technology architecture that improves data center networking and processing bandwidth, density, and power efficiency by placing optics and electronics closer together in a switching or processing system. CPO can take AI to the next level by enabling higher speeds, higher densities, and lower latency while improving the system’s overall power efficiency.

The importance of fiber reliability

A key challenge to the broad adoption of CPO is avoiding fiber damage or failure during the system’s assembly process that would impact the reliability of the overall CPO system. Tension cracks, nicks, or cuts to the glass surface of one or more fiber strands would cause the fiber to weaken which may lead to a failure in the affected optical path. As even the slightest contact within the bare glass can induce lifetime-limiting flaws tough polymer coatings are used to isolate the glass surface.

The most common way that stress is applied to a fiber is by bending it. Therefore, designers laying out the fiber infrastructure inside a CPO switch or processing box must consider how bend configurations will impact not only optical loss but also the reliability of the optical path. Silica-based glass fibers, which Corning specializes in, is both quantitively and qualitatively a reliable solution. Corning has conducted extensive tests to determine strength distributions (read the full research of Corning and Broadcom’s CPO whitepaper). As you will read in the paper, the failure rate rapidly increases as the bend radius of the fiber is reduced. Understanding this factor is particularly important for systems that use a high number of fibers, such as in high bandwidth switches using parallel optics, due to the aggregate reliability impact.

The concept shown below (Figure 1) is that of a ribbon-management device designed for a CPO fiber infrastructure. It provides a slack management capability to enable realistic cable length tolerances, a mechanism to control the bending radius of each ribbon to avoid over-bending and can be stacked to accommodate high fiber count configurations.

The reliability data and models have been used to calculate the five-year failure probability for different fiber-count ribbons deployed in such an accumulator where the minimum bend radii are set by the built-in flanges. The graph shown in Figure 2 below indicates that the probability of having a fiber break for a given bend radius increases with the number of fibers in the box.

The design of the fiber layout in the CPO box must consider both fiber stress limits and the thermal environment of the fiber coatings to ensure fiber protection, reliability, and longevity. Standard fiber coatings can withstand the high temperatures expected in an operating CPO assembly. Specialized higher-temperature coatings are available but potential hot spots should still be avoided to eliminate the chances of potential coating degradation as such designers should consider spacers or routing hardware to keep the fiber infrastructure in suitable microenvironments.

The mechanical environment within which the coatings are exposed is also an important consideration because loss of coating integrity may lead to glass surface damage and thus, a shortened fiber lifetime. Pinching, abrasion, and nicking must be avoided to prevent coating failures. Further, fibers should not be deployed against rough or sharp surfaces nor routed through narrow openings, which over time, and combined with mechanical shocks and vibrations, may lead to damage and eventual failures in the optical path. Strain relief structures for optical connections must observe manufacturers’ limits on stress applied to the fibers and their packages (cables, ribbons etc.). 

Fiber design handling for reliability

The design of the fiber configuration within a server rack can play a pivotal role in a CPO system’s capabilities, longevity, and reliability. Good design practices include avoiding the interweaving or entanglement of different optical subassemblies wherever possible (Figure 3), making each optical subassembly easily accessible, and providing a clear path for working on the infrastructure that minimizes the risks of snagging or pinching fibers.

During the implementation phase of deploying the fiber infrastructure within a CPO system, it’s important to avoid three factors that may compromise reliability: Damage to the glass surface, application of large tensile stresses, and damage to the integrity of the coating. Since contact with the fiber glass may cause microscopic surface flaws that can result in fiber failure, bare glass should not be exposed in the subassembly within the box. Additionally, it is critical to avoid sharp edges that may cause damage to the fiber coating (i.e. nicks, cuts, punctures, etc.). Particulates, such as tiny solid particles or liquid droplets, should be avoided, and electrostatic forces controlled, to avoid the damage to the fiber’s glass surface.

CPO systems will likely be built by incorporating optical subassemblies into the equipment’s enclosure. These subassemblies, typically built separately, will form part of the final assembly kit. The concept of pre-packaged fiber cassettes could minimize fiber breakages by box-level assemblers with limited fiber layout experience.

Examples of reliable fiber dense deployment

Corning has a long-standing history of configuring and deploying fibers to achieve tight bends inside confined spaces. For example, Corning’s EDGE™ solutions for data centers include 1RU housings that can accommodate 576 fibers, with the limitation being the connector density at the front panel, not the number of fibers in the box. Further, Corning’s various splice management enclosures also pack many fibers into tight spaces and protect PON splices which have a high risk of damage due to human error or extreme temperatures.

Conclusion

Fiber infrastructures for co-packaged optical systems can be designed and assembled to achieve high reliability at scale—a key to the broader adoption of CPO technology and high performing AI networks. By understanding and mitigating the failure mechanisms of optical fibers, and by following proper layout and handling guidelines, the overall reliability of the optical connections can be ensured. Also, leveraging fibers that accommodate smaller bend radii, pre-connectorized componentry, and pre-assembled fiber cassettes, ease system configuration, design, and assembly—can help to provide a successful long-term CPO network. The future of CPO is bright and is sure to take AI operations to the next level.

Read the full whitepaper by Corning Optical Communications and Broadcom Optical Systems. Learn more about Corning’s innovative data center or CPO solutions within the respective links.