Abstract
Data traffic is projected to increase exponentially for the foreseeable future, necessitating major advances in data communication technologies to meet ever more demanding network bandwidth, efficiency, scale, and cost targets. This review summarizes emerging applications, architectures and interconnect technologies across high performance computing (HPC) and hyperscale data centers. Optical interconnects offer clear advantages over electrical links but face integration barriers around reliability, thermal design, and backwards compatibility. Promising solutions including co-packaged optics modules and non-retimed interfaces are assessed. Additionally, strategic directions proposed to overcome critical cost barriers inhibiting mainstream adoption are explored. Successfully driving optical transceiver pricing down towards parity with copper cabling technologies will ultimately determine whether integrated photonics can become a ubiquitous pillar within future data center architectures[1].
Introduction
Global internet protocol (IP) traffic continues to show tremendous growth, approximately doubling every 2-3 years by most estimates [1]. This relentless increase is driven by new applications including social media, search, video streaming, e-commerce, cloud computing and storage along with emerging 5G wireless services, artificial intelligence, augmented/virtual reality platforms, and the exponential proliferation of Internet of Things (IoT) devices.
To keep pace with rapidly rising interconnection bandwidth demands, high performance computing (HPC) architects and hyperscale data center operators including Amazon, Google, Meta, and Microsoft face acute challenges delivering order-of-magnitude leaps in network capacity and efficiency while simultaneously minimizing total cost of ownership.
This review summarizes the evolution and deployment of interconnect architectures, technologies and applications across diverse HPC and data center segments. Key requirements, metrics, and workloads for each environment are explored, along with strengths and limitations of current electrical and optical interconnect approaches.
Emerging solutions including copackaged optics assemblies that integrate lasers alongside switch logic dies as well as non-retimed optical interfaces that eliminate power-hungry retimers are assessed. Additionally, strategic directions proposed to overcome critical cost barriers inhibiting mainstream adoption of integrated photonics are discussed.
Ultimately, successfully driving optical transceiver pricing down towards parity with incumbent high-volume copper cabling technologies will determine whether integrated photonics can transition from niche applications to become a ubiquitous foundational technology pillar across future data center architectures.
HPC and Hyperscale Data Center Trends
High performance computing (HPC) relies on low-latency adaptive routing networks linking massively parallel nodes to run communication-intensive computational workloads. Top ranked supercomputer installations require node injection bandwidths 5-10x higher than leading hyperscale cloud data centers in order to feed ever more powerful multi-CPU/GPU nodes.
This has historically fueled development of proprietary interconnect fabrics like Cray's Gemini and Slingshot or Fujitsu’s Tofu achieving over 1 Tb/s per link, enabled by leveraging emerging technologies sooner than commodity Ethernet solutions. HPC communication workloads feature predictable persistent traffic patterns, but congestion must be proactively avoided to maintain application scalability across thousands of nodes.
In contrast, hyperscale data centers operated by companies like Amazon (AWS), Google, Meta, and Microsoft focus intensely on cost efficiency, redundancy, and rapid elastic scaling of homogeneous infrastructure in order to maximize availability and quickly adapt to changes in user demand. These factors have encouraged adoption of commodity electrical Ethernet despite demonstrated efficiency advantages of HPC-optimized technologies like Infiniband historically.
However, machine learning training workloads are now forcing a convergence on common hyperscale and HPC interconnect requirements to support scaling to thousands of GPU accelerators. But datacenter traffic patterns remain dominated by “many-to-many” flows rather than structured point-to-point communication.
Across applications, optical interconnect technologies offer clear advantages over copper in bandwidth density (bandwidth/mm), reach (up to 10 km), and power efficiency. However adoption is often gated by total cost of ownership parity during initial deployment and multi-generational upgrade cycles.
Interconnect Technologies
Figure 1 below summarizes the efficiency versus reach tradeoffs for various electrical and optical interconnect technologies. As expected based on physical media properties, short electrical links achieve the highest efficiency but performance falls off rapidly beyond 1-10 meters. Optical solutions offer extended reach up to 10 kilometers but at reduced bit energy efficiency.
Within data centers, active electrical cables dominate rack lengths below 10 meters where efficiencies reach as low as 5 pJ/bit thanks to advanced equalization techniques. However, growing bandwidth requirements are driving adoption of optical links within racks, not just at longer distances.
Parallel single mode fibers currently scale capacity through spatial multiplexing by stacking multiple transmit and receive elements. Emerging wavelength division multiplexing (WDM) techniques further increase fiber bandwidth through frequency reuse. Direct detect modulation up to 100 Gbps lane rates is standard, but low-power coherent detection will likely be required beyond 400 Gbps.
Silicon photonics enables further scaling through integration of higher port counts and functional density. For optical transceivers, a mix of form factors and packaging strategies balance performance, serviceability and density. Pluggable modules like QSFP-DD offer replaceability while co-packaged optics maximize bandwidth density but sacrifices field repair.
Reliability, Thermal Design and Infrastructure Challenges
Several persistent challenges have slowed mainstream adoption of optical interconnect across data center architectures.
First, continued scaling has driven optics and switch package power densities forcing adoption of liquid cooling techniques while demanding co-optimization of thermal design.
Second, reliability of active optical components constructed from III-V materials, especially lasers, must reach acceptable levels exceeding 50 khrs mean time between failures under data center operating conditions up to 80°C or higher and rapid thermal transients. This requires extensive qualifications well beyond typical telecom bands.
Additionally, physical infrastructure constraints imposed by backwards compatibility with existing cabling plants, wavelength bands, and optical connector schemes limit deployments of radically new signaling approaches during upgrade cycles. Typical optical product generations focus on doubling lane rates while maintaining compatibility with prior generations. More flexible greenfield sites will be needed to insert disruptive technologies like low-power coherent links within the data center itself.
Finally, published analysis indicates current optical transceiver solutions still carry a 5-10x cost premium over ubiquitous passive copper cabling. Ultimately matching silicon CMOS cost reduction trajectories should be the end goal for integrated photonics technologies. This will require both further monolithic integration to improve yields and reduce packaging complexity as well as strategic engagement from high-volume customers to kickstart manufacturing investment.
Integrated Photonics Solutions
Integrated photonic approaches can address several of these challenges through advantages in density, efficiency, reliability and scale. Integration of higher port count switching and routing functions alongside lasers, modulators, photodetectors and transimpedance amplifiers enables scaling of aggregate bandwidth while simultaneously reducing fiber coupling requirements. Smaller feature sizes also translate to lower capacitance devices, reducing dynamic power consumption.
However, serious technology gaps must be addressed. Efficient on-chip lasers demand heterogeneous integration of mature III-V gain materials. Cost-effective zero-change CMOS processes still face challenges around temperature sensitivity and achievable loss budgets [10]. Finally, advanced integrated photonic packaging techniques are required to address fiber coupling, thermal design and optical power handling while avoiding Bandwidth × Distance bottlenecks encountered in legacy microbump-based co-packaged optics.
Promising current directions include co-packaging switch ASIC die alongside integrated photonic chiplets as shown in Figure 2. This architecture eliminates retimers otherwise required to drive signals across copper channels to traditional optical pluggables. The OIF 400G-ZR standard enables these chiplets to be mounted directly on the package substrate, avoiding costly pixelated microbump interposers required in previous generations. Locating driver circuitry adjacent to modulators improves efficiency while also avoiding electrical connectors in the path. The OSFP-LS form factor maintains replaceability for these co-packaged optics modules.
Thermal and reliability challenges do constrain modules to shorter reaches compared to standalone pluggables, providing motivation for further co-optimization. Extending integrated photonic manufacturing flows leveraged from silicon photonics research can help improve yields and repeatability as volumes scale up. Automated test, validation and control techniques used widely in CMOS fabs also translate effectively to augment photonics manufacturing.
Cost parity roadmap
Ultimately, published system cost analysis predicts optical transceivers still carry about a 10× cost premium over incumbent passive copper cabling technologies as shown in Figure 3. Therefore the cost status quo clearly remains the most critical obstacle inhibiting widespread optical adoption despite exponential traffic growth projections across HPC and cloud data center networks.
What steps can be taken to overcome this gap? Driving down integrated photonic pricing through superior manufacturing scale and efficiency should be treated as the end goal. Reaching copper cable costs below $0.05 per Gigabit per second of bandwidth will enable direct replacement of electrical links across rack interconnects and beyond. Component cost reduction trajectories should aim to mimic success in CMOS silicon chips, which have seen costs decrease dramatically over time from precision process improvement and economies of scale.
Initial niche applications exist today such as silicon photonics for LIDAR, consul interfaces, or analog links for radar and 5G. However, data communications must still be treated as the “killer app” for integrated photonics where volume economics can take effect. Call to action mandates around underserved applications like memory disaggregation have the potential to drive initial deployments past the cost premium barrier if value is compelling enough.
Standards organizations and industry collaborations also share an important role navigating technology transitions and fostering aligned investment across customers and the supplier ecosystem. Groups including the Optical Internetworking Forum (OIF), IEEE Industry Connections, and consortiums around Open Compute Project (OCP) have started these efforts for integrated photonics already. However, orders of magnitude broader participation will be needed from network architects and decision-makers within equipment vendors across the entire value chain.
Conclusion
Exponential traffic growth across high performance computing and cloud data center networks is driving interconnect bandwidth requirements to unprecedented emergent levels. Electrical signaling interfaces are rapidly approaching practical performance limits. Integrated optical interconnect solutions offer clear advantages thanks to physical media properties but still face barriers around reliability, thermal design and compatibility with existing infrastructure.
While today’s applications feature niche deployments, successfully driving integrated photonic pricing down cost parity with incumbent copper cabling technologies should be treated as an imperative by the industry based on future scaling projections. Overcoming this obstacle through manufacturing innovation and disruptive high-volume adoption vectors will allow integrated photonics to transform from leading indicator to foundational technology pillar across future data center architectures.
Reference
[1] M. Glick, L. Liao, and K. Schmidtke, Eds., Integrated Photonics for Data Communication Applications. Elsevier, 2023.
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