Why Co-Packaged Optics Is Becoming Essential for AI and HPC Multi-Die Designs

The rapid growth of artificial intelligence (AI) and high-performance computing (HPC) is driving unprecedented demand for compute capability and I/O bandwidth. As systems scale, power and thermal constraints are becoming the dominant barriers to continued performance growth. Even with advanced cooling technologies, there is a practical limit to how much heat modern systems can dissipate. As a result, interconnect energy efficiency has become a first-order design constraint.

At the same time, multi-die design has emerged as a cornerstone of advanced compute platforms. By partitioning functionality across multiple dies in an advanced package, designers can improve integration density, optimize yield, mix process technologies, and tailor architectures for increasingly specialized workloads. However, this architectural shift also increases the importance of interconnect technologies, as data movement increasingly dominates power consumption, performance, and scalability.

Copper interconnects have long served as the foundation for short-reach communication. However, as data rates continue to increase, copper no longer scales efficiently with distance. Signal loss, electromagnetic coupling, equalization complexity, and power consumption all rise significantly as bandwidth requirements grow. Data center networks are already transitioning from 1.6Tb/s toward 3.2Tb/s links, while lane speeds are advancing from 224Gb/s to 448Gb/s. These trends are pushing the industry toward optical connectivity that are placed much closer to compute and switching silicon.

Traditional approaches such as fully retimed optics (FRO), linear retimed optics (LRO), and linear pluggable optics (LPO) continue to rely on external optical transceivers located at the system edge. While these solutions extend communication reach, they still require long copper channels between the ASIC and the optical module, along with digital signal processors (DSPs) to overcome channel impairments. As data rates increase, these copper paths become increasingly inefficient from both power and performance perspectives.

Co-packaged optics (CPO) addresses this challenge by integrating optical engines directly within the same multi-die design as compute, switch, and I/O dies. Electrical-to-optical conversion occurs close to the signal source, reducing copper interconnect lengths from centimeters to just a few millimeters. By minimizing channel loss and reducing DSP overhead, CPO dramatically lowers connectivity power while delivering the bandwidth density required for next-generation AI and HPC systems.

CPO should not be viewed as a replacement for electrical die-to-die connectivity. Short-reach electrical interfaces remain essential inside the package, where latency and efficiency requirements are stringent. Standards such as UCIe provide a structured framework for connecting compute dies and I/O chiplets electrically within advanced multi-die designs.

Instead, CPO complements these electrical interconnects by extending connectivity far beyond what retimed electrical links can economically support. By constraining electrical channels to very low-loss environments before optical conversion, CPO enables efficient communication across racks and data centers while maintaining high bandwidth density and energy efficiency.

The industry is exploring several architectural approaches for implementing CPO, including side-by-side 2.5D integration, centralized optical I/O chiplets, advanced 3D-stacked photonic architectures, and bridge-assisted or glass-substrate implementations. Each approach balances integration complexity, thermal considerations, serviceability, and performance requirements differently.

Thermal Management

Photonic devices such as lasers, modulators, and photodiodes are highly sensitive to temperature variations. At the same time, they must often operate adjacent to extremely high-power switch ASICs and AI accelerators. Thermal coupling can affect wavelength stability, insertion loss, bit-error rates, and long-term reliability, making thermal management one of the most important design considerations for CPO implementations.

Yield and Manufacturability

CPO solutions integrate advanced-node logic dies, silicon photonics devices, analog electronics, and advanced packaging technologies. Managing yield across these heterogeneous components while ensuring known-good die presents significant manufacturing challenges. Optical testing and qualification also remain more complex than traditional electrical testing.

Serviceability and Ecosystem Readiness

Unlike pluggable optics, co-packaged optics can expand the replacement domain when failures occur. In some cases, an optical engine failure may require replacement of a larger system component. Additionally, industry-wide standards for interfaces, testing, qualification, and interoperability are still evolving, creating ecosystem considerations that must be addressed as adoption grows.

System-Level Co-Design Complexity

CPO requires simultaneous optimization across electrical, optical, thermal, and mechanical domains. Success increasingly depends on system-level integration and multiphysics analysis rather than the performance of individual devices alone.

Successfully deploying co-packaged optics requires more than individual technologies. It demands a comprehensive design environment that enables co-optimization across interface IP, photonics, packaging, implementation, verification, and multiphysics analysis.

Synopsys supports CPO adoption through an end-to-end design environment that combines silicon-proven interface IP, photonic IC design solutions, multiphysics analysis, and advanced implementation capabilities. At the connectivity layer, Synopsys provides high-speed PHYs and controllers, including 224G, PCIe, CXL, UALink, UEC, and UCIe solutions that enable efficient communication among logic dies, optical engines, and optical I/O chiplets.

For multi-die implementation, the Synopsys 3DIC Compiler platform provides a unified environment for exploration, implementation, analysis, and signoff. It enables automated UCIe and HBM routing, TSV and bump planning, and in-design multiphysics analysis, helping designers optimize die partitioning, interconnect topology, and physical realization throughout the development process.

In photonics design, Synopsys delivers comprehensive photonic IC design and simulation capabilities that support device-level, link-level, and end-to-end electro-optical co-simulation. Designers can evaluate complete optical links under realistic process, voltage, temperature, and wavelength conditions while maintaining consistency with foundry manufacturing requirements.

Beyond electrical and optical design, Synopsys provides multiphysics analysis capabilities to address the tightly coupled thermal, electrical, optical, and mechanical interactions that define CPO performance and reliability. These capabilities help designers identify issues such as wavelength drift, thermal crosstalk, signal integrity degradation, and power integrity challenges early in the design process.

Co-packaged optics is emerging as a practical and scalable response to the interconnect challenges facing advanced multi-die designs. By bringing optical engines closer to compute and switching silicon, CPO reduces electrical loss, improves energy efficiency, and enables bandwidth densities that are becoming increasingly difficult to achieve with copper-based approaches alone.

As the industry addresses challenges related to thermal management, reliability, manufacturability, and ecosystem maturity, momentum behind co-packaged optics continues to grow. Through silicon-proven interface IP, photonics design solutions, multiphysics analysis, and advanced multi-die implementation tools, Synopsys helps customers evaluate, design, and deploy co-packaged optics as part of a robust roadmap for next-generation AI and HPC systems.

Learn more: Read the full white paper, Co-Packaged Optics for Multi-Die Designs, to explore how co-packaged optics enables scalable, power-efficient multi-die designs for next-generation AI and HPC systems.