CPO vs On-Board Optics

Co-Packaged Optics (CPO) and On-Board Optics (OBO) are two approaches to moving optical engines closer to high-speed switch ASICs and computing devices. Both are designed to address the electrical loss, power consumption, signal integrity, and bandwidth-density challenges that become increasingly difficult as network speeds move from 800G toward 1.6T, 3.2T, and beyond.

The main difference is the level of integration. In an OBO architecture, the optical engine is mounted on the same printed circuit board as the packaged ASIC but remains a separate component. In CPO, the optical engine is integrated into the same package or package substrate environment as the switch ASIC.

This difference changes the electrical interconnect length, thermal architecture, serviceability, packaging requirements, and overall system design.

1. What Is On-Board Optics?

On-Board Optics, commonly abbreviated as OBO, places the optical engine on the system PCB near the switch ASIC or other high-speed electronic device.

The optical engine remains physically separate from the ASIC package. High-speed electrical signals therefore travel from the ASIC package across a relatively short PCB connection to the optical engine before being converted into optical signals.

Compared with front-panel pluggable optics, OBO significantly reduces the electrical distance between the ASIC and optical conversion point.

OBO is sometimes also described as board-mounted optics.

2. What Is Co-Packaged Optics?

Co-Packaged Optics places the optical engine much closer to the switch ASIC by integrating optical and electronic components within the same package or package-substrate architecture.

Instead of routing high-speed electrical signals across a large section of the system PCB, the electrical connection between the ASIC and optical engine is reduced to a much shorter package-level path.

This architecture is intended to address the increasing electrical-channel loss and power associated with very high-speed interfaces.

CPO can use silicon photonics, InP-based optical devices, VCSEL technologies, external laser sources, or combinations of these technologies depending on the implementation.

3. CPO vs OBO at a Glance

4. Why Optical Engines Are Moving Closer to the ASIC

As electrical signaling rates increase, copper traces and connectors become increasingly difficult to use efficiently over longer distances.

Higher-speed electrical channels experience greater insertion loss, reflection, crosstalk, and equalization requirements. Additional signal conditioning can increase power consumption and latency.

Moving the optical conversion point closer to the ASIC reduces the electrical distance over which the highest-speed signals must travel.

The overall evolution can be represented as:

Front-Panel Pluggable → On-Board Optics → Near-Package Optics → Co-Packaged Optics

Each step moves optical conversion closer to the switching or computing silicon.

5. Electrical Path: CPO vs OBO

The electrical path between the switch ASIC and optical engine is one of the most important differences between OBO and CPO.

OBO provides a significant improvement compared with conventional pluggable optics, but the electrical path is still longer than a true co-packaged implementation.

CPO minimizes the distance between electrical and optical conversion, which becomes increasingly valuable at very high lane rates.

6. Signal Integrity and High-Speed Scaling

Signal integrity becomes increasingly challenging as electrical lane speeds increase.

High-speed signals are affected by PCB trace length, vias, connectors, package transitions, impedance discontinuities, and crosstalk.

OBO shortens the electrical channel and can therefore improve signal integrity compared with front-panel optics.

CPO minimizes the electrical channel even further by placing the optical engine in close proximity to the ASIC.

This architecture can reduce the amount of electrical equalization required and provide a stronger path toward higher lane rates.

7. Power Consumption and Power per Bit

Power efficiency is a major reason for developing OBO and CPO architectures.

When high-speed electrical signals travel across longer PCB traces, more electrical power may be required for equalization, signal recovery, and driving the channel.

Shortening the electrical path can reduce these requirements.

Actual power depends on the ASIC, SerDes, DSP, optical engine, laser architecture, process node, cooling solution, and overall system implementation.

8. Bandwidth Density

Bandwidth density describes how much network capacity can be delivered within a limited physical area.

OBO increases bandwidth density by moving optical engines away from the front panel and closer to the ASIC.

CPO can provide an even higher level of integration because optical engines can be distributed around the ASIC package.

This becomes increasingly important as switch bandwidth moves toward 51.2T, 102.4T, and higher architectures.

9. Thermal Management: OBO vs CPO

Thermal management is one of the most important differences between OBO and CPO.

In OBO, the optical engine is separate from the ASIC package and can be cooled through the board-level thermal architecture.

In CPO, the optical engines are much closer to the high-power switch ASIC. The thermal systems for the ASIC and optical components must therefore be considered together.

This can create significant packaging and cooling challenges, especially as switch ASIC power increases.

10. Serviceability and Replacement

Serviceability is one of the strongest advantages of OBO compared with CPO.

Because the optical engine remains a separate board-mounted component, OBO can provide more opportunities for replacement, inspection, or modular maintenance.

CPO integrates optics much more deeply with the switch package. A failure in the optical subsystem may therefore require more complex service procedures and can potentially involve replacement of a larger system assembly.

This tradeoff is important for large data center operators that need rapid field maintenance.

11. Manufacturing Complexity

CPO requires highly accurate integration of electronic and photonic components within a compact package.

Optical coupling, fiber attachment, thermal expansion, package tolerances, assembly yield, testing, and laser integration all become important manufacturing considerations.

OBO is less tightly integrated because the optical engine and ASIC remain separate components on the board.

12. CPO and OBO Optical Engine Architecture

Both architectures can use an optical engine containing photonic integrated circuits, electrical integrated circuits, lasers, drivers, receivers, and other optical components.

The major difference is how the optical engine is positioned relative to the switching ASIC.

13. External Laser Sources in CPO

CPO systems do not necessarily need to place the laser inside the same package as the switch ASIC.

An External Laser Source can provide continuous-wave optical power to the photonic engine while the laser source remains physically separate from the hottest region of the package.

This architecture can simplify thermal management and improve laser serviceability while retaining a short electrical connection between the ASIC and optical engine.

External laser architectures are therefore one of the important approaches being considered for scalable CPO systems.

14. Silicon Photonics in CPO and OBO

Silicon photonics can be used in both OBO and CPO architectures.

Its ability to integrate optical waveguides, modulators, photodetectors, and related photonic functions on a compact silicon platform makes it well suited to high-density optical engines.

In OBO, silicon photonics can reduce the size and complexity of the board-mounted optical engine.

In CPO, silicon photonics can be integrated much closer to the switch ASIC to minimize the electrical interconnect distance.

Therefore, silicon photonics and CPO should not be treated as interchangeable terms. Silicon photonics is an optical technology, while CPO describes an integration architecture.

15. CPO vs OBO for 800G and 1.6T

Higher bandwidth interfaces increase the pressure on electrical channels and optical integration.

OBO can support high-speed optical architectures by shortening the electrical path without fully integrating the optical engine with the ASIC.

CPO takes the integration one step further and is especially relevant to future switching systems where the electrical interface between the ASIC and optical engine becomes increasingly difficult to scale.

16. CPO vs OBO for AI Data Centers

AI data centers place unusually high demands on network bandwidth because thousands of accelerators can communicate simultaneously.

High-radix switches need to provide enormous aggregate switching capacity while controlling power density and thermal load.

OBO can reduce the electrical path between switch ASICs and optical engines while maintaining some level of modularity.

CPO can further reduce electrical interconnect distance and increase optical integration, making it particularly relevant to future AI networking systems with extremely high switch bandwidth.

17. CPO vs OBO: Key Tradeoffs

18. How to Choose Between CPO and OBO

The choice depends on the switch bandwidth, electrical channel constraints, serviceability requirements, thermal architecture, manufacturing maturity, and deployment scale.

OBO and CPO should therefore be viewed as points on the optical integration roadmap rather than mutually exclusive technologies.

19. CPO vs OBO and the Future of Optical Networking

As switch bandwidth continues to increase, the industry is moving optical conversion progressively closer to the switching silicon.

OBO provides an intermediate architecture that reduces electrical distance while retaining more modularity than CPO.

CPO provides deeper integration and can address electrical-channel and bandwidth-density challenges that become increasingly difficult at very high speeds.

Current industry development is also expanding beyond CPO toward NPO, higher-speed pluggable optics, optical chiplets, external laser architectures, and other forms of optical integration.

The future is therefore unlikely to depend on a single architecture. Different parts of an AI data center may use pluggable optics, OBO, NPO, CPO, and other optical interconnect approaches according to their bandwidth, reach, serviceability, and system requirements.

20. Conclusion: CPO vs On-Board Optics

OBO and CPO both address the same fundamental problem: high-speed electrical connections between switching silicon and optical interfaces are becoming increasingly difficult to scale.

On-Board Optics: places the optical engine on the same PCB as the packaged ASIC. It shortens the electrical path, improves signal integrity and power efficiency compared with front-panel optics, and retains more modularity and serviceability.

Co-Packaged Optics: integrates the optical engine much more closely with the ASIC at the package level. It minimizes the electrical path and provides the highest potential for bandwidth density and power-per-bit improvements, but introduces greater thermal, packaging, manufacturing, testing, and serviceability challenges.

For current and near-term high-speed data center architectures, OBO can serve as an intermediate integration approach, while CPO is being developed for increasingly high-bandwidth switching and AI networking systems.

The broader evolution can be summarized as:

Pluggable Optics → OBO → NPO → CPO → More Integrated Optical I/O

As AI clusters and data center switch capacities continue to scale, the distance between electronics and optics will become an increasingly important design parameter for future optical interconnect systems.

21.CPO vs On-Board Optics Q&A