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Co-packaged optics (CPO) places optical engines next to a high-speed switch or compute ASIC, often in the same package assembly, instead of sending high-speed electrical signals across a circuit board to a front-panel optical module. Shorter electrical paths can improve signal integrity, power efficiency and bandwidth density, but CPO also requires tightly coordinated ASIC, SerDes, photonics, package, thermal, fiber and service designs.
In a CPO link, the ASIC sends electrical data over short SerDes connections to an optical engine. The engine converts the data into light for fiber transmission, while an optical engine at the receiving end converts incoming light back into electrical signals. The exact package, lane architecture, laser location and service model vary by implementation, so CPO describes an integration approach rather than one specific photonic technology.
Why co-packaged optics is different from conventional pluggable optics
Most data-center switches use pluggable optics. A switching ASIC sits on a circuit board, and electrical traces carry each high-speed channel from the ASIC across the board to a connector on the front panel. A removable optical transceiver plugs into that connector and performs the electrical-to-optical conversion. On the receive path, the module converts light back into electrical data and sends it across the board to the ASIC.
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A CPO assembly normally combines a switching or compute ASIC with one or more optical engines. Host-side SerDes lanes connect the ASIC to the engines over short package or substrate routes. The engines provide the line-side electrical interfaces and optical conversion, and fiber carries the signal outside the package. Receiving equipment performs the reverse conversion. Some designs put the laser source near the engine; others distribute light from a separate laser module.
How co-packaged optics works, step by step
- The ASIC creates or receives electrical data. A switch ASIC processes packets and presents very high-speed electrical SerDes lanes. In the transmit direction, those lanes carry data toward an optical engine; in the receive direction, they carry recovered electrical data back into the ASIC.
- Short host-side SerDes lanes connect the ASIC and optical engine. Because the engine is adjacent to the ASIC, these lanes can be much shorter than the traces leading to a front-panel module. A 2023 review discusses 56–112 Gb/s PAM4 host-side XSR SerDes as a design context; those figures describe lane requirements in that review, not a maximum throughput for every CPO system.
- The optical engine converts electrical signals to light. The engine contains the photonic and electronic functions needed to launch data into an optical fiber. The particular modulator, detector, laser arrangement and lane grouping depend on the vendor’s implementation.
- Fiber transports the optical signal. Fiber connections leave the package or an attached optical interface and carry the signal to another switch, accelerator, rack or data-center interconnect endpoint.
- The receiving optical engine converts light back to electrical data. Detection and electronic recovery produce SerDes signals that travel over a short electrical path to the receiving ASIC.
- Lasers may be separated from the hottest package. In NVIDIA’s described design, an external laser source (ELS) module sits in a separately thermally controlled environment and sends light by fiber to integrated optical engines. NVIDIA says those ELS modules are field replaceable. An ELS is an example of one implementation, not a requirement for all CPO products.
What CPO is—and what it is not
An integration and packaging approach
CPO specifies where optical conversion is integrated relative to the ASIC. It does not dictate one semiconductor material, one modulation format, one connector, or one switch architecture. A CPO product might use silicon-photonic components, another photonic platform, or a combination of technologies. Silicon photonics and CPO therefore overlap in many products but are not synonyms: silicon photonics describes a photonic technology, while CPO describes the physical and electrical integration strategy.
Not a claim that every laser is inside the switch package
Heat and service considerations can motivate a separate laser source. The ELS example demonstrates that an integrated optical engine and a removable laser subsystem can coexist. Other designs may use different laser placement, so the package diagram and field-service documentation for the actual product matter more than the CPO label.
Not automatically an unserviceable optical port
Putting the engine beside the ASIC can make that engine less like a conventional front-panel transceiver, but serviceability depends on the design. A replaceable ELS, a replaceable optical assembly, or a board-level replacement plan each creates a different failure-isolation and maintenance process.
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Why data centers are exploring CPO
Shorter electrical links
High-speed copper traces lose signal quality through attenuation, reflection and crosstalk. Shortening the ASIC-to-optics path can reduce the electrical channel burden and may reduce the amount of signal conditioning required. The benefit is architectural, not guaranteed: package materials, lane rate, reach and implementation determine the actual margin.
Power and bandwidth density
Removing or reducing some electrical reach and conditioning can lower link power in a particular design. Integrating many optical engines around one ASIC can also increase the number of optical lanes that fit in a given switch footprint. A peer-reviewed review identifies lower power consumption and higher signal density as potential CPO benefits. Those are engineering possibilities, not universal measured outcomes for every product.
Scaling to wider switch fabrics
As switch fabrics move to more ports and higher per-lane rates, the front-panel, board-routing and connector budget becomes restrictive. CPO can place many optical channels around the ASIC and shorten the most demanding electrical paths. The trade-off is that the package, fiber exits, thermal solution and manufacturing process become correspondingly more complex.
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NVIDIA states that its described CPO design does not require digital signal-processing retimers and says this reduces network latency. That is a statement about NVIDIA’s implementation; it should not be read as proof that every CPO system eliminates retimers or has lower end-to-end latency than every pluggable design. SerDes architecture, buffering, forwarding behavior and the rest of the network still affect latency.
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CPO compared with pluggable optics
Neither architecture is a universal winner. Pluggables provide a well-understood replacement model and a broad ecosystem, while CPO can address electrical reach and density limits at very high bandwidths. Compare a specific product and workload using the dimensions below.
| Decision area | Co-packaged optics | Pluggable optics |
|---|---|---|
| ASIC-to-optics electrical reach | Very short package or substrate connection; exact length is design-specific. | Board traces run from the ASIC to a front-panel connector and module. |
| Signal conditioning | May reduce the need for some retimers or conditioning because of the shorter path; verify the product design. | May require additional conditioning as lane rate and board reach increase. |
| Power per link | Potentially lower in a particular implementation; no neutral cross-vendor figure establishes a universal saving. | Depends on module generation, reach, DSP use and host electrical channel. |
| Bandwidth density | Can place many optical lanes close to the ASIC and reduce front-panel electrical constraints. | Constrained by panel space, module size, connector count and board routing. |
| Latency | Could avoid a retimer in a vendor’s design; end-to-end latency remains system-dependent. | Depends on module electronics, host channel and whether retimers are used. |
| Thermal design | ASIC, engines and package must be co-designed; a separate laser source may move some heat elsewhere. | Modules are thermally managed at the panel and can be replaced independently. |
| Failure isolation and replacement | Varies by engine, laser and package service model; obtain the vendor’s replacement procedure. | Individual modules are normally accessible at the front panel, subject to the platform’s procedures. |
| Interoperability | Shared mechanical, optical, electrical and management specifications are still an industry focus. | Benefits from established transceiver form factors and multi-vendor qualification, although compatibility still must be checked. |
| Total system cost | Can change package, fiber, cooling, manufacturing and spare-parts costs; no neutral universal cost threshold is established. | Includes modules, board design, power, cooling and replacement inventory. |
Engineering and operational challenges
ASIC and SerDes co-design
The ASIC’s electrical interface, equalization, lane mapping and clocking must match the optical engine. Host-side and line-side SerDes are both design constraints. Changing a lane rate or package escape can affect signal integrity, power and manufacturability across the whole assembly.
Thermal management
Switch ASICs are already major heat sources. Optical engines add electronics and photonic components in the same constrained area, while lasers have their own temperature and reliability requirements. Liquid cooling, airflow, heat spreaders and remote laser placement are possible responses. NVIDIA describes the Quantum-X800 Q3450-LD as liquid cooled, but that product detail should not be generalized to all CPO systems.
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Optical coupling and fiber attachment
Fibers must align with the optical engines and remain reliable through assembly, vibration, temperature changes and service operations. Fiber exits, connectors, bend radius, cleaning procedures and optical-loss budgets become package and rack-design concerns rather than issues confined to a replaceable front-panel module.
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Packaging yield and reliability
Combining expensive ASICs, photonics and high-density electrical connections creates a larger consequence when assembly yield or reliability is poor. Manufacturers must qualify bonding, alignment, thermal cycling, contamination control and long-term optical performance. A package failure can affect many ports at once, so operators should understand the platform’s fault containment and spare strategy.
Service access
Ask which elements can be replaced in the field: the laser source, an optical engine, a cable or the complete switch assembly. A separately housed ELS can provide a replacement point, but it does not describe every other failure mode. Maintenance documentation should specify optical cleaning, laser-safety procedures, diagnostics, expected downtime and whether replacement requires factory handling.
Interoperability and ecosystem maturity
CPO requires agreement across ASIC vendors, optical-engine suppliers, laser providers, package houses, fiber-interface makers, system manufacturers and test organizations. The Open Compute Project initiative announced by Lightmatter on March 16, 2026 proposes a shared CPO reference architecture, open specifications and interoperability testing and certification. It identifies integration, reliability and supply-chain scaling as challenges. The initiative was announced as proposed work; it should not be treated as proof that a completed, universally adopted standard already exists.
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Current commercial examples and roadmap claims
NVIDIA platforms
NVIDIA’s live product page, accessed October 13, 2026, lists Quantum-X InfiniBand Photonics and Spectrum-X Ethernet Photonics CPO platforms. It describes the Quantum-X800 Q3450-LD as a 144-port, 800 Gb/s product with liquid cooling. The same page says Spectrum-X Ethernet Photonics offers up to 409.6 Tb/s and is available in the second half of 2026. Availability dates can change, so confirm current status and regional delivery with NVIDIA before using those figures for procurement.
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NVIDIA also states that its described CPO design does not require DSP retimers and uses field-replaceable external laser source modules. These statements apply to the named NVIDIA architecture, not to CPO as a category.
Broadcom TH5-Bailly and generation claims
In a presentation dated May 15, 2025, Broadcom calls its TH5-Bailly offering Gen 2 CPO and says Gen 2 systems are shipping in volume. The presentation labels Gen 3 CPO as 200G per lane and lists 400G per lane as a Gen 4 development roadmap item. It also advertises more than 3.5 times less energy consumption and more than 30% cost reduction for its third-generation CPO comparison with pluggables. These are Broadcom-reported product and roadmap claims, not independent benchmarks or industry-wide results.
Open-architecture work
The Lightmatter announcement of March 16, 2026 describes a proposed OCP effort to create a shared reference architecture, open specifications and interoperability tests. Such work matters because operators need predictable interfaces, qualification methods and replacement processes when CPO components come from multiple suppliers. Until specifications and certifications are completed and adopted, treat the effort as an interoperability initiative rather than a finished market standard.
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How to evaluate CPO for a data-center deployment
- Define the traffic and reach. Record required port count, lane rate, fiber distance, rack topology, oversubscription and growth assumptions. CPO is most relevant when electrical reach or front-panel density is a limiting factor.
- Map the complete electrical channel. Request ASIC-to-engine lane details, SerDes rate and encoding, equalization, retimer use, loss budget and connector locations. Do not infer these from the CPO label.
- Measure the power boundary consistently. Decide whether comparisons include only optics, the switch, retimers, cooling and laser sources. A vendor percentage is meaningful only within the vendor’s stated product generation and comparison method.
- Review thermal and mechanical integration. Check heat load, liquid or air cooling, laser temperature control, fiber routing, rack airflow, bend radius and installation tooling.
- Test service and failure procedures. Identify replaceable units, diagnostics, optical cleaning requirements, laser-safety controls, spare quantities and the effect of one failed engine or laser on neighboring ports.
- Validate interoperability. Confirm qualified ASIC, engine, laser, fiber, connector, management and diagnostic combinations. Ask whether the platform supports open specifications or only a proprietary combination.
- Model total cost over the operating life. Include package and switch price, power, cooling, fiber plant, technician training, spares, repair logistics, downtime and upgrade paths. The available evidence does not establish a universal CPO-versus-pluggable cost crossover.
- Separate shipping products from roadmaps. Record the vendor, product name, announcement date, region and stated availability. Recheck volatile availability and roadmap information before signing a purchase agreement.
Key takeaways
- CPO places optical engines close to high-speed electronics, often in the same package assembly, to shorten the electrical path.
- The engine converts electrical data to light for fiber transport and converts received light back to electrical signals.
- Shorter links can improve power and bandwidth density, but numerical gains depend on the exact design and must be attributed to the measuring vendor.
- Thermal control, SerDes compatibility, fiber coupling, package yield, reliability, field service and interoperability are central parts of the architecture.
- Commercial products and proposed open-architecture efforts exist, but product availability, roadmap status and industry adoption vary by vendor and date.
CPO is best understood as a system-level response to the electrical and density limits of very high-speed interconnects. It can make optical bandwidth easier to scale around an ASIC, but it exchanges some front-panel modularity for deeper package, thermal and service integration. The right choice depends on the complete deployment—not on a single advertised power, cost or lane-rate number.
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