Co-Packaged Optics: How It Works in Data Centers

Co-packaged optics (CPO) is a technology that places optical engines—the components that convert electrical signals to light and back—directly on the same package as the switch chip, instead of in a separate pluggable module at the faceplate. In a data center, you enter the switch’s data rate and the number of ports, and the calculator below estimates the total optical bandwidth you can achieve with CPO versus traditional pluggable optics. This helps you understand the scale of the change, not the exact performance of any specific product.

Formula

The calculator compares two scenarios. For co-packaged optics, the aggregate bandwidth is simply the switch’s total data rate, because all ports are assumed to be used. For pluggable optics, the aggregate bandwidth is the number of ports multiplied by the per-port rate (here 1.6 Tbps per port, a common high-end pluggable rate).

CPO bandwidth (Tbps) = Switch data rate (Tbps)

Pluggable bandwidth (Tbps) = Number of ports × 1.6 Tbps

Variables:

• Switch data rate (Tbps): the total switching capacity of the ASIC, e.g., 51.2 Tbps.
• Number of ports: the number of optical ports on the switch.
• 1.6 Tbps: assumed per-port rate for pluggable optics (e.g., 1.6T-DR8 modules).

Worked Example

Using the defaults: a 51.2 Tbps switch with 64 ports.

1. CPO aggregate bandwidth = 51.2 Tbps (the switch’s full capacity).
2. Pluggable aggregate bandwidth = 64 × 1.6 = 102.4 Tbps.

So the pluggable scenario would need more than 51.2 Tbps of optics to match the switch’s capacity, which is why CPO is attractive: it can reduce the number of optical modules needed.

What Is Co-Packaged Optics?

Co-packaged optics (CPO) is a packaging approach where the optical transceivers (lasers, modulators, photodetectors) are integrated onto the same substrate or package as the switch ASIC (Application-Specific Integrated Circuit). This is in contrast to traditional pluggable optics, where the optical modules are separate, field-replaceable units that plug into the switch faceplate. The term “co-packaged� simply means “packaged together� – the optics and the silicon share a package. It emerged in the late 2010s as a response to the growing energy and cost of pluggable optics at higher data rates.

How Co-Packaged Optics Works

In a CPO implementation, the switch ASIC is mounted on a substrate that also carries one or more “optical engines� – small chips that contain the electrical-to-optical (E/O) and optical-to-electrical (O/E) conversion circuitry. These engines are placed very close to the ASIC, often within a few millimeters, and connected via short, high-density electrical traces. The optical fibers are then attached to the package via a fiber array unit (FAU) or a connector on the package edge. The key is that the electrical signal travels a very short distance before being converted to light, which reduces signal loss and power consumption compared to driving a signal across a circuit board to a faceplate module.

The main steps in operation:

• Electrical signals from the switch ASIC are routed to the optical engine.
• The optical engine modulates a laser (or uses a laser source) to encode the data as light.
• The light is coupled into optical fibers that leave the package.
• Incoming light is received by photodetectors in the optical engine and converted back to electrical signals for the ASIC.

This architecture reduces the electrical trace length from tens of centimeters to a few millimeters, which is the core benefit.

Why Data Centers Need Co-Packaged Optics

Data centers are facing a bandwidth crunch. As switch speeds increase from 12.8 Tbps to 51.2 Tbps and beyond, the number of pluggable modules needed grows, and each module consumes power and generates heat. Pluggable optics also require complex high-speed electrical signaling over the circuit board, which becomes less efficient at higher speeds. CPO addresses these issues by:

• Reducing power consumption: shorter electrical paths mean less energy lost as heat.
• Increasing bandwidth density: more optical lanes can be packed into a smaller area.
• Lowering cost: fewer components and less packaging complexity.
• Improving signal integrity: less distance for the electrical signal to degrade.

However, CPO is not a simple upgrade; it requires changes in how switches are built, how fibers are connected, and how maintenance is done.

Co-Packaged Optics vs. Pluggable Optics

The main comparison is between CPO and traditional pluggable optics (e.g., QSFP-DD, OSFP). Pluggables are hot-swappable, meaning you can replace a failed module without shutting down the switch. They also allow flexibility in choosing different reach or data rates per port. CPO, on the other hand, is fixed – you cannot change the optics after manufacturing. This is a major trade-off.

Key differences:

• Serviceability: Pluggables are field-replaceable; CPO is not.
• Power: CPO is generally more power-efficient.
• Density: CPO allows higher port density per rack unit.
• Cost: CPO may have lower material cost but higher upfront engineering cost.
• Reach: Both can support similar reaches, but CPO is often designed for short-reach (within a data center) initially.

In practice, many experts expect a hybrid approach, where some ports use CPO and others use pluggables for flexibility.

Common Problems and Challenges

CPO is not yet mainstream, and several challenges remain:

• Thermal management: The optical engines generate heat, and co-packaging with the ASIC makes cooling more difficult.
• Fiber management: Connecting hundreds of fibers to the package is complex and requires careful routing.
• Yield and testing: Manufacturing CPO packages with both electrical and optical components is harder than making either alone.
• Laser reliability: Lasers are often the most failure-prone component; in pluggables they can be replaced, but in CPO the whole package may need replacement.
• Standards: There is no single CPO standard yet, which slows adoption.

Despite these, major networking companies and cloud providers are investing heavily in CPO, indicating they see it as a long-term solution.

How to Choose Between CPO and Pluggable Optics

For a data center operator, the choice depends on several factors:

• Scale: If you are a hyperscale operator with predictable traffic, CPO may be worth the lack of flexibility.
• Maintenance model: If you need hot-swappable modules, pluggables are still necessary.
• Power and space: If you are power-constrained, CPO’s efficiency is attractive.
• Cost: CPO may have lower operating cost but higher capital cost initially.

In general, CPO is most suitable for fixed, high-bandwidth connections like spine-to-leaf links, while pluggables remain for edge ports or where flexibility is needed.

Frequently Asked Questions

Is co-packaged optics the same as silicon photonics? No. Silicon photonics is a technology for making optical components on a silicon chip. CPO is a packaging approach that can use silicon photonics, but it can also use other materials. CPO is about where the optics are placed, not how they are made.

Will co-packaged optics replace pluggable modules? Not entirely. In the near term, pluggables will remain for many applications. CPO will likely be used in high-density, high-bandwidth switches, while pluggables continue for edge and lower-speed ports. The two will coexist.

What is the power saving of co-packaged optics? Estimates vary, but CPO can reduce power consumption by 30-50% compared to pluggable optics for the same bandwidth, mainly due to shorter electrical traces and fewer components. Exact numbers depend on the implementation.

Can co-packaged optics be repaired if a laser fails? Generally no. Since the optics are integrated into the package, a failure often requires replacing the entire switch module. This is a key trade-off versus pluggables.

When will co-packaged optics be widely available? As of 2024, CPO is in early deployment, with some products announced. Widespread adoption is expected in the late 2020s as standards mature and costs decrease.

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in t | Switch data rate | Tbps | 51.2 | 0 | 1000
in p | Number of ports | - | 64 | 1 | 512
out cpobw | CPO aggregate bandwidth | Tbps | t | 2
out plugbw | Pluggable aggregate bandwidth | Tbps | p * 1.6 | 2