CPO vs NPO vs XPO: Architecture, Power, Cooling, and Maintainability in AI Data Center Optics

AI clusters are forcing switch bandwidth, optical-lane count, front-panel density, and system power to scale simultaneously. As electrical lane rates increase, the connection between a switch ASIC and its optical interfaces becomes progressively harder to design. Longer PCB channels introduce more loss and often require stronger equalization, retiming, or digital signal processing.

CPO, NPO, and XPO address this problem through three different optical-engine placement strategies:

• CPO moves optical conversion into the package-level environment of the switch ASIC.

• NPO places optical engines close to the ASIC but keeps them on the host PCB.

• XPO retains a front-panel pluggable module while increasing electrical-lane density and introducing module-level liquid cooling.

Their common objective is to reduce the limitations created by high-speed electrical transmission. However, each architecture distributes power, heat, packaging risk, fiber connectivity, and maintenance responsibility differently.

What Are CPO, NPO, and XPO?

CPO places optical engines within the package-level environment of the host ASIC, NPO mounts them on the system PCB close to the ASIC, and XPO retains a high-density front-panel pluggable module. The principal trade-off is between electrical reach, package integration, thermal design, and field serviceability.

The OIF CEI-448G Framework defines CPO as an electrical-to-optical device mounted on the host package. It defines NPO as a device mounted on the host PCB adjacent to the host silicon to minimize PCB traces and electrical signaling requirements.

                                               CPO vs NPO vs XPO Optical Engine Placement

Descriptions such as “micrometer-scale CPO,” “centimeter-scale NPO,” and “decimeter-scale pluggables” may be useful as conceptual illustrations, but they are not universal specification limits. Physical distance depends on the package, board, connector, and chassis design.

The Shared Objective: Shorten the Electrical Path

In a conventional switch, the ASIC is located on the system board while optical transceivers are installed at the front panel. High-speed electrical signals must travel through package transitions, PCB traces, vias, connectors, and the module electrical interface before optical conversion occurs.

At higher data rates, this channel becomes more difficult to manage. Dielectric loss, reflections, crosstalk, and impedance discontinuities reduce signal margin. The system may compensate through stronger transmitter and receiver equalization, clock recovery, retiming, forward error correction, or a retimed module DSP.

Moving the optical engine closer to the ASIC shortens the electrical portion of the link. More of the physical distance can then be covered optically rather than through high-speed PCB traces.

Three Optical-Engine Placement Models

• CPO: optical conversion occurs inside the package-level assembly.

• NPO: optical conversion occurs on the host PCB near the package.

• XPO: optical conversion remains inside a replaceable front-panel module.

This placement decision influences the system’s electrical loss, power distribution, cooling structure, fiber routing, manufacturing process, and repair strategy.

Why Electrical Reach Matters in High-Speed Switches

                              How Shorter Electrical Paths Reduce Signal-Conditioning Burden

The electrical link between an ASIC and an optical engine consumes part of the system’s signal-integrity, power, and thermal budgets.

As lane rates rise, PCB transmission becomes increasingly sensitive to:

• Trace length

• Package escape routing

• Board dielectric loss

• Vias and connector transitions

• Crosstalk

• Return loss

• Equalization capability

A longer channel generally requires more compensation. That compensation consumes power and creates heat, often in areas where airflow and panel space are already limited.

PCB Channel Loss, Equalization, and Power

A conventional optical module may contain a DSP that recovers and retimes the electrical signal before optical transmission. This creates a robust module boundary, but it also adds power inside the transceiver.

A shorter electrical path may support other interface arrangements:

• Linear optics, where more signal conditioning remains in the host ASIC

• Half-retimed optics, where only part of the interface is retimed

• Fully retimed optics, where the module provides a complete retiming boundary

The preferred design depends on host SerDes capability, channel loss, interoperability requirements, optical reach, thermal limits, and acceptable implementation risk.

The relevant engineering question is therefore not simply whether a DSP is present. It is:

Where are equalization, retiming, clock recovery, and FEC functions located, and what electrical channel must they compensate?

Why Shorter Electrical Links Do Not Automatically Create a Better System

Reducing electrical reach improves one part of the design but may complicate others.

• Concentrate additional heat around the system’s largest thermal source

• Increase package size and substrate complexity

• Make optical engines more difficult to replace

• Couple optical-engine yield to package yield

• Increase internal fiber density

• Require more precise fiber-to-chip alignment

• Complicate package-level testing

CPO, NPO, and XPO are therefore different ways of distributing engineering constraints rather than eliminating them.

CPO Architecture: Optical Engines Inside the ASIC Package

Co-Packaged Optics places optical engines within the package-level environment of the switch ASIC. Instead of routing every high-speed electrical lane to the front panel, the system performs optical conversion close to the ASIC and carries the signals toward the panel through fiber.

This is the most aggressive of the three architectures in reducing electrical reach.

Physical Integration with 2.5D and 3D Packaging

CPO is often associated with 2.5D and 3D packaging, but these terms are not interchangeable with CPO.

• A switch ASIC

• Multiple optical engines

• Silicon-photonics devices

• Electrical drivers and receivers

• Package substrates or interposers

• Fiber-attachment structures

• Thermal spreaders or cold plates

The optical engine does not have to be fabricated on the same semiconductor die as the ASIC. Separate electronic and photonic chiplets may be integrated within the same package-level assembly.

The OIF Co-Packaging Framework describes co-packaged assemblies containing socketed or soldered ASICs and optical or electrical engines on a high-performance substrate. It also discusses a socketed near-package arrangement intended to improve assembly and rework flexibility.

CPO Bandwidth Is Implementation-Specific

CPO is an integration architecture rather than a fixed bandwidth class.

The OIF 3.2 Tb/s Co-Packaged Module Implementation Agreement defines a 3.2 Tb/s building block for 51.2 Tb/s switch assemblies. Its optical variants include parallel-fiber and wavelength-multiplexed configurations, while the same mechanical concept can also support a passive copper attachment module.

This 3.2T module is one standardized implementation. It does not mean that every CPO engine must operate at 3.2 Tbps or that CPO is permanently limited to one bandwidth range.

• Electrical-lane count

• Per-lane data rate

• Optical wavelength count

• Modulation format

• Engine partitioning

• Fiber count

• Package topology

Power and Latency Benefits

The principal CPO power advantage comes from shortening the high-speed electrical connection between the ASIC and optical engine.

• High-swing electrical drivers

• Strong receive equalization

• Intermediate retimers

• Full module DSP processing

• Additional FEC stages

The total benefit depends on the baseline architecture. Power saved across the ASIC-to-optics interface should not automatically be presented as the same percentage of total switch power.

• The switch ASIC

• Optical modulators and receivers

• Laser sources

• Voltage conversion

• Cooling pumps and fans

• Management electronics

• Control-plane hardware

CPO can also reduce interface latency when it removes or simplifies retiming and signal-processing stages. There is no universal CPO latency figure because the result depends on whether the measurement covers the electrical interface, optical engine, FEC, complete optical link, switch pipeline, or end-to-end network.

Serviceability, Yield, and Failure Boundaries

Traditional pluggable modules create a clear maintenance boundary. A failed module can be removed from the front panel without replacing the switch ASIC.

CPO changes that boundary.

A soldered optical engine may be difficult to replace after package assembly. A failure inside a tightly integrated package can therefore enlarge the replacement domain and increase repair cost.

This does not mean every optical failure requires the ASIC to be discarded. Serviceability depends on whether the design uses:

• Soldered optical engines

• Socketed optical engines

• Replaceable external lasers

• Channel redundancy

• Engine redundancy

• Package-level rework

• Depot repair rather than field repair

Socketed engines can improve manufacturing rework, but they remain less accessible than front-panel transceivers. The design must therefore consider both initial manufacturing yield and long-term in-service reliability.

                                        CPO Package Architecture with External Laser Source

External Laser Sources as a Thermal and Maintenance Compromise

Lasers are temperature-sensitive components. Locating them next to a high-power ASIC can complicate thermal design and reduce the available reliability margin.

An external-laser architecture separates the continuous-wave laser source from the optical engine. Optical power is delivered through fiber to modulators inside the co-packaged assembly, while the laser remains in a cooler and more accessible location.

The OIF ELSFP Implementation Agreement defines the External Laser Small Form-Factor Pluggable as a field-replaceable source of continuous-wave light for optical transceivers co-packaged within a system. It uses a blind-mate electro-optical connection and is intended primarily for CPO applications.

• Separation of the laser thermal environment from the ASIC package

• Independent replacement of a failed light source

• Simplified laser cooling

• Centralized optical-power management

• Potential reuse or upgrading of laser modules

It also creates requirements for optical-power distribution, connector cleanliness, safety interlocks, redundancy, and monitoring.

ELSFP is not another name for XPO. ELSFP supplies external optical power to co-packaged engines, while XPO defines a different pluggable optical architecture.

NPO Architecture: Optical Engines Near the ASIC but Outside the Package

Near-Packaged Optics places optical engines on the host PCB close to the switch ASIC but outside the ASIC package.

NPO shortens electrical reach while maintaining greater physical separation between the optical engine and the host package.

                                           NPO Board-Level Optical Engine Architecture

Board-Level Placement and Intermediate Electrical Reach

• Beside the ASIC

• Around the perimeter of the ASIC cooling structure

• On a nearby daughterboard

• In an internal connectorized assembly

• Within a board-level socket

The exact placement and attachment method are implementation-dependent.

Compared with front-panel optics, NPO reduces PCB reach. Compared with CPO, electrical signals still cross the ASIC package boundary and travel across part of the host PCB.

NPO therefore retains some electrical-channel constraints while avoiding some package-level integration risks.

Optical-Electrical Separation and Repairability

Because the optical engine remains outside the ASIC package, NPO can provide a smaller failure domain than a tightly integrated CPO assembly.

A failed optical engine may be replaceable without replacing the switch ASIC. However, this should not be confused with front-panel hot swapping.

• Opening the chassis

• Removing a heat sink or cold plate

• Disconnecting internal fibers

• Releasing an internal connector or socket

• Replacing a daughterboard

• Performing board-level rework

NPO is therefore more separable than CPO but less accessible than XPO or a conventional front-panel module.

Packaging and Cooling Advantages over CPO

NPO avoids placing every optical engine directly inside the host package. This can reduce pressure on:

• Package-substrate area

• Package-level optical attachment

• Package assembly

• Coupled package yield

• Package rework

It can also provide greater freedom to establish separate thermal paths for the ASIC and optical engines.

• Air cooling

• Conductive heat spreaders

• Board-mounted heat sinks

• System cold plates

• Chassis-level liquid cooling

NPO still requires sophisticated manufacturing. The host board must integrate short high-speed electrical links, optical engines, internal fibers, power delivery, thermal structures, and service access within a constrained area.

Limits of NPO

NPO does not shorten the electrical path as aggressively as CPO. It may therefore require stronger equalization or retiming than a package-level optical engine.

• The ASIC package

• Host PCB traces

• Intermediate connectors

• Engine placement

• Electrical-lane rate

• Thermal design

• Internal fiber routing

NPO should not be defined by a fixed aggregate bandwidth. Its capacity depends on the number of electrical lanes, per-lane data rate, optical wavelength plan, and engine partitioning.

NPO as an Intermediate Architecture

• Front-panel electrical reach is becoming too difficult

• Full CPO integration is not acceptable

• Internal engine servicing is possible

• Board-level optical integration is available

• Front-panel hot replacement is not essential

This does not mean NPO must be temporary. It can remain useful wherever system designers value both shorter electrical reach and partial optical-engine independence.

XPO Architecture: Rebuilding the Pluggable Model for Extreme Density

XPO stands for eXtra-dense Pluggable Optics. It retains a front-panel replacement boundary while increasing electrical-lane density and introducing liquid cooling at the module level.

The official XPO MSA is developing a liquid-cooled pluggable form factor that supports 64 high-speed electrical lanes. The MSA is open to interested participants on a non-discriminatory basis.

Unlike CPO and NPO, XPO does not primarily solve the electrical-distance problem by moving optical conversion next to the ASIC. It focuses on increasing the density and cooling capability of a replaceable front-panel module.

                                               XPO Liquid-Cooled Pluggable Module

Front-Panel Pluggability and Module-Level Integration

An XPO module remains accessible from the front panel.

• Independent module replacement

• Field servicing

• Separate switch and optics lifecycles

• Module-level inventory

• Flexible optical-reach selection

• Clearer fault isolation

The cost is a larger and more complex module boundary. XPO must accommodate a high number of electrical lanes, substantial power delivery, dense optical connectivity, module management, liquid cooling, and a reliable insertion and ejection mechanism.

What 64 Electrical Lanes Mean for System Design

The XPO MSA currently identifies a 64-lane electrical interface. The aggregate optical capacity will depend on the final per-lane signaling rate, modulation method, encoding, retiming architecture, and optical implementation.

• Electrical connector density

• Host PCB escape routing

• Module power delivery

• Thermal load

• Module control and diagnostics

• Optical transmitter and receiver count

• Fiber or wavelength mapping

Until the complete MSA specification is published, exact module bandwidth, power limits, connector assignments, and mechanical dimensions should be treated as implementation-dependent rather than universal XPO specifications.

Integrated Liquid Cooling

XPO places liquid cooling inside the pluggable-module architecture.

This is a fundamental change from conventional air-cooled modules. The cooling system must operate together with:

• Electrical contacts

• Optical interfaces

• Module retention

• Management connections

• Insertion and removal procedures

• Service access

Liquid cooling introduces additional engineering requirements, including:

• Reliable fluid connections

• Leak prevention and detection

• Blind-mate alignment

• Coolant compatibility

• Pressure-drop control

• Module insertion force

• Maintenance procedures

The cooling interface becomes part of the module service model rather than only part of the switch chassis.

XPO Does Not Mean External Laser Pluggable

The official expansion of XPO is eXtra-dense Pluggable Optics.

An external laser may be used in a particular optical implementation, but it is not the defining feature of XPO.

The correct standardized term for the replaceable external laser used primarily with CPO is ELSFP, or External Laser Small Form-Factor Pluggable.

Serviceability Benefits and Added Complexity

XPO provides the clearest field-replacement model among the three architectures.

A failed module can be removed from the front panel without replacing the switch ASIC or accessing an internal optical engine.

However, liquid-cooled pluggability is mechanically more demanding than conventional module replacement. A completed design may need to connect and disconnect:

• High-speed electrical lanes

• Power contacts

• Management signals

• Optical fibers

• Liquid-cooling ports

• Mechanical retention features

All interfaces must remain reliable over repeated insertion and removal cycles.

CPO vs NPO vs XPO: Side-by-Side Engineering Comparison

Integration Location and Electrical Distance

CPO provides the shortest electrical path by placing optical conversion inside the package-level environment.

NPO allows a longer path between the package and a nearby board-mounted engine.

XPO retains the electrical connection between the ASIC and the front-panel module.

The actual distance varies by implementation, so architecture names should not be converted into universal physical-length specifications.

Power, Cooling, and Signal-Integrity Trade-Offs

CPO offers the strongest potential to reduce electrical-interface power, but it creates the highest thermal concentration around the ASIC package.

NPO provides more separation between the ASIC and optical engines while still reducing PCB reach.

XPO preserves module replacement but concentrates substantial functionality and heat inside the front-panel form factor.

Serviceability and Failure Boundaries

The replacement boundary differs significantly:

• CPO: package assembly or internal optical engine

• NPO: internal engine, socket, or daughterboard

• XPO: front-panel module

Engineers must evaluate not only whether a component is technically replaceable, but where the repair occurs, what tools are required, and how much of the system must be taken out of service.

Packaging Complexity and Manufacturing Ownership

• Semiconductor packaging

• Silicon photonics

• Package substrates

• Optical attachment

• Package-level thermal design

• Host-board design

• Short electrical interfaces

• Internal optical-engine attachment

• Fiber routing

• Board-level cooling

• High-density module packaging

• Liquid-cooling integration

• High-current power delivery

• Dense electrical and optical interfaces

• Front-panel mechanics

How the Manufacturing Ecosystem Changes

CPO: Advanced Packaging and Silicon Photonics

CPO requires close coordination among ASIC design, photonic integration, substrate design, electrical packaging, optical attachment, thermal management, and testing.

Multiple yield domains must be managed together. A completed assembly may contain a high-value switch ASIC, several optical engines, photonic integrated circuits, drivers, receivers, fiber couplers, and cooling structures.

Known-good-die testing, socketed engines, external lasers, redundancy, and package-level diagnostics can reduce risk, but they also add cost and complexity.

NPO: Board Integration and Internal Optical Alignment

NPO keeps the optical engine outside the package while moving it inside the switch.

Manufacturing priorities include short PCB channels, low-loss electrical transitions, internal engine connectors, fiber routing, board-level cooling, optical alignment, service access, and engine testability.

NPO reduces some package-level constraints but creates a more specialized system board.

XPO: Module Integration and Liquid Cooling

XPO retains the optical module as a separate product, but the required capabilities extend beyond conventional pluggables.

The module must combine a high-lane-count electrical interface, substantial power delivery, liquid cooling, dense optical connectivity, module management, and mechanical serviceability.

The central challenge is to preserve a replaceable module boundary while integrating significantly more electrical, optical, and thermal functionality into that boundary.

Implications for MPO, Fiber Arrays, and Chip-Level Optical Coupling

CPO, NPO, and XPO do not eliminate the need for fiber connectivity. They change where the connection occurs and what density, precision, and mechanical characteristics are required.

                                        How CPO, NPO and XPO Change Fiber Connectivity

XPO and High-Density Multi-Fiber Connectivity

A 64-lane pluggable electrical interface creates a strong need for organized, high-density optical routing.

• Wavelength multiplexing

• Duplex architecture

• Optical modulation

• Reach

• Lane mapping

• Connector design

Relevant connector and cable considerations include:

• Connector footprint

• Fiber polarity

• Insertion and return loss

• Cleaning access

• Cable-exit direction

• Routing around the cooling structure

• Mechanical strain during replacement

• Connector retention

MPO-type interfaces are well suited to standardized multi-fiber connectivity, but the final connector configuration must follow the completed XPO specification and the optical implementation.

Thermal and Mechanical Requirements Around Liquid-Cooled Modules

Fiber assemblies near a liquid-cooled module must coexist with fluid ports, cold plates, power contacts, high-speed electrical connectors, ejector mechanisms, and front-panel retention structures.

• Bend-radius management

• Cable routing

• Connector accessibility

• Service loops

• Strain relief

• Thermal expansion

• Mechanical clearance

Universal temperature classes or jacket-material requirements should not be assumed before final module and system specifications are available.

CPO and NPO Shift Optical Connections Inside the Switch

When optical engines move closer to the ASIC, part of the optical connection previously contained inside a front-panel transceiver becomes an internal optical interconnect.

• Internal fiber harnesses

• Compact multi-fiber connectors

• Fiber-array units

• Low-profile routing structures

• Optical-engine pigtails

• Chip-level coupling assemblies

CPO may require smaller or more package-compatible optical interfaces than conventional front-panel connectors. The preferred interface depends on available space, fiber count, loss budget, serviceability, and assembly process.

Fiber Arrays, V-Grooves, and Microlenses

A fiber array positions multiple fibers at a controlled pitch so that they can couple to a photonic integrated circuit.

A V-groove structure mechanically locates the fibers and helps maintain their relative alignment.

A microlens array may focus, collimate, or reshape optical beams between the fibers and the photonic chip.

• Edge coupling

• Grating coupling

• Expanded-beam interfaces

• Removable optical connections

• Permanently attached fiber-array units

Their required alignment tolerance and coupling performance depend on the optical mode, waveguide structure, lens geometry, attachment material, and operating temperature.

                               Fiber Array, V-Groove and Microlens Coupling to a Silicon Photonics Chip

How to Choose Between CPO, NPO, and XPO

No single architecture is optimal for every switch.

Choose by Electrical Performance and Power Budget

CPO is the strongest candidate when minimizing electrical reach and interface power is the dominant requirement.

NPO is relevant when the electrical path must be shortened but package-level integration is not acceptable.

XPO is appropriate when front-panel serviceability and increased pluggable density are prioritized over minimum electrical distance.

Choose by Serviceability

XPO provides the most direct replacement model for operators that require independent optics inventory and rapid field servicing.

NPO may be suitable when internal engine replacement can be performed during scheduled chassis maintenance.

CPO requires careful analysis of package repair, engine redundancy, laser placement, and replacement cost.

Choose by Cooling Readiness

CPO requires the ability to remove heat from optical and electrical components concentrated around the ASIC package.

NPO requires effective thermal paths for internal board-mounted optical engines.

XPO requires liquid-cooling infrastructure and reliable fluid interfaces at the module boundary.

Choose by Manufacturing Capability

CPO depends heavily on advanced semiconductor and photonic packaging.

NPO depends on specialized board design, internal optical-engine integration, and fiber alignment.

XPO depends on liquid-cooled module design, dense electrical connectivity, high-power delivery, and multi-fiber interfaces.

Engineering Decision Checklist

Before selecting an architecture, confirm:

• Required ASIC-to-optics electrical reach

• Maximum channel loss

• Total system-power budget

• Cooling architecture

• Optical-engine replacement strategy

• Acceptable failure domain

• Package and board manufacturing capability

• Internal fiber-routing space

• Connector density

• Optical-alignment requirements

• Test and rework strategy

• Expected switch and optics upgrade cycles

Common Misunderstandings About CPO, NPO, and XPO

They Are Not Three Bandwidth Levels

CPO, NPO, and XPO describe placement and integration architectures.

Their aggregate bandwidth depends on lane count, per-lane data rate, wavelength architecture, modulation format, and system generation.

Moving Optics Closer Does Not Remove Every Problem

Shorter electrical reach can reduce channel loss and signal-conditioning power, but it may increase package complexity, thermal concentration, yield coupling, and maintenance cost.

The shortest electrical path is not automatically the lowest-risk system.

NPO Is Not Automatically Hot-Swappable

NPO separates the optical engine from the ASIC package, but the engine normally remains inside the chassis.

Independent replacement should not be confused with front-panel hot swapping.

CPO Does Not Always Require Replacing the ASIC After an Optical Failure

The failure boundary depends on whether optical engines are soldered, socketed, redundant, or independently repairable.

CPO is less field-serviceable than front-panel optics, but its exact repair model is implementation-specific.

XPO Does Not Mean External Laser Pluggable

XPO means eXtra-dense Pluggable Optics.

ELSFP is the separate term for an External Laser Small Form-Factor Pluggable source used primarily with co-packaged optical systems.

Will CPO, NPO, and Pluggable Optics Coexist?

The three architectures solve different combinations of problems, so coexistence is technically plausible.

CPO offers the shortest electrical path and the highest package-integration level.

NPO reduces PCB reach while preserving greater separation between the ASIC and optical engines.

XPO preserves a field-replaceable front-panel module while increasing electrical-lane density and cooling capability.

Their adoption will depend on more than bandwidth. Important variables include:

• Interface power

• Total system power

• Cooling infrastructure

• Packaging yield

• Optical-engine reliability

• Field-maintenance requirements

• Internal fiber density

• Connector technology

• Manufacturing cost

• Deployment scale

CPO should not be treated as a predetermined universal endpoint. NPO may remain useful where both proximity and internal serviceability matter. XPO may become attractive where liquid cooling is available and operators want to preserve a pluggable maintenance model.

The likely outcome is a broader set of optical architectures matched to different switch designs, network layers, cooling systems, and operational priorities.

Frequently Asked Questions

What is the main difference between CPO, NPO, and XPO?

The main difference is optical-engine location. CPO places the engine within the ASIC package-level environment, NPO places it on the system PCB near the ASIC, and XPO keeps it in a front-panel liquid-cooled pluggable module.

Why can CPO reduce power compared with front-panel pluggable optics?

CPO shortens the electrical connection between the ASIC and optical conversion point. This can reduce the equalization, retiming, drive-power, and signal-processing burden. The total system benefit depends on the electrical interface and comparison baseline.

Can a CPO optical engine be replaced independently?

It depends on the package design. Socketed engines may permit manufacturing rework or specialized replacement, while soldered engines are more difficult to service. Neither normally provides the same accessibility as a front-panel module.

Is NPO hot-swappable?

Not necessarily. NPO engines remain inside the switch and may require chassis access, cooling-component removal, internal fiber disconnection, or board-level servicing.

What does XPO mean?

XPO means eXtra-dense Pluggable Optics. The XPO MSA is developing a liquid-cooled pluggable form factor supporting 64 high-speed electrical lanes.

How will these architectures affect MPO connectors and fiber arrays?

XPO supports continued demand for dense front-panel multi-fiber connectivity. CPO and NPO move more optical routing inside the switch, increasing the importance of compact fiber arrays, internal harnesses, V-groove alignment, microlenses, and package-compatible optical interfaces.