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Laser-based optical interconnects turn the electrical bit streams produced by AI accelerators into light, carry that light through fiber, and convert it back to electrical data at the destination. A modulator writes symbols onto a laser carrier; wavelength-division multiplexing can place several channels on one fiber; a photodetector and receiver electronics recover the stream. Silicon photonics integrates these functions with semiconductor manufacturing, while co-packaged optics (CPO) puts the photonic engine beside the switch or accelerator ASIC. That placement shortens the lossy electrical path to millimeters, reducing the need for retiming DSP and lowering energy per bit compared with long-reach electrical connections. The result is not an optical replacement for every trace: copper remains useful for short board distances, but fiber becomes increasingly important between GPUs, switches, racks and storage as lane rates and cluster size grow.
What happens when a laser link carries data between GPUs?
The link begins as an electrical SerDes stream from a GPU, CPU, switch ASIC or other accelerator. SerDes (serializer/deserializer) circuits serialize parallel data into very high-speed differential lanes. A photonic engine then performs three jobs:
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- Encode symbols: An electro-optic modulator varies the laser’s amplitude, phase or other property according to the electrical symbols. Multiple wavelengths can be multiplexed onto the same fiber, increasing capacity without adding a separate fiber for every lane.
- Recover the stream: At the far end, a photodetector converts the changing light into an electrical signal. Receiver electronics amplify, equalize and deserialize it for the destination ASIC or accelerator.
The optical portion is passive, low-loss fiber over its designed reach; the difficult electrical equalization is concentrated near each endpoint. That division is why an optical link can span a rack or adjacent racks while avoiding the attenuation and crosstalk that make very long copper SerDes channels difficult.
Why are optical links becoming important in AI data centers?
AI clusters move enormous volumes of parameter, activation and synchronization data between accelerators. Copper traces are efficient and inexpensive over short distances, but loss, crosstalk and equalization demands rise as lane rates increase and the channel extends from a package to a front-panel module or across a rack.
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Intel’s 2024 OCI demonstration characterized electrical I/O at roughly one meter or less and showed an optical connection over up to 100 meters of fiber. The figures describe that demonstration, not a universal limit for every optical module. NVIDIA positions optical links for switch-to-switch, server-to-switch and storage connections in AI factories, with CPO designs aimed at 1.6 Tbps and higher switch interfaces.
Fiber also lets a system designer separate physical location from electrical reach. GPUs can be distributed across racks while the optical cable handles the distance, leaving only a short electrical path on each board or package. That flexibility matters when a cluster is expanded: adding racks does not require extending a high-speed copper backplane through the entire fabric.
What is silicon photonics, and how is it different from a conventional transceiver?
Silicon photonics places waveguides, modulators, splitters, multiplexers and photodetectors on a silicon photonic integrated circuit, alongside a separate or integrated CMOS electrical die. It is a manufacturing and integration approach, not a single connector or network protocol. The electrical IC still performs functions such as driver amplification, clocking, monitoring and signal recovery.
A conventional pluggable transceiver packages optical and electrical components in a replaceable module at the front of a switch. Silicon photonics can be used inside such a module, on a board-mounted engine or in a co-packaged design. Therefore, “silicon photonics” describes where and how optical functions are built; “CPO” describes their physical relationship to the ASIC.
Intel describes OCI as a photonics IC paired with a CMOS electrical IC and designed for co-packaging with CPUs, GPUs, IPUs and SoCs. Its current product description lists 4 Tbps bidirectional support as a first-generation OCI figure. That is a vendor specification for the described product, not a guaranteed rate for every silicon-photonics implementation.
What is the difference between pluggable optics, on-board optics and co-packaged optics?
The architectures differ mainly in where optical conversion occurs and how much high-speed electrical distance remains between the ASIC and the photonic engine.
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| Architecture | Electrical path | Serviceability | Packaging and thermal considerations | Typical trade-off |
|---|---|---|---|---|
| Pluggable optics | ASIC-to-front-panel module path is the longest of the three; it may require DSP retiming and substantial equalization. | Module can be removed and replaced independently of the switch board. | Optical engines are away from the ASIC, simplifying package integration but concentrating module heat at the panel. | Best modularity and field replacement, with more electrical loss and power in the host path. |
| On-board optics | Engine sits on the board near the ASIC, shortening traces compared with a front-panel module. | More serviceable than full CPO, but replacement is generally board-level rather than a simple front-panel swap. | Requires board-level optical alignment, fiber attach and thermal planning. | Middle ground between electrical reach and modularity. |
| Co-packaged optics (CPO) | Photonic engines share a package or substrate with the switch ASIC; the high-speed electrical run is only millimeters long. | Not equivalent to replacing a pluggable; the platform’s package and board service model determines repairability. | Optical coupling, ASIC heat, laser reliability and fiber routing must be solved in one package-level design. | Shortest electrical path and potential power savings, but highest integration and service complexity. |
| Optical I/O chiplets | Photonics and electrical dies are designed as co-packaged chiplets beside a CPU, GPU, IPU or SoC. | Depends on the host package and system design. | Chiplet assembly, optical attach and thermal co-design are required. | Extends optical I/O directly to compute packages rather than only to network switches. |
NVIDIA reports 3.5-times lower power for its CPO comparison than traditional pluggables. That is a vendor comparison for the stated designs and conditions, not an industry-wide constant. A CPO design can reduce electrical loss and DSP power, but it also makes optics, packaging and thermal management part of the ASIC platform rather than an interchangeable module.
How does co-packaged optics reduce electrical loss and latency?
At very high lane rates, every centimeter of copper between an ASIC and a transceiver behaves like a transmission line that attenuates and distorts the signal. Equalizers and DSP can restore signal quality, but they consume power and add processing stages. Moving the optical engine next to the ASIC removes most of that channel before conversion.
The fiber then carries the signal over the intended optical reach with far less distance-dependent electrical loss. The conversion still has finite modulator, detector and receiver delays, so optics do not make propagation instantaneous. Their practical latency advantage comes from avoiding long electrical channels and some retiming work, not from eliminating all serialization or protocol latency.
CPO also changes failure boundaries. With a pluggable, a technician can replace the module while leaving the switch ASIC and board in place. With CPO, the photonic engine is part of the package or substrate, so a failed engine may require a board or package replacement unless the platform provides a separate service mechanism. That trade-off must be evaluated alongside watts per bit and bandwidth.
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The following numbers describe specific announcements or demonstrations and should not be treated as universal limits.
| Figure | What it describes | Qualification |
|---|---|---|
| 4 Tbps bidirectional | Intel’s current OCI product-description support figure. | First-generation vendor specification; accessed in 2026. |
| 64 channels × 32 Gbps in each direction over up to 100 m | Intel’s 2024 live OCI demonstration parameters. | Equivalent to 2.048 Tbps aggregate in each direction when multiplying the stated channels and lane rate; demonstration conditions do not define every product’s reach. |
| 1.6 Tbps per port | NVIDIA’s 2025 Spectrum-X and Quantum-X photonics switch announcement. | Port figure for the announced platform. |
| 115.2 Tbps full duplex over 144 ports | NVIDIA’s 2025 technical description of the Quantum-X Photonics Q3450 configuration. | Configuration-specific aggregate switching figure. |
| More than 5-times power reduction and up to 5-times faster LLM training | IBM’s 2024 CPO prototype research claims. | Research projections for a prototype, not deployed-product measurements. |
| Up to 80-times chip-to-chip bandwidth potential | IBM’s 2024 estimate using optical structures and multiple wavelengths. | Potential cited in research; it is not a shipping interconnect specification. |
Comparisons must keep their boundaries intact. A port-rate announcement, a live demonstration, a vendor power comparison and a research projection measure different things. Ask for the lane encoding, reach, fiber type, error-rate target, DSP assumptions and test configuration before comparing watts per bit or throughput.
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Do optical interconnects reduce AI data-center power?
They can, especially when they replace long, heavily equalized electrical paths. CPO’s short ASIC-to-optics connection reduces insertion loss and can reduce the amount of DSP and driver power needed to close the link. Fewer electrical retiming stages also simplify the signal path.
The net result is system-dependent. Lasers, modulators, photodetectors, drivers, monitoring circuits and cooling all consume power. A pluggable may use more host electrical power but offer easier replacement; a CPO design may save link power while increasing package and thermal complexity. IBM’s “more than 5-times” reduction is a prototype research claim, while NVIDIA’s 3.5-times comparison applies to its own CPO-versus-pluggable designs. Neither should be applied automatically to another switch, cable length or optical standard.
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Power should therefore be evaluated as joules per delivered bit for the complete link, including host SerDes, DSP, optical engine, laser and cooling—not by laser power alone.
How should you choose fiber and optical modules for an AI fabric?
The cleanest physical starting point is single-mode fiber, commonly specified as OS2 or SMF-28, when the optical engine and reach call for it. Intel explicitly specifies SMF-28 for OCI, and NVIDIA lists optical cables among its AI-networking products. The cable name alone is not enough; the connector, polarity, fiber count and reach must match the transceiver or optical engine.
Use this compatibility checklist
- Interface: Confirm whether the endpoint is Ethernet, InfiniBand or another electrical/optical interface and whether the switch supports the intended module.
- Rate: Match the port and module generation, such as 400G or 800G. A higher advertised module rate does not make an older host port compatible.
- Reach and fiber: Verify the supported distance and whether the design requires single-mode OS2/SMF-28 or another fiber class.
- Wavelength plan: For DWDM or other wavelength-multiplexed links, use the wavelength set and channel spacing specified by the platform.
- Connector and polarity: Check connector type, male/female gender where applicable, lane polarity and the required fiber count before ordering.
- Breakout behavior: If one high-rate port is split into multiple lower-rate links, verify that the switch ASIC, module firmware and cable support that breakout mode.
- Diagnostics: Ensure the platform exposes optical power, temperature, laser bias and error counters so a marginal fiber or engine can be isolated.
A 400G or 800G optical transceiver is a secondary path when a pluggable design is appropriate, but compatibility must be checked against the exact switch, standard, wavelength and reach. CPO and on-board optics may use a vendor-specific fiber attach rather than a front-panel module, so a standard cable cannot be assumed to fit.
How do Ethernet and InfiniBand fit into optical interconnects?
Optics carry the physical signal; Ethernet or InfiniBand defines the higher-level link behavior, framing and management expected by the endpoints. The same basic laser–modulator–fiber–detector chain can serve either ecosystem when the optical electrical interface, encoding, error targets and management functions match.
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Interoperability is therefore a system property. A cable with the right connector may still fail if its wavelength, lane mapping, forward-error-correction mode, firmware or host electrical interface differs. For a new fabric, validate one complete path—ASIC or NIC, optical engine or module, fiber assembly and remote endpoint—before buying large quantities.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What can go wrong, and how do you diagnose it?
Link will not train
Start with the physical path: confirm connector seating, polarity, lane count and the module’s supported rate. Then check whether the switch is configured for the selected Ethernet or InfiniBand mode and whether a breakout setting is required. A mismatch in wavelength or host electrical interface can prevent training even when optical power is present.
Intermittent errors or rising retransmissions
Read optical power, temperature and error counters at both ends. A contaminated or sharply bent fiber, an over-temperature engine, a marginal laser or an incorrect reach specification can produce errors without a complete loss of link. Compare the measured values with the platform’s limits rather than relying on the module label.
Power is higher than expected
Separate host SerDes and DSP power from optical-engine and cooling power. A CPO comparison may not match a pluggable in another lane rate or reach. Check whether the link is using unnecessary retiming, an incompatible module that forces a different operating mode, or thermal conditions that increase cooling demand.
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In a multiplexed design, a single laser, modulator or detector channel can remove only part of the aggregate capacity while the remaining channels continue to operate. Use per-channel telemetry and the platform’s protection or fallback policy to determine whether traffic should be rerouted or the entire link taken out of service.
When does CPO make sense for an AI data-center design?
CPO is most compelling when ASIC bandwidth is high enough that the electrical path to front-panel modules dominates power, signal integrity or board routing. It is less attractive when short copper traces already meet the reach and power target, or when field replacement is more important than package-level efficiency.
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- Choose pluggables when modular replacement, incremental upgrades and broad interoperability are priorities.
- Choose on-board optics when you need a shorter electrical path but still want more separation between the optical engine and the ASIC package.
- Choose CPO when the platform can absorb package, fiber-attach and thermal complexity in exchange for very high bandwidth density and lower electrical reach.
- Choose optical I/O chiplets when compute packages themselves—not only network switches—must expose very high-bandwidth links.
In every case, specify the complete link budget, service procedure, telemetry, fiber plant and protocol compatibility. Optical technology solves the distance and bandwidth problem only when the mechanical, electrical and operational interfaces are designed together.
What is the practical takeaway?
Lasers do not carry raw GPU data by themselves. Electrical SerDes drives a modulator, the modulator encodes one or more laser wavelengths, fiber carries the optical carriers, and photodetectors plus receiver electronics recreate the electrical stream. Silicon photonics integrates that conversion at semiconductor scale; CPO moves it beside the ASIC to minimize lossy electrical distance. Published bandwidth and power numbers are architecture-specific, so compare complete, qualified link designs rather than headline figures.
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Can I use any 800G transceiver with an optical AI switch?
No. The switch’s electrical lane interface, module form factor, Ethernet or InfiniBand mode, wavelength plan, fiber type, firmware and supported reach all have to match. Check the platform’s interoperability matrix and validate one full link before deployment.
Is a 100-meter optical reach a universal limit for AI links?
No. Up to 100 meters is the distance reported for Intel’s 2024 OCI demonstration. Actual reach depends on the optical engine, wavelength, loss budget, fiber and error-rate target of the specific implementation.
Does silicon photonics always put the laser on the same chip as the modulator?
Not necessarily. Silicon photonics refers to the integrated optical circuit and its manufacturing platform; lasers may be integrated, attached, or supplied by a separate optical source. Intel’s OCI description specifically includes on-chip DWDM lasers and SOAs, but that feature should not be assumed for every silicon-photonics product.
Are co-packaged optics easy to replace in the field?
They are not interchangeable with front-panel module service. Because the photonic engine shares a package or substrate with the ASIC, the platform vendor must define whether an engine, board or complete package is replaceable and how diagnostics identify the failed channel.
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