For decades, semiconductor engineers have worked to bring components closer together. Shorter connections mean less electrical loss and greater efficiency.
But in some of the most advanced AI networking systems, engineers are making a seemingly contradictory decision.
They’re moving the lasers farther away.
Broadcom’s 51.2-terabit co-packaged optics (CPO) switch illustrates why. While its optical engines sit alongside the switch ASIC, the lasers supplying their light remain outside the main chip package.
The reason reveals a growing challenge in AI infrastructure: integrating optics is only half the problem. Keeping the light source reliable is the other.
The Problem With Putting Lasers Next to a Hot Switch Chip
In March 2024, Broadcom announced Bailly, a 51.2 Tbps CPO Ethernet switch integrating eight silicon photonics optical engines with its Tomahawk 5 switching chip.
Broadcom reported more than 70% lower optical interconnect power consumption compared with conventional pluggable optics.
Yet the design kept its lasers separate.
Broadcom’s BCM78909 product documentation specifies 16 field-replaceable laser modules supplying light to the optical engines.
This separation is deliberate.
CPO moves optical engines closer to the switch ASIC to shorten the high-speed electrical connections that otherwise run across a circuit board. As signaling speeds increase, those connections become increasingly difficult and power-intensive to maintain.
But high-power switching chips also generate substantial heat.
Semiconductor lasers are temperature-sensitive. Elevated temperatures can reduce conversion efficiency, increase the current needed to produce light, and accelerate device degradation.
Placing lasers inside the same high-heat package introduces another problem: maintenance.
If a laser fails, replacing an entire expensive co-packaged assembly would be far more disruptive than exchanging a dedicated optical module.
Broadcom’s architecture keeps the optical engines close to the silicon while making the light sources accessible for replacement.
That solves an important reliability problem. But moving the lasers outside the package creates another engineering challenge.
Moving the Laser Solves One Problem—and Creates Another
Light still has to reach the optical engines.
An external laser supplies continuous-wave light through optical fibers and connectors. Inside the optical engine, silicon photonic devices modulate that light to transmit data.
Every connection introduces some optical loss.
As light passes through connectors, coupling interfaces, waveguides, and potentially optical splitters, less power remains available for the intended optical channels.
The problem becomes more demanding as systems support greater bandwidth and more optical paths.
Moving the laser away from the switch ASIC improves thermal isolation and serviceability, but it also requires careful management of the power delivered to each optical engine.
This is why seemingly ordinary components, including precision optical connectors, have become important to advanced semiconductor packaging.
The Optical Internetworking Forum (OIF) has standardized an External Laser Small Form Factor Pluggable interface, known as ELSFP, to support interoperable external light-source designs.
The engineering challenge is no longer simply whether optics can be integrated beside a switch chip.
It is whether sufficient optical power can be delivered reliably through the complete system.
The Next CPO Bottleneck May Be the Light Source Itself
The laser industry is responding.
In September 2025, Coherent announced sampling of 400 mW continuous-wave lasers designed for silicon photonics and CPO. The company reported output above 400 mW at 55°C, targeting applications requiring high optical power with low noise.
Lumentum is also developing ultra-high-power laser technology and external laser modules intended to supply multiple silicon photonics optical engines.
These developments reflect an important change in what advanced networking systems require from optical suppliers.
Higher laser output is useful, but power alone is insufficient. Light sources must also maintain stable output, low noise, thermal reliability, and manufacturability.
A more powerful laser that produces excessive heat or suffers reliability problems can simply move the bottleneck from the switch package into the external module.
Meanwhile, optical losses between the laser and its destination cannot be ignored.
This creates an increasingly interconnected engineering challenge involving laser manufacturers, optical connector suppliers, silicon photonics developers, and semiconductor packaging companies.
It does not mean every CPO architecture must use an external laser, nor that high-power lasers will inevitably become the industry’s dominant bottleneck.
But Broadcom’s design and the direction of optical supplier development point toward a broader conclusion.
The future of co-packaged optics may depend as much on the light delivered from outside the package as on the silicon inside it.
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TECH VISION MAGAZINE · AUGUST 11, 2026
CPO Moves Lasers Outside High-Heat Packaging as Optical Power Budgets Pose Manufacturing Challenge
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