Optical interconnects: free or bonded?
Wavelength division multiplexing sends multiple data streams through fiber by encoding them on different light wavelengths. What if you could multiplex spatially instead, a grid of pixels per pulse? That leads to Free-Space Optics. It uses VCSEL arrays to transmit through air and aims for very low latency. The deployable path today is switch-level co-packaged optics, while in-package optical I/O targets accelerator fabrics. FSO remains mostly in research labs because micron-level alignment is hard to maintain in real datacenters.
Electrical interconnects degrade beyond about 50cm at 100+ Gbps, and power per bit climbs quickly. As AI workloads push petabytes between accelerators, optical links become increasingly attractive. Switch CPO improves bandwidth density and power by moving optics closer to the switch ASIC. In-package optical I/O pushes that idea toward accelerator packages. FSO tries to remove fiber paths entirely, but depends on tight alignment.
The interconnect wall
Training trillion-parameter models requires moving petabytes between accelerators, but copper traces consume watts per Gbps and degrade beyond 50cm at 100+ Gbps rates. Torus topologies can make it worse, adding cable length and latency that builds up across synchronized operations.
Co-Packaged Optics (CPO) already has volume-production examples at the switch level. In-package optical I/O is the compute-package cousin: optical engines sit beside CPUs, GPUs, or XPUs to move data off package. Free-Space Optics (FSO), replacing cables with steerable laser beams transmitted through air, targets lower latency but faces alignment challenges that have kept it in research labs. FSO's challenges begin with alignment.
FSO vs CPO
FSO uses VCSEL arrays to transmit parallel data streams through air. One lab-scale result reports 1.6 Tb/s at 2.3 pJ/bit with sub-nanosecond latency. Steerable beams could reconfigure links without re-cabling, which is attractive for HPC systems with changing topology.
FSO requires maintaining ±5 µm alignment across meter-scale distances in datacenters subject to thermal expansion, floor vibration, and building settling. A 10 µm misalignment can cut power by over 50% and raise bit error rates. Active alignment systems employ piezoelectric actuators with sub-microsecond response, but every link needs dedicated tracking hardware running continuously.
Indoor atmospheric turbulence from equipment heat dissipation causes beam wander and intensity flickering beyond 1-2 meters. Datacenters are not controlled optical labs, so stability is hard to maintain at scale.
Dense VCSEL arrays generate heat causing thermal crosstalk, wavelength drift, and power fluctuations. Dust and condensation can disrupt beams. Hermetic packaging is often required for reliability. Production systems have generally chosen the option that costs latency but gains stability, while researchers continue working on FSO's open problems.
Tune the PID controller that drives MEMS mirrors, then inject thermal drift and dust disturbances. Watch alignment error spike beyond ±5 µm and bit error rate collapse. This is why FSO remains experimental: the control loop must fight continuous mechanical chaos while maintaining Gbps data integrity.
Switch CPO places optical engines close to the switch ASIC, shortening the high-speed electrical path before optical conversion. In-package optical I/O moves a similar idea beside compute dies for accelerator fabrics. Both couple into fiber and avoid some of the reach and power penalties of long electrical SerDes paths.
The fiber-coupled path leverages decades of fiber infrastructure, proven silicon photonics processes, and established packaging techniques. Alignment tolerances relax to ±50 µm, 10x looser than FSO, and hermetic sealing protects optics from contamination.
The cost can be latency. Current CPO modules, like pluggable optics, may use FEC to reach BER targets, adding 100-150 ns per hop in Ayar's comparison. That is acceptable for many network-switch paths, but expensive for memory-semantic fabrics or tightly synchronized HPC collectives where nanoseconds accumulate.
FSO and CPO make different bets:
| Metric | Free-Space Optics | Co-Packaged Optics |
|---|---|---|
| Latency | Sub-nanosecond | 100-150 ns (FEC overhead) |
| Power Efficiency | 2.3 pJ/bit | 5-10 pJ/bit |
| Alignment Tolerance | ±5 µm (critical) | ±50 µm (relaxed) |
| Environmental Sensitivity | High (dust, vibration, turbulence) | Low (enclosed fiber) |
| Manufacturing Maturity | Experimental | Production-ready |
FSO bets on lower latency and power if alignment can be kept stable at scale. Switch CPO bets on mature fiber packaging and accepts networking latency that is easier to deploy. In-package optical I/O tries to keep the fiber reliability story while moving latency closer to package-level electrical links.
Deployment path
Switch CPO met immediate bandwidth needs using mature fiber technology and reached volume-production examples by 2025. In-package optical I/O is the more relevant path for accelerator-to-accelerator fabrics, where latency and bandwidth density matter more than traditional switch economics.
FSO targets specialized applications such as supercomputing and quantum interfaces, where the latency gains may justify the complexity. MEMS advances, improved packaging, and AI-assisted alignment may enable broader FSO adoption, but datacenter deployments still require breakthroughs in alignment stability. In practice, teams usually pick what they can deploy with existing fiber, racks, and packaging while research continues to test whether FSO can survive real datacenter conditions.