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 led me to Free-Space Optics. Eliminate cables entirely, beam data through air with VCSEL arrays acting like optical pixel grids, achieve sub-nanosecond latency. CPO, the pragmatic approach integrating fiber inside processor packages, started volume production in 2025. FSO remains largely in research labs, due to micron-precision alignment requirements in thermally chaotic datacenters.
Electrical interconnects hit hard limits beyond 50cm at 100+ Gbps; signal integrity collapses, power consumption becomes unsustainable. AI workloads pushing petabytes between accelerators made optical interconnects mandatory. CPO accepts latency penalties to gain proven reliability, while FSO bets that micron-precision alignment can survive datacenter reality.
Tune the PID controller that drives MEMS mirrors, ramp die temperature, and inject dust pulses. The lab plots alignment error, estimated BER, and mirror drive power so you can feel how fragile a ±5 µm tolerance really is.
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 make it worse: wrap-around links span meters of cable for what should be single-hop connections, introducing latency that cascades across thousands of synchronized operations.
Co-Packaged Optics (CPO), integrating fiber-coupled photonics inside processor packages, already ships in volume production. Free-Space Optics (FSO), replacing cables entirely 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.
Free-space optics: cutting the cable
FSO eliminates cables by using VCSEL arrays to transmit parallel data streams through datacenter air. Research demonstrations achieve 1.6 Tb/s bandwidth at 2.3 pJ/bit with sub-nanosecond latency. Reconfigurable links establish connections by steering beams rather than re-cabling racks, ideal for HPC systems requiring dynamic topology reconfiguration.
Alignment precision challenge
FSO requires maintaining ±5 µm alignment across meter-scale distances in datacenters subject to thermal expansion, floor vibration, and building settling. Misalignment by 10 µm causes 50%+ power loss and bit error rate spikes. 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 aren't controlled optical labs; they're thermally chaotic environments where FSO beams must maintain Gbps error rates.
Highlight the thermal, mechanical, or optical layers inside a CPO module versus an FSO free-space array. Tooltips surface tolerances, heat flux, and yields pulled from IEEE ECTC and JLT packaging papers.
Dense VCSEL arrays generate heat causing thermal crosstalk, wavelength drift, and power fluctuations. Dust particles block streams, condensation scatters beams. Hermetic packaging becomes mandatory for reliability. While researchers work on taming these challenges, production datacenters deployed the alternative that trades latency for stability.
Co-packaged optics: the conservative bet
CPO integrates silicon photonic transceivers inside processor packages alongside the ASIC die. Electrical signals convert to optical at package edge, then couple into standard fiber. The approach drops I/O power from 10+ pJ/bit to 2-5 pJ/bit and eliminates external transceiver modules.
CPO 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 is latency. CPO transceivers need DSP and FEC to fight fiber dispersion, adding 100-150 ns per hop. For tightly synchronized HPC collectives (AllReduce across thousands of GPUs) or latency-sensitive real-time workloads, these nanoseconds accumulate.
The trade-off analysis
The choice between FSO and CPO represents a fundamental engineering trade-off:
| 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 |
Each approach optimizes different constraints. FSO offers near-zero latency and superior power efficiency but demands mechanical precision and environmental control not yet proven at datacenter scale. CPO provides robustness through mature fiber technology at the cost of 100-150 ns fixed latency overhead.
Deployment path
CPO addressed immediate bandwidth demands using mature fiber technology, shipping in volume by 2025 despite latency overhead. FSO targets specialized applications (supercomputing, quantum interfaces) where sub-nanosecond latency justifies the complexity. MEMS advances, improved packaging, and AI-assisted alignment may enable broader FSO adoption, but datacenter deployments require breakthroughs in alignment stability. Production systems needed a solution that worked within existing infrastructure; research continues exploring whether FSO's advantages can overcome its physical challenges.
Toggle DSP, FEC, fiber length, and optical switches to recompute cumulative latency for an FSO versus CPO link. Values cite Ayar Labs, IEEE JLT, and hyperscaler whitepapers.