ECOC 2025 Tech Focus: Broadcom × Corning on Design Considerations for a 102.4 Tb/s CPO Switch

Introduction

As AI factories and hyperscale data centers push toward 1.6T/3.2T modules and CPO (Co-Packaged Optics), the architectural challenge is expanding from the single module to the whole system. Broadcom and Corning's ECOC 2025 talk focused on a 102.4 Tb/s CPO Ethernet switch system, arguing that the design must simultaneously satisfy four core metrics: density, performance, reliability and ease of assembly.

Key Points

1. Three Core Design Goals

1. Density

   • Target: ≥1 Tb/s per mm shoreline density.

   • Fiber requirement: thousands of fibers must be neatly routed in a compact space.

   • Front-panel design: a single 1RU can support 64 high-density connectors (e.g., MMC), carrying 1024 signal fibers; at 200G/lane this reaches 102 Tb/s.

   • Future evolution: multi-core fiber (MCF) and glass-substrate waveguide structures could raise density by another 4×.

2. Performance

   • Insertion loss: must be below 3.5 dB — the lower the better.

   • Optical coupling challenge: PIC-to-fiber coupling loss is ~1–2 dB, depending on edge coupling or surface coupling.

   • Fiber bending: loss is negligible at radii >30 mm, but multipath interference (MPI) must be avoided.

   • Polarization extinction ratio (PER): alignment between the external laser and PM fiber must reach 10–15 dB to avoid degradation from rotation and mechanical stress.

3. Reliability

   • Principle: fiber and optical connections must be more reliable than the ASIC and must never become the system bottleneck.

   • FIT (Failure in Time) testing:

     • FIT for fiber and FAU <1 per 10^9 hours.

     • A single harness <1 FIT; the whole system (32 harnesses × 36 fibers) <40 FIT.

   • Bend radii that are too small (<10 mm) significantly increase failure rates.

   • FAUs have accumulated more than 1 billion hours of field data, confirming high reliability.

2. Liquid Cooling and Assembly Challenges

• Liquid cooling design:

  • Improves heat dissipation, but takes up space inside the chassis, making fiber routing more complex.

• Ease of assembly:

  • Use pre-assembled, pre-tested optical sub-assemblies (known-good sub-assemblies) to avoid handling individual fibers during final assembly.

  • Assembly must keep fiber bend radii within spec and ensure connectors remain easy to service.

3. System-Level Design Thinking

• Whole-system view: designers must consider optics, mechanics, thermals and assembly together, rather than optimizing a single component.

• The 102.4 Tb/s CPO switch case:

  • 1RU form factor integrating 1000+ fibers.

  • Optical engines (OE) arranged as 6.4 Tb/s units around the ASIC.

  • High-density fiber infrastructure co-designed with liquid cooling to ensure system performance and reliability.

Summary

Broadcom and Corning's perspective highlights that:

1. The real challenge of CPO lies not in the module but in whole-system design.

2. Density, performance, reliability and ease of assembly are the four core metrics, and all must be met at once.

3. ≥1 Tb/s/mm shoreline density and <3.5 dB loss are the design baselines for 102.4 Tb/s CPO.

4. Multi-core fiber, glass waveguide substrates and liquid cooling are the paths to higher density and reliability.

5. System-level integration (fiber, FAU, ASIC, cooling) is what will let CPO truly scale.