Coherent Pluggable Evolution: From CFP2-DCO to QSFP-DD ZR+

Five years ago, deploying a coherent optical link meant installing a 2RU transponder shelf with dedicated line cards. Today, the same capability fits in a QSFP-DD module that plugs directly into a router port. The journey from chassis-based transponders to pluggable coherent optics is one of the fastest technology transitions in networking — and it is not finished. Here is how coherent pluggables evolved and where they are going.

Generation 1: CFP2-DCO (2018–2022)

The CFP2-DCO (Digital Coherent Optics) was the first pluggable coherent transceiver. It packed a full coherent DSP, tunable laser, and optical front-end into a module roughly the size of a smartphone. At 200G and later 400G, it eliminated the need for a separate transponder shelf — the router or switch port itself became the coherent endpoint.

CFP2-DCO proved that coherent DSP could fit in a pluggable form factor. But it was large, drew 20–30 W, and required switch platforms designed for the CFP2 cage — limiting adoption to service provider routers. It was the proof-of-concept generation that showed coherent pluggables were viable.

Generation 2: QSFP-DD ZR/ZR+ (2023–Present)

The leap from CFP2 to QSFP-DD shrank the coherent module by roughly 60% by volume while doubling capacity to 800G. Key advances that made this possible:

• 7nm and 5nm DSP silicon. New process nodes cut DSP power from ~15 W to ~5 W, freeing thermal budget for the optical engine.
• Silicon photonics integration. Modulators, receivers, and MUX functions integrated on a single silicon die eliminated discrete optical components and shrank the optical engine.
• OpenZR+ standardization. oFEC, common management, and multi-vendor interoperability turned QSFP-DD ZR+ from a single-vendor curiosity into a competitive multi-source ecosystem.

The result: an 800G QSFP-DD ZR+ module that draws 20–24 W, reaches 500+ km with amplification, and plugs into the same QSFP-DD cage used by PAM4 DR8 and FR4 modules. A single switch port can serve intra-DC PAM4 today and metro DCI coherent tomorrow — same cage, same fiber plant.

What Generation 3 (1.6T ZR+) Brings

The next generation — sampling in 2025–2026 — doubles capacity again to 1.6 Tbps in the same QSFP-DD or OSFP form factor. This requires 224G SerDes on the host side, 3nm DSP silicon, and probabilistic constellation shaping at ~240+ Gbaud. The 1.6T ZR+ module will likely be the last generation that fits in a QSFP-DD cage before CPO architectures take over for extreme-density AI cluster applications.

Three practical implications for network architects:

• Fiber plant readiness. If your fiber plant supports the full C-band and flexible-grid DWDM, the upgrade from 800G ZR+ to 1.6T ZR+ is a module swap — no fiber changes, no MUX replacement.
• Switch power budget. A 1.6T module at 28 W in a 32-port switch pushes total optics power past 900 W. Verify switch power supplies and rack cooling.
• CFP2-DCO to QSFP-DD migration. If you still have CFP2-DCO in the network, plan migration now. New coherent innovation is happening exclusively in QSFP-DD and OSFP.
• Bottom line: The coherent pluggable has won. Dedicated transponder shelves are now a niche for ultra-long-haul and subsea — everything else converges on the router port. For network architects, this means coherent capability is no longer a separate transport layer. It is a port configuration choice — PAM4 or coherent, same cage, same switch.

APEX Group supplies the full coherent pluggable portfolio — 400G CFP2-DCO for existing transport platforms, 800G QSFP-DD ZR+ for current metro and regional DCI deployments, and 1.6T QSFP-DD ZR+ for next-generation capacity — with OpenZR+ compliance and multi-vendor interoperability across every generation.

APEX GROUP — www.apexallinone.com