For twenty years, moving 400G between two data centres 80 km apart meant buying a transponder. A separate chassis, its own power feed, its own management system, its own team who understood it. The router handed off a grey client signal, the transponder turned it into a coloured DWDM wavelength, and you paid for the privilege in rack units and operational overhead.
Coherent pluggables collapsed that. A 400ZR module is a QSFP-DD or OSFP transceiver that plugs straight into a router or switch port and emits a tuned DWDM wavelength with coherent detection built in. The transponder layer disappears. What replaces it is a decision: 400ZR or OpenZR+?
The short answer
- Choose 400ZR for point-to-point data centre interconnect up to around 120 km, where you control both ends and there is nothing but fibre and possibly an amplifier in between.
- Choose OpenZR+ when the light has to cross a ROADM, traverse multiple amplified spans, or run at 100G, 200G or 300G rather than a fixed 400G.
Both fit the same cages and draw from the same power budgets. The difference is in how far the signal survives and what it is allowed to cross on the way.
What makes a coherent optic different
Every optic covered in our guide to choosing 400G optics — SR, DR, FR, LR — uses direct detection. The transmitter switches light on and off, the receiver measures how much arrives, and that is the whole conversation. It is cheap and it works, but it throws away most of the information the light was carrying.
A coherent optic encodes data in the amplitude and the phase of the carrier, across two polarisations at once, then mixes the received signal against a local oscillator laser to recover all of it. A digital signal processor then undoes chromatic dispersion and polarisation mode dispersion electronically — the impairments that force direct-detect links to stop at 80 km.
That DSP is why a coherent module can cross hundreds of kilometres of ordinary ITU-T G.652 fibre with no dispersion compensation in the path at all. It is also why the module is more expensive, hotter and slower to lock than an LR4.
400ZR: the DCI specification
400ZR is an implementation agreement from the Optical Internetworking Forum. It was written for one job: carrying 400GbE between two routers up to about 120 km apart, over passive single-channel or amplified DWDM, with enough interoperability that modules from different manufacturers light up against each other.
Its choices all follow from that job. A single carrier at a fixed 400G line rate using dual-polarisation 16QAM. A concatenated forward error correction scheme — CFEC — chosen to be cheap to implement in silicon rather than to squeeze out the last decibel of coding gain. A power envelope that fits a pluggable cage. A tunable laser covering the C-band, so one part number serves every channel in the plan, which is the same inventory argument we make for tunable transceivers in DWDM networks.
What it deliberately does not do is survive a complicated optical line system. 400ZR assumes the path is simple. Put a couple of ROADMs and four amplified spans in front of it and the margin runs out.
OpenZR+: the same form factor, more reach
The OpenZR+ MSA took the 400ZR pluggable and asked what would happen if you spent the available power differently. The answer was oFEC — a stronger, higher-overhead forward error correction with considerably more coding gain — plus support for multiple line rates rather than one.
That buys three things:
- Multi-span, amplified, ROADM-based paths. OpenZR+ modules are specified to work through a real optical line system, not just a dark fibre pair. The MSA’s own interoperability testing carried 400G across roughly 430 km of multi-span metro fibre.
- Rate flexibility. 100G, 200G, 300G and 400G line rates, so you can trade capacity for distance on the links that need it rather than failing to close them at all.
- OTN framing. Inherited from Open ROADM, which matters if the transport layer between your sites speaks ITU-T G.709 rather than pure Ethernet.
The MSA’s promoter list — Arista, Cisco, Fujitsu, Innolight, Juniper, Lumentum and NTT Innovative Devices — tells you why interoperability is taken seriously here. These are the platforms the modules have to work in.
Side by side
| 400ZR | OpenZR+ | |
|---|---|---|
| Specified by | OIF implementation agreement | OpenZR+ MSA |
| Line rates | 400G | 100G, 200G, 300G, 400G |
| FEC | CFEC | oFEC (higher coding gain) |
| Target reach | Up to ~120 km | Regional and long-haul, multi-span |
| Optical line system | Point-to-point, passive or single amplifier | Amplified, multi-span, ROADM |
| Client framing | Ethernet | Ethernet, with OTN support |
| Typical use | Two data centres, dark fibre between them | Metro and regional transport, carrier networks |
| Form factors | QSFP-DD, OSFP | QSFP-DD, OSFP |
The thermal question nobody asks early enough
A coherent pluggable is the hottest thing you will ever put in a QSFP-DD cage. The DSP, the tunable laser and the local oscillator together push power consumption several times past an FR4 module in the same footprint, and that heat has to leave through the cage.
In practice this means switch vendors publish population rules for coherent modules: which ports accept them, how many per line card, what airflow direction is required, and sometimes a maximum ambient temperature that is lower than the switch’s general rating. None of that is an optics problem you can buy your way around — check it before you design the link, not after the modules arrive.
It is also the clearest argument for OSFP at these speeds. As our comparison of QSFP-DD vs OSFP sets out, OSFP’s slightly larger body and integrated heatsink exist precisely for modules like these.
Where coherent replaces a transponder, and where it does not
IP-over-DWDM is genuinely cheaper when it fits. You remove a chassis, its power draw, its spares holding and a management system. Our article on data centre interconnect covers the architecture, and the real TCO of 400G works through the numbers.
But coherent pluggables are not always the right answer:
- Under 80 km on dark fibre with few channels? A direct-detect ER4 or ZR4 module is cheaper, cooler and simpler. Our comparison of 100G ZR4 vs LR4 vs ER4 covers that end of the range, and for modest channel counts a passive CWDM or DWDM plan may be all you need.
- Very high channel counts on one fibre pair? A purpose-built line system with a proper optical control plane still beats a rack of router ports each managing its own wavelength.
- No optical engineering capability in the network team? Coherent pluggables move optical layer decisions into the routing team’s remit. That is a real organisational change, not just a procurement one.
Five checks before you order
- Confirm the host supports coherent modules on that specific port. Support is usually restricted by port group, line card and software version.
- Work the optical path end to end. Span loss, amplifier noise, filter penalties and channel spacing all decide whether the link closes — our guide to optical link budget covers the arithmetic, and coherent links need more of it, not less.
- Check the channel plan and spacing. 75 GHz spacing is increasingly common for 400G coherent, and your existing mux or ROADM may be built for a 50 GHz grid.
- Match the far end exactly. A 400ZR module and an OpenZR+ module can interoperate only in a mode both support. Do not assume.
- Get the coding right. The host has to accept the module before any of the optics matter — see transceiver coding explained.
Monitoring a coherent link once it is live
Coherent modules expose far more than transmit and receive power. Pre-FEC bit error rate, chromatic dispersion, optical signal-to-noise ratio and Q-factor are all readable from the module, and the pre-FEC BER is the single most useful number you will ever trend on a DWDM link. It degrades long before errors reach the traffic, which gives you weeks of warning rather than none. The same principles as our guide to spotting a failing fibre link before it drops apply, with better instrumentation.
Getting the right coherent optics
Carritech Optics supplies coherent and direct-detect modules across the full range. For 400G, our 400G QSFP-DD range covers DR4, FR4, LR4 and coherent variants, with the 400G transceiver range as a starting point. At 800G we stock ZR and ZR+ modules in both form factors — the CT-8QDD-ZR-P in QSFP-DD and the CT-8OSFP-ZR-P in OSFP — alongside the wider 800G OSFP and 1.6T OSFP ranges. If you are planning further ahead, 800G and 1.6T migration is the place to start.
Every module is coded for the platform it is going into and tested before despatch — the 22 checks a module passes explains what that involves — and carries a lifetime warranty, with UK and EU stock and support behind it.
Have OEM part numbers in hand? Our compatibility checker returns the tested Carritech equivalent in seconds. Or send us the two sites, the fibre route and what sits in between, and we will tell you whether 400ZR closes it or you need OpenZR+ — request a quote.
