400ZR and ZR+ (coherent pluggables)

Until the late 2010s, coherent transmission lived in line cards and transponders, and a router reached another data center across town through a separate optical box. 400ZR put the whole coherent transceiver, the narrow-linewidth tunable laser, the dual-polarization IQ modulator, the coherent receiver and the DSP, into a QSFP-DD or OSFP module that plugs into the same port as a short-reach optic. The OIF published the implementation agreement in 2020 so that modules from different vendors interoperate.

The format is single-carrier dual-polarization 16QAM at 59.84375 GBd, 8 bits per symbol across the two polarizations, a gross line rate of 478.75 Gb/s that carries 400 Gb/s Ethernet plus the error-correction overhead. The FEC is a concatenated code (CFEC) with a hard-decision staircase outer code and a soft-decision Hamming inner code. Application codes cover amplified DWDM on 100 GHz and 75 GHz grids and a single-wavelength unamplified link; for the amplified case the target is 120 km or less, with at least 80 km expected in noise-limited conditions. On a 75 GHz grid, 400 Gb/s per channel is a spectral efficiency of 5.3 b/s/Hz.

The distance is set by OSNR rather than by dispersion. The DSP removes chromatic dispersion electronically, so no compensating fiber is needed; what limits reach is the noise that amplifiers add, against the OSNR the receiver needs for the FEC threshold, and the module's modest transmit power, a consequence of the pluggable's power budget.

"ZR+" is not one standard but a family of higher-performance modes in the same module formats. The most widely adopted is OpenZR+, a multi-source agreement that replaces CFEC with the stronger open FEC (oFEC) used in metro transponders and adds lower-rate modes (300, 200 and 100 Gb/s with QPSK and 8QAM) that trade capacity for reach, extending pluggable coherent links to hundreds of kilometers over multi-span ROADM networks. Vendors also offer higher-power variants whose stronger transmit signal survives the losses of ROADM filters. The same architecture continues at 800 Gb/s (800ZR) with higher symbol rates.

For the measurement side, the quantities that matter are the ones the DSP cannot hide: required OSNR at the FEC threshold, tolerance to CD and PMD across the specified range, transmitter laser linewidth and frequency accuracy on the grid, and interoperability of the DSP modes between vendors, which the OIF agreement exists to guarantee. What the coherent DSP does covers the processing chain in these modules.

References: OIF, Implementation Agreement 400ZR, OIF-400ZR-01.0 (2020) and revisions; OpenZR+ MSA, Technical Specification (2020).