Detection that recovers amplitude, phase and polarisation — allowing far more bits per symbol and DSP compensation of fibre impairments.
Four Dimensions Instead of One
A direct-detect receiver has a photodiode, and a photodiode measures one thing: how much light arrived. Phase and polarisation are discarded at that point and cannot be recovered afterwards. A coherent receiver instead mixes the incoming signal with a local oscillator laser in an optical hybrid, so amplitude, phase and both polarisation states all survive into the electrical domain.
Two consequences follow, and the second matters more than the first. Data can be encoded in all four dimensions, and everything the fibre did to the field is still present as recoverable information.
Bits per Symbol, and What They Cost
Dual-polarisation QPSK carries four bits per symbol, DP-8QAM six and DP-16QAM eight. A 400G wavelength is DP-16QAM at 59.84 GBd; an 800G one is the same constellation at about 118.2 GBd. Denser constellations pack more into the same spectrum and demand more OSNR to be read correctly.
The published OpenZR+ modes make the price explicit: 12.5 dB for 100G DP-QPSK, 16 dB at 200G, 21 dB for 300G DP-8QAM, 24 dB for 400G DP-16QAM. Roughly 8 dB separates QPSK from 16QAM at the same symbol rate, and 8 dB of OSNR is several amplified spans.
The DSP Undoes the Fibre
Because the receiver has the full optical field, linear impairments can be inverted arithmetically. Chromatic dispersion is a known filter and its inverse is applied digitally; PMD is tracked adaptively as the fibre moves. A 400ZR module tolerates 2400 ps/nm and 10 ps of mean DGD without a single spool of compensating fibre in the route.
This is why long-haul design changed shape. Dispersion maps, compensation modules and their insertion loss disappeared from the span budget, and the remaining constraints became OSNR and fibre nonlinearity.
What It Costs in the Box
The DSP that does all this is a large, hot chip, and a tunable ITLA laser covering roughly 191.3 to 196.1 THz sits beside it. A 400G coherent pluggable draws about 18 W where a 400G DR4 draws around 9, and 800G coherent modules run 20 to 30 W.
That has practical consequences at the switch: not every port will host one, faceplate airflow matters, and the port map is worth checking before the design depends on it. See ZR vs ZR+ for how the standardised modes differ, and the 400ZR and OpenZR+ guide for the DCI case.
Coherent reference
| Recovers | amplitude, phase and both polarisation states |
|---|---|
| Bits per symbol | DP-QPSK 4, DP-8QAM 6, DP-16QAM 8 |
| 400G line rate | DP-16QAM at 59.84 GBd |
| 800G line rate | DP-16QAM at about 118.2 GBd |
| OSNR needed | 12.5 dB at 100G QPSK, 24 to 26 dB at 400G 16QAM |
| CD handled in DSP | 2400 ps/nm at 400ZR, far more at lower rates |
| DGD handled in DSP | 10 ps mean, 33 ps instantaneous |
| Laser | tunable ITLA, roughly 191.3 to 196.1 THz |
| Pluggable power | about 18 W at 400G, 20 to 30 W at 800G |
Related Terms
DCO and ACO, 400ZR and ZR+, OSNR, DWDM and chromatic dispersion.