Digital Coherent Optics Tutorial#

Michel Lanners, LU-CIX

• Why: Coherent pluggables (400ZR/800ZR-class) are becoming the default way to light DC-to-DC and metro dark fiber, and spec sheets for them are notoriously hard to read. This is a rare vendor-neutral, operator-led on-ramp to the technology.
• What: A full tutorial (grown out of a Euro-IX workshop, co-presented with Flexoptix and Arista) covering fundamentals, choosing optics from standards and datasheets, host compatibility, power and cooling, link/network design, commissioning, monitoring, troubleshooting, and deployment case studies.
• How: The material goes deep enough to be useful: NRZ→PAM4 trade-offs (half the spectrum, ~9.5 dB less SNR margin per eye, KP4 FEC RS(544,514)), coherent modulation tables (DP-QPSK for long-haul 100G/200G ZR+, DP-16QAM for 400ZR/800ZR/1600ZR), dual-polarization MIMO equalization tracking PMD and polarization transients, and the line-rate math (a 59.84 GBd 400ZR fits a 75 GHz DWDM slot; 800ZR at 118.2 GBd needs 150 GHz).
• Where applicable: Anyone buying or deploying coherent optics for router-to-router or DCI links, on any vendor's platform — the physics and the standards are the same. The form-factor/host-compatibility sections matter for mixed-estate DCs.