Coherent optics & long-haul design

Beyond about 80 km at 10G — and for any 100G+ wavelength — DWDM links stop being a matter of power budget alone. Dispersion, noise accumulation and modulation format take over, and the technology that handles them is coherent transmission, now available in pluggable modules.

Direct detect vs coherent

Direct detect (10G DWDM SFP+/XFP)	Coherent (100G–800G)
Modulation	on/off keying	QPSK, 8/16/64-QAM with polarisation multiplexing
Receiver	photodiode	local-oscillator laser + DSP
Chromatic dispersion	tolerates ~1 600 ps/nm (≈80–100 km)	compensated digitally — thousands of km
PMD	limits long spans	compensated digitally
Reach without regeneration	~80 km, more with DCM/EDFA	120 km (400ZR) to >1 000 km (transport)

The two limits of a long link

Chromatic dispersion — different wavelengths within the signal travel at

slightly different speeds and smear the pulse. ~17 ps/nm/km in the C-band on G.652. Direct-detect 10G runs out near 80–100 km; DCMs extend it; coherent removes the limit.

OSNR — every EDFA adds noise. The optical signal-to-noise ratio at the

receiver must exceed the format's requirement (roughly: 10G OOK ~20 dB; 100G QPSK ~13–15 dB; 400G 16-QAM ~24 dB, all per 0.1 nm). Higher-order modulation carries more bits but needs more OSNR — hence 400ZR reaches ~120 km while 100G QPSK on the same plant goes much further.

Coherent pluggables

400ZR (OIF) — 400G 16-QAM in QSFP-DD/OSFP, designed for amplified

point-to-point DCI up to ~120 km, 75/100 GHz grid.

ZR+ / OpenZR+ — the same modules with stronger FEC and additional modes

(100G/200G/300G/400G, QPSK to 16-QAM) for metro/regional reach of several hundred km.

Tunable across the C-band, managed through CMIS

(frequency, output power, modulation/application selection).

These modules draw 15–20 W and are the hottest optics in a router — thermal design of the host matters (CMIS issues).

Designing a span

Loss budget per span with mux/demux, connectors and fibre

(the same arithmetic as CWDM, with EDFA gain added).

Amplifier placement every ~80–120 km; keep launch power per channel within the

nonlinear limit (typically −3…+3 dBm/ch).

OSNR budget end-to-end for the chosen modulation; add margin for ageing.
Dispersion — DCM for 10G direct-detect; none needed for coherent.
Channel plan on the ITU grid, with guard

channels for the OSC.

In CodingBox

Coherent and 10G DWDM pluggables are read like any module: identity, wavelength/channel, DDM and — for tunables — the channel setting itself. CodingBox sets the channel on tunable SFP+/QSFP modules and reads CMIS-managed coherent modules' state and applications, giving the bench view of what will go into the line system.