Multiplying a fiber
Wavelength-division multiplexing sends many independent channels down one fiber. CWDM (G.694.2): 18 channels, 20 nm spacing, 1271–1611 nm, unamplified economics for metro access up to ~70 km. DWDM (G.694.1): 40–96+ channels at 100/50 GHz (and flex-grid) in the C-band, amplified by EDFAs every 60–100 km, powering backbone and submarine capacity.
The impairments a designer manages
Chromatic dispersion accumulates ~17 ps/nm·km on G.652 at 1550 nm — compensated by DCF/FBG in legacy 10G, absorbed by coherent DSP at 100G+. OSNR degrades with every amplifier; the design tracks an OSNR budget alongside the power budget. Non-linearities (SPM/XPM/FWM) cap per-channel launch power — more power is not always better. PMD matters on older fiber at high rates.
Coherent changed the rules
Modern 100/400/800G transponders use coherent detection: dispersion and PMD largely handled in silicon, reach set by OSNR and non-linear limits, capacity tunable (e.g., 400G short reach vs 100G long reach on the same card). Design becomes a table of reach vs modulation vs channel plan.
Commissioning a WDM span
Verify per-channel power flatness, OSNR per channel at the receive end, and pre-FEC BER margin under stress. Archive the channel plan and amplifier gain settings — the transport network's baseline records are as vital as the OTDR traces of the plant beneath it.