Wavelength bands & channel plans

Every optical module lives at a wavelength, and every wavelength belongs to a named band with its own fibre loss, amplifier availability and typical uses. This page is the lookup table: bands, the wavelengths you will meet on module labels, and the WDM grids.

The named bands (single-mode)

Multi-mode fibre uses 850 nm (VCSEL) and, on legacy links, 1300 nm; its loss is ~3 dB/km, so reach is set by bandwidth rather than attenuation (Connectors & fibre).

Wavelengths you meet on labels

BiDi pairs

Single-fibre bi-directional modules use two wavelengths; the two ends must be the mirror pair (often labelled -U/-D or -A/-B):

PON wavelength plans (GPON, XGS-PON, NG-PON2, coexistence) are detailed in PON wavelength plans & coexistence.

CWDM grid (ITU-T G.694.2)

18 channels, 20 nm apart. Centre wavelengths in nm:

1271  1291  1311  1331  1351  1371  1391  1411  1431
1451  1471  1491  1511  1531  1551  1571  1591  1611

Common 8-channel systems use 1471–1611 nm; 16-channel systems add 1311–1451 on low-water-peak fibre. Colour codes, muxes and budget: CWDM channel plan, CWDM mux/demux & link budget.

DWDM grid (ITU-T G.694.1)

Defined in frequency, anchored at 193.1 THz. On the 100 GHz grid a channel number N maps to frequency as:

f (THz)  = 190 + N / 10          e.g. C31 → 193.1 THz → 1552.52 nm
N        = (f − 190) × 10
λ (nm)   = 299 792.458 / f (THz)

The 50 GHz grid interleaves half-channels (e.g. C31.5 = 193.15 THz). Channel labels vary by vendor — always confirm by frequency. Full treatment: DWDM and the ITU grid.

Amplification windows

CWDM channels outside the C/L bands cannot use EDFAs — the root of CWDM's reach limit.

Where the wavelength lives in memory

A module reports its nominal wavelength in the identity block — SFF-8472 A0h bytes 60–61 (nm), SFF-8636 upper page 00h bytes 186–187 (÷20 nm) — and tunable modules expose channel/frequency registers (Tunable memory maps). CodingBox shows it on the Check transceiver screen; when recoding, keep it equal to the laser's real wavelength.

How wavelengths are printed on labels and encoded in latch colours (CWDM channel colours, BiDi pairs): Labels & colour codes; the ITU-T recommendations behind the grids: Standards map.

Why these windows exist — Rayleigh scattering, absorption, the water peak — and attenuation per fibre type and wavelength: Attenuation and transmission windows.