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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)

Band	Range	Name	Loss (G.652.D)	Typical use
O	1260–1360 nm	Original	~0.33–0.35 dB/km	1310 nm LR/DR/FR optics, PON upstream, LAN-WDM, CWDM 1271–1351
E	1360–1460 nm	Extended	0.3–0.5 dB/km, water peak ~1383 nm	CWDM 1371–1451 on low-water-peak fibre only
S	1460–1530 nm	Short	~0.25 dB/km	CWDM 1471–1511, GPON downstream 1490, OSC 1510
C	1530–1565 nm	Conventional	~0.18–0.22 dB/km (minimum)	DWDM, EDFA band, 1550 nm ER/ZR, CWDM 1531/1551
L	1565–1625 nm	Long	~0.2–0.25 dB/km	L-band DWDM, XGS-PON downstream 1577, CWDM 1571–1611, NG-PON2
U	1625–1675 nm	Ultra-long	rising	monitoring / OTDR test wavelengths (1625, 1650)

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

Wavelength	Where
850 nm	SX, SR, SR4, SR8, AOC — multi-mode
1270 / 1330 nm	10G BiDi pair; XGS-PON upstream (1270)
1310 nm	LX, LR, DR, PSM4, FR — the single-mode workhorse
1295–1310 nm (4 λ)	LAN-WDM: 100GBASE-LR4/ER4, 400GBASE-FR8/LR8
1271–1331 nm (4 λ)	CWDM4, 40GBASE-LR4, 400GBASE-FR4/LR4
1490 nm	GPON downstream; 1G BiDi pair with 1310
1510 nm	optical supervisory channel in DWDM systems
1550 nm	EX/ZX, ER, ZR; RF video overlay in PON
1577 nm	XG-PON / XGS-PON / 10G-EPON downstream
1528–1565 nm	DWDM C-band channels
1596–1603 / 1524–1544 nm	NG-PON2 TWDM downstream / upstream

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):

Pair	Rate	Standard
1310 / 1490	1G	1000BASE-BX10
1310 / 1550	1G	1000BASE-BX (long)
1270 / 1330	10G	10GBASE-BX
1270 / 1330	25G	25G BiDi
1490 / 1550	1G/10G	vendor pairs

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)

Channel	Frequency	Wavelength
C17	191.7 THz	1563.86 nm
C21	192.1 THz	1560.61 nm
C31	193.1 THz	1552.52 nm
C41	194.1 THz	1544.53 nm
C51	195.1 THz	1536.61 nm
C61	196.1 THz	1528.77 nm

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

Amplifier	Band	Note
EDFA (C)	~1530–1565 nm	standard DWDM amplifier
EDFA (L)	~1570–1610 nm	doubles fibre capacity
Raman	any, pump-defined	distributed, extends spans
SOA	O-band and others	in-module pre-amplification

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.

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