Attenuation and transmission windows

Light gets weaker as it travels through glass, and how much weaker depends on the wavelength — a curve with two humps and three valleys that decided which wavelengths the whole industry uses. This page explains where the loss comes from (scattering, absorption, the water peak), quantifies it per kilometre for each fibre type and window, adds the discrete losses of connectors and splices, and shows how the total turns into the "dB" column of every link budget.

Where the loss comes from

Mechanism	Physics	Wavelength dependence	Share at 1550 nm
Rayleigh scattering	density fluctuations frozen into the glass scatter light in all directions	∝ 1/λ⁴ — dominates at short wavelengths	~0.15 dB/km of the 0.2
Infrared absorption	silica lattice vibrations	rises steeply above 1 600 nm	small at 1550, closes the window beyond 1 700
OH⁻ (water) absorption	hydroxyl ions from manufacturing	peaks at 1 383 nm (and 1 240, 950 nm)	0 in low-water-peak fibre; up to 1–2 dB/km in old fibre
Metal-ion absorption	Fe, Cu, Cr impurities	—	negligible in modern fibre
Bending	macro- and microbends	grows with wavelength	design-dependent (Light in fibre)
UV absorption	electronic transitions	far below 800 nm	irrelevant for telecom

The minimum of the sum is at ≈ 1 550 nm (0.17–0.20 dB/km in the best fibre) — the reason long-haul and DWDM live there.

The windows

Window	Wavelength	Typical attenuation (SMF)	Typical attenuation (MMF)	Who uses it
1st, 850 nm	770–910	— (multi-mode only)	2.5–3.5 dB/km	VCSEL-based SR optics, 1000BASE-SX to 800G-SR8; SWDM
2nd, 1310 nm (O-band)	1 260–1 360	0.32–0.40 dB/km	0.6–1.5 (MMF at 1300)	LX/LR/DR/FR, CWDM low band, PON upstream, zero dispersion
E-band	1 360–1 460	0.35 (only low-water-peak fibre)	—	CWDM channels 1371–1451
S-band	1 460–1 530	0.25	—	PON XG-PON upstream? no — GPON downstream 1490, CWDM 1471–1511
3rd, 1550 nm (C-band)	1 530–1 565	0.18–0.22 dB/km	—	ER/ZR, DWDM, EDFA gain window, PON video/1550
L-band	1 565–1 625	0.20–0.25	—	DWDM extension, PON XG-PON downstream 1577, monitoring 1625
U-band	1 625–1 675	0.3+	—	OTDR monitoring of live systems

Band names and channel plans: Wavelength bands.

Attenuation by fibre type

Fibre	850 nm	1310 nm	1383 nm	1550 nm	1625 nm	Standard maximums
OM1 (62.5 µm)	3.5	1.5 (1300)	—	—	—	TIA-568
OM2–OM5 (50 µm)	3.0 (2.3 typ.)	1.5 (0.7 typ.)	—	—	—	TIA-568
OS1 (indoor tight buffer)	—	1.0	—	1.0	—	ISO 11801
OS2 / G.652.D (low water peak)	—	0.35 (0.4 max)	≤ 0.4	0.20 (0.3 max)	0.23	ITU-T G.652.D
G.652.B (older, water peak)	—	0.35	1–2	0.22	0.3	G.652.B
G.655 NZ-DSF	—	—	—	0.22	0.25	G.655
G.654.E ultra-low-loss	—	—	—	0.17	0.19	G.654
G.657 bend-insensitive	—	0.35	—	0.20	0.23	G.657

Field rule: use the cable manufacturer's specified maximum for design (0.35/0.22), the measured value for troubleshooting.

Discrete losses

Element	Typical	Design allowance	Notes
Connector pair, single-mode UPC, clean	0.1–0.3 dB	0.5 dB (TIA), 0.75 dB (old designs)	grade B connectors ≤ 0.25 dB typical
Connector pair, APC	0.1–0.3 dB	0.5 dB	plus return loss ≥ 60 dB
Connector pair, multi-mode	0.1–0.5 dB	0.75 dB	—
MPO-12/16 pair	0.2–0.5 dB (low-loss ≤ 0.35)	0.5–0.75 dB	per mated pair, all fibres
Fusion splice, single-mode	0.02–0.1 dB	0.1–0.3 dB	measured by OTDR
Fusion splice, multi-mode	0.05–0.2 dB	0.3 dB	—
Mechanical splice	0.2–0.5 dB	0.5 dB	temporary repairs
Dirty connector	0.5–3 dB	—	the most common loss of all (Plant faults)
1:2 splitter	3.5 dB	3.9 dB	each doubling adds ≈ 3.2–3.5 dB
1:32 splitter	17 dB	17.5–18 dB	PON (ODN classes)
CWDM mux (8-ch)	1.0–2.0 dB	2.5 dB	per pass; ×2 for mux + demux (CWDM budget)
DWDM AWG (40-ch)	3–5 dB	5–7 dB	per pass
Fixed attenuator	1–20 dB nominal	± 0.5 dB	to prevent receiver overload

Putting it together

Total loss = Σ(fibre length × dB/km) + Σ(connector pairs) + Σ(splices) + Σ(passive devices).

Example link	Fibre	Connectors	Splices	Devices	Total
10 km SMF at 1310 nm, 4 connector pairs, 4 splices	3.5	2.0 (design) / 0.8 (typ)	0.4 / 0.2	—	5.9 design / 4.5 typical
40 km SMF at 1550 nm, 4 pairs, 12 splices	8.8	2.0	1.2	—	12.0
300 m OM4 at 850 nm, 2 pairs	0.9	1.5	—	—	2.4
20 km GPON at 1490 nm, 1:32 split, 4 pairs, 6 splices	5.0	2.0	0.6	17.5	25.1 (class B+ 28 dB)
40 km CWDM at 1550 with mux/demux	8.8	2.0	1.0	5.0	16.8

Compare with the module's budget: Link budget engineering.

Wavelength matters twice

Loss per kilometre — a 1550 nm ER module loses 0.2 dB/km where a 1310 nm LR loses 0.35; over 40 km that is 6 dB of difference, the reason ER and ZR sit at 1550.
Bend sensitivity — losses from bends grow with wavelength; a plant that passes at 1310 can fail at 1550 or 1577 (XG-PON), and a 1625 nm test wavelength is used to find such bends (Testing).

In CodingBox

The module's DDM gives the two ends of the equation — Tx power here, Rx power there — and the difference, read on the bench with a known-good jumper and then in the rack, is the plant loss; comparing it with the table above tells you whether the plant is what the design assumed (DDM in the app, Rx power & budget).

Light that comes back instead of going forward — Fresnel reflection, reflectance and return loss per component and per technology: Reflections & return loss; how attenuation grows with age — hydrogen, water, radiation, microbending: Reliability & ageing.