Fiber Wavelengths & Bands
The wavelength map behind every optic label: the working wavelengths and the physics that picked them, the six ITU single-mode bands, and the ITU-T G.694.1 / G.694.2 channel grids for CWDM and DWDM. Band boundaries and grids are ITU-T; the working-wavelength facts are physics plus universal practice.
Working wavelengths
| Wavelength (nm) | Use | Why |
|---|---|---|
| 850 | Multimode window 1 (VCSEL) — SR optics | The low-cost window; MM loss ≈ 3 dB/km |
| 1300 | Multimode window 2 | Lower loss (≈ 1 dB/km), near zero dispersion |
| 1310 | Single-mode window (O-band) — LR optics, PON upstream | Zero-dispersion point of standard SMF |
| 1490 | GPON downstream (S-band) | Clear of 1310 upstream and 1550 overlay |
| 1550 | Single-mode window (C-band) — ER/ZR, DWDM | Lowest silica attenuation (≈ 0.2 dB/km); EDFA band |
| 1625 / 1650 | Live-fiber monitoring, in-service OTDR (U-band) | Out-of-band vs traffic; extra bend-sensitive, exposes stress |
ITU single-mode bands
| Band | Name | Range (nm) | Note |
|---|---|---|---|
| O | Original | 1260–1360 | Zero dispersion; PON upstream |
| E | Extended | 1360–1460 | Historic water-peak band; usable on G.652.D (OS2) |
| S | Short | 1460–1530 | GPON downstream (1490) |
| C | Conventional | 1530–1565 | Lowest loss; DWDM + EDFA amplifiers |
| L | Long | 1565–1625 | DWDM expansion band |
| U | Ultra-long | 1625–1675 | Monitoring only |
The two WDM grids
CWDM (ITU-T G.694.2): 18 channels, 1271–1611 nm on 20 nm spacing — coarse enough that lasers need no cooling, which is the entire economics of it. DWDM (ITU-T G.694.1) anchors at 193.1 THz in the C-band: the 100 GHz grid (≈ 0.8 nm) carries ~40 (up to ~48) channels and the 50 GHz grid (≈ 0.4 nm) roughly doubles that, with a flexible grid (12.5 GHz slot widths on 6.25 GHz centers (2012 revision)) for coherent systems. Channel counts vary by system design — treat the counts as capacity classes, not inventory. The PON plan rides the same map: GPON 1490 nm down / 1310 nm up; XGS-PON 1577 nm down / 1270 nm up — interleaved so both share one strand.
These bands explain optic names one page over — LR's 1310, ER's 1550, FR4's CWDM lanes — see the transceiver decoder, and the attenuation each band sees on the loss limits chart.
Common questions
Why does fiber use 850, 1310, and 1550 nm?
They sit at attenuation minima with hardware to match. 850 nm is where cheap VCSELs work — the multimode window. 1310 nm is standard single-mode fiber's zero-dispersion point. 1550 nm is silica's absolute loss minimum (~0.2 dB/km) and the band erbium amplifiers serve — which is why everything long-haul and DWDM lives there.
What is the difference between CWDM and DWDM?
Channel spacing and cost. CWDM spaces 18 channels 20 nm apart (1271–1611 nm) — wide enough for cheap uncooled lasers, typically 8–18 channels on metro links. DWDM packs channels 0.8 nm (100 GHz) or 0.4 nm (50 GHz) apart in the C-band with temperature-stabilized lasers — ~40 to ~96 channels, amplifiable with EDFAs. CWDM is the budget multiplier; DWDM is the capacity machine.
What is the water peak in fiber?
A hydroxyl (OH⁻) absorption spike near 1383 nm that historically made the E-band unusable. Modern low-water-peak fiber (ITU G.652.D — what OS2 cable contains) suppresses it, opening the full 1260–1625 nm range. It is the practical difference between legacy single-mode plants and anything pulled in the last two decades.
What wavelengths does GPON use?
GPON transmits downstream at 1490 nm and upstream at 1310 nm; XGS-PON uses 1577 down / 1270 up. The plans interleave deliberately so both generations coexist on one fiber through a wavelength splitter — which is also why PON plants are so reflection-sensitive and require APC (green) connectors throughout.
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