Telecom wavelength bands
The six standardized wavelength bands used in optical telecommunications: O, E, S, C, L, and U. Each is named for its spectral position and defined by ITU-T G.692.
The ITU-T standardizes six wavelength bands for fiber-optic telecommunications, each spanning approximately 40–50 nm in the 1260–1675 nm range. Bands are positioned at the silica fiber low-loss spectral window.
| Band | Name | Wavelength range | Common use |
|---|---|---|---|
| O | Original | 1260 – 1360 nm | Datacenter, short-reach (zero-dispersion of standard SMF) |
| E | Extended | 1360 – 1460 nm | Limited use (OH absorption peak in older fibers) |
| S | Short | 1460 – 1530 nm | Some CWDM, increasingly used for amplified links |
| C | Conventional | 1530 – 1565 nm | Long-haul DWDM (EDFA gain peak) |
| L | Long | 1565 – 1625 nm | Capacity extension to C-band |
| U | Ultra-long | 1625 – 1675 nm | Experimental, network monitoring |
Why these bands? The silica fiber loss spectrum has a broad minimum near 1550 nm (0.18–0.22 dB/km) and a secondary minimum near 1310 nm (0.32–0.38 dB/km). Between these, a historical OH absorption peak near 1380 nm produced excess loss that excluded the E-band from most telecom deployments. Modern "low water peak" fibers (ITU-T G.652.D) have eliminated this peak, opening the E-band for future use.
Why C-band dominates long-haul. Three factors converge:
- Lowest fiber loss — minimizes per-span attenuation
- EDFA gain window — efficient amplification with low noise figure
- Mature components — decades of optimization of lasers, modulators, detectors, and filters at 1550 nm
Why O-band dominates datacenter. Two factors:
- Zero dispersion of standard SMF at 1310 nm — no dispersion compensation needed for short-reach links
- Lower-cost lasers — uncooled DFBs in the O-band are cheaper than C-band cooled DFBs
DWDM and CWDM. The C-band hosts dense wavelength division multiplexing (DWDM) on 50 GHz or 100 GHz channel grids — typically 40 to 80 channels per fiber. CWDM (coarse WDM, 20 nm channel spacing) typically uses 8 channels spanning O, S, and C bands for shorter, simpler systems.
For DFB laser characterization across telecom bands, the per-band gain medium and packaging vary: InGaAsP/InP at C-band, InGaAsP/InP optimized for O-band, and AlGaInAs/InP for higher-T designs across bands.