Telecom wavelength bands (O-band, C-band, L-band)
The six standardized wavelength bands used in optical telecommunications: O, E, S, C, L, and U. Each is named for its spectral position; the band edges are tabulated in ITU-T G-series Supplement 39.
The ITU-T standardizes six wavelength bands for fiber-optic telecommunications, each spanning 35–100 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 largely removed 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 from 1471 to 1611 nm, spanning the S, C, and L 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.
The Photonics Unit Converter names the band of any wavelength and, in the S, C and L bands, the nearest channels of the ITU-T G.694.1 100 GHz and 50 GHz grids.
References: ITU-T G-series Recommendations, Supplement 39, Optical system design and engineering considerations. ITU-T Recommendation G.652, Characteristics of a single-mode optical fibre and cable. G. P. Agrawal, Fiber-Optic Communication Systems, 5th ed. (Wiley, 2021).