C+L band transmission
Carrying WDM channels in both the C-band (1530 to 1565 nm) and the L-band (1565 to 1625 nm) of one fiber, with separate amplifiers for each band. Raises capacity per fiber by using spectrum the C-band alone leaves idle.
Long-haul and submarine systems were built around the C-band because the conventional erbium-doped fiber amplifier provides gain there and fiber loss is near its minimum. Standard fiber is almost as transparent in the neighboring L-band, and erbium can amplify it too, so adding L-band channels is the most direct way to raise the capacity of a fiber already in the ground. By the ITU band definitions the C-band, 1530 to 1565 nm, spans 4.38 THz of optical frequency and the L-band, 1565 to 1625 nm, 7.07 THz; amplifiers in practice cover somewhat less than either definition, and a guard band is lost where the two meet. At the same spectral efficiency, C+L roughly doubles the capacity of a C-band system.
The L-band amplifier is a different design from the C-band one. Erbium's gain at longer wavelengths comes from a lower average inversion, so an L-band EDFA uses a much longer erbium-doped fiber, several times the C-band length, and converts pump power to signal power less efficiently. A C+L system therefore splits the signal into two bands at every amplifier site, amplifies each in its own amplifier, and recombines them, so each span carries two amplifiers and a pair of band filters, and the filters' edge sets the guard band.
The two bands also interact in the fiber. Stimulated Raman scattering transfers power from shorter to longer wavelengths across the whole occupied spectrum, and over a wide band the effect is large: C-band channels lose power to L-band channels, and the transfer changes whenever channels are added or dropped. Systems counter the resulting tilt with pre-emphasis of the launch powers, dynamic gain equalization, and in some designs distributed Raman amplification, and the transient response when one band fails is a design case in its own right.
The L-band has a separate history: on dispersion-shifted fiber, whose zero-dispersion wavelength lies in the C-band, four-wave mixing between WDM channels made the C-band nearly unusable, and the L-band's nonzero dispersion made it the practical choice. On standard fiber, C+L is now a routine option for new submarine and terrestrial line systems. Beyond it lie the S- and U-bands, which need thulium-doped or Raman amplifiers, and space-division multiplexing, which adds spatial paths instead of spectrum. The band definitions themselves are in telecom wavelength bands.
References: E. Desurvire, Erbium-Doped Fiber Amplifiers: Principles and Applications (Wiley, 1994); ITU-T G-series Supplement 39, Optical system design and engineering considerations.