The WDM answer to line-rate increase within the terrestrial fiber-based networks

The line-rate, supported by a single optical fiber, is likely to be extended in the near future beyond 2.5 Gbit/s. Considerable attention is being paid to the wavelength division multiplexed (WDM) approach. This is due to the fact that the terrestrial optical backbones predominantly contain standard, 1310-nm optimized, G.652 fibers, while signal wavelength near 1550 nm is preferentially used in order to take advantage of both the lowest fiber attenuation and the only one mature optical amplification technology (erbium-doped fiber amplification). This paper examines high-capacity line systems relying on a single optical channel. We discuss the limitation caused by chromatic dispersion for achieving long nonregenerated fiber spans at 10 Gbit/s over standard G.652 fibers. We also describe another source of capacity-dependent limitation, the polarization mode dispersion. The paper also deals with the wavelength division multiplexing alternative, using a channel rate of 2.5 Gbit/s. Among the new technical issues raised by the use of several wavelength-multiplexed channels (such as transmitters wavelength selection and stabilization, propagation through the line fiber, demultiplexing on the receiver side), we particularly stress on the WDM amplification topic which is seen to date as the most limiting and challenging issue.