Figure 1. Schematic diagram of a typical WDM 10Gb/s system. 40Gb/s transmission can be done over that system just by replacing 10Gb/s transponders by 40Gb/s ones (see text).

corresponding to that WDM channel. At the transmitter side, 10Gb/s optical signals from different WDM channels are first optically multiplexed by an optical WDM multiplexer. The WDM signal is then sent to a transmission fiber link. To compensate for the loss of the fiber, optical amplifiers (in most cases, these are Erbium-doped fiber amplifiers (EDFA), in some cases it is Raman-assisted EDFAs, and in some cases it could be pure Raman amplifiers) are installed after each span; a typical span length is 60-120 km. Also, to compensate for the chromatic dispersion of the transmission fiber, dispersion compensating fiber spools (also called dispersion compensating modules, DCM) are installed in between the stages of the in-line optical amplifiers. After transmission through the link, the WDM signal is optically demultiplexed and each WDM channel is detected by the receiver part of a corresponding transponder.

It is important to emphasize that during the previous technological transition from 2.5Gb/s to 10Gb/s ETDM rates, in addition to the obvious replacement of 2.5 Gb/s transponders by 10Gb/s ones, all the major parts of the optical transmission line had to be replaced (like optical amplifiers, multiplexers and demultiplexers) or even totally newly introduced (dispersion compensation fiber). Surprisingly enough, none of these parts will have to be replaced in order to migrate from 10Gb/s to 40Gb/s systems. Below we consider the technological advances that make this transition so simple for the carriers.