The widespread deployment of single-mode fiber has encouraged the investigation of WDM on this transmission medium. In particular, developments concerned with single-mode fiber WDM transmission can be distinguished into two broad categories, namely coarse WDM (CDWM) and dense WDM (DWDM).

$\Rightarrow $ Although both categories use the same concept of multiple-wavelength channels on a single fiber, they differ in the channel spacing they employ. CWDM as implied by the terminology uses wider channel spacing and hence provides significantly fewer channels than DWDM.

$\Rightarrow $ Dense WDM was originally concerned with optical signals multiplexed in the 1.55 μ m wavelength region using the capabilities of erbium-doped fiber amplifiers (EDFAs) to increase system capacity and therefore to reduce system cost.

$\Rightarrow $ Figure 4.14 shows a block schematic for a DWDM system where a large number of channels N, each utilizing a single wavelength (i.e. from λ1 to λN), are multiplexed onto a single-fiber transmission medium.

$\Rightarrow $ Both the deployment of EDFAs and dispersion compensation are required for long-haul DWDM systems to offset any optical signal power losses caused by optical wavelength multiplexers and other passive optical devices. Finally, a wavelength demultiplexer distributes each channel to the corresponding receiver.

$\Rightarrow $ Dense WDM systems use narrow channel spacing and can therefore accommodate several hundred wavelength channels on a single optical fiber. The three possible channel spacing specified for DWDM systems are 1.6 nm (200 GHz), 0.8 nm (100 GHz) and 0.4 nm (50 GHz) while an even smaller channel spacing of 0.1 nm (12.5 GHz) is feasible in which case the system may also be referred to as super-DWDM.

DWDM has again advantages over general WDM:

• Channels are nearer together which leads to a higher possible capacity.

• If the different DWDM channels are chosen in a way that they lie all around the 1550 nm communication window, all channels can be amplified simultaneously using optical amplifiers as EDFA’s (erbium doped fiber amplifier). This saves additional equipment and therefore money.