1.1 Wavelength-division Multiplexing
In fibre optic telecommunications, wavelength-division multiplexing (WDM) is a technology which multiplexes multiple optical carrier signals on a single optical fibre by using different wavelengths (colors) of laser light to carry different signals. This allows for a multiplication in capacity, in addition to making it possible to perform bidirectional communications over one strand of fiber. (Tomlinson & Lin, 2000, p.345-347)
1.2 Systems involve in Wavelength-division Multiplexing technology
A WDM system uses a multiplexer at the transmitter to join the signals together and a demultiplexer at the receiver to split them apart. With the right type of fiber it is possible to have a device that does both simultaneously, and can function as an optical add-drop multiplexer. The optical filtering devices used in the modems are usually etalons, stable solid-state single-frequency Fabry-Perot interferometers in the form of thin-film-coated optical glass. Figure1.1 shows the structure of a WDM system
Figure 1.1 Wavelength-division Multiplexing System (Tomlinson & Lin, 2000, p.349)
The concept was first published in 1970, and by 1978 WDM systems were being realized in the laboratory. The first WDM systems only combined two signals. Modern systems can handle up to 160 signals and can thus expand a basic 10 Gbit/s fibre system to a theoretical total capacity of over 1.6 Tbit/s over a single fibre pair. (Ishio et al, 2004, p.448-463)
1.3 Benefits of Wavelength-division Multiplexing technology
WDM offers an attractive solution to increasing LAN bandwidth without disturbing the existing embedded fiber, which populates most buildings and campuses, and continue to be the cable of choice for the near future. By multiplexing several relatively coarsely spaced wavelengths over a single, installed multimode network, the aggregate bandwidth can be increased by the multiplexing factor.
WDM systems are popular with telecommunications companies because they allow them to expand the capacity of the network without laying more fiber. By using WDM and optical amplifiers, they can accommodate several generations of technology development in their optical infrastructure without having to overhaul the backbone network. Capacity of a given link can be expanded by simply upgrading the multiplexers and demultiplexers at each end. (Ishio et al, 2004, p.448-463)
1.4 Coarse and Dense WDM
WDM systems are divided into two market segments, coarse and dense WDM. Systems with more than 8 active wavelengths per fibre are generally considered Dense WDM (DWDM) systems, while those with fewer than eight active wavelengths are classed as Coarse WDM (CWDM). CWDM and DWDM technology are based on the same concept of using multiple wavelengths of light on a single fibre, but the two technologies differ in the spacing of the wavelengths, number of channels, and the ability to amplify signals in the optical space.
1.4.1 Coarse WDM
One common meaning for Coarse WDM meant two (or possibly more) signals multiplexed onto a single fiber, where one signal was in the 1550-nm band, and the other in the 1310-nm band. The main characteristic of the recent ITU CWDM standard is that the signals are not spaced appropriately for amplification by EDFAs. This therefore limits the total CWDM optical span to somewhere near 60 km for a 2.5 Gb/s signal, which is more than enough for the vast majority of metropolitan applications. The significantly reduced optical requirements also allow the associated costs of CWDM to be greatly reduced to very nearly that of non-CWDM optical components.
CWDM is also being used in cable television networks, where different wavelengths are used for the downstream and upstream signals. In these systems, the wavelengths used are often widely separated, for example the downstream signal might be at 1310 nm while the upstream signal...