WDM technology

The majority of optical networks are currently based on spectral multiplexing WDM
(Wavelength Division Multiplexing), is injected in a same fiber signals dedicated to several applicationsor different users and it performs a correspondence between the colors of beams light and interfaces Client ends
optical links.
Each node in the network, it has the possibility to extract or insert at channel of choice.
Recommendations of the ITU (International Telecommunications) define very precisely the characteristics of channels to use, including their central wavelength and width of each beam.

Two main types of WDM exist:
- The CWDM (Coarse WDM) is used to multiplex simple and inexpensive way to eighteen
lambdas on access networks, or metropolitan on a spectrum ranging from 1270 to 1610 nm.
The spacing between each lambda is important (20 nm), the temperature of each laser is not controlled and the wavelengths plants can therefore suffer no significant deviations from about 6 or 7 nm.
The scope of such a system has limited the lack of function of re-amplification, 80 km at most.
- The DWDM (Dense WDM) is a solution to transport of several dozen high lambdas
flow over long distances, the spacing between lambdas can vary from 1.6 nm (200 GHz) to 0.1 nm (12.5 GHz).
An enslavement of laser temperature necessary to avoid any deviation of the lengths
wave.
The use of electronics and photonics accuracy that results justify the cost differences between CWDM and DWDM.

The DWDM channels are located on the C and L bands, it is known technically amplifier, distances transmission can reach several thousand km.