Wavelength division multiplexing

Dense wavelenght division multiplexing (DWDM) is generally held to be WDM with more than 8 active wavelengths per fibre.

The device that joins the signals together is known as a multiplexor, and the one that splits them apart is a demultiplexor. With the right type of fibre you can have a device that does both.

The first WDM systems combined two signals and appeared around 1985. Modern systems can handle up to 128 signals and can expand a basic 9.6 Gbps fibre system to a capacity of over 1000 Gbps.

WDM systems are popular with telecommunications companies because they allow them to expand the capacity of their fibre networks without digging up roads again more than necessary which is extremely costly. All they have to do is to upgrade the (de)multiplexors at each end. However these systems are expensive and complicated to run.

The introduction of the ITU-T G.694.1 frequency raster in 2002 has made it easier to integrate WDM with older but more standard SONET systems. (I don't have the details to hand, but I believe it specifies a 200 GHz frequency raster[?], with 100 GHz channel spacing as a refinement)

Note that this term applies to an optical carrier (which is typically described by its wavelength), whereas frequency division multiplexing typically applies to a radio carrier (which is more often described by frequency). However, since wavelength and frequency are inversely proportional, and since radio and light are both forms of electromagnetic radiation, the distinction is somewhat arbitrary.

See also time division multiplexing, code division multiplexing.

Based on FOLDOC