wavelength division multiplexing
DWDM
WDM
(WDM) Multiplexing several Optical Carrier n signals on a single optical fibre by using different wavelengths (colours) of laser light to carry different signals.
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 and that ought to be called a "mudem" but isn't.
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 the road again. All they have to do is to upgrade the (de)multiplexors at each end. However these systems are expensive and complicated to run. There is currently no standard, which makes it awkward to integrate with older but more standard SONET systems.
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.
[Is "wave division multiplexing", as in "dense wave division multiplexing" (DWDM) just a trendy abbreviation?]