The Free On-line Dictionary of Computing (30 December 2018):

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?]

   (2002-07-16)