Source : Free On-Line Dictionary of Computing

wavelength division multiplexing
     
         (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)