wavelength division multiplexing
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.
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.