WDM: Wavelength-division multiplexing, transmission of separate signals simultaneously at separate wavelengths through the same transmission medium, usually an optical fiber.
Multiplexing combines two or more signals for simultaneous transmission through the same medium. It was first used to increase capacity of 19th century telegraph wires. Later, frequency-division multiplexing divided the radio spectrum into separate broadcast channels. Each station was assigned a fixed transmission frequency, and listeners tuned the frequency of their receiver to select a station. Frequency-division multiplexing let cable television networks pack many video channels into frequency slots for transmission through copper cable.
Optics specialists think of wavelength rather than frequency, so when Bell Labs tried sending multiple laser wavelengths through the same hollow light pipe in the 1960s, they called it wavelength-division multiplexing. The appeal was understandable, and the Bell System tried WDM again in 1980 when it designed its first high-capacity fiber-optic system along the Northeast Corridor from Boston to Washington. But that system had a fatal flaw; it used multimode fibers, so every seven kilometers it needed a repeater to separate the wavelengths, detect the signals separately, amplify each one electronically, and drive separate transmitters that had to be multiplexed together. Single-mode fiber won hands down.
Wideband erbium-doped fiber amplifiers (EDFAs) revived interest in WDM because they could amplify many separate laser signals near 1550 nanometers. For long-haul, high-speed transmission, optical channels were packed close together for dense-WDM or DWDM systems. However, a committee from the International Telecommunications Union, apparently dominated by radio engineers, specified DWDM channels in frequency units. Typically 50 gigahertz wide, those channels initially transmitted 2.5 or 10 gigabits per second, and now can carry up to 100 GHz using coherent transmission.
Coarse-WDM (CWDM) came later, allowing the use of lower-cost multiplexing and demultiplexing optics in lower-speed, shorter-distance WDM systems, such as dividing 10-Gigabit Ethernet traffic among four lower-speed CWDM channels. Optical engineers apparently won on the ITU committee handling CWDM, because they specified CWDM channels in wavelength, setting center wavelengths every 20 nm from 1270 to 1610 nm.
CWDM and DWDM grids. The wavelengths for the DWDM grid are approximate; the specification are defined in terahertz.