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Wavelength division multiplexing (WDM) is a technology or technique modulating numerous data streams, i.e. optical carrier signals of varying wavelengths (colors) of laser light, onto a single optical fiber. WDM enables bi-directional communication as well as multiplication of signal capacity.
WDM is actually frequency division multiplexing (FDM) but referencing the wavelength of light as opposed to the frequency of light. However, since wavelength and frequency have an inverse relationship (shorter wavelength means higher frequency), the WDM and FDM terms actually describe the same technology – light in optical cable used to carry data and communication signals.
Wavelength division multiplexing systems can combine signals with multiplexing and split them apart with a demultiplexer. And with the proper fiber cable, the two can be done simultaneously; moreover, these two devices can also function as an add/drop multiplexer (ADM), i.e. simultaneously adding light beams while dropping other light beams and rerouting them to other destinations and devices. Formerly, such filtering of light beams was done with etalons, devices called Fabry–Pérot interferometers using thin-film-coated optical glass. The first WDM technology was conceptualized in the early 1970s and realized in the laboratory in the late 1970s; but these only combined two signals, and many years later were still very expensive.
As of 2011, WDM systems can handle 160 signals, which will expand a 10 Gbit/second system with a single fiber optic pair of conductors to more than 1.6 Tbit/second (i.e. 1,600 Gbit/s).
Typical WDM systems use single-mode optical fiber (SMF); this is optical fiber for only a single ray of light and having a core diameter of 9 millionths of a meter (9 µm). Other systems with multi-mode fiber cables (MM Fiber; also called premises cables) have core diameters of about 50 µm. Standardization and extensive research have brought down system costs significantly.
WDM systems are divided according to wavelength categories, generally course WDM (CWDM) and dense WDM (DWDM). CWDM operates with 8 channels (i.e., 8 fiber optic cables) in what is known as the “C-Band” or “erbium window” with wavelengths about 1550 nm (nanometers or billionths of a meter, i.e. 1550 x 10-9 meters). DWDM also operates in the C-Band but with 40 channels at 100 GHz spacing or 80 channels at 50 GHz spacing. Even newer technology, called Raman amplification, is using light in the L-Band (1565 nm to 1625 nm), approximately doubling these capacities.