Wavelength division multiplexing(WDM) is the combining of different optical wavelengths into one optical fiber. This comining or coupling of the wavelengths can be very useful in increasing the bandwidth of a fiber to couple the different wavelength inputs and one at the end of the fiber to decouple the wavelengths. Figure 1 shows a simple WDM system composed of multiple light sources, a multiplexer or combiner that combines the wavelengths into one optical fiber, and a demultiplexer or splitter that separates the wavelengths to their respective receivers.
WDM multiplexer and DWDM Multiplexing classifications are defined in this, Transmission Characteristics of Optical Components and Subsystems. Awide WDM device has channel wavelength spacing greater than or equal to 50nm. WDM devices typically separate a channel in one conventional transmission window such as 1310nm from another such as 1550nm. These types of devices are typically referred to as wideband or crossband. Figure 2 shows a basic wideband or crossband WDM system.
A coarse WDM(CWDM) device has channel spacing less than 50nm but greater than 1000Chz. However, these types of devices typically operate with 20nm spacing as defined. Spectral Grids for WDM Applications: CWDM Multiplexer: Wavelength Grid. lists the nominal central wavelengths for 20nm spacing. Note that the lower and upper wavelengths defined in we do not represent the end of either spectrum; these wavelengths are illustrative. Does not define the minimum or maximum spectral wavelengths.