WDM

WDM (Wavelength-division multiplexing) is the basic technology combining multiplexing signals on laser beams at various infared (IR) wavelengths for transmission along fiber optic media. In early WDM systems, there were two IR channels per fiber. At the destination, the IR channels were demultiplexed by a dichroic (two-wavelength) filter with a cutoff wavelength approximately midway between the wavelengths of the two channels. It soon became clear that more than two multiplexed IR channels could be demultiplexed using cascaded dichroic filters, thus giving rise to coarse wavelength-division multiplexing (CWDM) and dense wavelength-division multiplexing (DWDM). 

WDM