As DWDM systems send signals from several sources over a single fiber, they must be able to combine the incoming signals. This is done with a multiplexer, which takes optical wavelengths from multiple fibers and converges them into one beam. At the receiving end, the system must be able to separate out the components of the light so that they can be discreetly detected. Demultiplexers perform this function by separating the received beam into its wavelength components and coupling them to individual fibers. Demultiplexing must be done before the light is detected, because photo-detectors are inherently broadband devices that cannot selectively detect a single wavelength.
Multiplexers and demultiplexers can be either passive or active in design. Passive designs are based on prisms, diffraction gratings or filters, while active designs combine passive devices with tunable filters. The primary challenges in these devices is to minimize cross-talk and maximize channel separation. Cross-talk is a measure of how well the channels are separated, while channel separation refers to the ability to distinguish each wavelength
