A WDM system uses a multiplexer at the transmitter to join the several signals together and a demultiplexer at the receiver to split them apart. There are multiplexer(MUX) and demultiplexer(DEMUX). The main function of MUX is to combine multiple signal wavelengths in one fiber for transmission at the sending end. And the main function of the demultiplexer DEMUX is to separate multiple wavelength signals transmitted in an optical fiber at the receiving end https://www.glsunmall.com/fiber-optic-articles/what-s-the-difference-of-cwdm-dwdm-and-ccwdm-.html