Coarse wavelength division multiplexer (CWDM) is a low-cost WDM transmission technology for metro and access networks. In principle, CWDM is to use optical multiplexer to multiplex optical signals of different wavelengths into a single fibre for transmission. At the receiving end of the link, with the help of demultiplexer, the mixed signals in the fibre are decomposed into signals of different wavelengths and connected to the corresponding receiving equipment.