The Coarse Wavelength Division Multiplexer (CWDM) is a low-cost WDM transmission technology for metro and access networks. In principle, CWDM uses an optical multiplexer to transmit optical signals of different wavelengths into a single optical fibre for transmission. At the receiving end of the link, the mixed signals in the optical fibre are decomposed into signals of different wavelengths with the help of optical demultiplexer, which is connected to the corresponding receiving device. Using thin film filter technology, it has characteristics such as high isolation and high reliability.