Coarse Wavelength Division Multiplexing (CWDM) is a low-cost WDM transmission technology for the access layer of the metropolitan area network. Mainly divided into four types.

1. Coarse wavelength division multiplexer (CWDM for short), currently there are 1*4 channel coarse wavelength division multiplexer, 1*8 channel coarse wavelength division multiplexer, 1*16 coarse wavelength division multiplexer, 1*18 Coarse wavelength division multiplexer. Customers can choose coarse wavelength division multiplexers with different channels according to their own wavelength needs. According to the different packaging forms, it is divided into mini coarse wavelength division multiplexer (MINI CWDM for short), plug-in box type coarse wavelength division multiplexer, and chassis type coarse wavelength division multiplexer.

2. Three-port wavelength division multiplexer (FWDM for short), can also be called three-wavelength wavelength division multiplexer. Three-wavelength wavelength division multiplexer refers to a wavelength division multiplexer device that fixes three specific wavelengths (1310/1490/1550). The single-wavelength wavelength division multiplexer is the simplest one in the series of wavelength division multiplexers. The structure is the same as the three-wavelength (FWDM). It refers to a total of three ports, namely the transmission end, the reflection end and the common end. The wavelength is Wavelengths other than 1310/1490/1550nm.

3. Pull cone wavelength division multiplexer (abbreviated as FBT WDM), which realizes the separation or synthesis of two wavelengths. At present, the most commonly used wavelength is 1310nm and 1550nm wavelength division multiplexing.

4. The main function of OADM is to drop or insert one or more wavelengths from a multi-wavelength channel. There are two types: fixed and reconfigurable. The fixed type can only add one or more fixed wavelengths, and the routing of the node is determined; it lacks flexibility, but has reliable performance and small delay. The reconfigurable type can dynamically adjust the wavelength of the upper and lower channels of the OADM node, and can realize the dynamic reconstruction of the optical network, so that the wavelength resources of the network can be well allocated. However, the structure is complex and can be constructed with various filter components such as arrayed wave guides and fiber gratings. Structural OADMs can also be constructed entirely with optical fiber technology to construct all-fiber OADMs.