From:eefocus.com

WDM, the full English name is wavelength division multiplexing. It is a technology that combines a variety of optical signals with different wavelengths through a combiner and couples them to the same optical fiber for data transmission.
wavelength × Frequency = speed of light (constant value), so wavelength division multiplexing is actually frequency division multiplexing
In short, we can also think of WDM as a highway - different types of vehicles flow into the highway and go their separate ways when they arrive at their destination.
The role of WDM is to improve the transmission capacity of optical fiber and improve the utilization efficiency of optical fiber resources.
In the early days, the technical conditions were limited, and the wavelength interval would be controlled at tens of nm. This relatively decentralized wavelength division multiplexing is called sparse wavelength division multiplexing, that is, CWDM (coarse WDM).
Later, the technology became more and more advanced, and the wavelength spacing became shorter and shorter. At the level of several nm, it became a compact WDM, which is called dense wavelength division multiplexing (DWDM)
The wavelength interval of CWDM is 20nm, the wavelength range is from 1270nm to 1610nm, and there are 18 bands. Because there is an obvious increase in attenuation in the 1270-1470nm band, many old optical fibers can not be used normally, so CWDM generally gives priority to 8 bands of 1470-1610nm. The wavelength interval of DWDM can be 1.6nm, 0.8nm, 0.4nm and 0.2nm, and can accommodate 40, 80 and 160 waves (up to 192 waves). The wavelength range of DWDM is 1525nm to 1565nm (C-band) and 1570nm to 1610nm (L-band).

Mwdm is medium wavelength division multiplexing. This is the favorite of China Mobile, which was proposed together with its semi-active prequel scheme (also known as open WDM).
At present, 5g forward transmission requires at least 12 wavelength channels. Therefore, the schemes of the three operators aim to achieve 12 waves. The principle of mwdm is to reuse the first 6 waves of 25g CWDM. By adding tec (thermal electronic cooler) temperature control, the left and right wavelengths are shifted by 3.5Nm to form 12 wavelengths