WDM (Wavelength Division Multiplexing ) is a transmission technology that uses one optical fiber to simultaneously transmit multiple optical carriers of different wavelengths. The transmission loss in optical fiber varies with the wavelength of light. In order to reduce loss as much as possible and ensure transmission effect, it is necessary to find the most suitable wavelength for transmission. After a long time of exploration and testing, light in the wavelength range of 1260nm~1625nm has the lowest signal distortion and loss caused by dispersion, and is most suitable for transmission in optical fiber. https://www.glsun.com/article-p103-what-is-c-band-and-l-band-in-wdm.html