WDM is a communication technology that combines a series of optical signals carrying information but different wavelengths into one bundle and transmits them along a single optical fiber. At the receiving end, the optical signals of different wavelengths are separated by some method. This technology can simultaneously transmit multiple signals on a single fiber. Each signal is transmitted by a specific wavelength of light. This is a wavelength channel.
In the same fiber, two or more optical wavelength signals are simultaneously transmitted through different optical channels, which is called optical wavelength division multiplexing (WDM). Optical wavelength division multiplexing includes frequency division multiplexing and wavelength division multiplexing. There is no significant difference between frequency-division multiplexing (FDM) technology and optical wavelength division multiplexing (WDM) technology, because light waves are part of electromagnetic waves, and the frequency of light has a single correspondence with wavelength. It is also generally understood that optical frequency division multiplexing refers to the subdivision of optical frequencies, and the optical channels are very dense. Optical wavelength division multiplexing refers to the coarse division of the optical frequency, and the optical multiples are far apart, even in different windows of the optical fiber.