In the same fiber, two or more optical wavelength signals transmit information through different optical channels at the same time, which is called optical wavelength division multiplexing technology, or
WDM for short. Optical wavelength division multiplexing includes frequency division multiplexing and wavelength division multiplexing. There is no obvious difference between optical 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. Usually it can also be understood in this way, 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 rough division of optical frequencies, and the optical channels are far apart, even in different windows of the optical fiber.
Optical wavelength division multiplexing generally uses wavelength division multiplexers and demultiplexers (also called multiplexers/demultiplexers) to be placed at both ends of the fiber to realize the coupling and separation of different light waves. The principle of these two devices is the same. The main types of optical wavelength division multiplexers are fused taper type, dielectric film type, grating type and flat type. Its main characteristic indicators are insertion loss and isolation. Usually, the increase in the loss of the optical link is called the insertion loss of the wavelength division multiplexing due to the use of wavelength division multiplexing equipment in the optical link. When the wavelengths 11 and l2 are transmitted through the same fiber, the difference between the power at the input end l2 of the demultiplexer and the power mixed in the fiber at the output end of 11 is called isolation.
