In the early 1980s, the rise of fiber-optic communications, people think and first used in the optical fiber in the two low-loss window 1310nm and 1550nm window each transmits 1 optical wavelength signal, that is, 1310nm/1550nm two-wave WDM system, this system also has practical applications in China. The system is relatively simple, generally using fused wavelength-division multiplexing devices, insertion loss is small; there is no optical amplifier, in each relay station, the two wavelengths are demultiplexed and optical/electrical/optical regeneration relay, and then re-multiplexed together and transmitted to the next station. For a long time in people's understanding, WDM system is the wavelength interval of tens of nm system, such as 1310nm/1550nm two-wavelength system (interval of more than 200nm). Because in the conditions at that time, the realization of a few nm wavelength spacing is unlikely.

With the commercialization of 1550nm window EDFA, the application of WDM systems entered a new period. Instead of utilizing the 1310 nm window, people transmitted multiple optical carrier signals only in the 1550 nm window. Because these WDM systems have a narrow adjacent wavelength spacing (typically 1.6 nm) and work in a window sharing EDFA optical amplifiers, in order to differentiate them from the traditional WDM systems, people call this WDM system with tighter wavelength spacing a dense wavelength division multiplexing system. The term dense is used for the adjacent wavelength spacing. In the past, WDM systems were tens of nm wavelength spacing, but now the wavelength spacing is much smaller, only (0.8-2) nm, or even <0.8 nm. Dense wavelength-division multiplexing technology is in fact a specific manifestation of wavelength-division multiplexing. Because the interval of DWDM optical carriers is very dense, and thus must be selected using high-resolution WDM devices, such as planar waveguide-type or fiber grating-type and other new types of optical devices, and can no longer be used to utilize the fused WDM devices.

In DWDM long-haul fiber optic cable systems, multiple optical signals with small wavelength intervals can share EDFA optical amplifiers. Between two WDM terminals, one EDFA is used instead of multiple conventional electrically regenerative repeaters to simultaneously amplify multiple optical signals and extend the optical transmission distance. In the DWDM system, EDFA optical amplifier and ordinary optical/electrical/optical regenerative repeater will co-exist, EDFA is used to compensate for the loss of optical fiber, while the conventional optical/electrical/optical regenerative repeater is used to compensate for the signal distortion brought about by dispersion and noise accumulation.

Nowadays, people like to call DWDM system by WDM. Essentially, DWDM is only a form of WDM, WDM is more universal, DWDM lacks a clear and precise definition, and with the development of technology, the original so-called dense wavelength spacing, in the technical realization of more and more easily, has become less "dense". In general, if you do not specifically refer to the 1310nm/1550nm two-wavelength-division WDM system, people talk about the WDM system is the DWDM system.

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