Wavelength Division Multiplexing (WDM) is a technology that transmits multiple wavelengths of optical signals simultaneously in a single fiber. Compared with single-wavelength technology, WDM can save more fiber resources. The basic principle of WDM is to use a wavelength division multiplexer (combiner) at the transmitter side to combine different wavelengths of information optical carriers together into one fiber for transmission, and at the receiver side, another wavelength division multiplexer (splitter) to separate different optical carriers.

　　WDM systems all work in the 1550nm window. Quartz fiber has three bands available in the 1550 wavelength region, namely S-band, C-band and L-band. The S-band wavelength range is from 1460 to 1530nm, the C-band wavelength range is from 1530 to 1565nm, and the L-band wavelength range is from 1570 to 1605nm.

　　Wavelength division multiplexing (WDM) with 1310nm/1550nm window is used for access networks, but rarely for long-distance transmission. dense wavelength division multiplexing (DWDM) with 1550nm window: can be widely used for long-distance transmission and used to build all-optical networks. because the multiplexing of 1310/1550nm is beyond the gain range of EDFA, it is only applied in some specialized occasions, so it is often used WDM is a broader name to replace DWDM. 1.25G DWDM and 10G DWDM optical modules are currently used in long-distance transmission of backbone networks.

　　WDM technology can make full use of the huge bandwidth resources of optical fiber, in a single fiber, WDM transmission capacity is single wavelength several times, tens of times, hundreds of times. In addition, WDM technology signal transparent transmission, the channels of each wavelength are independent of each other, can transmit signals with completely different characteristics and rates, to complete the integrated transmission of various telecommunications services signals, such as PDH signals and SDH signals, digital signals and analog signals, mixed transmission of multiple services (audio, video, data, etc.), etc.