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Silica, as a fabrication material of fiber, has two major low-loss windows, i.e., the windows at 1.3um and 1.55um. In bandwidth, the 1.3um window covers 80nm, and the 1.5um window covers 180nm. Thus, in terms of optical frequency, each fiber has a total bandwidth of about 35,000 GHz.
Specifically, under DWDM, the 1.55um window is further divided into three sub-bands, including C-band, L-band, and S-band. C-band stands for conventional band, ranging from 1530nm to 1565nm, covered by the amplification bandwidth of conventional EDFA optical amplifiers. The band from 1565nm to 1625nm, which consists of wavelengths longer than those in the C-band, is called the L-band and today being used in high-capacity WDM system, with the deployment of gain-shifted EDFA amplifiers. Lastly, the band below 1530nm, consisting of wavelengths shorter than those in the C-band, is called S-band, which is amplified by fiber Raman amplifiers.
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