Hello,
Typically, the following bands have been defined in Optical Industry:
O-Band: 1260-1360nm
E-Band: 1360-1460nm
S-Band: 1460-1530nm
C-Band: 1530-1565nm
L-Band: 1565-1625nm
U-Band: 1625-1675nm
Earlier fibers had lowest attenuation in O- and E-Bands so most of the systems were devised to work at 850 nm (Multimode) or 1310nm (Singlemode). In the last decade or so, extensive work was done on DWDM and optical fiber was also improved so that at 1550 nm it gave the lowest attenuation. But more important reason to use C-Band (and surrounding bandwidth) is that the attenuation curve is relatively flat here so all the wavelengths are amplified in almost the same manner.
However, as C-Band is limited to only 35 nm, only a certain number of wavelengths can fit here. Different Channel Spacings has been defined by ITU, like 200 GHz, 100 GHz, 50 GHz, 25 GHz and even 12.5 GHz. But most of the systems available today operate in 100 GHz or 50 GHz. Just remember that reducing the channel spacing, no doubt provides more room for additional wavelengths, but it also adds in the complexity of Muxing and Demuxing devices. Various technologies are available for Muxing but only a few can be used for channel counts of 100 or higher.
Hence once of the ways to add more channels to go beyond C-Band; L-Band is a good option as it lies just adjacent to C-Band and its attenuation is almost the same as for C-Band. However, as I mentioned earlier, EDFA can only operate in one band; for L-Band a different type of OA is required.
Due to above complexities, use of other bands in DWDM is limited.
And one more thing; current trend is NOT to add more channels, but to enhance the channel speed. It started with 1 Gbps and currently we have 10 Gbps, ,40 Gbps and 100 Gbps
Thanks
