What is C band and L band in WDM (Wavelength Division Multiplexing)?

WDM (Wavelength Division Multiplexing) is a transmission technology that uses asingle optical fiber to simultaneously transmit multiple optical carriers of differentwavelengths in optical fiber communications. When the wavelength of light isdifferent, the transmission loss in the fiber is also different. In order to reduce thefiber loss, it’s important to find the most suitable transmission wavelength. After along period of exploration and testing, light with a wavelength of 1260nm~1625nmhas the smallest dispersion signal distortion with low loss, which is the most suitablefor transmission in optical fiber.

The wavelength of fiber can be divided into several bands, each band is used as anindependent channel to transmit predetermined wavelength. According to the ITU-Tstandards, singlemode fiber with the band over 1260nm is divided into O/E/S/C/L/Usix bands.

Band Description Wavelength range Bandwidth

O band original 1260-1360nm 100

E band extended 1360-1460nm 100

S band Short wavelength 1460-1530nm 65

C band conventional 1530-1565nm 40

L band Long wavelength 1565-1625nm 60

U bandUltralongwavelength

1625-1675nm 50

What is O band?The O band is the original band with wavelength from 1260 to 1360nm. The O-bandis the first wavelength band used in optical communications in history, and the signaldistortion (due to dispersion) is minimal.

What is E band?The E-band (extended wavelength band: 1360-1460 nm) is the least common ofthese bands. The E-band is mainly used for the expansion of the O-band, but it israrely used, mainly because many existing optical cables show high attenuation inthe E-band and the manufacturing process is very energy-intensive, so the use inoptical communication is limited.

What is S band?The optical fiber loss in the S-band (Short-wavelength Band, 1460-1530 nm) is lowerthan the loss in the O-band. The S-band is used as many PON (passive opticalnetwork) systems.

What is C band?The C-band (Conventional Band) ranges from 1530 nm to 1565nm and represents theconventional band. Optical fiber shows the lowest loss in the C-band and occupies alarge advantage in long-distance transmission systems. It is usually used in manymetropolitan areas combined with WDM, long-distance, ultra-long-distance andsubmarine optical transmission systems and EDFA technology. As the transmissiondistance becomes longer, and fiber optic amplifiers are used instead ofoptical-to-electronic-to-optical repeaters, the C-band becomes more and moreimportant. With the advent of DWDM (Dense Wavelength Division Multiplexing) thatallows multiple signals to share a single fiber, the use of the C-band has beenexpanded.

What is L band?The L-band (Long-wavelength Band, 1565-1625nm) is the second lowest-losswavelength band, and is often used when the C-band is insufficient to meet thebandwidth requirements. With the wide availability of b-doped fiber amplifiers(EDFAs), DWDM systems have expanded upward to the L-band, and were initiallyused to expand the capacity of terrestrial DWDM optical networks. Now, it has beenintroduced to submarine cable operators to do the same thing-to expand the totalcapacity of submarine cables.

Due to the its low transmission attenuation loss, C-band and L-band is usuallyselected to use in the DWDM system. Except for the O-band and L-band, there aretwo other bands, 850nm band and the U band (ultra-long band: 1625-1675 nm). The850nm band is the main wavelength of the multimode optical fiber communicationsystem, which combines VCSEL (Vertical Cavity Surface Emitting Laser). The Ufrequency band is mainly used for network monitoring.

WDM technology can be divided into WDM, CWDM, DWDM according to differentwavelength modes. The wavelength range stipulated by ITU for CWDM (ITU-TG.694.2) is 1271 to 1611nm, but considering the attenuation of the 1270-1470nmband in the application, the band of 1470~1610nm is usually used. The channelspace of DWDM is more closeness, so choose the C-band (1530 nm-1565 nm) andL-band (1570 nm-1610 nm) transmission windows. Ordinary WDM generally uses1310 and 1550nm wavelengths.

With the growth of FTTH applications, the C-band and L-band will play anincreasingly important role in optical transmission systems.

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