With the rapid development of the 5G network, the demand for network data transmission is increasing exponentially. As the underlying bearer network, the transmission capacity of the optical network is crucial to the development of the 5G network. A magic weapon for expanding the transmission capacity of an optical network is to keep digging deep into the available band resources of the optical fiber, that is, to continuously expand the width of the transmission path of the optical network. The wider the transmission path, the better the transmission capacity of the optical network. Recently, new bands of CE, Cpp, and C+L bands have emerged in optical networks, contributing to expanding the transmission capacity of optical networks. The following small series will tell you about these frequency bands of optical fiber.
Traditional band
Optical fiber communication, as the name suggests, is communication in which light is used as an information carrier and optical fiber is used as a transmission medium. However, not all light is suitable for fiber optic communications. The wavelength of light is different (it can be simply understood as light with different colors), and the transmission loss in the fiber is different. Light with high transmission loss cannot carry information in the optical fiber. After long-term research by scientists, it was first discovered that light with a wavelength of 850nm can be used as light for optical communication, and this band is also directly called the 850nm band. However, the transmission loss in the wavelength region of the 850nm band is relatively large, and there is no suitable fiber amplifier. Therefore, the 850nm band is only suitable for short-range transmission. Then, scientists explored the "low-loss wavelength region" light band, that is, light in the 1260nm~1625nm region, which is most suitable for transmission in optical fibers.
The 1260nm~1625nm region is further subdivided into five bands: O-band, E-band, S-band, C-band, and L-band.
O band: The wavelength range of the O band is: 1260nm~1360nm. The optical dispersion in this band causes the least signal distortion and the lowest loss, which is the early optical communication band. Hence the name O-band, where O stands for "Orignal".
E-band: The wavelength range of the E-band is: 1360nm~1460nm, and the E-band is the least common of the five bands. E stands for "extended". From the above diagram of the relationship between transmission loss and the optical band, it can be seen that there is an obvious irregular transmission loss bump in the E-band. This transmission loss bump is due to the absorption of light with wavelengths from 1370nm to 1410nm by hydroxide ions (OH-), resulting in a sharp increase in transmission loss. This bump is also known as the water peak.
Due to the limitation of early optical fiber technology, water (OH group) impurities are often left in the optical fiber glass fiber, resulting in the highest attenuation of the E-band light transmitted in the optical fiber, which cannot be used for normal transmission and communication. With the improvement of optical fiber processing technology, ITU-T G.652.D optical fiber appeared, which made the transmission attenuation of E-band light lower than that of O-band and solved the water peak problem of E-band light.
S-band: The wavelength range of the S-band is: 1460nm~1530nm. S stands for "short-wavelength". The transmission loss of S-band light is lower than that of the O-band, and it is often used for the downstream wavelength of PON (Passive Optical Network) systems.
C-band: The wavelength range of the C-band is: 1530nm~1565nm. C stands for "conventional". C-band light has the lowest transmission loss and is widely used in metropolitan area networks, long-haul, ultra-long-haul, and submarine cable systems. The C-band is also frequently used in WDM networks.
L-band: The wavelength range of the L-band is: 1565nm~1625nm. L stands for "long wavelength". The L-band light has the second lowest transmission loss. When C-band light is insufficient to meet bandwidth requirements, L-band light is used as a supplement for optical networks.
U-band: In addition to the above five bands, there is actually one more band that will be used, which is the U band. The wavelength range of the U-band is 1625nm~1675nm. U stands for "ultra-long-wavelength". U-band is mainly used for network monitoring.
CE / Cpp / C+L band: The commonly used wavelength bands for optical communication are: the wavelength range of 1529.16nm ~ 1560.61nm in the traditional C-band. The rookie band CE / Cpp / C+L mentioned here refers to the new band resources introduced by the current optical communication to expand the traditional C-band transmission resources.
From the previous analysis of traditional bands, it can be seen that in order to broaden the C-band used in optical communication, support can be sought from the adjacent short-wavelength band (S-band) and long-wavelength band (L-band). This is like, if you want to expand an existing road, you can only see if the wasteland on both sides of the road is available, and the road can be widened if there is a wasteland.
Next, let's take a look at the rookie bands CE / Cpp / C+L. What resources are borrowed from the S and L bands?
CE band: CE (C Extended) band is also called C+ band. What wavelength range is the CE band "+" compared to the C band? We can divide the C-band resources into 80 channels to transmit information, where each channel occupies a 0.4 nm band range resource. Therefore, the C-band is also known as the C80-band. The CE band borrows some wavelength resources from the L band (ie long wavelength band), and the wavelength range is extended to 1529.16nm ~ 1567.14nm. The CE band resources can be divided into 96 channels to transmit information, that is, the C96 band. The transmission capacity of the CE band is increased by 20% compared to the C band.
Cpp band: The Cpp (C++) band is also known as the C++ band. The Cpp band not only borrows wavelength resources from the L-band like the CE band, but also borrows resources from the S-band at the same time, and the wavelength range is extended to 1524.30nm ~ 1572.27nm. According to the resource division of the band range each channel occupies 0.4 nm, the band resource can be divided into 120 channels to transmit information. Therefore the Cpp band is also called the C120 band. The transmission capacity of the Cpp band is increased by 50% compared to the C band.
C+L band: C+L Band Literally, both C-band and L-band resources are used for optical communications. Similarly, according to the 0.4 nm band range resources occupied by each channel, there are three common transmission schemes in the C+L band as follows.
C120+L80: Cpp band (120 channels) + L band (80 channels), realizing 200-wave system. Among them, the L band is actually the L + band, and the wavelength range is 1575.16nm ~ 1617.66nm. The transmission capacity of the C120+L80 transmission scheme is 1.5 times higher than that of the C-band.
C96+L96: CE band (96 channels) + L band (96 channels), realizing a 192-wave system. The L band is actually the L++ band, and the wavelength range is 1575.16nm ~ 1626.43nm. The transmission capacity of the C96+L96 transmission scheme is more than double that of the C-band.
C120+L96: Cpp band (120 channels) + L band (96 channels), realizing a 216-wave system. The L band is actually the L++ band, and the wavelength range is 1575.16nm ~ 1626.43nm. The transmission capacity of the C120+L96 transmission scheme is approximately 2 times higher than that of the C-band.
In conclusion, scientists have expanded the available wavelength resources of optical fibers to a very large range. However, these band resources can be really applied to communication systems such as 5G, and are also affected by the following factors. Due to the limitation of optical components, for example, the following optical components cannot directly support the newly expanded wavelength range and need to be upgraded.
Erbium Doped Fiber Amplifier (EDFA)
Active devices such as modulators
Wavelength Selective Switch (WSS) Passive Devices
For the L-band, the transmission performance is degraded, which will increase the complexity of operation and maintenance, and thus increase the cost investment. The good news is that operators have made full use of existing optical fiber resources, expanded available optical fiber band resources, and improved transmission capacity. As the goal of future optical communication network development, some operators have begun to deploy Cpp band optical networks.n With the rapid advancement of technology, we will definitely see optical communication networks using C+L band solutions in the future.
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