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With the rapid development of 5G network, the demand for network data transmission increases exponentially. As the underlying bearer network, the transmission capacity of optical network is very important for the development of 5G network.

One of the magic weapons to expand the transmission capacity of optical networks is to constantly dig deep into the available band resources of optical fibers, that is, to continuously expand the transmission path width of optical networks. As the transmission path widened, the transmission capacity of the optical network naturally increased.

Recently, optical networks have emerged new bands CE, Cpp, C+L bands, which have increased the transmission capacity of optical networks.

The following small series will talk to you about these bands of optical fiber.

Traditional band optical fiber communication is, as its name implies, 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. Different wavelengths of light (which can be simply understood as different colors of light), the transmission loss in the optical fiber is different. Light with large transmission loss cannot carry information in optical fiber transmission.

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 wavelength band is also directly referred to as the 850nm band. However, the transmission loss in the wavelength region of 850nm is relatively large, and there is no suitable fiber amplifier. Therefore, the 850nm band is only suitable for short-range transmission.

Later, scientists explored the "low-loss wavelength region" optical wavelength band, that is, light in the region of 1260nm to 1625nm, which is most suitable for transmission in optical fibers. Transmission loss and optical wavelength relationship is shown in the figure below.

The 1260nm~1625nm region is subdivided into five bands: O band, E band, S band, C band and L band.

O-band O-band wavelength range: 1260nm~1360nm. This band of optical dispersion results in the smallest signal distortion and the lowest loss, which is the early optical communication band. Therefore, it is named O-band, where O stands for "Orignal."

E band E band wavelength range: 1360nm~1460nm, E band is the least common band of the five bands. E means "extended." From the transmission loss vs. optical wavelength graph above, it can be seen that there is a distinct irregular transmission loss bump in the E band. This transmission loss bump is due to the absorption of hydroxyl ions (OH-) in the wavelength of 1370nm~1410nm, which leads to 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 residual in optical fiber glass fiber, resulting in the highest attenuation of E-band light transmission in optical fiber, which cannot be normally used for transmission communication.

With the improvement of fiber processing technology, ITU-T G.652.D fiber appeared, which made the transmission attenuation of E band light lower than that of O band, and solved the problem of water peak of E band light.

S-band S-band wavelength range: 1460nm~1530nm. S stands for "short-wavelength." The transmission loss of S-band light is lower than that of O-band light and is often used for downstream wavelengths of PON (Passive Optical Network) systems.

The wavelength range of C-band is 1530nm~1565nm. C means "conventional." C-band light has the lowest transmission loss and is widely used in metropolitan area networks, long-distance, ultra-long-distance and submarine cable systems. C-band is also often used in wavelength division networks.

The wavelength range of L band is 1565nm~1625nm. L stands for "long-wavelength." 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.

In addition to the above five bands, there is actually another band that will be used, that is, the U band. The wavelength range of U-band is 1625nm~1675nm. U stands for "ultra-long-wavelength." U-band is mainly used for network monitoring.

Here's a summary of these traditional bands.

CE / Cpp / C+L wavelength band optical communication commonly used bands are: traditional C band 1529.16nm ~ 1560.61nm wavelength range. The new band CE / Cpp / C+L mentioned here refers to the new band resources introduced by current optical communication to expand the traditional C band transmission resources.

As can be seen from the conventional wavelength band analysis above, in order to broaden the C band used for optical communication, support can be sought from the adjacent short wavelength band (S band) and the 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 usable. If there is wasteland, you can widen the road.

Let's look at the rookie band 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 does CE band "+" have compared to C band? We can divide the C-band resources into 80 channels to transmit information, where each channel occupies 0.4 nm of the band range resources. Therefore, the C band is also known as the C80 band. CE band borrows part of the wavelength resources of L band (i.e. long wavelength band), and the wavelength range is extended to 1529.16nm ~ 1567.14nm. CE band resources can be divided into 96 channels to transmit information, i.e. C96 band. The transmission capacity of CE band is increased by 20% compared with C band.

The Cpp (C plus plus) band is also known as the C++ band. Cpp band borrows wavelength resources from L band as CE band does, and borrows wavelength resources from S band at the same time. The wavelength range extends to 1524.30nm ~ 1572.27nm. According to the division of band resources occupied by 0.4 nm for each channel, the band resources can be divided into 120 channels for transmitting information. The Cpp band is also known as the C120 band. The transmission capacity of Cpp band is increased by 50% compared with 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 band range resources occupied by 0.4 nm for each channel, there are three common transmission schemes in C+L band as follows.

C120+ L80: Cpp band (120 channels)+L band (80 channels), achieving a 200-wave system. The L band is actually L + band, and the wavelength range is 1575.16nm ~ 1617.66nm. The transmission capacity of C120+ L80 transmission scheme is increased by 1.5 times compared with C-band.

C96+L96: CE band (96 channels)+L band (96 channels), achieving a 192-wave system. The L band is actually L++ band, and the wavelength range is 1575.16nm ~ 1626.43nm. The transmission capacity of C96+L96 transmission scheme is more than double that of C-band.

C120+L96: Cpp band (120 channels)+L band (96 channels), achieving a 216-wave system. The L band is actually L++ band, and the wavelength range is 1575.16nm ~ 1626.43nm. The transmission capacity of C120+L96 transmission scheme is increased by about 2 times compared with C-band.

Finally, a picture shows these three rookie bands.

In summary, scientists have expanded the wavelength resources available to optical fibers to a very large range. However, these band resources can be really applied to communication systems such as 5G, which are also affected by the following factors.

The limitations of photodetectors, such as the following photodetectors, 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 Device

For L-band, transmission performance degradation will increase operation and maintenance complexity, thus increasing cost investment.

Fortunately, operators have made full use of existing optical fiber resources, expanded optical fiber available band resources, and improved transmission capacity as the goal of future optical communication network development. At present, some operators have begun to deploy Cpp band optical networks.

With the rapid development of technology, we will definitely see optical communication networks using C+L band solutions in the future.

This article comes from Weixin Official Accounts: ZTE Document (ID: ztedoc), Author: ZTE Document

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